Patent Publication Number: US-6342342-B2

Title: Photothermograhpic image forming element

Description:
This invention relates to a photothermographic image forming element, and more particularly, to a photothermographic image forming element suitable for use in a photomechanical process and especially adapted for scanners and image setters. More specifically, it relates to such a photothermographic element featuring low Dmin and consistent manufacture. 
     BACKGROUND OF THE INVENTION 
     There are known a number of photosensitive elements having a photosensitive layer on a support wherein images are formed by imagewise exposure. Among these, a technique of forming images through heat development is known as a system capable of simplifying image forming means and contributing to the environmental protection. 
     From the contemporary standpoints of environmental protection and space saving, it is strongly desired in the photomechanical process field to reduce the quantity of spent solution. Needed in this regard is a technology relating to—photothermographic elements for use in photomechanical process which can be effectively exposed by means of laser scanners or laser image setters and produce distinct black images having a high resolution and sharpness. These photothermographic elements offer to the customer a simple thermographic system that eliminates a need for solution type chemical agents and is not detrimental to the environment. 
     The technology of forming images through heat development is disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, “Thermally Processed Silver Systems” in “Imaging Processes and Materials,” Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969. These photothermographic elements generally contain a reducible non-photosensitive silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), and a reducing agent for silver, typically dispersed in an organic binder matrix. Photothermographic elements are stable at room temperature. When they are heated at an elevated temperature (e.g., 80° C. or higher) after exposure, redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the reducible silver salt in exposed regions provides black images in contrast to unexposed regions, forming an image. 
     For the preparation of organic acid silver salts, it is well known in the art to treat an organic acid such as an aliphatic carboxylic acid with an alkali to form the organic acid alkali metal salt, and add silver nitrate thereto, whereupon exchange reaction takes place between the alkali metal of the organic acid alkali metal salt and silver ion to thereby form the organic acid silver salt. This process, however, is difficult to obtain homogeneous organic acid silver because the organic acid alkali metal salt becomes solid, which hinders uniform proceeding of exchange reaction with silver ion. 
     EP 762,196 and JP-A 90550/1997 disclose to introduce metal ions or metal complex ions belonging to Group VII or VIII (Groups 7 to 10) in the Periodic Table into photosensitive silver halide grains to be used in thermographic image forming elements and to introduce hydrazine derivatives into photosensitive elements to achieve high contrast photographic properties. Since the photosensitive elements containing such nucleating agents as hydrazine derivatives tend to fog, there is a desire to produce high contrast images while suppressing the fog. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the invention is to provide a photothermographic image forming element suitable for use in a photomechanical process and exhibiting excellent photographic properties including a high contrast and low fog so that it may especially comply with scanners and image setters. 
     According to the invention, there is provided a photothermographic image forming element comprising a non-photosensitive silver salt and a photosensitive silver halide on a support. The non-photosensitive silver salt has been formed by simultaneously adding a previously prepared aqueous solution of silver nitrate and a previously prepared solution or suspension of an organic acid alkali metal salt to a reactor and mixing them therein. A photosensitive layer containing the photosensitive silver halide or a layer disposed adjacent thereto or both contain a nucleating agent. 
     Preferably, the solution or suspension of an organic acid alkali metal salt contains a tertiary alcohol. 
     Also preferably, the nucleating agent is a substituted alkene derivative of the following formula (1), a substituted isoxazole derivative of the following formula (2), an acetal compound of the following formula (3), or a hydrazine derivative.                    
     R 1 , R 2 , and R 3  are independently hydrogen or substituents, and Z is an electron attractive group or silyl group, and at least one pair of R 1  and Z, R 2  and R 3 , R 1  and R 2 , and R 3  and Z, taken together, may form a cyclic structure;                    
     R 4  is a substituent;                    
     X and Y are independently hydrogen or substituents, A and B are independently alkoxy, alkylthio, alkylamino, aryloxy, arylthio, anilino, heterocyclic oxy, heterocyclic thio, or heterocyclic amino groups, or X and Y, and A and B, taken together, may form a cyclic structure. 
     In one preferred embodiment, the photosensitive layer has been formed by applying a coating solution containing the photosensitive silver halide and a binder in a solvent in which water constitutes at least 60% by weight of the solvent, the photosensitive silver halide has been formed independent from the non-photosensitive silver salt and added during preparation of the coating solution, and the binder contains at least 50% by weight of a polymer latex having a glass transition temperature of −30° C. to 40° C. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The only FIGURE, FIG. 1 is a schematic view of one exemplary heat developing apparatus for use in the processing of the photothermographic image forming element according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The photothermographic (or photosensitive heat developable) image forming element of the present invention has a photosensitive layer containing a photosensitive silver halide on a support and further contains a non-photosensitive silver salt and a nucleating agent. The non-photosensitive silver salt has been formed by simultaneously adding a previously prepared aqueous solution of silver nitrate and a previously prepared solution or suspension of an organic acid alkali metal salt to a reactor and mixing them therein. The nucleating agent is added to the photosensitive layer or a layer disposed adjacent thereto or both. By using the organic acid silver formed as above as the non-photosensitive silver salt and the nucleating agent in combination therewith, high contrast images are produced at a high sensitivity and low fog (or Dmin). In contrast, if an organic acid silver formed by a method other than the simultaneous addition and mixing is used, the fog increases and the contrast is insufficiently enhanced or rather becomes low or soft despite the combined use of the nucleating agent. On the other hand, if the nucleating agent is omitted, high contrast images meeting the requirements of photomechanical process are not obtained. Although the problem of increased fog arises when a nucleating agent is added in order to product high contrast images, the use of the organic acid silver formed as above solves the problem. Although the use of a nucleating agent can cause soft gradation or low contrast when combined with an organic acid silver formed by a particular method, the organic acid silver formed as above ensures a high contrast. 
     Organic Silver Salt 
     The non-photosensitive silver salt used herein is an organic silver salt which is relatively stable to light, but forms a silver image when heated at 80° C. or higher in the presence of an exposed photocatalyst (as typified by a latent image of photosensitive silver halide) and a reducing agent. The organic silver salt may be of any desired organic compound containing a source capable of reducing silver ion. Preferred are silver salts of organic acids, typically long chain aliphatic carboxylic acids having 10 to 30 carbon atoms, especially 15 to 28 carbon atoms. Also preferred are complexes of organic or inorganic silver salts with ligands having a stability constant in the range of 4.0 to 10.0. The silver-providing substance preferably constitutes about 5 to 70% by weight of the image forming layer (or photosensitive layer). Preferred organic silver salts include silver salts of organic compounds having a carboxyl group. Examples include silver salts of aliphatic carboxylic acids and silver salts of aromatic carboxylic acids though not limited thereto. Preferred examples of the silver salt of aliphatic carboxylic acid include silver behenate, silver arachidate, silver stearate, silver oleate, silver laurate, silver caproate, silver myristate, silver palmitate, silver maleate, silver fumarate, silver tartrate, silver linolate, silver butyrate, silver camphorate and mixtures thereof. 
     The organic acid silver used herein is formed by reacting silver nitrate with a solution or suspension of an alkali metal salt (e.g., sodium, potassium or lithium salt) of an organic acid. Specifically, the organic acid silver is formed by simultaneously adding a previously prepared aqueous solution of silver nitrate and a previously prepared solution or suspension of an organic acid alkali metal salt to a reactor and mixing them therein. In this step, one solution may be added to the reactor in advance of the other solution as will be described later. However, the procedure of charging the reactor with the entirety of one solution and then adding the other solution is outside the scope of the invention. At least 50% by volume of the respective solutions should preferably be added at the same time. The organic acid alkali metal salt used herein is obtained by treating the above-described organic acid with an alkali. The preparation of the organic acid silver according to the invention may be carried out in any suitable reactor in a batchwise or continuous manner. Agitation in the reactor may be carried out by any desired method depending on the characteristics required for organic acid silver grains. 
     As to the addition of the silver nitrate aqueous solution and the organic acid alkali metal salt solution or suspension, both the solutions may have any suitable concentrations for the desired grain size of the organic acid silver grains to be formed therefrom. They may be added at any desired rates. A constant addition method of adding them at a constant rate or an accelerated or decelerated addition method of accelerating or decelerating the addition rate as a function of time may be employed. The solutions may be added to or below the surface of the reaction solution. Either one of the silver nitrate aqueous solution and the organic acid alkali metal salt solution or suspension may be partially added in advance. Preferably the silver nitrate aqueous solution is added in advance. An appropriate amount of one solution added in advance of the other solution is 0 to 50%, more preferably 0 to 25% by volume of the entire amount. As described in JP-A 127643/1997, it is also preferable to add both the solutions while controlling the pH or silver potential of the reaction solution. 
     The silver nitrate aqueous solution and the organic acid alkali metal salt solution or suspension may be adjusted to suitable pH levels depending on the desired characteristics required for the organic acid silver grains. For pH adjustment, any suitable acid or alkali may be added. Depending on the characteristics required for the organic acid silver grains, for example, for controlling the size of organic acid silver grains, the temperature in the reactor may be set at a suitable level. Similarly, the temperatures of the silver nitrate aqueous solution and the organic acid alkali metal salt solution or suspension to be added may also be set at suitable levels. Typically, the organic acid alkali metal salt solution or suspension is heated and maintained at or above 50° C., preferably at about 65 to about 95° C., in order to keep them well flowing. 
     Preferably, the organic acid silver used herein is prepared in the presence of a tertiary alcohol. The tertiary alcohols used herein are preferably those of up to 15 carbon atoms in total, more preferably up to 10 carbon atoms in total. Tert-butanol is the preferred tertiary alcohol although the invention is not limited thereto. 
     The tertiary alcohol may be added at any stage during preparation of the organic acid silver. Preferably the tertiary alcohol is added during preparation of an organic acid alkali metal salt whereby the organic acid alkali metal salt is dissolved in the alcohol. The amount of the tertiary alcohol used is such that the weight ratio of tertiary alcohol to water may fall in the range from 0.01 to 10 provided that water (H 2 O) is used as the solvent during preparation of the organic acid silver. The preferred weight ratio of tertiary alcohol to water falls in the range from 0.03 to 1. 
     The organic silver salt which can be used herein may take any desired shape although needle crystals having a minor axis and a major axis are preferred. In the practice of the invention, grains should preferably have a minor axis or breadth of 0.01 μm to 0.20 μm and a major axis or length of 0.10 μm to 5.0 μm, more preferably a minor axis of 0.01 μm to 0.15 μm and a major axis of 0.10 μm to 4.0 μm. The grain size distribution of the organic silver salt is desirably monodisperse. The monodisperse distribution means that a standard deviation of the length of minor and major axes divided by the length, respectively, expressed in percent, is preferably up to 100%, more preferably up to 80%, most preferably up to 50%. It can be determined from the measurement of the shape of organic silver salt grains using an image of a grain dispersion obtained through a transmission electron microscope. Another method for determining a monodisperse distribution is to determine a standard deviation of a volume weighed mean diameter. The standard deviation divided by the volume weighed mean diameter, expressed in percent, which is a coefficient of variation, is preferably up to 100%, more preferably up to 80%, most preferably up to 50%. It may be determined by irradiating laser light, for example, to organic silver salt grains dispersed in liquid and determining the autocorrelation function of the fluctuation of scattering light relative to a time change, and obtaining the grain size (volume weighed mean diameter) therefrom. 
     The organic silver salt used herein is preferably desalted. The desalting method is not critical. Any well-known method may be used although well-known filtration methods such as centrifugation, suction filtration, ultrafiltration, and flocculation/water washing are preferred. 
     For the purpose of obtaining a solid particle dispersion of an organic silver salt having a high S/N ratio and a small particle size and free of agglomeration, use is preferably made of a dispersion method involving the steps of converting a water dispersion containing an organic silver salt as an image forming medium, but substantially free of a photosensitive silver salt into a high pressure, high speed flow, and causing a pressure drop to the flow. Thereafter, the dispersion is mixed with an aqueous solution of a photosensitive silver salt, thereby preparing a photosensitive image forming medium coating solution. 
     When a photothermographic image forming element is prepared using this coating solution, the resulting photothermographic image forming element has a low haze, low fog and high sensitivity. In contrast, if a photosensitive silver salt is co-present when an organic silver salt is dispersed in water by converting into a high pressure, high speed flow, then there result a fog increase and a substantial sensitivity decline. If an organic solvent is used instead of water as the dispersing medium, then there result a haze increase, a fog increase and a sensitivity decline. If a conversion technique of converting a portion of an organic silver salt in a dispersion into a photosensitive silver salt is employed instead of mixing a photosensitive silver salt aqueous solution, then there results a sensitivity decline. 
     The water dispersion which is dispersed by converting into a high pressure, high speed flow should be substantially free of a photosensitive silver salt. The content of photosensitive silver salt is less than 0.1 mol % based on the non-photosensitive organic silver salt. The positive addition of photosensitive silver salt is avoided. 
     With respect to the solid dispersing technology and apparatus employed in carrying out the above-described dispersion method of the invention, reference should be made to Kajiuchi and Usui, “Dispersed System Rheology and Dispersing Technology,” Shinzansha Publishing K.K., 1991, pp. 357-403; and Tokai Department of the Chemical Engineering Society Ed., “Progress of Chemical Engineering, Volume 24,” Maki Publishing K.K., 1990, pp. 184-185. According to the dispersion method recommended above, a water dispersion liquid containing at least an organic silver salt is pressurized by a high pressure pump or the like, fed into a pipe, and passed through a narrow slit in the pipe whereupon the dispersion liquid is allowed to experience an abrupt pressure drop, thereby accomplishing fine dispersion. 
     Such a high pressure homogenizer which is used in the practice of the invention is generally believed to achieve dispersion into finer particles under the impetus of dispersing forces including (a) “shear forces” exerted when the dispersed phase is passed through a narrow gap under high pressure and at a high speed and (b) “cavitation forces” exerted when the dispersed phase under high pressure is released to atmospheric pressure. As the dispersing apparatus of this type, Gaulin homogenizers are known from the past. In the Gaulin homogenizer, a liquid to be dispersed fed under high pressure is converted into a high-speed flow through a narrow slit on a cylindrical surface and under that impetus, impinged against the surrounding wall surface, achieving emulsification and dispersion by the impact forces. The pressure used is generally 100 to 600 kg/cm 2  and the flow velocity is from several meters per second to about 30 m/sec. To increase the dispersion efficiency, improvements are made on the homogenizer as by modifying a high-flow-velocity section into a saw-shape for increasing the number of impingements. Apart from this, apparatus capable of dispersion at a higher pressure and a higher flow velocity were recently developed. Typical examples of the advanced dispersing apparatus are available under the trade name of Micro-Fluidizer (Microfluidex International Corp.) and Nanomizer (Tokushu Kika Kogyo K.K.). 
     Examples of appropriate dispersing apparatus which are used in the practice of the invention include Micro-Fluidizer M-110S-EH (with G10Z interaction chamber), M-110Y (with H10Z interaction chamber), M-140K (with G10Z interaction chamber), HC-5000 (with L30Z or H230Z interaction chamber), and HC-8000 (with E230Z or L30Z interaction chamber), all available from Microfluidex International Corp. 
     Using such apparatus, a water dispersion liquid containing at least an organic silver salt is pressurized by a high pressure pump or the like, fed into a pipe, and passed through a narrow slit in the pipe for applying a desired pressure to the liquid and thereafter, the pressure within the pipe is quickly released to atmospheric pressure whereby the dispersion liquid experiences an abrupt pressure drop, thereby accomplishing the fine dispersion effect of the invention. 
     Prior to the dispersing operation, the starting liquid is preferably pre-dispersed. For such pre-dispersion, there may be used any of well-known dispersing means, for example, high-speed mixers, homogenizers, high-speed impact mills, Banbury mixers, homomixers, kneaders, ball mills, vibrating ball mills, planetary ball mills, attritors, sand mills, bead mills, colloid mills, jet mills, roller mills, trommels, and high-speed stone mills. Rather than such mechanical dispersion, the pre-dispersion may be carried out by controlling the pH of the starting liquid for roughly dispersing particles in a solvent, and then changing the pH in the presence of dispersing agents for fine graining. The solvent used in the rough dispersing step may be an organic solvent although the organic solvent is usually removed after the completion of fine graining. 
     According to the invention, the organic silver salt dispersion can be dispersed to a desired particle size by adjusting a flow velocity, a differential pressure upon pressure drop, and the number of dispersing cycles. From the standpoints of photographic properties and particle size, it is preferable to use a flow velocity of 200 to 600 m/sec and a differential pressure upon pressure drop of 900 to 3,000 kg/cm 2 , and especially a flow velocity of 300 to 600 m/sec and a differential pressure upon pressure drop of 1,500 to 3,000 kg/cm 2 . The number of dispersing cycles may be selected as appropriate although it is usually 1 to 10. From the productivity standpoint, the number of dispersing cycles is 1 to about 3. It is not recommended from the standpoints of dispersibility and photographic properties to elevate the temperature of the water dispersion under high pressure. High temperatures above 90° C. tend to increase the particle size and the fog due to poor dispersion. Accordingly, in the preferred embodiment of the invention, a cooling step is provided prior to the conversion step and/or after the pressure drop step whereby the water dispersion is maintained at a temperature in the range of 5 to 90° C., more preferably 5 to 80° C. and most preferably 5 to 65° C.. It is effective to use the cooling step particularly when dispersion is effected under a high pressure of 1,500 to 3,000 kg/cm 2 . The cooling means used in the cooling step may be selected from various coolers, for example, double tube type heat exchangers, static mixer-built-in double tube type heat exchangers, multi-tube type heat exchangers, and serpentine heat exchangers, depending on the necessary quantity of heat exchange. For increasing the efficiency of heat exchange, the diameter, gage and material of the tube are selected as appropriate in consideration of the pressure applied thereto. Depending on the necessary quantity of heat exchange, the refrigerant used in the heat exchanger may be selected from well water at 20° C., cold water at 5 to 10° C. cooled by refrigerators, and if necessary, ethylene glycol/water at −30° C. 
     In the dispersing operation according to the invention, the organic silver salt is preferably dispersed in the presence of dispersants or dispersing agents soluble in an aqueous medium. The dispersing agents used herein include synthetic anionic polymers such as polyacrylic acid, acrylic acid copolymers, maleic acid copolymers, maleic acid monoester copolymers, and acryloylmethylpropanesulfonic acid copolymers; semi-synthetic anionic polymers such as carboxymethyl starch and carboxymethyl cellulose; anionic polymers such as alginic acid and pectic acid; the compounds described in JP-A 350753/1995; well-known anionic, nonionic and cationic surfactants; well-known polymers such as polyvinyl alcohol, polyvinyl pyrrolidone, carboxymethyl cellulose, hydroxypropyl cellulose and hydroxypropylmethyl cellulose; and naturally occurring polymers such as gelatin. Of these, polyvinyl alcohol and water-soluble cellulose derivatives are especially preferred. 
     In general, the dispersant is mixed with the organic silver salt in powder or wet cake form prior to dispersion. The resulting slurry is fed into a dispersing machine. Alternatively, a mixture of the dispersant with the organic silver salt is subject to heat treatment or solvent treatment to form a dispersant-bearing powder or wet cake of the organic silver salt. It is acceptable to effect pH control with a suitable pH adjusting agent before, during or after dispersion. 
     Rather than mechanical dispersion, fine particles can be formed by roughly dispersing the organic silver salt in a solvent through pH control and thereafter, changing the pH in the presence of dispersing aids. An organic solvent can be used as the solvent for rough dispersion although the organic solvent is usually removed at the end of formation of fine particles. 
     The thus prepared dispersion may be stored while continuously stirring for the purpose of preventing fine particles from settling during storage. Alternatively, the dispersion is stored after adding hydrophilic colloid to establish a highly viscous state (for example, in a jelly-like state using gelatin). An antiseptic agent may be added to the dispersion in order to prevent the growth of bacteria during storage. 
     The grain size (volume weighed mean diameter) of the solid particle dispersion of the organic silver salt obtained by the present invention may be determined by irradiating laser light, for example, to organic silver salt grains dispersed in liquid and determining the autocorrelation function of the fluctuation of scattering light relative to a time change. Preferably, the solid particle dispersion has a mean grain size of 0.05 μm to 10.0 μm, more preferably 0.1 μm to 5.0 μm, and most preferably 0.1 μm to 2.0 μm. 
     The grain size distribution of the organic silver salt is desirably monodisperse. Illustratively, the standard deviation of a volume weighed mean diameter divided by the volume weighed mean diameter, expressed in percent, which is a coefficient of variation, is preferably up to 80%, more preferably up to 50%, most preferably up to 30%. 
     The shape of the organic silver salt may be determined by observing a dispersion of the organic silver salt under a transmission electron microscope (TEM). 
     The dispersion liquid used herein is composed of at least the organic silver salt and water. The ratio of the organic silver salt to water is not critical although it is preferred that the organic silver salt accounts for 5 to 50% by weight, especially 10 to 30% by weight, of the entire system. It is preferred to use the dispersing agent as mentioned above and more preferably, in a minimum amount necessary to minimize the particle size. The dispersing agent is preferably used in an amount of 1 to 30% by weight, especially 3 to 15% by weight of the organic silver salt. 
     According to the invention, image forming elements can be prepared by mixing the water dispersion of the organic silver salt with a water dispersion of a photosensitive silver salt. The mixing ratio of organic silver salt to photosensitive silver salt is determined in accordance with a particular purpose. The proportion of the photosensitive silver salt is preferably 1 to 30 mol %, more preferably 3 to 20 mol % and most preferably 5 to 15 mol %, based on the moles of the organic silver salt. With respect to this mixing, a method of mixing two or more organic silver salt water dispersions with two or more photosensitive silver salt water dispersions is preferably employed for the purpose of adjusting photographic properties. 
     The organic silver salt is used in any desired amount, preferably about 0.1 to 5 g/m 2 , more preferably about 1 to 3 g/m 2 , as expressed by a silver coverage per square meter of the element. 
     Photosensitive Silver Halide 
     The halogen composition of the photosensitive silver halide used herein is not critical and may be any of silver chloride, silver chlorobromide, silver bromide, silver iodobromide, and silver iodochlorobromide. The halogen composition in grains may have a uniform distribution or a non-uniform distribution wherein the halogen concentration changes in a stepped or continuous manner. Silver halide grains of the core/shell structure are also useful. Such core/shell grains preferably have a multilayer structure of 2 to 5 layers, more preferably 2 to 4 layers. Silver chloride or silver chlorobromide grains having silver bromide localized at the surface thereof are also preferably used. 
     A method for forming the photosensitive silver halide according to the invention is well known in the art. Any of the methods disclosed in  Research Disclosure  No. 17029 (June 1978) and U.S. Pat. No. 3,700,458, for example, may be used. Specifically, use is made of a method of adding a silver-providing compound and a halogen-providing compound to a solution of gelatin or another polymer to form photosensitive silver halide grains and mixing the grains with an organic silver salt. 
     The photosensitive silver halide should preferably have a smaller grain size for the purpose of minimizing white turbidity after image formation. Specifically, the grain size is up to 0.20 μm, preferably 0.01 μm to 0.15 μm, most preferably 0.02 μm to 0.12 μm. The term grain size designates the length of an edge of a silver halide grain where silver halide grains are regular grains of cubic or octahedral shape. Where silver halide grains are tabular, the grain size is the diameter of an equivalent circle having the same area as the projected area of a major surface of a tabular grain. Where silver halide grains are not regular, for example, in the case of spherical or rod-shaped grains, the grain size is the diameter of an equivalent sphere having the same volume as a grain. 
     The shape of silver halide grains may be cubic, octahedral, tabular, spherical, rod-like and potato-like, with cubic and tabular grains being preferred in the practice of the invention. Where tabular silver halide grains are used, they should preferably have an average aspect ratio of from 100:1 to 2:1, more preferably from 50:1 to 3:1. Silver halide grains having rounded corners are also preferably used. No particular limit is imposed on the face indices (Miller indices) of an outer surface of silver halide grains. Preferably silver halide grains have a high proportion of {100} face featuring high spectral sensitization efficiency upon adsorption of a spectral sensitizing dye. The proportion of {100} face is preferably at least 50%, more preferably at least 65%, most preferably at least 80%. Note that the proportion of Miller index {100} face can be determined by the method described in T. Tani, J. Imaging Sci., 29, 165 (1985), utilizing the adsorption dependency of {111} face and {100} face upon adsorption of a sensitizing dye. 
     The photosensitive silver halide grains used herein may contain any of metals or metal complexes belonging to Groups VII and VIII (or Groups 7 to 10) in the Periodic Table. Preferred metals or central metals of metal complexes belonging to Groups VII and VIII in the Periodic Table are rhodium, rhenium, ruthenium, osmium, and iridium. The metal complexes may be used alone or in admixture of complexes of a common metal or different metals. The content of metal or metal complex is preferably 1×10 −9  mol to 1×10 −3  mol, more preferably 1×10 −8  mol to 1×10 −4  mol, per mol of silver. Illustrative metal complexes are those of the structures described in JP-A 225449/1995. 
     The rhodium compounds which can be used herein are water-soluble rhodium compounds, for example, rhodium (III) halides and rhodium complex salts having halogen, amine or oxalato ligands, such as hexachlororhodium(III) complex salt, pentachloroaquorhodium(III) complex salt, tetrachlorodiaquorhodium(III) complex salt, hexabromorhodium(III) complex salt, hexamminerhodium(III) complex salt, and trioxalatorhodium(III) complex salt. On use, these rhodium compounds are dissolved in water or suitable solvents. They are preferably added by a method commonly employed for stabilizing a solution of a rhodium compound, that is, a method of adding an aqueous solution of a hydrogen halide (e.g., hydrochloric acid, hydrobromic acid or hydrofluoric acid) or an alkali halide (e.g., KCl, NaCl, KBr or NaBr). Instead of using the water-soluble rhodium, it is possible to add, during preparation of silver halide, separate silver halide grains previously doped with rhodium, thereby dissolving rhodium. 
     An appropriate amount of the rhodium compound added is 1×10 −8  to 5×10 −6  mol, especially 5×10 −8  to 1×10 −6  mol, per mol of silver halide. 
     The rhodium compounds may be added at an appropriate stage during preparation of silver halide emulsion grains or prior to the coating of the emulsion. Preferably, the rhodium compound is added during formation of the emulsion so that the compound is incorporated into silver halide grains. 
     In the practice of the invention, rhenium, ruthenium and osmium are added in the form of water-soluble complex salts as described in JP-A 2042/1988, 285941/1989, 20852/1990 and 20855/1990. Especially preferred are hexa-coordinate complexes represented by the formula: 
     
       
         [ML 6 ] n−   
       
     
     wherein M is Ru, Re or Os, L is a ligand, and letter n is equal to 0, 1, 2, 3 or 4. The counter ion is not critical although it is usually an ammonium or alkali metal ion. Preferred ligands are halide ligands, cyanide ligands, cyanate ligands, nitrosil ligands, and thionitrosil ligands. 
     Illustrative, non-limiting, examples of the complex used herein are given below. 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 [ReCl 6 ] 3−   
                 [ReBr 6 ] 3−   
                 [ReCl 5 (NO)] 2−   
               
               
                   
                 [Re(NS)Br 5 ] 2−   
                 [Re(NO) (CN) 5 ] 2−   
                 [Re(O) 2 (CN) 4 ] 3−   
               
               
                   
                 [RuCl 6 ] 3−   
                 [RuCl 4 (H 2 O) 2 ] −   
                 [RuCl 5 (H 2 O)] 2−   
               
               
                   
                 [RuCl 5 (NO)] 2−   
                 [RuBr 5 (NS)] 2−   
               
               
                   
                 [Ru(CO) 3 Cl 3 ] 2−   
                 [Ru(CO)Cl 5 ] 2−   
                 [Ru(CO)Br 5 ] 2−   
               
               
                   
                 [OsCl 6 ] 3−   
                 [OsCl 5 (NO)] 2−   
                 [Os(NO) (CN) 5 ] 2−   
               
               
                   
                 [Os(NS)Br 5 ] 2−   
                 [Os(O) 2 (CN) 4 ] 4−   
               
               
                   
                   
               
            
           
         
       
     
     An appropriate amount of these compounds added is 1×10 −9  to 1×10 −5  mol, especially 1×10 −8  to 1×10 −6  mol, per mol of silver halide. 
     These compounds may be added at an appropriate stage during preparation of silver halide emulsion grains or prior to the coating of the emulsion. Preferably, the compound is added during formation of the emulsion so that the compound is incorporated into silver halide grains. 
     In order that the compound be added during formation of silver halide grains so that the compound is incorporated into silver halide grains, there can be employed a method of adding a powder metal complex or an aqueous solution of a powder metal complex dissolved together with NaCl or KCl, to a water-soluble salt or water-soluble halide solution during formation of grains; a method of preparing silver halide grains by adding an aqueous solution of a metal complex as a third solution when silver salt and halide solutions are simultaneously mixed, thereby simultaneously mixing the three solutions; or a method of admitting a necessary amount of an aqueous solution of a metal complex into a reactor during formation of grains. Of these, the method of adding a powder metal complex or an aqueous solution of a powder metal complex dissolved together with NaCl or KCl to a water-soluble halide solution is especially preferred. 
     For addition to surfaces of grains, a necessary amount of an aqueous solution of a metal complex can be admitted into a reactor immediately after formation of grains, during or after physical ripening or during chemical ripening. 
     As the iridium compound, a variety of compounds may be used. Examples include hexachloroiridium, hexammineiridium, trioxalatoiridium, hexacyanoiridium, and pentachloronitrosiliridium. These iridium compounds are used by dissolving in water or suitable solvents. They are preferably added by a method commonly employed for stabilizing a solution of an iridium compound, that is, a method of adding an aqueous solution of a hydrogen halide (e.g., hydrochloric acid, hydrobromic acid or hydrofluoric acid) or an alkali halide (e.g., KCl, NaCl, KBr or NaBr). Instead of using the water-soluble iridium, it is possible to add, during preparation of silver halide, separate silver halide grains previously doped with iridium, thereby dissolving iridium. 
     The silver halide grains used herein may contain metal atoms such as cobalt, iron, nickel, chromium, palladium, platinum, gold, thallium, copper, and lead. Preferred compounds of cobalt, iron, chromium and ruthenium are hexacyano metal complexes. Illustrative, non-limiting, examples include ferricyanate, ferrocyanate, hexacyanocobaltate, hexacyanochromate and hexacyanoruthenate ions. The distribution of the metal complex in silver halide grains is not critical. That is, the metal complex may be contained in silver halide grains uniformly or at a high concentration in either the core or the shell. 
     An appropriate amount of the metal added is 1×10 −9  to 1×10 −4  mol per mol of silver halide. The metal may be contained in silver halide grains by adding a metal salt in the form of a single salt, double salt or complex salt during preparation of grains. 
     Photosensitive silver halide grains may be desalted by any of well-known water washing methods such as noodle and flocculation methods although silver halide grains may be either desalted or not according to the invention. 
     When the silver halide emulsion according to the invention is subject to gold sensitization, there may be used any of gold sensitizers whose gold may have an oxidation number of +1 or +3. Conventional gold sensitizers are useful. Typical examples include chloroaurates such as potassium chloroaurate, auric trichloride, potassium auric thiocyanate, potassium iodoaurate, tetracyanoauric acid, ammonium aurothiocyanate, and pyridyl trichlorogold. The amount of the gold sensitizer added varies with various conditions although it is typically 1×10 −7  to 10 −3  mol, preferably 10 −6  to 5×10 −4  mol per mol of the silver halide. 
     The silver halide emulsion used herein should preferably be subject to gold sensitization and another chemical sensitization in combination. The chemical sensitization methods which can be used herein are sulfur, selenium, tellurium, and noble metal sensitization methods which are well known in the art. When they are used in combination with gold sensitization, preferred combinations are a combination of sulfur sensitization with gold sensitization, a combination of selenium sensitization with gold sensitization, a combination of sulfur sensitization and selenium sensitization with gold sensitization, a combination of sulfur sensitization and tellurium sensitization with gold sensitization, and a combination of sulfur sensitization, selenium sensitization, and tellurium sensitization with gold sensitization. 
     Sulfur sensitization that is preferably employed in the invention is generally carried out by adding a sulfur sensitizer to an emulsion and agitating the emulsion at an elevated temperature above 40° C. for a certain time. The sulfur sensitizers used herein are well-known sulfur compounds, for example, sulfur compounds contained in gelatin as well as various sulfur compounds such as thiosulfates, thioureas, thiazoles, and rhodanines. Preferred sulfur compounds are thiosulfate salts and thiourea compounds. The amount of the sulfur sensitizer added varies with chemical ripening conditions including pH, temperature and silver halide grain size although it is preferably 10 −7  to 10 −2  mol, more preferably 10 −5  to 10 −3  mol per mol of silver halide. 
     It is also useful to use selenium sensitizers which include well-known selenium compounds. Specifically, selenium sensitization is generally carried out by adding an unstable selenium compound and/or non-unstable selenium compound to an emulsion and agitating the emulsion at elevated temperature above 40° C. for a certain time. Preferred examples of the unstable selenium compound include those described in JP-B 15748/1969, JP-B 13489/1968, JP-A 25832/1992, JP-A 109240/1992 and JP-A 121798/1991. Especially preferred are the compounds represented by general formulae (VIII) and (IX) in JP-A 324855/1992. 
     The tellurium sensitizers are compounds capable of forming silver telluride, which is presumed to become sensitization nuclei, at the surface or in the interior of silver halide grains. The production rate of silver telluride in a silver halide emulsion can be determined by the test method described in JP-A 313284/1993. Exemplary tellurium sensitizers include diacyltellurides, bis(oxycarbonyl)tellurides, bis(carbamoyl)tellurides, bis(oxycarbonyl)ditellurides, bis(carbamoyl)ditellurides, compounds having a P═Te bond, tellurocarboxylic salts, Te-organyltellurocarboxylic esters, di(poly)tellurides, tellurides, telluroles, telluroacetals, tellurosulfonates, compounds having a P—Te bond, Te-containing heterocycles, tellurocarbonyl compounds, inorganic tellurium compounds, and colloidal tellurium. Examples are described in U.S. Pat. Nos. 1,623,499, 3,320,069, 3,772,031, BP 235,211, 1,121,496, 1,295,462, 1,396,696, Canadian Patent No. 800,958, JP-A 204640/1992, Japanese Patent Application Nos. 53693/1991, 131598/1991, 129787/1992, J. Chem. Soc. Chem. Commun., 635 (1980), ibid., 1102 (1979), ibid., 645 (1979), J. Chem. Soc. Perkin. Trans., 1, 2191 (1980), S. Patai Ed., The Chemistry of Organic Selenium and Tellurium Compounds, Vol. 1 (1986), ibid., Vol. 2 (1987). Especially preferred are the compounds represented by general formulae (II), (III) and (IV) in JP-A 313284/1993. 
     The amounts of the selenium and tellurium sensitizers used vary with the type of silver halide grains, chemical ripening conditions and other factors although they are preferably about 10 −8  to 10 −2  mol, more preferably about 10 −7  to 10 −3  mol per mol of silver halide. The chemical sensitizing conditions are not particularly limited although preferred conditions include a pH of 5 to 8, a pAg of 6 to 11, more preferably 7 to 10, and a temperature of 40 to 95° C., more preferably 45 to 85° C. 
     In the preparation of the silver halide emulsion used herein, any of cadmium salts, sulfite salts, lead salts, and thallium salts may be co-present in the silver halide grain forming step or physical ripening step. 
     Reduction sensitization may also be used in the practice of the invention. Illustrative examples of the compound used in the reduction sensitization method include ascorbic acid, thiourea dioxide, stannous chloride, aminoiminomethanesulfinic acid, hydrazine derivatives, borane compounds, silane compounds, and polyamine compounds. Reduction sensitization may also be accomplished by ripening the emulsion while maintaining it at pH 7 or higher or at pAg 8.3 or lower. Reduction sensitization may also be accomplished by introducing a single addition portion of silver ion during grain formation. 
     To the silver halide emulsion according to the invention, thiosulfonic acid compounds may be added by the method described in EP-A 293,917. 
     The silver halide emulsion in the image forming element according to the invention may be a single emulsion or a mixture of two or more emulsions which are different in mean grain size, halogen composition, crystal habit or chemical sensitizing conditions. 
     According to the invention, the photosensitive silver halide is preferably used in an amount of 0.01 to 0.5 mol, more preferably 0.02 to 0.3 mol, most preferably 0.03 to 0.25 mol per mol of the organic silver salt. With respect to a method and conditions of admixing the separately prepared photosensitive silver halide and organic silver salt, there may be used a method of admixing the separately prepared photosensitive silver halide and organic silver salt in a high speed agitator, ball mill, sand mill, colloidal mill, vibratory mill or homogenizer or a method of preparing an organic silver salt by adding the already prepared photosensitive silver halide at any timing during preparation of an organic silver salt. Any desired mixing method may be used insofar as the benefits of the invention are fully achievable. 
     Reducing Agent 
     The photothermographic image forming element according to the preferred embodiment of the invention contains a reducing agent for the organic silver salt. The reducing agent for the organic silver salt may be any of substances, preferably organic substances, that reduce silver ion into metallic silver. Conventional photographic developing agents such as Phenidone®, hydroquinone and catechol are useful although hindered phenols are preferred reducing agents. The reducing agent should preferably be contained in an amount of 5 to 50 mol %, more preferably 10 to 40 mol % per mol of silver on the image forming layer-bearing side. The reducing agent may be added to any layer on the image forming layer-bearing side. Where the reducing agent is added to a layer other than the image forming layer, the reducing agent should preferably be contained in a slightly greater amount of about 10 to 50 mol % per mol of silver. The reducing agent may take the form of a precursor which is modified so as to exert its effective function only at the time of development. 
     For thermographic image forming elements using organic silver salts, a wide range of reducing agents are disclosed, for example, in JP-A 6074/1971, 1238/1972, 33621/1972, 46427/1974, 115540/1974, 14334/1975, 36110/1975, 147711/1975, 32632/1976, 1023721/1976, 32324/1976, 51933/1976, 84727/1977, 108654/1980, 146133/1981, 82828/1982, 82829/1982, 3793/1994, U.S. Pat. Nos. 3,667,958, 3,679,426, 3,751,252, 3,751,255, 3,761,270, 3,782,949, 3,839,048, 3,928,686, 5,464,738, German Patent No. 2321328, and EP 692732. Exemplary reducing agents include amidoximes such as phenylamidoxime, 2-thienylamidoxime, and p-phenoxyphenylamidoxime; azines such as 4-hydroxy-3,5-dimethoxybenzaldehydeazine; combinations of aliphatic carboxylic acid arylhydrazides with ascorbic acid such as a combination of 2,2′-bis(hydroxymethyl)propionyl-β-phenylhydrazine with ascorbic acid; combinations of polyhydroxybenzenes with hydroxylamine, reductone and/or hydrazine, such as combinations of hydroquinone with bis(ethoxyethyl)hydroxylamine, piperidinohexosereductone or formyl-4-methylphenylhydrazine; hydroxamic acids such as phenylhydroxamic acid, p-hydroxyphenylhydroxamic acid, and β-anilinehydroxamic acid; combinations of azines with sulfonamidophenols such as a combination of phenothiazine with 2,6-dichloro-4-benzenesulfonamidephenol; α-cyanophenyl acetic acid derivatives such as ethyl-α-cyano-2-methylphenyl acetate and ethyl-α-cyanophenyl acetate; bis-β-naphthols such as 2,2′-dihydroxy-1,1′-binaphthyl, 6,6′-dibromo-2,2′-dihydroxy-1,1′-binaphthyl, and bis(2-hydroxy-1-naphthyl)methane; combinations of bis-β-naphthols with 1,3-dihydroxybenzene derivatives such as 2,4-dihydroxybenzophenone and 2′,4′-dihydroxyacetophenone; 5-pyrazolones such as 3-methyl-1-phenyl-5-pyrazolone; reductones such as dimethylaminohexosereductone, anhydrodihydroaminohexosereductone and anhydrodihydropiperidonehexosereductone; sulfonamidephenol reducing agents such as 2,6-dichloro-4-benzenesulfonamidephenol and p-benzenesulfonamidephenol; 2-phenylindane-1,3-dione, etc.; chromans such as 2,2-dimethyl-7-t-butyl-6-hydroxychroman; 1,4-dihydropyridines such as 2,6-dimethoxy-3,5-dicarboethoxy-1,4-dihydropyridine; bisphenols such as bis(2-hydroxy-3-t-butyl-5-methylphenyl)methane, 2,2-bis(4-hydroxy-3-methylphenyl)propane, 4,4-ethylidene-bis(2-t-butyl-6-methylphenol), 1,1-bis(2-hydroxy-3,5-dimethylphenyl)-3,5,5-trimethylhexane, and 2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane; ascorbic acid derivatives such as 1-ascorbyl palmitate and ascorbyl stearate; aldehydes and ketones such as benzil and diacetyl; 3-pyrazolidones and certain indane-1,3-diones; and chromanols (tocopherols). Preferred reducing agents are bisphenols and chromanols. 
     The reducing agent may be added in any desired form such as solution, powder or solid particle dispersion. The solid particle dispersion of the reducing agent may be prepared by well-known comminuting means such as ball mills, vibrating ball mills, sand mills, colloidal mills, jet mills, and roller mills. Dispersing aids may be used for facilitating dispersion. 
     Toner 
     A higher optical density is sometimes achieved when an additive known as a “toner” for improving images is contained. The toner is also sometimes advantageous in forming black silver images. The toner is preferably used in an amount of 0.1 to 50 mol %, especially 0.5 to 20 mol % per mol of silver on the image forming layer-bearing side. The toner may take the form of a precursor which is modified so as to exert its effective function only at the time of development. 
     For thermographic image forming elements using organic silver salts, a wide range of toners are disclosed, for example, in JP-A 6077/1971, 10282/1972, 5019/1974, 5020/1974, 91215/1974, 2524/1975, 32927/1975, 67132/1975, 67641/1975, 114217/1975, 3223/1976, 27923/1976, 14788/1977, 99813/1977, 1020/1978, 76020/1978, 156524/1979, 156525/1979, 183642/1986, and 56848/1992, JP-B 10727/1974 and 20333/1979, U.S. Pat. Nos. 3,080,254, 3,446,648, 3,782,941, 4,123,282, 4,510,236, BP 1,380,795, and Belgian Patent No. 841,910. Examples of the toner include phthalimide and N-hydroxyphthalimide; cyclic imides such as succinimide, pyrazolin-5-one, quinazolinone, 3-phenyl-2-pyrazolin-5-one, 1-phenylurazol, quinazoline and 2,4-thiazolidinedione; naphthalimides such as N-hydroxy-1,8-naphthalimide; cobalt complexes such as cobaltic hexammine trifluoroacetate; mercaptans as exemplified by 3-mercapto-1,2,4-triazole, 2,4-dimercaptopyrimidine, 3-mercapto-4,5-diphenyl-1,2,4-triazole, and 2,5-dimercapto-1,3,4-thiadiazole; N-(aminomethyl)aryldicarboxyimides such as (N,N-dimethylaminomethyl)phthalimide and N,N-(dimethylaminomethyl)-naphthalene-2,3-dicarboxyimide; blocked pyrazoles, isothiuronium derivatives and certain photo-bleach agents such as N,N′-hexamethylenebis(1-carbamoyl-3,5-dimethylpyrazole), 1,8-(3,6-diazaoctane)bis(isothiuroniumtrifluoroacetate) and 2-tribromomethylsulfonyl-benzothiazole; 3-ethyl-5-{(3-ethyl-2-benzothiazolinylidene)-1-methylethylidene}-2-thio-2,4-oxazolidinedione; phthalazinone, phthalazinone derivatives or metal salts, or derivatives such as 4-(1-naphthyl)-phthalazinone, 6-chlorophthalazinone, 5,7-dimethoxyphthalazinone and 2,3-dihydro-1,4-phthalazinedione; combinations of phthalazinones with phthalic acid derivatives (e.g., phthalic acid, 4-methylphthalic acid, 4-nitrophthalic acid and tetrachlorophthalic anhydride); phthalazine, phthalazine derivatives or metal salts such as 4-(1-naphthyl)-phthalazine, 6-chlorophthalazine, 5,7-dimethoxyphthalazine, 6-isobutylphthalazine, 6-tert-butylphthalazine, 5,7-dimethylphthalazine, and 2,3-dihydrophthalazine; combinations of phthalazine or derivatives thereof with phthalic acid derivatives (e.g., phthalic acid, 4-methylphthalic acid, 4-nitrophthalic acid and tetrachlorophthalic anhydride); quinazolinedione, benzoxazine or naphthoxazine derivatives; rhodium complexes which function not only as a tone regulating agent, but also as a source of halide ion for generating silver halide in situ, for example, ammonium hexachlororhodinate (III), rhodium bromide, rhodium nitrate and potassium hexachlororhodinate (III); inorganic peroxides and persulfates such as ammonium peroxide disulfide and hydrogen peroxide; benzoxazine-2,4-diones such as 1,3-benzoxazine-2,4-dione, 8-methyl-1,3-benzoxazine-2,4-dione, and 6-nitro-1,3-benzoxazine-2,4-dione; pyrimidine and asymtriazines such as 2,4-dihydroxypyrimidine and 2-hydroxy-4-aminopyrimidine; azauracil and tetraazapentalene derivatives such as 3,6-dimercapto-1,4-diphenyl-1H,4H-2,3a,5,6a-tetraazapentalene, and 1,4-di(o-chlorophenyl)-3,6-dimercapto-lH,4H- 2,3a,5,6a-tetraazapentalene. 
     The toner may be added in any desired form, for example, as a solution, powder and solid particle dispersion. The solid particle dispersion of the toner is prepared by well-known finely dividing means such as ball mills, vibrating ball mills, sand mills, colloid mills, jet mills, and roller mills. Dispersing aids may be used in preparing the solid particle dispersion. 
     Polymer Latex 
     At least one layer of the photosensitive layers or image forming layers used herein is preferably an image forming layer wherein a polymer latex constitutes at least 50% by weight of the entire binder. This image forming layer is sometimes referred to as “inventive image forming layer” and the polymer latex used as the main binder therefor is referred to as “inventive polymer latex,” hereinafter. Beside the image forming layer, the polymer latex may also be used in a protective layer or back layer. Particularly when the photothermographic image forming element of the invention is used in a printing application where dimensional changes are a problem, it is necessary to use the polymer latex in the protective layer and back layer too. The “polymer latex” is a dispersion of a microparticulate water-insoluble hydrophobic polymer in a water-soluble dispersing medium. With respect to the dispersed state, a polymer emulsified in a dispersing medium, an emulsion polymerized polymer, a micelle dispersion, and a polymer having a hydrophilic structure in a part of its molecule so that the molecular chain itself is dispersed on a molecular basis are included. With respect to the polymer latex, reference is made to Okuda and Inagaki Ed., “Synthetic Resin Emulsion,” Kobunshi Kankokai, 1978; Sugimura, Kataoka, Suzuki and Kasahara Ed., “Application of Synthetic Latex,” Kobunshi Kankokai, 1993; and Muroi, “Chemistry of Synthetic Latex,” Kobunshi Kankokai, 1970. Dispersed particles should preferably have a mean particle size of about 1 to 50,000 nm, more preferably about 5 to 1,000 nm. No particular limit is imposed on the particle size distribution of dispersed particles, and the dispersion may have either a wide particle size distribution or a monodisperse particle size distribution. 
     The polymer latex used herein may be either a latex of the conventional uniform structure or a latex of the so-called core/shell type. In the latter case, better results are sometimes obtained when the core and the shell have different glass transition temperatures. 
     Polymers of polymer latexes used as the binder according to the invention have glass transition temperatures (Tg) whose preferred range differs among the protective layer, the back layer and the image-forming layer. For the image forming layer, polymers having a Tg of up to 40° C., typically −30° C. to 40° C., especially 0° C. to 40° C. are preferred in order to promote the diffusion of photographically effective addenda upon heat development. For the protective layer and the back layer which are to come in contact with various equipment, polymers having a Tg of 25° C. to 70° C. are especially preferred. 
     The polymer latex should preferably have a minimum film-forming temperature (MFT) of about −30° C. to 90° C., more preferably about 0° C. to 70° C. A film-forming aid may be added in order to control the minimum film-forming temperature. The film-forming aid is also referred to as a plasticizer and includes organic compounds (typically organic solvents) for lowering the minimum film-forming temperature of a polymer latex. It is described in Muroi, “Chemistry of Synthetic Latex,” Kobunshi Kankokai, 1970. 
     Polymers used in the polymer latex according to the invention include acrylic resins, vinyl acetate resins, polyester resins, polyurethane resins, rubbery resins, vinyl chloride resins, vinylidene chloride resins, polyolefin resins, and copolymers thereof. The polymer may be linear, branched or crosslinked. The polymer may be either a homopolymer or a copolymer having two or more monomers polymerized together. The copolymer may be either a random copolymer or a block copolymer. The polymer preferably has a number average molecule weight Mn of about 5,000 to about 1,000,000, more preferably about 10,000 to about 100,000. Polymers with a too lower molecular weight would generally provide a low mechanical strength as the binder whereas polymers with a too higher molecular weight are difficult to form films. 
     Illustrative examples of the polymer latex which can be used as the binder in the image forming layer or photosensitive layer of the thermographic image forming element of the invention include latexes of methyl methacrylate/ethyl acrylate/methacrylic acid copolymers, latexes of methyl methacrylate/2-ethylhexyl acrylate/styrene/acrylic acid copolymers, latexes of styrene/butadiene/acrylic acid copolymers, latexes of styrene/butadiene/divinyl benzene/methacrylic acid copolymers, latexes of methyl methacrylate/vinyl chloride/acrylic acid copolymers, and latexes of vinylidene chloride/ethyl acrylate/acrylonitrile/methacrylic acid copolymers. These polymers or polymer latexes are commercially available. Exemplary acrylic resins are Sebian A-4635, 46583 and 4601 (Daicell Chemical Industry K.K.), Nipol LX811, 814, 820, 821, and 857 (Nippon Zeon K.K.), and Jurimer ET-410 and 530 (Nippon Junyaku K.K.). Exemplary polyester resins are FINETEX ES650, 611, 675, and 850 (Dai-Nippon Ink &amp; Chemicals K.K.) and WD-size and WMS (Eastman Chemical Products, Inc.). Exemplary polyurethane resins are HYDRAN AP10, 20, 30 and 40 (Dai-Nippon Ink &amp; Chemicals K.K.). Exemplary rubbery resins are LACSTAR 7310K, 3307B, 4700H, and 7132C (Dai-Nippon Ink &amp; Chemicals K.K.) and Nipol Lx416, 410, 438C, and 2507 (Nippon Zeon K.K.). Exemplary vinyl chloride resins are G351 and G576 (Nippon Zeon K.K.). Exemplary vinylidene chloride resins are L502 and L513 (Asahi Chemicals K.K.) and Aron D7020, D5040 and D5071 (Mitsui-Toatsu K.K.). Exemplary olefin resins are Chemipearl S120 and SA100 (Mitsui Petro-Chemical K.K.). These polymers may be used alone or in admixture of two or more. 
     In the image forming layer according to the invention, the above-described polymer latex is used in an amount of at least 50%, preferably at least 70% by weight of the entire binder. 
     In the image forming layer, a hydrophilic polymer is added to the binder in an amount of up to 50% by weight of the entire binder, if desired. Such hydrophilic polymers are gelatin, polyvinyl alcohol, methyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, and hydroxypropyl methyl cellulose. The amount of the hydrophilic polymer added is preferably less than 30%, more preferably less than 15% by weight of the entire binder in the image-forming layer. 
     In the practice of the invention, the image forming layer is preferably formed by applying an aqueous coating solution followed by drying. By the term “aqueous”, it is meant that water accounts for at least 60% by weight of the solvent or dispersing medium of the coating solution. The component other than water of the coating solution may be a water-miscible organic solvent such as methyl alcohol, ethyl alcohol, isopropyl alcohol, methyl cellosolve, ethyl cellosolve, dimethylformamide, and ethyl acetate. Beside water, exemplary solvent compositions include a 90/10 mixture of water/methanol, a 70/30 mixture of water/methanol, a 90/10 mixture of water/ethanol, a 90/10 mixture of water/isopropanol, a 95/5 mixture of water/dimethylformamide, a 80/15/5 mixture of water/methanol/dimethylformamide, and a 90/5/5 mixture of water/methanol/dimethylformamide, all expressed in a weight ratio. 
     In the image forming layer according to the invention, the total amount of binder is preferably 0.2 to 30 g/m 2 , more preferably 1.0 to 15 g/m 2 . To the image forming layer, crosslinking agents for crosslinking, surfactants for ease of application, and other addenda may be added. 
     Nucleating Agent 
     In order to produce high contrast images, the photothermographic image forming element of the invention preferably contains a nucleating agent in the photosensitive layer or a layer disposed adjacent thereto or both. The nucleating agents which can be used herein are preferably selected from among substituted alkene derivatives, substituted isoxazole derivatives, specific acetal compounds, and hydrazine derivatives. 
     The substituted alkene derivatives, substituted isoxazole derivatives, and specific acetal compounds used herein are of the following formulas (1), (2), and (3), respectively.                    
     In formula (1), R 1 , R 2 , and R 3  are independently hydrogen or substituents, and Z is an electron attractive group or silyl group. At least one pair of (R 1  and Z), (R 2  and R 3 ), (R 1  and R 2 ), and (R 3  and Z), taken together, may form a cyclic structure.                    
     In formula (2), R 4  is a substituent.                    
     In formula (3), X and Y are independently hydrogen or substituents, A and B are independently alkoxy, alkylthio, alkylamino, aryloxy, arylthio, anilino, heterocyclic oxy, heterocyclic thio, or heterocyclic amino groups. X and Y, or A and B, taken together, may form a cyclic structure. 
     First, the substituted alkene derivatives of formula (1) are described in detail. In formula (1), R 1 , R 2 , and R 3  are independently hydrogen or substituents, and Z is an electron attractive group or silyl group. At least one pair of R 1  and Z, R 2  and R 3 , R 1  and R 2 , and R 3  and Z, taken together, may form a cyclic structure. 
     When R 1 , R 2 , and R 3  represent substituents, substituents are halogen atoms (e.g., fluorine, chlorine, bromine and.iodine atoms), alkyl groups (including aralkyl, cycloalkyl and active methine groups), alkenyl groups, alkynyl groups, aryl groups, heterocyclic groups (inclusive of N-substituted nitrogenous heterocyclic groups), quaternized nitrogen atom-containing heterocyclic groups (such as pyridinio), acyl groups, alkoxycarbonyl groups, aryloxycarbonyl groups, carbamoyl groups, carboxy groups or salts thereof, imino groups, N-substituted imino groups, thiocarbonyl groups, sulfonylcarbamoyl groups, acylcarbamoyl groups, sulfamoylcarbamoyl groups, carbazoyl groups, oxalyl groups, oxamoyl groups, cyano groups, thiocarbamoyl groups, hydroxy groups or salts thereof, alkoxy groups (including groups containing recurring ethylenoxy or propylenoxy units), aryloxy groups, heterocyclic oxy groups, acyloxy groups, (alkoxy or aryloxy) carbonyloxy groups, carbamoyloxy groups, sulfonyloxy groups, amino groups, (alkyl, aryl or heterocyclic) amino groups, acylamino groups, sulfonamide groups, ureido groups, thioureido groups, imide groups, (alkoxy or aryloxy) carbonylamino groups, sulfamoylamino groups, semicarbazide groups, thiosemicarbazide groups, hydrazino groups, quaternary amino groups, oxamoylamino groups, (alkyl or aryl) sulfonylureido groups, acylureido groups, acylsulfamoylamino groups, nitro groups, mercapto groups, (alkyl, aryl or heterocyclic) thio groups, acylthio groups, (alkyl or aryl) sulfonyl groups, (alkyl or aryl) sulfinyl groups, sulfo groups or salts thereof, sulfamoyl groups, acylsulfamoyl groups, sulfonylsulfamoyl groups or salts thereof, phosphoryl groups, phosphoramide or phosphate structure-bearing groups, silyl groups, and stannyl groups. These substituents may be further replaced by other substituents selected from the foregoing examples. 
     In formula (1), Z is an electron attractive group or silyl group. The electron attractive group is a substituent whose Hammett substituent constant σp has a positive value. Exemplary electron attractive groups are cyano groups, alkoxycarbonyl groups, aryloxycarbonyl groups, carbamoyl groups, imino groups, N-substituted imino groups, thiocarbonyl groups, sulfamoyl groups, alkylsulfonyl groups, arylsulfonyl groups, nitro groups, halogen atoms, perfluoroalkyl groups, perfluoroalkaneamide groups, sulfonamide groups, acyl groups, formyl groups, phosphoryl groups, carboxy groups (or salts thereof), sulfo groups (or salts thereof), heterocyclic groups, alkenyl groups, alkynyl groups, acyloxy groups, acylthio groups, sulfonyloxy groups, and aryl groups having such electron attractive groups substituted thereon. The heterocyclic groups include saturated or unsaturated heterocyclic groups, for example, pyridyl, quinolyl, pyrazinyl, quinoxalinyl, benzotriazolyl, imidazolyl, benzimidazolyl, hydantoin-1-yl, succinimide and phthalimide groups. 
     The electron attractive group represented by Z in formula (1) may have a substituent or substituents which are selected from the same substituents that the substituents represented by R 1 , R 2  and R 3  in formula (1) may have. 
     In formula (1), at least one pair of R 1  and Z, R 2  and R 3 , R 1  and R 2 , and R 3  and Z, taken together, may form a cyclic structure, which is a non-aromatic carbocyclic or non-aromatic heterocyclic one. 
     Described below is the preferred range of the compounds of formula (1). Preferred examples of the silyl group represented by Z in formula (1) include trimethylsilyl, t-butyldimethylsilyl, phenyldimethylsilyl, triethylsilyl, triisopropylsilyl, and trimethylsilyldimethylsilyl groups. 
     Preferred examples of the electron attractive group represented by Z in formula (1) include groups having 0 to 30 carbon atoms in total, for example, cyano, alkoxycarbonyl, aryloxycarbonyl, carbamoyl, thiocarbonyl, imino, N-substituted imino, sulfamoyl, alkylsulfonyl, arylsulfonyl, nitro, perfluoroalkyl, acyl, formyl, phosphoryl, acyloxy, and acylthio groups, and phenyl groups having an electron attractive group substituted thereon. More preferred examples include cyano, alkoxycarbonyl, carbamoyl, imino, sulfamoyl, alkylsulfonyl, arylsulfonyl, acyl, formyl, phosphoryl, and trifluoromethyl groups, and phenyl groups having an electron attractive group substituted thereon. Further preferred examples include cyano, formyl, acyl, alkoxycarbonyl, imino and carbamoyl groups. 
     The preferred groups represented by Z in formula (1) are electron attractive groups. 
     The substituents represented by R 1 , R 2  and R 3  in formula (1) are preferably groups having 0 to 30 carbon atoms in total, for example, the same groups as the electron attractive groups represented by Z in formula (1), as well as alkyl, hydroxy (or salts thereof), mercapto (or salts thereof), alkoxy, aryloxy, heterocyclic oxy, alkylthio, arylthio, heterocyclic thio, amino, alkylamino, arylamino, heterocyclic amino, ureido, acylamino, sulfonamide, and substituted or unsubstituted aryl groups. 
     In formula (1), R 1  is preferably an electron attractive group, aryl group, alkylthio group, alkoxy group, acylamino group, hydrogen atom or silyl group. 
     When R 1  represents electron attractive groups, they are preferably groups of 0 to 30 carbon atoms, including cyano, nitro, acyl, formyl, alkoxycarbonyl, aryloxycarbonyl, thiocarbonyl, imino, N-substituted imino, alkylsulfonyl, arylsulfonyl, carbamoyl, sulfamoyl, trifluoromethyl, phosphoryl, carboxy (or salts thereof), and saturated or unsaturated heterocyclic groups; more preferably cyano, acyl, formyl, alkoxycarbonyl, carbamoyl, imino, N-substituted imino, sulfamoyl, carboxy (or salts thereof), and saturated or unsaturated heterocyclic groups; most preferably cyano, formyl, acyl, alkoxycarbonyl, carbamoyl, and saturated or unsaturated heterocyclic groups. 
     When R 1  represents aryl groups, they are preferably substituted or unsubstituted phenyl groups having 6 to 30 carbon atoms in total wherein the substituents, if any, are arbitrary although electron attractive substituents are preferred. 
     More preferably, R 1  in formula (1) is an electron attractive group or aryl group. 
     The substituents represented by R 2  and R 3  in formula (1) are preferably the same groups as the electron attractive groups represented by Z in formula (1), as well as alkyl, hydroxy (or salts thereof), mercapto (or salts thereof), alkoxy, aryloxy, heterocyclic oxy, alkylthio, arylthio, heterocyclic thio, amino, alkylamino, anilino, heterocyclic amino, acylamino, and substituted or unsubstituted phenyl groups. 
     More preferably, one of R 2  and R 3  in formula (1) is hydrogen and the other is a substituent. In this case, preferred substituents are alkyl, hydroxy (or salts thereof), mercapto (or salts thereof), alkoxy, aryloxy, heterocyclic oxy, alkylthio, arylthio, heterocyclic thio, amino, alkylamino, anilino, heterocyclic amino, acylamino (especially perfluoroalkaneamide), sulfonamide, substituted or unsubstituted phenyl and heterocyclic groups; more preferably hydroxy (or salts thereof), mercapto (or salts thereof), alkoxy, aryloxy, heterocyclic oxy, alkylthio, arylthio, heterocyclic thio and heterocyclic groups; and most preferably hydroxy (or salts thereof), alkoxy or heterocyclic groups. 
     It is also preferred that Z and R 1 , or R 2  and R 3  in formula (1) form a cyclic structure together. The cyclic structures formed are non-aromatic carbocyclic or non-aromatic heterocyclic structures, preferably 5- to 7-membered cyclic structures having 1 to 40 carbon atoms, more preferably 3 to 30 carbon atoms in total inclusive of the carbon atoms in substituents. 
     Especially preferred of the compounds of formula (1) are those wherein Z is a cyano, formyl, acyl, alkoxycarbonyl, imino or carbamoyl group, R 1  is an electron withdrawing group or aryl group, one of R 2  and R 3  is hydrogen and the other is a hydroxy (or salts thereof), mercapto (or salts thereof), alkoxy, aryloxy, heterocyclic oxy, alkylthio, arylthio, heterocyclic thio or heterocyclic group. Also especially preferred of the compounds of formula (1) are those wherein Z and R 1  form a non-aromatic, 5- to 7-membered cyclic structure together, one of R 2  and R 3  is hydrogen and the other is a hydroxy (or salts thereof), mercapto (or salts thereof), alkoxy, aryloxy, heterocyclic oxy, alkylthio, arylthio, heterocyclic thio or heterocyclic group. In this case, Z which forms a non-aromatic cyclic structure with R 1  is preferably an acyl, carbamoyl, oxycarbonyl, thiocarbonyl or sulfonyl group while R 1  is preferably an acyl, carbamoyl, oxycarbonyl, thiocarbonyl, sulfonyl, imino, N-substituted imino, acylamino or carbonylthio group. 
     Secondly, the substituted isoxazole derivatives of formula (2) are described in detail. In formula (2), R 4  is a substituent. The definition and examples of the substituent represented by R 4  are the same as described for the substituents represented by R 1  to R 3  in formula (1). 
     In formula (2), the substituents represented by R 4  are preferably electron attractive groups or aryl groups. Preferred examples of the electron attractive groups include groups having 0 to 30 carbon atoms in total, such as cyano, nitro, acyl, formyl, alkoxycarbonyl, aryloxycarbonyl, alkylsulfonyl, arylsulfonyl, carbamoyl, sulfamoyl, trifluoromethyl, phosphoryl, imino, and saturated or unsaturated heterocyclic groups; more preferably cyano, acyl, formyl, alkoxycarbonyl, carbamoyl, sulfamoyl, alkylsulfonyl, arylsulfonyl, and heterocyclic groups; most preferably cyano, formyl, acyl, alkoxycarbonyl, carbamoyl, and heterocyclic groups. 
     When R 4  represents aryl, preferred aryl groups are substituted or unsubstituted phenyl groups having 6 to 30 carbon atoms in total. The substituents on the aryl groups are the same as described for the substituents represented by R 1  to R 3  in formula (1). 
     Preferably in formula (2), R 4  represents cyano, alkoxycarbonyl, carbamoyl, heterocyclic, or substituted or unsubstituted phenyl groups, and especially cyano, heterocyclic or alkoxycarbonyl groups. 
     Thirdly, the acetal compounds of formula (3) are described in detail. In formula (3), X and Y are independently hydrogen or substituents, A and B are independently alkoxy, alkylthio, alkylamino, aryloxy, arylthio, anilino, heterocyclic thio, heterocyclic oxy, or heterocyclic amino groups. X and Y, or A and B, taken together, may form a cyclic structure. 
     The substituents represented by X and Y are the same as described for the substituents represented by R 1  to R 3  in formula (1). Exemplary substituents are alkyl (inclusive of perfluoroalkyl and trichloromethyl), aryl, heterocyclic, halogen, cyano, nitro, alkenyl, alkynyl, acyl, formyl, alkoxycarbonyl, aryloxycarbonyl, imino, N-substituted imino, carbamoyl, thiocarbonyl, acyloxy, acylthio, acylamino, alkylsulfonyl, arylsulfonyl, sulfamoyl, phosphoryl, carboxy (or salts thereof), sulfo (or salts thereof), hydroxy (or salts thereof), mercapto (or salts thereof), alkoxy, aryloxy, heterocyclic oxy, alkylthio, arylthio, heterocyclic thio, amino, alkylamino, anilino, heterocyclic amino, and silyl groups. These groups may further have substituents. X and Y may bond together to form a cyclic structure, which may be either a non-aromatic carbocyclic or non-aromatic heterocyclic ring. 
     In formula (3), the substituents represented by X and Y are preferably groups having 1 to 40 carbon atoms in total, more preferably 1 to 30 carbon atoms in total, and include cyano, alkoxycarbonyl, aryloxycarbonyl, carbamoyl, imino, N-substituted imino, thiocarbonyl, sulfamoyl, alkylsulfonyl, arylsulfonyl, nitro, perfluoroalkyl, acyl, formyl, phosphoryl, acylamino, acyloxy, acylthio, heterocyclic, alkylthio, alkoxy, and aryl groups. 
     In formula (3), more preferred substituents represented by X and Y are cyano, nitro, alkoxycarbonyl, carbamoyl, acyl, formyl, acylthio, acylamino, thiocarbonyl, sulfamoyl, alkylsulfonyl, arylsulfonyl, imino, N-substituted imino, phosphoryl, trifluoromethyl, heterocyclic, and substituted phenyl groups. Especially preferred are cyano, alkoxycarbonyl, carbamoyl, alkylsulfonyl, arylsulfonyl, acyl, acylthio, acylamino, thiocarbonyl, formyl, imino, N-substituted imino, heterocyclic groups and phenyl groups having an electron attractive group substituted thereon. 
     It is also preferred that X and Y bond together to form a non-aromatic carbocyclic or non-aromatic heterocyclic ring. In this case, the cyclic structures are preferably 5- to 7-membered rings and have 1 to 40 carbon atoms, especially 3 to 30 carbon atoms in total. X and Y forming a cyclic structure are preferably acyl, carbamoyl, oxycarbonyl, thiocarbonyl, sulfonyl, imino, N-substituted imino, acylamino, and carbonylthio groups. 
     In formula (3), A and B are independently alkoxy, alkylthio, alkylamino, aryloxy, arylthio, anilino, heterocyclic thio, heterocyclic oxy or heterocyclic amino groups. A and B, taken together, may form a ring. The groups represented by A and B in formula (3) are preferably groups having 1 to 40 carbon atoms in total, more preferably 1 to 30 carbon atoms in total, and may further have substituents.. 
     It is more preferred in formula (3) that A and B bond together to form a cyclic structure. In this case, the cyclic structures are preferably 5- to 7-membered non-aromatic heterocycles and have 1 to 40 carbon atoms, especially 3 to 30 carbon atoms in total. Examples of A bonded to B (that is, —A—B—) include —O—(CH 2 ) 2 —O—, —N(CH 3 )—(CH 2 ) 2 —S—, —O—(CH 2 ) 2 —S—, —O—(CH 2 ) 3 —O—, —S—(CH 2 ) 2 —S—, —S—(CH 2 ) 3 —S—, —S—Ph—S—, —N(CH 3 )—(CH 2 ) 2 —O—, —O—(CH 2 ) 3 —S—, —N(CH 3 )—Ph—O—, —N(CH 3 )—Ph—S—, and —N(Ph)—(CH 2 ) 2 —S—. 
     The compounds of formulas (1), (2), and (3) may have incorporated therein a group capable of adsorbing to silver halide. Such adsorptive groups include alkylthio, arylthio, thiourea, thioamide, mercapto heterocyclic and triazole groups as described in U.S. Pat. Nos. 4,385,108 and 4,459,347, JP-A 195233/1984, 200231/1984, 201045/1984, 201046/1984, 201047/1984, 201048/1984, 201049/1984, 170733/1986, 270744/1986, 948/1987, 234244/1988, 234245/1988, and 234246/1988. These adsorptive groups to silver halide may take the form of precursors. Such precursors are exemplified by the groups described in JP-A 285344/1990. 
     The compounds of formulas (1), (2), and (3) may have incorporated therein a ballast group or polymer commonly used in immobile photographic additives such as couplers. The incorporation of a ballast group is one of the preferred embodiments of the present invention. The ballast group is a group having at least 8 carbon atoms and relatively inert with respect to photographic properties. It may be selected from, for example, alkyl, aralkyl, alkoxy, phenyl, alkylphenyl, phenoxy, and alkylphenoxy groups. The polymer is exemplified in JP-A 100530/1989, for example. 
     The compounds of formulas (1), (2), and (3) may contain a cationic group (e.g., a group containing a quaternary ammonio group and a nitrogenous heterocyclic group containing a quaternized nitrogen atom), a group containing recurring ethylenoxy or propylenoxy units, an (alkyl, aryl or heterocyclic) thio group, or a group which is dissociable with a base (e.g., carboxy, sulfo, acylsulfamoyl, and carbamoylsulfamoyl). The incorporation of groups containing recurring ethylenoxy or propylenoxy units or (alkyl, aryl or heterocyclic) thio groups is one of the preferred embodiments of the present invention. Exemplary compounds containing such a group are described in, for example, in JP-A 234471/1995, 333466/1993, 19032/1994, 19031/1994, 45761/1993, 259240/1991, 5610/1995, and 244348/1995, U.S. Pat. Nos. 4,994,365 and 4,988,604, and German Patent No. 4006032. 
     Illustrative examples of the compounds of formulas (1), (2), and (3) are given below although the invention is not limited thereto.                  
                 
                 
                 
                 
                 
                 
                   
     The compounds of formulas (1), (2), and (3) can be readily synthesized by well-known methods, for example, the methods described in U.S. Pat. Nos. 5,545,515, 5,635,339, and 5,654,130, WO 97/34196, and Japanese Patent Application Nos. 354107/1997, 309813/1997, and 272002/1997. 
     In the practice of the invention, the compound of formula (1) to (3) is used as solution in water or a suitable organic solvent. Suitable solvents include alcohols (e.g., methanol, ethanol, propanol, and fluorinated alcohols), ketones (e.g., acetone and methyl ethyl ketone), dimethylformamide, dimethyl sulfoxide and methyl cellosolve. 
     A well-known emulsifying dispersion method may be used for dissolving the compound of formula (1) to (3) with the aid of an oil such as dibutyl phthalate, tricresyl phosphate, glyceryl triacetate or diethyl phthalate or an auxiliary solvent such as ethyl acetate or cyclohexanone whereby an emulsified dispersion is mechanically prepared. Alternatively, a method known as a solid dispersion method is used for dispersing the compound of formula (1) to (3) in powder form in a suitable solvent, typically water, in a ball mill, colloidal mill or ultrasonic mixer. 
     The compound of formula (1) to (3) may be added to a layer on the photosensitive layer-bearing side of the support, that is, a photosensitive layer or any other layer on that side of the support, and preferably to the photosensitive layer or a layer disposed adjacent thereto. 
     The compound of formula (1) to (3) is preferably used in an amount of 1×10 −6  mol to 1 mol, more preferably 1×10 −5  mol to 5×10 −1  mol, and most preferably 2×10 −5  mol to 2×10 −1  mol per mol of silver. 
     The compounds of formulas (1) to (3) may be used alone or in admixture of two or more. In combination with the compounds of formulas (1) to (3), there may be used any of the compounds described in U.S. Pat. Nos. 5,545,515, 5,635,339, 5,654,130, and 5,686,228, WO 97/34196, and Japanese Patent Application Nos. 279962/1996, 228881/1997, 273935/1997, 354107/1997, 309813/1997, 296174/1997, 282564/1997, 272002/1997, 272003/1997, and 332388/1997. 
     The hydrazine derivatives used herein as the nucleating agent are preferably of the following formula (H).                    
     In formula (H), R 12  is an aliphatic, aromatic or heterocyclic group. R 11  is hydrogen or a block group. G 1  is —CO—, —COCO—, —C(═S)—, —SO 2 —, —SO—, —PO(R 13 )— or iminomethylene group. R 13  is selected from the same groups as defined for R 11  and may be different from R 11 . Both A 1  and A 2  are hydrogen, or one of A 1  and A 2  is hydrogen and the other is a substituted or unsubstituted alkylsulfonyl, substituted or unsubstituted arylsulfonyl or substituted or unsubstituted acyl group. Letter ml is equal to 0 or 1. R 11  is an aliphatic, aromatic or heterocyclic group when m1 is 0. 
     In formula (H), the aliphatic groups represented by R 12  are preferably substituted or unsubstituted, normal, branched or cyclic alkyl, alkenyl and alkynyl groups having 1 to 30 carbon atoms. 
     In formula (H), the aromatic groups represented by R 12  are preferably monocyclic or fused ring aryl groups, for example, phenyl and naphthyl groups. The heterocyclic groups represented by R 12  are preferably monocyclic or fused ring, saturated or unsaturated, aromatic or non-aromatic heterocyclic groups while the heterocycles in these groups include pyridine, pyrimidine, imidazole, pyrazole, quinoline, isoquinoline, benzimidazole, thiazole, benzothiazole, piperidine, triazine, morpholine, and piperazine rings. 
     Aryl and alkyl groups are most preferred as R 12 . 
     The groups represented by R 12  may have substituents. Exemplary substituents include halogen atoms (e.g., fluorine, chlorine, bromine and iodine), alkyl groups (inclusive of aralkyl, cycloalkyl and active methine groups), alkenyl groups, alkynyl groups, aryl groups, heterocyclic groups, heterocyclic groups containing a quaternized nitrogen atom (e.g., pyridinio), acyl groups, alkoxycarbonyl groups, aryloxycarbonyl groups, carbamoyl groups, carboxy groups or salts thereof, sulfonylcarbamoyl groups, acylcarbamoyl groups, sulfamoylcarbamoyl groups, carbazoyl groups, oxalyl groups, oxamoyl groups, cyano groups, thiocarbamoyl groups, hydroxy groups, alkoxy groups (inclusive of groups having recurring ethylenoxy or propylenoxy units), aryloxy groups, heterocyclic oxy groups, acyloxy groups, (alkoxy or aryloxy)carbonyloxy groups, carbamoyloxy groups, sulfonyloxy groups, amino groups, (alkyl, aryl or heterocyclic) amino groups, N-substituted nitrogenous heterocyclic groups, acylamino groups, sulfonamide groups, ureido groups, thioureido groups, imide groups, (alkoxy or aryloxy)carbonylamino groups, sulfamoylamino groups, semicarbazide groups, thiosemicarbazide groups, hydrazino groups, quaternary ammonio groups, oxamoylamino groups, (alkyl or aryl)sulfonylureido groups, acylureido groups, acylsulfamoylamino groups, nitro groups, mercapto groups, (alkyl, aryl or heterocyclic) thio groups, (alkyl or aryl)sulfonyl groups, (alkyl or aryl)sulfinyl groups, sulfo groups or salts thereof, sulfamoyl groups, acylsulfamoyl groups, sulfonylsulfamoyl groups or salts thereof, and groups containing a phosphoramide or phosphate structure. These substituents may be further substituted with such substituents. 
     Preferred substituents that R 12  may have include, where R 12  is an aromatic or heterocyclic group, alkyl (inclusive of active methylene), aralkyl, heterocyclic, substituted amino, acylamino, sulfonamide, ureido, sulfamoylamino, imide, thioureido, phosphoramide, hydroxy, alkoxy, aryloxy, acyloxy, acyl, alkoxycarbonyl, aryloxycarbonyl, carbamoyl, carboxy (inclusive of salts thereof), (alkyl, aryl or heterocyclic) thio, sulfo (inclusive of salts thereof), sulfamoyl, halogen, cyano, and nitro groups. 
     Where R 12  is an aliphatic group, preferred substituents include alkyl, aryl, heterocyclic, amino, acylamino, sulfonamide, ureido, sulfamoylamino, imide, thioureido, phosphoramide, hydroxy, alkoxy, aryloxy, acyloxy, acyl, alkoxycarbonyl, aryloxycarbonyl, carbamoyl, carboxy (inclusive of salts thereof), (alkyl, aryl or heterocyclic) thio, sulfo (inclusive of salts thereof), sulfamoyl, halogen, cyano, and nitro groups. 
     In formula (H), R 11  is hydrogen or a block group. Illustrative block groups are aliphatic groups (e.g., alkyl, alkenyl and alkynyl groups), aromatic groups (monocyclic or fused ring aryl groups), heterocyclic groups, alkoxy, aryloxy, amino and hydrazino groups. 
     The alkyl groups represented by R 11  are preferably substituted or unsubstituted alkyl groups having 1 to 10 carbon atoms, for example, methyl, ethyl, trifluoromethyl, difluoromethyl, 2-carboxytetrafluoroethyl, pyridiniomethyl, difluoromethoxymethyl, difluorocarboxymethyl, 3-hydroxypropyl, 3-methanesulfonamidopropyl, phenylsulfonylmethyl, o-hydroxybenzyl, methoxymethyl, phenoxymethyl, 4-ethylphenoxymethyl, phenylthiomethyl, t-butyl, dicyanomethyl, diphenylmethyl, triphenylmethyl, methoxycarbonyldiphenylmethyl, cyanodiphenylmethyl, and methylthiodiphenylmethyl groups. The alkenyl groups are preferably those having 1 to 10 carbon atoms, for example, vinyl, 2-ethoxycarbonylvinyl, and 2-trifluoro-2-methoxycarbonylvinyl groups. The alkynyl groups are preferably those having 1 to 10 carbon atoms, for example, ethynyl and 2-methoxycarbonylethynyl groups. The aryl groups are preferably monocyclic or fused ring aryl groups, especially those containing a benzene ring, for example, phenyl, perfluorophenyl, 3,5-dichlorophenyl, 2-methanesulfonamidophenyl, 2-carbamoylphenyl, 4,5-dicyanophenyl, 2-hydroxymethylphenyl, 2,6-dichloro-4-cyanophenyl, and 2-chloro-5-octylsulfamoylphenyl groups. 
     The heterocyclic groups represented by R 11  are preferably 5- and 6-membered, saturated or unsaturated, monocyclic or fused ring, heterocyclic groups containing at least one of nitrogen, oxygen and sulfur atoms, for example, morpholino, piperidino (N-substituted), imidazolyl, indazolyl (e.g., 4-nitroindazolyl), pyrazolyl, triazolyl, benzimidazolyl, tetrazolyl, pyridyl, pyridinio (e.g., N-methyl-3-pyridinio), quinolinio, and quinolyl groups. 
     The alkoxy groups are preferably those having 1 to 8 carbon atoms, for example, methoxy, 2-hydroxyethoxy, benzyloxy, and t-butoxy groups. The aryloxy groups are preferably substituted or unsubstituted phenoxy groups. The amino groups are preferably unsubstituted amino, alkylamino having 1 to 10 carbon atoms, arylamino, and saturated or unsaturated heterocyclic amino groups (inclusive of nitrogenous heterocyclic amino groups containing a quaternized nitrogen atom). Examples of the amino group include 2,2,6,6-tetramethylpiperidin-4-ylamino, propylamino, 2-hydroxyethylamino, anilino, o-hydroxyanilino, 5-benzotriazolylamino, and N-benzyl-3-pyridinioamino groups. The hydrazino groups are preferably substituted or unsubstituted hydrazino groups and substituted or unsubstituted phenylhydrazino groups (e.g., 4-benzenesulfonamidophenylhydrazino). 
     The groups represented by R 11  may be substituted ones, with examples of the substituent being as exemplified for the substituent on R 12 . 
     In formula (H), R 11  may be such a group as to induce cyclization reaction to cleave a G 1 —R 11  moiety from the remaining molecule to generate a cyclic structure containing the atoms of the —G 1 —R 11  moiety. Such examples are described in JP-A 29751/1988, for example. 
     The hydrazine derivative of formula (H) may have incorporated therein a group capable of adsorbing to silver halide. Such adsorptive groups include alkylthio, arylthio, thiourea, thioamide, mercapto heterocyclic and triazole groups as described in U.S. Pat. Nos. 4,385,108 and 4,459,347, JP-A 195233/1984, 200231/1984, 201045/1984, 201046/1984, 201047/1984, 201048/1984, 201049/1984, 170733/1986, 270744/1986, 948/1987, 234244/1988, 234245/1988, and 234246/1988. These adsorptive groups to silver halide may take the form of precursors. Such precursors are exemplified by the groups described in JP-A 285344/1990. 
     R 11  and R 12  in formula (H) may have incorporated therein a ballast group or polymer commonly used in immobile photographic additives such as couplers. The ballast group is a group having at least 8 carbon atoms and relatively inert with respect to photographic properties. It may be selected from, for example, alkyl, aralkyl, alkoxy, phenyl, alkylphenyl, phenoxy, and alkylphenoxy groups. The polymer is exemplified in JP-A 100530/1989, for example. 
     R 11  or R 12  in formula (H) may have a plurality of hydrazino groups as substituents. In this case, the compounds of formula (H) are polymeric with respect to hydrazino groups. Exemplary polymeric compounds are described in JP-A 86134/1989, 16938/1992, 197091/1993, WO 95-32452 and 95-32453, Japanese Patent Application Nos. 351132/1995, 351269/1995, 351168/1995, 351287/1995, and 351279/1995. 
     R 11  or R 12  in formula (H) may contain a cationic group (e.g., a group containing a quaternary ammonio group and a nitrogenous heterocyclic group containing a quaternized nitrogen atom), a group containing recurring ethylenoxy or propylenoxy units, an (alkyl, aryl or heterocyclic) thio group, or a group which is dissociable with a base (e.g., carboxy, sulfo, acylsulfamoyl, and carbamoylsulfamoyl). Exemplary compounds containing such a group are described in, for example, in JP-A 234471/1995, 333466/1993, 19032/1994, 19031/1994, 45761/1993, 259240/1991, 5610/1995, and 244348/1995, USP 4,994,365 and 4,988,604, and German Patent No. 4006032. 
     In formula (H), each of A 1  and A 2  is a hydrogen atom, a substituted or unsubstituted alkyl- or arylsulfonyl group having up to 20 carbon atoms (preferably a phenylsulfonyl group or a phenylsulfonyl group substituted such that the sum of Hammett substituent constants may be −0.5 or more), or a substituted or unsubstituted acyl group having up to 20 carbon atoms (preferably a benzoyl group, a benzoyl group substituted such that the sum of Hammett substituent constants may be −0.5 or more, or a linear, branched or cyclic, substituted or unsubstituted, aliphatic acyl group wherein the substituent is selected from a halogen atom, ether group, sulfonamide group, carbonamide group, hydroxyl group, carboxy group and sulfo group) Most preferably, both A 1  and A 2  are hydrogen atoms. 
     The preferable range of the hydrazine derivatives of formula (H) is described. 
     In formula (H), R 12  is preferably phenyl or substituted alkyl of 1 to 3 carbon atoms. 
     Where R 12  represents phenyl groups, preferred substituents thereon include nitro, alkoxy, alkyl, acylamino, ureido, sulfonamide, thioureido, carbamoyl, sulfamoyl, carboxy (or salts thereof), sulfo (or salts thereof), alkoxycarbonyl, and chloro groups. 
     Where R 12  represents substituted phenyl groups, it is preferred that the substituent have attached thereto directly or through a linking group at least one group selected from among ballast groups, adsorptive groups to silver halide, groups containing a quaternary ammonio group, nitrogenous heterocyclic groups containing a quaternized nitrogen atom, groups containing recurring ethylenoxy units, (alkyl, aryl or heterocyclic) thio groups, nitro groups, alkoxy groups, acylamino groups, sulfonamide groups, dissociable groups (e.g., carboxy, sulfo, acylsulfamoyl and carbamoylsulfamoyl), and hydrazino groups capable of forming a polymer (as represented by —NHNH—G 1 —R 11 ). 
     Where R 12  represents substituted alkyl groups of 1 to 3 carbon atoms, it is more preferably substituted methyl groups, and further preferably di- or tri-substituted methyl groups. Exemplary preferred substituents on these methyl groups include methyl, phenyl, cyano, (alkyl, aryl or heterocyclic) thio, alkoxy, aryloxy, chloro, heterocyclic, alkoxycarbonyl, aryloxycarbonyl, carbamoyl, sulfamoyl, amino, acylamino, and sulfonamide groups, and especially, substituted or unsubstituted phenyl groups. 
     Where R 12  represents substituted methyl groups, preferred examples thereof are t-butyl, dicyanomethyl, dicyanophenylmethyl, triphenylmethyl (trityl), diphenylmethyl, methoxycarbonyldiphenylmethyl, cyanodiphenylmethyl, methylthiodiphenylmethyl, cyclopropyldiphenylmethyl groups, with trityl being most preferred. 
     Most preferably, R 12  in formula (H) represents substituted phenyl groups. 
     In formula (H), m1 is equal to 0 or 1. When m1 is 0, R 11  represents aliphatic, aromatic or heterocyclic groups. When m1 is 0, R 11  more preferably represents phenyl groups or substituted alkyl groups of 1 to 3 carbon atoms. These phenyl or substituted alkyl groups are the same as the preferred groups of R 12  mentioned above. 
     Preferably m1 is equal to 1. 
     Where R 12  is a phenyl group and G 1  is —CO—, the groups represented by R 11  are preferably selected from hydrogen, alkyl, alkenyl, alkynyl, aryl and heterocyclic groups, more preferably from hydrogen, alkyl and aryl groups, and most preferably from hydrogen atoms and alkyl groups. Where R 11  represents alkyl groups, preferred substituents thereon are halogen, alkoxy, aryloxy, alkylthio, arylthio, and carboxy groups. 
     Where R 12  is a substituted methyl group and G 1  is —CO—, the groups represented by R 11  are preferably selected from hydrogen, alkyl, aryl, heterocyclic, alkoxy, and amino groups (including unsubstituted amino, alkylamino, arylamino and heterocyclic amino groups), more preferably from hydrogen, alkyl, aryl, heterocyclic, alkoxy, alkylamino, arylamino and heterocyclic amino groups. Where G 1  is —COCO—, independent of R 12 , R 11  is preferably selected from alkoxy, aryloxy, and amino groups, more preferably from substituted amino groups, specifically alkylamino, arylamino and saturated or unsaturated heterocyclic amino groups. 
     Where G 1  is —SO 2 —, independent of R 12 , R 11  is preferably selected from alkyl, aryl and substituted amino groups. 
     In formula (H), G 1  is preferably —CO— or —COCO—, and most preferably —CO—. 
     Illustrative, non-limiting, examples of the compound represented by formula (H) are given below. 
     
       
         
           
               
             
               
                   
               
             
            
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 R= 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 X= 
                 —H 
                 —C 2 F 4 —COOH or (—C 2 F 4 —COO ⊖ K ⊕ ) 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 1 
                 3-NHCO—C 9 H 19 (n) 
                 1a 
                 1b 
                 1c 
                 1d 
               
               
                   
               
               
                 2 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 2a 
                 2b 
                 2c 
                 2d 
               
               
                   
               
               
                 3 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 3a 
                 3b 
                 3c 
                 3d 
               
               
                   
               
               
                 4 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 4a 
                 4b 
                 4c 
                 4d 
               
               
                   
               
               
                 5 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 5a 
                 5b 
                 5c 
                 5d 
               
               
                   
               
               
                 6 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 6a 
                 6b 
                 6c 
                 6d 
               
               
                   
               
               
                 7 
                 2,4-(CH 3 ) 2 -3-SC 2 H 4 —(OC 2 H 4 ) 4 —OC 8 H 17   
                 7a 
                 7b 
                 7c 
                 7d 
               
               
                   
               
            
           
           
               
            
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 R= 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 X= 
                 —H 
                 —CF 2 H 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                   
               
               
                  8 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 8a 
                 8e 
                 8f 
                 8g 
               
               
                   
               
               
                  9 
                 6-OCH 3 -3-C 5 H 11 (t) 
                 9a 
                 9e 
                 9f 
                 9g 
               
               
                   
               
               
                 10 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 10a 
                 10e 
                 10f 
                 10g 
               
               
                   
               
               
                 11 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 11a 
                 11e 
                 11f 
                 11g 
               
               
                   
               
               
                 12 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 12a 
                 12e 
                 12f 
                 12g 
               
               
                   
               
               
                 13 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 13a 
                 13e 
                 13f 
                 13g 
               
               
                   
               
               
                 14 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 14a 
                 14e 
                 14f 
                 14g 
               
               
                   
               
               
                   
               
            
           
           
               
            
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 X= 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Y= 
                 —CHO 
                 —COCF 3   
                 —SO 2 CH 3   
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                   
               
               
                 15 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 15a 
                 15h 
                 15i 
                 15j 
               
               
                   
               
               
                 16 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 16a 
                 16h 
                 16i 
                 16j 
               
               
                   
               
               
                 17 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 17a 
                 17h 
                 17i 
                 17j 
               
               
                   
               
               
                 18 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 18a 
                 18h 
                 18i 
                 18j 
               
               
                   
               
               
                 19 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 19a 
                 19h 
                 19i 
                 19j 
               
               
                   
               
               
                 20 
                 3-NHSO 2 NH—C 8 H 17   
                 20a 
                 20h 
                 20i 
                 20j 
               
               
                   
               
               
                 21 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 21a 
                 21h 
                 21i 
                 21j 
               
               
                   
               
               
                   
               
            
           
           
               
               
            
               
                   
                 R= 
               
            
           
           
               
               
               
               
               
            
               
                   
                 —H 
                 —CF 3   
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                   
               
               
                 22 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 22a 
                 22h 
                 22k 
                 22l 
               
               
                   
               
               
                 23 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 23a 
                 23h 
                 23k 
                 23l 
               
               
                   
               
               
                 24 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 24a 
                 24h 
                 24k 
                 24l 
               
               
                   
               
               
                 25 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 25a 
                 25h 
                 25k 
                 25l 
               
               
                   
               
               
                 26 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 26a 
                 26h 
                 26k 
                 26l 
               
               
                   
               
               
                 27 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 27a 
                 27h 
                 27k 
                 27l 
               
               
                   
               
               
                 28 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 28a 
                 28h 
                 28k 
                 28l 
               
               
                   
               
               
                   
               
            
           
           
               
            
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 R= 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Y= 
                 —H 
                 —CH 2 OCH 3   
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                   
               
               
                 29 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 29a 
                 29m 
                 29n 
                 29f 
               
               
                   
               
               
                 30 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 30a 
                 30m 
                 30n 
                 30f 
               
               
                   
               
               
                 31 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 31a 
                 31m 
                 31n 
                 31f 
               
               
                   
               
               
                 32 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 32a 
                 32m 
                 32n 
                 32f 
               
               
                   
               
               
                 33 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 33a 
                 33m 
                 33n 
                 33f 
               
               
                   
               
               
                 34 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 34a 
                 34m 
                 34n 
                 34f 
               
               
                   
               
               
                 35 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 35a 
                 35m 
                 35n 
                 35f 
               
               
                   
               
               
                   
               
            
           
           
               
            
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 R= 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Y= 
                 —H 
                 —CF 2 SCH 3   
                 —CONHCH 3   
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                   
               
               
                 36 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 36a 
                 36o 
                 36p 
                 36q 
               
               
                   
               
               
                 37 
                 2-OCH 3 -4-NHSO 2 Cl 2 H 25   
                 37a 
                 37o 
                 37p 
                 37q 
               
               
                 38 
                 3-NHCOC 11 H 23 -4-NHSO 2 CF 3   
                 38a 
                 38o 
                 38p 
                 38q 
               
               
                   
               
               
                 39 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 39a 
                 39o 
                 39p 
                 39q 
               
               
                   
               
               
                 40 
                 4-OCO(CH 2 ) 2 COOC 6 H 13   
                 40a 
                 40o 
                 40p 
                 40q 
               
               
                   
               
               
                 41 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 41a 
                 41o 
                 41p 
                 41q 
               
               
                   
               
               
                 42 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 42a 
                 42o 
                 42p 
                 42q 
               
               
                   
               
            
           
           
               
               
            
               
                   
               
               
                 43 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 44 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 45 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 46 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 47 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                   
                 X:Y = 3:97 
               
               
                   
                 average molecular weigh ˜100,000 
               
               
                   
               
               
                 48 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 49 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 50 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 51 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 52 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 53 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                   
               
            
           
           
               
            
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 R= 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Y= 
                 —H 
                 —CH 2 OCH 3   
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 —CONHC 3 H 7   
               
               
                   
               
               
                 54 
                 2-OCH3 
                 54a 
                 54m 
                 54r 
                 54s 
               
               
                 55 
                 2-OCH 3   
                 55a 
                 55m 
                 55r 
                 55s 
               
               
                   
                 5-C 8 H 17 (t) 
               
               
                 56 
                 4-NO 2   
                 56a 
                 56m 
                 56r 
                 56s 
               
               
                 57 
                 4-CH 3   
                 57a 
                 57m 
                 57r 
                 57s 
               
               
                   
               
               
                 58 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 58a 
                 58m 
                 58r 
                 58s 
               
               
                   
               
               
                 59 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 59a 
                 59m 
                 59r 
                 59s 
               
               
                   
               
               
                   
               
            
           
           
               
            
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 R= 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Y= 
                 —H 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 60 
                 2-OCH 3   
                 60a 
                 60c 
                 60f 
                 60g 
               
               
                   
                 5-OCH3 
               
               
                 61 
                 4-C 6 H 17 (t) 
                 61a 
                 61c 
                 61f 
                 61g 
               
               
                 62 
                 4-OCH 3   
                 62a 
                 62c 
                 62f 
                 62g 
               
               
                 63 
                 3-NO 2   
                 63a 
                 63c 
                 63f 
                 63g 
               
               
                   
               
               
                 64 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 64a 
                 64c 
                 64f 
                 64g 
               
               
                   
               
               
                 65 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 65a 
                 65c 
                 65f 
                 65g 
               
               
                   
               
               
                   
               
            
           
           
               
            
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 R B = 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 R A = 
                 —H 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                   
               
               
                 66 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 66a 
                 66u 
                 66v 
                 66t 
               
               
                   
               
               
                 67 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 67a 
                 67u 
                 67v 
                 67t 
               
               
                   
               
               
                 68 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 68a 
                 68u 
                 68v 
                 68t 
               
               
                   
               
               
                 69 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 69a 
                 69u 
                 69v 
                 69t 
               
               
                   
               
               
                 70 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 70a 
                 70u 
                 70v 
                 70t 
               
               
                   
               
               
                 71 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 71a 
                 71u 
                 71v 
                 71t 
               
               
                   
               
               
                   
               
            
           
           
               
            
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 R B = 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 R A = 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 —OC 4 H 9 (t) 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                   
               
               
                 72 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 72s 
                 72x 
                 72y 
                 72w 
               
               
                   
               
               
                 73 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 73s 
                 73x 
                 73y 
                 73w 
               
               
                   
               
               
                 74 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 74s 
                 74x 
                 74y 
                 74w 
               
               
                   
               
               
                 75 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 75s 
                 75x 
                 75y 
                 75w 
               
               
                   
               
               
                 76 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 76s 
                 76x 
                 76y 
                 76w 
               
               
                   
               
               
                   
               
            
           
           
               
            
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                   
                 R= 
               
               
                   
                   
               
               
                   
                   
               
               
                   
                 77 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 78 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 79 
                 —CH 2 OCH 2 CH 2 SCH 2 CH 2 OCH 3   
               
               
                   
                 80 
                 —CF 2 CF 2 COOH 
               
               
                   
                   
               
               
                   
                 81 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 82 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 83 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 84 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 85 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 86 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 87 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 88 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 89 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 90 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 91 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 92 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 93 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 94 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                   
               
            
           
           
               
            
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 R= 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Y= 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 —CH 2 —Cl 
               
               
                   
               
               
                   
               
               
                  95 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 95-1 
                 95-2 
                 95-3 
                 95-4 
               
               
                   
               
               
                  96 
                 4-COOH 
                 96-1 
                 96-2 
                 96-3 
                 96-4 
               
               
                   
               
               
                  97 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 97-1 
                 97-2 
                 97-3 
                 97-4 
               
               
                   
               
               
                  98 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 98-1 
                 98-2 
                 98-3 
                 98-4 
               
               
                   
               
               
                  99 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 99-1 
                 99-2 
                 99-3 
                 99-4 
               
               
                   
               
               
                 100 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 100-1 
                 100-2 
                 100-3 
                 100-4 
               
               
                   
               
               
                   
               
            
           
           
               
            
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 X= 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Y= 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 101 
                 4-NO 2   
                 101-5 
                 101-6 
                 101-7 
                 101y 
               
               
                 102 
                 2,4-OCH 3   
                 102-5 
                 102-6 
                 102-7 
                 102y 
               
               
                   
               
               
                 103 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 103-5 
                 103-6 
                 103-7 
                 103y 
               
               
                   
               
               
                   
               
            
           
           
               
               
            
               
                   
                 X= 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Y= 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                   
               
               
                 104 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 104-8 
                 104-9 
                 104w′ 
                 104x 
               
               
                   
               
               
                 105 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 105-8 
                 105-9 
                 105w′ 
                 105x 
               
               
                   
               
               
                   
               
            
           
           
               
            
               
                 Y—NHNH—X 
               
            
           
           
               
               
            
               
                   
                 X= 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Y= 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                   
               
               
                 106 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 106-10 
                 106a 
                 106m 
                 106y 
               
               
                   
               
               
                 107 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 107-10 
                 107a 
                 107m 
                 107y 
               
               
                   
               
               
                 108 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 108-10 
                 108a 
                 108m 
                 108y 
               
               
                   
               
               
                 109 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 109-10 
                 109a 
                 109m 
                 109y 
               
               
                   
               
               
                 110 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 110-10 
                 110a 
                 110m 
                 110y 
               
               
                   
               
               
                 111 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 111-10 
                 111a 
                 111m 
                 111y 
               
               
                   
               
               
                   
               
            
           
           
               
            
               
                 Y—NHNH—X 
               
            
           
           
               
               
            
               
                   
                 X= 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Y= 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                   
               
               
                 112 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 112-11 
                 112-12 
                 112-13 
                 112-14 
               
               
                   
               
               
                 113 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 113-11 
                 113-12 
                 113-13 
                 113-14 
               
               
                   
               
               
                 114 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 114-11 
                 114-12 
                 114-13 
                 114-14 
               
               
                   
               
               
                 115 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 115-11 
                 115-12 
                 115-13 
                 115-14 
               
               
                   
               
               
                 116 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 116-11 
                 116-12 
                 116-13 
                 116-14 
               
               
                   
               
               
                 117 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 117-11 
                 117-12 
                 117-13 
                 117-14 
               
               
                   
               
               
                   
               
            
           
           
               
               
            
               
                   
               
               
                 118 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 119 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 120 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 121 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 122 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 123 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                   
               
            
           
           
               
            
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 X= 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Ar= 
                 —OH 
                 —SH 
                 —NHCOCF 3   
                 —NHSO 2 CH 3   
                 —NHSO 2 ph 
                 —N(CH 3 ) 2   
               
               
                   
               
               
                   
               
               
                 124 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 124a 
                 124b 
                 124c 
                 124d 
                 124e 
                 124f 
               
               
                   
               
               
                 125 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 125a 
                 125b 
                 125c 
                 125d 
                 125e 
                 125f 
               
               
                   
               
               
                 126 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 126a 
                 126b 
                 126c 
                 126d 
                 126e 
                 126f 
               
               
                   
               
               
                 127 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 127a 
                 127b 
                 127c 
                 127d 
                 127e 
                 127f 
               
               
                   
               
               
                 128 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 128a 
                 128b 
                 128c 
                 128d 
                 128e 
                 128f 
               
               
                   
               
               
                 129 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 129a 
                 129b 
                 129c 
                 129d 
                 129e 
                 129f 
               
               
                   
               
               
                 130 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 130a 
                 130b 
                 130c 
                 130d 
                 130e 
                 130f 
               
               
                   
               
               
                 131 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 131a 
                 131b 
                 131c 
                 131d 
                 131e 
                 131f 
               
               
                   
               
               
                 132 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 132a 
                 132b 
                 132c 
                 132d 
                 132e 
                 132f 
               
               
                   
               
               
                 133 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 133a 
                 133b 
                 133c 
                 133d 
                 133e 
                 133f 
               
               
                   
               
               
                 134 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 134a 
                 134b 
                 134c 
                 134d 
                 134e 
                 134f 
               
               
                   
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                   
               
               
                   
                 135 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 136 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
               
                   
                 137 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
                   
               
            
           
         
       
     
     The hydrazine derivatives of formula (H) may be used alone or in admixture of two or more. 
     In addition to the above-described ones, the following hydrazine derivatives are also preferable for use in the practice of the invention. If desired, any of the following hydrazine derivatives may be used in combination with the hydrazine derivatives of formula (H). The hydrazine derivatives which are used herein can be synthesized by various methods as described in the following patents. 
     Exemplary hydrazine derivatives which can be used herein include the compounds of the chemical formula [1] in JP-B 77138/1994, more specifically the compounds described on pages 3 and 4 of the same; the compounds of the general formula (I) in JP-B 93082/1994, more specifically compound Nos. 1 to 38 described on pages 8 to 18 of the same; the compounds of the general formulae (4), (5) and (6) in JP-A 230497/1994, more specifically compounds 4-1 to 4-10 described on pages 25 and 26, compounds 5-1 to 5-42 described on pages 28 to 36, and compounds 6-1 to 6-7 described on pages 39 and 40 of the same; the compounds of the general formulae (1) and (2) in JP-A 289520/1994, more specifically compounds 1-1 to 1-17 and 2-1 described on pages 5 to 7 of the same; the compounds of the chemical formulae [2] and [3] in JP-A 313936/1994, more specifically the compounds described on pages 6 to 19 of the same; the compounds of the chemical formula [1] in JP-A 313951/1994, more specifically the compounds described on pages 3 to 5 of the same; the compounds of the general formula (I) in JP-A 5610/1995, more specifically compounds I-1 to I-38 described on pages 5 to 10 of the same; the compounds of the general formula (II) in JP-A 77783/1995, more specifically compounds II-1 to II-102 described on pages 10 to 27 of the same; the compounds of the general formulae (H) and (Ha) in JP-A 104426/1995, more specifically compounds H-1 to H-44 described on pages 8 to 15 of the same; the compounds having an anionic group in proximity to a hydrazine group or a nonionic group capable of forming an intramolecular hydrogen bond with the hydrogen atom of hydrazine described in EP 713131A, especially compounds of the general formulae (A), (B), (C), (D), (E), and (F), more specifically compounds N-1 to N-30 described therein; and the compounds of the general formula (1) in EP 713131A, more specifically compounds D-1 to D-55 described therein. 
     Also useful are the hydrazine derivatives described in “Known Technology,” Aztech K.K., Mar. 22, 1991, pages 25-34 and Compounds D-2 and D-39 described in JP-A 86354/1987, pages 6-7. 
     In the practice of the invention, the hydrazine nucleating agent is used as solution in a suitable organic solvent. Suitable solvents include alcohols (e.g., methanol, ethanol, propanol, and fluorinated alcohols), ketones (e.g., acetone and methyl ethyl ketone), dimethylformamide, dimethyl sulfoxide and methyl cellosolve. 
     A well-known emulsifying dispersion method may be used for dissolving the hydrazine derivative with the aid of an oil such as dibutyl phthalate, tricresyl phosphate, glyceryl triacetate or diethyl phthalate or an auxiliary solvent such as ethyl acetate or cyclohexanone whereby an emulsified dispersion is mechanically prepared. Alternatively, a method known as a solid dispersion method is used for dispersing the hydrazine derivative in powder form in water in a ball mill, colloidal mill or ultrasonic mixer. 
     The hydrazine nucleating agent may be added to a layer on the photosensitive layer-bearing side of the support, that is, a photosensitive layer or any other layer on that side of the support, and preferably to the photosensitive layer or a layer disposed adjacent thereto. 
     The nucleating agent is preferably used in an amount of 1×10 −6  mol to 1×10 −2  mol, more preferably 1×10 −5  mol to 5×10 −3  mol, and most preferably 2×10 −5  mol to 5×10 −3  mol per mol of silver. 
     Also in the practice of the invention, contrast promoting agents may be used in combination with the aforementioned nucleating agents (or contrast enhancers) for forming high contrast images. Such ultrahigh contrast promoting agents include the amine compounds described in U.S. Pat. No. 5,545,505, specifically Compounds AM-1 to AM-5 therein, the hydroxamic acids described in U.S. P 5,545,507, specifically HA-1 to HA-11 therein, the acrylonitriles described in U.S. Pat. No. 5,545,507, specifically CN-1 to CN-13 therein, the hydrazine compounds described in U.S. Pat. No. 5,558,983, specifically CA-1 to CA-6 therein, the onium salts described in Japanese Patent Application No. 132836/1996, specifically A-1 to A-42, B-1 to B-27 and C-1 to C-14. 
     The synthesis methods, addition methods, and addition amounts of these nucleating agents (or contrast enhancers) and contrast promoting agents are as described in the above-listed patents. 
     Sensitizing Dye 
     A sensitizing dye may be used in the practice of the invention. There may be used any of sensitizing dyes which can spectrally sensitize silver halide grains in a desired wavelength region when adsorbed to the silver halide grains. The sensitizing dyes used herein include cyanine dyes, merocyanine dyes, complex cyanine dyes, complex merocyanine dyes, holopolar cyanine dyes, styryl dyes, hemicyanine dyes, oxonol dyes, and hemioxonol dyes. Useful sensitizing dyes which can be used herein are described in  Research Disclosure , Item 17643 IV-A (December 1978, page 23), ibid., Item 1831 X (August 1979, page 437) and the references cited therein. It is advantageous to select a sensitizing dye having appropriate spectral sensitivity to the spectral properties of a particular light source of various laser imagers, scanners, image setters and process cameras. 
     Exemplary dyes for spectral sensitization to red light include compounds I-1 to I-38 described in JP-A 18726/1979, compounds I-1 to I-35 described in JP-A 75322/1994, compounds I-1 to I-34 described in JP-A 287338/1995, dyes 1 to 20 described in JP-B 39818/1980, compounds I-1 to I-37 described in JP-A 284343/1987, and compounds I-1 to I-34 described in JP-A 287338/1995 for red light sources such as He-Ne lasers, red laser diodes, and LED. 
     For compliance with laser diode light sources in the wavelength range of 750 to 1,400 nm, it is advantageous to spectrally sensitize silver halide grains. Such spectral sensitization may be advantageously done with various known dyes including cyanine, merocyanine, styryl, hemicyanine, oxonol, hemioxonol, and xanthene dyes. Useful cyanine dyes are cyanine dyes having a basic nucleus such as a thiazoline, oxazoline, pyrroline, pyridine, oxazole, thiazole, selenazole or imidazole nucleus. Preferred examples of the useful merocyanine dye contain an acidic nucleus such as a thiohydantoin, rhodanine, oxazolidinedione, thiazolinedione, barbituric acid, thiazolinone, malononitrile or pyrazolone nucleus in addition to the above-mentioned basic nucleus. Among the above-mentioned cyanine and merocyanine dyes, those having an imino or carboxyl group are especially effective. A suitable choice may be made of well-known dyes as described, for example, in U.S. Pat. Nos. 3,761,279, 3,719,495, and 3,877,943, BP 1,466,201, 1,469,117, and 1,422,057, JP-B 10391/1991 and 52387/1994, JP-A 341432/1993, 194781/1994, and 301141/1994. 
     Especially preferred dye structures are cyanine dyes having a thioether bond-containing substituent, examples of which are the cyanine dyes described in JP-A 58239/1987, 138638/1991, 138642/1991, 255840/1992, 72659/1993, 72661/1993, 222491/1994, 230506/1990, 258757/1994, 317868/1994, and 324425/1994, Publication of International Patent Application No. 500926/1995, and U.S. Pat. No. 5,541,054; dyes having a carboxylic group, examples of which are the dyes described in JP-A 163440/1991, 301141/1994 and U.S. Pat. No. 5,441,899; and merocyanine dyes, polynuclear merocyanine dyes, and polynuclear cyanine dyes, examples of which are the dyes described in JP-A 6329/1972, 105524/1974, 127719/1976, 80829/1977, 61517/1979, 214846/1984, 6750/1985, 159841/1988, 35109/1994, 59381/1994, 146537/1995, Publication of International Patent Application No. 50111/1993, BP 1,467,638, and USP 5,281,515. 
     Also useful in the practice of the invention are dyes capable of forming the J-band as disclosed in U.S. Pat. Nos. 5,510,236, 3,871,887 (Example 5), JP-A 96131/1990 and 48753/1984. 
     These sensitizing dyes may be used alone or in admixture of two or more. A combination of sensitizing dyes is often used for the purpose of supersensitization. In addition to the sensitizing dye, the emulsion may contain a dye which itself has no spectral sensitization function or a compound which does not substantially absorb visible light, but is capable of supersensitization. Useful sensitizing dyes, combinations of dyes showing supersensitization, and compounds showing super-sensitization are described in  Research Disclosure , Vol. 176, 17643 (December 1978), page 23, IV J and JP-B 25500/1974 and 4933/1968, JP-A 19032/1984 and 192242/1984. 
     The sensitizing dye may be added to a silver halide emulsion by directly dispersing the dye in the emulsion or by dissolving the dye in a solvent and adding the solution to the emulsion. The solvent used herein includes water, methanol, ethanol, propanol, acetone, methyl cellosolve, 2,2,3,3-tetrafluoropropanol, 2,2,2-trifluoroethanol, 3-methoxy-1-propanol, 3-methoxy-1-butanol, 1-methoxy-2-propanol, N,N-dimethylformamide and mixtures thereof. 
     Also useful are a method of dissolving a dye in a volatile organic solvent, dispersing the solution in water or hydrophilic colloid and adding the dispersion to an emulsion as disclosed in U.S. Pat. No. 3,469,987, a method of dissolving a dye in an acid and adding the solution to an emulsion or forming an aqueous solution of a dye with the aid of an acid or base and adding it to an emulsion as disclosed in JP-B 23389/1969, 27555/1969 and 22091/1982, a method of forming an aqueous solution or colloidal dispersion of a dye with the aid of a surfactant and adding it to an emulsion as disclosed in U.S. Pat. Nos. 3,822,135 and 4,006,025, a method of directly dispersing a dye in hydrophilic colloid and adding the dispersion to an emulsion as disclosed in JP-A 102733/1978 and 105141/1983, and a method of dissolving a dye using a compound capable of red shift and adding the solution to an emulsion as disclosed in JP-A 74624/1976. It is also acceptable to apply ultrasonic waves to form a solution. 
     The time when the sensitizing dye is added to the silver halide emulsion according to the invention is at any step of an emulsion preparing process which has been ascertained effective. The sensitizing dye may be added to the emulsion at any stage or step before the emulsion is coated, for example, 15 at a stage prior to the silver halide grain forming step and/or desalting step, during the desalting step and/or a stage from desalting to the start of chemical ripening as disclosed in U.S. Pat. Nos. 2,735,766, 3,628,960, 4,183,756, and 4,225,666, JP-A 184142/1983 and 196749/1985, and a stage immediately before or during chemical ripening and a stage from chemical ripening to emulsion coating as disclosed in JP-A 113920/1983. Also as disclosed in U.S. Pat. No. 4,225,666 and JP-A 7629/1983, an identical compound may be added alone or in combination with a compound of different structure in divided portions, for example, in divided portions during a grain forming step and during a chemical ripening step or after the completion of chemical ripening, or before or during chemical ripening and after the completion thereof. The type of compound or the combination of compounds to be added in divided portions may be changed. 
     The amount of the sensitizing dye used may be an appropriate amount complying with sensitivity and fog although the preferred amount is about 10 −6  to 1 mol, more preferably 10 −4  to 10 −1  mol per mol of the silver halide in the photosensitive layer. 
     Antifoggant 
     With antifoggants, stabilizers and stabilizer precursors, the silver halide emulsion and/or organic silver salt according to the invention can be further protected against formation of additional fog and stabilized against lowering of sensitivity during shelf storage. Suitable antifoggants, stabilizers and stabilizer precursors which can be used alone or in combination include thiazonium salts as described in U.S. Pat. Nos. 2,131,038 and 2,694,716, azaindenes as described in U.S. Pat. Nos. 2,886,437 and 2,444,605, mercury salts as described in U.S. Pat. No. 2,728,663, urazoles as described in U.S. Pat. No. 3,287,135, sulfocatechols as described in U.S. Pat. No. 3,235,652, oximes, nitrons and nitroindazoles as described in BP 623,448, polyvalent metal salts as described in U.S. Pat. No. 2,839,405, thiuronium salts as described in U.S. Pat. No. 3,220,839, palladium, platinum and gold salts as described in U.S. Pat. Nos. 2,566,263 and 2,597,915, halogen-substituted organic compounds as described in U.S. Pat. Nos. 4,108,665 and 4,442,202, triazines as described in U.S. Pat. Nos. 4,128,557, 4,137,079, 4,138,365 and 4,459,350, and phosphorus compounds as described in U.S. Pat. No. 4,411,985. 
     Preferred antifoggants are organic halides, for example, the compounds described in JP-A 119624/1975, 120328/1975, 121332/1976, 58022/1979, 70543/1981, 99335/1981, 90842/1984, 129642/1986, 129845/1987, 208191/1994, 5621/1995, 2781/1995, 15809/1996, U.S. Pat. Nos. 5,340,712, 5,369,000, and 5,464,737. 
     The antifoggant may be added in any desired form such as solution, powder or solid particle dispersion. The solid particle dispersion of the antifoggant may be prepared by well-known comminuting means such as ball mills, vibrating ball mills, sand mills, colloidal mills, jet mills, and roller mills. Dispersing aids may be used for facilitating dispersion. 
     It is sometimes advantageous to add a mercury (II) salt to an emulsion layer (or photosensitive layer) as an antifoggant though not necessary in the practice of the invention. Mercury (II) salts preferred to this end are mercury acetate and mercury bromide. The mercury (II) salt is preferably added in an amount of 1×10 −9  mol to 1×10 −3  mol, more referably 1×10 −8  mol to 1×10 −4  mol per mol of silver coated. 
     Still further, the thermographic image forming element of the invention may contain a benzoic acid type compound for the purposes of increasing sensitivity and restraining fog. Any of benzoic acid type compounds may be used although examples of the preferred structure are described in U.S. Pat. Nos. 4,784,939 and 4,152,160, Japanese Patent Application Nos. 98051/1996, 151241/1996, and 151242/1996. The benzoic acid type compound may be added to any site in the image forming element, preferably to a layer on the same side as the photosensitive layer, and more preferably an organic silver salt-containing layer. The benzoic acid type compound may be added at any step in the preparation of a coating solution. Where it is contained in an organic silver salt-containing layer, it may be added at any step from the preparation of the organic silver salt to the preparation of a coating solution, preferably after the preparation of the organic silver salt and immediately before coating. The benzoic acid type compound may be added in any desired form including powder, solution and fine particle dispersion. Alternatively, it may be added in a solution form after mixing it with other additives such as a sensitizing dye, reducing agent and toner. The benzoic acid type compound may be added in any desired amount, preferably 1×10 −6  to 2 mol, more preferably 1×10 −3  to 0.5 mol per mol of silver. 
     In the element of the invention, mercapto, disulfide and thion compounds may be added for the purposes of retarding or accelerating development to control development, improving spectral sensitization efficiency, and improving storage stability before and after development. 
     Where mercapto compounds are used herein, any structure is acceptable. Preferred are structures represented by Ar—S—M and Ar—S—S—Ar wherein M is a hydrogen atom or alkali metal atom, and Ar is an aromatic ring or fused aromatic ring having at least one nitrogen, sulfur, oxygen, selenium or tellurium atom. Preferred hetero-aromatic rings are benzimidazole, naphthimidazole, benzothiazole, naphthothiazole, benzoxazole, naphthoxazole, benzoselenazole, benzotellurazole, imidazole, oxazole, pyrrazole, triazole, thiadiazole, tetrazole, triazine, pyrimidine, pyridazine, pyrazine, pyridine, purine, quinoline and quinazolinone rings. These hetero-aromatic rings may have a substituent selected from the group consisting of halogen (e.g., Br and Cl), hydroxy, amino, carboxy, alkyl groups (having at least 1 carbon atom, preferably 1 to 4 carbon atoms), and alkoxy groups (having at least 1 carbon atom, preferably 1 to 4 carbon atoms), and aryl groups (optionally substituted). Illustrative, non-limiting examples of the mercapto-substituted hetero-aromatic compound include 2-mercaptobenzimidazole, 2-mercaptobenzoxazole, 2-mercapto-benzothiazole, 2-mercapto-5-methylbenzimidazole, 6-ethoxy-2-mercaptobenzothiazole, 2,2′-dithiobis(benzothiazole), 3-mercapto-1,2,4-triazole, 4,5-diphenyl-2-imidazolethiol, 2-mercaptoimidazole, 1-ethyl-2-mercaptobenzimidazole, 2-mercaptoquinoline, 8-mercaptopurine, 2-mercapto-4(3H)-quinazolinone, 7-trifluoromethyl-4-quinolinethiol, 2,3,5,6-tetrachloro-4-pyridinethiol, 4-amino-6-hydroxy-2-mercaptopyrimidine monohydrate, 2-amino-5-mercapto-1,3,4-thiadiazole, 3-amino-5-mercapto-1,2,4-triazole, 4-hydroxy-2-mercaptopyrimidine, 2-mercaptopyrimidine, 4,6-diamino-2-mercaptopyrimidine, 2-mercapto-4-methylpyrimidine hydrochloride, 3-mercapto-5-phenyl-1,2,4-triazole, 1-phenyl-5-mercaptotetrazole, sodium 3-(5-mercaptotetrazole)-benzenesulfonate, N-methyl-N′-{3-(5-mercaptotetrazolyl)-phenyl}urea, and 2-mercapto-4-phenyloxazole. 
     These mercapto compounds are preferably added to the emulsion layer (or photosensitive layer) in amounts of 0.0001 to 1.0 mol, more preferably 0.001 to 0.3 mol per mol of silver. 
     In the photosensitive layer, polyhydric alcohols (e.g., glycerin and diols as described in U.S. Pat. No. 2,960,404), fatty acids and esters thereof as described in U.S. Pat. Nos. 2,588,765 and 3,121,060, and silicone resins as described in BP 955,061 may be added as a plasticizer and lubricant. 
     Protective Layer 
     A surface protective layer may be provided in the image forming element of the present invention for the purpose of preventing sticking of the photosensitive layer or image forming layer. 
     The surface protective layer is based on a binder which may be any desired polymer, although the layer preferably contains 100 mg/m 2  to 5 g/m 2  of a polymer having a carboxylic acid residue. The polymers having a carboxylic acid residue include natural polymers (e.g., gelatin and alginic acid), modified natural polymers (e.g., carboxymethyl cellulose and phthalated gelatin), and synthetic polymers (e.g., polymethacrylate, polyacrylate, polyalkyl methacrylate/acrylate copolymers, and polystyrene/polymethacrylate copolymers). The content of the carboxylic acid residue is preferably 10 mmol to 1.4 mol per 100 grams of the polymer. The carboxylic acid residue may form a salt with an alkali metal ion, alkaline earth metal ion or organic cation. 
     In the surface protective layer, any desired anti-sticking material may be used. Examples of the anti-sticking material include wax, silica particles, styrene-containing elastomeric block copolymers (e.g., styrene-butadiene-styrene and styrene-isoprene-styrene), cellulose acetate, cellulose acetate butyrate, cellulose propionate and mixtures thereof. Crosslinking agents for crosslinking, surfactants for ease of application, and other addenda are optionally added to the surface protective layer. 
     In the photosensitive layer or a protective layer therefor according to the invention, there may be used light absorbing substances and filter dyestuffs as described in U.S. Pat. Nos. 3,253,921, 2,274,782, 2,527,583, and 2,956,879. The dyestuffs may be mordanted as described in U.S. Pat. No. 3,282,699. The filer dyestuffs are used in such amounts that the layer may have an absorbance of 0.1 to 3, especially 0.2 to 1.5 at the exposure wavelength. 
     In the photosensitive layer, a variety of dyestuffs and pigments may be used from the standpoints of improving tone and preventing irradiation. Any desired dyestuffs and pigments may be used in the invention. Useful pigments and dyestuffs include those described in Colour Index and both organic and inorganic, for example, pyrazoloazole dyes, anthraquinone dyes, azo dyes, azomethine dyes, oxonol dyes, carbocyanine dyes, styryl dyes, triphenylmethane dyes, indoaniline dyes, indophenol dyes, and phthalocyanine dyes. The preferred dyes used herein include anthraquinone dyes (e.g., Compounds 1 to 9 described in JP-A 341441/1993 and Compounds 3-6 to 3-18 and 3-23 to 3-38 described in JP-A 165147/1993), azomethine dyes (e.g., Compounds 17 to 47 described in JP-A 341441/1993), indoaniline dyes (e.g., Compounds 11 to 19 described in JP-A 289227/1993, Compound 47 described in JP-A 341441/1993 and Compounds 2-10 to 2-11 described in JP-A 165147/1993), and azo dyes (e.g., Compounds 10 to 16 described in JP-A 341441/1993). The dyes and pigments may be added in any desired form such as solution, emulsion or solid particle dispersion or in a form mordanted with polymeric mordants. The amounts of these compounds used are determined in accordance with the desired absorption although the compounds are generally used in amounts of 1 μg to 1 g per square meter of the element. 
     In one preferred embodiment, the thermographic image forming element of the invention is a one-side image forming element having at least one photosensitive layer containing a silver halide emulsion on one side and a back layer on the other side of the support. 
     The back layer preferably exhibits a maximum absorbance of about 0.3 to 2.0 in the desired wavelength range. When the desired wavelength range is from 750 to 1,400 nm, the back layer is preferably an antihalation layer having an optical density of 0.005 to less than 0.5, especially 0.001 to less than 0.3, in the wavelength range of 750 to 360 nm. When the desired wavelength range is up to 750 nm, the back layer is preferably an antihalation layer having a maximum absorbance of 0.3 to 2.0 at the desired range before image formation and an optical density of 0.005 to less than 0.3 at 360 to 750 nm after image formation. The method of reducing the optical density after image formation to the above-defined range is not critical. For example, the density given by a dye can be reduced by thermal decolorization as described in Belgian Patent No. 733706, or the density is reduced through decolorization by light irradiation as described in JP-A 17833/1979. 
     Where an anti-halation dye is used in the invention, it may be selected from various compounds insofar as it has the desired absorption in the wavelength range, is sufficiently low absorptive in the visible region after processing, and provides the back layer with the preferred absorbance profile. Exemplary antihalation dyes are given below though the dyes are not limited thereto. Useful dyes which are used alone are described in JP-A 56458/1984, 216140/1990, 13295/1995, 11432/1995, U.S. Pat. No. 5,380,635, JP-A 68539/1990, page 13, lower-left column, line 1 to page 14, lower-left column, line 9, and JP-A 24539/1991, page 14, lower-left column to page 16, lower-right column. It is further preferable in the practice of the invention to use a dye which will decolorize during processing. Illustrative, non-limiting, examples of decolorizable dyes are disclosed in JP-A 139136/1977, 132334/1978, 501480/1981, 16060/1982, 68831/1982, 101835/1982, 182436/1984, 36145/1995, 199409/1995, JP-B 33692/1973, 16648/1975, 41734/1990, U.S. Pat. Nos. 4,088,497, 4,283,487, 4,548,896, and 5,187,049. 
     In the practice of the invention, the binder used in the back layer is preferably transparent or translucent and generally colorless. Exemplary binders are naturally occurring polymers, synthetic resins, polymers and copolymers, and other film-forming media, for example, gelatin, gum arabic, poly(vinyl alcohol), hydroxyethyl cellulose, cellulose acetate, cellulose acetate butyrate, poly(vinyl pyrrolidone), casein, starch, poly(acrylic acid), poly(methyl methacrylate), polyvinyl chloride, poly(methacrylic acid), copoly(styrene-maleic anhydride), copoly(styrene-acrylonitrile), copoly(styrene-butadiene), polyvinyl acetals (e.g., polyvinyl formal and polyvinyl butyral), polyesters, polyurethanes, phenoxy resins, poly(vinylidene chloride), polyepoxides, polycarbonates, poly(vinyl acetate), cellulose esters, and polyamides. The binder may be dispersed in water, organic solvent or emulsion to form a dispersion which is coated to form a layer. 
     In the one-side image forming element of the invention, a matte agent may be added to a surface protective layer for the photosensitive emulsion layer and/or the back layer or a surface protective layer therefor for improving transportation. The matte agents used herein are generally microparticulate water-insoluble organic or inorganic compounds. There may be used any desired one of matte agents, for example, well-known matte agents including organic matte agents as described in U.S. Pat. Nos. 1,939,213, 2,701,245, 2,322,037, 3,262,782, 3,539,344, and 3,767,448 and inorganic matte agents as described in U.S. Pat. Nos. 1,260,772, 2,192,241, 3,257,206, 3,370,951, 3,523,022, and 3,769,020. Illustrative examples of the organic compound which can be used as the matte agent are given below; exemplary water-dispersible vinyl polymers include polymethyl acrylate, polymethyl methacrylate, polyacrylonitrile, acrylonitrile-α-methylstyrene copolymers, polystyrene, styrene-divinylbenzene copolymers, polyvinyl acetate, polyethylene carbonate, and polytetrafluoroethylene; exemplary cellulose derivatives include methyl cellulose, cellulose acetate, and cellulose acetate propionate; exemplary starch derivatives include carboxystarch, carboxynitrophenyl starch, urea-formaldehyde-starch reaction products, gelatin hardened with well-known curing agents, and hardened gelatin which has been coaceruvation hardened into microcapsulated hollow particles. Preferred examples of the inorganic compound which can be used as the matte agent include silicon dioxide, titanium dioxide, magnesium dioxide, aluminum oxide, barium sulfate, calcium carbonate, silver chloride and silver bromide desensitized by a well-known method, glass, and diatomaceous earth. The aforementioned matte agents may be used as a mixture of substances of different types if necessary. The size and shape of the matte agent are not critical. The matte agent of any particle size may be used although matte agents having a particle size of 0.1 μm to 30 μm are preferably used in the practice of the invention. The particle size distribution of the matte agent may be either narrow or wide. Nevertheless, since the haze and surface luster of coating are largely affected by the matte agent, it is preferred to adjust the particle size, shape and particle size distribution of a matte agent as desired during preparation of the matte agent or by mixing plural matte agents. 
     In one preferred embodiment of the invention, the matte agent is added to the back layer. The back layer should preferably have a degree of matte as expressed by a Bekk smoothness of 10 to 1,200 seconds, more preferably 50 to 700 seconds. 
     In the image forming element of the invention, the matte agent is preferably contained in an outermost surface layer, a layer functioning as an outermost surface layer, a layer close to the outer surface or a layer functioning as a so-called protective layer. The emulsion layer side protective layer may have any degree of matte insofar as no star dust failures occur although a Bekk smoothness of 500 to 10,000 seconds, especially 500 to 2,000 seconds is preferred. 
     The photothermographic emulsion used in the photothermographic element according to the invention is contained in one or more layers on a support. In the event of single layer construction, it should contain an organic silver salt, silver halide, developing agent, and binder, and other optional additives such as a toner, coating aid and other auxiliary agents. In the event of two-layer construction, a first emulsion layer which is generally a layer disposed adjacent to the support should contain an organic silver salt and silver halide and a second emulsion layer or both the layers contain other components. Also envisioned herein is a two-layer construction consisting of a single emulsion layer containing all the components and a protective topcoat. In the case of multi-color sensitive photothermographic element, a combination of such two layers may be employed for each color. Also a single layer may contain all necessary components as described in U.S. Pat. No. 4,708,928. In the case of multi-dye, multi-color sensitive photothermographic element, emulsion (or photosensitive) layers are distinctly supported by providing a functional or non-functional barrier layer therebetween as described in U.S. Pat. No. 4,460,681. 
     A backside resistive heating layer as described in U.S. Pat. Nos. 4,460,681 and 4,374,921 may be used in a photothermographic image forming system according to the present invention. 
     According to the invention, a hardener may be used in various layers including a photosensitive layer, protective layer, and back layer. Examples of the hardener include polyisocyanates as described in U.S. Pat. No. 4,281,060 and JP-A 208193/1994, epoxy compounds as described in U.S. Pat. No. 4,791,042, and vinyl sulfones as described in JP-A 89048/1987. 
     A surfactant may be used for the purposes of improving coating and electric charging properties. The surfactants used herein may be nonionic, anionic, cationic and fluorinated ones. Examples include fluorinated polymer surfactants as described in JP-A 170950/1987 and U.S. Pat. No. 5,380,644, fluorochemical surfactants as described in JP-A 244945/1985 and 188135/1988, polysiloxane surfactants as described in U.S. Pat. No. 3,885,965, and polyalkylene oxide and anionic surfactants as described in JP-A 301140/1994. 
     Support 
     According to the invention, the photothermographic emulsion may be coated on a variety of supports. Typical supports include polyester film, subbed polyester film, poly(ethylene terephthalate) film, polyethylene naphthalate film, cellulose nitrate film, cellulose ester film, poly(vinyl acetal) film, polycarbonate film and related or resinous materials, as well as glass, paper, metals, etc. Often used are flexible substrates, typically paper supports, specifically baryta paper and paper supports coated with partially acetylated α-olefin polymers, especially polymers of α-olefins having 2 to 10 carbon atoms such as polyethylene, polypropylene, and ethylene-butene copolymers. The supports are either transparent or opaque, preferably transparent. Of these, biaxially oriented polyethylene terephthalate (PET) films of about 75 to 200 μm thick are preferred. 
     When plastic film is passed through a thermographic processor where it will encounter a temperature of at least 80° C., the film experiences dimensional shrinkage or expansion. When the thermographic element as processed is intended for printing plate purposes, this dimensional shrinkage or expansion gives rise to a serious problem against precision multi-color printing. Therefore, the invention favors the use of a film experiencing a minimal dimensional change, that is, a film which has been biaxially stretched and then properly treated for mitigating the internal distortion left after stretching and for preventing distortion from being generated by thermal shrinkage during subsequent heat development. One exemplary material is polyethylene terephthalate (PET) film which has been heat treated at 100 to 210° C. prior to the coating of a photothermographic emulsion. Also useful are materials having a high glass transition temperature (Tg), for example, polyether ethyl ketone, polystyrene, polysulfone, polyether sulfone, polyarylate, and polycarbonate. 
     For antistatic purpose, the thermographic image forming element of the invention may be provided with a layer containing soluble salts (e.g., chlorides and nitrates), an evaporated metal layer, or a layer containing ionic polymers as described in U.S. Pat. Nos. 2,861,056 and 3,206,312, insoluble inorganic salts as described in U.S. Pat. No. 3,428,451, or tin oxide microparticulates as described in JP-A 252349/1985 and 104931/1982. 
     A method for producing color images using the thermographic image forming element of the invention is as described in JP-A 13295/1995, page 10, left column, line 43 to page 11, left column, line 40. Stabilizers for color dye images are exemplified in BP 1,326,889, U.S. Pat. Nos. 3,432,300, 3,698,909, 3,574,627, 3,573,050, 3,764,337, and 4,042,394. 
     In the practice of the invention, the photothermographic emulsion can be applied by various coating procedures including dip coating, air knife coating, flow coating, and extrusion coating using a hopper of the type described in U.S. Pat. No. 2,681,294. If desired, two or more layers may be concurrently coated by the methods described in U.S. Pat. No. 2,761,791 and BP 837,095. 
     In the thermographic image forming element of the invention, there may be contained additional layers, for example, a dye accepting layer for accepting a mobile dye image, an opacifying layer when reflection printing is desired, a protective topcoat layer, and a primer layer well known in the photothermographic art. The image forming element of the invention is preferably such that only a single sheet of the image forming element can form an image. That is, it is preferred that a functional layer necessary to form an image such as an image receiving layer does not constitute a separate member. 
     The thermographic image forming element of the invention may be developed by any desired method although it is generally developed by heating after imagewise exposure. Preferred examples of the heat developing machine used include heat developing machines of the contact type wherein the thermographic image forming element is contacted with a heat source in the form of a heat roller or heat drum as described in JP-B 56499/1993, Japanese Patent No. 684453, JP-A 292695/1997, 297385/1997, and WO 95/30934; and heat developing machines of the non-contact type as described in JP-A 13294/1995, WO 97/28489, 97/28488, and 97/28487. The heat developing machines of the non-contact type are especially preferred examples. The preferred developing temperature is about 80 to 250° C., more preferably 100 to 140° C. The preferred developing time is about 1 to 180 seconds, more preferably about 10 to 90 seconds. 
     One effective means for preventing the thermographic image forming element from experiencing process variations due to dimensional changes during heat development is a method (known as a multi-stage heating method) of heating the element at a temperature of 80° C. to less than 115° C. (preferably up to 113° C.) for at least 5 seconds so that no images are developed and thereafter, heating at a temperature of at least 110° C. (preferably up to 130° C.) for heat development to form images. 
     Any desired technique may be used for the exposure of the image forming element of the invention. The preferred light source for exposure is a laser, for example, a gas laser, YAG laser, dye laser or semiconductor laser. A semiconductor laser combined with a second harmonic generating device is also useful. 
     Owing to low haze upon exposure, the image forming element of the invention tends to generate interference fringes. Known techniques for preventing generation of interference fringes are a technique of obliquely directing laser light to a image forming element as disclosed in JP-A 113548/1993 and the utilization of a multi-mode laser as disclosed in WO 95/31754. Exposure is preferably carried out in combination with these techniques. 
     Upon exposure of the image forming element of the invention, exposure is preferably made by overlapping laser light so that no scanning lines are visible, as disclosed in SPIE, Vol. 169, Laser Printing 116-128 (1979), JP-A 51043/1992, and WO 95/31754. 
     Developing Apparatus 
     Referring to FIG. 1, there is schematically illustrated one exemplary heat developing apparatus for use in the processing of the thermographic image forming element according to the invention. FIG. 1 is a side elevation of the heat developing apparatus which includes a cylindrical heat drum  2  having a halogen lamp  1  received therein as a heating means, and an endless belt  4  trained around a plurality of feed rollers  3  so that a portion of the belt  4  is in close contact with the drum  2 . A length of thermographic image forming element  5  is fed and guided by pairs of guide rollers to between the heat drum  2  and the belt  4 . The element  5  is fed forward while it is clamped between the heat drum  2  and the belt  4 . While the element  5  is fed forward, it is heated to the developing temperature whereby it is heat developed. In the heat developing apparatus of the drum type, the luminous intensity distribution of the lamp is optimized so that the temperature in the transverse direction may be precisely controlled. 
     The element  5  exits at an exit  6  from between the heat drum  2  and the belt  4  where the element is released from bending by the circumferential surface of the heat drum  2 . A correcting guide plate  7  is disposed in the vicinity of the exit  6  for correcting the element  5  into a planar shape. A zone surrounding the guide plate  7  is temperature adjusted so that the temperature of the element  5  may not lower below the predetermined level. 
     Disposed downstream of the exit  6  are a pair of feed rollers  8 . A pair of planar guide plates  9  are disposed downstream of and adjacent to the feed rollers  8  for guiding the element  5  while keeping it planar. Another pair of feed rollers  10  are disposed downstream of and adjacent to the guide plates  9 . The planar guide plates  9  have such a length that the element  5  is fully cooled, typically below 30° C., while it passes over the plates  9 . The means associated with the guide plates  9  for cooling the element  5  are cooling fans  11 . 
     Although the belt conveyor type heat developing apparatus has been described, the invention is not limited thereto. Use may be made of heat developing apparatus of varying constructions such as disclosed in JP-A 13294/1995. In the case of a multi-stage heating mode which is preferably used in the practice of the invention, two or more heat sources having different heating temperatures are disposed in the illustrated apparatus so that the element may be continuously heated to different temperatures. 
     EXAMPLE 
     Examples of the invention are given below by way of illustration and not by way of limitation. 
     Example 1 
     Silver Halide Emulsion A 
     In 700 ml of water were dissolved 11 g of phthalated gelatin, 30 mg of potassium bromide, and 10 mg of sodium benzenethiosulfonate. The solution was adjusted to pH 5.0 at a temperature of 55° C. To the solution, 159 ml of an aqueous solution containing 18.6 g of silver nitrate and an aqueous solution containing 1 mol/liter of potassium bromide were added over 6-½ minutes by the controlled double jet method while maintaining the solution at pAg 7.7. Then, 476 ml of an aqueous solution containing 55.5 g of silver nitrate and an aqueous halide solution containing  1  mol/liter of potassium bromide were added over 28-½ minutes by the controlled double jet method while maintaining the solution at pAg 7.7. Thereafter, the pH of the solution was lowered to cause flocculation and sedimentation for desalting. Further, 0.17 g of Compound A and 23.7 g of deionized gelatin (calcium content below 20 ppm) were added to the solution, which was adjusted to pH 5.9 and pAg 8.0. There were obtained cubic grains of silver halide having a mean grain size of 0.11 μm, a coefficient of variation of the projected area of 8%, and a (100) face proportion of 93%. 
     The thus obtained silver halide grains were heated at 60° C., to which 76 μmol of sodium benzenethiosulfate was added per mol of silver. After  3  minutes, 154 μmol of sodium thiosulfate was added and the emulsion was ripened for 100 minutes. 
     Thereafter, the emulsion was maintained at 40° C., and with stirring, 6.4×10 −4  mol of Sensitizing Dye A and 6.4×10 −3  mol of Compound B were added per mol of silver halide. After 20 minutes, the emulsion was quenched to 30° C., completing the preparation of a silver halide emulsion A.                    
     Preparation of Organic Acid Silver Dispersion 
     Organic acid silver A 
     While a mixture of 4.4 g of arachic acid, 39.4 g of behenic acid, and 770 ml of distilled water was stirred at 85° C., 103 ml of 1N NaOH aqueous solution was added over 60 minutes. Reaction was carried out for 240 minutes. The solution was cooled to 75° C. Next, 112.5 ml of an aqueous solution containing 19.2 g of silver nitrate was added over 45 seconds to the solution, which was left to stand for 20 minutes and cooled to 30° C. Thereafter, the solids were separated by suction filtration and washed with water until the water filtrate reached a conductivity of 30 μS/cm. The thus obtained solids were handled as a wet cake without drying. To 100 g as dry solids of the wet cake, 5 g of polyvinyl alcohol PVA-205 (Kurare K.K.) and water were added to a total weight of 500 g. This was pre-dispersed in a homomixer. 
     The pre-dispersed liquid was processed three times by a dispersing machine Micro-Fluidizer M-110S-EH (with G10Z interaction chamber, manufactured by Microfluidex International Corporation) which was operated under a pressure of 1,750 kg/cm 2 . There was obtained an organic acid silver dispersion A. The organic acid silver grains in this dispersion were acicular grains having a mean minor axis (or breadth) of 0.04 μm, a mean major axis (or length) of 0.8 μm, and a coefficient of variation of 30%. It is noted that particle dimensions were measured by Master Sizer X (Malvern Instruments Ltd.). The desired dispersion temperature was set by mounting serpentine heat exchangers at the front and rear sides of the interaction chamber and adjusting the temperature of refrigerant. 
     Organic acid silver B 
     A mixture of 4.4 g of arachic acid, 39.4 g of behenic acid, 700 ml of distilled water, 70 ml of tert-butanol, and 123 ml of 1N NaOH aqueous solution was stirred at 75° C. for one hour for reaction. The solution was cooled to 65° C. Next, 112.5 ml of an aqueous solution containing 19.2 g of silver nitrate was added over 45 seconds to the solution, which was left to stand for 5 minutes and cooled to 30° C. Thereafter, the solids were separated by suction filtration and washed with water until the water filtrate reached a conductivity of 30 μS/cm. Thereafter, the solids were dispersed as in organic acid silver A, obtaining an organic acid silver dispersion B. The organic acid silver grains in this dispersion were acicular grains having a mean breadth of 0.04 μm, a mean length of 0.8 μm, and a coefficient of variation of 30%. 
     Organic acid silver C 
     A mixture of 4.4 g of arachic acid, 39.4 g of behenic acid, 700 ml of distilled water, 70 ml of tert-butanol, and 123 ml of 1N NaOH aqueous solution was stirred at 85° C. for 120 minutes for reaction. Cooling to 75° C. gave an organic acid alkali metal salt solution. A reactor was charged with 600 ml of distilled water and 25 ml of tert-butanol and kept at 35° C. The organic acid alkali metal salt solution prepared just above and 112.5 ml of a silver nitrate aqueous solution containing 19.2 g of silver nitrate and 20 cc of iN nitric acid were added to the reactor over 5 minutes. In this step, the addition of the silver nitrate solution was started 30 seconds prior to the addition of the organic acid alkali metal salt solution (precedence 10%) and at a constant rate, and the addition of the organic acid alkali metal salt solution was completed after 30 seconds from the end of addition of the silver nitrate solution. After the completion of addition, the reaction solution was allowed to stand for 20 minutes and then cooled to 30° C. Thereafter, the solids were separated by suction filtration and washed with water until the water filtrate reached a conductivity of 30 μS/cm. Thereafter, the solids were dispersed as in organic acid silver A, obtaining an organic acid silver dispersion C. The organic acid silver grains in this dispersion were flake grains having a mean breadth of 0.04 μm, a mean length of 0.4 μm, and a coefficient of variation of 20%. 
     It is noted that the precedence is an amount of one solution added in advance, expressed in percent by volume based on the entire amount. 
     Organic acid silver D 
     A mixture of 4.4 g of arachic acid, 39.4 g of behenic acid, 700 ml of distilled water, 70 ml of tert-butanol, and 123 ml of 1N NaOH aqueous solution was stirred at 85° C. for 120 minutes for reaction. Cooling to 75° C. gave an organic acid alkali metal salt solution. A reactor was charged with 600 ml of distilled water and 25 ml of tert-butanol and kept at 35° C. The organic acid alkali metal salt solution prepared just above and 112.5 ml of an aqueous solution containing 22 g of silver nitrate were added to the reactor over 5 minutes. In this step, the addition of the silver nitrate solution was started 30 seconds prior to the addition of the organic acid alkali metal salt solution (precedence 10%) and at a constant rate, and the addition of the organic acid alkali metal salt solution was completed after 30 seconds from the end of addition of the silver nitrate solution. After the completion of addition, the reaction solution was allowed to stand for 20 minutes and then cooled to 30° C. Thereafter, the solids were separated by suction filtration and washed with water until the water filtrate reached a conductivity of 30 μS/cm. Thereafter, the solids were dispersed as in organic acid silver A, obtaining an organic acid silver dispersion D. The organic acid silver grains in this dispersion were flake grains having a mean breadth of 0.04 μm, a mean length of 0.4 μm, and a coefficient of variation of 20%. 
     Solid Particle Dispersion of 1,1-bis(2-hydroxy-3,5-dimethylphenyl)-3,5,5-trimethylhexane 
     To 20 g of 1,1-bis(2-hydroxy-3,5-dimethylphenyl)-3,5,5-trimethylhexane were added 3.0 g of polyvinyl alcohol MP-203 (Kurare K.K.) and 77 ml of water. They were thoroughly agitated to form a slurry, which was allowed to stand for 3 hours. A vessel was charged with the slurry together with 360 g of zirconia beads having a mean diameter of 0.5 mm. A dispersing machine 1/4G Sand Grinder Mill (Imex K.K.) was operated for 3 hours for dispersion, obtaining a solid particle dispersion of the reducing agent in which particles with a diameter of 0.3 to 1.0 μm accounted for 80% by weight. 
     Solid Particle Dispersion of Tribromomethylphenylsulfone 
     To 30 g of tribromomethylphenylsulfone were added 0.5 g of hydroxypropylmethyl cellulose, 0.5 g of Compound C, and 88.5 g of water. They were thoroughly agitated to form a slurry, which was allowed to stand for 3 hours. Following the steps used in the preparation of the solid particle dispersion of the reducing agent, a solid particle dispersion of the antifoggant was prepared in which particles with a diameter of 0.3 to 1.0 μm accounted for 80% by weight. 
     Emulsion Layer Coating Solution 
     To each of the above-prepared organic acid silver microcrystalline dispersions (corresponding to 1 mol of silver) were added the above-prepared silver halide emulsion A and the binder and addenda described below. Water was added thereto to form an emulsion layer coating solution. 
     
       
         
           
               
               
               
             
               
                   
               
             
            
               
                 LACSTAR 3307B binder (SBR latex, Tg 17° C., 
                 470 
                 g 
               
               
                 Dai-Nippon Ink &amp; Chemicals K. K.) as solids 
               
               
                 1,1-bis(2-hydroxy-3,5-dimethylphenyl)- 
                 110 
                 g 
               
               
                 3,5,5-trimethylhexane as solids 
               
               
                 Tribromomethylphenylsulfone as solids 
                 25 
                 g 
               
               
                 Sodium benzenethiosulfonate 
                 0.25 
                 g 
               
               
                 Polyvinyl alcohol MP-203 (Kurare K. K.) 
                 46 
                 g 
               
               
                 6-Isobutylphthalazine 
                 0.12 
                 mol 
               
            
           
           
               
               
            
               
                 Nucleating agent (Table 23) 
                 (Table 23) 
               
            
           
           
               
               
               
            
               
                 Dyestuff A 
                 0.62 
                 g 
               
               
                 Silver halide emulsion A as Ag 
                 0.05 
                 mol 
               
               
                   
               
            
           
         
       
     
     
       
         
         
             
             
         
       
     
     Emulsion Surface Protective Layer Coating Solution 
     A surface protective layer coating solution was prepared by adding 3.75 g of H 2 O to 109 g of a polymer latex having a solids content of 27.5% (methyl methacrylate/styrene/2-ethylhexyl acrylate/2-hydroxyethyl methacrylate/methacrylic acid=59/9/26/5/1 copolymer, Tg 55° C.), then adding 4.5 g of benzyl alcohol as a film-forming aid, 0.45 g of Compound D, 0.125 g of Compound E, 0.0125 mol of Compound F, and 2.25 g of polyvinyl alcohol PVA-217 (Kurare K.K.), and diluting with water to a total weight of 150 g.                    
     PET Supports With Back and Undercoat Layers 
     (1) Support 
     Using terephthalic acid and ethylene glycol, a polyethylene terephthalate (PET) having an intrinsic viscosity of 0.66 as measured in a phenol/tetrachloroethane 6/4 (weight ratio) mixture at 25° C. was prepared in a conventional manner. After the PET was pelletized and dried at 130° C. for 4 hours, it was melted at 300° C., extruded through a T-shaped die, and quenched to form an unstretched film having a thickness sufficient to give a thickness of 120 μm after thermosetting. 
     The film was longitudinally stretched by a factor of 3.3 by means of rollers rotating at different circumferential speeds and then transversely stretched by a factor of 4.5 by means of a tenter. The temperatures in these stretching steps were 110° C. and 130° C., respectively. Thereafter, the film was thermoset at 240° C. for 20 seconds and then transversely relaxed 4% at the same temperature. Thereafter, with the chuck of the tenter being slit and the opposite edges being knurled, the film was taken up under a tension of 4.8 kg/cm 2 . In this way, a film of 2.4 m wide, 3,500 m long and 120 μm thick was obtained in a roll form. 
     (2) Undercoat layer 
     
       
         
           
               
               
               
             
               
                   
               
             
            
               
                 Undercoat layer (a) 
                   
                   
               
               
                 Polymer latex (styrene/butadiene/ 
                 160 
                 mg/m 2   
               
               
                 hydroxyethyl methacrylate/ 
               
               
                 divinyl benzene = 
               
               
                 67/30/2.5/0.5 wt %) 
               
               
                 2,4-dichloro-6-hydroxy-s-triazine 
                 4 
                 mg/m 2   
               
               
                 Matte agent (polystyrene, 
                 3 
                 mg/m 2   
               
               
                 mean particle size 2.4 μm) 
               
               
                 Undercoat layer (b) 
               
               
                 Alkali-treated gelatin 
                 50 
                 mg/m 2   
               
               
                 (Ca 2+  content 30 ppm, 
               
               
                 jelly strength 230 g) 
               
            
           
           
               
               
            
               
                 Dyestuff A 
                 a coverage to give an optical 
               
               
                   
                 density of 0.7 at 780 nm 
               
            
           
           
               
               
               
            
               
                 (3) Conductive layer 
                   
                   
               
               
                 Jurimer ET410 (Nippon Junyaku K. K.) 
                 38 
                 mg/m 2   
               
               
                 SnO 2 /Sb (9/1 weight ratio, 
                 120 
                 mg/m 2   
               
               
                 mean particle size 0.25 μm) 
               
               
                 Matte agent (polymethyl methacrylate, 
                 7 
                 mg/m 2   
               
               
                 mean particle size 5 μm) 
               
               
                 Melamine 
                 13 
                 mg/m 2   
               
               
                 (4) Protective layer 
               
               
                 Chemipearl S-120 (Mitsui 
                 500 
                 mg/m 2   
               
               
                 Petro-Chemical K. K.) 
               
               
                 Snowtex C (Nissan Chemical K. K.) 
                 40 
                 mg/m 2   
               
               
                 Denacol EX614B (Nagase 
                 30 
                 mg/m 2   
               
               
                 Chemicals K. K.) 
               
               
                   
               
            
           
         
       
     
     The undercoat layer (a) and the undercoat layer (b) were successively coated on each surface of the PET support and respectively dried at 180° C. for 4 minutes. Then, the conductive layer and the protective layer were successively coated on one surface of the support where undercoat layers (a) and (b) had been coated, and respectively dried at 180° C. for 4 minutes, completing the PET support having the back and undercoat layers. 
     The thus prepared PET support having back coated and undercoated sides was passed through a heat treating zone having an overall length of 200 m and set at 200° C. at a feed speed of 20 m/min under a tension of 3 kg/m 2 . Thereafter, the support was passed through a zone set at 40° C. for 15 seconds and taken up into a roll under a tension of 10 kg/cm 2 . 
     Thermographic Image Forming Element 
     The emulsion layer coating solution was applied onto the undercoat layer of the PET support having back-coated and undercoated sides to a silver coverage of 1.6 g/m 2 . The emulsion surface protective layer coating solution was applied thereon so that the coverage of the polymer latex (as solids) was 2.0 g/m 2 , obtaining thermographic image forming element samples. 
     Photographic Properties 
     The coated samples were exposed to xenon flash light for an emission time of 10 −6  sec through an interference filter having a peak at 780 nm and a step wedge. 
     Using the heat developing apparatus shown in FIG. 1, the exposed samples were heat developed at 120° C. for 15 seconds. In the heat developing apparatus of the drum type, the luminous intensity distribution of the lamp was optimized so that the temperature in the transverse direction might be controlled to a variation within ±1° C. A zone surrounding the guide plate  7  was temperature adjusted so that the temperature of the image forming element  5  might not be below 90° C. 
     The resulting images were measured for visible density by a Macbeth TD904 densitometer. The results of measurement were evaluated in terms of Dmin, sensitivity and contrast. The sensitivity is the reciprocal of a ratio of the exposure providing a density of Dmin+1.0 and expressed in a relative value based on a sensitivity of 100 for image forming element sample No. 1. The contrast was expressed by the gradient of a straight line connecting density points 0.3 and 3.0 in a graph wherein the logarithm of the exposure is on the abscissa. 
     The results are shown in Table 23. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 23 
               
             
            
               
                   
               
               
                   
                   
                 Nucleating 
                   
               
               
                 Sam- 
                 Organic 
                 agent 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 ple 
                 acid 
                   
                 Amount 
                   
                   
                   
                   
               
               
                 No. 
                 silver 
                 Type 
                 (mg/m 2 ) 
                 Sensitivity 
                 Contrast 
                 Dmin 
                 Remarks 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 1 
                 A 
                 — 
                 — 
                 100 
                  3 
                 0.15 
                   
               
               
                   
                   
                   
                   
                 (standard) 
               
               
                 2 
                 B 
                 — 
                 — 
                 102 
                  4 
                 0.12 
               
               
                 3 
                 C 
                 — 
                 — 
                 102 
                  6 
                 0.10 
               
               
                 4 
                 D 
                 — 
                 — 
                 105 
                  6 
                 0.10 
               
               
                 5 
                 A 
                 C-1 
                 50 
                 200 
                 unrated 
                 0.50 
               
               
                 6 
                 B 
                 C-1 
                 50 
                 210 
                  8 
                 0.25 
               
               
                 7 
                 C 
                 C-1 
                 50 
                 210 
                 15 
                 0.12 
                 Invention 
               
               
                 8 
                 D 
                 C-1 
                 50 
                 220 
                 17 
                 0.12 
                 Invention 
               
               
                 9 
                 A 
                 C-42 
                 50 
                 190 
                 unrated 
                 0.60 
               
               
                 10 
                 B 
                 C-42 
                 50 
                 200 
                  6 
                 0.28 
               
               
                 11 
                 C 
                 C-42 
                 50 
                 200 
                 15 
                 0.12 
                 Invention 
               
               
                 12 
                 D 
                 C-42 
                 50 
                 210 
                 17 
                 0.12 
                 Invention 
               
               
                 13 
                 A 
                 C-8 
                 50 
                 180 
                 unrated 
                 0.35 
               
               
                 14 
                 B 
                 C-8 
                 50 
                 190 
                  7 
                 0.20 
               
               
                 15 
                 C 
                 C-8 
                 50 
                 190 
                 14 
                 0.11 
                 Invention 
               
               
                 16 
                 D 
                 C-8 
                 50 
                 200 
                 16 
                 0.11 
                 Invention 
               
               
                 17 
                 A 
                 C-57 
                 50 
                 180 
                 unrated 
                 0.35 
               
               
                 18 
                 B 
                 C-57 
                 50 
                 190 
                  8 
                 0.25 
               
               
                 19 
                 C 
                 C-57 
                 50 
                 190 
                 15 
                 0.11 
                 Invention 
               
               
                 20 
                 D 
                 C-57 
                 50 
                 200 
                 17 
                 0.11 
                 Invention 
               
               
                 21 
                 A 
                 54a 
                 20 
                 200 
                 unrated 
                 0.50 
               
               
                 22 
                 B 
                 54a 
                 20 
                 210 
                  7 
                 0.25 
               
               
                 23 
                 C 
                 54a 
                 20 
                 210 
                 13 
                 0.11 
                 Invention 
               
               
                 24 
                 D 
                 54a 
                 20 
                 220 
                 16 
                 0.11 
                 Invention 
               
               
                   
               
            
           
         
       
     
     The samples within the scope of the invention show favorable characteristics including a low fog, high contrast, and high sensitivity. 
     Example 2 
     Samples were prepared as in Example 1, using the following silver halide emulsion B instead of silver halide emulsion A in Example 1. 
     Silver Halide Emulsion B 
     In 1000 ml of water were dissolved 27 g of phthalated gelatin, 1.8 g of sodium chloride, and 10 mg of sodium thiosulfonate. The solution was adjusted to pH 5.0 at a temperature of 40° C. To the solution, 120 ml of an aqueous solution containing 60 g of silver nitrate and 120 ml of an aqueous halide solution A containing 21.6 g of sodium chloride and 1×10 −6  mol per mol of the silver nitrate of K 2 Rh(H 2 O)Cl 5  were added over 4 minutes by the controlled double jet method. Then, 30 ml of an aqueous solution containing 15 g of silver nitrate and 30 ml of an aqueous halide solution B containing 4.35 g of sodium chloride, 2.13 g of potassium bromide were added over 2 minutes. Thereafter, 2 g of 4-hydroxy-6-methyl-1,3,3a,7-tetrazaindene was added thereto and the pH of the solution was lowered to cause flocculation and sedimentation for desalting. Further, 0.1 g of phenoxyethanol was added to the solution, which was adjusted to pH 5.9 and pAg 7.6. There were obtained cubic grains of silver chlorobromide having a silver bromide content of 6 mol %, a mean grain size of 0.12 μm, a coefficient of variation of the projected area of 8%. 
     The thus obtained silver halide grains were heated at 60° C., to which 85 μmol of sodium thiosulfate, 11 μmol of 2,3,4,5,6-pentafluorophenyldiphenylphosphine selenide, 15 μmol of a tellurium compound, and 120 μmol of chloroauric acid were added per mol of silver. The emulsion was ripened for 120 minutes and quenched to 40° C. Further, 100 μmol of Sensitizing Dye A, 500 μmol of 2-mercapto-5-methylbenzimidazole, 500 μmol of N-methyl-N′-{3-(5-mercaptotetrazolyl)phenyl}urea, 500 μmol of Compound A, and 500 μmol of Compound B were added to the emulsion, which was quenched to 30° C., obtaining a silver halide emulsion B. 
     The samples were similarly rated as in Example 1. The samples within the scope of the invention showed favorable characteristics. 
     Example 3 
     The heat developing apparatus used in Example 1 was modified by incorporating two heat sources in accordance with the construction of the heat developing apparatus shown in FIG. 3 of JP-A 13294/1995 such that the sample might be heated in two continuous stages. The exposed samples were developed through this heat developing apparatus in accordance with the following schedule. 
     Schedule (1): 
     The sample was heated at 70° C. for 10 seconds and then at 115° C. for 30 seconds. 
     Schedule (2): 
     The sample was heated at 90° C. for 10 seconds and then at 115° C. for 30 seconds. 
     Schedule (3): 
     The sample was heated at 105° C. for 10 seconds (conditions under which no images were developed) and then at 115° C. for 30 seconds. 
     Schedule (4): 
     The sample was heated at 105° C. for 2 seconds and then at 115° C. for 30 seconds. 
     Schedule (5): 
     The sample was heated at 115° C. for 15 seconds (conditions under which images were developed) and then at 115° C. for 15 seconds. 
     The samples were similarly rated as in Example 1. The samples within the scope of the invention showed favorable characteristics. 
     There have been described photothermographic image forming elements containing a non-photosensitive silver salt which has been formed by substantially simultaneous addition of a silver nitrate solution and an organic acid alkali metal salt solution and further containing a nucleating agent. The elements produce low fog, high contrast images. 
     Japanese Patent Application No. 94074/1998 is incorporated herein by reference. 
     Reasonable modifications and variations are possible from the foregoing disclosure without departing from either the spirit or scope of the present invention as defined by the claims.