| 摘要 |
A method has been described of preparing an ultrathin tabular grain emulsion rich in silver bromide, having ä111ü major faces, wherein tabular grains having a thickness of less than 0.08 mu m exhibit an average aspect ratio of more than 5:1 and account for at least 75 %, and more preferably at least 80 %, by number of hexagonal grains and a coefficient of variation on average equivalent surface area of less than 0.50, said process comprising: introducing into a temperature-controlled reaction vessel an aqueous dispersing medium and silver, bromide, and, optionally, iodide and/or chloride, and maintaining pBr values within said reaction vessel during grain nucleation and growth that favors the formation and growth of tabular grains, characterized in that at the time said tabular grains are being formed, (a) pH in said reaction vessel is maintained at a value ranging from 0.8 to 10; (b) a gelatino-peptizer is present in the reaction vessel in a concentration of from 0 to 50 g per liter of said dispersing medium, and (c) pBr within said reaction vessel having a value of at least 1.8 is maintained during grain nucleation, wherein nucleation is performed in a reaction vessel at a temperature in the range from 10-60 DEG C and a pBr value within said reaction vessel is maintained at a value of less than 2.4 during growth, wherein growth is performed in a reaction vessel at a temperature in the range from 40-80 DEG C, and more preferably in the range between 40 and 50 DEG C in order to reach a grain thickness of less than 0.06 mu m, provided that a gelatin peptizer which is free from calcium ions and has a methionine content of less than 30 micromoles per gram of gelatino-peptizer is present in said dispersing medium during grain nucleation and growth. Moreover a photographic material comprising an emulsion prepared according to the method of the present invention has been claimed.
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