Photothermographic material

1 . A photothermographic material comprising an image forming layer, on at least one side of a support, comprising at least a photosensitive silver halide, a non-photosensitive silver salt, a reducing agent, and a binder, wherein 50% by weight or more of the binder is formed by a linear polymer, and wherein a mean grain size (D 0.5 ) of developed silver in an image portion having a density of 0.5 and a mean grain size (D 3.0 ) of developed silver in an image portion having a density of 3.0 satisfy a relationship represented by the following equation (1):

D 0.5 /D 3.0 ≧1.1  Equation (1)

2 . The photothermographic material according to claim 1 , wherein the linear polymer is soluble in an organic solvent.

3 . The photothermographic material according to claim 2 , wherein the polymer which is soluble in an organic solvent is poly(vinyl butyral).

4 . The photothermographic material according to claim 1 further comprising a development accelerator.

5 . The photothermographic material according to claim 4 , wherein the development accelerator is at least one compound selected from the group consisting of hydrazine derivative, a phenol derivative, and a naphthol derivative.

6 . The photothermographic material according to claim 5 , wherein the development accelerator is a hydrazine derivative represented by the following formula (A-1):

Q 1 -NHNH-Q 2   Formula (A-1)

wherein, Q 1 represents an aromatic group or a heterocyclic group which bonds to —NHNH-Q 2 at a carbon atom; and Q 2 represents one selected from the group consisting of a carbamoyl group, an acyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, a sulfonyl group, and a sulfamoyl group.

7 . The photothermographic material according to claim 5 , wherein the development accelerator is a compound represented by the following formula (A-2):

wherein R 1 represents one selected from an alkyl group, an acyl group, an acylamino group, a sulfonamide group, an alkoxycarbonyl group, or a carbamoyl group; R 2 represents one selected from a hydrogen atom, a halogen atom, an alkyl group, an alkoxy group, an aryloxy group, an alkylthio group, an arylthio group, an acyloxy group, or a carbonate ester group; R 3 and R 4 each independently represent a group capable of substituting for a hydrogen atom on a benzene ring; R 3 and R 4 may link together to form a condensed ring.

8 . The photothermographic material according to claim 1 further comprising a nucleator, wherein a gradation is in a range of from 1.8 to 4.3.

9 . The photothermographic material according to claim 1 , wherein the reducing agent is a compound represented by the following formula (R):

wherein R 11 and R 11′ each independently represent an alkyl group having 1 to 20 carbon atoms; R 12 and R 12′ each independently represent a hydrogen atom or a substituent capable of substituting for a hydrogen atom on a benzene ring; L represents an —S— group or a —CHR 13 — group; R 13 represents a hydrogen atom or an alkyl group having 1 to 20 carbon atoms; and X 1 and X 1′ each independently represent a hydrogen atom or a group capable of substituting for a hydrogen atom on a benzene ring.

10 . The photothermographic material according to claim 9 , wherein R 11 and R 11′ of formula (R) are a secondary or tertiary alkyl group having 3 to 15 carbon atoms.

11 . The photothermographic material according to claim 1 further comprising a compound represented by the following formula (PH), wherein a molar ratio of the compound represented by formula (PH) to the reducing agent is 0.2 to 2.0:

wherein R 21 to R 26 each independently represent a hydrogen atom or a substituent.