Material classification by Fast Neutron Scattering Analysis

Nuclear Instruments and Methods in Physics Research B 173 (2001) 483±502 

www.elsevier.nl/locate/nimb 

Material classi®cation by fast neutron scattering 

A. Bu‚er a , F.D. Brooks a, * 

, M.S. Allie a , K. Bharuth-Ram b , M.R. Nchodu c 

a Physics Department, University of Cape Town, Rondebosch 7700, South Africa 

b Physics Department, University of Durban-Westville, Durban 4000, South Africa 

c Physics Department, University of the Western Cape, Bellville 7535, South Africa 

Received 10 May 2000; received in revised form 22 August 2000 

Abstract 

The scattering of a beam of fast monoenergetic neutrons is used to determine elemental compositions of bulk 

samples (0.2±0.8 kg) of materials composed from one or more of the elements H, C, N, O, Al, S, Fe and Pb. Scattered 

neutrons are detected by liquid scintillators placed at forward and at backward angles. Di€erent elements are identi®ed 

by their characteristic scattering signatures derived either from a combination of time-of-¯ight and pulse height 

measurements, or from pulse height measurements alone. Scattering signatures measured for multi-element samples are 

analysed to determine atom fractions for H, C, N, O and other elements in the sample. Atom fractions determined from 

scattering signatures are insensitive to neutron interactions in material surrounding the scattering sample, provided the 

amount of material is not excessive. The atom fraction data are used to classify scattering material into categories 

including `èxplosives'', `ìllicit drugs'' and `òther materials'' for the purpose of contraband detection. Ó 2001 Elsevier 

Science B.V. All rights reserved. 

PACS: 87.53Pb; 29.30Hs 

Keywords: HCNO analysis; Fast neutron scattering; Non-intrusive contraband detection 

1. Introduction 

Neutron methods for examining materials in 

bulk are currently under investigation and a variety 

of di€erent approaches [1±19] are being explored. 

There is particular interest in the 

possibility of using neutrons for the non-intrusive 

* Corresponding author. Tel.: +27-21-6503325; fax: +27-21- 

6503342. 

E-mail addresses: abu‚er@physci.uct.ac.za (A. Bu‚er), 

brooks@physci.uct.ac.za (F.D. Brooks). 

detection of hidden contraband, especially explosives 

or illicit drugs, in packages ranging in size 

from carry-on airline baggage to cargo containers. 

Among the neutron-based techniques that have 

been proposed for baggage or cargo inspection are 

pulsed fast neutron transmission spectroscopy 

(PFNTS) [8±11], thermal neutron analysis (TNA) 

[12], pulsed fast neutron analysis (PFNA) [13,14] 

and pulsed fast-thermal neutron analysis 

(PFTNA) [15,16]. PFNTS is based on measurements 

of the attenuation of a pulsed fast neutron 

beam as it passes through the object or material 

0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. 

PII: S 0 168-583X(00)00425-0

484 A. Bu‚er et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 483±502 

being interrogated. TNA, PFNA and PFTNA 

identify di€erent chemical elements via characteristic 

gamma rays which are excited either by 

thermal neutron capture (TNA, PFTNA) or by 

inelastic neutron scattering (PFNA, PFTNA) in 

the interrogated material. The technique of fast 

neutron scattering analysis (FNSA) is an alternative 

approach [4,17±19] in which neutrons scattered 

out of the interrogated material are detected. 

The nuclides responsible for the scattering are 

determined from measurements of the dependence 

of scattered neutron intensity and energy on scattering 

angle and the incident neutron energy. 

The FNSA technique described in [18] consists 

of bombarding the sample of material being examined 

with a beam of monoenergetic neutrons, 

alternating the incident neutron energy between 

two carefully chosen values, and using two detectors 

to observe scattered neutrons, one detector at 

a forward angle (45°) and the other at a backward 

angle (150°). The pulse height resolution of the 

neutron detectors should ideally, but not necessarily, 

be sucient to resolve the small energy 

di€erences (4%) between neutrons backscattered 

by the elements C, N and O. Time-of-¯ight can 

also be used to resolve larger energy di€erences, 

for example to separate elastically and inelastically 

scattered neutrons. Measurements from the two 

detectors at two di€erent incident neutron energies 

are combined to form a ``scattering signature'' [18]. 

Recent tests of FNSA [18] were concentrated on 

materials composed exclusively from the elements 

H, C, N and O, which are the most important 

atomic constituents of contraband materials such 

as explosives and illicit drugs. Measurements on 

pure elemental samples such as carbon (graphite), 

liquid nitrogen or liquid oxygen and on a water 

sample provide scattering signatures for these four 

elements. The signatures are strongly characteristic 

of the scattering element and provide a reference 

matrix which can be used to analyze the scattering 

signature measured for a multi-element sample. 

The signature measured for a scattering sample of 

unknown material is unfolded into components 

corresponding to elemental scattering signatures 

(H, C, N and O) to determine the atom fraction of 

each element in the sample. Atom fractions are 

then used to identify the scattering material. It has 

been shown [18] that atom fractions of the elements 

in a small sample (0.2±0.8 kg) of a HCNO 

material can be measured to an accuracy of a few 

percent by means of FNSA and that explosive or 

illicit drug material can be reliably identi®ed from 

these measurements. 

In this paper we describe further experimental 

studies of FNSA and present a new method for 

using the atom fractions determined from FNSA 

measurements to classify materials into categories 

appropriate for contraband detection, such as 

`èxplosive'', `ìllicit drug'' and `òther''. The new 

experimental studies aimed (a) to extend the 

FNSA method to materials containing a wider 

range of elements, namely H, C, N, O, Al, S, Fe 

and Pb, (b) to investigate alternative FNSA scattering 

signatures, and (c) to investigate the e€ects 

of neutron interactions in material close to the 

scattering sample, especially material in the incident 

neutron beam, or between the sample and the 

detectors. Information obtained from these studies 

provides a further proof-of-principle test of FNSA 

and can be used to assess the feasibility of possible 

practical applications of this technique. 

2. Experiments 

The experimental arrangement used for the 

FNSA studies is shown schematically in Fig. 1. 

Monoenergetic neutrons were obtained from the 

2 H…d; n† 3 He reaction by bombarding a deuterium 

gas target (30 mm long, 1.0 atm) with a pulsed 

beam (repetition frequency 2 MHz, FWHM 2 ns) 

of 4.7 MeV deuterons, and selecting neutrons 

emitted at 0° by means of the aperture in the shield 

SH. The pulsed deuteron beam was provided by 

the 5.5 MV pulsed Van de Graa€ accelerator of the 

South African National Accelerator Centre, Faure, 

Western Cape. The energy E n of the neutron 

beam was alternated between 6.8 and 7.5 MeV 

[18,19] at intervals of about 2 min by moving an 

energy-degrading Havar foil HF (11 lm) in and 

out of the deuteron beam. Neutrons scattered by 

the sample S were detected by the liquid scintillators 

F and B at forward and backward scattering 

angles h F ˆ 45° and h B ˆ 150°, respectively. Detectors 

F and B were NE213 liquid scintillators of

A. Bu‚er et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 483±502 485 

Fig. 1. Schematic diagram of the geometry used for FNSA test measurements, showing: NE213 neutron detectors, F, B and M; degrader 

foil HF; neutron shielding SH; copper scatterer CS; scattering sample S; and additional scatterer positions S 1 ±S 4 . Distances are 

indicated in mm. 

dimensions 130 130 70 mm and 40 mm 

diam: 30 mm, respectively, subtending solid 

angles of 89 and 13 msr at S. The neutron beam 

was monitored by an NE213 liquid scintillator 

detector M (50 mm diam: 50 mm) embedded in 

the shield (Fig. 1) and positioned to detect neutrons 

scattered by a copper plate CS (1 mm thick) 

mounted inside the collimator. Detectors B, F 

and M were connected to scintillation pulse shape 

discriminators to select neutrons and reject gamma 

rays. 

The scattering samples studied included 

graphite (carbon), liquid nitrogen, liquid oxygen, 

aluminium, sulphur, iron and lead, and compounds 

of these elements with one-another and 

with the elements hydrogen, lithium and boron. 

For scattering measurements on liquid nitrogen or 

liquid oxygen, the liquid was contained in a 

spherical Dewar ¯ask of inner diameter 100 mm. 

The other scattering samples were cylinders 

(60 mm diam: 120 mm), which were suspended 

so that their axes were aligned along the neutron 

beam. Solid cylinders were used for the metal and 

graphite samples. Samples of the other materials 

were contained in light (0.03 kg) aluminium canisters. 

Backgrounds were measured using either no 

sample or an empty sample container. All measurements 

were normalized to the same incident 

neutron ¯uence based on the counts recorded by 

the monitor M (Fig. 1). The regions labelled S 1 ±S 4 

in Fig. 1 indicate positions at which one or more 

additional scatterers were placed for the experiments 

described in Section 4.3. These experiments 

were carried out to study the e€ects of neutron 

scattering outside the sample (in the incident beam 

or between the sample and the detectors) on 

FNSA signatures. 

Pulse height L and time-of-¯ight T were measured 

for neutrons detected by F and B in the 

scattering experiments. Data were acquired on 

disk in multiparameter event mode, at 12-bit (4096 

channels) resolution for each parameter. In the o€line 

analysis adjacent channels were binned in 

groups of between 2 and 32 channels, in order to 

achieve appropriate dispersions in L and T, as 

shown in Figs. 2±6. A pattern register was used to 

record whether the Havar foil HF (Fig. 1) was in 

or out of the beam, and which detector (F, B or M) 

was active in the event. 

Figs. 2 and 3 show numbers of events as a 

function of L and T, recorded using graphite and 

water scattering samples, respectively: (a) and (b) 

by detector F; (c) and (d) by detector B; (a) and (c) 

for 6.8 MeV incident neutrons; and (b) and (d) for 

7.5 MeV incident neutrons. Pulse height scales L 

are calibrated in terms of the recoil proton energy 

E p recorded by the liquid scintillator detector. 

Time-of-¯ight scales T are calibrated in terms of


Fig. 2. Counts (vertical) as a function of pulse height L and time-of-¯ight T, recorded using a graphite scattering sample: (a) and (b) by detector F (45°); (c) and (d) by 

detector B (150°); (a) and (c) for incident neutrons of energy 6.8 MeV; and (b) and (d) for incident neutrons of energy 7.5 MeV. Ridges attributed to elastic and inelastic 

neutron scattering are indicated by EC and I4, respectively. Ridge B is attributed to neutron background. LC1±LC4 and TC1±TC4 indicate pulse height and time-of-¯ight 

cuts, respectively. LTF and LBF indicate pulse height thresholds for the F and B detectors. Count scales are normalized to a standard number of neutron beam monitor 

counts and to a sample mass corresponding to 100 NA carbon nuclei, where NA ˆ Avogadro's number.


Fig. 3. Counts (vertical) as a function of pulse height L and time-of-¯ight T, recorded using a water scattering sample. Ridges attributed to elastic neutron scattering on 

hydrogen and oxygen are indicated by EH and EO, respectively. Count scales are normalized to a standard number of neutron beam monitor counts and to a sample mass 

corresponding to 100 NA oxygen nuclei. Other details are the same as for Fig. 2.


Fig. 4. S D0 scattering signatures for the elements hydrogen (H), 

carbon (C), nitrogen (N), oxygen (O), aluminium (Al), sulphur 

(S), iron (Fe) and lead (Pb). Each signature shows counts S…n† 

as a function of channel number n, and is a combination of 

projected-L and projected-T spectra, as detailed in Table 1(a). 

The signatures are normalized to the same number of incident 

neutrons and the same number (100 N A ) of target nuclei in the 

scatterer. The numbers 1±10 at the top of the ®gure indicate the 

components of the S D0 signature as listed in Table 1(a). 

the energy E n of the scattered neutron detected. 

Prominent features in the plots are the ridges E C , 

E O and E H which are attributed to recoil protons 

associated with neutrons detected after elastic 

scattering on carbon, oxygen and hydrogen, respectively. 

The ridge I 4 in Fig. 2 is attributed to the 

lower energy neutrons originating from inelastic 

neutron scattering in which the 4.43 MeV level of 

12 C is populated. The ridge labelled B in panels (c) 

and (d) of Figs. 2 and 3 is a background component 

attributed to source neutrons which either 

penetrate the shield SH (Fig. 1) or are scattered in 

SH near the collimator exit and thus arrive at 

detector B ahead of the neutrons backscattered by 

the sample. 

The features (other than B) observed in L±T 

spectra such as Figs. 2 and 3 clearly depend 

strongly on the elements contained in the scattering 

sample. For carbon (Fig. 2(c) and (d)), for 

example, the intensity of backscattered components, 

both elastic (E C ) and inelastic (I 4 ), increases 

markedly with the increase of incident neutron 

energy from 6.8 to 7.5 MeV, whereas for oxygen 

(Fig. 3(c) and (d)) an opposite and smaller energydependence 

is observed for the elastic scattering 

components (E O ). The incident neutron energies of 

6.8 and 7.5 MeV were in fact selected [18] to take 

advantage of these features. The presence of the 

inelastic group I 4 (Fig. 2) is also a unique and 

prominent feature in the carbon signature. Two 

features in Fig. 3 indicate the presence of hydrogen 

in the measurement made using the water sample: 

®rstly the strong group E H (Fig. 3(a) and (b)) at 

the forward scattering angle of 45°, which extends 

to a proton recoil energy of half the incident 

neutron energy; and secondly the absence of any 

associated contribution from hydrogen to the yield 

at backward angles (Fig. 3(c) and (d)), as expected 

from the kinematics of n±p elastic scattering. The 

L±T spectra for all of the elements investigated in 

this study each displayed unique features of the 

type seen in Figs. 2 and 3, con®rming that FNSA 

can provide information which is highly elementspeci®c 

and can therefore be used to identify different 

elements and measure their concentrations. 

3. Scattering signatures 

The L±T spectra measured for single-element 

samples, such as those measured using a carbon 

scatterer (Fig. 2), incorporate information which 

characterizes the scattering element and therefore 

constitute a signature for that element. The raw 

signatures shown in Figs. 2 and 3 each consist of 

16k channels of numerical data. In order to simplify 

the analysis of FNSA measurements made 

using multi-element samples it is convenient to 

introduce a compact ``scattering signature'' which 

summarizes the important features of the raw 

signature [18]. The scattering signature for an


Fig. 5. (a) S L0 scattering signatures for the elements H, C, N, O, Al, S, Fe and Pb; (b) S DZ scattering signatures for the elements H, C, N 

and O; and (c) S LZ scattering signatures for the elements H, C, N and O. All signatures are normalized as described in Fig. 4. The 

numbers 1±4 at the top of panel (a) indicate the components of the S L0 signature, as listed in Table 1(b). The numbers 1±8 on the bar 

above panel (a) indicate the regions which are summed to form the S LZ signature (c). 

element (or other) sample is obtained by projecting 

certain selected regions of the raw 2-parameter 

data (e.g. Fig. 2 for carbon) onto either the L-axis 

or the T-axis and then assembling the resulting 

projected spectra (L and T) serially, into a single, 

spectrum-like, distribution (see Fig. 4). Each region 

that is selected for projection is de®ned by a 

T-cut (time-of-¯ight window) and an L-cut (pulse 

height window) or a pulse height threshold. The 

cuts and thresholds used to de®ne the selected regions 

are indicated in Figs. 2 and 3, in which TC1± 

TC4 are T-cuts, LC1±LC4 are L-cuts and LTB 

and LTF are L-thresholds. Reference signatures 

measured for elemental samples are corrected for 

background by subtracting a background signature 

that is constructed by applying the same selection 

procedure to the appropriate background 

measurement. 

Several alternative types of scattering signature 

were investigated in these studies. Some of these 

signatures included both projected T-components 

and projected L-components. Other types of signature 

included only projected L-components and 

used the time-of-¯ight information T only for 

background suppression. Signatures based on L- 

components only are of special interest because


Fig. 6. Scattering signatures for an acetamide scattering sample of mass 0.212 kg: (a) measured (histogram) and refolded (full curve) S D0 

signature, and background component (dashed curve); and (b) measured (histogram) and refolded (full curve) S L0 signature (background 

subtracted). The peaks labelled B in panel (a) correspond to projections of the background ridges B in Fig. 2(c) and (d). 

they can be used in conjunction with a neutron 

source which does not incorporate nanosecond 

beam pulsing, provided other means for suppressing 

background are available. 

Two basic types of signature were used in the 

studies presented here: ®rstly, a dual parameter 

signature, S D0 , which incorporates projections of 

both L- and T-components; and secondly a single 

parameter signature, S L0 , which is based exclusively 

on projections of the pulse height parameter 

L. The structures of the two basic signatures are 

summarized in Table 1 and discussed below. Some 

analyses made using variations of the two basic 

signatures are also presented. 

3.1. The S D0 signature 

The S D0 signature is de®ned by a procedure 

similar to that described in [18], but modi®ed so as 

to obtain additional useful information from the 

two neutron detectors F and B. This signature 

consists of ten components, as shown in Table 

1(a). Components 1 and 2 (see Table 1(a)) are 

projected L-spectra from detector B and provide 

channels 1±150 and 151±300, respectively, of the 

signature (see Fig. 4). Both components are selected 

by the L-cut LC1 and the T-cut TC1 (see 

Figs. 2 and 3). Component 1 is derived from data 

taken at 6.8 MeV, and component 2 from data 

taken at 7.5 MeV. In the S D0 signature for carbon 

(panel C of Fig. 4), for example, the distribution 

from n ˆ 1±150 is therefore the projection of the 

rectangle bounded by cuts LC1 and TC1 of Fig. 

2(c) onto the L-axis. Channels 1±150 of Fig. 4 

(panel C), in other words, show the L-spectrum 

(within the window LC1) that is obtained by integrating 

the data in Fig. 2(c) over T, between the 

T-limits de®ned by cut TC1. Channels 151±300 of 

the same panel show the L-spectrum obtained by 

applying the same procedure to the data of Fig. 

2(d). 

Components 3±10 of the S D0 signature (Table 

1(a)) are projected T-spectra (projections onto the 

T-axis) and provide the remaining channels, 301±


Table 1 

Components of the S D0 and S L0 scattering signatures 

Signature No. Channels Detector E n (MeV) Parameter L-cut T-cut Weight 

(a) S D0 1 1±150 B 6.8 L LC1 TC1 1 

2 151±300 B 7.5 L LC1 TC1 1 

3 301±375 B 6.8 T LC1 TC3 0.25 

4 376±450 B 7.5 T LC1 TC3 0.25 

5 451±525 F 6.8 T LC2 TC4 0.01 

6 526±600 F 7.5 T LC2 TC4 0.01 

7 601±675 F 6.8 T LC3 TC4 0.01 

8 676±750 F 7.5 T LC3 TC4 0.01 

9 751±825 F 6.8 T LC4 TC4 0.01 

10 826±900 F 7.5 T LC4 TC4 0.01 

(b) S L0 1 1±150 B 6.8 L LTB TC2 1 

2 151±300 B 7.5 L LTB TC2 1 

3 301±450 F 6.8 L LTF TC4 0.1 

4 451±600 F 7.5 L LTF TC4 0.1 

900, of the signature. Components 3 and 4 are 

derived from the B detector, and components 5±10 

from the F detector. The cuts and incident neutron 

energies used to extract these components are 

listed in Table 1(a). The L-cuts LC2-LC4 select 

events according to pulse height from detector F as 

follows: LC2, a low pulse height window, selects 

inelastically scattered neutrons as well as elastically 

scattered neutrons from both hydrogen and 

heavier nuclides; LC3, an intermediate pulse 

height window, excludes most inelastically scattered 

neutrons, due to their low pulse height; and 

LC4, a higher pulse height window, excludes both 

inelastically scattered neutrons and neutrons elastically 

scattered by hydrogen, retaining only higher 

energy neutrons from elastic scattering on A P 2 

nuclei. 

The S D0 scattering signatures are thus combinations 

of projected L-spectra and projected T- 

spectra. Each signature embodies, in a more 

compact form (S…n† for n ˆ 1±900), salient characteristics 

of the raw two-parameter data (e.g. 

Figs. 2 or 3) measured for the corresponding 

scatterer. In order to ensure that the di€erent L- 

and T-components in the signatures are accorded 

approximately equal weight in subsequent ®tting 

procedures [18,19] each component has been 

multiplied by the weighting factor shown in the 

right-hand column of Table 1(a). 

The S D0 signatures derived as described above 

for the eight elements used in this study and corrected 

for background are shown in the eight 

panels of Fig. 4. The hydrogen signature (panel H) 

was derived from measurements made using the 

water scatterer (Fig. 3), after applying a correction 

for the oxygen contribution, based on the data 

measured using the liquid oxygen scatterer. In 

addition to being adjusted as described above and 

normalized to the same incident neutron ¯uence, 

as noted earlier, each signature shown in Fig. 4 has 

also been normalized so as to correspond to the 

same number of target element nuclides, 100 N A , 

in the scatterer, where N A ˆ Avogadro's number. 

The eight elemental signatures shown in Fig. 4 

exhibit distinct individual characteristics and are 

therefore well suited for unfolding signatures 

measured for multi-element compounds or materials 

into elemental components [18]. For example, 

the hydrogen signature is unique in having negligible 

intensity in channels 1±450 and 751±900, a 

consequence of the kinematics of np elastic scattering. 

Components 1 and 2 (channels 1±300) for 

the A P 12 elements display upper limits which 

increase systematically with incident beam energy 

and with target mass number A, as expected from 

the kinematics of neutron backscattering. Components 

1±4 of the carbon signature show a strong 

dependence on the incident neutron energy, in-


creasing in intensity with the energy change from 

6.8 to 7.5 MeV. The intensities of the corresponding 

components in the oxygen signature exhibit 

a weaker energy dependence in the opposite 

direction. The low energy forward-scattered components 

(channels 451±600) of all the elements 

shown, except hydrogen and oxygen, display 

characteristic inelastic scattering contributions at 

low pulse height. The four heavier elements studied 

(Al, S, Fe, Pb) also show characteristic contributions 

from inelastic scattering in the 

backscattered components (channels 1±450) of 

their signatures. All of these features are consistent 

with expectations based on nuclear data such as 

di€erential cross-sections [20] and excited state 

energies [21], which were important guiding considerations 

in the choice [18] of the incident neutron 

energies used, 6.8 and 7.5 MeV. 

3.2. The S L0 signature 

The S L0 signature, shown in Fig. 5(a), consists 

of four components (Table 1(b)), each of which is 

a projected L-spectrum. Components 1 and 2 

(channels 1±150 and 151±300) of this signature are 

derived from the B detector, at E n ˆ 6:8 and 7.5 

MeV, respectively, and selected by T-cut TC2 and 

pulse height threshold LTB (see Figs. 2 and 3). 

Components 3 and 4 (channels 301±450 and 451± 

600) are derived from the F detector at the same 

two energies, and selected by T-cut TC4 and pulse 

height threshold LTF. The weights assigned to the 

B and F components are shown in Table 1(b). The 

S L0 signatures include contributions from both 

elastically and inelastically scattered neutrons. The 

signatures for the eight elements, shown in Fig. 

5(a), have been corrected for background and 

normalized in the same way as described in Section 

3.1 for the S D0 signatures. 

Results obtained using the S L0 signature provide 

a guide to the quality of FNSA that should be 

attainable using a monoenergetic neutron source 

without nanosecond beam pulsing. The T-cuts TC2 

and TC4 employed to derive the signatures shown 

in Fig. 5(a) were used only for the purpose of 

suppressing background components such as ridge 

B in Figs. 2 and 3. If backgrounds were eciently 

suppressed by other means, for example by 

improving the shielding and collimator used in the 

experiments (Fig. 1), then these T-cuts (and hence 

T-measurements) would be unnecessary. 

3.3. The S D1 and S D2 signatures 

The S D1 and S D2 signatures are subsets of the 

dual-parameter signature S D0 . The S D1 signature 

consists of the S D0 signature reduced from 10 

components (900 channels) to 5 components (450 

channels) by including only the components associated 

with the 6.8 MeV incident neutron energy, 

that is the odd-numbered components in Table 

1(a). The S D2 signature is derived similarly by using 

only the 7.5 MeV components, in other words the 

even-numbered components in Table 1(a). Results 

obtained using the S D0 , S D1 and S D2 signatures were 

compared in order to determine the extent to 

which the use of two incident neutron energies, 

rather than one, improved the quality of the 

FNSA result obtained. 

3.4. The S DZ and S LZ signatures 

The two signatures denoted S DZ and S LZ are 

compact signatures consisting of integrals derived 

from the S D0 and S L0 signatures. Each signature 

consists of an 8-channel distribution (S…n† for 

n ˆ 1±8) obtained by summing selected sections of 

the parent signature, S D0 or S L0 . The eight channels 

of the S DZ signature are integrals of components 3± 

10, respectively, of the S D0 signature (Table 1(a)). 

Channels 1 and 2 of the S LZ signature are integrals 

of components 1 and 2 of the S L0 signature (Table 

1(b)). Channels 3±8 of this signature are obtained 

by summing the following subsections (channels) 

of the S L0 signature (Fig. 5(a)): (3) 301±325; (4) 

326±350; (5) 351±450; (6) 451±475; (7) 476±500 and 

(8) 501±600. Thus, channels 1 and 2 of the S LZ 

signature are a measure of the backscattered neutron 

yields for 6.8 and 7.5 MeV incident neutrons, 

respectively, and channels 3±5 and 6±8 measure 

forward-scattered neutrons for the same two energies. 

The summation limits speci®ed for channels 

3±8 are chosen so as to emphasize the contributions 

of inelastic neutron scattering (3 and 6), n±p 

elastic scattering (4 and 7) and elastic scattering on 

A P 2 nuclides (6 and 8), as discussed in Section


3.1. Studies of the S DZ and S LZ signatures have been 

con®ned to scattering samples composed exclusively 

from the elements H, C, N and O. The S DZ 

and S LZ signatures for these elements are shown in 

Figs. 5(b) and (c), respectively. 

3.5. The Q-signatures 

The signatures described above can be modi®ed 

as outlined below to deal with problems that arise 

if neutron scattering materials are present in signi®cant 

amounts between the scattering sample 

and one or both of detectors B and F. For example, 

neutrons on paths (Fig. 1) leading directly 

from the scatterer to detector B (F) may interact 

with material present in region S 3 …S 4 †, thus reducing 

the intensity of the B (F) components of the 

scattering signature. If this attenuation is signi®cant 

then the ratio of B and F intensities will be 

a€ected and the signature will be distorted. This 

problem may be expected to occur in the envisaged 

practical applications of FNSA, for example when 

screening packages in which contraband items are 

hidden amongst other scattering material. It can 

be overcome by introducing Q-signatures, which 

are identical to the S-signatures listed in Table 1, 

except for the fact that the Q-signatures include an 

additional parameter k FB , which represents the 

ratio of the attenuation factors for neutrons scattered 

towards detectors B and F, respectively. 

Each Q-signature is derived from the corresponding 

S-signature, for example Q D0 is obtained from 

S D0 , by multiplying the F components of the S 

signature by the factor k FB . Thus Q-signatures are 

the same as corresponding S-signatures (Table 1) 

apart from the additional parameter k FB . When a 

Q-signature is used k FB is a free parameter which is 

determined in the unfolding procedure (Section 4) 

used to ®t the raw scattering signature measured 

for the sample under analysis. Experiments carried 

out to test the use of Q-signatures are described in 

Section 4.3.1. 

4. Analyses of scattering measurements 

The analysis of the FNSA signatures obtained 

in these test measurements (Section 2) followed a 

procedure similar to that described in [18]. Signatures 

were unfolded into elemental scattering signatures 

and background components, in order to 

determine atom fractions for the eight (or four, in 

certain cases) candidate elements in the scattering 

sample. The procedures used are summarized below, 

followed by a description of the least squares 

procedure used to assign materials to categories 

such as explosive, illicit drug or other. 

4.1. Unfolding of scattering signatures 

The scattering signature measured for each 

sample was unfolded into N ‡ 4 components, 

where N is the number of candidate elements 

considered in the analysis. The additional four 

components represent two measured backgrounds 

and two neutron multiple scattering components, 

respectively, one for E n ˆ 6:8 MeV and the other 

for E n ˆ 7:5 MeV in each case [18]. For analyses 

based on the S D0 (S L0 ) signature (N ˆ 8) the elemental 

signatures were those shown in Fig. 4 (Fig. 

5(a)). For analyses based on the S DZ or S LZ signatures 

(N ˆ 4) the elemental signatures were those 

shown in Figs. 5(b) and (c), respectively. For analyses 

based on the other signatures the elemental 

signatures were appropriate subsets of those 

shown in Figs. 4 or 5. Background and multiple 

scattering components were determined or speci- 

®ed as described in [18]. 

Unfolding was carried out using the MIEKE 

program [22,23] which forms part of the HEPRO 

spectrum unfolding package [24,25]. The MIEKE 

code makes use of a Monte Carlo importance 

sampling algorithm which draws spectra at random 

from a probability density distribution [25] in 

order to calculate expectation values and uncertainties 

for the N ‡ 4 components which are ®tted 

to the unknown scattering signature. The unfolding 

procedure using the S D0 signature is described 

below. The same description is valid for all S-signatures 

and Q-signatures described in Section 3. 

When a Q-signature is used, the factor k FB (Section 

3.5) is an additional parameter which is also determined 

in the unfolding procedure. 

Using notation similar to that introduced previously 

[18], the scattering signature S…n† measured 

for a sample composed from N elements in un-


known proportions can be represented by the 

equation 

S…n† ˆXN‡4 

iˆ1 

f i S i …n† 

…1† 

in which the functions S i …n† represent the following: 

i ˆ 1toN represent the elemental scattering 

signatures (as in Fig. 4) for the N elements; 

i ˆ N ‡ 1 and N ‡ 2 represent smoothed distributions 

based on the background measurements 

made at 6.8 and 7.5 MeV, respectively; and 

i ˆ N ‡ 3andN ‡ 4 denote linear functions which 

are used [18,19] to represent small multiple scattering 

contributions at 6.8 and 7.5 MeV, respectively. 

The unfolding analysis determines the 

values and standard deviations for the N ‡ 4, coecients 

f i which give the best ®t to the scattering 

signature S…n†. The atom fraction a i of element i in 

the sample is given [18] by 

, 

X N 

a i ˆ f i f i 

…2† 

iˆ1 

and the mass M of the scattering sample, in kg, is 

given [18] by 

M ˆ 0:1 XN 

iˆ1 

f i A i ; 

…3† 

where A i is the mass number of nuclide i. Masses 

determined from Eq. (3) will be underestimated if 

there is signi®cant attenuation of neutrons between 

the scattering sample and either of the 

neutron detectors, B or F, in other words if a Q- 

signature has to be used in the unfolding analysis. 

However, the atom fractions obtained from Eq. (2) 

should be insensitive to such scattering e€ects. 

Fig. 6(a) shows, for example, the raw S D0 signature 

(histogram) measured for a sample of 

acetamide (H 5 C 2 NO, mass 0.212 kg), together 

with the refolded distribution (curve) obtained 

from the MIEKE unfolding of this signature. The 

atom fractions derived (Eq. (2)) from the unfolding 

are plotted in panel (iii) of Fig. 7(a). Points 

show the experimentally determined values for the 

elements H, C, N, O, Al, S, Fe and Pb, corresponding 

to i ˆ 1±8, respectively, and the histogram 

shows the atom fractions calculated from the 

chemical formula of acetamide. The results of 

similar measurements and analyses made on ®ve 

other compounds, using the S D0 signature, are 

shown in the other panels of Fig. 7(a). Masses of 

the six samples determined from the same measurements 

(Eq. (3)) are in good agreement with 

values obtained by weighing (see Table 2). 

The S L0 signature measured for acetamide is 

shown in Fig. 6(b), together with the refolded 

distribution obtained from MIEKE unfolding 

analysis. Atom fractions and masses obtained 

from the unfolding of the S L0 signatures obtained 

for the same six compounds as in Fig. 7(a) are 

shown in Fig. 7(b) and Table 2, respectively. The 

atom fractions obtained using the S L0 signature are 

less accurate than those obtained using the S D0 

signature, but both signatures lead to results (Fig. 

7) which are consistent with the known values for 

the samples used. Masses determined (Eq. (3)) 

using the S L0 signatures are less reliable than those 

obtained using the S D0 signatures (Table 2). Previous 

work [18] demonstrated that FNSA was 

successful in determining atom fractions for H, C, 

N and O in samples composed from these elements 

alone. The present results (Fig. 7) show that the 

atom fractions of these important light elements 

can still be determined accurately when a larger set 

of candidate elements (8 instead of 4, and including 

heavy elements) is used in the samples and the 

analysis. 

4.2. Identi®cation of scattering materials 

The identi®cation of speci®c compounds or 

materials from atom fractions determined in the 

unfolding analysis can be facilitated by introducing 

a weighted chi-square coecient 

v 2 …x; k† ˆXN 

iˆ1 

, 

2 

a i b ik 

N XN 

Da i 

iˆ1 

…Da i † 2 ; …4† 

where a i (Da i ) are the atom fractions measured 

for the N elements in an unknown sample x and 

b ik are the corresponding atom fractions for a 

candidate material k of known chemical composition. 

Eq. (4) is used to calculate v 2 …x; k† for each 

member k of a set of M candidate materials. If a


Fig. 7. Atom fractions a i , measured (points) and calculated (histograms), for six di€erent compounds, (i)±(vi). The measured values 

were determined using: (a) the S D0 ; and (b) the S L0 signatures. Points indicate di€erent elements, plotted in order of increasing atomic 

number, as follows: crosses, H; solid circles, C; open triangles, N; open circles, O; solid triangles, Al; diamonds, S; open squares, Fe; 

solid squares, Pb. The six compounds were: (i) methanol; (ii) ammonium nitrate: (iii) acetamide; (iv) ammonium acetate; (v) aluminium 

oxide; and (vi) crystalline iron sulphate. 

Table 2 

Masses (in kg) determined for the test samples 

Sample 

Method Methanol Ammonium 

nitrate 

Acetamide 

Ammonium 

acetate 

Aluminium 

oxide 

Iron sulphate 

S D0 signature 0.286 (15) 0.305 (17) 0.208 (8) 0.272 (17) 0.601 (39) 0.318 (15) 

S L0 signature 0.348 (25) 0.350 (19) 0.223 (11) 0.265 (15) 0.584 (45) 0.615 (60) 

Weighing 0.288 (1) 0.298 (1) 0.212 (1) 0.272 (1) 0.618 (1) 0.284 (1) 

deep minimum value of v 2 …x; k† is found for one 

member k of the set, showing that x has atom 

fractions similar to those of k, it can be argued 

that x is most likely to be identi®ed with material 

k. For a convenient presentation of the screening 

tests, the reciprocal R…x; k† ˆ1=v 2 …x; k† is used 

and a screening function P…x; k† is introduced by 

normalizing R…x; k† to a percentage value as follows: 

, 

X M 

P…x; k† ˆ100 R…x; k† R…x; k† : …5† 

kˆ1


Thus P…x; k† is a v 2 -based, percentage measure of 

the degree to which sample x matches member k of 

the set of M candidate materials. The distribution 

P…x; k† versus k displays a pro®le of the screening 

of x against the M candidate materials and the 

values of k corresponding to maxima in this pro®le 

indicate the candidate materials whose elemental 

compositions most closely resemble that of the 

unknown material x. 

Fig. 8 shows pro®les P…x; k† derived from 

screening of the scattering measurements made 

using the methanol sample. The pro®les presented 

in panels (a)±(f) of this ®gure were determined 

from atom fractions obtained using the scattering 

signatures S D0 , S L0 , S D1 , S D2 , S DZ and S LZ , respectively, 

with N ˆ 8 (4) for the ®rst four (last two) 

signatures. For example, the pro®les shown in 

panels (a) and (b) of Fig. 8 were calculated from 

the atom fractions shown in panels (i) of Figs. 7(a) 

and (b), respectively. Forty-®ve materials 

(M ˆ 45) were selected for using as candidates in 

the screening process. These materials are listed in 

Table 3, together with their assigned k-numbers 

and atomic compositions. The candidates are 

classi®ed in four groups, numbered g ˆ 1±4 as 

follows: g ˆ 1, element samples (k ˆ 1±8), comprising 

the eight elements used in this study; g ˆ 2, 

explosives (k ˆ 9±18), ten well-known explosives; 

g ˆ 3, illicit drugs (k ˆ 19±23), ®ve well-known 

drug materials; and g ˆ 4(k ˆ 24±45), 22 other 

materials, including chemical compounds used in 

the test measurements and some other commonly 

occurring materials. 

All pro®les obtained from the screening of the 

methanol data (Fig. 8) show a clear peak at the 

expected candidate number, k ˆ 27. The 

``strength'' P…x; k† of this peak varies between 43% 

and 80% depending on the signature used in the 

analysis. These results demonstrate the capability 

of FNSA to identify this material clearly from the 

other 44 candidates considered in the screening 

process. As expected, the S D0 signature provides 

the most positive identi®cation (P ˆ 0:80). 

In the screening of packages for contraband it 

will be more important to determine whether a 

suspect item or sample belongs to either of the 

contraband groups, g ˆ 2 or 3, than to identify the 

speci®c material k of the item. The group identi- 

®cation is facilitated by introducing a group pro®le 

function G…x; g† for unknown sample x, which is 

obtained by summing P…x; k† over the members k 

of each group g, 

G…x; g† ˆXq 

kˆp 

P…x; k†; 

…6† 

Fig. 8. Pro®le functions P…x; k† determined for the methanol 

sample (k ˆ 27) using six di€erent scattering signatures: (a) S D0 , 

(b) S L0 , (c) S D1 , (d) S D2 , (e) S DZ and (f) S LZ . Also shown are the 

boundaries (in k) which de®ne the four groups g of candidate 

materials (see Table 3). 

where p and q represent the ®rst and last members 

of group g. 

The four histograms plotted in Fig. 9(a)±(d) 

show group pro®les G…x; g† obtained from the 

methanol data shown in panels (a), (b), (e) and (f), 

respectively, of Fig. 8. The point plotted below 

each histogram indicates the value of P…x; k† for 

methanol (k ˆ 27) for the corresponding signatures 

and is plotted at the value g ˆ 4, corresponding 

to the group in which methanol is 

classi®ed in Table 3. This presentation is used to 

illustrate the proportion of G…x; 4† strength that is 

allocated in the screening process to the correctly 

identi®ed material, in this example methanol. The


Table 3 

Candidate materials and group classi®cations g 

g a k Material Atomic proportions 

H C N O X a 

1 1 Hydrogen 1 0 0 0 0 

2 Carbon 0 1 0 0 0 

3 Nitrogen 0 0 1 0 0 

4 Oxygen 0 0 0 1 0 

5 Aluminium 0 0 0 0 1 

6 Sulphur 0 0 0 0 1 

7 Iron 0 0 0 0 1 

8 Lead 0 0 0 0 1 

2 9 Ammonium nitrate 4 0 2 3 0 

10 C-4 16 8 11 11 0 

11 RDX/HMX 2 1 2 2 0 

12 EGDN 4 2 2 6 0 

13 PETN 8 5 4 12 0 

14 Nitrocellulose 7 6 3 10 0 

15 Nitroglycerine 5 3 3 9 0 

16 TNT 5 7 3 6 0 

17 Tetryl 5 7 5 8 0 

18 Picric acid 3 6 3 7 0 

3 19 Heroin 20 17 1 1 0 

20 LSD 25 20 3 1 0 

21 Cocaine 21 17 1 4 0 

22 Morphine 19 17 1 3 0 

23 Mandrax 14 16 2 1 0 

4 24 Paran wax 37 18 0 0 0 

25 Polyethylene 2 1 0 0 0 

26 Ethanol 6 2 0 1 0 

27 Methanol 4 1 0 1 0 

28 Water 2 0 0 1 0 

29 Ammonium acetate 7 2 1 2 0 

30 Nylon 11 6 1 1 0 

31 Lucite 4 2 0 1 0 

32 Polyurethane 10 5 2 1 0 

33 Acetamide 5 2 1 1 0 

34 Benzene 6 6 0 0 0 

35 Sugar 24 13 0 12 0 

36 Fe 2 …SO 4 † 3 

15H 2 O 30 0 0 27 5 

37 Wood 31 22 0 12 0 

38 Paper 10 6 0 5 0 

39 Cotton 27 18 0 13 0 

40 Silk 13 8 5 4 0 

41 Orlon 6 6 2 0 0 

42 Wool 9 6 3 3 0 

43 Melamine 2 1 2 0 0 

44 Polyester 2 3 0 1 0 

45 Aluminium oxide 0 0 0 3 2 

a Groups g are: 1, elements; 2, explosives; 3, illicit drugs; and 4, other materials. 

a Element X ˆ Al, S, Fe or Pb.


Fig. 9. Group pro®le functions G…x; g† (histograms) determined for the following scattering samples: (a)±(d) methanol; (e)±(h) and (n)± 

(p) ammonium nitrate; (i)±(l) acetamide; and (m) an illict drug simulant. The scattering signatures used in the analyses were: S D0 for (a), 

(e), (i) and (m); S L0 for (b), (f) and (j); S DZ for (c), (g) and (k); S LZ for (d), (h) and (l); Q D0 for (n); Q L0 for (o) and S D1 for (p). The points 

plotted beneath the histograms indicate the pro®le strength P…x; k† allocated to the correct sample material k in each case (see text). 

points in Fig. 9(a)±(d) therefore show the same 

data as the associated peaks at k ˆ 27 in Fig. 8. 

While the point values vary between 43% and 80%, 

as already noted with reference to Fig. 8, the associated 

g ˆ 4 strengths, G…x; 4†, exceed 90% for 

all four signatures, as can be seen from the histograms 

in Fig. 9(a)±(d). The group identi®cation for 

methanol is therefore emphatic even when the 

pro®le strength P…x; 27† measured for the compound 

itself is as low as 43%. 

Fig. 9(e)±(h) and Fig. 9(i)±(l) show results obtained 

from similar analyses of the data measured 

using samples of ammonium nitrate and acetamide, 

respectively. These results assign ammonium 

nitrate and acetamide unambiguously to the correct 

groups, explosives (g ˆ 2) and `òther materials'' 

(g ˆ 4), respectively, for all signatures. For 

ammonium nitrate (Fig. 9(e)±(h)) the pro®le 

strength assigned to the compound is a large 

fraction of that assigned to the group, for all signatures. 

For acetamide (Fig. 9(i)±(l)) this fraction 

is smaller and more variable, probably because 

group 4 contains a wider selection of candidate 

materials with atomic compositions similar to 

acetamide. Similar results have been obtained 

from the measurements and analyses made for 

many other samples. Without exception the correct 

group is indicated by the maximum in the 

G…x; g† distribution. 

The total number of neutrons incident on the 

sample during each of the measurements represented 

in Fig. 9(a)±(l) was about 10 9 . A series of 

analyses were also carried out using various different 

subsets (di€erent numbers of events) of the 

data measured for acetamide. These analyses 

showed that a measurement for a minimum number 

of 10 8 neutrons incident on the sample would 

provide sucient data (using the arrangement of 

Fig. 1) to identify the group g of this typical 

scatterer (mass 0.2 kg) unambiguously. The minimum 

number of incident neutrons required for a 

successful analysis can be reduced simply by extending 

the F and B detectors so as to increase 

their solid angles. If the detectors shown in Fig. 1


were extended into rings coaxial with the beam 

axis, for example, the minimum number of neutrons 

required would be reduced by a factor of 25. 

Fig. 9(m) shows the group pro®le G…x; g† obtained 

from a measurement made using a 0.11 kg 

sample of a material X, not listed in Table 3, which 

has H:C:N:O atom ratios of 15:11:1:1, similar to 

those of the g ˆ 3 group (illicit drugs) of Table 3. 

This screening of X, which was based on the S D0 

signature, points clearly to the illicit drugs group 

(g ˆ 3) and identi®es heroin (k ˆ 19) as the candidate 

of composition closest to that of X. The 

point plotted in Fig. 9(m) corresponds to the value 

of P…x; k† determined from the measurements on 

X, for k ˆ 19. This result con®rms that FNSA 

should be e€ective for detecting illicit drugs. 

4.3. E€ects of neutron interactions in material other 

than the scattering sample 

The S-signatures described in Sections 3.1±3.4 

are designed for analyzing measurements made in 

the open laboratory geometry depicted in Fig. 1, in 

which no additional neutron scattering material is 

present either in the neutron beam, e.g. in the regions 

S 1 or S 2 , or between the scattering sample S 

and neutron detectors F or B, e.g. in regions S 3 or 

S 4 . In any practical implementation of FNSA it is 

very likely that additional scattering material will 

be present in the package to be interrogated, 

possibly in the regions represented by S 1 ±S 4 in the 

laboratory geometry (Fig. 1). To investigate possible 

consequences of this situation, scattering experiments 

were carried out with additional 

scattering material placed in one or more of these 

regions. The e€ects of additional material in the 

neutron beam (regions S 1 and S 2 ) or between the 

scattering sample S and the neutron detectors 

(regions S 3 and S 4 ) were investigated independently. 

4.3.1. Material between sample and detectors (regions 

S 3 and S 4 ) 

The Q-signatures described in Section 3.5 are 

designed to deal with problems caused by neutron 

interactions in material between the scattering 

sample and either or both of the neutron detectors 

F and B (Fig. 1). The e€ectiveness of these signatures 

was experimentally tested, for example in a 

measurement made using an ammonium nitrate 

scattering sample at S in Fig. 1, with slabs of lucite, 

60 mm thick, placed at positions S 3 and S 4 , between 

the scattering sample and detectors B and F. 

Independent experimental checks con®rmed that 

the lucite blocks reduced the intensities of scattered 

neutrons (3±8 MeV) reaching the detectors 

by about 30±50%. Pro®les G…x; g† obtained from 

the screening of the results obtained in the test 

measurement, using signatures Q D0 and Q L0 , are 

shown in Figs. 9(n) and (o), respectively. The 

corresponding k FB factors determined in the analysis 

(see Section 3.5) were 1.26 and 1.38, respectively. 

The analyses based on Q-signatures give 

G…x; 2† P 0:7 and thus assign this sample unambiguously 

to the correct group, g ˆ 2. These results, 

together with those of several other tests 

made using slabs of various materials and thickness 

in positions S 3 and/or S 4 (Fig. 1), con®rm that 

the procedures used to assign the sample group g 

can be rendered insensitive to moderate amounts 

of neutron scattering material between scatterer S 

and the detectors if Q-signatures are used in the 

analysis. 

4.3.2. Material in the neutron beam (regions S 1 and 

S 2 ) 

The introduction of additional scattering material 

in the neutron beam, for example at positions 

S 1 or S 2 (Fig. 1) will obviously increase the 

numbers of neutrons detected by the scintillators F 

and B and therefore distorts the signature recorded 

for the scatterer S. Two possible solutions are 

suggested for this problem. Firstly, if the distribution 

of scattering material around S is known, 

or can be monitored, then it may be possible to 

orient the package being interrogated so as to 

position S in the neutron beam while also reducing 

other scattering material in the beam to a tolerable 

minimum. Alternatively, and assuming that a 

pulsed neutron beam is used, neutron time-of- 

¯ight information can be used to distinguish the 

scattering signatures from di€erent regions (volume 

elements) along the neutron beam. Experiments 

were undertaken as follows to test this 

approach. Two additional scattering samples were 

placed in the neutron beam (Fig. 1) at positions S 1


and S 2 adjacent to S, thus forming an extended 

scattering sample consisting of three components 

or volume elements. One such test was carried out 

using an ammonium nitrate sample at S, an 

acetamide sample at S 1 and a methanol sample at 

S 2 . Before measuring the scattering signature of 

this triple sample arrangement two additional sets 

of scattering signatures were determined for the 

elements H, C, N and O, one set for samples at 

position S 1 and the other for samples at position 

S 2 . Together with the elemental signatures (Figs. 4 

and 5) measured for samples at the standard position 

S, this provided three signatures per element, 

one signature per element for each of the three 

positions or volume elements. The scattering signature 

measured for the triple scattering sample 

was then unfolded (Section 4.1) in the standard 

way, as expressed in Eq. (1), but summing to 

3N ‡ 4 terms instead of N ‡ 4 terms, to accommodate 

the additional two positions per element. 

The factors f i obtained from this analysis were 

then grouped into three sets, one for each position, 

and processed separately through Eqs. (2)±(6) to 

obtain atom fractions, pro®les and group pro®les 

for the three volume elements. 

Results obtained for sample S (ammonium nitrate) 

in the triple sample measurement are shown 

in Fig. 9(p). The ammonium nitrate sample is still 

clearly assigned (G…x; 2† > 0:6) to the g ˆ 2 group 

(explosive), regardless of the presence of two other 

samples in the incident neutron beam, one upstream 

and the other downstream of it. The group 

assignments for the two additional samples used at 

S 1 and S 2 in this test were also correct. These results 

demonstrate that reliable group assignments 

are obtained, even when signi®cant amounts of 

additional material are present in the neutron 

beam, if the appropriately modi®ed (multi-position) 

FNSA analysis procedure is used. 

5. Discussion and conclusions 

The nuclear characteristics on which the elemental 

scattering signatures used in FNSA are 

based are: (a) the elastic and inelastic di€erential 

scattering cross sections of the nuclides of di€erent 

elements, which determine the relative intensities 

of the elemental signature components (Figs. 4 and 

5) at di€erent incident neutron energies and scattering 

angles; and (b) the dependence of backscattered 

neutron energy on target mass number 

A, analogous to that underlying the well-known 

charged particle technique of Rutherford backscattering 

analysis. The FNSA test measurements 

presented here explored six di€erent scattering 

signatures of varying complexity, based on these 

properties. In the analyses of the FNSA test 

measurements each test sample was classi®ed in 

one of the four groups, element, explosive, illicit 

drug or other. All the signatures considered 

proved reliable for determining group aliations g 

(Table 3) of the test samples. In every test carried 

out, without exception, the correct group g was 

assigned. 

The signature which should be best-suited for 

detecting contraband in a given situation will depend 

on factors such as the size, nature and shape 

of the package to be screened, the energy and type 

of neutron beam available and the time available 

for carrying out the screening. Other signatures 

superior to, or more convenient than, those considered 

here might be devised. For example, signatures 

incorporating 2, 3 or 4 multiplexed 

neutron energies in the range 1±6 MeV appear 

promising because this region is rich in resonance 

structure for the nuclides 12 C, 14 N and 16 O and 

might therefore o€er distinctive signatures for 

these nuclides at conveniently lower neutron energies. 

The success achieved using pulse height only 

signatures, such as S L0 and S LZ (Sections 3.2 and 

3.4) should be particularly noted, in view of the 

associated implication that nanosecond pulsed 

neutron beams are not essential for FNSA. This 

success implies that it should be possible to construct 

FNSA systems, capable of identifying contraband 

such as illicit drugs or explosives with high 

reliability, from less expensive accelerator-based 

neutron sources. 

We have presented data and analyses from 

measurements in which an NE213 scintillator was 

used as the backward-angle detector B (Fig. 1). 

Parallel measurements were made using a deuterated 

scintillator (NE230) as detector B and gave 

results which achieved slightly better discrimina-


tion between di€erent elements than the results 

shown here for the NE213 detector. However, the 

improved performance obtained by using NE230 

is not considered sucient to justify the substantial 

extra cost of this detector for FNSA applications. 

Results obtained using an NE230 detector 

were presented in an earlier report [18]. 

An important feature of the FNSA technique is 

the fact that it measures the elements which are 

essential for contraband detection (H, C, N and O) 

with similar sensitivity. This ensures that the 

atomic proportions of these di€erent elements in 

the sample are determined with similar accuracy, 

which leads to a more reliable identi®cation of 

contraband materials (groups 2 or 3 in Table 2). 

The potential of FNSA for identifying the lighter 

elements, lithium to boron, remains to be properly 

explored but looks promising, since the energy 

transfer in neutron backscattering increases as the 

mass number of the scattering nuclide is reduced. 

The tests described in Section 4.3 show that 

FNSA is insensitive to interference caused by 

neutron interactions in materials other than the 

sample, provided the amounts of such materials 

present between sample and detectors or in the 

neutron beam are not excessive. Interference 

caused by limited amounts of material between 

sample and detectors is counteracted by using Q- 

signatures (Sections 3.5 and 4.3.1). Extended dual 

parameter signatures (Section 4.3.2) are successful 

in counteracting the e€ects of additional scattering 

material in the neutron beam. 

In conclusion, these laboratory studies provide 

further proof-of-principle con®rmation that 

FNSA o€ers a reliable, non-intrusive method for 

determining whether a sample material of mass 0.2 

kg, or more, is an illicit drug or an explosive. In all 

tests carried out, without exception, the test sample 

was assigned to the correct group (Table 3). 

Test measurements also demonstrated that FNSA 

is e€ective even when additional scattering material 

is present in the neutron beam or between the 

scattering sample and the neutron detectors. The 

tests also provided count rate information (see 

Section 4.2) from which inspection times for 

FNSA screening can be estimated. Further work 

should be directed towards the development of a 

pilot system that can be tested and evaluated in an 

environment similar to that in which screening for 

contraband would take place. 

Acknowledgements 

We thank the following for their interest in this 

work and their support: Dr. H Klein and Dr. M 

Matzke of PTB, Braunschweig, Germany, for 

valuable discussions and for making the HEPRO 

spectrum unfolding package available to us; M 

Herbert for assistance in the experimental work; 

D. Momsen, D. Boulton, G. Fowle and L. van 

Heerden for their technical assistance; the NAC 

Van de Graa€ group for their support, interest and 

assistance in the accelerator experiments; and the 

International Atomic Energy Agency, Vienna, and 

the South African Foundation for Research Development, 

for ®nancial support. 

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Material classification by Fast Neutron Scattering Analysis

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