Title of Invention: Thermally developable emulsions and materials containing triazine-thione compounds Abstract: Thermally developable compositions such thermographic and photothermographic emulsions include certain triazine-thione compounds. These emulsions can be used in thermally developable materials such as thermographic and photothermographic materials to provide increased image density and shortened development time, and to allow development at lower temperatures. Such materials can have imaging layers on one or both sides of the support. Detailed Description: Part general_description_paragraphs: Part CROSS-REFERENCE TO RELATED APPLICATION: This is a Divisional of application Ser. No. 10/341,754, filed Jan. 14, 2003, now, U.S. Pat. No. 6,703,191. Part FIELD OF THE INVENTION: This invention relates to thermally developable compositions and imaging materials comprising certain triazine-thione compounds. In particular, the invention relates to thermographic and photothermographic materials containing the triazine-thione compounds. The invention also relates to methods of imaging the thermally developable materials. Part BACKGROUND OF THE INVENTION: Silver-containing thermographic and photothermographic imaging materials (that is, thermally developable imaging materials) that are imaged and/or developed using heat and without liquid processing have been known in the art for many years. Silver-containing thermographic imaging materials are non-photosensitive materials that are used in a recording process wherein images are generated by the use of thermal energy. These materials generally comprise a support having disposed thereon (a) a relatively or completely non-photosensitive source of reducible silver ions, (b) a reducing composition (usually including a developer) for the reducible silver ions, and (c) a suitable hydrophilic or hydrophobic binder. In a typical thermographic construction, the image-forming layers are based on silver salts of long chain fatty acids. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. At elevated temperatures, the silver of the silver carboxylate is reduced by a reducing agent for silver ion such as methyl gallate, hydroquinone, substituted-hydroquinones, hindered phenols, catechols, pyrogallol, ascorbic acid, and ascorbic acid derivatives, whereby an image of elemental silver is formed. Some thermographic constructions are imaged by contacting them with the thermal head of a thermographic recording apparatus such as a thermal printer or thermal facsimile. In such constructions, an anti-stick layer is coated on top of the imaging layer to prevent sticking of the thermographic construction to the thermal head of the apparatus utilized. The resulting thermographic construction is then heated to an elevated temperature, typically in the range of from about 60 to about 225° C., resulting in the formation of an image. Silver-containing photothermographic imaging materials are photosensitive materials that are used in a recording process wherein an image is formed by imagewise exposure of the photothermographic material to specific electromagnetic radiation (for example, X-radiation, or ultraviolet, visible, or infrared radiation) and developed by the use of thermal energy. These materials, also known as “dry silver” materials, generally comprise a support having coated thereon: (a) a photocatalyst (that is, a photosensitive compound such as silver halide) that upon such exposure provides a latent image in exposed grains that are capable of acting as a catalyst for the subsequent formation of a silver image in a development step, (b) a relatively or completely non-photosensitive source of reducible silver ions, (c) a reducing composition (usually including a developer) for the reducible silver ions, and (d) a hydrophilic or hydrophobic binder. The latent image is then developed by application of thermal energy. In such materials, the photosensitive catalyst is generally a photographic type photosensitive silver halide that is considered to be in catalytic proximity to the non-photosensitive source of reducible silver ions. Catalytic proximity requires intimate physical association of these two components either prior to or during the thermal image development process so that when silver atoms (Ag0)n, also known as silver specks, clusters, nuclei, or latent image, are generated by irradiation or light exposure of the photosensitive silver halide, those silver atoms are able to catalyze the reduction of the reducible silver ions within a catalytic sphere of influence around the silver atoms [D. H. Klosterboer, Imaging Processes and Materials, (Neblette's Eighth Edition), J. Sturge, V. Walworth, and A. Shepp, Eds., Van Nostrand-Reinhold, New York, 1989, Chapter 9, pp. 279-291]. It has long been understood that silver atoms act as a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed in catalytic proximity with the non-photosensitive source of reducible silver ions in a number of different ways (see, for example, Research Disclosure, June 1978, item 17029). Other photosensitive materials, such as titanium dioxide, cadmium sulfide, and zinc oxide have also been reported to be useful in place of silver halide as the photocatalyst in photothermographic materials [see for example, Shepard, J. Appl. Photog. Eng. 1982, 8(5), 210-212, Shigeo et al., Nippon Kagaku Kaishi, 1994, 11, 992-997, and FR 2,254,047 (Robillard)]. The photosensitive silver halide may be made “in situ,” for example by mixing an organic or inorganic halide-containing source with a source of reducible silver ions to achieve partial metathesis and thus causing the in situ formation of silver halide (AgX) grains throughout the silver source [see, for example, U.S. Pat. No. 3,457,075 (Morgan et al.)]. In addition, photosensitive silver halides and sources of reducible silver ions can be coprecipitated [see Yu. E. Usanov et al., J. Imag. Sci. Tech. 1996, 40, 104]. Alternatively, a portion of the reducible silver ions can be completely converted to silver halide, and that portion can be added back to the source of reducible silver ions (see Yu. E. Usanov et al., International Conference on Imaging Science, 7-11 Sep. 1998). The silver halide may also be “preformed” and prepared by an “ex situ” process whereby the silver halide (AgX) grains are prepared and grown separately. With this technique, one has the possibility of controlling the grain size, grain size distribution, dopant levels, and composition much more precisely, so that one can impart more specific properties to both the silver halide grains and the photothermographic material. The preformed silver halide grains may be introduced prior to and be present during the formation of the source of reducible silver ions. Co-precipitation of the silver halide and the source of reducible silver ions provides a more intimate mixture of the two materials [see for example U.S. Pat. No. 3,839,049 (Simons)]. Alternatively, the preformed silver halide grains may be added to and physically mixed with the source of reducible silver ions. The non-photosensitive source of reducible silver ions is a material that contains reducible silver ions. Typically, the preferred non-photosensitive source of reducible silver ions is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms, or mixtures of such salts. Such acids are also known as “fatty acids” or “fatty carboxylic acids.” Silver salts of other organic acids or other organic compounds, such as silver imidazoles, silver tetrazoles, silver benzotriazoles, silver benzotetrazoles, silver benzothiazoles and silver acetylides may also be used. U.S. Pat. No. 4,260,677 (Winslow et al.) discloses the use of complexes of various inorganic or organic silver salts. In photothermographic materials, exposure of the photographic silver halide to light produces small clusters containing silver atoms (Ag0)n. The imagewise distribution of these clusters, known in the art as a latent image, is generally not visible by ordinary means. Thus, the photosensitive material must be further developed to produce a visible image. This is accomplished by the reduction of silver ions that are in catalytic proximity to silver halide grains bearing the silver-containing clusters of the latent image. This produces a black-and-white image. The non-photosensitive silver source is catalytically reduced to form the visible black-and-white negative image while much of the silver halide, generally, remains as silver halide and is not reduced. In photothermographic materials, the reducing agent for the reducible silver ions, often referred to as a “developer,” may be any compound that, in the presence of the latent image, can reduce silver ion to metallic silver and is preferably of relatively low activity until it is heated to a temperature sufficient to cause the reaction. A wide variety of classes of compounds have been disclosed in the literature that function as developers for photothermographic materials. At elevated temperatures, the reducible silver ions are reduced by the reducing agent. In photothermographic materials, upon heating, this reaction occurs preferentially in the regions surrounding the latent image. This reaction produces a negative image of metallic silver having a color that ranges from yellow to deep black depending upon the presence of toning agents and other components in the imaging layer(s). Differences between Photothermography and Photography The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing with aqueous processing solutions. As noted above, in photothermographic imaging materials, a visible image is created by heat as a result of the reaction of a developer incorporated within the material. Heating at 50° C. or more is essential for this dry development. In contrast, conventional photographic imaging materials require processing in aqueous processing baths at more moderate temperatures (from 30° C. to 50° C.) to provide a visible image. In photothermographic materials, only a small amount of silver halide is used to capture light and a non-photosensitive source of reducible silver ions (for example a silver carboxylate) is used to generate the visible image using thermal development. Thus, the imaged photosensitive silver halide serves as a catalyst for the physical development process involving the non-photosensitive source of reducible silver ions and the incorporated reducing agent. In contrast, conventional wet-processed, black-and-white photographic materials use only one form of silver (that is, silver halide) that, upon chemical development, is itself at least partially converted into the silver image, or that upon physical development requires addition of an external silver source (or other reducible metal ions that form black images upon reduction to the corresponding metal). Thus, photothermographic materials require an amount of silver halide per unit area that is only a fraction of that used in conventional wet-processed photographic materials. In photothermographic materials, all of the “chemistry” for imaging is incorporated within the material itself. For example, such materials include a developer (that is, a reducing agent for the reducible silver ions) while conventional photographic materials usually do not. Even in so-called “instant photography,” the developer chemistry is physically separated from the photosensitive silver halide until development is desired. The incorporation of the developer into photothermographic materials can lead to increased formation of various types of “fog” or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems during the preparation of the photothermographic emulsion as well as during coating, use, storage, and post-processing handling. Moreover, in photothermographic materials, the unexposed silver halide generally remains intact after development and the material must be stabilized against further imaging and development. In contrast, silver halide is removed from conventional photographic materials after solution development to prevent further imaging (that is in the aqueous fixing step). In photothermographic materials, the binder is capable of wide variation and a number of binders (both hydrophilic and hydrophobic) are useful. In contrast, conventional photographic materials are limited almost exclusively to hydrophilic colloidal binders such as gelatin. Because photothermographic materials require dry thermal processing, they present distinctly different problems and require different materials in manufacture and use, compared to conventional, wet-processed silver halide photographic materials. Additives that have one effect in conventional silver halide photographic materials may behave quite differently when incorporated in photothermographic materials where the chemistry is significantly more complex. The incorporation of such additives as, for example, stabilizers, antifoggants, speed enhancers, supersensitizers, and spectral and chemical sensitizers in conventional photographic materials is not predictive of whether such additives will prove beneficial or detrimental in photothermographic materials. For example, it is not uncommon for a photographic antifoggant useful in conventional photographic materials to cause various types of fog when incorporated into photothermographic materials, or for supersensitizers that are effective in photographic materials to be inactive in photothermographic materials. These and other distinctions between photothermographic and photographic materials are described in Imaging Processes and Materials (Neblette's Eighth Edition), noted above, Unconventional Imaging Processes, E. Brinckman et al. (Eds.), The Focal Press, London and New York, 1978, pp. 74-75, in Zou et al., J. Imaging Sci. Technol. 1996, 40, pp. 94-103, and in M. R. V. Sahyun, J. Imaging Sci. Technol. 1998, 42, 23. Problem to be Solved Photothermographic materials known in the art generally include one or more “toners” in an attempt to provide desired black tone and maximum image density (Dmax). Conventional compounds used for this purpose include phthalimide, N-hydroxyphthalimide, cyclic imides, pyrazoline-5-ones, naphthalimides, cobalt complexes, N-(aminomethyl)aryldicarboximides, a combination of blocked pyrazoles, isothiuronium derivatives, merocyanine dyes, phthalazine and derivatives thereof, phthalazinone and phthalazinone derivatives, a combination of phthalazine (or derivatives thereof) plus one or more phthalic acid derivatives, quinazolinediones, benzoxazine or naphthoxazine derivatives, benzoxazine-2,4-diones, pyrimidines and asym-triazines, and tetraazapentalene derivatives. Phthalazine or derivatives thereof have become the most common toners in photothermographic materials as described in U.S. Pat. No. 6,413,710 (Shor et al.) and U.S. Pat. No. 6,146,822 (Asamuma et al.). U.S. Pat. No. 4,105,451 (Smith et al.) describes certain mercaptans such as 2,4-dimercaptopyrimidine as toners in photothermographic materials. U.S. Pat. No. 5,149,620 (Simpson et al.) similarly describes 3-mercapto-4,5-diphenyl-1,2,4-triazole compounds. U.S. Pat. No. 4,201,582 (White) describes 2,5-dimercapto-1,3,4-thiadiazole, 3-mercapto-1H-1,2,4-triazole, and 5-methyl-4-phenyl-3-mercapto-1,2,4-triazole as useful toners, while 4-phenyl-3-mercapto-1,2,4-triazole and 5-ethyl-4-phenyl-1,2,4-triazole are described to have disadvantages. U.S. Pat. No. 3,832,186 (Masuda et al.) describes the use of various mercaptotriazoles in combination with silver benzotriazole. 4-Phenyl-3-mercapto-1,2,4-triazole is also found in JP Kokai 44-026582 (Okubo) in a film that requires the use of a compound that releases base by heating. Amino and amido substituted mercaptotriazole toners are described in JP Kokai [1990] 2-179236 (Masukawa et al.) and U.S. Pat. No. 4,451,561 (Hirabayshi et al.). There remains a need for toners that contribute to image density and shorter development time and that allow for development at lower processing temperature, especially in aqueous-based photothermographic materials. Part SUMMARY OF THE INVENTION: The present invention provides a thermally developable composition comprising a non-photosensitive source of reducible silver ions, a reducing agent composition for the reducible silver ions, and a triazine-thione compound represented by the following Structure (I): wherein R1, R2, R3, R4, and R5 individually represent a substituent attached to the triazine-thione ring by a single bond. This invention also provides a thermally developable material comprising a support and having thereon at least one thermally developable layer, and comprising a triazine-thione compound represented by the Structure (I) noted above. Moreover, a black-and-white thermographic material of the present invention comprises a support having thereon one or more thermally-developable imaging layers comprising a binder and in reactive association, a non-photosensitive source of reducible silver ions, and a reducing composition for the non-photosensitive source of reducible silver ions, and a triazine-thione compound represented by the Structure (I) noted above. This invention also provides a photothermographic material that comprises a support having thereon one or more thermally developable imaging layers comprising a binder and in reactive association, a photosensitive silver halide, a non-photosensitive source of reducible silver ions, a reducing composition for the non-photosensitive source reducible silver ions, and a triazine-thione compound represented by Structure (I) noted above. Preferred embodiments of the present invention include a black-and-white aqueous-based photothermographic material that comprises a transparent support having a front side thereof: a) one or more thermally developable imaging layers each comprising a hydrophilic binder, and in reactive association, a preformed photosensitive silver bromide or silver iodobromide provided in predominantly as tabular grains, a non-photosensitive source of reducible silver ions that includes one or more silver salts of a compound containing an imino group at least one of which is silver salt of benzotriazole, a reducing composition for the non-photosensitive source reducible silver ions that includes at least one hindered phenol or an ascorbic acid, and b) a protective overcoat disposed over the one or more thermally developable imaging layers, wherein the one or more thermally developable imaging layers further comprises a triazine-thione compound represented by Structure (I) noted above. Other embodiments of the present invention include photothermographic materials that comprise a support having on a frontside thereof, one or more frontside thermally developable imaging layers comprising a binder and in reactive association, a photosensitive silver halide, a non-photosensitive source of reducible silver ions, a reducing composition for the non-photosensitive source reducible silver ions, and a triazine-thione compound represented by the following Structure (I): wherein R1, R2, R3, R4, and R5, independently represent a substituent attached to the triazine-thione ring by a single bond, the materials comprising on the backside of the support, one or more backside thermally developable imaging layers comprising a binder and in reactive association, a photosensitive silver halide, a non-photosensitive source of reducible silver ions, a reducing composition for the non-photosensitive source reducible silver ions, and a triazine-thione compound represented by the following Structure (I): wherein R1, R2, R3, R4, and R5, independently represent a substituent attached to the triazine-thione ring by a single bond, the frontside and backside thermally developable layers and compounds of Structure (I) in the frontside and backside layers having the same or different compositions. In addition, the present invention provides a method of forming a visible image comprising: A) thermal imaging of the thermally developable material of the present invention. Where the thermally developable material comprises a transparent support, this image-forming method can further comprise: B) positioning the thermally imaged thermally developable material between a source of imaging radiation and an imageable material that is sensitive to the imaging radiation, and C) exposing the imageable material to the imaging radiation through the visible image in the thermally imaged thermographic material to provide an image in the imageable material. In addition, the present invention provides a method of forming a visible image comprising: A) imagewise exposing a photothermographic material of the present invention to electromagnetic radiation to form a latent image, and B) simultaneously or sequentially, heating the exposed photothermographic material to develop the latent image into a visible image. Where the photothermographic material comprises a transparent support, this image-forming method can further comprise: C) positioning the exposed and heat-developed photothermographic material with the visible image therein between a source of imaging radiation and an imageable material that is sensitive to the imaging radiation, and D) exposing the imageable material to the imaging radiation through the visible image in the exposed and heat-developed photothermographic material to provide an image in the imageable material. In addition, the present invention provides an imaging assembly comprising the photothermographic material of the present invention that is arranged in association with one or more phosphor intensifying screens. In these embodiments, the photothermographic material may include one or more thermally developable layers on both sides of the support. The present invention provides a number of advantages with the use of the triazine-thione compounds defined herein. They can be used in a variety of thermally developable materials including aqueous-based and solvent-based thermographic and photothermographic materials. They are particularly useful in aqueous-based photothermographic materials wherein the organic silver salt is a salt of a compound containing an imino group (such as silver benzotriazole) and have been observed to provide increased image density and shortened development time, and to allow development at relatively lower temperatures. Part DETAILED DESCRIPTION OF THE INVENTION: The thermally developable materials of this invention include both thermographic and photothermographic materials. While the following discussion will often be directed to the preferred photothermographic embodiments, it would be readily understood by one skilled in the imaging arts that thermographic materials can be similarly constructed (using one or more imaging layers) and used to provide black-and-white or color images using non-photosensitive silver salts, reducing compositions, binders, and other components known to be used in such embodiments. The thermographic and photothermographic materials of this invention can be used in black-and-white or color thermography and photothermography and in electronically generated black-and-white or color hardcopy recording. They can be used in microfilm applications, in radiographic imaging (for example digital medical imaging), X-ray radiography, and in industrial radiography. Furthermore, the absorbance of these thermally developable materials between 350 and 450 nm is desirably low (less than 0.5), to permit their use in the graphic arts area (for example, imagesetting and phototypesetting), in the manufacture of printing plates, in contact printing, in duplicating (“duping”), and in proofing. The thermographic and photothermographic materials of this invention are particularly useful for medical imaging of human or animal subjects in response to visible or X-radiation. Such applications include, but are not limited to, thoracic imaging, mammography, dental imaging, orthopedic imaging, general medical radiography, therapeutic radiography, veterinary radiography, and auto-radiography When used with X-radiation, the photothermographic materials of this invention may be used in combination with one or more phosphor intensifying screens, with phosphors incorporated within the photothermographic emulsion, or with a combination thereof. The materials of this invention are also useful for non-medical uses of visible or X-radiation (such as X-ray lithography and industrial radiography). The photothermographic materials of this invention can be made sensitive to radiation of any suitable wavelength. Thus, in some embodiments, the materials are sensitive at ultraviolet, visible, infrared, or near infrared wavelengths, of the electromagnetic spectrum. In other embodiments, they are sensitive to X-radiation. Increased sensitivity to a particular region of the spectrum is imparted through the use of various sensitizing dyes. The photothermographic materials of this invention are also useful for non-medical uses of visible or X-radiation (such as X-ray lithography and industrial radiography). In such imaging applications, it is particularly desirable that the photothermographic materials be “double-sided” and have photothermographic coatings on both sides of the support. In the photothermographic materials of this invention, the components needed for imaging can be in one or more layers. The layer(s) that contain the photosensitive photocatalyst (such as a photosensitive silver halide) or the non-photosensitive source of reducible silver ions, or both, are referred to herein as photothermographic emulsion layer(s). The photocatalyst and the non-photosensitive source of reducible silver ions are in catalytic proximity (that is, in reactive association with each other) and preferably are in the same emulsion layer. Similarly, in the thermographic materials of this invention, the components needed for imaging can be in one or more layers. The layer(s) that contain the non-photosensitive source of reducible silver ions are referred herein as thermographic emulsion layer(s). Where the materials contain imaging layers on one side of the support only, various non-imaging layers are usually disposed on the “backside” (non-emulsion or non-imaging side) of the materials, including antihalation layer(s), protective layers, antistatic layers, conducting layers, and transport enabling layers. In such instances, various non-imaging layers can also be disposed on the “frontside” or imaging or emulsion side of the support, including protective topcoat layers, primer layers, interlayers, opacifying layers, antistatic layers, antihalation layers, acutance layers, auxiliary layers, and other layers readily apparent to one skilled in the art. For some embodiments of photothermographic materials containing imaging layers on both sides of the support, such material can also include one or more protective topcoat layers, primer layers, interlayers, antistatic layers, acutance layers, antihalation layers, auxiliary layers, anti-crossover layers, and other layers readily apparent to one skilled in the art on either or both sides of the support. When the thermographic and photothermographic materials of this invention are heat-developed as described below in a substantially water-free condition after, or simultaneously with, imagewise exposure, a silver image (preferably a black-and-white silver image) is obtained. Definitions As used herein: In the descriptions of the photothermographic materials of the present invention, “a” or “an” component refers to “at least one” of that component [for example, the triazine-thione compounds of Structure (I)]. Heating in a substantially water-free condition as used herein, means heating at a temperature of from about 50° C. to about 250° C. with little more than ambient water vapor present. The term “substantially water-free condition” means that the reaction system is approximately in equilibrium with water in the air and water for inducing or promoting the reaction is not particularly or positively supplied from the exterior to the material. Such a condition is described in T. H. James, The Theory of the Photographic Process, Fourth Edition, Eastman Kodak Company, Rochester, N.Y., 1977, p. 374. “Thermographic material(s)” means a construction comprising at least one thermographic emulsion or imaging layer or a set of imaging layers (wherein the source of reducible silver ions is in one layer and the other essential components or desirable additives are distributed, as desired, in an adjacent coating layer) and any supports, topcoat layers, image-receiving layers, blocking layers, and subbing or priming layers. These materials also include multilayer constructions in which one or more imaging components are in different layers, but are in “reactive association” so that they readily come into contact with each other during thermal imaging and development. For example, one layer can include the non-photosensitive source of reducible silver ions and another layer can include the reducing composition, but the two reactive components are in reactive association with each other. “Photothermographic material(s)” means a construction comprising at least one photothermographic emulsion layer or a photothermographic set of layers (wherein the photosensitive silver halide and the source of reducible silver ions are in one layer and the other essential components or desirable additives are distributed, as desired, in the same layer or in an adjacent coating layer) as well as any supports, topcoat layers, image-receiving layers, blocking layers, antihalation layers, subbing or priming layers. These materials also include multilayer constructions in which one or more imaging components are in different layers, but are in “reactive association” so that they readily come into contact with each other during imaging and/or development. For example, one layer can include the non-photosensitive source of reducible silver ions and another layer can include the reducing composition, but the two reactive components are in reactive association with each other. When used in photothermography, the term, “imagewise exposing” or “imagewise exposure” means that the material is imaged using any exposure means that provides a latent image using electromagnetic radiation. This includes, for example, by analog exposure where an image is formed by projection onto the photosensitive material as well as by digital exposure where the image is formed one pixel at a time such as by modulation of scanning laser radiation. When used in thermography, the term, “imagewise exposing” or “imagewise exposure” means that the material is imaged using any means that provides an image using heat. This includes, for example, by analog exposure where an image is formed by differential contact heating through a mask using a thermal blanket or infrared heat source, as well as by digital exposure where the image is formed one pixel at a time such as by modulation of thermal print-heads. “Catalytic proximity” or “reactive association” means that the materials are in the same layer or in adjacent layers so that they readily come into contact with each other during thermal imaging and development. “Emulsion layer,” “imaging layer,” “thermographic emulsion layer,” or “photothermographic emulsion layer,” means a layer of a thermographic or photothermographic material that contains the photosensitive silver halide (when used) and/or non-photosensitive source of reducible silver ions. It can also mean a layer of the thermographic or photothermographic material that contains, in addition to the photosensitive silver halide (when used) and/or non-photosensitive source of reducible ions, additional essential components and/or desirable additives. These layers are usually on what is known as the “frontside” of the support. In addition, “frontside” also generally means the side of a thermally developable material that is first exposed to imaging radiation, and “backside” generally means the opposite side of the thermally developable material. The terms “double-sided” and “double-faced coating” are used to define photothermographic materials having one or more of the same or different thermally developable emulsion layers disposed on both sides (frontside and backside) of the support. “Photocatalyst” means a photosensitive compound such as silver halide that, upon exposure to radiation, provides a compound that is capable of acting as a catalyst for the subsequent development of the image-forming material. Many of the materials used herein are provided as a solution. The term “active ingredient” means the amount or the percentage of the desired material contained in a sample. All amounts listed herein are the amount of active ingredient added. “Ultraviolet region of the spectrum” refers to that region of the spectrum less than or equal to 410 mn, and preferably from about 100 nm to about 410 nm, although parts of these ranges may be visible to the naked human eye. More preferably, the ultraviolet region of the spectrum is the region of from about 190 to about 405 nm. “Visible region of the spectrum” refers to that region of the spectrum of from about 400 nm to about 700 nm. “Short wavelength visible region of the spectrum” refers to that region of the spectrum of from about 400 nm to about 450 nm. “Red region of the spectrum” refers to that region of the spectrum of from about 600 nm to about 700 nm. “Infrared region of the spectrum” refers to that region of the spectrum of from about 700 nm to about 1400 nm. “Non-photosensitive” means not intentionally light sensitive. The sensitometric terms Dmin and Dmax have conventional definitions known in the imaging arts. In photothermographic materials, Dmin is considered herein as image density achieved when the photothermographic material is thermally developed without prior exposure to radiation. It is the average of eight lowest density values on the exposed side of the fiducial mark. In thermographic materials, Dmin is considered herein as image density in the non-thermally imaged areas of the thermographic material. The sensitometric term “absorbance” is another term for optical density (OD). “Transparent” means capable of transmitting visible light or imaging radiation without appreciable scattering or absorption. As used herein, the phrase “organic silver coordinating ligand” refers to an organic molecule capable of forming a bond with a silver atom. Although the compounds so formed are technically silver coordination compounds they are also often referred to as silver salts. In the compounds described herein, no particular double bond geometry (for example, cis or trans) is intended by the structures drawn. Similarly, in compounds having alternating single and double bonds and localized charges their structures are drawn as a formalism. In reality, both electron and charge delocalization exists throughout the conjugated chain. As is well understood in this art, for the chemical compounds herein described, substitution is not only tolerated, but is often advisable and various substituents are anticipated on the compounds used in the present invention unless otherwise stated. Thus, when a compound is referred to as “having the structure” of, or as “a derivative” of, a given formula, any substitution that does not alter the bond structure of the formula or the shown atoms within that structure is included within the formula, unless such substitution is specifically excluded by language (such as “free of carboxy-substituted alkyl”). For example, where a triazine-thione ring structure is shown (including fused ring structures), substituent groups may be placed on the triazine-thione ring structure to form triazine-thione derivatives, but the atoms making up the triazine-thione ring structure may not be replaced. As a means of simplifying the discussion and recitation of certain substituent groups, the term “group” refers to chemical species that may be substituted as well as those that are not so substituted. Thus, the term “group,” such as “alkyl group” is intended to include not only pure hydrocarbon alkyl chains, such as methyl, ethyl, n-propyl, t-butyl, cyclohexyl, iso-octyl, and octadecyl, but also alkyl chains bearing substituents known in the art, such as hydroxyl, alkoxy, phenyl, halogen atoms (F, Cl, Br, and I), cyano, nitro, amino, and carboxy. For example, alkyl group includes ether and thioether groups (for example CH3—CH2—CH2—O—CH2— and CH3—CH2—CH2—S—CH2—), haloalkyl, nitroalkyl, alkylcarboxy, carboxyalkyl, carboxamido, hydroxyalkyl, sulfoalkyl, and other groups readily apparent to one skilled in the art. Substituents that adversely react with other active ingredients, such as very strongly electrophilic or oxidizing substituents, would, of course, be excluded by the ordinarily skilled artisan as not being inert or harmless. Research Disclosure is a publication of Kenneth Mason Publications Ltd., Dudley House, 12 North Street, Emsworth, Hampshire PO10 7DQ England (also available from Emsworth Design Inc., 147 West 24th Street, New York, N.Y. 10011). Other aspects, advantages, and benefits of the present invention are apparent from the detailed description, examples, and claims provided in this application. The Photocatalyst As noted above, the photothermographic materials of the present invention include one or more photocatalysts in the photothermographic emulsion layer(s). Useful photocatalysts are typically silver halides such as silver bromide, silver iodide, silver chloride, silver bromoiodide, silver chlorobromoiodide, silver chlorobromide, and others readily apparent to one skilled in the art. Mixtures of silver halides can also be used in any suitable proportion. In preferred embodiments, the silver halide comprises at least 70 mol % silver bromide with the remainder being silver chloride and silver iodide. More preferably, the amount of silver bromide is at least 90 mol %. Silver bromide and silver bromoiodide are more preferred silver halides, with the latter silver halide having up to 10 mol % silver iodide based on total silver halide. Typical techniques for preparing and precipitating silver halide grains are described in Research Disclosure, 1978, item 17643. In some embodiments of aqueous-based photothermographic materials, higher amounts of iodide may be present in the photosensitive silver halide grains, and particularly from about 20 mol % up to the saturation limit of iodide, to increase image stability and to reduce “print-out,” as described for example in copending and commonly assigned U.S. Ser. No. 10/246,265 (filed Sep. 18, 2002 by Maskasky and Scaccia). The shape of the photosensitive silver halide grains used in the present invention is in no way limited. The silver halide grains may have any crystalline habit including, but not limited to, cubic, octahedral, tetrahedral, orthorhombic, rhombic, dodecahedral, other polyhedral, tabular, laminar, twinned, or platelet morphologies and may have epitaxial growth of crystals thereon. If desired, a mixture of these crystals can be employed. Silver halide grains having cubic and tabular morphology are preferred. The silver halide grains may have a uniform ratio of halide throughout. They may have a graded halide content, with a continuously varying ratio of, for example, silver bromide and silver iodide or they may be of the core-shell type, having a discrete core of one halide ratio, and a discrete shell of another halide ratio. For example, the central regions of the tabular grains may contain at least 1 mol % more iodide than the outer or annular regions of the grains. Core-shell silver halide grains useful in photothermographic materials and methods of preparing these materials are described for example in U.S. Pat. No. 5,382,504 (Shor et al.), incorporated herein by reference. Iridium and/or copper doped core-shell and non-core-shell grains are described in U.S. Pat. No. 5,434,043 (Zou et al.) and U.S. Pat. No. 5,939,249 (Zou), both incorporated herein by reference. Mixtures of preformed silver halide grains having different compositions or dopants grains may be employed. The photosensitive silver halide can be added to (or formed within) the emulsion layer(s) in any fashion as long as it is placed in catalytic proximity to the non-photosensitive source of reducible silver ions. It is preferred that the silver halide grains be preformed and prepared by an ex-situ process. The silver halide grains prepared ex-situ may then be added to and physically mixed with the non-photosensitive source of reducible silver ions. In some formulations it is useful to form the source of reducible silver ions in the presence of ex-situ-prepared silver halide. In this process, the source of reducible silver ions, such as a long chain fatty acid silver carboxylate (commonly referred to as a silver “soap”), is formed in the presence of the preformed silver halide grains. Co-precipitation of the reducible source of silver ions in the presence of silver halide provides a more intimate mixture of the two materials [see, for example U.S. Pat. No. 3,839,049 (Simons)]. Materials of this type are often referred to as “preformed soaps.” In general, the non-tabular silver halide grains used in the imaging formulations can vary in average diameter of up to several micrometers (μm) depending on their desired use. Usually, the silver halide grains have an average particle size of from about 0.01 to about 1.5 μm. In some embodiments, the average particle size is preferable from about 0.03 to about 1.0 μm, and more preferably from about 0.05 to about 0.8 μm. Those of ordinary skill in the art understand that there is a finite lower practical limit for silver halide grains that is partially dependent upon the wavelengths to which the grains are spectrally sensitized. Such a lower limit, for example, is typically from about 0.01 to about 0.005 μm. The average size of the photosensitive doped silver halide grains is expressed by the average diameter if the grains are spherical, and by the average of the diameters of equivalent circles for the projected images if the grains are cubic, tabular, or other non-spherical shapes. Grain size may be determined by any of the methods commonly employed in the art for particle size measurement. Representative methods are described by in “Particle Size Analysis,” ASTM Symposium on Light Microscopy, R. P. Loveland, 1955, pp. 94-122, and in C. E. K. Mees and T. H. James, The Theory of the Photographic Process, Third Edition, Macmillan, New York, 1966, Chapter 2. Particle size measurements may be expressed in terms of the projected areas of grains or approximations of their diameters. These will provide reasonably accurate results if the grains of interest are substantially uniform in shape. In most preferred embodiments of this invention, the silver halide grains are tabular silver halide grains that are considered “ultrathin” and have an average thickness of at least 0.02 μm and up to and including 0.10 μm. Preferably, these ultrathin grains have an average thickness of at least 0.03 μm and more preferably of at least 0.04 μm, and up to and including 0.08 μm and more preferably up to and including 0.07 μm. In addition, these ultrathin tabular grains have an equivalent circular diameter (ECD) of at least 0.5 μm, preferably at least 0.75 μm, and more preferably at least 1 μm. The ECD can be up to and including 8 μm, preferably up to and including 6 μm, and more preferably up to and including 4 μm. The aspect ratio of the useful tabular grains is at least 5:1, preferably at least 10:1, and more preferably at least 15:1. For practical purposes, the tabular grain aspect is generally up to 50:1. The grain size of ultrathin tabular grains may be determined by any of the methods commonly employed in the art for particle size measurement, such as those described above. The ultrathin tabular silver halide grains can also be doped using one or more of the conventional metal dopants known for this purpose including those described in Research Disclosure item 38957, September, 1996 and U.S. Pat. No. 5,503,970 (Olm et al.), incorporated herein by reference. Preferred dopants include iridium (III or IV) and ruthenium (II or III) salts. Preformed silver halide emulsions used in the material of this invention can be prepared by aqueous or organic processes and can be unwashed or washed to remove soluble salts. In the latter case, the soluble salts can be removed by ultrafiltration, by chill setting and leaching, or by washing the coagulum [for example, by the procedures described in U.S. Pat. No. 2,618,556 (Hewitson et al.), U.S. Pat. No. 2,614,928 (Yutzy et al.), U.S. Pat. No. 2,565,418 (Yackel), U.S. Pat. No. 3,241,969 (Hart et al.), and U.S. Pat. No. 2,489,341 (Waller et al.)]. It is also effective to use an in-situ process in which a halide-containing compound is added to an organic silver salt to partially convert the silver of the organic silver salt to silver halide. The halogen-containing compound can be inorganic (such as zinc bromide or lithium bromide) or organic (such as N-bromosuccinimide). Additional methods of preparing these silver halide and organic silver salts and manners of blending them are described in Research Disclosure, June 1978, item 17029, U.S. Pat. No. 3,700,458 (Lindholm) and U.S. Pat. No. 4,076,539 (Ikenoue et al.), JP Kokai 49-013224, (Fuji), JP Kokai 50-017216 (Fuji), and JP Kokai 51-042529 (Fuji). Mixtures of both in-situ and ex-situ silver halide grains may be used. In some instances, it may be helpful to prepare the photosensitive silver halide grains in the presence of a hydroxytetraazaindene (such as 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene) or an N-heterocyclic compound comprising at least one mercapto group (such as 1-phenyl-5-mercaptotetrazole) to provide increased photospeed. Details of this procedure are provided in U.S. Pat. No. 6,413,710 (Shor et al.), that is incorporated herein by reference. The one or more light-sensitive silver halides used in the photothermographic materials of the present invention are preferably present in an amount of from about 0.005 to about 0.5 mole, more preferably from about 0.01 to about 0.25 mole, and most preferably from about 0.03 to about 0.15 mole, per mole of non-photosensitive source of reducible silver ions. Chemical Sensitizers The photosensitive silver halides used in photothermographic features of the invention may be employed without modification. However, one or more conventional chemical sensitizers may be used in the preparation of the photosensitive silver halides to increase photospeed. Such compounds may contain sulfur, tellurium, or selenium, or may comprise a compound containing gold, platinum, palladium, ruthenium, rhodium, iridium, or combinations thereof, a reducing agent such as a tin halide or a combination of any of these. The details of these materials are provided for example, in T. H. James, The Theory of the Photographic Process, Fourth Edition, Eastman Kodak Company, Rochester, N.Y., 1977, Chapter 5, pp. 149-169. Suitable conventional chemical sensitization procedures are also described in U.S. Pat. No. 1,623,499 (Sheppard et al.), U.S. Pat. No. 2,399,083 (Waller et al.), U.S. Pat. No. 3,297,447 (McVeigh), U.S. Pat. No. 3,297,446 (Dunn), U.S. Pat. No. 5,049,485 (Deaton), U.S. Pat. No. 5,252,455 (Deaton), U.S. Pat. No. 5,391,727 (Deaton), U.S. Pat. No. 5,912,111 (Lok et al.), U.S. Pat. No. 5,759,761 (Lushington et al.), U.S. Pat. No. 6,296,998 (Eikenberry et al), and EP 0 915 371 A1 (Lok et al.). In addition, mercaptotetrazoles and tetraazaindenes as described in U.S. Pat. No. 5,691,127 (Daubendiek et al.), incorporated herein by reference, can be used as suitable addenda for tabular silver halide grains. When used, sulfur sensitization is usually performed by adding a sulfur sensitizer and stirring the emulsion at an appropriate temperature for a predetermined time. Various sulfur compounds can be used. Some examples of sulfur sensitizers include thiosulfates, thioureas, thioamides, thiazoles, rhodanines, phosphine sulfides, thiohydantoins, 4-oxo-oxazolidine-2-thiones, dipolysulfides, mercapto compounds, polythionates, and elemental sulfur. Certain tetrasubstituted thiourea compounds are also useful in the present invention. Such compounds are described, for example in U.S. Pat. No. 6,296,998 (Eikenberry et al.), U.S. Pat. No. 6,322,961 (Lam et al.) and U.S. Pat. No. 6,368,779 (Lynch et al.). Also useful are the tetrasubstituted middle chalcogen (that is, sulfur, selenium, and tellurium) thiourea compounds disclosed in U.S. Pat. No. 4,810,626 (Burgmaier et al.). All of the above publications are incorporated herein by reference. The amount of the sulfur sensitizer to be added varies depending upon various conditions such as pH, temperature and grain size of silver halide at the time of chemical ripening, it is preferably from 10−7 to 10−2 mole per mole of silver halide, and more preferably from 10−6 to 10−4 mole per mold of silver halide. In one embodiment, chemical sensitization is achieved by oxidative decomposition of a sulfur-containing spectral sensitizing dye in the presence of a photothermographic emulsion. Such sensitization is described in U.S. Pat. No. 5,891,615 (Winslow et al.), incorporated herein by reference. Still other useful chemical sensitizers include certain selenium-containing compounds. When used, selenium sensitization is usually performed by adding a selenium sensitizer and stirring the emulsion at an appropriate temperature for a predetermined time. Some specific examples of useful selenium compounds can be found in U.S. Pat. No. 5,158,892 (Sasaki et al.), U.S. Pat. No. 5,238,807 (Sasaki et al.), U.S. Pat. No. 5,942,384 (Arai et al.) and in co-pending and commonly assigned U.S. Ser. No. 10/082,516 (filed Feb. 25, 2002 by Lynch, Opatz, Gysling, and Simpson). All of the above documents are incorporated herein by reference. Still other useful chemical sensitizers include certain tellurium-containing compounds. When used, tellurium sensitization is usually performed by adding a tellurium sensitizer and stirring the emulsion at an appropriate temperature for a predetermined time. Tellurium compounds for use as chemical sensitizers can be selected from those described in J. Chem. Soc., Chem. Commun. 1980, 635, ibid., 1979, 1102, ibid., 1979, 645, J. Chem. Soc. Perkin. Trans, 1980, 1, 2191, The Chemistry of Organic Selenium and Tellurium Compounds, S. Patai and Z. Rappoport, Eds., Vol. 1 (1986), and Vol. 2 (1987), U.S. Pat. No. 1,623,499 (Sheppard et al.), U.S. Pat. No. 3,320,069 (Illingsworth), U.S. Pat. No. 3,772,031 (Berry et al.), U.S. Pat. No. 5,215,880 (Kojima et al.), U.S. Pat. No. 5,273,874 (Kojima et al.), U.S. Pat. No. 5,342,750 (Sasaki et al.), U.S. Patent 5,677,120 (Lushington et al.), British Patent 235,211 (Sheppard), British Patent 1,121,496 (Halwig), British Patent 1,295,462 (Hilson et al.) British Patent 1,396,696 (Simons), JP Kokai 04-271341 A (Morio et al.), in co-pending and commonly assigned U.S. Ser. No. 09/975,909 (filed Oct. 11, 2001 by Lynch, Opatz, Shor, Simpson, Willett, and Gysling), and in co-pending and commonly assigned U.S. Ser. No. 09/923,039 (filed Aug. 6, 2001 by Gysling, Dickinson, Lelental, and Boettcher). All of the above documents are incorporated herein by reference. The amount of the selenium or tellurium sensitizer used in the present invention varies depending on silver halide grains used or chemical ripening conditions. However, it is generally from 10−8 to 10−2 mole per mole of silver halide, preferably on the order of from 10−7 to 10−3 mole of silver halide. Noble metal sensitizers for use in the present invention include gold, platinum, palladium and iridium. Gold sensitization is particularly preferred. When used, the gold sensitizer used for the gold sensitization of the silver halide emulsion used in the present invention may have an oxidation number of 1 or 3, and may be a gold compound commonly used as a gold sensitizer. U.S. Pat. No. 5,858,637 (Eshelman et al.) describes various Au (I) compounds that can be used as chemical sensitizers. Other useful gold compounds can be found in U.S. Pat. No. 5,759,761 (Lushington et al.). Useful combinations of gold (I) complexes and rapid sulfiding agents are described in U.S. Pat. No. 6,322,961 (Lam et al.). Combinations of gold (III) compounds and either sulfur- or tellurium-containing compounds are useful as chemical sensitizers and are described in U.S. Pat. No. 6,423,481 (Simpson et al.). All of the above references are incorporated herein by reference. Reduction sensitization may also be used. Specific examples of compounds useful in reduction sensitization include, but are not limited to, stannous chloride, hydrazine ethanolamine, and thioureaoxide. Reduction sensitization may be performed by ripening the grains while keeping the emulsion at pH 7 or above, or at pAg 8.3 or less. The chemical sensitizers can be used in making the silver halide emulsions in conventional amounts that generally depend upon the average size of the silver halide grains. Generally, the total amount is at least 10−10 mole per mole of total silver, and preferably from about 10−8 to about 10−2 mole per mole of total silver. The upper limit can vary depending upon the compound(s) used, the level of silver halide, and the average grain size and grain morphology, and would be readily determinable by one of ordinary skill in the art. Spectral Sensitizers The photosensitive silver halides used in the photothermographic features of the invention may be spectrally sensitized with various spectral sensitizing dyes that are known to enhance silver halide sensitivity to ultraviolet, visible, and/or infrared radiation. Non-limiting examples of sensitizing dyes that can be employed include cyanine dyes, merocyanine dyes, complex cyanine dyes, complex merocyanine dyes, holopolar cyanine dyes, hemicyanine dyes, styryl dyes, and hemioxanol dyes. Cyanine dyes, merocyanine dyes and complex merocyanine dyes are particularly useful. Spectral sensitizing dyes are chosen for optimum photosensitivity, stability, and synthetic ease. They may be added at any stage in chemical finishing of the photothermographic emulsion. Suitable sensitizing dyes such as those described in U.S. Pat. No. 3,719,495 (Lea), U.S. Pat. No. 4,396,712 (Kinoshita et al.), U.S. Pat. No. 4,439,520 (Kofron et al.), U.S. Pat. No. 4,690,883 (Kubodera et al.), U.S. Pat. No. 4,840,882 (Iwagaki et al.), U.S. Pat. No. 5,064,753 (Kohno et al.), U.S. Pat. No. 5,281,515 (Delprato et al.), U.S. Pat. No. 5,393,654 (Burrows et al), U.S. Pat. No. 5,441,866 (Miller et al.), U.S. Pat. No. 5,508,162 (Dankosh), U.S. Pat. No. 5,510,236 (Dankosh), U.S. Pat. No. 5,541,054 (Miller et al.), JP Kokai 2000-063690 (Tanaka et al.), JP Kokai 2000-112054 (Fukusaka et al.), JP Kokai 2000-273329 (Tanaka et al.), JP Kokai 2001-005145 (Arai), JP Kokai 2001-064527 (Oshiyama et al.), and JP Kokai 2001-154305 (Kita et al.), can be used in the practice of the
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