Silver halide emulsion

Details Silver halide emulsion Silver halide emulsion

2001-09-06

2003-05-06

Letscher, Geraldine (Department: 1752)

Radiation imagery chemistry: process, composition, or product th

Radiation sensitive product

Silver compound sensitizer containing

C430S569000, C430S599000, C430S604000, C430S605000

Reexamination Certificate

active

06558891

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a silver halide emulsion, a preparation method of the silver halide emulsion and a silver halide photographic material by use of the silver halide emulsion.
BACKGROUND OF THE INVENTION
Silver halide photographic materials (hereinafter, also referred to as photographic materials) are said to be a mature product having extremely high completeness. On the other hand, various enhanced performance is still required, such as enhanced sensitivity, superior image quality and minimized variation in performance after storage. Further, enhancement of suitability for rapid access by accelerating progression of development is also required and required levels thereof recently have become higher. Specifically with regard to enhancement of sensitivity, to maintain superiority of silver halide photographic materials over recent technical progress of digital cameras, further enhanced sensitivity compatible with storage stability is desired, while fogging is maintained at low levels.
To achieve further enhanced sensitivity and superior image quality, a technique for enhancing the ratio of sensitivity/grain size per silver halide grain has been explored in silver halide emulsions (hereinafter, also referred to as emulsions). As is commonly known, silver halide grains contained in a silver halide emulsion have various grain shapes, such as cubic, octahedral, or tetradecahedral, regular crystal silver halide grains, tabular silver halide grains having a single twin plane or plural parallel twin planes, and tetrapod-like or needle-like silver halide grains having non-parallel twin planes. Specifically, tabular silver halide grains (hereinafter, also referred to as tabular grains) are supposed to have the following advantages of photographic performance.
1. The ratio of grain surface area to grain volume (hereinafter, denoted as specific surface area) is relatively large, thereby causing a relatively large amount of a sensitizing dye to be adsorbed onto the grain surface, leading to enhanced spectral sensitivity relative to inherent sensitivity.
2. When tabular grain emulsion is coated and dried, tabular grains are arranged parallel to the surface of the support, thereby reducing the coating layer thickness and leading to enhanced sharpness of the photographic material.
3. Light scattering by silver halide grains is reduced, resulting in images having enhanced resolution.
4. Sensitivity to blue light (inherent sensitivity) is relatively low, so that in cases where a green-sensitive layer or red-sensitive layer is concurrently included, the density of a yellow filter layer can be decreased or the yellow filter can be removed from the photographic material.
5. A given level of sensitivity can be achieved by a low silver coverage relative to conventional grains, resulting in an enhanced sensitivity/graininess ratio and superior resistance to natural radiation.
Prior arts concerning tabular grains, specifically concerning manufacturing methods and techniques for usage thereof are described in, for example, U.S. Pat. Nos. 4,434,226, 4,439,520, 4,414,310, 4433,048, 4,414,306, 4,459,353; JP-B No. 4-36347, 5-16015, 6-44132 (hereinafter, the term, JP-B refers to published Japanese Patent); JP-A No. 6-43605, 6-43606, 6-214331, 6-222488, 6-230493 and 6-258745 (hereinafter, the term, JP-A refers to unexamined, published Japanese Patent Application).
To effectuate the foregoing advantages of the tabular grains, it is effective to employ tabular grains having relatively high aspect ratio. As is known in the art, increasing the iodide content makes it more difficult to prepare tabular grains having a high aspect ratio, so that most of tabular grains prepared by the foregoing prior arts were silver bromide or relatively low iodide silver iodobromide. However, low iodide silver halide grains exhibit relatively high development activity, and in addition thereto, high aspect ratio tabular grains further promote development, due to their grain shape factors. As a result, deterioration in graininess or influence by natural radiation easily occurs, making it difficult for tabular grains having a relative high aspect ratio to effectuate their inherent advantages. Further, tabular grains having a relatively high aspect ratio tend to increase fluctuation in grain size, making it difficult to optimize chemical sensitization or spectral sensitization and resulting in reduction in contrast or color density.
JP-A No. 6-230491 discloses tabular grains having an iodide content in the fringe portions of 1.5 to 50 times that in the central portion of the grain and an aspect ratio of 8 to 100. In this technique, however, allowing a high iodide phase to be arranged in the fringe portions led to lowering suitability for chemical sensitization, resulting in reduction in sensitivity and contrast. JP-A No. 6-235988 discloses tabular grains having a multiple structure comprising an inner shell, an intermediate shell containing relative high iodide and an outermost shell, having an aspect ratio of 3 to 100. However, it was proved that the relatively low iodide outermost shell external to the intermediate shell accounted for a relatively high fraction of the grain produced problems such that accelerated development due to tabular grains having a high aspect ratio caused deteriorated graininess. In view of the foregoing, advantages achievable by tabular grains having a relatively high aspect ratio could not be effectuated.
In general, tabular grains are formed by a process comprising the stages of nucleation, ripening and growth. It is substantially infeasible to selectively form tabular nucleus grains alone in the nucleation stage, so that grains other than tabular nucleus grains need to be allowed to disappear in the ripening stage. Thus, the nucleation and ripening stages largely affect grain size homogeneity or aspect ratio of the tabular grains.
JP-A No. 6-230491 and 6-230493 describe a preparation method of tabular grains having a relatively high aspect ration, with attention given in the nucleation stage, in which low molecular weight gelatin is used in the nucleation stage, and nucleation time “t” (sec) and temperature “T” (° C.) within the reaction vessel at the nucleation stage satisfy the following relationship: 1<t<−T+90. In this disclosure, however, although the nucleation temperature is defined as 20 to 60° C., and preferably 30 to 60° C., nothing is taught therein with respect to nucleation at lower temperature of less than 20° C.
As disclosed in JP-A Nos. 63-11928, 1-131541, 2-838 and 2-28638, it is commonly known that the use of silver halide solvents such as ammonia and thioethers in the ripening stage enhances monodispersibility of tabular grains. Although this technique is effective for preparation of tabular grains having a relatively low aspect ratio, the use of such silver halide solvents increases the thickness of tabular nucleus grains so that this technique is taught not to be applicable in the preparation of relatively high aspect ratio tabular grains. In cases when silver halide solvents are not used in the ripening stage, it becomes difficult to allow twinned nuclei having non-parallel twin planes to be disappeared. As a result, it was proved that in a silver halide emulsion obtained by the foregoing technique were concurrently present high aspect ratio-having tabular grains and non-parallel-twinned crystal grains, leading to increased fogging and deteriorated graininess caused by such non-parallel-twinned crystal grains. Accordingly, technical development is urgently desired.
SUMMARY OF THE INVENTION
In view of the foregoing problems, the present invention was achieved. Thus, it is an object of the invention to provide a silver halide emulsion exhibiting enhanced relationship between sensitivity and fog density, a superior graininess, improved radiation resistance and improvements in contrast and color forming property, and silver halide photographic material by use of the emulsion. In addition thereto, it is an object of the invention to

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