Understanding Neutron Radiography Reading II-TNR of Materials

Understanding 

Neutron Radiography 

Reading II 

My ASNT Level III, 

Pre-Exam Preparatory 

Self Study Notes 

27 June 2015 

Charlie Chong/ Fion Zhang 

http://homework55.com/apphysicsb/ap5-28-08/

Nuclear Applications 

Charlie Chong/ Fion Zhang

Nuclear Applications 


The Magical Book of Neutron Radiography 



ASNT Certification Guide 

NDT Level III / PdM Level III 

NR - Neutron Radiographic Testing 

Length: 4 hours Questions: 135 

1. Principles/Theory 

• Nature of penetrating radiation 

• Interaction between penetrating radiation and matter 

• Neutron radiography imaging 

• Radiometry 

2. Equipment/Materials 

• Sources of neutrons 

• Radiation detectors 

• Non-imaging devices 


3. Techniques/Calibrations 

• Blocking and filtering 

• Multifilm technique 

• Enlargement and projection 

• Stereoradiography 

• Triangulation methods 

• Autoradiography 

• Flash Radiography 

• In-motion radiography 

• Fluoroscopy 

• Electron emission radiography 

• Micro-radiography 

• Laminography (tomography) 

• Control of diffraction effects 

• Panoramic exposures 

•Gaging 

• Real time imaging 

• Image analysis techniques 


4. Interpretation/Evaluation 

• Image-object relationships 

• Material considerations 

• Codes, standards, and specifications 

5. Procedures 

• Imaging considerations 

• Film processing 

• Viewing of radiographs 

• Judging radiographic quality 

6. Safety and Health 

• Exposure hazards 

• Methods of controlling radiation exposure 

• Operation and emergency procedures 

Reference Catalog Number 

NDT Handbook, Third Edition: Volume 4, 

Radiographic Testing 144 

ASM Handbook Vol. 17, NDE and QC 105 



Fion Zhang at Shanghai 

27th June 2015 

http://meilishouxihu.blog.163.com/ 


Greek 

Alphabet 



http://greekhouseoffonts.com/


Why Neutron Radiography? 

"finding lead in a paraffin block (or a needle in a haystack) would work for x 

rays while looking for paraffin in a lead block or a straw in a needle-stack 

would work for neutrons." 












■ 

http://minerals.usgs.gov/minerals/pubs/commodity/ 


Neutron Cross Section of the elements 

■ 

http://periodictable.com/Properties/A/NeutronCrossSection.html 


IVONA TTS Capable. 


http://www.naturalreaders.com/

Reading II 

Content 

• Reading One: E748 

• Reading Two: ASMHB17-NRT 

• Reading Three: E1316 

• Reading Four: Neutrons provide unique penetrating radiation 


Reading-1 

E748 


1. Scope 

1.1 Purpose - Practices to be employed for the radiographic examination of 

materials and components with thermal neutrons are outlined herein. They 

are intended as a guide for the production of neutron radiographs that 

possess consistent quality characteristics, as well as aiding the user to 

consider the applicability of thermal neutron radiology (radiology, radiographic, 

and related terms are defined in Terminology E 1316). Statements concerning 

preferred practice are provided without a discussion of the technical 

background for the preference. The necessary technical background can be 

found in Refs (1-16). 


1.2 Limitations - Acceptance standards have not been established for any 

material or production process (see Section 5 on Basis of Application). 

Adherence to the practices will, however, produce reproducible results that 

could serve as standards. Neutron radiography, whether performed by means 

of a reactor, an accelerator, subcritical assembly, or radioactive source, will 

be consistent in sensitivity and resolution only if the consistency of all details 

of the technique, such as neutron source, collimation, geometry, film, etc., is 

maintained through the practices. These practices are limited to the use of 

photographic or radiographic film in combination with conversion screens for 

image recording; other imaging systems are available. Emphasis is placed on 

the use of nuclear reactor neutron sources. 

1.3 Interpretation and Acceptance Standards - Interpretation and acceptance 

standards are not covered by these practices. Designation of accept-reject 

standards is recognized to be within the cognizance 认定 of product 

specifications. 


1.4 Safety Practices - General practices for personnel protection against 

neutron and associated radiation peculiar to the neutron radiologic process 

are discussed in Section 17. For further information on this important aspect 

of neutron radiology, refer to current documents of the National Committee on 

Radiation Protection and Measurement, the Code of Federal Regulations, the 

U.S. Nuclear Regulatory Commission, the U.S. Department of Energy, the 

National Institute of Standards and Technology, and to applicable state and 

local codes. 

1.5 Other Aspects of the Neutron Radiographic Process - For many 

important aspects of neutron radiography such as technique, files, viewing of 

radiographs, storage of radiographs, film processing, and record keeping, 

refer to Guide E 94. (See Section 2.) 

1.6 The values stated in either SI or inch-pound units are to be regarded as 

the standard. 


1.7 This standard does not purport to address all of the safety concerns, if 

any, associated with its use. It is the responsibility of the user of this standard 

to establish appropriate safety and health practices and determine the 

applicability of regulatory limitations prior to use. (For more specific safety 

information see 1.4.) 


2. Referenced Documents 

2.1 ASTM Standards: 

• E 94 Guide for Radiographic Testing 

• E 543 Practice for Evaluating Agencies that Perform Nondestructive 

Testing E 545 Method for Determining Image Quality in Direct Thermal 

Neutron Radiographic Examination 

• E 803 Method for Determining the L/D Ratio of Neutron Radiography 

Beams E 1316 Terminology for Nondestructive Examinations 

• E 1496 Test Method for Neutron Radiographic Dimensional 

Measurements 


2.2 ASNT Standard: 

• SNT-TC-1A Recommended Practice for Personnel Qualification and 

Certification 

2.3 ANSI Standard: 

• ANSI/ASNT- P- 89 Standard for Qualification and Certification of 

Nondestructive Testing Personnel 

2.4 Military Standard: 

• MIL-STD-410 Nondestructive Testing Personnel Qualification and 

Certification 


3. Terminology 

3.1 Definitions - For definitions of terms used in these practices, see 

Terminology E 1316, Section H. 

4. Significance and Use 

4.1 These practices include types of materials to be examined, neutron 

radiographic examination techniques, neutron production and collimation 

methods, radiographic film, and converter screen selection. Within the 

present state of the neutron radiologic art, these practices are generally 

applicable to specific material combinations, processes, and techniques. 


5. Basis of Application 

5.1 Personnel Qualification - Nondestructive testing (NDT) personnel shall be 

qualified in accordance with a nationally recognized NDT personnel 

qualification practice or standard such as ANSI/ASNT-CP-189, SNT-TC-1A, 

MIL-STD-410, or a similar document. The practice or standard used and its 

applicable revision shall be specified in the contractual agreement between 

the using parties. 

5.2 Qualification of Nondestructive Agencies - If specified in the contractual 

agreement, NDT agencies shall be qualified and evaluated as described in 

Practice E 543. The applicable edition of Practice E 543 shall be specified in 

the contractual agreement. 

5.3 Procedures and Techniques - The procedures and techniques to be used 

shall be as described in these practices unless otherwise specified. Specific 

techniques may be specified in the contractual agreement. 


5.4 Extent of Examination - The extent of examination shall be in accordance 

with Section 16 unless otherwise specified. 

5.5 Reporting Criteria/Acceptance Criteria - Reporting criteria for the 

examination results shall be in accordance with 1.3 unless otherwise 

specified. Acceptance criteria (for example, for reference radiographs) shall 

be specified in the contractual agreement. 

5.6 Reexamination of Repaired/Reworked Items - Reexamination of 

repaired/reworked items is not addressed in these practices and, if required, 

shall be specified in the contractual agreement. 


6. Neutron Radiography 

6.1 The Method - Neutron radiography is basically similar to X radiography in 

that both techniques employ radiation beam intensity modulation by an object 

to image macroscopic object details. X rays or gamma rays are replaced by 

neutrons as the penetrating radiation in a through-transmission examination. 

Since the absorption characteristics of matter for X rays and neutrons differ 

drastically, the two techniques in general serve to complement one another. 

6.2 Facilities - The basic neutron radiography facility consists of a source of 

fast neutrons, a moderator, a gamma filter, a collimator, a conversion screen, 

a film image recorder or other imaging system, a cassette, and adequate 

biological shielding and interlock systems. A schematic diagram of a 

representative neutron radiography facility is illustrated in Fig. 1. 

6.3 Thermalization - The process of slowing down neutrons by permitting the 

neutrons to come to thermal equilibrium with their surroundings; see definition 

of thermal neutrons in Terminology E 1316, Section H. 


FIG. 1 Typical Neutron Radiography Facility with Divergent Collimator 


7. Neutron Sources 

7.1 General - The thermal neutron beam may be obtained from: 

■ 

■ 

■ 

■ 

a nuclear reactor, 

a subcritical assembly, 

a radioactive neutron source, 

or an accelerator. 

Neutron radiography has been achieved successfully with all four sources. In 

all cases the initial neutrons generated possess high energies and must be 

reduced in energy (moderated) to be useful for thermal neutron radiography. 

This may be achieved by surrounding the source with light materials such as: 

■ 

■ 

■ 

■ 

■ 

■ 

water, 

oil, 

plastic, 

paraffin, 

beryllium, or 

graphite. 


The preferred moderator will be dependent on the constraints dictated by the 

energy of the primary neutrons, which will in turn be dictated by neutron beam 

parameters such as thermal neutron yield requirements, cadmium ratio, and 

beam gamma ray contamination. The characteristics of a particular system for 

a given application are left for the seller and the buyer of the service to decide. 

Characteristics and capabilities of each type of source are referenced in the 

References section. A general comparison of sources is shown in Table 1. 


TABLE 1 Comparison of Thermal Neutron Sources 


7.2 Nuclear Reactors - Nuclear reactors are the preferred thermal neutron 

source in general, since high neutron fluxes are available and exposures can 

be made in a relatively short time span. The high neutron intensity makes it 

possible to provide a tightly collimated beam; therefore, high-resolution 

radiographs can be produced. 

U g = Dt/L 


7.3 Subcritical Assembly - A subcritical assembly is achieved by the addition 

of sufficient fissionable material surrounding a moderated source of neutrons, 

usually a radioisotope source. Although the total thermal neutron yield is 

smaller than that of a nuclear reactor, such a system offers the attractions of 

adequate image quality in a reasonable exposure time, relative ease of 

licensing, adequate neutron yield for most industrial applications, and the 

possibility of transportable operation. 


Subcritical Assembly 

Critical mass 

A critical mass is the smallest amount of fissile material needed for a 

sustained nuclear chain reaction. The critical mass of a fissionable material 

depends upon its nuclear properties (specifically, the nuclear fission crosssection), 

its density, its shape, its enrichment, its purity, its temperature, and 

its surroundings. The concept is important in nuclear weapon design. 

Explanation of criticality 

When a nuclear chain reaction in a mass of fissile material is self-sustaining, 

the mass is said to be in a critical state in which there is no increase or 

decrease in power, temperature, or neutron population. 

A numerical measure of a critical mass is dependent on the effective neutron 

multiplication factor k, the average number of neutrons released per fission 

event that go on to cause another fission event rather than being absorbed or 

leaving the material. When k = 1, the mass is critical, and the chain reaction is 

barely self-sustaining. 


https://en.wikipedia.org/wiki/Critical_mass

A subcritical mass is a mass of fissile material that does not have the ability to 

sustain a fission chain reaction. A population of neutrons introduced to a 

subcritical assembly will exponentially decrease. In this case, k < 1. A steady 

rate of spontaneous fissions causes a proportionally steady level of neutron 

activity. The constant of proportionality increases as k increases. 

A supercritical mass is one where there is an increasing rate of fission. The 

material may settle into equilibrium (i.e. become critical again) at an elevated 

temperature/power level or destroy itself, by which equilibrium is reached. In 

the case of supercriticality, k > 1 


https://en.wikipedia.org/wiki/Critical_mass

7.4 Accelerator Sources - Accelerators used for thermal neutron radiography 

have generally been of the low-voltage type which utilize the 3 H(d,n) 4 He 

reaction, high-energy X-ray machines in which the (x,n) reaction is applied 

and Van de Graaff and other high-energy accelerators which employ 

reactions such as 9 Be(d,n) 10 B. In all cases, the targets are surrounded by a 

moderator to reduce the neutrons to thermal energies. The total neutron 

yields of such machines can be on the order of 10 12·n·s -1 ; the thermal neutron 

flux of such sources before collimation can be on the order of 10 9 n·cm -2·s -1 , 

for example, the yield from a Van de Graaff accelerator. 

Total flux Ф 1012·n·s-1 

D 

I 

L 

I = Ф/16(L/D) 


Accelerator Sources-Linear Accelerator 


http://atomic.lindahall.org/what-is-an-atom-smasher.html

Accelerator Sources-Cyclotron 


http://atomic.lindahall.org/what-is-an-atom-smasher.html

7.5 Isotopic Sources - Many isotopic sources have been employed for 

neutron radiologic applications. Those that have been most widely utilized are 

outlined in Table 2. Radioactive sources offer the best possibility for portable 

operation. However, because of the relatively low neutron yield, the exposure 

times are usually long for a given image quality. The isotopic source252Cf 

offers a number of advantages for thermal neutron radiology, namely, low 

neutron energy and small physical size, both of which lead to efficient neutron 

moderation, and the possibility for high total neutron yields. 

TABLE 2 Radioactive Sources Employed for Thermal Neutron Radiography 

A: These comments compare sources in the table. 


8. Imaging Methods and Conversion Screens 

8.1 General - Neutrons are nonionizing particulate radiation that have little 

direct effect on radiographic film. To obtain a neutron radiographic image on 

film, a conversion screen is normally employed; upon neutron capture, 

screens emit prompt and delayed decay products in the form of nuclear 

radiation or light. In all cases the screen should be placed in intimate contact 

with the radiographic film in order to obtain sharp images. 

8.2 Direct Method - In the direct method, a film is placed on the source side of 

the conversion screen (front film) and exposed to the neutron beam together 

with the conversion screen. Electron emission upon neutron capture is the 

mechanism by which the film is exposed in the case of gadolinium conversion 

screens. 


The screen is generally one of the following types: 

1. a free-standing gadolinium metal screen accessible to film on both sides; 

2. a sapphire coated, vapordeposited gadolinium screen on a substrate 

such as aluminum; or 

3. a light-emitting fluorescent screen such as gadolinium oxysulfide or 

6 

LiF/ZnS. Exposure of an additional film (without object) is often useful to 

resolve artifacts that may appear in radiographs. 

Such artifacts could result from screen marks, excess pressure, light leaks, 

development, or nonuniform film. In the case of light-emitting conversion 

screens, it is recommended that the spectral response of the light emission 

be matched as closely as possible to that of the film used for optimum results. 

The direct method should be employed whenever high-resolution radiographs 

are required, and high beam contamination of low-energy gamma rays or 

highly radioactive objects do not preclude its use. 


8.3 Indirect Method - This method makes use of conversion screens that can 

be made temporarily radioactive by neutron capture. The conversion screen 

is exposed alone to the neutronimaging beam; the film is not present. 

Candidate conversion materials include (1) rhodium, (2) gold, (3) indium, and 

(4) dysprosium. 

Indium and dysprosium are recommended with dysprosium yielding the 

greater speed and emitting less energetic gamma radiation. 

It is recommended that the conversion screens be activated in the neutron 

beam for a maximum of three half-lives (3 x T ½ ) . Further neutron irradiation 

will result in a negligible amount of additional induced activity. After irradiation, 

the conversion screens should be placed in intimate contact with a 

radiographic film in a vacuum cassette, or other light-tight assembly in which 

good contact can be maintained between the radiographic film and 

radioactive screen. 

X- ay intensification screens may be used to increase the speed of the 

autoradiographic process if desired. 


For the indirect type of exposure, the material from which the cassette is 

fabricated is immaterial as there are no neutrons to be scattered in the 

exposure process. In this case, as in the activation process, there is little to 

be gained for conversion screen-film exposures extending beyond three halflives. 

It is recommended that this method be employed whenever the neutron 

beam is highly contaminated with gamma rays, which in turn cause film 

fogging and reduced contrast sensitivity, or when highly radioactive objects 

are to be radiographed. In short, this method is beam gamma-insensitive. 

8.4 Other Imaging Systems - The scope of these practices is limited to film 

imaging (see 1.2). However, other imaging systems such as track-etch or 

radioscopic systems are available. 


Track-etch 

Ion tracks are damage-trails created by swift heavy ions penetrating through 

solids, which may be sufficiently-contiguous for chemical etching in a variety 

of crystalline, glassy, and/or polymeric solids.[1][2] They are associated with 

cylindrical damage-regions several nanometers in diameter[3][4] and can be 

studied by Rutherford backscattering spectrometry (RBS), transmission 

electron microscopy (TEM), small-angle neutron scattering (SANS), smallangle 

X-ray scattering (SAXS) or gas permeation. 

"Fresh" (latent or unetched) 

Californium-252 fission tracks[1] in 

a chromite (FeCr 2 O 4 ) grain from the 

Allende meteorite, showing up in a 

weak-beam darkfield TEM image 

which lights up the strain-fields 

around the 40Å-diameter trackdamage 

cores. This work confirmed 

chromite's ability to record nuclear 

particle tracks in spite of its 

relatively low resistivity. 


Track-etch 


More Reading on Radioscopy 

■ http://www.ndt.net/article/wcndt00/papers/idn284/idn284.htm 

■ http://www.nationalboard.org/Index.aspx?pageID=164&ID=199 


9. Neutron Collimators 

9.1 General - Neutron sources for thermal neutron radiology generally involve 

a sizeable moderator region in which the neutron motion is highly 

multidirectional. Collimators are required to produce a beam and thereby 

produce adequate image resolution capability in a neutron radiology facility. It 

should be noted that in the definitions of collimator parameters, it is assumed 

that the object under examination is placed as close to the imaging system as 

possible to decrease both magnification and image unsharpness due to the 

finite neutron source size. Several types of collimators are available. These 

include the widely used divergent type, multichannel, pinhole, and straight 

collimators. The image spatial resolution properties of the beams are 

generally set in part by the diameter or longest dimension of the collimator 

entrance port (D) and the distance between that aperture and the imaging 

system (L). An exception is the multichannel collimator in which D is the 

diameter of a channel and L is the length of the collimator. It should be noted 

that the detection system used in conjunction with a multichannel collimator 

will register the collimator pattern. 


Registry can be eliminated by empirically adjusting the distance between the 

collimator and the imaging system until the pattern disappears. Ratios of L/D 

as low as 10 are not unusual for low neutron yield sources, while higher 

resolution capability systems often will display L/ D values of several hundred 

or more. Method E 803 details the method of measuring the L/D ratio for 

neutron radiography systems. The actual spatial resolution or image 

unsharpness in a particular radiologic examination will depend, of course, on 

factors additional to the beam characteristics. These include the object size, 

the geometry of the system, and scatter conditions. For the typical calculation 

of geometric unsharpness, the size of the X-radiologic source, F, would be 

replaced by the size of the effective thermal neutron radiologic source (D) as 

discussed in Guide E 94. 

Keywords: 

radiologic source 


9.2 Divergent Collimator - The divergent collimator is a tapered reentrant port 

into the point of highest thermal neutron flux in the moderator. The walls of 

the collimator are lined with a thermal neutron absorbing material to permit 

only unscattered neutrons from the source to reach the object and the image 

plane. This type of collimator is preferred when larger objects will be 

radiographed in a single exposure. It is recommended that the divergent 

collimator be lined with a neutron absorber which produces neutron capture 

decay products that will not result in background fogging of the film, such as 

6 

Li carbonate. A typical divergent collimating system is illustrated in the 

schematic diagram of Fig. 1. 


9.3 Multichannel Collimator - The multichannel collimator is an array of 

tubular collimators stacked within a larger collimator envelope. It is 

recommended as a means of achieving a high degree of collimation within a 

short collimation length. When this type of collimator is employed, a suitable 

collimator to detector distance should be maintained to avoid registry of the 

collimator pattern on the radiologic image. 

9.4 Straight Collimator - A straight-tube reentrant port can also be used 

instead of the tapered assembly described in 9.2. Although such collimators 

were widely used in early neutron radiologic work, the need to examine larger 

objects and to achieve higher resolution has fostered the use of divergent 

collimators. 

a straight collimator when it is employed in conjunction with a pinhole iris. The 

pinhole is generally fabricated from a neutron-opaque material such as Cd, 

Gd, or 10B. The resolution attainable will be dependent on the pinhole 

diameter D. A schematic diagram of this system is illustrated in Fig. 2. 


FIG. 2 Pinhole Collimator 


Parallel & Divergent Collimator - 

Fig. 2 Thermalization and collimation of beam in neutron radiography. Neutron collimators can be of the 

parallel-wall (a) or divergent (b) type. The transformation of fast neutrons to slow neutrons is achieved by 

moderator materials such as paraffin, water, graphite, heavy water, or beryllium. Boron is a typically used 

neutron-absorbing layer. The L/D ratio, where L is the total length from the inlet aperture to the detector 

(conversion screen) and D is the effective dimension of the inlet of the collimator, is a significant geometric 

factor that determines the angular divergence of the beam and the neutron intensity at the inspection plane 


ASMV17 Neutron Radiography



10. Beam Filters 

10.1 Thermal Neutron Radiography - In general, filters may not be necessary. 

However, it may be desirable to employ Pb or Bi filters in the neutron beam to 

minimize beam gamma-ray contamination. Whenever Bi gamma-ray filters 

are employed in a high neutron flux environment, the filter should be encased 

in a sealed aluminum can to contain alpha particle contamination due to the 

210 

Po produced by the neutron capture reaction in 209 Bi. Gamma rays can 

cause film fogging and reduced contrast sensitivity. In particular, some 

scintillator converter screens exhibit sensitivity to beam gamma-ray 

contamination. This effect can be minimized by careful selection of the 

screen/film combination. 

Keywords: 

gamma-ray contamination 


11. Masking 

11.1 General - In general, masking is not often used in thermal neutron 

radiology. Where it is desirable to reduce scatter or to reduce unusual 

contrasts, the choice of masking materials should be made carefully. 

Materials that scatter readily, such as those containing hydrogen or materials 

that emit radiation that may be readily detected, for example, as indium, 

dysprosium, or cadmium, should be avoided or used with exceptional care. 

Lithium-containing materials may be useful for masking purposes. 

Background fogging may result from the 470 keV gamma ray from boron. 


Fig. 1 Mass attenuation coefficients for the elements as a function of atomic number for thermal (4.0 × 10-21 J, or 0.025 eV) 

neutrons and x-rays (energy 125 kV). The mass attenuation coefficient is the ratio of the linear attenuation coefficient, μ, to the 

density, ρ, of the absorbing material. 



12. Effect of Materials Surrounding Object and Cassette 

12.1 Backscatter - As in the case of X radiography, effects of back-scattered 

radiation, for example, from walls, etc., can be reduced by masking the 

radiation beam to the smallest practical exposure area. Effects of backscatter 

can be determined by placing a neutron-absorbing marker of a material such 

as gadolinium and a gamma-absorbing marker of a material such as lead on 

the back of the exposure cassette. If problems with backscatter are shown, 

one should minimize in the exposure area materials that scatter or emit 

radiation as discussed in Section 11. Backscatter can be minimized by 

placing a neutron absorber such as gadolinium behind the cassette. 


13. Cassettes 

13.1 Material of Construction - The cassette frame and back may be 

fabricated of aluminum or magnesium as employed in standard X-ray film 

cassettes. Aluminum or magnesium entrance window X-ray cassettes can be 

used directly for neutron radiography. Special vacuum cassettes designed 

specifically for neutron radiography are preferred to conventional X-ray 

cassettes. Plastic window X-ray cassettes should not be used. The plastic 

entrance face may be replaced with thin, 0.25 to 1.7-mm thick 1100 reactor 

grade, or 6061T6 aluminum, or magnesium to eliminate image resolution 

degradation due to scattering; use of hydrogenous materials in the 

construction of a cassette can lead to image degradation and the use of these 

materials should be considered carefully. 

13.2 Vacuum Cassettes - Whenever possible, vacuum cassettes should be 

employed to hold the converter foil or scintillator screen in intimate contact 

with the film both in the direct and indirect exposure methods. Cassettes of 

the type that maintain vacuum during the exposure or that must be pumped 

continuously during the exposure are equally applicable. Vacuum storage 

minimizes atmospheric corrosion of converters such as dysprosium and 

substantially increases their useful life. 


14. Thermal Neutron Radiographic Image Quality 

14.1 Image Quality Indicators - Image quality indicators for thermal neutron 

radiography are described in Method E 545. The devices and methods 

described therein permit: 

(1) the measurement of beam composition, including relative thermal neutron 

to higher energy neutron composition and relative gamma-ray content; and 

(2) devices for indicating the sensitivity of detail visible on the neutron 

radiograph. 


15. Contrast Agents 

15.1 Improved Contrast - Contrast agents are useful in thermal neutron 

radiology for demonstrating improved contrast of a tagged material or 

component. For thermal neutron radiography even simple liquids such as 

water or oil can serve as effective contrast agents. Additional useful marker 

materials can be chosen from neutron-attenuating materials such as boron, 

cadmium, and gadolinium. Of course, the deleterious effect of the contrast 

agent employed upon the test object should be considered. 


16. Types of Materials To Be Examined with Thermal 


16.1 General - This section provides a categorization of applications 

according to the characteristics of the object being examined. The following 

paragraphs provide a general list of four separate categories for which 

thermal neutron radiographic examination is particularly useful. Additional 

details concerning neutron attenuation are discussed in Appendix X1. 


16.2 Detection of Similar Density Materials - Thermal neutron radiography 

can offer advantages in cases of objects of similar-density materials, that can 

represent problems for X-radiography. Some brazing materials, such as 

cadmium and silver, for example, are readily shown by thermal neutron 

radiography. Contrast agents can help show materials such as ceramic 

residues in investment-cast turbine blades. Inspection of castings for voids or 

uniformity and of cladding materials can often be accomplished with thermal 

neutron radiography. Material migration in solid-state electroniccomponents, 

electrolyte migration in batteries, diffusion between light and heavy water, and 

movement of moisture through concrete are examples in which thermal 

neutron radiography has proveduseful. 


16.3 The Detection of Low-Density Components and Materials in High- 

DensityContainments - This recommended category includes the 

examination of metal-jacketed explosive devices, location andmeasurement 

of hydrogen in cladding materials and weldments, and of moisture in 

assemblies, location of fluids and lubricants in metal containmentsystems, 

examination of adhesive bonds in metal parts including honeycomb, location 

of liquid metals in metal parts, location of corrosion products in aluminum 

airframe components, examination of boron-filament composites, studies of 

fluid migration in sealed metal systems, and the determination of poison 

distribution in nuclear reactor fuel rods or control plates. 


16.4 The Examination of Highly Radioactive Objects - The technique of 

indirect neutron imaging is insensitive to gamma radiation in the imaging 

beam or from a radioactive object that could produce fogging of the film with 

the resulting loss in contrast sensitivity. This category of recommended 

examinations includes the inspection of irradiated reactor fuel capsules and 

plates for cracking and swelling, the determination of highly enriched nuclear 

fuel distribution in assemblies, and the inspection of weld and braze joints in 

irradiated subassemblies. 

16.5 Differentiation Between Isotopes of the Same Element - Neutron 

attenuation is a function of the particular isotope rather than the element 

involved. There are certain isotopes that have either very high or very low 

attenuation and, therefore, are subject to detection by thermal neutron 

radiology. For example, it is possible to differentiate between isotopes such 

as 1 H and 2 H or 235 U and 238 U. 


17. Activation of Objects and Exposure Materials 

17.1 Objects - Certain objects placed in the neutron beam may be activated, 

depending upon the: 

■ incident neutron energy (Mev), 

■ intensity (n/cm 2 ) and 

■ exposure time (s), and 

■ the material activation cross section (cm -2 ) and 

■ half-life (T ½ ). 

Therefore, objects under examination may become radioactive. In extreme 

cases this could produce film fogging, thereby reducing contrast. Safety is a 

strong consideration; radiation monitoring of objects should be performed 

after each exposure. Objects that exhibit a radiation level too high for 

handling should be set aside to allow the radiation to decay to acceptable 

levels. In practice, since neutron exposure times are normally short, a short 

decay period will usually be satisfactory. 


17.2 Cassettes - Radiographic cassettes containing materials such as 

aluminum and steel can become activated, particularly on multiple exposures. 

Monitoring of radiation to determine safe handling levels can alleviate safety 

problems and minimize film fogging. Activated cassettes, screens, and 

objects should be kept away from unexposed or unprocessed film. Converted 

X-radiography cassettes are virtually worthless for high-resolution industrial 

neutron radiography. Vacuum cassettes should be employed whenever 

possible to maintain the film and converter foil in intimate contact during the 

exposure. This holds for both the direct and indirect methods. 


17.3 Conversion Screens - Conversion screens used for direct exposure 

methods are usually chosen for low activation properties. Conversion screen 

materials such as gadolinium, boron, or lithium seldom cause problems. 

(Gd, B, Li) 

However, conversion screens for the indirect exposure method are chosen for 

high-activation potential. Therefore, exposed and activated screens such as 

indium, dysprosium, rhodium, or gold should be handled with care. Screens 

should be handled with gloves or tongs and should be moved in a shield. 

High-radiation exposures to the fingers are a potential hazard. 

(Dy, Rh, Au) 

A cassette will shield much of the beta radiation emitted by the commonly 

used indirect exposure converter screens. Conversion screens should 

normally be allowed at least a three half-life decay period ( 3 x T1/2 )before 

reuse to prevent double exposures. 


18. Keywords 

18.1 neutron attenuation; neutron collimator; neutron radiography; neutron 

sources 


APPENDIXES 

(Nonmandatory Information) 


X1. Attenuation Of Neutrons By Matter 

X1.1 A major advantage of using neutrons for radiography is that radiologic 

observation of certain material combinations is easily accomplished with slow 

neutrons where, because of attenuation differences, problems will arise with 

X rays. For example, the high attenuation of slow neutrons by elements such 

as hydrogen, lithium, boron, cadmium, and several rare earths means that 

these materials can readily be shadowed with neutrons even when they are 

combined in an assembly with some high atomic weight material such as 

steel, lead, bismuth, or depleted uranium. Although the heavy material would 

make X radiography difficult, neutron radiography should yield a successful 

inspection. Further, the differences in slow neutron attenuation often found 

between neighboring materials in the periodic table offer an advantage for 

neutron radiologic discrimination between materials that have similar X-ray 

attenuation characteristics. 


X1.2 This advantage is illustrated in Fig. X1.1 in which the mass attenuation 

coefficients μ/r are plotted as a function of atomic number of the attenuating 

element for both X rays (about 120 kVp energy) and slow neutrons. There are 

many apparent attenuation differences. The coefficient μ/r is normally used in 

attenuation calculations in the exponential relationship: 

I/I o = e –(μ/ρ)ρx 

(X1.1) 

where: 

I/Io 

μ 

r 

x 

= ratio of emergent radiation intensity to the intensity incident on a 

material, 

= linear attenuation coefficient, 

= density, and 

= thickness. 

μ = σ total , total cross section area cm 2 x Number of nuclei in cm 2 

Number of nuclei in 1 gram of material = N/gram atomic weight (A), 

Number of gram of material in 1 cm 2 = density, ρ 

Number of nuclei in 1 cm 2 = ρN/A 

μ = (ρN/A)∙σ total 


X1.3 For neutrons, it is more convenient to have the relationship between 

attenuation coefficient and cross section, as follows: 

μ = P∙σ total = p∙(σ abs + σ scatt ) 

where: 

P = number of nuclei per cm 3 of attenuating material, 

σ total = total cross section (cm 2 ), equal to the sum of absorption σ abs and 

scattering σ scatt cross sections, and 

μ = the linear attenuation coefficient (cm -1 ). 

A tabular listing of linear attenuation coefficients is shown in Table X1.1 and a 

comparative plot is given in Fig. X1.2; these values should be considered only 

as general guides. The data presented in Fig. X1.3 give half-value-layer 

thicknesses for thermal neutrons for many materials. 


X1.4 In radiologic situations, radiation that is transmitted through the object 

being examined is recorded so that those areas in which radiation has been 

removed, either by absorption or by scattering, may be observed. (Eq X1.1) 

and (Eq X1.2) are valuable in assessing the relative change in transmitted 

radiation intensity for several materials and thicknesses within an object of 

interest. 


FIG. X1.1 Approximate Mass Attenuation Coefficients μ/ρ as a Function of 

Atomic Number 


FIG. X1.2 Calculated Thermal Neutron and 100 and 500 KEV X-Ray Linear 

Attenuation Coefficients (μ) as a Function of Atomic Number (A) 


FIG. X1.3 Half-Value Layers of Selected Materials for Thermal Neutrons 


FIG. X1.3 Half-Value Layers of Selected Materials for Thermal Neutrons 


TABLE X1.1 Thermal Neutron Linear Attenuation Coefficients Using Average 

Scattering and Thermal Absorption Cross Sections for the Naturally Occurring 

Elements 


Neutron Cross Section of the elements 

■ 

http://periodictable.com/Properties/A/NeutronCrossSection.html 


X2. Calculation Of The Linear Attenuation Coefficient Of A Compound 

■ Element’s μ 

X2.1 If the material under examination contains only one element, then the 

linear attenuation coefficient is as follows: 

μ = (ρN/A)∙σ 

(X2.1) 

where: 

μ = linear attenuation coefficient, cm -1 , 

ρ= material density, gm·cm -3 , 

N = Avogadro’s number = 6.023 x 10 23 atoms·g-mol -1 , 

σ = total cross section, cm 2 , and 

A = gram atomic weight of material. 

(ρN/A) = numbers of nuclei in 1 cm -3 of material. 


■ Compound's μ 

X2.2 If, on the other hand, the material under examination contains several 

elements, or is in the form of a compound, then the linear attenuation 

coefficient is as follows: 

μ = (ρN/M)∙(ѵ 1 σ 1 + ѵ 2 σ 2 +..... + ѵ i σ i ) 

(X2.2) 

where: 

μ = linear attenuation coefficient of the compound, cm −1 , 

ρ = compound density, g·cm −3 , 

N = Avogadro’s number = 6.023 x 10 23 atoms·g-mol −1 , 

M = gram molecular weight of the compound, 

σ i = total cross section of the i th atom, cm 2 . 

(ρN/M) = numbers of nuclei in 1 cm -3 of material (compound). 


X2.3 As an example, consider the calculation of the linear attenuation 

coefficient, μ, for the compound polyethylene CH2: 

μ = (ρN/M)∙(1σ C + 2σ H ) 

ρ = 0.91 g·cm −3 , 

N = 6.023 x 10 23 atoms·g-mol −1 , 

M = 2(1.0079) + 14.011 = 14.0268 

σ H = (20.49 + 0.333)10 -24 cm 2 = 20.823 x 10 -24 cm 2 

σ C =(4.74 + 0.0035)10 -24 cm 2 = 4.744 x 10-24 cm 2 

μ = 1.8126 cm -2 


■ 



TABLE X1.1 Thermal Neutron Linear Attenuation Coefficients Using Average 

Scattering and Thermal Absorption Cross Sections for the Naturally Occurring 

Elements 


References 



End Of Reading 



ASMHB-17 NRT 


1.0 Principles of Neutron Radiography 

Neutron radiography is similar to conventional radiography in that both 

techniques employ radiation beam intensitymodulation by an object to image 

macroscopic object details. X-rays or -rays are replaced by neutrons as the 

penetrating radiation in a through-transmission inspection. The absorption 

characteristics of matter for x-rays and neutrons differ drastically; the two 

techniques in general serve to complement one another. Neutrons are 

subatomic particles that are characterized by relatively large mass and a 

neutral electric charge. The attenuation of neutrons differs from the 

attenuation of x-rays in that the processes of attenuation are nuclear rather 

than ones that depend on interaction with the electron shells surrounding the 

nucleus. Neutrons are produced by nuclear reactors, accelerators, and 

certain radioactive isotopes, all of which emit neutrons of relatively high 

energy (fast neutrons). Because most neutron radiography is performed with 

neutrons of lower energy (thermal neutrons), the sources are usually 

surrounded by a moderator, which is a material that reduces the kinetic 

energy of the neutrons. 


2.0 Neutron Versus Conventional Radiography. 

Neutron radiography is not accomplished by direct imaging on film, 

because neutrons do not expose x-ray emulsions efficiently. 

■ In one form of neutron radiography, the beam of neutrons impinges on a 

conversion screen or detector made of a material such as dysprosium or 

indium, which absorbs the neutrons and becomes radioactive, decaying with 

a short half-life. In this method, the conversion screen alone is exposed in the 

neutron beam, then immediately placed in contact with film to expose it by 

autoradiography. 

■ In another common form of imaging, a conversion screen that 

immediately emits secondary radiation is used with film directly in the neutron 

beam. Neutron radiography differs from conventional radiography in that the 

attenuation of neutrons as they pass through the testpiece is more related to 

the specific isotope present than to density or atomic number. 


X-rays are attenuated more by elements of high atomic number than by 

elements of low atomic number, and this effect varies relatively smoothly with 

atomic number. Thus, x-rays are generally attenuated more by materials of 

high density than by materials of low density. For thermal neutrons, 

attenuation generally tends to decrease with increasing atomic number, 

although the trend is not a smooth relationship. In addition, certain light 

elements (hydrogen, lithium, and boron), certain medium-to-heavy elements 

(especially cadmium, samarium, europium, gadolinium, and dysprosium), and 

certain specific isotopes have an exceptionally high capability of attenuating 

thermal neutrons (Fig. 1). This means that neutron radiography can detect 

these highly attenuating elements or isotopes when they are present in a 

structure of lower attenuation. 


Fig. 1 Mass attenuation coefficients for the elements as a function of atomic number for thermal (4.0 × 10- 

21 J, or 0.025 eV) neutrons and x-rays (energy 125 kV). The mass attenuation coefficient is the ratio of the 

linear attenuation coefficient, , to the density, , of the absorbing material. 


Thermal (slow) neutrons permit the radiographic visualization of low atomic 

number elements even when they are present in assemblies with high atomic 

number elements such as iron, lead, or uranium. Although the presence of 

the heavy metals would make detection of the light elements virtually 

impossible with x-rays, the attenuation characteristics of the elements for slow 

neutrons are different, which makes detection of light elements feasible. 

Practical applications of neutron radiography include the inspection of metaljacketed 

explosives, rubber O-ring assemblies, investment cast turbine 

blades to detect residual ceramic core, and the detection of corrosion in 

metallic assemblies. 

Using neutrons, it is possible to detect radiographically certain isotopes: for 

example, certain isotopes of hydrogen, cadmium, or uranium. Some neutron 

image detection methods are insensitive to background γ-rays or x-rays and 

can be used to inspect radioactive materials such as reactor fuel elements. In 

the nuclear field, these capabilities have been used to image highly 

radioactive materials and to show radiographic differences between different 

isotopes in reactor fuel and control materials. The characteristics of neutron 

radiography complement those of conventional radiography; one radiation 

provides a capability lacking or difficult for the other. 


3.0 Neutron Sources 

The excellent discrimination capabilities of neutrons generally refer to 

neutrons of low energy, that is, thermal neutrons. The characteristics of 

neutron radiography corresponding to various ranges of neutron energy are 

summarized in Table 1. Although any of these energy ranges can be used for 

radiography, this article emphasizes the thermal-neutron range, which is the 

most widely used for inspection. In thermal-neutron radiography, an object 

(testpiece) is placed in a thermal-neutron beam in front of an image detector. 

The neutron beam may be obtained from a nuclear reactor, a radioactive 

source, or an accelerator. Several characteristics of these sources are 

summarized in Table 2. For thermal-neutron radiography, fast neutrons 

emitted by these sources must first be moderated and then collimated (Fig. 2). 

The radiographic intensities listed in Table 2 typically do not exceed 10 -5 

times the total fast-neutron yield of the source. 


Part of this loss is incurred in moderating the neutrons, and the remainder in 

bringing a collimated beam out of a large-volume moderator. Collimation is 

necessary for thermal-neutron radiography because there are no useful point 

sources of low-energy neutrons. Good collimation in thermal-neutron 

radiography is comparable to small focal-spot size in conventional 

radiography; the images of thick objects will be sharper with good collimation. 

On the other hand, it should be noted that available neutron intensity 

decreases with increasing collimation. 


Table 1 Characteristics of neutron radiography at various neutron-energy ranges 


Table 2 Properties and characteristics of thermal-neutron sources 


Fig. 2 Thermalization and collimation of beam in neutron radiography. 

Neutron collimators can be of the parallel-wall (a) or divergent (b) type. The 

transformation of fast neutrons to slow neutrons is achieved by moderator 

materials such as paraffin, water, graphite, heavy water, or beryllium. Boron is 

a typically used neutron-absorbing layer. The L/D ratio, where L is the total 

length from the inlet aperture to the detector (conversion screen) and D is the 

effective dimension of the inlet of the collimator, is a significant geometric 

factor that determines the angular divergence of the beam and the neutron 

intensity at the inspection plane. 



3.1 Nuclear Reactors. 

Many types of reactors have been used for thermal-neutron radiography. The 

high neutron flux generally available provides high-quality radiographs and 

short exposure times. Although truck-mounted reactors are technically 

feasible, a reactor normally must be considered a fixed-site installation, and 

testpieces must be taken to the reactor for inspection. Investment costs are 

generally high, but small medium-cost reactors can provide good results. 

When costs are compared on the basis of available neutron flux (typically, 

10 12 n/cm 2 · s flux is often available at collimator entrance, and 10 6 to 10 7 

n/cm 2 · s flux is available at the film plane), reactor sources can be less costly 

than other sources. 


3.2 Accelerators. The accelerators most often used for thermal-neutron 

radiography are: 

• The low-voltage type employing the reaction 3 H + 2 H → 4 He + 1 n , a(d,T) 

generator, where n, d, and T represent the neutron, deuteron (the nucleus 

of a deuterium atom, D or 2 H, that consists of one neutron and one proton), 

and tritium ( 3 H), respectively 

• High-energy x-ray machines, in which (x,n) reactions are used, where x 

represents x-ray radiation 

• Van de Graaff accelerators 

• More recently, high-energy linear accelerators and cyclotrons to generate 

neutrons by charged-particle reactions on beryllium or lithium targets 


■ Low-Voltage Accelerators. A (d,T) generator provides fast-neutron 

yields in the range of 10 10 to 10 12 n/s. Target lives in sealed neutron tubes are 

reasonable (100 to 1000 h, depending on yield), and the sealed-tube system 

presents a source similar to that of certain types of x-ray machines. 

■ High- nergy X-Ray Machines. An (x,n) neutron source is a high-energy 

x-ray source such as a linear accelerator that can be converted for the 

production of neutrons by adding a suitable secondary target: 

for example, beryllium. X- ays having energies above an energy threshold 

level cause the secondary target to emit neutrons; in beryllium, the threshold 

x-ray energy for neutron production is 2.67 × 10 -13 J (1.66 MeV). Useful 

neutron radiography has been performed with an 8.8 × 10 -13 J (5.5 MeV) 

linear accelerator having an x-ray output of 0.17 C/kg · min (650 R/min) at 1 

m (3 ft). Changeover time from neutron emission to x-ray emission for this 

source was only 1 h. Beam intensities for neutron radiography with this 

source were about 5 × 10 4 n/cm 2 · s. with reasonable beam collimation. 


■ Van de Graaff Accelerators. 

Much higher beam intensities have been obtained by the acceleration of 

deuterons onto a beryllium target in a 3.2 × 10 -13 J (2.0 MeV) Van de Graaff 

generator. An intensity of 1.2 × 10 6 n/cm 2 · s was achieved (with medium 

collimation), and it is estimated that an acceleration voltage of 4.8 × 10 -13 J 

(3.0 MeV) would improve beam intensity by a factor of approximately six. The 

principle of the Van de Graaff machine is illustrated in Fig. 3. A rotating belt 

transports the charge from a supply to a high-voltage terminal. An ion source 

within the terminal is fed deuterium gas from a reservoir frequently located 

within the terminal. A radio-frequency system ionizes the gas, and positive 

ions are extracted into the accelerator tube. The terminal voltage of about 3 

MV is distributed by a resistor chain over about 80 gaps forming the 

accelerator tube, all of which is enclosed in a pressure vessel filled with 

insulating gas (N 2 and CO 2 at 2.0 MPa, or 290 psi). 


Fig. 3 Cross section showing Van de Graaff principle as it is applied to 

neutron radiography. Source: Ref 6 


The particle beam is extracted along flight tubes. In a typical neutron reaction, 

the beam bombards a water-cooled beryllium target in the center of the water 

moderator tank, which also serves as a partial shield. The higher-energy 

accelerators indicated above can provide neutron yields of 10 13 n/s and 

moderated, well-collimated beam intensities of the order of 10 6 n/cm 2 ·s. 

A few 4.8 × 10 -13 J (3.0 MeV) Van de Graaff generators have recently been 

placed in service for thermal-neutron radiography. In one such Van de Graaff 

system designed for neutron radiography, deuterons (4.8 × 10 -13 J, or 3 MeV; 

280 A) are accelerated onto a disk-shaped, water-cooled beryllium metal 

target. Neutrons in the range of 3.2 to 9.6 × 10 -13 J (2 to 6 MeV) are emitted 

preferentially in the forward direction and are moderated in water. The 4 (solid 

angle) yield of 5 × 10 11 n/s produces a peak thermal neutron flux of 2 × 10 9 

n/cm 2 · s. At a collimator ratio of 36:1, the typical exposure time for highquality 

film (3 × 10 9 n/cm 2 ) is about 2 h. 

Keywords: 

Neutron Yield 

Collimated beam intensity/ Neutron flux 


The accelerator tank for the 4.8 × 10 -13 J (3 MeV) machine measures 5.2 m 

(17 ft) in length and 1.5 m (5 ft) in diameter. The weight is 6100 kg (13,500 lb). 

The dimensions of the water tank are approximately 1 m (3 ft) on each side. 

Neutron beams can be extracted through three horizontal beam collimators. 

Unlike reactors, subcritical multipliers, or (d, T) accelerators, the Van de 

Graaff accelerators utilize no radioactive source material and sometimes 

require less stringent license processes. Other acceleration machines or 

reactions can be used for thermal-neutron radiography. However, those 

described above have been most widely used. 


3.3 Radioactive Sources. 

There are many possible radioactive sources. The characteristics of several 

radioisotopes that are commonly used are summarized in Table 3. 

Table 3 Properties and characteristics of several radioisotopes used for 

thermal-neutron radiography 

(γ, n) 

γ 

(α, n) 

γ 

(α, n) 

γ 




(α, n) 

(α, n) 

γ 


■ Radioisotopes 

offer the best prospect for a portable neutron-radiographic facility, but it 

should be recognized that the thermal-neutron intensity is only about 10 -5 of 

the total fast-neutron yield from the source. Consequently, neutron 

radiography using a radioisotope as a neutron source normally requires long 

exposure times and fast films. For example, a typical 3.7 × 10 11 Bq (10 Ci) 

source would provide a total fast-neutron yield of the order of 10 7 n/s. The 

radiographic intensity would be about 10 2 n/cm 2 · s, and a typical exposure 

time using a fast film/converter-screen combination would be about 1 h. 

Californium-252, usually purchased in the form shown in Fig. 4, has been the 

most frequently used radioactive source for neutron radiography. 


Fig. 4 Cross section of doubly encapsulated 252 Cf source. Source: Ref 6 


3.4 Subcritical Assembly. 

Another type of source that has received some attention is a subcritical 

assembly. This type of source is similar to a reactor, except that the neutron 

flux is less and the design is such that criticality cannot be achieved. A 

subcritical assembly offers some of the same neutron multiplication features 

as a reactor. It is somewhat easier to operate, and safety precautions are less 

stringent, because it is not capable of producing a self-sustaining neutron 

chain reaction. 

Interesting reading: 

https://en.wikipedia.org/wiki/Critical_mass 


4.0 Attenuation of Neutron Beams 

Unlike electrons and electromagnetic radiation, which interact with orbital 

electrons surrounding an atomic nucleus, neutrons interact only with atomic 

nuclei. Usually, neutrons are deflected by interaction with the nuclei, but 

occasionally a neutron is absorbed into a nucleus. When a neutron collides 

with the nucleus of an atom and is merely deflected, the neutron imparts 

some of its kinetic energy to the atom. Both the neutron and the atom move 

off in different directions from the original direction of motion of the neutron. 

This process, known as scattering, reduces the kinetic energy of the neutron 

and the probability that the neutron will pass through the object (testpiece) in 

a direction that will permit it to be detected by a device placed behind the 

object. True absorption of neutrons occurs when they are captured by nuclei. 

The capture of a neutron transforms the nucleus to the next-higher isotope of 

the target nucleus and sometimes produces an unstable nucleus that then 

undergoes radioactive decay. 


The probability that a collision between a neutron and a nucleus will result in 

capture is known as the capture cross section and is expressed as an 

effective area per atom. (The capture cross section is usually measured in 

barns, 1 barn equaling 10 -24 cm 2 or 1.6 × 10 -5 in. 2 .) The capture cross section 

varies with neutron energy, atomic number, and mass number. (the 

probability of neutron/ nuclei collision that results in capture) 

For thermal neutrons (energy of about 4.0 × 10 -21 J, or 0.025 eV), the 

average capture cross section varies randomly with atomic number, being 

high for certain elements and relatively low for other elements. The cross 

section actually varies by isotope rather than element. However, 

radiographers usually consider an average cross section for an element. For 

intermediate neutrons (energies of 8.0 × 10 -20 to 1.6 × 10 -15 J, or 0.5 eV to 

10 keV) and for fast neutrons (energies exceeding 1.6 × 10 -15 J, or 10 keV), 

the capture cross section is normally smaller than that for thermal neutrons, 

and there is much less variation with atomic number. 

For fast neutrons, most elements are similarly absorbing, and scattering is the 

dominant process of attenuation. 


In relation to other types of penetrating radiation, many materials interact less 

with neutrons. Therefore, neutrons can sometimes be used to inspect greater 

thicknesses than can be conveniently inspected with electromagnetic 

radiation. The combined effect of scattering and capture can be expressed as 

a mass-absorption coefficient; this coefficient is used to determine the 

exposure factor for the neutron radiography of a given object (testpiece). For 

a given material, attenuation varies exponentially with thickness, and the 

basic law of radiation absorption (discussed in the article "Radiographic 

Inspection" in this Volume) applies to neutron attenuation as well as to the 

attenuation of electromagnetic radiation. 

μ = (ρN/A)∙σ 

I = I o e -μt 


5.0 Neutron Detection Methods 

Detection methods for neutron radiography generally use photographic or x- 

ray films. In the so-called direct-exposure method, film is exposed directly to 

the neutron beam, with a conversion screen or intensifying screen providing 

the secondary radiation that actually exposes the film (Fig. 5a). 

Alternatively, film can be used to record an autoradiographic image from a 

radioactive image-carrying screen in a technique called the transfer method 

(Fig. 5b). 


Fig. 5 Schematics of neutron radiography with film using the direct-exposure 

method (a) and the transfer method (b). The cassette is a light-tight device for 

holding film or conversion screens and film in close contact during exposure. 


5.1 Direct-Exposure Method. 

Conversion screens of thin gadolinium foil or a scintillator have been most 

widely used in the direct-exposure method. When bombarded with a beam of 

neutrons, some of the gadolinium atoms absorb some of the neutrons and 

then promptly emit γ-rays. The γ-rays in turn produce internal conversion 

electrons that actually expose the film; these are directly related in intensity to 

the intensity of the neutron beam. Scintillators, on the other hand, are 

fluorescent materials often made of zinc sulfide crystals that also contain a 

specific isotope, such as 6 Li 3 or 10 B 5 . In a neutron beam, these isotopes react 

with neutrons as follows: 

6 

Li 3 + 1 n 0 → 3 H 1 + α ( 4 He 2 ) 

10 

B 5 + 1 n 0 → 7 Li 3 + α ( 4 He 2 ) 


The α particles emitted as a result of these reactions cause the zinc sulfide to 

fluoresce, which in turn exposes the film. Gadolinium oxysulfide, a scintillator 

originally developed for conventional radiography, is now widely used for 

neutron radiography. Scintillators provide useful images with total exposures 

as low as 5 × 10 5 n/cm 2 . The high speed and favorable relative 

neutron/gamma response of scintillators make them attractive for use with 

nonreactor neutron sources. For high-intensity sources, gadolinium screens 

are widely used. Gadolinium screens provide greater uniformity and image 

sharpness (high contrast resolution of 10 μm, or 400 μin., has been reported), 

but an exposure about 30 or more times that of a scintillator is required, even 

with fast films. Excessive background radiation should be kept to a minimum 

because it can have a detrimental effect on image quality. 

Keywords: 

Zinc Sulfide-Scintillator 

Gadolinium oxysulfide-Scintillator 

Gadolinium screens-Non Scintillator 


5.2 In the transfer method, 

a thin sheet of metal called a transfer screen, which is usually made of indium 

or dysprosium, is exposed to the neutron beam transmitted through the 

specimen. Neutron capture by the isotope 115 In 49 or 164 Dy 66 (Dysprosium) 

induces radioactivity, indium having a half-life of 54 min and dysprosium a 

half-life of 2.35 h. The intensity of radioactive emission from each area of the 

transfer screen is directly related to the intensity of the portion of the 

transmitted neutron beam that induced radioactivity in that area. The 

radiograph to be interpreted is made by placing the radioactive transfer 

screen in contact with a sheet of film. The particle β and γ-ray emissions from 

the transfer screen expose the film, with film density in various portions of the 

developed image being proportionally related to the intensity of radioactive 

emission. 

Keywords: 

Transfer screen-indium or dysprosium, In, Dy. 

Thermal neutron filter using Cadmium for epithermal neutron radiography, Cd. 

Converter screen uses gadolinium which emit beta particles, Gd. 


The transfer method is especially valuable for inspecting a radioactive 

specimen. Although the radiation emitted by the specimen (especially γ-rays) 

causes heavy film fogging during conventional radiography or direct-exposure 

neutron radiography, the same radiation will not induce radioactivity in a 

transfer screen. Therefore, a clear image of the specimen can be obtained 

even when there is a high level of background radiation. In comparing the two 

primary detection methods, the direct-exposure method offers high speed, 

unlimited image integration time, and the best spatial resolution. 

The transfer method offers insensitivity to the γ-rays emitted by the specimen 

and greater contrast because of lower amounts of scattered and secondary 

radiation. 

Keypoints: 

Direct Method: Best spatial resolution 

Transfer Method; Greater contrast 


The direct-exposure method offers: 

• high speed, 

• unlimited image integration time, and 

• the best spatial resolution.(?) 

The transfer method offers: 

• insensitivity to the γ-rays emitted by the specimen and 

• greater contrast because of lower amounts of scattered and secondary 

radiation. 


5.3 Real-time imaging, 

in which light from a scintillator is observed by a television camera, can also 

be used for neutron radiography. Because of low brightness, most real-time 

neutron radiographic images are enhanced by an image-intensifier tube, 

which may be separate or integral with the scintillator screen. This method 

can be used for such applications as the study of fluid flow in a closed system 

or the study of metal flow in a mold during casting. The lubricants moving in 

an operating engine have been observed with the real-time neutron imaging 

method. 


Real Time Radiography Set-Up 


http://crisasantos.com.br/com/neutron-radiography

6.0 Applications 

Various applications concerning the inspection of ordnance 军备物资 , 

explosive, aerospace, and nuclear components. The presence, absence, or 

correct placement of explosives, adhesives, O- ings, plastic components, and 

similar materials can be verified. Nuclear fuel and control materials can be 

inspected to determine the distribution of isotopes and to detect foreign or 

imperfect material. Ceramic residual core in investment cast turbine blades 

can be detected. Observations of corrosion in metal assemblies are possible 

because of the excellent neutron sensitivity to the hydrogenous corrosion 

product. Hydride deposition in metals and diffusion of boron in heat treated 

boron-fiber composites can be observed. The following examples illustrate 

the application of neutron radiography to the inspection of radioactive 

materials and several assemblies of metallic and nonmetallic components. 


Example 1: Thermal-Neutron Radiography Used to Determine Size of 

Highly Radioactive Nuclear Fuel Elements. 

Highly radioactive nuclear fuel elements required size measurements to 

determine the extent of dimensional changes that may have occurred during 

irradiation. Generally, inspection is done in a hot cell, but because hot-cell 

inspection is a relatively long, tedious, and costly procedure, neutron 

radiography was selected. The fuel elements to be inspected consisted of 6.4 

mm ( ¼ in.) diam cylindrical pellets of UO 2 -PuO 2 ; the plutonium content was 

20%, and the uranium was enriched in 235 U. The pellets had been irradiated 

to 10% burnup, which resulted in a level of radioactivity of 3 × 10 -2 Ci/kg∙h 

(10 KR/h) at 0.3 m (1 ft). Five elements were selected for inspection. A 

neutron radiograph was taken by activating 0.25 mm (0.010 in.) thick 

dysprosium foil with a transmitted beam of thermal neutrons. 

An autoradiograph of the activated-dysprosium transfer screen on a mediumpeed 

x-ray film yielded the result shown in the positive print in Fig. 6. 


Both 235U and plutonium have high attenuation coefficients for thermal 

neutrons. The high contrast of the fuel pellets made it possible to measure 

pellet diameter directly from the neutron radiographs. These measurements 

were both repeatable and statistically significant within 0.013 mm (0.0005 in.). 

Later, radiographic measurements were compared with physical 

measurements made in a hot cell. The two sets of values corresponded within 

0.038 mm (0.0015 in.). 


Fig. 6 Positive print of a thermal-neutron radiograph of five irradiated nuclear 

fuel elements, taken to determine if dimensional changes occurred during 

irradiation. Radiograph was made using a dysprosium transfer-screen 

method. Dark squares in middle element are voids. 

voids. 

Rods 


Example 2: Indium-Resonance Technique for Determining Internal 

Details of Highly Radioactive Nuclear Fuel Elements. 

The five nuclear fuel elements inspected for dimensional changes in Example 

1 were further inspected for internal details. This was necessary because the 

thermal-neutron inspection procedure did not reveal any internal details; it 

only shadowed the pellets, as shown by the positive print in Fig. 6. To inspect 

for internal details, an indium-resonance technique, which utilizes epithermal 

neutrons, was used. 

In this technique, a collimated neutron beam was filtered by 0.5 mm (0.02 in.) 

of cadmium to remove most of the thermal neutrons. Filtering produced a 

neutron beam with a nominal average energy somewhat above thermal. The 

epithermal neutron beam passed through the fuel elements and activated a 

sheet of indium foil. Neutrons with an energy of about 2.34×10 -19 J (1.46 eV), 

which is the resonance-absorption energy for indium (not the object!) , were 

primarily involved in activation. 

Comments: Indium-Resonance Technique is a type of transfer technique. 


The positive print of a radiograph made with epithermal neutrons shown in 

Fig. 7 reveals considerable internal details, in contrast to the lack of internal 

details in Fig. 6. With epithermal neutrons, there was less attenuation by the 

fuel elements than with thermal neutrons. Therefore, internal details that were 

not revealed by thermal neutron radiography: such as cracking or chipping of 

fuel pellets, and dimensional features of the central void in the fuel pellets 

(including changes in size and accumulation of fission products)- were 

revealed with the indium-resonance technique. 


Fig. 7 Positive print of a neutron radiograph of the same five nuclear fuel 

elements shown in Fig. 6. Radiograph was made with epithermal neutrons 

and an indium-resonance technique, and it reveals internal details not shown 

in the thermal-neutron radiograph in Fig. 6 


Example 3: Use of Conventional and Neutron Radiography to Inspect an 

Explosive Device for Correct Assembly. 

Small explosive devices assembled from both metallic and nonmetallic 

components required inspection to ensure correct assembly. The explosive 

and the components made of paper, plastic, or other low atomic number 

materials, which are less transparent to thermal neutrons than to x-rays, could 

be readily observed with thermal-neutron radiography. Metallic components 

were inspected by conventional x-ray radiography. A positive print of a 

thermal-neutron, direct-exposure radiograph of a 50 mm (2 in.) long explosive 

device is shown in Fig. 8(a). The radiograph was made on Industrex R film 

(Eastman Kodak), using a gadolinium-foil screen. Total exposure was 3 × 

10 9 n/cm 2 , which was achieved with an exposure time of 4 to 5 min. 


At the top in Fig. 8(a), just inside the stainless steel cap, can be seen a line 

image that corresponds to a moisture absorbent made of chemically treated 

paper. Below the paper is a mottled image, which is the explosive charge. 

Below the explosive charge are plastic components and, at the very bottom, 

epoxy. A conventional radiograph of the same device is shown in Fig. 8(b). 

The metallic components, which were poorly delineated in the thermalneutron 

radiograph, are more clearly seen in Fig. 8(b). Together, the two 

radiographs verified that both metallic and nonmetallic components were 

correctly assembled. 


Fig. 8 Comparison of positive prints of a thermal-neutron radiograph (a) and a 

conventional radiograph (b) of a 50 mm (2 in.) long explosive device. Neutron 

radiograph reveals details of paper, explosive compound, and plastic 

components not revealed by x-rays. 


Example 4: Use of Neutron Radiography to Detect Corrosion in Aircraft 

Components. 

Aluminum honeycomb components are extensively used for aircraft 

construction. The aluminum material is subject to corrosion if exposed to 

water or humid environments. Thermal-neutron radiography is an excellent 

method of detecting hidden corrosion in these assemblies. The corrosion 

products are typically hydroxides or water-containing oxides; these corrosion 

products contain hydrogen, a material that strongly attenuates thermal 

neutrons. The aluminum metal, on the other hand, is essentially transparent 

to the neutrons. Therefore, a thermal-neutron radiograph of a corroded 

aluminum honeycomb assembly shows the corrosion product and other 

attenuating components such as adhesives and sealants. Figure 9 depicts a 

thermal-neutron radiograph of an aluminum honeycomb assembly showing 

the beginnings of corrosion. 


The white line image the middle of the radiograph represents the adhesive 

coupling together two core sections. The faint white smears in the upper half 

of the image and the double dot in the lower left area are images of corrosion 

as disclosed by the thermal-neutron radiograph. Developmental work has 

shown that thermal-neutron imaging techniques are capable of detecting the 

corrosion product buildup represented by an aluminum metal loss of 25 μm 

(1000 μin.). The neutron method, therefore, is a very sensitive technique for 

the detection of corrosion. 

adhesive coupling 

corrosion 


Fig. 9 Thermal-neutron radiograph of aluminum honeycomb aircraft 

component showing early evidence of hydrogen corrosion. See text for 

discussion. Courtesy of D. Froom, U.S. Air Force, McClellan Air Force Base 


Example 5: Use of Neutron Radiography to Detect Corrosion in 

Adhesive - Bonded Aluminum Honeycomb Structures. 

Aluminum corrosion of aircraft surfaces has plagued both military and civilian 

aircraft. Identification of this corrosion has been difficult, at best, usually being 

detected after the corrosion has caused the part to fail. Of the nondestructive 

testing methods used to detect aluminum corrosion, thermal neutron 

radiography has proved the most sensitive method to date. The detection of 

aluminum corrosion is based on the attenuation properties of hydrogen 

associated with the corrosion products rather than aluminum and aluminum 

oxide with their low attenuation coefficients. Depending on the environment, 

the corrosion products include aluminum trihydrates, monohydrates, and 

various other aluminum salts. Because the linear attenuation coefficient for 

aluminum is similar to that of water and about 28 times greater than that for 

aluminum, a 0.13 mm (0.005 in.) corrosion layer should be detectable under 

optimum conditions. 


The sensitivity standard plate for aluminum corrosion fabricated by the 

Aeronautical Research Laboratories (Australia) contains corrosion products 

varying from 0.13 to 0.61 mm (0.005 to 0.024 in.) thick (Fig. 10). Aluminum 

corrosion of honeycomb structures is complicated by the bonding adhesives 

that may appear similar in a neutron radiograph (Fig. 11a) (see the article 

"Adhesive-Bonded Joints" in this Volume). Tilting of the honeycomb structure 

will alleviate this problem by allowing adhesive found along the bond lines to 

be distinguished from the randomly distributed corrosion products (Fig. 11b). 


Fig. 10 Standard plate for aluminum corrosion detection contains 0.13 to 0.61 

mm (0.005 to 0.024 in.) thick corrosion products. Courtesy of R. Tsukimura, 

Aerotest Operations Inc. 


Fig. 11 Effect of bonding adhesives on the quality of neutron radiographs 

obtained when checking for aluminum corrosion in honeycomb structures. 

Radiograph taken (a) normal to specimen surface and (b) tilted at any angle 

other than 90° to specimen surface. Courtesy of R. Tsukimura, Aerotest 

Operations Inc. 


Example 6: Use of Neutron Radiography to Verify Welding of Dissimilar 

Materials (Titanium and Niobium). 

Exotic metal welded joints are a product of the extremely cold environment of 

space and man's desire to explore the vast emptiness of space. For space 

vehicles, attitude control rockets provide the fine touch for proper vehicle 

alignment. 

For one application, a titanium-niobium welded joint was required between 

the light-weight propellant tank and the nozzle section. Attempts to verify weld 

integrity using conventional radiography were not productive. Thermal 

neutron radiography provided the image required to ensure quality welds. 

This defect standard weld shows the porosity at the seam and the similar 

thermal neutron attenuation for both titanium (Ti) and niobium (formerly 

known as columbium, Cb) (Fig. 12a). For comparison, the x-ray radiograph 

image is also shown (Fig. 12b). 


Fig. 12 Comparison of thermal neutron (a) and x-ray (b) radiographs of a 

titanium-niobium welded joint. Courtesy of R. Tsukimura, Aerotest Operations 

Inc. 


Example 7: Use of Neutron Radiography to Detect Core Material Still 

Remaining in the Interior Cooling Passages of Air-Cooled Turbine 

Blades. 

Investment casting of turbine blades using the lost wax process results in 

relatively clean castings. As the demand for higher-powered turbine engines 

has increased, the interior cooling passages for air-cooled turbine blades 

have become more and more complex. Concurrently, the removal of the core 

material has become increasingly more difficult. Incomplete removal of the 

core results in restricted flow through the cooling passages and possible 

failure of the overheated blade. 

Previously, visual inspection was the nondestructive inspection method of 

choice for residual core detection. However, current designs preclude the use 

of borescopes and other visual means for the interior passages. X- 

adiography has proved rather ineffective in detecting residual core material. 

Thermal neutron radiography is the nondestructive testing method of choice, 

especially when gadolinium oxide (Gd 2 O 3 ) is used to dope the core material 

(1 to 3% by weight) (Fig. 13) prior to casting the blade. 


When concerns about the possible detrimental effects of Gd 2 O 3 during the 

casting process prevents its use in the core material, a procedure was 

developed to tag the residual core material after the core removal process. 

The castings are dipped in a gadolinium solution [Gd(NO 3 ) 2 in solution] to 

impregnate any residual core, which is then imaged and subsequently 

detected by neutron radiography. The blades shown in Fig. 14 have been 

tagged. The neutron radiograph shows any residual core material greater 

than 0.38 mm (0.015 in.) in diameter. Figure 15 is a schematic of typical core 

fragments in investment cast turbine blades detected by thermal neutron 

radiography. Image clarity of gadolinium tagged or doped cores is much 

greater than that of normal cores. 


Fig. 13 Residual core material in a gas-cooled aircraft-engine turbine blade 

as detected by thermal neutron radiography. The excess core material, 

tagged with 1.5% Gd 2 O 3 , is shown circled in the second photo from the right. 

Courtesy of R. Tsukimura, Aerotest Operations Inc. 


Fig. 14 Thermal neutron radiograph of 12 turbine blades tagged with Gd 2 

O 3 

solution. 

One of the 12 blades (located in the top row and second from the left) contains 

residual core material in its upper right-hand corner cooling passage. Courtesy of R. 

Tsukimura, Aerotest Operations Inc. 


Fig. 15 Schematic of turbine blade core standards: gadolinium [Gd(NO 3 ) 3 in 

solution] tagged core, normal core (no gadolinium tagging or doping), and 

Gd 2 O 3 doped core. Typical core fragments of various thicknesses are shown. 

Source: R. Tsukimura, Aerotest Operations Inc. 



Example 8: Use of Neutron Radiography to Verify Position of Explosive 

Charges and Seating of O-Ring Seals in Explosive Bolt Assemblies. 

There are many critical applications of explosive release devices in aircraft, 

space, and missile systems. Nondestructive testing is an important step in the 

quality control portion of the production cycle for these units. 

Thermal neutron radiography has proved an indispensable tool in the 

nondestructive testing arsenal, particularly for thick-walled, metal devices, 

such as explosive bolts (Fig. 16). The inner details of explosive bolts can be 

imaged only by thermal neutron radiography methods (Fig. 17). This 

particular type of bolt from a missile system is activated from the bottom by 

actuating the firing pin onto the primer. The short section of mild detonating 

cord carries the energy to the output charge, which fractures the bolt and 

allows the bolt to be severed. In addition to the explosive charges, the internal 

O-ring seals, including the concentric pair around the firing pin and for the 

body are readily visible. For safety's sake, determining the presence of the 

shear pin can also be accomplished through the use of thermal neutron 

radiography. 


Fig. 16 Schematic showing location of critical components that comprise an 

explosive bolt. Source: R. Tsukimura, Aerotest Operations Inc. 


Fig. 17 Thermal neutron radiograph showing two sample bolts identical to the 

workpiece shown schematically in Fig. 16. Courtesy of R. Tsukimura, 

Aerotest Operations Inc. 


Example 9: Application of Neutron Radiography to Determine Potting 

Fill Levels in Encapsulated Electronic Filters. 

Electronic filters are an integral component of all space and satellite systems. 

Because the cost of these satellites is very high and the cost to repair them 

even more prohibitive, high reliability filters are necessary. A common mode 

of filter failure is that caused by inadequate potting of the internal components 

and the subsequent physical breakdown of the filter during periods of high 

vibration, such as that encountered during vehicle launch. Thermal neutron 

radiography is the method of choice for determining potting fill levels in 

encapsulated filters. The potting material attenuates the thermal neutrons and 

appears as the light density area. Voids in the potting material, the fill level, 

and the distribution can readily be detected with neutron radiography. 


Applications of Neutron Imaging in Earth Sciences and Biosciences 

2D & 3D Neutron radiography & Tomography 

Non-destructive imaging techniques are extremely powerful diagnostic tools in the 

study of internal structures without the need to break open the test sample. The 

requirement of non-destructive testing chiefly has two sources; the specimen may be 

rare or even unique (e.g. geological and archaeological artefacts), or breaking may 

defeat the purpose of the investigation (e.g. bonding between surfaces and internal 

fluid flows). 

The analytical information provided by neutron radiography (2-D) and tomography (3- 

D) are entirely complementary to their better-known x-ray and gamma-ray 

counterparts, but have important advantages; in particular, neutrons can penetrate 

many materials, including metals, relatively easily, whilst being highly sensitive to 

hydrogen. This has lead to application in fields as wide-ranging as archaeology, 

engineering, biomaterials, biology and earth sciences. 

Neutrograph at the ILL exploits the most intense neutron beam in world in use for this 

technique, giving it the highest time resolution. In the case of radiography, fast 

processes can be resolved below 1 ms, while 3-D tomographic imaging can be carried 

out in 10 s. Neutrograph is also highly sensitive in distinguishing between low 

contrasting materials. Currently, the spatial resolution is ~150 microns, but it is 

foreseen that this will improve in the near future. 


http://see.leeds.ac.uk/ebi/studentship-neutron.htm

Real Time Imaging: Combustion Chamber 



Tomography of Antarctic Conifer Fossils 

3-D tomographic reconstruction of a late Early Eocene age conifer fossil (~53 million years old), 

discovered on Seymour Island, West Antarctica reveals a beautifully preserved new flora, 

Araucariaceae, much resembling Araucaria araucana, the modern-day Monkey Puzzle tree. In 

this case, neutron tomography has replaced the time-consuming and utterly destructive serial 

thin-sections method. The 3-D structure has been held in tact with the leaves spiraling around a 

central woody stem. 



Dynamic Radiography of Fluid Flow Through Sandstone 

In situ 2-D dynamic radiographic imaging of consecutive fluid flows through 

an initially water-saturated cylindrical sandstone core, Ф 5cm. Internal 

structures and fault zones are clearly visible, and appear to behave quite 

differently for different fluid combinations. 

nitrogen-water 

oil-water 


water-nitrogen 

water-oil 


Neutron graph 



Neutron graph 






E1316 


E1316 Section H: Neutron Radiologic Testing (NRT) Terms 

The terms defined in Section H are the direct responsibility of Subcommittee 

E07.05 on the Radiology (Neutron) Method. Additional radiological terms can 

be found in Section D. 

activation - the process of causing a substance to become artificially 

radioactive by subjecting it to bombardment by neutrons or other particles. 

attenuation coefficient - related to the rate of change in the intensity of a 

beam of radiation as it passes through matter. (See linear and mass 

attenuation coefficient.) 

attenuation cross section - the probability, expressed in barns, that 

a neutron will be totally absorbed by the atomic nucleus. 

barn - a unit of area used for expressing the area of nuclear cross sections. 

1 barn = 10 -24 cm 2 (3) 


cadmium ratio - the ratio of the neutron reaction rate measured with a given 

bare neutron detector to the reaction rate measured with an identical neutron 

detector enclosed by a particular cadmium cover and exposed in the same 

neutron field at the same or an equivalent spatial location. 

NOTE 27 - In practice, meaningful experimental values can be obtained in an 

isotropic neutron field by using a cadmium filter approximately 1 mm thick. 

Cassette - light-tight device for holding film or conversion screens and film 

in close contact during exposure. 

contrast agent - a material added to a component to enhance details by 

selective absorption of the incident radiation. 


conversion screen - a device that converts the imaged neutron beam to 

radiation or light that exposes the radiographic film. 

cross section -theapparent cross-sectional area of the nucleus as 

calculated on the basis of the probability of occurrence of a reaction by 

collision with a particle. It does not necessarily coincide with the geometrical 

cross-sectional area πr 2 . It is given in units of area, 1 barn = 10 −24 cm 2 . 

direct exposure imaging - in the direct exposure imaging method, the 

conversion screen and image recorder are simultaneously exposed to the 

neutron beam. 

electron volt - the kinetic energy gained by an electron after passing through 

a potential difference of 1 V. 


facility scattered neutrons - neutrons scattered in the facility that contribute 

to the film exposure. 

γ - effective gamma content. γ is the percent background film darkening 

caused by low-energy photon radiation absorbed by pair production in 2 mm 

of lead. 


Pair Production 

hν = E - + E + = (m 0 c 2 + K - ) + (m 0 c 2 + K + ) = K - + K + + 2m 0 c 2 


http://electrons.wikidot.com/pair-production-and-annihilation

gamma ray - electromagnetic radiation having its origin in an atomic nucleus. 

half-life T ½ - the time required for one half a given number of radioactive 

atoms to undergo decay. 

half-value layer - the thickness of an absorbing material required to reduce 

the intensity of a beam of incident radiation to one-half of its original intensity. 

image quality indicator - a device or combination of devices whose image or 

images on a neutron radiograph provide visual or quantitative data, or both, 

concerning the radiographic sensitivity of the particular neutron radiograph. 


indirect exposure - a method in which only a gamma-insensitive conversion 

screen is exposed to the neutron beam. After exposure, the conversion 

screen is placed in contact with the image recorder. 

L/D ratio - one measure of the resolution capability of a neutron radiographic 

system. It is the ratio of the distance between the entrance aperture and the 

image plane (L) to the diameter of the entrance aperture (D). 

U g = Dt/L , I = Ф /[16∙(L/D) 2 ] 

Linear attenuation coefficient - a measure of the fractional decrease in 

radiation beam intensity per unit of distance traveled in the material (cm -1 ). 

low-energy photon radiation - Gamma- and X-ray photon radiation having 

energy less than 200 keV (0.2 Mev) (excluding visible and ultraviolet light). 


mass attenuation coefficient - a measure of the fractional decrease in 

radiation beam intensity per unit of surface density cm 2 ∙gm −1 . (cm 2 ∙g -1 ) 

Mass attenuation coefficient 

The mass attenuation coefficient or mass narrow beam attenuation coefficient 

of the volume of a material characterizes how easily it can be penetrated by a 

beam of light, sound, particles, or other energy or matter. In addition to visible 

light, mass attenuation coefficients can be defined for other electromagnetic 

radiation (such as X-rays), sound, or any other beam that attenuates. The SI 

unit of mass attenuation coefficient is the square meter per kilogram (m 2 /kg). 

Other common units include cm 2 /g (the most common unit for X-ray mass 

attenuation coefficients) and mL∙ g −1 ccm −1 (sometimes used in solution 

chemistry). "Mass extinction coefficient" is an old term for this quantity. 

The mass attenuation coefficient can be thought of as a variant of absorption 

cross section where the effective area is defined per unit mass instead of per 

particle. 

https://en.wikipedia.org/wiki/Mass_attenuation_coefficient 


moderator - a material used to slow fast neutrons. Neutrons are slowed 

down when they collide with atoms of light elements such as hydrogen, 

deuterium, beryllium, and carbon. 

Why the 

heavier 

elements 

were not 

used as 

moderator? 


NC - effective thermal neutron content or neutron radiographic contrast. NC is 

the percent background film exposure due to unscattered thermal neutrons. 

neutron - a neutral elementary particle having an atomic mass close to 1. In 

the free state outside of the nucleus, the neutron is unstable having a half-life 

of approximately 10 min. A neutron undergoes spontaneous beta decay to form a proton, a high 

energy electron (β particle) and an electron antineutrino. Whilst atomic and mass numbers are conserved in the 

process the combined mass of the products is slightly less than the original neutron mass. This accounts for the 

energy released. http://www.atnf.csiro.au/outreach//education/senior/cosmicengine/sun_nuclear.html 

http://scienceblogs.com/startswithabang/2013/07/0 

5/why-did-the-universe-start-off-with-hydrogenhelium-and-not-much-else/ 


neutron radiography - the process of producing a radiograph using neutrons 

as the penetrating radiation. 

object scattered neutrons - neutrons scattered by the test objects that 

contribute to the film exposure. 

P - effective pair production content. P is the percent background exposure 

caused by pair production in 2 mm of lead. 

pair production - the process whereby a gamma photon with energy greater 

than 1.02 MeV is converted directly into matter in the form of an electronpositron 

pair. Subsequent annihilation of the positron results in the production 

of two 0.511 MeV gamma photons. 

process control radiograph - a radiograph which images a beam purity 

indicator and sensitivity indicator under identical exposure and processing 

procedures as the test object radiograph. A process control radiograph may 

be used to determine image quality parameters in circumstances of large or 

unusual test object geometry. 


Pair Production 

hν = E - + E + = (m 0 c 2 + K - ) + (m 0 c 2 + K + ) = K - + K + + 2m 0 c 2 


http://electrons.wikidot.com/pair-production-and-annihilation

adiograph - a permanent, visible image on a recording medium produced by 

penetrating radiation passing through the material being tested. 

radiographic inspection - the use of X rays or nuclear radiation, or both, to 

detect discontinuities in material, and to present their images on a recording 

medium. 

radiography - the process of producing a radiograph using penetrating 

radiation. 

radiological examination - the use of penetrating ionizing radiation to 

display images for the detection of discontinuities or to help ensure integrity of 

the part. 


adiology - the science and application of X rays, gamma rays, neutrons, and 

other penetrating radiations. 

radioscopic inspection - the use of penetrating radiation and radioscopy to 

detect discontinuities in material. 

radioscopy - the electronic production of a radiological image that follows 

very closely the changes with time of the object being imaged. 

real-time radioscopy - radioscopy that is capable of following the motion of 

the object without limitation of time. 

S - effective scattered neutron content. S is the percent background film 

darkening caused by scattered neutrons. 

scattered neutrons - neutrons that have undergone a scattering collision but 

still contribute to film exposure. 


sensitivity value - the value determined by the smallest standard 

discontinuity in any given sensitivity indicator observable in the radiographic 

image. Values are defined by identification of type of indicator, size of defect, 

and the absorber thickness on which the discontinuity is observed. 

thermalization - the process of slowing neutron velocities by permitting the 

neutrons to come to thermal equilibrium with a moderating medium. 

thermalization factor - the inverse ratio of the thermal neutron flux obtained 

in a moderator, per source neutron. 

thermal neutrons - neutrons having energies ranging between 0.005 eV and 

0.5 eV; neutrons of these energies are produced by slowing down fast 

neutrons until they are in equilibrium with the moderating medium at a 

temperature near 20°C. 


total cross section - the sum of the absorption and scattering cross sections. 

vacuum cassette - a light-tight device having a flexible entrance window, 

which when operated under a vacuum, holds the film and conversion screen 

in intimate contact during exposure. 


Thermalization of Neutrons 

the last stage of the moderation of neutrons in various mediums, in which the 

role of chemical bonding and the thermal motion of the atoms of the medium 

become essential. 

When the kinetic energy of neutrons is reduced to values less than 1 electron 

volt, the neutron velocity becomes comparable to the velocity of thermal 

motion of atoms and molecules. Energy exchange arises between these 

species and the neutrons, leading to the establishment of an equilibrium 

Maxwellian velocity distribution of the neutrons. However, because of several 

factors, such as the motion and binding of atoms, absorption, and the finite 

size of the system, the energy spectra of neutrons in moderators differ from 

the equilibrium spectra. The study of the thermalization of neutrons is 

required for the calculation and prediction of the behavior of thermal reactors. 

Research in this area has been the source of new methods for the study of 

the physics of solids and liquids. 


http://encyclopedia2.thefreedictionary.com/Neutrons%2c+Thermalization+of

Neutron temperature 

The neutron detection temperature, also called the neutron energy, indicates 

a free neutron's kinetic energy, usually given in electron volts. The term 

temperature is used, since hot, thermal and cold neutrons are moderated in a 

medium with a certain temperature. The neutron energy distribution is then 

adopted to the Maxwellian distribution known for thermal motion. Qualitatively, 

the higher the temperature, the higher the kinetic energy is of the free neutron. 

Kinetic energy, speed and wavelength of the neutron are related through the 

De Broglie relation. 


https://en.wikipedia.org/wiki/Neutron_temperature

Maxwellian distribution 


http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/kintem.html

Neutron energy distribution ranges 

Neutron energy range names 

But different ranges with different names 

are observed in other sources. For 

example, Epithermal neutrons have 

energies between 1 eV and 10 keV and 

smaller nuclear cross sections than 

thermal neutrons. 



Thermal 

Neutrons in thermal equilibrium with their surroundings 

Most probable energy at 20 degrees (C) - 0.025 eV; Maxwellian distribution 

of 20 degrees (C) extends to about 0.2 eV. 

Epithermal 

Neutrons of energy greater than thermal 

Greater than 0.2 eV 

Cadmium 

Neutrons which are strongly absorbed by cadmium 

Less than 0.4 eV 

Epicadmium 

Neutrons which are not strongly absorbed by cadmium 

Greater than 0.6 eV 



Slow 

Neutrons of energy slightly greater than thermal 

Less than 1 to 10 eV (sometimes up to 1 keV) 

Resonance 

In pile neutron physics, usually refers to neutrons which are strongly captured 

in the resonance of U-238, and of a few commonly used detectors (e.g., 

Indium, Gold, etc.) 

1 eV to 300 eV 

Intermediate 

Neutrons that are between slow and fast 

Few hundred eV to 0.5 MeV 

Fast 

Greater than 0.5 MeV 



Ultrafast 

Relativistic 

Greater than 20 MeV 

Pile 

Neutrons of all energies present in nuclear reactors 

0.001 eV to 15 MeV 

Fission 

Neutrons formed during fission 

100 keV to 15 MeV (Most probable: 0.8 MeV; Average: 2.0 MeV) 



Ultracold neutrons (UCN) 

Ultracold neutrons are free neutrons which can be stored in traps made from 

certain materials. 



Thermal neutrons 

A thermal neutron is a free neutron with a kinetic energy of about 0.025 eV 

(about 4.0×10 -21 J or 2.4 MJ/kg, hence a speed of 2.2 km/s), which is the 

energy corresponding to the most probable velocity at a temperature of 290 K 

(17 °C or 62 °F), the mode of the Maxwell–Boltzmann distribution for this 

temperature. 

After a number of collisions with nuclei (scattering) in a medium (neutron 

moderator) at this temperature, neutrons arrive at about this energy level, 

provided that they are not absorbed. 

Thermal neutrons have a different and often much larger effective neutron 

absorption cross-section for a given nuclide than fast neutrons, and can 

therefore often be absorbed more easily by an atomic nucleus, creating a 

heavier, often unstable isotope of the chemical element as a result (neutron 

activation). 



Fast neutrons 

A fast neutron is a free neutron with a kinetic energy level close to 1 MeV 

(100 TJ/kg), hence a speed of 14,000 km/s, or higher. They are named fast 

neutrons to distinguish them from lower-energy thermal neutrons, and highenergy 

neutrons produced in cosmic showers or accelerators. 

Fast neutrons are produced by nuclear processes: 

nuclear fission produces neutrons with a mean energy of 2 MeV (200 TJ/kg, 

i.e. 20,000 km/s), which qualifies as "fast". However the range of neutrons 

from fission follows a Maxwell–Boltzmann distribution from 0 to about 14 MeV 

in the center of momentum frame of the disintegration, and the mode of the 

energy is only 0.75 MeV, meaning that fewer than half of fission neutrons 

qualify as "fast" even by the 1 MeV criterion. nuclear fusion: deuterium–tritium 

fusion produces neutrons of 14.1 MeV (1400 TJ/kg, i.e. 52,000 km/s, 17.3% 

of the speed of light) that can easily fission uranium-238 and other non-fissile 

actinides. 

Fast neutrons can be made into thermal neutrons via a process called 

moderation. This is done with a neutron moderator. In reactors, typically 

heavy water, light water, or graphite are used to moderate neutrons. 



other non-fissile actinides. 

■ 




other non-fissile actinides. 



Fast reactor and thermal reactor compared 

Most fission reactors are thermal reactors that use a neutron moderator to 

slow down ("thermalize") the neutrons produced by nuclear fission. 

Moderation substantially increases the fission cross section for fissile nuclei 

such as uranium-235 or plutonium-239. In addition, uranium-238 has a much 

lower capture cross section for thermal neutrons, allowing more neutrons to 

cause fission of fissile nuclei and propagate the chain reaction, rather than 

being captured by 238U. The combination of these effects allows light water 

reactors to use low-enriched uranium. Heavy water reactors and graphitemoderated 

reactors can even use natural uranium as these moderators have 

much lower neutron capture cross sections than light water. (moderation 

without capture?) 

An increase in fuel temperature also raises U-238's thermal neutron 

absorption by Doppler broadening, providing negative feedback to help 

control the reactor. Also, when the moderator is also a circulating coolant 

(light water or heavy water), boiling of the coolant will reduce the moderator 

density and provide negative feedback (a negative void coefficient). 



Intermediate-energy neutrons have poorer fission/capture ratios than either 

fast or thermal neutrons for most fuels. An exception is the uranium-233 of 

the thorium cycle, which has a good fission/capture ratio at all neutron 

energies. 

Fast reactors use unmoderated fast neutrons to sustain the reaction and 

require the fuel to contain a higher concentration of fissile material relative to 

fertile material U-238. However, fast neutrons have a better fission/capture 

ratio for many nuclides, and each fast fission releases a larger number of 

neutrons, so a fast breeder reactor can potentially "breed" more fissile fuel 

than it consumes. 

Fast reactor control cannot depend solely on Doppler broadening or on 

negative void coefficient from a moderator. However, thermal expansion of 

the fuel itself can provide quick negative feedback. Perennially expected to be 

the wave of the future, fast reactor development has been nearly dormant 

with only a handful of reactors built in the decades since the Chernobyl 

accident due to low prices in the uranium market, although there is now a 

revival with several Asian countries planning to complete larger prototype fast 

reactors in the next few years. 






Neutrons provide unique penetrating 

radiation 


http://spie.org/x18990.xml

In the short-wavelength world of matter waves -- past the UV portion of the 

spectrum and beyond x rays -- materials look very different and the rules of 

imaging are different than with photons. "Neutrons show you things that x 

rays will never be able to," said Wade Richards of the Univ. of 

California/Davis research nuclear reactor in Sacramento, CA (Figure 1). 

Figure 1. Neutron radiograph of a flower corsage shows the method's ability 

to image thin biological samples. Photo courtesy of Nray Services. 



Neutron basics 

The ways that neutrons interact with matter are very different from the way x 

rays interact with matter. 

• X rays interact with the electron cloud surrounding the nucleus of an atom. 

• Neutrons interact with the nucleus itself. 

• In general, x-ray attenuation increases as the atomic number of the target 

material increases; usually, the attenuation is greater for lower energy x 

rays. 

• For Neutron radiography, some light elements (such as hydrogen, boron, 

and carbon) have high thermal neutron attenuation coefficients, while 

some heavier elements (such as lead) have relatively small attenuation 

coefficients. 



The methods can be used in a complementary fashion. And while the imaging 

materials are different, neutron radiography is similar to x-ray radiography: a 

beam of particles penetrates the target, and the shadow of the device is 

captured. 

Patrick Doty, Senior Scientist at Sandia National Labs. (Albuquerque, NM) 

said that because the intensity of neutron scattering varies irregularly with 

atomic number and neutron energies vary over a large range (and thus a 

wide range of useful wavelengths), neutrons can probe the structure of matter 

over many scales of length. 



FIG. X1.1 Approximate Mass Attenuation Coefficients μ/ρ as a Function of 

Atomic Number 


FIG. X1.2 Calculated Thermal Neutron and 100 and 500 KEV X-Ray Linear 

Attenuation Coefficients (μ) as a Function of Atomic Number (A) 


Neutrons can be divided into several energy groups: 

■ cold, 

■ thermal, 

■ epithermal, 

■ fast, and 

■ relativistic. 

This article concentrates on thermal and fast neutrons. Thermal neutrons 

have energies of roughly 0.03 eV or less, whereas fast neutrons are 10 to 15 

MeV. Fission reactors produce neutrons with a wide range of energies, but 

because most reactors also have large volumes of moderator (to slow the 

neutrons down so they will be more efficient at initiating new fissions), 

reactors are excellent sources of thermal neutrons. 

Beams of fast neutrons can (also) be generated indirectly from particle 

accelerators (which accelerate charged particles into a target that gives off 

neutrons). "Fast neutrons are very highly penetrating," Doty said. "You can 

tailor the energies to look at a material, or you can look at shielded things -- 

you can look through lead shielding." Radioisotopes such as californium and 

americium-beryllium can generate fast neutrons as well, although this is not 

generally used for radiography. 



3.3 Radioactive Sources. 

There are many possible radioactive sources. The characteristics of several 

radioisotopes that are commonly used are summarized in Table 3. 



(γ, n) 

γ 

(α, n) 

γ 

(α, n) 

γ 




(α, n) 

(α, n) 

γ 

252 

Cf 

■ 



Thermal neutrons from reactors 

At the McClellan Nuclear Radiation Center (MNRC) located just north of 

Sacramento, CA, Wade Richards, Tom Majchrowski and others use thermal 

neutrons from the small nuclear reactor for a variety of applications, including 

inspecting aircraft parts for corrosion. 

The MNRC is a small nuclear reactor: whereas a power-generating reactor 

produces 2000 to 3000 MW, the McClellan reactor produces 2 MW. In 

nuclear reactors, neutrons are generated by fission in uranium. One neutron 

hits a 235 U and two neutrons emerge. The neutrons emerge isotropically 各向 

同性的 , heading in all directions. To use the neutrons for imaging, part of the 

shielding (10-ft.-thick concrete walls outside the tank of water in which the 

uranium sits) is removed. Pipes lined with neutron-absorbing and -scattering 

materials remove the particles that are too far out of line, providing a 

collimated beam of neutrons to the imaging bays. The reactor provides a flux 

of about 4 X 10 6 neutrons/cm 2 /s at the radiography plane. 



Richards said that in the 2D and 3D computer tomography neutron 

radiography systems in his facility, a converter screen uses gadolinium (which 

has a high cross-section for thermal neutrons), which emits beta particles 

when bombarded with neutrons, and the beta particles are caught by a 

fluorescing zinc sulfide material. Once the image has been converted to 

visible light, it can be captured on film or by a video camera sitting outside the 

neutron-beam axis. (Other converters can also be used, including lithium 

carbonate and plastic scintillators, depending on the application and neutron 

energies used.) A major application of neutron radiography at the reactor is 

for real-time imaging of military aircraft wings as a standard maintenance 

method to detect corrosion. 

Keywords: 

Converter screen: Gadolinium, Indium, Dysprosium 

Scintillator screen: (1) Zinc sulfide, (2) Lithium carbonate, (3) plastid 

scintillator 

Question: Is Cadmium used as converter screen? 



The McClellan site is the only facility in the world equipped with a robot stage 

and a video-camera radiography system that allows real-time imaging of 

objects 34 ft. long by 12 ft. high and as heavy as 5000 pounds. 

There is always some concern about how much one can bombard the object 

before the nucleus becomes radioactive. For aluminum, Richards said there 

is a 2.5-minute decay time. "Within 10 minutes, it's all gone." 



McClellan Nuclear Radiation Center (MNRC) 







http://mnrc.ucdavis.edu/ 



McClellan Nuclear Radiation 

Center (MNRC) 




McClellan Nuclear Radiation 

Center (MNRC) 




Thermal neutron radiography 

Glen MacGillivray at Nray Services (Petawawa, ON, Canada) described the 

complementary relationship between x-ray and neutron radiography: "When 

one is looking for a flaw, the contrast between the flaw and the unflawed 

material is paramount. If the attenuation in the material is too large, then 

insufficient beam penetrates to allow inspection." For applications that require 

distinguishing materials that attenuate differently, usually one wants to find 

the higher attenuator within a bulk material of lower attenuation. "So," 

MacGillivray said, "finding lead in a paraffin block (or a needle in a haystack) 

would work for x rays while looking for paraffin in a lead block (or a straw in a 

needlestack) would work for neutrons." 

In addition to imaging corrosion in aircraft wings, neutron radiography's ability 

to detect hydrogen and carbon yield other applications. Neutron computer 

tomography has been used to analyze flaws and misalignments in o-rings, 

which are used for sealing joints in rockets and other heavy machinery. 



The method allows researchers to look through the outer (usually metallic) 

materials to see how artifacts from the fabrication process sometimes put 

crimps into the rings when they are in place (work of this sort for NASA has 

been done at McClellan). 

In a similar way, neutron radiography can be used for viewing lubricants or 

other details of interest within a metal structure (Figure 2), or for looking at 

explosives contained within metals. 

"Thermal neutron radiography is limited to reactor-based sources for highresolution 

production-rate work," MacGillivray said. Accelerator and isotopic 

sources of neutrons can be used for imaging, but with sacrifices in speed 

and/or resolution. Images are obtained using a variety of techniques, 

including direct to film from gadolinium foil, indirect to film from activated foils 

of dysprosium (Dy, atomic number 66) or indium, to a CCD device from a 

scintillator, or to an imaging plate from a scintillator. "Imaging speeds of 

several thousand frames per second have been obtained," MacGillivray said. 



Figure 2. Neutron radiograph of a handgun shows a wide range of contrasts 

within a complex mechanical structure. 



MacGillivray said the neutron flux required for imaging depends on the 

application. "We have successfully used image-plane neutron fluxes ranging 

from 5 X 10 4 to 4 X 10 7 neutrons/cm 2 /s for film imaging," he said. 

MacGillivray will be presenting his invited paper, "Imaging with neutrons: the 

other penetrating radiation," at the Penetrating Radiation Systems and 

Applications conference at SPIE's Annual Meeting in July. 

In addition to the applications mentioned above, neutron radiography can be 

used with a contrast agent to look for residual ceramic core material inside jet 

engine turbine blades. Another application is an indirect method of inspecting 

nuclear fuel -- a very radioactive material that would, if inspected directly, fog 

the film (by the gamma ray) . 



Fast neutron radiography 

Beams of fast neutrons, with energies from 10 to 15 MeV, offer different 

imaging capabilities than thermal neutrons. These energetic particles can be 

produced by linear accelerators. 

Robert Hamm of AccSys Technology (Pleasanton, CA) said his company 

generates neutrons from linear accelerators by knocking an electron off a 

proton or deuteron (usually from hydrogen gas), accelerating the positively 

charged particle, then bombarding a target (usually beryllium). The target 

then emits secondary radiation (neutrons), mostly moving in the same 

direction as the ion beam. (Figure 3). "Fast neutrons penetrate matter very 

easily," Hamm said. 



Figure 3. In addition to being gathered from nuclear reactors, neutrons can 

be generated by linear accelerators. This machine generates neutrons by 

accelerating protons into a beryllium target. The systems can generate from 

10 8 to 10 13 neutrons/second. Photo courtesy of AccSys Technology. 

Keypoint: 

Neutron Accelerator Target material: Beryllium, Be. 



Fast neutron radiography can be used in prospecting for diamonds or other 

minerals, Hamm said. It also shows mineral inclusions in rocks. For this 

application, users shine two beams at the target, one after the other. One 

neutron beam is at a resonant energy for the mineral, the other is offfrequency. 

The difference between the images provides information about the 

interior of the rock. 

Meanwhile, James Hall and others at Lawrence Livermore National Labs 

(Livermore, CA) produce fast neutrons using a "DD source." This is a system 

in which a deuterium beam is accelerated to the desired energies and hits 

deuterium with a pressurized gas cell (at 2 to 3 atm). The interaction creates 

neutrons mostly going in the same direction as the incoming beam. 

Hall uses fast neutrons to penetrate steel, lead, or uranium. "We can look at 

voids of a few cubic millimeters in size behind up to four inches of uranium," 

he said, "and we can see it well." 



Hall wants to create a system that will fit into a small laboratory and can 

image cubic-millimeter-scale voids or other structural defects in heavily 

shielded thick materials. The system should also be able to acquire 

tomographic image data sets, allowing users to gain a 3D image of the object. 

To detect fast neutrons, Hall's group is using a rigid 4-cm-thick plastic 

scintillator indirectly viewed by a single commercial CCD camera. A thin 

mirror made of front-surfaced-aluminized Pyrex glass reflects light from the 

scintillator to the camera, which is well out of the neutron beam path. The 

camera's CCD is a cryogenically cooled thinned, back-illuminated 1024- X 

1024-pixel CCD with an antireflective coating on its active area. The detector 

can be used for image integration times as long as an hour. 



Applications include looking at thick objects not well penetrated by thermal 

neutrons. Hall's group has imaged a 6-in.-thick uranium and polyethylene slab 

assembly (with features machined into the polyethylene). The group also 

imaged a set of nine conventional step wedges. The step wedges were 

fabricated from lead, Lucite®, mock high explosive, aluminum, beryllium, 

graphite, brass, polyethylene, and stainless steel. All were 0.5-in.-thick pieces 

with uniform steps ranging from 0.5 in. to 5 in. in width. All of steps in each of 

the materials within the detector's field of view could be discerned in the final 

processed image. A series of other imaging experiments, including 

tomographic imaging, have also been performed. 



End Of Reading 4 



Neutrons Radiography mini article 




Similar to x-ray radiography, neutron radiography is a very efficient tool to 

enhance investigations in the field of non-destructive testing (NDT) as well as 

in many fundamental research applications. Neutron radiography is, however, 

suitable for a number of tasks impossible for conventional x-ray radiography. 

The advantage of neutrons compared to x-rays is the ability to image light 

elements (i.e. with low atomic numbers) such as hydrogen, water, carbon etc. 

In addition, neutrons penetrate heavy elements (i.e. with high atomic numbers) 

such as lead, titanium etc. allowing the study of materials in complex sample 

environments, for example water accumulation in hydrogen fuel cells: 

see Fig. 1. 

Because neutrons interact with the nucleus rather than with the electron shell, 

they can also distinguish between different isotopes of the same element. 

This makes neutron radiography an important tool in various research 

applications and in the field of NDT. The MNRC high neutron intensity beams 

permit short exposure times, high spatial resolution and high sample 

throughput. 


Fig. 1. Left: Photograph of a hydrogen fuel cell. Right: False colored neutron 

radiograph of a fuel cell showing the water content of the cell during operation. 


Methods of neutron radiography 

The detection of neutrons relies on a conversion into visible light; to achieve 

this, conversion screens containing either Gd or 6 Li and a fluorescence 

material are commonly used. After the conversion the emitted light can be 

detected by different media such as: 

• Film, that is then developed in a dark room and results in a permanent 

image, 

• Imaging Plates, that can be re-used after being processed by an image 

reader. (The technology is very similar to x-ray imaging plates used at 

medical offices), 

• Digital cameras (CCD, CMOS), allow to capture the image digitally. 


Differences between neutron and x-ray radiography 

Neutron radiography is based on the principal that neutrons interact with the 

nucleus of the atom, rather than the electrons. Therefore neutrons are 

absorbed in matter very differently from x-rays and gamma rays. This means 

that, contrary to x-rays, neutrons are attenuated by some light materials, such 

as hydrogen, boron and lithium, but penetrate many heavy materials such as 

titanium and lead. This allows for some unique applications of neutron 

radiography. 

The figures 2 below impressively demonstrate how neutron radiography can 

yield different yet complementary information to x-ray radiography. 


Fig.2. X-ray and Neutron radiographs of a 35 mm film SLR camera. Dark elements in the x-ray 

radiographs are caused by metal components; comparison shows that they are almost 

transparent for neutrons. Dark componets in the neutron radiograph are due to plastic 

components which in turn are almost transparent to x-rays. 

X-Ray Radiograph 


Fig.2. X-ray and Neutron radiographs of a 35 mm film SLR camera. Dark elements in the x-ray 

radiographs are caused by metal components; comparison shows that they are almost 

transparent for neutrons. Dark componets in the neutron radiograph are due to plastic 

components which in turn are almost transparent to x-rays. 

Neutron Radiograph 


Applications 

Neutron Radiography has a wide range of uses, including: 

• Imaging casting to ensure that the mold materials don't carry into the 

castings as impurities. 

• Validating the proper fill of pyrotechnical in actuators 

• Studying the flow of oil in automobile transmissions 

• Facilitate Fluid flow analysis 

• Analyze O-ring placements 

• Image carbon, gun powder grain structure, plastics, lead, and other heavy 

metals. 


End Of Reading 5 


Screen Types 

1. Transfer screen-indium or dysprosium, In, Dy. 

2. Thermal neutron filter using Cadmium for epithermal neutron radiography, 

Cd. 

3. Converter screen uses gadolinium which emit beta particles, Gd. 

4. the beta particles are caught by a fluorescing zinc sulfide material 

5. Scintillator screen: Zinc sulfide, Lithium carbonate, plastid scintillator 

6. Neutron Accelerator Target material: Beryllium, Be. 


■ωσμ∙Ωπ∆º≠δ≤>ηθφФρ|β≠Ɛ∠ ʋ λαρτ√ ≠≥ѵФ 


Peach – 我爱桃子 


Good Luck 


Good Luck 


Charlie https://www.yumpu.com/en/browse/user/charliechong 

Chong/ Fion Zhang

Understanding Neutron Radiography Reading II-TNR of Materials

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