CA1196733A

CA1196733A - Radiographic emulsions

Info

Publication number: CA1196733A
Application number: CA000401289A
Authority: CA
Inventors: Thomas D. Lyons; Peter B. Jamieson
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1981-05-26
Filing date: 1982-04-20
Publication date: 1985-11-12
Also published as: JPS57198456A; ZA823627B; JPH0473136B2; ES8308075A1; AU8413882A; AU550866B2; ES512479A0; EP0065877A1; EP0065877B1; DE3272443D1; MX157370A; AR241831A1; BR8203024A

Abstract

ABSTRACT OF THE DISCLOSURE

Industrial radiographic systems having low grainines and high information density may be constructed with intensifying screens sandwiching radiation sensitive elements having emulsions wherein the average size of the silver halide grains is less than 0.4 micrometers.

Description

;733 Field OE The Invention This invention relates to a novel, high deEini~
tion, industrial radiographic system. The sy~tem uniquely combines fine grain silver halide emulsion photographic film and a light-emitting phosphor screen.

Background Of The Art Nondestructive testing of articles and materials has become an integral part of quality control in modern manufacturing industries. This type of testing enables on-line and intensive evaluation o~ the struc~ural soundness of products. One of the most commonly used Eorms o~ nondestructive testing is radiographic images taken on industrial materials. Industrial X-rays have been used for many years in the testing oE support beams used in the construction of buildings, bridges and the like. They are particularly useful in the evaluation of welds and in te.~ting metal plates for minute flaws which could affect performance.
As industrial demands on materials become more stringent anrl the tolerance for Elaws becomes reduced, more precise testing methc~s are required. In all imaging processes, including photography and radiography, there is an inherent limit in the re~solution available through the process because of the physical elements used. In the practice of modern industrial X-ray procedures, the use of intensifying screens adds a further limit on the resolution available in radiographs. It has heretofore been generally accepted that the phosphor grc~ins in intensifying screens 3 ancl the screens themselves were the limiting Eactor in the graininess or resolution availahle in radiographs used in nondestructive testing (cf. Nondestructive Testing, 2d Ed.
Warren J. McGonnagle, Science Publishers, 1971, pages 119-123, Radiography in Modern Industry, 3d Ed.~ Eas-t~an 35 Kodak, 1969, pages 34-38, and Physics of Industrial Radiology, R. I-lalmshaw, London, Heywood Books, 1966, pp. 110 and 176). This limitation was believed to be a result oE th~ ~act that visib]e radiation emitted Erom the phosphor grain is spread out r~ther than projected in a linear path like the incident X-rays.
Radiographic emulsions used in industrial screen/film X-ray procedures typically have emulsions where the average grain size is above 0.5 micrometers (e.g., U.S~
Patent No. 3~922,545, col. 13, lines 25-46) and generally over 1 rnicrometer te.g., UOS. Patent No. 3,753,714, col. 4, lines 34-40). U.S. Patent Nos. 4,177,071 and 4,130,~28 discloses a range of 0.25 to 1.2 micrometers for the grain size, b~t the examples are only of emulsions having average grain sizes of 0.5, 0.6, 0.7 and 0.8 micrometers.

Summary Of The Invention An imageable system particularly useful for industrial X-ray procedures comprises at least two X-ray intensifying screens having a radiation sensitive photo-~raphic film between the screens. The film comprises a base with a decolorizable (e.g., bleachable or solvent removable in aqueous alkaline solvent) dye underlayer on at least one side of the base and two radiation sensitive silver halide emulsion layers, one on each side of the base (with at least one over the dye underlayer), The silver halide emulsions are comprised of dye sensitized silver halide grains having a number average size oE less than 0.40 micrometers and greater than 0.05 micrometers.
The grains are preferably sensitized to a portion oE the spectral region near that of the light emitted by the phosphor screen.

Detailed Description Of The Invention The present invention concerns itself with radio-graphic imaging systems comprising two X-ray intensifying screens sandwiching a radiation sensitive element, said element comprising:

1) a base,

2) a decolorizable dye underlayer on at least one side o~ said ~ase,

3) a f~rst silver halide emulsion over said dye underlayer, and ~) a second silver halide emulsion on the other side of said base.
Both of the silver halide emulsions, although not necessarily identical, must have silver halide grains with an average size of less than 0.40 micrometers and larger than 0.05 micrometers. Preferably the average size is between 0.075 and 0.35 micrometers and most preferably between 0.10 and 0.25 or even 0.20 micrometers. The silver halide grains should be sensitized to light emitted by the 1~ intensifying screens when struck by X-rays. ~ye sensi-tization oE the silver halide is well understood in the art. Upon determination of the emission spectrum of the particular phosphor selected, one can readily select sensitizing dyes which are known to sensitize silver halide crystals to the appropriate region of the spectrum, usual]y between 40~ and 780 nanometers. PreEerably the silver halide is sensitized to a spectral range within 25 nano-meters of the maximum wat~^length emission of the screen t max~ more preferably within 15 nm, and most preferably within 10 nm.
The present invention also relates to a process Eor taking industrial radiographic images of industrial materials. In the practice of the present invention, 'industrial materialsl are defined as all items or arti-Eacts other than life forms. Industrial materials ofmetals, alloys, ceramics, glass, and polymeric resins (or~anic and inorganic) in the form oE sheets, Eilms, art Eorms, staple articles, intermediate and completed struc-tures~ and other forms are contemplated in khe practice of the present invention.
Conventional industrial radiographic processes and materials utilize emulsions having a high concenkration 73~

of silver which is used to absorb X-rays. Some of the consequences o~ using these higll concentrations o~ silver include lony processing times ~e.g., in the neighborhood of ten to -twelve minutes), long drying times, and hlgh material costs.
Raclioqraphic emulsions used in the practice o~
the present invention should have silver coating weights less th~n 10 g Ag/rn2 and pre~erably between 3 and 8 grams of silver per square meter. The most preferred range is between 3 and 7 g/m2 of silver. These films have enabled complete processing times to be reduced to as little as ninety seconds.
The process would b~ performed hy ~lsiny a conven-tional X-ray projection source or other high energy parti-cle radiation sources including gamma and neutron sources.As well known in the art, the particular phosphor used should have a high absorption coefficient for the radiation emitted from the source. ~sually this radiation is hiyh energy particle radiation which is defined as any of X-rays/ newtrons and gamma radiation. The industrial material would be placed between the controllable source of X-rays and the industrial radiographic system of the present invention~ A controlled exposure oE X-rays would be directed from the source and through the industrial ~aterial so as to enter and impact the radiographic system at an angle approximately perpendicular to the plane or surface of the intensiEying screen and the photographic filJn contiguous to the inside surface of the screen. The radiation absorbed by the phosphors of the screen would cause light to be emitted by the screen which in turn would generate a latent irnage in the two silver halide imaging layers. Conventional development processes including stop baths, washes, fixing, bleaching and the like would then be used on the exposed film.
The silver halide grains may be selected frorn amongst any of the known photographic silver hal;~e materials such as silver chloride, silver brornide, silver 3;~
. ~;

iodide, silver bromoiodide, silver chlorobromoiodide, silver chlorobromide, and the li]ce and mixtures tilereof.
The vast list of known photographic adjuvants and processing aids may be used in the practice o~ the present invention. These materials include gelatin extenders, chelnical sensitizers (including sulEur and gold compounds), development accelerators (e.g., onium and polyonium compoullds), alkylene oxide polymer accelerators, antiEoggant compounds, stabilizers ~e.g., azaindenes especially the tetra- and pentaazaindenes), surface active agents (particularly fluorlnated surfactants), antistatic agents tparticularly fluorinated compounds), plasticizers, matting agents, hardening agents, hardening accelerators, and the like.
The base may be any one of the well known photographic support materials such as glass, polymeric Eilms such as cellulose acetate (and triacetate), polyesters (particularly polyethylenetereplltha]ate), polycarbonates, polystyrene, and polyvinyl acetal film base. Many other materials may also be used.
The dye underlayer must contain a decolori~ab]e dye. By the term 'deco orizable', it is meant that the light absorbing ability of the dye must be substantially diminishable or capable of being completely removed. For example, the dye in the binder which forms tl-e underlayer may be readily soluble in aqueous alkaline solutions used in the processing (developing) of the film element so that the dye would be washed out of the element. The dye coul~
be alkaline solution bleachable~ heat bleachable, sulfite bleachable, or removable in any other manner which would not require destruction of the image in the Eilm. There are many ways of accomplishing removability known in the art, but the two preferred means are using dyes which are bleachable in conventional developing solutions, such as those disclosed in Photographic Chemistry, Vol. II, P. Glafkides, 1960, pages 703-704. Heat bleaching oE the dyes may be accomplished by selecting dyes which are 67~33 ., themselves thermolabile or by combining them with materials which can bleach the dye~ when heated. The combination of bleachable dyes with nitrate salts capable of liberating HNO3 or nitorgen oxides when heated to 160~200C (as taught in U.S. Patent No. 4,336,323) are particularly desirable.
The dye underlayer is particularly important because it preven-ts cross-talk within the radiographic element. Cross-talk occurs when light emitted from one screen passes through one silver halide emulsion and the base into the second silver halide emulsion and forrns a latent image there. Because the second emulsion (i.e., the emulsion on the side of the base away from the emitting screen under consideration) is relatively far removed from the screen, the light image is greatly dispersed and the resolution would be greatly reduced. It is, thereEore, essential that the dye underlayer absorb radiation of the wavelength emitted by the phosphors.

Examples 1-6 A series of silver halide emulsions with narrow grain size distribution was made in which the grain size was varied from 0.22 to 0.6 micrometers. The emulsions were made using a double jet procedure under controlled pAg conditions.
The grains in all cases were iodobromide in composition containing 2.75 mole ~ iodide and were of cubic habi~. The emulsions were handled in the normal manner for coagulating, washing and reconstituting them. The reconstituted emulsions were treated with conventional sulfur and gold sensitizers and were digested at 55C to increase their sensitivity, cooled to 40C, and treated with post sensitization additives and stabilizers (namely, tetraazaindines, additional halides, antifoggants, and a spectral sensitizer chosen to provide maximum sensitivity at 550 nm which matches the maximum emission characteristics of 3M's TrimaxR intensifying screen) as is common to the art.

..

i733 The photographic films were prepared by separately coating the above emulsion onto both sides of a polyester film base which had previously been coated wlth an aqueous alkallne solu~le dye in a gelatin layer. The Eilm base was 7 mil photogra~e polyester. The emu]s ions were applied using a precision photographic coating machine . The f inal coatings contained 5.1g Ag/m2 These films were then exposed to 125 kVp X-rays at a distance of 48 inches (104 cm) in a cassette con~a1ning 3M Trimax~ intensifying screens which are gadolinium, terbium doped oxysulfide phosphor screens.
After conventional development, various data were recorded and are shown below in the Table. The noise power was determined by taking a Weiner spectrum (cf. J Optical Soc. Am. 45, 709-808 (1955)). The results recorded below are given at a frequency of 1 cycle per millimeter in units of microns density.

Grain Size Example(~m) Noise Power MTF
1 0.~ 14.5 0.~3 2 0.50 15.5 0.41 3 0.42 13.~ 0.35

4 0.30 7.8 0.41

5 0.~2 7.4 0.~4 ~ 0.20 4.8 0.42 The dramatic and unexpected irprovement in the reduction in graininess can be seen in the greatly reduced noise level achieved according to the practice of the present invention. Further measurement of the images by modula-tion transfer function (MTF) at 4 cycles/mm revealed thatresolution was not sacrificed in the emulsions with reduced graininess. This means that the information content of -the film has been substantially increased.

3~;~

Example 7 This example shows the use of the ma-terials of the present invention in commercial industrial radio-graphic situations.
5 Specimen: Two low carbon steel plates joined toyether with a bukt weld. The overall piece measured 12" x 8" x 1". An ~STM-E142 penetrameter 2.0 was located near the weld joint.
10 Film: Seven mil polyester coated two sides with a silver iodobromide emulsion optlcally sensitized to 550 nm. The silver coating weight was 5.4 g/m2. A bleachable dye underlayer was coated on one side. The average grain size was 0.247 microns as determined by electron microscopy.
Screens: Trimax~ 12 Front, Trimax~ 12 Back Technique 300 KVp, 48 inch (122 cm) film-focus-distance, 300 milliamp seconds.
20 Processing: The exposed film was processed in a Kodak X-OMAT automatic processer in 90 seconds dry-to-dry D
Results: The 2-lT penetrameter hole was clearly visible, indicating an E~uivalent Penetrameter Sensi-tivity 25 ~ 104% as defined in ASTM E142. A small region of incom-plete fusion in the weld area was clearly visible.

Example 3 This is an example of field radiography using a radioactive isotope source.
3o Specimen: Steel girder weld, 3.76 cm thick, containing an ASTM-E142 penetrameter, 3Ø
Film: Same as Example 7.
Screens: Trimax~ 12 Front, Trimax~ 12 Back.
Source: IR-192, 49 curies.
35 Technique Film-focus-distance 13.5 inches (3~.2 cm) Time 10 seconds.

Process. Exposed Eilm processed in Kodak X-OMAT Type B, 12 minutes dry-to-dry.
Res~lt.s: ~he 2-2T penel-rameter hole wc,s cl~a~ly visible at a density oE 1.88. Tllis provides an Equlvalent Sensitivity o ~.O defined in ~STM~E142. ~ small crack within the weld was also clearly visible.

Example 9 This is an example of aluminum radiography.
Specimen: Aluminum stepwed~e, 7.5 inches (18.79 cm) in length, 2.75 inches in depth containing 10 steps in 0.25 inch (0.63 cm) increments.
The minimum step thickness was 0.5 inches (1.27 cm). Each level of thickness contained the appropriate MIL.STD.271D Al penetrameter.
Film: The film was a silver bromoiodide oE
average grain size 0.24 microns coated both sides onto seven mil polyes-ter film base.
The film ~ase was previously coated one side with a bleachable dye layer. The silver coating weiyht was 5.7 g/m2.
Screen: Trimax~ 2 Front, Trimax~ 2 Back.
Technique: 125 KVp, 48 inch (122 cm) film-focus-distance, 40 mas.
Processing: Exposed film was developed in a Kodak X-OMAT automatlc processor 90 seconds dry-to-dry.
Results: The 2-lT penetrameter holes were clearly visible on all thicknesses from 0.75 (1.88 cm) to 105 inches (3.15 cm) aluminum. This corresponcls to an Equivalent Sensitivity of 1.4%.

Example 10 This is an example oE multipLe film radiography.
Specimen: Same as Example 9.
35 Film: Same as Example 9.

Screens: Trimax 2 Eront, Trimax 12F sack.
Procedure: A Elexible vinyl cassette is loaded with the two s~-~eens and two pieces of ilm were inserted between the screens.
5 Technique: 125 KVp, film-focus-distance 44 inches, 30 milliamp sèconds.
Processing: The exposed films were developed in a Kodak X-OMAT automatic processor 90 seconds dry-to-dry.
I() l~es~lts: Film No. l, clcsest to the X-ray source, clearly revealed the 2-2T penetrameter holes on all thicknesses between 0.5 (1.27 cm) and 1.2 inches (3.06 cm) aluminum. Film No. 2 clearly revealed the 2-2r penetrameter holes for all thicknesses between ].2 ~3.06 l~ cm) and 2.5 inches (6.26 cm) aluminum. The two films combine to provide an Equivalent Sensitivity of 2% for all steps in the aluminum stepwedge.
The X-ray intensifying screens used in the practice of the present invention are phosphor screens well known in the art. These phosphors are materials which absorb incident X-rays and emit radiation in a different portion of the electromagnetic spectrum, particularly visible and ultraviolet radiation. Calcium tungstate and rare earth ~gadolinium and lanthanum) ~5 oxysulides and gadolinium or lanthanum oxybromides are particularly useful phosphors. The gadolinium oxysulfides and the lanthanum oxysulfides and the phosphates and arsenates can be doped to control the emussion wavelengths and improve their efficiency. Many of these phosphors are shown in V.S. Patent No. 3,725,704 and U.K. Patent No.
1,565,811. The phosphate and ~rsenate phosphors may be generally represented by the formula La(l-a-b-c~d-e)GdaCebEUcTbdThexO~
wherein a is 0.01 to 0.50, b is 0 to 0.50, c is 0 to 0.02, 3~ d is o to 0.10, e is 0 to 0.02 and X represents phosphorous or arsenic atoms or mixtures thereof.
Preferable, c is 0, a is 0.05 to 0.30 and d is 0 to 0.02.

73~3 The sum oE b, c, d ancl e should be greater tllan %ero ancl shoul.d most preEerably be at least 0.005.
The oxysulfide rare earth phosphors rnay be repre~ented by the formula La(2_g_f)GdaL~lh%~O2s wherein Z is the dopant element or elements, g is 0 to 1.99, h is 0 to 1.99 and f is 0.0005 to 0.16.
PreEerably b is 0, a is 0.15 to l~OOr E i.s 0.0010 -to 0.05 and Z is terbium.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An industrial radiographic system comprising two high energy particle radiation intensifying screens sandwiching a radiation sensitive element which comprises: 1) a base, 2) a decolorizable dye underlayer on at least one side of the base, 3) a first silver halide emulsion layer over said dye underlayer, and 4) a second silver halide emulsion layer on the other side of said base, wherein both of said silver halide emulsion layers are spectrally sensitized to the wavelength of radiation emitted by said screens when struck by high energy particle radiation and wherein the average size of the silver halide grains in the emulsions is less than 0.4 micrometers and larger than 0.05 micro-meters.

2. The system of claim 1 wherein said screens are X-ray intensifying screens.

3. The system of claim 2 wherein the average grain size is between 0.075 and 0.35 micrometers.

4. The system of claims 2 or 3 wherein the emulsions are sensitized by at least one sensitizing dye so that the maximum sensitivity of the emulsions is within 50 nanometers of the maxi-mum intensity wavelength emission of the screens.

5. The system of claims 2 or 3 wherein the emulsions are sensitized by at least one sensitizing dye so that the maximum sensitivity of the emulsions is within 25 nanometers of the maxi-mum intensity wavelength emission of the screens.

6. The system of claims 2 and 3 wherein the phosphors of said screens comprise gadolinium oxysulfides, lathanum oxy-sulfides, gadolinium-lanthanum oxysulfides, gadolinium oxybromides, lanthanum oxybromides, or lanthanum-gadolinium oxybromides.