Radiography in Modern Industry - Kodak

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The mark of the success of a theory is its ability to provide an understanding of previouslyinexplicable phenomena. The Gurney-Mott theory and those derived from it have been notablysuccessful in explaining a number of photographic effects. One of these effects--reciprocity-lawfailure--will be considered here as an illustration.Low-intensity reciprocity-law failure (See left branch of the curve in Figure 120) results from thefact that several atoms of silver are required to produce a stable latent image. A single atom ofsilver at a latent-image site (See Figure 130D) is relatively unstable, breaking down rather easilyinto an electron and a positive silver ion. Thus, if there is a long interval between the formation ofthe first silver atom and the arrival of the second conduction electron (See Figure 130E), the firstsilver atom may have broken down, with the net result that the energy of the light photon thatproduced it has been wasted. Therefore, increasing light intensity from very low to higher valuesincreases the efficiency, as shown by the downward trend of the left-hand branch of the curve inFigure 120, as intensity increases.High-intensity reciprocity-law failure (See right branch of the curve of Figure 120) is frequently aconsequence of the sluggishness of the ionic process in latent-image formation (See Figure 130).According to the Gurney-Mott mechanism, a trapped electron must be neutralized by themovement of an interstitial silver ion to that spot (See Figure 130D) before a second electron canbe trapped there (See Figure 130E); otherwise, the second electron is repelled and may betrapped elsewhere. Therefore, if electrons arrive at a particular sensitivity center faster than theions can migrate to the center, some electrons are repelled, and the center does not build up withmaximum efficiency. Electrons thus denied access to the same traps may be trapped at others,and the latent image silver therefore tends to be inefficiently divided among several latent-imagesites. (This has been demonstrated by experiments that have shown that high-intensity exposureproduces more latent image within the volume of the crystal than do either low- or optimumintensityexposures.) Thus, the resulting inefficiency in the use of the conduction electrons isresponsible for the upward trend of the right-hand branch of the curve.X-Ray Latent ImageIn industrial radiography, the photographic effects of x-rays and gamma rays, rather than those oflight, are of the greater interest.At the outset it should be stated that the agent that actually exposes a photographic grain, that is,a silver bromide crystal in the emulsion, is not the x-ray photon itself, but rather the electrons--photoelectric and Compton--resulting from the absorption event. It is for this reason that direct x-ray exposures and lead foil screen exposures are similar and can be considered together.The most striking differences between x-ray and visible-light exposures to grains arise from thedifference in the amounts of energy involved. The absorption of a single photon of light transfersa very small amount of energy to the crystal. This is only enough energy to free a single electronfrom a bromide (Br-) ion, and several successive light photons are required to render a singlegrain developable. The passage through a grain of an electron, arising from the absorption of anx-ray photon, can transmit hundreds of times more energy to the grain than does the absorptionof a light photon. Even though this energy is used rather inefficiently, in general the amount issufficient to render the grain traversed developable--that is, to produce within it, or on it, a stablelatent image.As a matter of fact, the photoelectric or Compton electron, resulting from absorption or interactionof a photon, can have a fairly long path in the emulsion and can render several or many grainsdevelopable. The number of grains exposed per photon interaction can vary from 1 grain for x-radiation of about 10 keV to possibly 50 or more grains for a 1 meV photon. However, for 1 meVand higher energy photons, there is a low probability of an interaction that transfers the totalenergy to grains in an emulsion. Most commonly, high photon energy is imparted to severalRadiography in Modern Industry 204
electrons by successive Compton interactions. Also, high-energy electrons pass out of anemulsion before all of their energy is dissipated. For these reasons there are, on the average, 5 to10 grains made developable per photon interaction at high energy.For comparatively low values of exposure, each increment of exposure renders on the averagethe same number of grains developable, which, in turn, means that a curve of net density versusexposure is a straight line passing through the origin (See Figure 131). This curve departssignificantly from linearity only when the exposure becomes so great that appreciable energy iswasted on grains that have already been exposed. For commercially available fine-grain x-rayfilms, for example, the density versus exposure curve may be essentially linear up to densities of2.0 or even higher.Figure 131: Typical net density versus exposure curves for direct x-ray exposures.The fairly extensive straight-line relation between exposure and density is of considerable use inphotographic monitoring of radiation, permitting a saving of time in the interpretation of densitiesobserved on personnel monitoring films.It the D versus E curves shown in Figure 131 are replotted as characteristic curves (D versus logE), both characteristic curves are the same shape (See Figure 132) and are merely separatedalong the log exposure axis. This similarity in toe shape has been experimentally observed forconventional processing of many commercial photographic materials, both x-ray films and others.Radiography in Modern Industry 205
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RadiographyinModernIndustry
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RadiographyinModernIndustryFOURTH E
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ContentsIntroduction...............
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Chapter 1: The Radiographic Process
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Intensifying ScreensX-ray and other
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makes it a very suitable material f
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Figure 6: Typical voltage waveforms
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Table I - Typical X-ray Machines an
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The wavelengths (or energies of rad
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Table III - Industrial Gamma-Ray So
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1. The source of light should be sm
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B and H in the Figure 13 show the e
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Figure 14: Geometric construction f
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Figure 17: Pinhole pictures of the
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The kilovoltage applied to the x-ra
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Figure 21: Schematic diagram of som
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kind of material radiographed, the
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instance, the kilovoltage may be fi
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The technique need not be limited t
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Chapter 5: Radiographic ScreensWhen
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Contact between the film and the le
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Figure 29: The number of electrons
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lead foil screens ran be retained w
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Figure 33: The sharpness of the rad
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Figure 34: Low density (right) is a
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such as a wall or floor, on the fil
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from this source. Since scatter als
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A filter reduces excessive subject
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Definite rules as to filter thickne
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0.010-inch front screen of value be
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Example: Suppose that with a given
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If the milliamperage remains consta
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espectively. In other words, a cons
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Any given exposure chart applies to
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Figure 46: Typical gamma-ray exposu
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where the slope of the characterist
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Figure 49: Characteristic curves of
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Figure 51: Characteristic curve of
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Nomogram MethodsIn Figure 54, the s
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Figure 56: Transparent overlay posi
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Figure 58: Overlay positioned so as
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The problem of radiographing a part
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Figure 62: System of lines drawn on
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Chapter 8: Radiographic Image Quali
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Film contrast refers to the slope (
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Hole Type PenetrametersThe common p
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of the same thickness as the specim
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Chapter 9: Industrial X-ray FilmsMo
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Figure 70 indicates the direction t
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the lead letters on a radiation-abs
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Therefore, protection requirements
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5. Avoid pressure damage caused by
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Paddles or plunger-type agitators a
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slow, and the development time reco
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ubbles make their way to the surfac
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When development is complete, the f
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soften considerably with prolonged
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Figure 77: The roller transport sys
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Rapid Access to Processed Radiograp
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Figure 78: Film-feeding procedures
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Chapter 11: Process ControlUsers of
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3. Age of the developer replenisher
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Figure 80: Control chart below for
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DiscussionDensitometric data and pr
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Figure 82: Plan of a manual x-ray p
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Figure 83: A schematic diagram of a
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loading-bench activities are carrie
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KODAK Quinone-Thiosulfate Intensifi
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Methylene-Blue MethodTwo variations
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KODAK Hypo Test Solution HT-2Avoird
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In summary, use of the test papers
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narrow angle would be very thick, e
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When radiation passes through a spe
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Figure 89: Demonstration of the eff
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Figure 90: The amount of gamma radi
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The radiograph exposed in the right
Page 153 and 154: To illustrate, let us assume that t
Page 155 and 156: Figure 95: High-speed x-ray picture
Page 157 and 158: Figure 97: Two methods of neutron r
Page 159 and 160: Duplicating RadiographsSimultaneous
Page 161 and 162: Sometimes, as when sets of referenc
Page 163 and 164: PhotofluorographyIn photofluorograp
Page 165 and 166: discontinuities or of segregation i
Page 167 and 168: from the camera or by reaching down
Page 169 and 170: Figure 106: Schematic diagram of th
Page 171 and 172: valuable technique, for instance, i
Page 173 and 174: The position of the spots is determ
Page 175 and 176: Powder Diffraction File, Internatio
Page 177 and 178: Processing TechniquesRadiographs on
Page 179 and 180: Since this formula applies only to
Page 181 and 182: such that it does not distort the i
Page 183 and 184: Figure 113: A: Representation of a
Page 185 and 186: Figure 115: Characteristic curve of
Page 187 and 188: film fairly well. If high densities
Page 189 and 190: Density = 1.5 Density = 2.5Film Rel
Page 191 and 192: In most industrial radiography, the
Page 193 and 194: e noted here. Although the average
Page 195 and 196: Chapter 17: Film Graininess; Signal
Page 197 and 198: The ratio of signal to noise has a
Page 199 and 200: Chapter 18: The Photographic Latent
Page 201 and 202: Thus, the change that makes an expo
Page 203: Figure 130: Stages in the developme
Page 207 and 208: Development is essentially a chemic
Page 209 and 210: Chapter 19: ProtectionOne of the mo
Page 211 and 212: duct is brought into the x-ray room
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