Process for treating photothermographic dry imaging material Number:7,150,964 from the United States Patent and Trademark Office (PTO) owispatent

Title: Process for treating photothermographic dry imaging material

Abstract: Disclosed is an image forming process having the steps of exposing by an exposure device a photothermographic dry imaging material with a support having thereon an image forming layer containing photosensitive silver halide, a reducing agent for silver ions, a binder and a light-insensitive organic silver salt, and developing the photothermographic dry imaging material by a developing device, while the photothermographic dry imaging material is transported, wherein a surface having the image forming layer is brought into contact with sticky rollers during or before each of exposing and developing so as to make an amount of peel-off static electrification between the photothermographic dry imaging material and the sticky roller to be from -5 to +5 kV.

Patent Number: 7,150,964 Issued on 12/19/2006 to Yanagisawa

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Inventors:	Yanagisawa; Hiroyuki (Hino, JP)
Assignee:	Konica Minolta Medical & Graphic, Inc. (Tokyo, JP)
Appl. No.:	11/141,846
Filed:	June 1, 2005

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Foreign Application Priority Data

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Jun 07, 2004 [JP]			2004-168637

Current U.S. Class:	430/617 ; 430/618; 430/619; 430/620
Current International Class:	G03C 1/498 (20060101)
Field of Search:	430/617,618,619,620

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References Cited [Referenced By]

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U.S. Patent Documents

	2003/0207216		November 2003		Fukui et al.
	2004/0033449		February 2004		Inoue

Other References

Abstract of JP 2006023717 A. cited by examiner . Derwent abstract 2006-009685. cited by examiner.

Primary Examiner: Le; Hoa Van
Attorney, Agent or Firm: Lucas & Mercanti, LLP

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Claims

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What is claimed is:

1. An image forming process comprising the steps of: (a) exposing by an exposure device a photothermographic dry imaging material comprising a support having thereon an image forming layer containing photosensitive silver halide, a reducing agent for silver ions, a binder and a light-insensitive organic silver salt, and (b) developing the photothermographic dry imaging material by a developing device, while the photothermographic dry imaging material is transported, wherein a surface having the image forming layer is brought into contact with sticky rollers during or before each of exposing and developing so as to make an amount of peel-off static electrification between the photothermographic dry imaging material and the sticky roller to be from -5 to +5 kV.

2. The image forming process of claim 1, wherein exposure is conducted with an exposure device located below where the photothermographic dry imaging material is exposed.

3. The image forming process of claim 1, wherein an air cleanliness class defined by ISO 14644-1 at the portion of an exposure device is not more than 5.

4. The image forming process of claim 1, wherein the air cleanliness class defined by ISO 14644-1 at the portion of a developing device is not more than 5.

5. The image forming process of claim 1, wherein sticky rollers comprise a function to remove static electrification.

6. The image forming process of claim 1, wherein static electrification is removed when the photothermographic dry imaging material is brought into contact with sticky rollers.

7. The image forming process of claim 1, wherein static electrification is removed before the photothermographic dry imaging material is brought into contact with sticky rollers.

8. The image forming process of claim 1, wherein a transporting speed at the developing device is from 30 to 60 mm/second.

9. The image forming process of claim 1, wherein the photothermographic dry imaging material comprises a light-sensitive layer containing silver halide particles and aliphatic carboxylic acid silver, and the content ratio of silver behenate in the aliphatic carboxylic acid silver is from 80 to 100 percent by mol.

10. The image forming process of claim 1, wherein the photothermographic dry imaging material comprises a light-sensitive layer containing silver halide particles and reducing agents for silver ions, and the reducing agents for silver ions are compounds represented by the following General Formula (RED). ##STR00017## wherein X.sub.1 represents a chalcogen atom or CHR.sub.1; R.sub.1 being a hydrogen atom, a halogen atom, an alkyl group, an alkenyl group, an aryl group or a heterocyclic group; R.sub.2 represents an alkyl group; R.sub.3 represents a hydrogen atom or a substituent capable of substituting a hydrogen atom on a benzene ring; R.sub.4 represents a substituent; and m2 and n2 each represents an integer of 0 to 2.

11. The image forming process of claim 1, wherein the photothermographic dry imaging material comprises a light-sensitive layer containing photosensitive silver halide particles, and the photosensitive silver halide particles are chemically sensitized employing organic sensitizers containing chalcogen atoms.

12. The image forming process of claim 1, wherein color image forming agents are contained which increase absorbance between 360 and 450 nm via oxidation.

13. The image forming process of claim 1, wherein color image forming agents are contained which increase absorbance between 600 and 700 nm via oxidation.

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Description

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This application claims priority from Japanese Patent Application No. JP2004-168637 filed on Jun. 7, 2004, which is incorporated hereinto by reference.

TECHNICAL FIELD

The present invention relates to a process for treating photothermographic dry imaging materials (hereinafter occasionally referred to simply as photothermographic materials), employing a thermal development apparatus.

BACKGROUND

In recent years, in the medical and graphic arts fields, a decrease in the processing effluent of image forming materials has increasingly been demanded from the viewpoint of environmental protection as well as space saving.

As a result, techniques have been sought which relate to photothermographic materials which can be effectively exposed, employing laser imagers and laser image setters, and can form clear black-and-white images exhibiting high resolution.

Silver salt photothermographic dry imaging materials are composed of a support having thereon organic silver salts, photosensitive silver halide and reducing agents (for example, refer to Patent Documents 1 and 2, and Non-Patent Document 1.). Since no solution-based processing chemicals are employed for the aforesaid silver salt photothermographic dry imaging materials, they exhibit advantages in that it is possible to provide a simpler environmentally friendly system.

High image quality, based on enhanced sharpness, and excellent graininess and in-plane evenness, is desired to obtain sensitive delineation in medical images. Performance of high image quality has especially been demanded in order to photographically capture tumor mass shadows inside mammary glands, especially for early detection of breast cancer, employing mammography. Major improvement in this technique has long been desired, specifically since dust and foreign matter in the air or which adhere to the image film can early be misdiagnosed as calcification-like negative image (being a false image). To overcome this problem, a significant amount of dust and foreign matter is still a problem, even though commonly known removal means, such as sticky rollers are employed.

Though a technique of eliminating dust and foreign matter has improved by increasing contact pressure of the sticky rollers onto the photothermographic dry imaging materials is for example described in Patent Document 3, adhesion of dust and foreign matter recurs, since static electrification is generated when photothermographic dry imaging materials are peeled from the sticky rollers. As a result, it is easily to be understood that insufficient elimination of dust and foreign matter is obtained via this technique. (Patent Document 1) U.S. Pat. No. 3,152,904 (Scope of Patent Claims) (Patent Document 2) U.S. Pat. No. 3,487,075 (Scope of Patent Claims) (Non-Patent Document 1) D. Morgan, B. Shely; Thermally Processed Silver Systems A; Imagining Processes and Materials: Neblette, 8.sup.th edition, Sturge, V. Walworth, A. Shepp edition, page 2, 1969 (Patent Document 3) Japanese Patent O.P.I. Publication No. 2003-107625 (Scope of Patent Claims)

SUMMARY

The present invention was accomplished in view of the above unresolved items, and it is an object of the present invention to provide a process for treating photothermographic dry imaging materials, and a thermal development apparatus capable of producing high quality diagnostic images, especially high quality images desired for mammary diagnosis.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements numbered alike in several Figures, in which:

FIG. 1 shows schematic drawings of a laser imager which is a thermal development apparatus of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The aforesaid object can be accomplished via the following structures.

(Structure 1) An image forming process having the steps of: (a) exposing by an exposure device a photothermographic dry imaging material with a support having thereon an image forming layer containing photosensitive silver halide, a reducing agent for silver ions, a binder and a light-insensitive organic silver salt, and (b) developing the photothermographic dry imaging material by a developing device, while the photothermographic dry imaging material is transported, wherein a surface having the image forming layer is brought into contact with sticky rollers during or before each of exposing and developing so as to make an amount of peel-off static electrification between the photothermographic dry imaging material and the sticky roller to be from -5 to +5 kV.

(Structure 2) The image forming process of Structure 1, wherein exposure is conducted with an exposure device located below where the photothermographic dry imaging material is exposed.

(Structure 3) The image forming process of Structure 1 or 2, wherein an air cleanliness class defined by ISO 14644-1 at the portion of an exposure device is not more than 5.

(Structure 4) The image forming process of Structure 1 or 2, wherein the air cleanliness class defined by ISO 14644-1 at the portion of a developing device is not more than 5.

(Structure 5) The image forming process of any one of Structures 1 4, wherein sticky rollers possess a function to remove static electrification.

(Structure 6) The image forming process of any one of Structures 1 5, wherein static electrification is removed when the photothermographic dry imaging material is brought into contact with sticky rollers.

(Structure 7) The image forming process of any one of Structures 1 6, wherein static electrification is removed before the photothermographic dry imaging material is brought into contact with sticky rollers.

(Structure 8) An image forming process having the steps of: (a) exposing by an exposure device a photothermographic dry imaging material possessing a support having thereon an image forming layer containing photosensitive silver halide, a reducing agent for silver ions, a binder and a light-insensitive organic silver salt, and (b) developing the photothermographic dry imaging material by a developing device, while the photothermographic dry imaging material is transported, wherein the exposure device is located below the photothermographic dry imaging material when the photothermographic dry imaging material is exposed.

(Structure 9) The image forming process of Structure 8, wherein one or both surfaces having the image forming layer composed of the photothermographic dry imaging material, are brought into contact with sticky rollers at or before each of the exposure and developing devices.

(Structure 10) The image forming process of Structure 8 or 9, wherein the amount of peel-off static electrification between the photothermographic dry imaging material and the sticky roller is from -5 to +5 kV.

(Structure 11) The image forming process of any one of Structures 8 10, wherein the air cleanliness class defined by ISO 14644-1 at the portion of an exposure device is not more than 5.

(Structure 12) The image forming process of any one of Structures 8 11, wherein the air cleanliness class defined by ISO 14644-1 at the portion of a developing device is not more than 5.

(Structure 13) The image forming process of any one of Structures 8 12, wherein the sticky rollers possess a function to remove static electrification.

(Structure 14) The image forming process of any one of Structures 8 13, wherein static electrification is removed, before the photothermographic dry imaging material is brought into contact with the sticky rollers.

(Structure 15) The image forming process of any one of Structures 1 14, wherein a transporting speed at the developing device is from 30 to 60 mm/second.

(Structure 16) The image forming process of any one of Structures 1 15, wherein the photothermographic dry imaging material comprises a light-sensitive layer containing silver halide particles and aliphatic carboxylic acid silver, and the content ratio of silver behenate in the aliphatic carboxylic acid silver is from 80 to 100 percent by mol.

(Structure 17) The image forming process of any one of Structures 1 16, wherein the photothermographic dry imaging material comprises a light-sensitive layer containing silver halide particles and reducing agents for silver ions, and the reducing agents for silver ions are compounds represented by the following General Formula (RED).

##STR00001## wherein X.sub.1 represents a chalcogen atom or CHR.sub.1; R.sub.1 being a hydrogen atom, a halogen atom, an alkyl group, an, alkenyl group, an aryl group or a heterocyclic group; R.sub.2 represents an alkyl group; R.sub.3 represents a hydrogen atom or a substituent capable of substituting a hydrogen atom on a benzene ring; R.sub.4 represents a substituent; and m2 and n2 each represents an integer of 0 to 2.

(Structure 18) The image forming process of any one of Structures 1 17, wherein the photothermographic dry imaging material comprises a light-sensitive layer containing photosensitive silver halide particles, and the photosensitive silver halide particles are chemically sensitized employing organic sensitizers containing chalcogen atoms.

(Structure 19) The image forming process of any one of Structures 1 18, wherein color image forming agents are contained which increase absorbance between 360 and 450 nm via oxidation.

(Structure 20) The image forming process of any one of Structures 1 19, wherein color image forming agents are contained which increase absorbance between 600 and 700 nm via oxidation.

While the preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be detailed. It is a feature in the present invention that one or both surfaces having an image forming layer (hereinafter occasionally referred to as a light-sensitive surface) composed of a photothermographic dry imaging material (occasionally referred to simply as a photothermographic material or a thermally developable light-sensitive material) are brought into contact with sticky rollers so as to make an amount of peel-off static electrification between the photothermographic dry imaging material and the sticky roller to be from -5 to +5 kV, preferably from -3 to +3 kV, or more preferably from -2 to +2 kV. In the case of the amount of peel-off static electrification being less than -5 kV or more than 5 kV, the desired effect of the present invention can not be attained, and a decline of image quality is observed. Desired effects of the present invention can also not be attained, when a light-insensitive surface is merely brought into contact with the sticky rollers.

No special technique is specifically required in the present invention to make the peel-off static electrification to be from -5 to +5 kV. However, it is preferred that the static electrification is simply removed with sticky rollers having a function of removing the static electrification, though a surface active agent is added into the photothermographic dry imaging material, or an electrically conductive support is employed.

The adhesive force of sticky rollers in the present invention is preferably in the range of 10 65 hPa, or more preferably 10 30 hPa, and excellent cleaning function is achieved in this range. In the case of the adhesive force of sticky rollers being at least 65 hPa, the adhesive force is too strong so that an image forming layer composed of a photothermographic dry imaging material or a backing layer is ripped off, and as a result the image quality frequently drops drastically. On the other hand, in the case of the adhesive force of the sticky rollers being at most 10 hPa, the adhesive force is too weak so that the desired effect of removing foreign matter can not be realized.

An adhesive force between a metal plate and rubber is expressed by the following formula, based on "samples in which two metal plates adhere to each other via rubber" in the physical test method of rubber vulcanization defined by JIS-K6301 for the adhesive force measurement. Adhesive force=Maximum peel-off load/Area of adhesion

In the recording apparatus of the present invention, hardness (JIS A) is preferably in the range of 10 70.degree., whereby an excellent cleaning function is ensured. In the case of the hardness being at most 10.degree., the sticky rollers are too soft so that the sticky rollers tend to be easily damaged, and also resulting in problems of transportability of photothermographic dry imaging materials. On the other hand, in the case of the hardness being at least 70.degree., the sticky rollers are too hard so that the sticky rollers are not transformable, the contact area between the photothermographic dry imaging material and the sticky rollers decreases, or no contact area exists in the direction of the axis of the sticky roller, and the desired effect of removing foreign matter can not be obtained.

Commonly known materials for roller surfaces used for removing dust and foreign matter may be composed of urethane rubber, silicone rubber, or butyl rubber. Materials of the roller surface can be appropriately selected in response to the support, the subbing layer, and the type of foreign matter. It is also preferred that the diameter of the sticky roller is approximately 1.0 10.0 cm, and the roller width is determined to match the width of the light-sensitive materials.

It is preferred that an air cleanliness class defined by ISO 14644-1 at the portion of the exposure device or the developing device in the recording apparatus of the present invention is not more than 5. Though the pressure at the portion of the exposure device or the developing device is increased so as to result in the peripheral portion to be at a negative pressure, and dust and foreign matter are removed via filters by recirculating air within the apparatus, no specific technique is required as a special air cleaning means in the present invention.

It is a feature of the recording apparatus of the present invention that the static electrification is removed before or when the photothermographic dry imaging material is brought into contact with the sticky roller. Though for removing static electrification the photothermographic dry imaging material may be brought into contact with a bar or a brush prior to sticky rollers, it is preferred that the static electrification is simply removed via the rollers incorporating such a function.

It is a feature of another embodiment concerning the image forming process of the present invention that the photothermographic dry imaging material located above the exposure device is exposed from the lower side of the photothermographic dry imaging material. Even though dust and foreign matter once adhere to the light-sensitive surface of the photothermographic dry imaging material, they are easily removed due to gravity by incorporating the previous technique. Lowering specific resistance of the light-sensitive surface is further effective for easily removing dust and foreign matter because of gravity. For this purpose, it is preferred that surface active agents, to be described later, are employed, a subbing layer composed of tin oxide or titanium oxide, whose surface is covered with antimony, is provided, and a protective layer employing electrically conductive polymers, such as polythiophene or polyaniline, is also provided. The image quality is further improved, since dust and foreign matter which adhere to the photothermographic dry imaging material are more effectively removed via these means. In the case of using a conventional type of technique in which the exposure device is located above the photothermographic dry imaging material, and the photothermographic dry imaging material is exposed from the upper side of the photothermographic dry imaging material, dust and foreign matter which adhere to the light-sensitive surface can not be removed, and accumulated dust and foreign matter frequently cause image defects after development. In order to sufficiently obtain effect of this invention, the exposure device is desired to be located below where the photothermographic dry imaging material is exposed, and the angle between the scanning surface of the photothermographic dry imaging material and the scanning laser beam is commonly from 55 to 90 degrees, preferably from 5.5 to 88 degrees, more preferably from 60 to 86 degrees, still more preferably from 65 to 84 degrees, but most preferably from 70 to 82 degrees.

In the case of using sticky rollers for an extended period of time, foreign matter starts to adhere to the surfaces of the sticky rollers, and a decline of adhesive performance tends to occur. In this case, adhesive performance can be recovered, whereby the sticky rollers are removed at regular intervals, and any foreign matter adhering to the sticky rollers is removed by washing the roller surface with pure water. It is possible that sticky rollers may be reused. Cleaning rollers being brought into contact with the surfaces of sticky rollers may also be used. Adhesive performance of the sticky rollers can be continuously maintained, since dust and foreign matter on the surfaces of sticky rollers adhere to the more tacky surfaces of cleaning rollers in such case.

Though the transporting speed of photosensitive material at the exposure and developing devices is appropriately determined, higher speed is desired to improve not only quick processing but also higher throughput. However, the transporting speed is preferably from 10 to 15 mm/second, more preferably from 23 to 60 mm/second, and still more preferably from 30 to 60 mm/second.

<Silver Halide Grains>

Photosensitive silver halide grains (hereinafter simply referred to as silver halide grains) will be described which are employed in the silver salt photothermographic dry imaging material of the present invention (hereinafter simply referred to as the photosensitive material of the present invention).

The photosensitive silver halide grains, as described in the present invention, refer to silver halide crystalline grains which can originally absorb light as an inherent quality of silver halide crystals, can absorb visible light or infrared radiation through artificial physicochemical methods and are treatment-produced so that physicochemical changes occur in the interior of the silver halide crystal and/or on the crystal surface, when the crystals absorb any radiation from ultraviolet to infrared.

Silver halide grains employed in the present invention can be prepared in the form of silver halide grain emulsions, employing methods described in P. Glafkides, "Chimie et Physique Photographiques" (published by Paul Montel Co., 1967), G. F. Duffin, "Photographic Emulsion Chemistry" (published by The Focal Press, 1955), and V. L. Zelikman et al., "Making and Coating Photographic Emulsion", published by The Focal Press, 1964). Namely, any of an acidic method, a neutral method, or an ammonia method may be employed. Further, employed as methods to allow water-soluble silver salts to react with water-soluble halides may be any of a single-jet precipitation method, a double-jet precipitation method, or combinations-thereof. However, of these methods, the so-called controlled double-jet precipitation method is preferably employed in which silver halide grains are prepared while controlling formation conditions.

Halogen compositions are not particularly limited. Any of silver chloride, silver chlorobromide, silver chloroiodobromide, silver bromide, silver iodobromide, or silver iodide may be employed. Of these, silver bromide or silver iodobromide is particularly preferred.

The content ratio of iodine in silver iodobromide is. preferably in the range of 0.02 to 16 mol percent per Ag mol. Iodine may be incorporated so that it is distributed into the entire silver halide grain. Alternatively, a core/shell structure may be formed in which, for example, the concentration of iodine in the central portion of the grain is increased, while the concentration near the grain surface is simply decreased or substantially decreased to zero.

Grain formation is commonly divided into two stages, that is, the formation of silver halide seed grains (being nuclei) and the growth of the grains. Either method may be employed in which two stages are continually carried out, or in which the formation of nuclei (seed grains) and the growth of grains are carried out separately. A controlled double-jet precipitation method, in which grains are formed while controlling the pAg and pH which are grain forming conditions, is preferred, since thereby it is possible to control grain shape as well as grain size. For example, when the method, in which nucleus formation and grain growth are separately carried out, is employed, initially, nuclei (being seed grains) are formed by uniformly and quickly mixing water-soluble silver salts with water-soluble halides in an aqueous gelatin solution. Subsequently, under the controlled pAg and pH, silver halide grains are prepared through a grain growing process which grows the grains while supplying water-soluble silver salts as well as water-soluble halides.

In order to minimize milkiness (or white turbidity) as well as coloration (yellowing) after image formation and to obtain excellent image quality, the average grain diameter of the silver halide grains, employed in the present invention, is preferably rather small. The average grain diameter, when grains having a grain diameter of less than 0.02 .mu.m is beyond practical measurement, is preferably 0.030 to 0.055 .mu.m.

Incidentally, grain diameter, as described herein, refers to the edge length of silver halide grains which are so-called regular crystals such as a cube or an octahedron. Further, when silver halide gains are planar, the grain diameter refers to the diameter of the circle which has the same area as the projection area of the main surface.

In the present invention, silver halide grains are preferably in a state of monodispersion. Monodispersion, as described herein, means that the variation coefficient, obtained by the formula described below, is not more than 30 percent. The aforesaid variation coefficient is preferably not more than 20 percent, and is more preferably not more than 15 percent. Variation coefficient (in percent) of grain diameter=standard deviation of grain diameter/average of grain diameter.times.100

Cited as shapes of silver halide grains may be cubic, octahedral and tetradecahedral grains, planar grains, spherical grains, rod-shaped grains, and roughly elliptical-shaped grains. Of these, cubic, octahedral, tetradecahedral, and planar silver halide grains are particularly preferred.

When the aforesaid planar silver halide grains are employed, their average aspect ratio is preferably 1.5 to 100, and is more preferably 2 to 50. These are described in U.S. Pat. Nos. 5,264,337, 5,314,798, and 5,320,958, and incidentally it is possible to easily prepare the aforesaid target planar grains. Further, it is possible to preferably employ silver halide grains having rounded corners.

The crystal habit of the external surface of silver halide grains is not particularly limited. However, when spectral sensitizing dyes, which exhibit crystal habit (surface) selectiveness are employed, it is preferable that silver halide grains are employed which have the crystal habit matching their selectiveness in a relatively high ratio. For example, when sensitizing dyes, which are selectively adsorbed onto a crystal plane having a Miller index of (100), it is preferable that the ratio of the (100) surface on the external surface of silver halide grains is high. The ratio is preferably at least 50 percent, is more preferably at least 70 percent, and is most preferably at least 80 percent. When sensitizing dyes, which are selectively adsorbed onto a crystal plane having a Miller index of (111), it is also preferable that the ratio of the (111) surface on the external surface of silver halide grains is high. Incidentally, it is possible to obtain a ratio of the surface having a Miller index of (100), based on T. Tani, J. Imaging Sci., 29, 165 (1985), utilizing adsorption dependence of sensitizing dye in a (111) plane as well as a (100) surface.

The silver halide grains, employed in the present invention, are preferably prepared employing low molecular weight gelatin, having an average molecular weight of not more than 50,000 during the formation of the grains, which are preferably employed during formation of nuclei. The low molecular weight gelatin refers to gelatin having an average molecular weight of not more than 50,000. The molecular weight is preferably from 2,000 to 40,000, and is more preferably from 5,000 to 25,000. It is possible to measure the molecular weight of gelatin employing gel filtration chromatography.

The concentration of dispersion media during the formation of nuclei is preferably not more than 5 percent by weight. It is more effective to carry out the formation at a low concentration of 0.05 to 3.00 percent by weight.

During formation of the silver halide grains employed in the present invention, it is possible to use polyethylene oxides represented by the general formula described below. YO(CH.sub.2CH.sub.2O).sub.m(CH(CH.sub.3)CH.sub.2O).sub.p(CH.sub.2CH.sub.2- O).sub.nY General Formula wherein Y represents a hydrogen atom, --SO.sub.3M.sup.1, or --CO-B-COOM.sup.1; M.sup.1 represents a hydrogen atom, an alkali metal atom, an ammonium group, or an ammonium group substituted with an alkyl group having not more than 5 carbon atoms; B represents a chained or cyclic group which forms an organic dibasic acid; m and n each represents 0 through 50; and p represents 1 through 100.

When silver halide photosensitive photographic materials are produced, polyethylene oxides, represented by the above general formula, have been preferably employed as anti-foaming agents to counter marked foaming which occurs while stirring and transporting emulsion raw materials in a process in which an aqueous gelatin solution is prepared, in the process in which water-soluble halides as well as water-soluble silver salts are added to the gelatin solution, and in a process in which the resultant emulsion is applied onto a support. Techniques to employ polyethylene oxides as an anti-foaming agent are disclosed in, for example, Japanese Patent O.P.I. Publication No. 44-9497. The polyethylene oxides represented by the above general formula function as an anti-foaming agent during nuclei formation.

The content ratio of polyethylene oxides, represented by the above general formula, is preferably not more than 1 percent by weight with respect to silver, and is more preferably from 0.01 to 0.10 percent by weight.

It is desired that polyethylene oxides, represented by the above general formula, are present during nuclei formation. It is preferable that they are previously added to the dispersion media prior to nuclei formation. However, they may also be added during nuclei formation, or they may be employed by adding them to an aqueous silver salt solution or an aqueous halide solution which is employed during nuclei formation. However, they are preferably employed by adding them to an aqueous halide solution, or to both aqueous solutions in an amount of 0.01 to 2.00 percent by weight. Further, it is preferable that they are present during at least 50 percent of the time of the nuclei formation process, and it is more preferable that they are present during at least 70 percent of the time of the same. The polyethylene oxides, represented by the above general formula, may be added in the form of powder or they may be dissolved in a solvent such as methanol and then added.

Incidentally, temperature during nuclei formation is commonly from 5 to 60.degree. C., and is preferably from 15 to 50.degree. C. It is preferable that the temperature is controlled within the range, even when a constant temperature, a temperature increasing pattern (for example, a case in which temperature at the initiation of nuclei formation is 25.degree. C., subsequently, temperature is gradually increased during nuclei formation and the temperature at the completion of nuclei formation is 40.degree. C.), or a reverse sequence may be employed.

The concentration of an aqueous silver salt solution and an aqueous halide solution, employed for nuclei formation, is preferably not more than 3.5 M/L, and is more preferably in the lower range of 0.01 to 2.50 M/L. The silver ion addition rate during nuclei formation per liter of reaction liquid is preferably from 1.5.times.1.sup.-3 to 3.0.times.10.sup.-1 mol/minute, and is more preferably from 3.0.times.10.sup.-3 to 8.0.times.10.sup.-2 mol/minute.

The pH during nuclei formation can be set in the range of 1.7 to 10.0. However, since the pH on the alkali side broadens the particle size distribution of the formed nuclei, the preferred pH is from 2 to 6. Further, the pBr during nuclei formation is usually from about 0.05 to about 3.00, is preferably from 1.0 to 2.5, and is more preferably from 1.5 to 2.0.

<Silver Halide Grains of Internal Latent Formation After Thermal Development>

The photosensitive silver halide grains according to the present invention are characterized in that they have a property to change from a surface latent image formation type to an internal latent image formation type after subjected to thermal development. This change is caused by decreasing the speed of the surface latent image formation by the effect of thermal development.

When the silver halide grains are exposed to light prior to thermal development, latent images capable of functioning as a catalyst of development reaction are formed on the surface of the aforesaid silver halide grains. "Thermal development" is a reduction reaction by a reducing agent for silver ions. On the other hand, when exposed to light after the thermal development process, latent images are more formed in the interior of the silver halide grains than the surface thereof. As a result, the silver halide grains result in retardation of latent image formation on the surface. It was not known in the field of a photothermographic material to employ the above-mentioned silver halide grains which largely change their latent image formation function before and after thermal development.

Generally, when photosensitive silver halide grains are exposed to light, silver halide grains themselves or spectral sensitizing dyes, which are adsorbed on the surface of photosensitive silver halide grains, are subjected to photo-excitation to generate free electrons. Generated electrons are competitively trapped by electron traps (sensitivity centers) on the surface or interior of silver halide grains. Accordingly, when chemical sensitization centers (chemical sensitization specks) and dopants, which are useful as an electron trap, are much more located on the surface of the silver halide grains than the interior thereof and the number is appropriate, latent images are dominantly formed on the surface, whereby the resulting silver halide grains become developable. Contrary to this, when chemical sensitization centers (chemical sensitization specks) and dopants, which are useful as an electron trap, are much more located in the interior of the silver halide grains than the surface thereof and the number is appropriate, latent images are dominantly formed in the interior, whereby it becomes difficult to develop the resulting silver halide grains. In other words, in the former, the surface speed is higher than interior speed, while in the latter, the surface speed is lower than the interior speed. The former type of latent image is called "a surface latent image", and the latter is called "an internal latent image". Examples of the references are:

(1) T. H. James ed., "The Theory of the Photographic Process" 4.sup.th edition, Macmillan Publishing Co., Ltd. 1977; and

(2) Japan Photographic Society, "Shashin Kogaku no Kiso" (Basics of Photographic Engineering), Corona Publishing Co. Ltd., 1998.

The photosensitive silver halide grains of the present invention are preferably provided with dopants which act as electron trapping in the interior of silver halide grains at least in a stage of exposure to light after thermal development. This is desired so as to achieve high photographic speed grains as well as high image keeping properties.

It is especially preferred that the dopants act as a hole trap during an exposure step prior to thermal development, and the dopants change after a thermal development step resulting in functioning as an electron trap.

Electron trapping dopants, as described herein, refer to silver, elements except for halogen or compounds constituting silver halide, and the aforesaid dopants themselves which exhibit properties capable of trapping free electron, or the aforesaid dopants are incorporated in the interior of silver halide grains to generate electron trapping portions such as lattice defects. For example, listed are metal ions other than silver ions or salts or complexes thereof, chalcogen (such as elements of oxygen family) sulfur, selenium, or tellurium, inorganic or organic compounds comprising nitrogen atoms, and rare earth element ions or complexes thereof.

Listed as metal ions, or salts or complexes thereof may be lead ions, bismuth ions, and gold ions, or lead bromide, lead carbonate, lead sulfate, bismuth nitrate, bismuth chloride, bismuth trichloride, bismuth carbonate, sodium bismuthate, chloroauric acid, lead acetate, lead stearate, and bismuth acetate.

Employed as compounds comprising chalcogen such as sulfur, selenium, and tellurium may be various chalcogen releasing compounds which are generally known as chalcogen sensitizers in the photographic industry. Further, preferred as organic compounds comprising chalcogen or nitrogen are heterocyclic compounds which include, for example, imidazole, pyrazole, pyridine, pyrimidine, pyrazine, pyridazine, triazole, triazine, idole, indazole, purine, thiazole, oxadiazole, quinoline, phthalazine, naphthylizine, quinoxaline, quinazoline, cinnoline, pteridine, acrydine, phenanthroline, phenazine, tetrazole, thiazole, oxazole, benzimidazole, benzoxazole, benzthiazole, indolenine, and tetraazaindene. Of these, preferred are imidazole, pyrazine, pyrimidine, pyrazine, pyridazine, triazole, triazine, thiadiazole, oxadiazole, quinoline, phthalazine, naphthylizine, quinoxaline, quinazoline, cinnoline, tetrazole, thiazole, oxazole, benzimidazole, benzoxazole, benzthiazole, and tetraazaindene.

Incidentally, the aforesaid heterocyclic compounds may have substituent(s). Preferable substituents include an alkyl group, an alkenyl group, an aryl group, an alkoxy group, an aryloxy group, an acyloxy group, an acyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, an acyloxy group, an acylamino group, an alkoxycarbonylamino group, an aryloxycarbonylamino group, a sulfonylamino group, a sulfamoyl group, a carbamoyl group, a sulfonyl group, a ureido group, a phosphoric acid amide group, a halogen atom, a cyano group, a sulfo group, a carboxyl group, a nitro group, a heterocyclic group. Of these, more preferred are an alkyl group, an aryl group, an alkoxy group, an aryloxy group, an acyl group, an acylamino group, an alkoxycarbonylamino group, an aryloxycarbonylamino group, a sulfonylamino group, a sulfamoyl group, a carbamoyl group, a ureido group, a phosphoric acid amido group, a halogen atom, a cyano group, a nitro group, and a heterocyclic group. More preferred are an alkyl group, an aryl group, an alkoxy group, an aryloxy group, an acyl group, an acylamino group, a sulfonylamino group, a sulfamoyl group, a carbamoyl group, a halogen atom, a cyano group, a nitro group, and a heterocyclic group.

Incidentally, ions of transition metals which belong to Groups 6 through 11 in the Periodic Table may be chemically modified to form a complex employing ligands of the oxidation state of the ions and incorporated in silver halide grains employed in the present invention so as to function as an electron trapping dopant, as described above, or as a hole trapping dopant. Preferred as aforesaid transition metals are W, Fe, Co, Ni, Cu, Ru, Rh, Pd, Re, Os, Ir, and Pt.

In the present invention, aforesaid various types of dopants may be employed individually or in combination of at least two of the same or different types. It is preferred that at least one of the dopants act as an electron trapping dopant during an exposure time after being thermal developed. They may be incorporated in the interior of the silver halide grains in any forms of chemical states.

It is not recommended to use a complex or a salt of Ir or Cu as a single dopant without combining with other dopant.

The content ratio of dopants is preferably in the range of 1.times.10.sup.-9 to 1.times.10 mol per mol of silver, and is more preferably 1.times.10.sup.-6 to 1.times.10.sup.-2 mol.

However, the optimal amount varies depending the types of dopants, the diameter and shape of silver halide grains, and ambient conditions. Accordingly, it is preferable that addition conditions are optimized taking into account these conditions.

In the present invention, preferred as transition metal complexes or complex ions are those represented by the general formula described below. [ML.sub.6].sup.m General Formula wherein M represents a transition metal selected from the elements of Groups 6 through 11 in the Periodic Table; L represents a ligand; and m represents 0, -, 2-, 3-, or 4-. Listed as specific examples of ligands represented by L arena halogen ion (a fluoride ion, a chloride ion, a bromide ion, or an iodide ion), a cyanide, a cyanate, a thiocyanate, a selenocyanate, a tellurocyanate, an azide, and an aqua ligand, and nitrosyl and thionitrosyl. Of these, aqua, nitrosyl, and thionitrosyl are preferred. When the aqua ligand is present, one or two ligands are preferably occupied by the aqua ligand. L may be the same or different.

It is preferable that compounds, which provide ions of these metals or complex ions, are added during formation of silver halide grains so as to be incorporated in the silver halide grains. The compounds may be added at any stage of, prior to or after, silver halide grain preparation, namely nuclei formation, grain growth, physical ripening or chemical ripening. However, they are preferably added at the stage of nuclei formation, grain growth, physical ripening, are more preferably added at the stage of nuclei formation and growth, and are most preferably added at the stage of nuclei formation. They may be added over several times upon dividing them into several portions. Further, they may be uniformly incorporated in the interior of silver halide grains. Still further, as described in Japanese Patent O.P.I. Publication Nos. 63-29603, 2-306236, 3-167545, 4-76534, 6-110146, and 5-273683, they may be incorporated so as to result in a desired distribution in the interior of the grains.

These metal compounds may be dissolved in water or suitable organic solvents (for example, alcohols, ethers, glycols, ketones, esters, and amides) and then added. Further, addition methods include, for example, a method in which either an aqueous solution of metal compound powder or an aqueous solution prepared by dissolving metal compounds together with NaCl and KCl is added to a water-soluble halide solution, a method in which silver halide grains are formed by a silver salt solution, and a halide solution together with a the compound solution as a third aqueous solution employing a triple-jet precipitation method, a method in which, during grain formation, an aqueous metal compound solution in a necessary amount is charged into a reaction vessel, or a method in which, during preparation of silver halide, other silver halide grains which have been doped with metal ions or complex ions are added and dissolved. Specifically, a method is preferred in which either an aqueous solution of metal compound powder or an aqueous solution prepared by dissolving metal compounds together with NaCl and KCl is added to a water-soluble halide solution. When added onto the grain surface, an aqueous metal compound solution in a necessary amount may be added to a reaction vessel immediately after grain formation, during or after physical ripening, or during chemical ripening.

Incidentally, it is possible to introduce non-metallic dopants into the interior of silver halide employing the same method as the metallic dopants.

In the imaging materials in accordance with the present invention, it is possible to evaluate whether the aforesaid dopants exhibit electron trapping properties or not, while employing a method which has commonly employed in the photographic industry. Namely a silver halide emulsion composed of silver halide grains, which have been doped with the aforesaid dopant or decomposition product thereof so as to be introduced into the interior of grains, is subjected to photoconduction measurement, employing a microwave photoconduction measurement method. Subsequently, it is possible to evaluate the aforesaid electron trapping properties by comparing the resulting decrease in photoconduction to that of the silver halide emulsion comprising no dopant as a standard. It is also possible to evaluate the same by performing experiments in which the internal speed of the aforesaid silver halide grains is compared to the surface speed.

Further, a method follows which is applied to a finished photothermographic dry imaging material to evaluate the electron trapping dopant effect in accordance with the present invention. For example, prior to exposure, the aforesaid imaging material is heated under the same conditions as the commonly employed thermal development conditions. Subsequently, the resulting material is exposed to white light or infrared radiation through an optical wedge for a definite time (for example, 30 seconds), and thermally developed under the same thermal development conations as above, whereby a characteristic curve (or a densitometry curve) is obtained. Then, it is possible to evaluate the aforesaid electron trapping dopant effect by comparing the speed obtained based on the characteristic curve to that of the imaging material which is composed of the silver halide emulsion which does not comprise the aforesaid electron trapping dopant. Namely, it is preferred to confirm that the speed of the former sample composed of the silver halide grain emulsion comprising the dopant in accordance with the present invention is lower than the latter sample which does not comprise the aforesaid dopant.

Speed of the aforesaid material is obtained based on the characteristic curve which is obtained by exposing the aforesaid material to white light or infrared radiation through an optical wedge for a definite time (for example 30 seconds) followed by developing the resulting material under common thermal development conditions. Further, speed of the aforesaid material is obtained based on the characteristic curve which is obtained by heating the aforesaid material under common thermal development conditions prior to exposure and giving the same definite exposure as above to the resulting material for the same definite time as above followed by thermally developing the resulting material under common thermal development conditions. The ratio of the latter speed to the former speed is preferably at most 1/10, and is more preferably at most 1/20. When the silver halide emulsion is chemically sensitized, the preferred photographic speed ratio is as low as not more than 1/50.

The silver halide grains of the present invention may be incorporated in a photosensitive layer employing an optional method. In such a case, it is preferable that the aforesaid silver halide grains are arranged so as to be adjacent to reducible silver sources (being aliphatic carboxylic silver salts) in order to get an imaging material having a high covering power.

The silver halide of the present invention is previously prepared and the resulting silver halide is added to a solution which is employed to prepare aliphatic carboxylic acid silver salt particles. By so doing, since a silver halide preparation process and an aliphatic carboxylic acid silver salt particle preparation process are performed independently, production is preferably controlled. Further, as described in British Patent No. 1,447,454, when aliphatic carboxylic acid silver salt particles are formed, it is possible to almost simultaneously form aliphatic carboxylic acid silver salt particles by charging silver ions to a mixture consisting of halide components such as halide ions and aliphatic carboxylic acid silver salt particle forming components. Still further, it is possible to prepare silver halide grains utilizing conversion of aliphatic carboxylic acid silver salts by allowing halogen-containing components to act on aliphatic carboxylic acid silver salts. Namely, it is possible to convert some of aliphatic carboxylic acid silver salts to photosensitive silver halide by allowing silver halide forming components to act on the previously prepared aliphatic carboxylic acid silver salt solution or dispersion, or sheet materials comprising aliphatic carboxylic acid silver salts.

Silver halide grain forming components include inorganic halogen compounds, onium halides, halogenated hydrocarbons, N-halogen compounds, and other halogen containing compounds.

Specific examples are disclosed in; U.S. Pat. Nos. 4,009,039, 3,4757,075, 4,003,749; G.B. Pat. No. 1,498,956; and Japanese Patent O.P.I. Publication Nos. 53-27027, 53-25420.

Further, silver halide grains may be employed in combination which are produced by converting some part of separately prepared aliphatic carboxylic acid silver salts.

The aforesaid silver halide grains, which include separately prepared silver halide grains and silver halide grains prepared by partial conversion of aliphatic carboxylic acid silver salts, are employed commonly in an amount of 0.001 to 0.7 mol per mol of aliphatic carboxylic acid silver salts and preferably in an amount of 0.03 to 0.5 mol.

The separately prepared photosensitive silver halide particles are subjected to desalting employing desalting methods known in the photographic art, such as a noodle method, a flocculation method, an ultrafiltration method, and an electrophoresis method, while they may be employed without desalting.

<Light-insensitive Aliphatic Carboxylic Acid Silver Salt>

The light-insensitive aliphatic carboxylic acid silver salts according to the present invention are reducible silver sources which are preferably silver salts of long chain aliphatic carboxylic acids, having from 10 to 30 carbon atoms and preferably from 15 to 25 carbon atoms. Listed as examples of appropriate silver salts are those described below.

For example, listed are silver salts of gallic acid, oxalic acid, behenic acid, stearic acid, arachidic acid, palmitic acid, and lauric acid. Of these, listed as preferable silver salts are silver behenate, silver arachidate, and silver stearate.

Further, in the present invention, it is preferable that at least two types of aliphatic carboxylic acid silver salts are mixed since the resulting developing ability is enhanced and high contrast silver images are formed. Preparation is preferably carried out, for example, by mixing a mixture consisting of at least two types of aliphatic carboxylic acid with a silver ion solution.

On the other hand, from the viewpoint of enhancing retaining properties of images, the melting point of aliphatic carboxylic acids, which are employed as a raw material of aliphatic carboxylic acid silver, is commonly at least 50.degree. C., and is preferably at least 60.degree. C. The content ratio of aliphatic carboxylic acid silver salts is commonly at least 50 percent by mol, is preferably at least 70 percent by mol, and still more preferably from 80 to 100 percent by mol. From this viewpoint, specifically, it is preferable that the content ratio of silver behenate in the aliphatic carboxylic acid silver is higher.

Aliphatic carboxylic acid silver salts are prepared by mixing water-soluble silver compounds with compounds which form complexes with silver. When mixed, a normal precipitation method, a reverse precipitating method, a double-jet precipitation method, or a controlled double-jet precipitation method, described in Japanese Patent O.P.I. Publication No. 9-127643, are preferably employed. For example, after preparing a metal salt soap (for example, sodium behenate and sodium arachidate) by adding alkali metal salts (for example, sodium hydroxide and potassium hydroxide) to organic acids, crystals of aliphatic carboxylic acid silver salts are prepared by mixing the soap with silver nitrate. In such a case, silver halide grains may be mixed together with them.

The kinds of alkaline metal salts employed in the present invention include sodium hydroxide, potassium hydroxide, and lithium hydroxide, and it is preferable to simultaneously use sodium hydroxide and potassium hydroxide. When simultaneously employed, the mol ratio of sodium hydroxide to potassium hydroxide is preferably in the range of 10:90 75:25. When the alkali metal salt of aliphatic carboxylic acid is formed via a reaction with an aliphatic carboxylic acid, it is possible to control the viscosity of the resulting liquid reaction composition within the desired range.

Further, in the case in which aliphatic carboxylic acid silver is prepared in the presence of silver halide grains at an average grain diameter of at most 0.050 .mu.m, it is preferable that the ratio of potassium among alkaline metals in alkaline metal salts is higher than the others, since dissolution of silver halide grains as well as Ostwald ripening is retarded. Further, as the ratio of potassium salts increases, it is possible to decrease the size of fatty acid silver salt particles. The ratio of potassium salts is preferably 50 100 percent with respect to the total alkaline metal salts, while the concentration of alkaline metal salts is preferably 0.1 0.3 mol/1,000 ml.

<Silver Salt Particles at a High Silver Ratio>

An emulsion containing aliphatic carboxylic acid silver salt particles according to the present invention is a mixture consisting of free aliphatic carboxylic acids which do not form silver salts, and aliphatic carboxylic acid silver salts. In view of storage stability of images, it is preferable that the ratio of the former is lower than the latter. Namely, the aforesaid emulsion according to the present intention preferably-contains aliphatic carboxylic acids in an amount of 3 10 mol percent with respect to the aforesaid aliphatic carboxylic acid silver salt particles, and most preferably 4 8 mol percent.

Incidentally, in practice, each of the amount of total aliphatic carboxylic acids and the amount of free aliphatic carboxylic acids is determined employing the methods described below. Whereby, the amount of aliphatic carboxylic acid silver salts and free aliphatic carboxylic acids, and each ratio, or the ratio of free carboxylic acids to total aliphatic carboxylic acids, are calculated.

(Quantitative Analysis of the Amount of Total Aliphatic Carboxylic Acids (the Total Amount of These Being Due to Both of the Aforesaid Aliphatic Carboxylic Acid Silver Salts and Free Acids))

(1) A sample in an amount (the weight when peeled from a photosensitive material) of approximately 10 mg is accurately weighed and placed in a 200 ml ovid flask. (2) Subsequently, 15 ml of methanol and 3 ml of 4 mol/L hydrochloric acid are added and the resulting mixture is subjected to ultrasonic dispersion for one minute. (3) Boiling stones made of Teflon (registered trade name) are placed and refluxing is performed for 60 minutes. (4) After cooling, 5 ml of methanol is added from the upper part of the cooling pipe and those adhered to the cooling pipe are washed into the ovoid flask (this is repeated twice). (5) The resulting liquid reaction composition is subjected to extraction employing ethyl acetate (separation extraction is performed twice by adding 100 ml of ethyl acetate and 70 ml of water). (6) Vacuum drying is then performed at normal temperature for 30 minutes. (7) Placed in a 10 ml measuring flask is 1 ml of a benzanthorone solution as an internal standard (approximately 100 mg of benzanthrone is dissolved in toluene and the total volume is made to 100 ml by the addition of toluene). (8) The sample is dissolved in toluene and placed in the measuring flask described in (7) and the total volume is adjusted by the addition of toluene. (9) Gas chromatography (GC) measurements are performed under the measurement conditions below.

Apparatus: HP-5890+HP-Chemistation Column: HP-1 30 m.times.0.32 mm.times.0.25 .mu.m (manufactured by Hewlett-Packard) Injection inlet: 250.degree. C. Detector: 280.degree. C. Oven: maintained at 250.degree. C. Carrier gas: He Head pressure: 80 kPa (Quantitative Analysis of Free Aliphatic Carboxylic Acids) (1) A sample in an amount of approximately 20 mg is accurately weighed and placed in a 200 ml ovoid flask. Subsequently, 100 ml of methanol was added and the resulting mixture is subjected to ultrasonic dispersion (free organic carboxylic acids are extracted). (2) The resulting dispersion is filtered. The filtrate is placed in a 200 ml ovoid flask and then dried up (free organic carboxylic acids are separated). (3) Subsequently, 15 ml of methanol and 3 ml of 4 mol/L hydrochloric acid are added and the resulting mixture is subjected to ultrasonic dispersion for one minute. (4) Boiling stones made of Teflon (registered trade mark) were added, and refluxing is performed for 60 minutes. (5) Added to the resulting liquid reaction composition are 60 ml of water and 60 ml of ethyl acetate, and a methyl-esterificated product of organic carboxylic acids is then extracted to an ethyl acetate phase. Ethyl acetate extraction is performed twice. (6) The ethyl acetate phase is dried, followed by vacuum drying for 30 minutes. (7) Placed in a 10 ml measuring flask is 1 ml of a benzanthorone solution (being an internal standard and prepared in such a manner that approximately 100 mg of benzanthrone is dissolved in toluene and the total volume is made to 100 ml by the addition of toluene). (8) The product obtained in (6) is dissolved in toluene and placed in the measuring flask described in (7) and the total volume is adjusted by the addition of more toluene. (9) GC measurement carried out using the conditions described below.

Apparatus: HP-5890+HP-Chemistation Column: HP-1 30 m.times.0.32 mm.times.0.25 .mu.m (manufactured by Hewlett-Packard) Injection inlet: 250.degree. C. Detector: 280.degree. C. Oven: maintained at 250.degree. C. Carrier gas: He Head pressure: 80 kPa <Morphology of Aliphatic Carboxylic Acid Silver Salts>

Aliphatic carboxylic acid silver salts according to the present invention may be crystalline grains which have the core/shell structure disclosed in European Patent No. 1168069A1 and Japanese Patent O.P.I. Publication No. 2002-023303. Incidentally, when the core/shell structure is formed, organic silver salts, except for aliphatic carboxylic acid silver, such as silver salts of phthalic acid and benzimidazole may be employed wholly or partly in the core portion or the shell portion as a constitution component of the aforesaid crystalline grains.

In the aliphatic carboxylic acid silver salts according to the present invention, it is preferable that the average circle equivalent diameter is from 0.05 to 0.80 .mu.m, and the average thickness is from 0.005 to 0.070 .mu.m. It is more preferable that the average circle equivalent diameter is from 0.2 to 0.5 .mu.m, and the average thickness is from 0.01 to