Blended grain direct-positive emulsions and photographic elements and processes for their use

Abstract

Blended emulsions for use in forming a direct-positive image are disclosed. The emulsions are comprised of a first, radiation-sensitive core-shell grain population having a relatively low coefficient of variation and a second, substantially smaller size grain population capable of .Iadd.internally forming a latent image by .Iaddend.internally trapping photolytically generated electrons. Photographic elements incorporating the blended emulsions exhibit improved covering power and can exhibit enhanced speed.

What is claimed is: 
     
       1. A radiation-sensitive emulsion particularly adapted to forming a direct-positive image comprised of a dispersing medium   a first, core-shell silver halide grain population having a coefficient of variation of less than 20%, and   a second silver halide grain population capable of .Iadd.internally forming a latent image by .Iaddend.internally trapping photolytically generated electrons and substantially incapable of forming a surface latent image within the direct-positive exposure latitude of the first grain population,   said second grain population having an average diameter less than 70% that of said first grain population, and   said first and second silver halide grain populations being present in a weight ratio of from 5:1 to 1:5.   
     
     
       2. A radiation-sensitive emulsion according to claim 1 additionally including a nucleating agent incorporated therein. 
     
     
       3. A radiation-sensitive emulsion according to claim 2 wherein said nucleating agent is chosen from the class consisting of aromatic hydrazide nucleating agents, N-substituted cycloammonium quaternary salt nucleating agents, and mixtures thereof. 
     
     
       4. A radiation-sensitive emulsion according to claim 2 wherein said nucleating agent is a hydrazide of the formula ##STR25## wherein D is an acyl group; φ is a phenylene or a halo-, alkyl, or alkoxy-substituted phenylene group; and   M is a moiety capable of restricting mobility.   
     
     
       5. A radiation-sensitive emulsion according to claim 1 wherein said first, core-shell grain population is sensitized with at least one of sulfur, selenium, and gold. 
     
     
       6. A radiation-sensitive emulsion according to claim 1 wherein said first, core-shell grain population contains a divalent or trivalent metal dopant capable of reducing rereversal. 
     
     
       7. A radiation-sensitive emulsion according to claim 1 wherein said second silver halide grain population is comprised of converted-halide silver halide grains. 
     
     
       8. A radiation-sensitive emulsion according to claim 1 wherein said second silver halide grain population is comprised of core-shell silver halide grains. 
     
     
       9. A radiation-sensitive emulsion according to claim 8 wherein said core-shell grains of said second grain population are substantially free of surface chemical sensitization. 
     
     
       10. A radiation-sensitive emulsion according to claim 8 wherein said core-shell grains of said second grain population are internally chemically sensitized. .[.11. A radiation-sensitive emulsion according to claim 1 wherein said second silver halide grain population is internally 
     
     
        fogged..]. 12. A radiation-sensitive emulsion according to claim 1 wherein at least said core-shell grains of said first grain population are 
     
     
        comprised of bromide. 13. A radiation-sensitive emulsion according to claim 12 wherein said bromide containing silver halide grains are 
     
     
        additionally comprised of iodide. 14. A radiation-sensitive emulsion according to claim 1 wherein said second grain population has an average 
     
     
        diameter less than 50% that of said first grain population. 15. A radiation-sensitive emulsion according to claim 14 wherein said second grain population has an average diameter less than 40% that of said first 
     
     
        grain population. 16. A radiation-sensitive emulsion according to claim 1 wherein said first and second silver halide grain populations are present 
     
     
        in a weight ratio of from 2:1 to 1:3. 17. A radiation-sensitive emulsion according to claim 1 wherein said first, core-shell grain population is 
     
     
        comprised of tabular grains. 18. A radiation-sensitive emulsion according to claim 17 and wherein said grains of said first grain population having a thickness of less than 0.5 micron and a diameter greater then 0.6 micron have an average aspect ratio greater than 8:1 and account for greater than 50% of the total projected surface area of said first grain population. 
     
     
            In a direct-positive photographic element comprised of a support and at least one radiation-sensitive emulsion layer, the improvement wherein said emulsion layer is comprised of an emulsion according to claim 1, 2, 
     
     
        3, 4, 5, 6, 7, 8, 9, 10, .[.11.]., 12, 13, 14, 15, 16, 17, or 18. 20. Processing in a surface developer an imagewise exposed photographic element according to claim 19 (a) in the presence of a nucleating agent, or   (b) with light flashing of the exposed photographic element during   
     
     
        processing. 21. In a multicolor direct-positive photographic element comprised of a support and, located thereon, p1 emulsion layers for separately recording blue, green, and red light each comprised of a dispersing medium, core-shell silver halide grains, and a nucleating agent, the improvement comprising, said core-shell silver halide grains in at least one of said emulsion layers being comprised of   a first core-shell grain population having a coefficient of variation of less than 20% and chemically sensitized wih at least one of sulfur, selenium, and gold, and   a second core-shell grain population substantially free of surface chemical sensitization.Iadd., internally chemically sensitized, capable of internally forming a latent image, .Iaddend.and substantially incapable of forming a surface latent image within the direct-positive exposure latitude of the first grain population,   said second grain population having an average diameter less than 40% that of said first grain population, and   said first and second grain populations being present in a weight ratio of   
     
     
        from 2:1 to 1:3. 22. In a photographic element according to claim 21 the further improvement in which said core-shell grains are comprised of 
     
     
        bromide. 23. In a photographic element according to claim 21 the further improvement in which said second core-shell grain population increases the minimum density of the photographic element to no more than 10% of its 
     
     
        maximum density. 24. In a photographic element according to claim 21 the further improvement in which said first core-shell grain population contains a divalent or trivalent metal ion chosen from the group consisting of cadmium, lead, and erbium in an amount sufficient to reduce 
     
     
        rereversal. 25. In a photographic image transfer film unit comprising a support,   at least one emulsion layer located on said support containing a dispersing medium, core-shell silver halide grains, and a nucleating agent,   a dye-image-providing material present in said emulsion layer or a layer adjacent thereto, and   a receiving layer for providing a viewable transferred dye image following imagewise exposure and processing of said emulsion layer,   the improvement comprising, said core-shell silver halide grains in at least one of said emulsion layers being comprised of   a first core-shell grain population having a coefficient of variation of less than 20% and chemically sensitized with at least one of sulfur, selenium, and gold, and   a second core-shell grain population substantially free of surface chemical sensitization.Iadd., internally chemically sensitized, capable of internally forming a latent image, .Iaddend.and substantially incapable of forming a surface latent image within the direct-positive exposure latitude of the photographic element,   said second grain population having an average diameter less than 40% that of said first grain population, and   said first and second populations being present in a weight ratio of from   
     
     
        2:1 to 1:3. 26. In a photographic element according to claim 25 the further improvement in which said core-shell grains are comprised of 
     
     
        bromide. 27. In a photographic element according to claim 25 the further improvement in which said second core-shell grain population increases the minimum density of said emulsion layers to no more than 20% of their 
     
     
        maximum density. 28. In a photographic element according to claim 15 the further improvement in which said first core-shell grain population contains a divalent or trivalent metal ion chosen from the group consisting of cadmium, lead, and erbium in an amount sufficient to reduce 
     
     
        rereversal. 29. In a black-and-white photographic element capable of producing a direct-positive silver image and comprised of a support and, located thereon, at least one emulsion layer comprised of a dispersing medium, core-shell silver halide grains, and a nucleating agent,   the improvement comprising, said core-shell silver halide grains in at least one emulsion layer being comprised of   a first core-shell grain population having a coefficient of variation of less than 20% and chemically sensitized with at least one of sulfur, selenium, and gold, and   a second core-shell grain population capable of .Iadd.internally forming a latent image by .Iaddend.internally trapping photolytically generated electrons and substantially incapable of forming a surface latent image within the direct-positive exposure latitude of the photographic element,   said second grain population having an average diameter less than 40% that of said first grain population, and   said first and second silver halide grain populations being present in a   
     
     
        weight ratio of from 2:1 to 1:3. 30. In a photographic element according to claim 29, the further improvement in w hich said photographic element 
     
     
        is capable of producing an image contrast of greater than 8. 31. In a photographic element according to claim 29, the further improvement in which said first core-shell grain population exhibits a coefficient of variation of less than 10% and said photographic element is capable of 
     
     
        producing a contrast of greater than 10. 32. In a photographic element according to claim 29, the further improvement in which said second core-shell grain population is substantially free of surface chemical 
     
     
        sensitization. 33. In a photographic element according to claim 29, the further improvement in which said core-shell grains are comprised of 
     
     
        bromide. 34. In a photographic element according to claim 29, the further improvement in which said second core-shell grain population increases the minimum density of the photographic element to no more than 5% of its 
     
     
        maximum density. 35. In a photographic element according to claim 29, the further improvement in which said first core-shell grain population contains a divalent or trivalent metal ion chosen from the group consisting of cadmium, lead, and erbium in an amount sufficient to reduce rereversal.