US 3597610 A
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United States Patent Inventor Raymond Marius Augustin Bayol Vincenncs, France Appl..No, 724,256
Filed Apr. 25, 1968 Patented Aug. 3, 1971 Assignee Eastman Kodak Company Rochester, N.Y.
Priority Feb. 22, 1968 France 140896 lN'IENSll-ICATION SCREEN FOR RADIOGRAPHIC FILM  Field of Search 250/65; 96/82  References Cited UNITED STATES PATENTS 2,739,243 1/1953 Sheldon 250/65 2,906,881 11/1958 Mittelstaedt 250/65 2,950,972 8/1960 Mueller 250/65 Primary Examiner-Norman G. Torchin Assistant Examiner-John L. Goodrow Attorneys-W. H. J. Kline, Bernard D. Wiese and Gordon L.
Hart 1 ABSTRACT: A layer comprising particles of a low melting point metal alloy of lead, bismuth, tin and cadmium and a binder is provided as an intensification screen. Good intensification without fog is obtained using particles of alloys, such as Wood metal, which melt lower than 80C.
HNTENSHFHCATHON SCREEN FOR lRADllOGlRAPlHlC FILM The present invention relates to intensification screens for radiographic films. More particularly, the invention relates to coated layers as intensification screens for radiographic films and to radiographic films including such layers, and i to a method for making such layers.
Radiation such as X and y rays, which include much more energy than light rays, have a powerful effect on photographic layers when the X and 'y rays are absorbed. However, only a very small proportion of X or 7 rays are absorbed by the photographic layer and for this reason, the layers are generally richer in silver halide and thicker than light-sensitive photographic layers. To obtain thick layers, an emulsion is usually provided on both sides ofa radiographic film base since a single thick layer could not be conveniently developed and fixed. it has long been known to markedly increase the effective sensitivity of radiographic film by providing an intensification screen adjacent to the silver halide emulsion. Upon irradiation, an intensifying screen of the type we are dealing with absorbs high energy X-rays which causes secondary electron emission to which the emulsion is highly sensitive. Variousintensification screens of this type are known, employing heavy metals such as lead, usually in the form ofa metal plate or foil.
It is an object of the present invention to provide a new intensification screen for radiographic films which greatly increases the effective X and y rays sensitivity of the film without causing emulsion fogging. This and other objects are achieved according to the invention by providing an intensification screen for radiography and the like comprising a layer with particles of a low melting point alloy which comprises a heavy metal that emits secondary electrons when exposed to X or y radiation, dispersed in a film-forming binder.
A preferred alloy has a low melting point, below 95 C. and preferably less than 80 C. The major ingredient is bismuth, which generally constitutes about one-half of the weight ofthe alloy. Lead makes up about one-fourth and cadmium and tin about one-eighth each. Wood alloy, having the composition Pb 25 percent, Bi 50 percent, Sn 12.5 percent and Cd 12.5 percent, and melting point of 70 C. is most preferred, but the constitution of each of the ingredients can be varied provided the melting point of the alloy remains low. The melting point.
should not exceed the boiling point of water and, for that reason, melting points of less than about 95C. are preferred since a pressurized process would berequired to reduce water losses to a controllable degree. Still more preferably, the melting point is below 80C.
The alloy is incorporated in an intensification screen layer by simply heating a mixture of the alloy and a suitable binder to melt the alloy, and dispersing the molten alloy in the binder and forming the dispersion into a layer or film. The layer can be self-supporting and applied as such to a radiographic film or it can be coated on a support which is applied to a radio graphic film or it can be coated directly on a radiographic film all as will become more apparent in light of the following detailed description which includes a preferred embodiment of the invention. 7
A radiographic film for use withan intensification screen according to the invention can be any film plate or the like containing-one-or more silver halide or other radiation sensb tive layers provided on a support. The silver content of such radiation-sensitive silver halide emulsions is generally higher than for other light-sensitive emulsions, reaching up to 3.6 grams per square foot. The iodide content is relatively high and the binder content is generally low. Gelatin is a preferred binder and hardened gelatin is preferred. It is quite'important that the film be resistant to chemical fog.
This invention can be further illustrated by the following example of a preferred embodiment thereof although it will be understood that this example is included merelyfor purposes of illustration and is not intended to limit the scope of the in vention unless otherwise specifically indicated.
EXAMPLE An intensifying screen composition is prepared by heating a mixture of 300 grams of Wood alloy and 30milliliters of dibutyl phthalate, which is used to stabilize the final dispersion, to C. to melt the alloy. The molten alloy is then added to 200 grams of a l0 percent gelatin solution maintained at a temperature of 80 C. and vigorously mixed for 2 minutes to disperse the molten alloy at that temperature. The mixture is then cooled, with continued stirring, below the melting point of the alloy to 40 C. which causes the alloy to form small solid spherical particles having a diameter ofabout 6 to 7 microns.
The resultant dispersion is coated. directly on a polyester (polyethylene terephthalate) support 0.06-millimeters thick at a coverage of 3 grams alloy per square decimeter. The gelatin binder sets on cooling. Examination of the dried layers shows that the alloy beads have settled by gravity and are concentrated mostly at the polyester support interface leaving mostly gelatin binder at the outer surface. This settling effect is helpful to improve resolution by bringing the alloy-rich side of the layer into contact with the silver emulsion.
In various embodiments this can be accomplished in several ways. The coated layer can be wet-transferred from a temporary support on which it has been coated to a permanent receiving supporting sheet, with the alloy-rich side outward. For an integral support the layer can be coated and settled directly on the surface ofa radiographic recording element, or a layer can be wet-transferred from a temporary coating support, on which the layer has been coated and settled, to the surface having a substratum, after stripping off the temporary support, the resulting material is applied onto the radiographic element, with the alloy-rich face inward against the recording element.
in some embodiments, if the settling effect is not preferred, sedimentation of alloy particles can be inhibited by coating a more viscous dispersion or by rapidly settling the coated layer, as by rapid cooling, or both.
Two screens are made: One by the settling technique described, followed by wet-transfer to a permanent support of 0.06-millimeter polyester film base, alloy side out; the second by coating the dispersion at a lower temperature and with rapid cooling to quickly set the gel and minimize settling. Both coatings contain 3 grams of alloy per square decimeter and 0.06-millimeter polyester film base. The screens are tested with a conventional silver halide radiographic film with exposure to X-rays at 240 KV, 3 ma. using a 6-millimeter copper filter, conventional processing. A control test is run with a conventional lead screen.- Results are tabulated showing density of the developed films after exposure with the respective screens.
Wood Alloy Screen (concentrated at Wood Alloy Screen Lead Screen (dispersed through (Control) outer surface) layer) 2.03 "1.9a 1.44 2.04 L89 L46 2.04 L89 1.46
2.04 Average D varying the temperature, agitation, binder content and nature and in several other ways as will be apparent to one of ordinary skill in the art. Generally speaking, however, the average particle size is less than microns and in the most preferred embodiments between I and 12 microns.
The nature of the binder of the screen can vary. Unhardened gelatin is a preferred binder, but others, including self-supporting resin sheets can be used depending primarily upon the manner in which the screen is utilized. Where the screen is provided in a layer distinct from the radiographic film and not integral therewith, the alloy can be provided in virtually any settable binder such as a thermoplastic natural or synthetic resin, gum or gel. The screen is merely placed next to the radiographic silver halide emulsion prior to exposure. In the event that long contact between some particular binder and the silver halide emulsion would cause some difficulty such as the formation of fog, the screen can be kept away until immediately before exposure. ln embodiments where the screen is provided in a layer integral with the film, it will usually be preferably that the screen be removed prior to developing and this can be accomplished by providing a screen layer which can be removable in any of several ways. The layer, for example, can be melted and/or dissolved off, such as be treating in hot water. Alternatively, the layer can be applied over a water-soluble adhesive layer. Gelatin, polyvinyl alcohol, polyvinyl pyrrolidone and the like are watersoftenable and are suitable for such uses as binders and adhesive sublayers.
The relative concentrations of alloy and binder in the intensification screen can vary quite widely. The layer is preferably richer in the alloy but can contain low amounts ifa low degree of intensification is desired. Generally, however, the layer will contain at least percent by weight of the alloy and up to 95 percent or more. The function of the screen is to provide the particles of intensifying material close to the silver halide emulsion during exposure thereofto X-rays and the amount of alloy in the layer is conveniently defined in terms of the amount of alloy provided per unit area since, when in operation, each unit area of the screen will'correspond to the same size unit area of the silver halide layer. The amount of alloy calculated in this manner will generally be between 0.l and I0 grams per square decimeter.
The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.
1. In a method of recording a radiographic image on a silver halide emulsion during exposure thereof to X-ray radiation in the presence of an adjacent intensification screen, the improvement comprising employing in said screen an alloy, having a melting point below C. of lead, bismuth, tin and cadmium.
2. A radiographic film comprising a support, at least one light-sensitive silver halide emulsion layer, and an intensification screen layer above said silver halide emulsion layer, said intensification screen layer comprising a binder and particles of an alloy of lead, bismuth, tin and cadmium having a melting point ofless than 95C.
3. A radiographic film according to claim 2 wherein said intensification layer is removable from said film.
4. A radiographic film according to claim 2 wherein said intensification layer comprises a water-softenable binder.
S. A radiographic film according to claim 2 wherein said intensification layer is secured to said emulsion layer by a watersoluble adhesive layer.
6. A radiographic film according to claim 2 wherein said intensification screen layer comprises from about 25 to about 95 percent by weight of said alloy particles based on the weight of the layer.
7. A radiographic film according to claim 6 wherein said alloy particles have an average particle size of from 1 to 12 microns.
8. A radiographic film according to claim 7 wherein the binder of said intensification layer comprises gelatin,
9. A radiographic film according to claim 8 wherein the alloy is provided in the intensification layer in an amount of from 0.! to 10 grams per square decimeter.
10. A radiographic film according to claim 9 wherein said alloy has the following composition:
Lead 25 weight percent Bismuth 50 weight percent Tin l2.5 weight percent Cadmium IILS welghl percent