Cubic regular grain photographic reversal emulsions

Abstract

Direct-positive photographic emulsions comprising silver halide grains which have a substantially regular crystal shape in contrast to substantially irregular crystal shape, are disclosed. Such emulsions can contain electron acceptors and the grains can be fogged with both reduction and gold fogging agents. Photographic elements employing such emulsions and processes for preparing them are also disclosed.

We claim: 
     
       1. A regular grain direct-positive photographic silver halide emulsion comprising .Iadd.reduction and gold .Iaddend.fogged silver halide grains wherein the halide of said silver halide grains is at least 50 mole percent bromide and wherein said emulsion comprises an organic desensitizer compound having an anodic polarographic half-wave potential and a cathodic half-wave potential which, when added together, give a positive sum. 
     
     
       2. A regular grain direct-positive photographic silver halide emulsion comprising .Iadd.reduction and gold .Iaddend.fogged silver halide grains wherein the halide of said silver halide grains comprises less than about 10 mole percent iodide and wherein said emulsion comprises an organic desensitizer compound having an anodic polarographic half-wave potential and a cathodic half-wave potential which, when added together, give a positive sum. 
     
     
       3. An emulsion according to claim 2 wherein at least 80% by weight, of said grains are regular. .[.4. An emulsion according to claim 2 wherein said 
     
     
        silver halide grains are reduction and gold fogged..]. 5. The photographic emulsion of claim 2 in which said silver halide grains are fogged with about 0.00005 to about 0.06 milliequivalent per mole of silver halide of a reduction fogging agent and about 0.001 to about 0.01 millimole per mole 
     
     
        of silver halide of a gold fogging agent. 6. The photographic emulsion of claim 2 in which said desensitizer is an organic compound which spectrally 
     
     
        sensitizes in the range of about 480 to about 800 mμ. 7. The photographic emulsion of claim 5 in which the ratio of gold fogging agent 
     
     
        to reduction fogging agent is in the range of about 1:3 to about 20:1. 8. The photographic emulsion of claim 2 in which the silver halide comprises at least 50 mole percent bromide and said desensitizer is a 
     
     
        cyanine dye. 9. The photographic emulsion of claim 2 which contains a 
     
     
        spectral sensitizing dye in addition to said desensitizer. 10. The direct-positive photographic silver halide emulsion of claim 2 in which no more than about 20%, by number, of any silver halide grains larger in size 
     
     
        than the mean grain size are not regular. 11. A photographic element comprising a support and at least one direct-positive photographic silver halide layer which comprises .Iadd.reduction and gold .Iaddend.fogged silver halide grains wherein at least 80%, by weight, of said grains are regular, wherein the halide of said silver halide grains is at least 50 mole percent bromide and wherein said emulsion comprises an organic desensitizer compound having an anodic polarographic half-wave potential and a cathodic half-wave potential which, when added together, give a 
     
     
        positive sum. 12. A photographic element comprising a support and at least one direct-positive photographic silver halide layer which comprises .Iadd.reduction and gold .Iaddend.fogged silver halide grains wherein the halide of said silver halide grains is less than 10 mole percent iodide and wherein said emulsion comprises an organic desensitizer compound having an anodic polarographic half-wave potential and a cathodic half-wave potential which, when added together, give a positive sum. .[.13. A photographic element according to claim 11 wherein said fogged 
     
     
        silver halide grains are reduction and gold fogged..]. 14. The photographic element of claim 11 in which said desensitizer is an 
     
     
        imidazoquinoxaline dye. 15. The photographic element of claim 11 in which said silver halide grains are fogged with about 0.0005 to about 0.06 milliequivalent per mole of silver halide of a reduction fogging agent and about 0.001 to about 0.01 millimole per mole of silver halide of a gold fogging agent, the ratio of gold fogging agent to reduction fogging agent 
     
     
        being in the range of about 1:3 to about 20:1. 16. The photographic element of claim 12 in which the silver halide grains are cubic-regular grains having a mean grain size in the range of about 0.1 to about 2 microns, and said grains in said emulsion being such that a test portion thereof, when coated as a photographic silver halide emulsion on a support to give a maximum density of at least about 1 upon processing for 6 minutes at about 68° F. in Kodak DK-50 developer, has a maximum density which is at least about 30% greater than the maximum density of an identical coated test portion which is processed for 6 minutes at about 68° F. in Kodak DK-50 developer after being bleached for about 10 minutes at about 68° F. in a bleach composition of: potassium cyanide--50 mg.   acetic acid (glacial)--3.47 cc.   sodium acetate--11.49 g.   potassium bromide--119 mg.   
     
     
       water to 1 liter. 17. The photographic element of claim .[.13.]. .Iadd.11 .Iaddend.in which the reduction fogging agent is thiourea dioxide and the 
     
     
        gold fogging agent is potassium chloroaurate. 18. The photographic element of claim 15 in which the reduction fogging agent is thiourea dioxide and 
     
     
        the gold fogging agent is potassium chloroaurate. 19. The process which comprises contacting (1) silver halide grains, at least 80%, by weight, of which are regular with (2) about 0.0005 to about 0.06 milliequivalent per mole of silver halide of a reduction fogging agent and about 0.001 to about 0.01 millimole per mole of silver halide of a gold fogging agent (3) at a temperature in the range of about 40 to about 100° C., the ratio of gold fogging agent to reduction fogging agent being in the range 
     
     
        of about 1:3 to about 20:1. 20. The process of claim 19 in which the silver halide comprises at least 50 mole percent bromide and has a mean 
     
     
        grain diameter in the range of about 0.1 to about 2 microns. 21. The process of claim 19 in which the silver halide grains are contacted with the reduction fogging agent and are subsequently contacted with the gold 
     
     
        fogging agent. 22. The process of claim 9 in which the reduction fogging agent is thiourea dioxide and the gold fogging agent is potassium 
     
     
        chloroaurate. 23. The process of claim 19 comprising adding a desensitizer having an anodic polarographic half-wave potential and a cathodic half-wave potential which, when added together, give a positive sum, to the silver grains after they have been contacted with the reduction and gold fogging agents. .Iadd. 24. A cubic regular grain direct-positive silver halide emulsion comprising reduction and gold fogged silver halide grains wherein the halide of said cubic regular silver halide grains is at least 50 mole percent bromide and wherein said emulsion comprises an organic desensitizer compound having an anodic polarographic half-wave potential and a cathodic half-wave potential which, when added together, give a positive sum. .Iaddend..Iadd. 25. A cubic regular grain direct-positive silver halide emulsion according to claim 24 wherein the cubic regular silver halide grains are less than 10 mole percent iodide. .Iaddend..Iadd. 26. A cubic regular grain direct-positive silver halide emulsion according to claim 25 wherein the cubic regular silver halide grains are silver bromoiodide grains. .Iaddend..Iadd. 27. A cubic regular grain direct-positive silver halide emulsion according to claim 24 in which no more than about 20 percent, by number, of any silver halide grains larger in size than the mean grain size are not regular. .Iaddend..Iadd. 28. A cubic regular grain direct-positive silver halide emulsion according to claim 24 wherein at least 80 percent by weight of said silver halide grains are regular. .Iaddend..Iadd. 29. A cubic regular grain direct-positive silver halide emulsion according to claim 24 wherein said desensitizer is a cyanine dye. .Iaddend..Iadd. 30. A cubic regular grain direct-positive silver halide emulsion according to claim 24 wherein said desensitizer is an imidazoquinoxaline dye. .Iaddend. .Iadd. 31. A photographic element comprising a support and at least one direct-positive photographic silver halide layer which comprises reduction and gold fogged silver halide grains wherein at least 80 percent, by weight, of said grains are cubic regular, wherein the halide of said cubic regular silver halide grains is at least 50 mole percent bromide and less than 10 mole percent iodide and wherein said emulsion comprises an organic desensitizer compound having an anodic polarographic half-wave potential and a cathodic half-wave potential which, when added together, give a positive sum. .Iaddend..Iadd. 32. A photographic element according to claim 31 wherein the cubic regular silver halide grains are silver bromoiodide grains. .Iaddend..Iadd. 33. A photographic element comprising a support and at least one direct-positive photographic silver halide layer which comprises reduction and gold fogged cubic regular silver halide grains wherein no more than 20 percent, by number, of any silver halide grains larger in size than the mean grain size are not cubic regular, the halide of said cubic regular silver halide grains is at least 50 mole percent bromide and less than 10 mole percent iodide and wherein said emulsion comprises an organic desensitizer compound having an anodic polarographic half-wave potential and a cathodic half-wave potential which, when added together, give a positive sum. .Iaddend. .Iadd.34. A photographic element according to claim 33 wherein the cubic regular silver halide grains are silver bromoiodide grains. .Iaddend..Iadd. 35. A photographic element according to claim 33 wherein said desensitizer is a cyanine dye. .Iaddend.