Method to spectrally sensitize tabular silver halide grains

Details Method to spectrally sensitize tabular silver halide grains Method to spectrally sensitize tabular silver halide grains

1999-03-18

2001-10-09

Chea, Thorl (Department: 1752)

Radiation imagery chemistry: process, composition, or product th

Radiation sensitive product

Silver compound sensitizer containing

C430S572000, C430S577000, C430S581000, C430S583000

Reexamination Certificate

active

06300051

ABSTRACT:

1. FIELD OF THE INVENTION
The present invention relates to a method of preparing a photosensitive emulsion having spectrally sensitized {111} tabular silver halide crystals showing an improvement in sensitivity (speed) to (preferably) visible green light, emitted by intensifying screens having green-light emitting luminescent phosphors, wherein said emulsion is suitable for use in a light-sensitive silver halide photographic film material applicable in a radiographic screen-film combination comprising at least one X-ray intensifying screen in operative association with the said film material.
2. BACKGROUND OF THE INVENTION
The effects of various precipitation conditions on the formation of silver halide emulsions comprising tabular crystals have been studied extensively as the said tabular grains are known in the photographic art for quite some time. As early as 1961 Berry et al. described the preparation and growth of tabular silver bromoiodide grains in Photographic Science and Engineering, Vol 5, No 6. A discussion of tabular grains appeared in Duffin, Photographic Emulsion Chemistry, Focal Press, 1966, p. 66-72. Early patent literature includes U.S. Pat. Nos. 4,063,951; 4,067,739; 4,150,994; 4,184,877 and 4,184,878. However the tabular grains described herein cannot be regarded as showing a high diameter to thickness ratio, commonly termed aspect ratio. In a number of U.S. applications filed in 1981 and issued in 1984 tabular grains with high aspect ratio and their advantages in photographic applications are described as, e.g., in U.S. Pat. Nos. 4,434,226; 4,439,520; 4,425,425; 4,425,426 and 4,433,048. A survey on high aspect ratio silver halide emulsions appeared in Research Disclosure, Vol 225, January 1983, Item 22534.
The above cited references on tabular grains are mainly concerned with high sensitive silver bromide or silver bromoiodide emulsions. Tabular grain emulsions having a high aspect ratio are known to provide several advantages over more conventional spherical grains as e.g. a high covering power, a high (spectral) sensitivity and a lower coating weight, which saves costs in manufacturing. Said lower coating weight is especially preferred if rapid processing applications are required, which is nowadays an ever more returning demand.
Spectrally sensitizing dyes are well known in the art of photography, especially for green and red sensitization of flat tabular grains, whereas for blue and/or ultraviolet sensitization the number of examples is rather limited. Further it is known to use in radiography combinations of green-emitting phosphor screens with film materials containing green sensitized tabular grain emulsions. After processing of exposed emulsion grains residual amounts of dyes may be present, especially due to the presence of huge amounts of spectral sensitizing dyes as tabular grains have a large specific surface capable of adsorbing said huge amounts. Those huge amounts are further in favour of high speed and image quality (especially sharpness), required in diagnostic imaging where it is further of utmost importance to reduce irradiation of the patient to minimum levels.
Specific layer arrangements developed in order to get the desired features mentioned hereinbefore have been described e.g. in JP-A's 08006192, 08076305 and 09197625; in EP-A's 0 661 592, 0 693 190 and 0 770 909 and in U.S. Pat. Nos. 4,585,729; 5,380,636; 5,718,994 and 5,460,916.
Especially in mammographic applications, wherein, for reasons of good image definition light-sensitive layers are present on only one side of the film support, and where image formation proceeds with a system consisting of only one intensifying screen, a high speed, a high contrast (preferably a high “toe contrast”) and low residual dye stain are desired. Specific measures taken therefor have e.g. been described in U.S. Pat. No. 5,290,655; in EP-A's 0 264 788 and 0 577 027 and in Research Disclosure No. 33487 (1992), p. 161.
3. OBJECTS OF THE INVENTION
Therefore it is an object of the present invention to provide emulsions and a method to prepare such emulsions, comprising spectrally sensitized {111} tabular silver halide grains showing an improvement in sensitivity (speed) to visible green light, emitted by intensifying screens having green-light emitting luminescent phospors.
It is another object to provide radiographic screen-film combinations comprising at least one X-ray intensifying screen in operative association with a photographic element comprising tabular grains spectrally sensitized to the green spectral region, wherein said grains are sensitive to low irradiation levels and wherein after exposure thereto, fog is low and dye stain is absent after processing.
It is a further object to provide, especially in mammographic applications, a desired high contrast in favour of image definition after processing, in combination with a high speed, a low fog and the absence of dye stain.
Other objects will become apparent from the description hereinafter.
4. SUMMARY OF THE INVENTION
According to the present invention a method is provided for preparing a tabular silver halide emulsion wherein tabular silver halide grains rich in silver bromide having an aspect ratio of at least 2:1, an average crystal diameter between 0.3 &mgr;m and 3.0 &mgr;m and an average thickness of less than 0.50 &mgr;m account for at least 50% of the total projective surface area of said grains, and wherein said method comprises the steps of precipitating, spectrally sensitizing and chemically ripening said grains, characterized by spectrally sensitizing the emulsion grains by adding before chemically ripening at least one J-band aggregating and at least one non-J-band aggregating dye.
A light-sensitive silver halide emulsion comprising tabular grains having an aspect ratio of at least 2:1, an average crystal diameter between 0.3 &mgr;m and 3.0 &mgr;m and an average thickness of less than 0.50 &mgr;m accounting for at least 50% of the total projective surface area of said grains, further having at least one J-band aggregating spectrally sensitizing dye and at least one non-J-band aggregating dye is also provided by the present invention, wherein said material preferably is an X-ray material comprising a support, at least one hydrophilic colloid layer on at least one side of said support and one or more photosensitive layer(s) comprising a silver halide photographic emulsion as disclosed above.
Further according to a preferred embodiment of the present invention a radiographic screen-film combination comprising at least one X-ray intensifying screen in operative association with the material as set forth has also been claimed.
5. DETAILED DESCRIPTION
Spectrally sensitizing dyes, more preferably those of the J-band aggregating sensitizing type, are adsorbed on the surface of tabular silver halide grains in an amount sufficient to optimally sensitize said tabular silver halide grains. As is well known in the art of photography, e.g. from F. M. Hamer, “Cyanine Dyes and Related Compounds” (1964), p. 710, J.Wiley, Interscience and from T. H. James, “The Theory of the Photographic Process”, 4th Edition, MacMillan 1977, Chapter 8, the sensitization maximum of a sensitizing dye in a silver halide emulsion, also called M-band sensitization due to molecular absorption, is situated near the absorption maximum of the free dye in an aqueous solution. Certain sensitizing dyes have a sharp sensitization band, or even peak, at a slightly longer wavelength (about 20-50 nm) than the wavelength corresponding to the M-band sensitization maximum. This is known in the art as “J-band sensitization” and the dyes are known as “J-band spectrally sensitizing dyes” or “J-band aggregating dyes”. Opposite thereto are the so-called “non-J-band aggregating dyes” which do not show the phenomenon of showing the sharp band or peak as described.
From indirect data there is a general acceptance that J-band aggregating spectrally sensitizing dyes consist of the nearly planar dye molecules adsorbed to silver halide surfaces along their l

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