EP0503700A1

EP0503700A1 - Method for the preparation of iodobromide emulsions containing monodisperse tabular grains

Info

Publication number: EP0503700A1
Application number: EP92200498A
Authority: EP
Inventors: Ann Leni Verbeeck
Original assignee: Agfa Gevaert NV
Current assignee: Agfa Gevaert NV
Priority date: 1991-03-08
Filing date: 1992-02-20
Publication date: 1992-09-16
Anticipated expiration: 2012-02-20
Also published as: EP0503700B1

Abstract

A method is disclosed for the preparation of a silver bromide or iodobromide emulsion containing monodisperse tabular grains accounting for at least 50 % of the total projected area of all grains, wherein the coëfficiënt of variation of the grain size distribution of said tabular grains is at most 15 %, characterized in that an aminoazaindene is added at any stage of the emulsion preparation before 50 % of the total silver halide is precipitated.

Preferred classes of aminoazaindenes are adenine derivatives and 4-amino-pyrazolo-pyrimidine derivatives. Most preferred substances are adenine and 4-amino-pyrazolo[3,4d]pyrimidine.

Description

FIELD OF THE INVENTION

The present invention relates to silver halide emulsion technology and more particularely to a method for the preparation of silver bromide or silver iodobromide emulsions containing tabular grains which show a monodisperse grain size distribution.

BACKGROUND OF THE INVENTION

As is known to those skilled in the art monodisperse emulsions show some interesting photographic properties. Monodispersity is characterized by a low coefficient of variation (or variance), defined as the quotient of the standard deviation on the size distribution divided by the average grain size. This value is often expressed as a percentage. Because of the influence of the specific grain surface on the chemical reactions occuring during chemical sensitization the latter process leads in the case of heterodisperse emulsions to a heterogeneously distributed degree of progress of said sensitization. In this way only a fraction of the total grain population is optimally sensitized. On the contrary chemical ripening of monodisperse emulsions leads to a homogeneous distribution of sensivity specks resulting in a homogeneous photographic response. As a consequence materials containing such monodisperse grain show a steep gradation after development which is much desired in various kinds of photographic applications. On the other hand monodisperse emulsions of different average size can be blended for those applications where a lower contrast is required since such a mixture retains advantages over a polydisperse emulsion of the same average grain size.
Further, monodisperse emulsions favourably influence overall picture quality. Fine grain populations which contribute disproportionately to light scattering resulting in sharpness reduction are restricted; populations of relative larger grains which contribute mainly to image granularity are restricted too.
On the other hand tabular grains with a high diameter to thickness ratio, commonly termed aspect ratio, exhibit several pronounced photographic advantages. Although tabular grains are known for quite some time the early publications cannot be regarded as dealing with high aspect ratio tabular grains. In a number of US applications filed in 1981 and issued in 1984 tabular grains with high aspect ratio and their advantages in photographic applications are extensively described. So Wilgus US 4,434,226 discloses tabular silver bromoiodide grains having a thickness less than 0.3 micron, a diameter of at least 0.6 micron and an average aspect ratio greater than 8:1 and accounting for at least 50 percent of the total projected area of all the emulsion grains. Kofron US 4,439,520 discloses similar grains which are spectrally sensitized. Abbott US 4,425,425 describes radiographic materials containing tabular grains with aspect ratio of at least 8:1 and Abbott US 4,425,426 discloses similar grains with an aspect ratio between 5:1 and 8:1. Solberg US 4,433,048 protects tabular silver bromoiodide grains with inhomogeneously distributed iodide. A survey on high aspect ratio silver halide emulsions appeared in Research Disclosure, Vol 225, Jan 1983, Item 22534.
The benefits of high aspect ratio tabular grains can be summarized as follows. Thanks to their particular morphology, greater amounts of spectral sensitizers can be adsorbed per mole silver halide compared to classical globular grains. As a consequence such spectrally sensitized tabular grains show an improved speed-granularity relationship and a wide separation between their blue speed and minus blue speed. Sharpness of photographic images can be improved using tabular grains thanks to their lower light scattering properties again compared to conventional globular emulsion grains. In color negative materials the conventional sequence of the light sensitive layers can be altered and the yellow filter layer can be omitted. In developed black-and-white images high covering power is obtained even at high hardening levels; alternatively reduced silver halide coverages can be achieved if wanted resulting again in improved sharpness. In double coated radiographic materials the presence of tabular grains reduces the so-called cross-over which is the dominant factor for sharpness in such materials.
Because of the photographic benefits of on one hand monodisperse emulsions and on the other hand tabular grain emulsions, it was obvious that emulsion technologist tried and still try to combine the advantages of both classes. So Mignot US 4,386,156 describes a method for the preparation of tabular grains with a variance of less than 30 % by transforming cubic seed crystals into tabular grains. Saitou DE 3 707 135 discloses hexagonal tabular grains with a low coefficient of variation by taking certain defined measures in the precipition and physical ripening stages. Nottorf US 4,722,866 discloses a preparation method for tabular grains with narrow size distribution by a rather complex precipitation process comprising at least five distinct stages. Zola EP 0 362 699 describes silver bromoiodide grains with an average aspect ratio greater than 12 characterized in that the quotient of the average aspect ratio of the tabular grains divided by the coefficient of variation of the total grain population is greater than 0.7. In this way the monodispersity is correlated with the aspect ratio because of the greater difficulty of preparing monodisperse tabular grains with very high aspect ratios. An essential feature of the preparation method consists in a nucleation stage characterized by a very high flow rate and concentrated solutions.
Since the prior art methods are rather complex and cumbersome there is permanent need for an uncomplicated method for preparing monodisperse tabular silver (iodo)bromide emulsions
It is an object of the present invention to provide a new and simple method for the preparation of emulsions containing tabular silver (iodo)bromide grains accounting for at least 50 % of the total projected area of all grains, wherein the coëfficiënt of variation of the grain size distribution of said tabular grains is at most 15 %.
It is a further object of the present invention to provide a new and simple method for the preparation of emulsions containing tabular silver (iodo)bromide grains with improved developability.
It is a still further object of the present invention to provide photographic materials containing monodisperse tabular silver (iodo)bromide grains prepared by this new method.
Other objects will become apparent from the description hereafter.

SUMMARY OF THE INVENTION

The object of the present invention of preparing emulsions containing tabular silver iodobromide or silver bromide grains showing a coefficient of variation of at most 15 % is realized by adding an aminoazaindene to the reaction vessel at any stage of the emulsion preparation before 50 % of the total silver halide is precipitated.
In a preferred embodiment of the invention said aminoazaindene is an adenine derivative according to formula Ia or a 4-amino-pyrazolo-pyrimidine derivative according to formula Ib :

wherein each of R¹ and R² which may be the same or different represents hydrogen, alkyl , alkenyl, aryl, alkoxy, hydroxy, mercapto, carboxy, amino or halogen, and
each of R³ and R⁴ which may be the same or different represents hydrogen or alkyl;
Although aminoazaindenes in general or particular classes thereof were known in the prior art in connection with the preparation of tabular grains rich in chloride, e.g. as disclosed in Maskasky US 4,400,463, Maskasky US 4,713,323 and Tufano US 4,804,621, it is the first time that their use in the preparation of tabular (iodo)bromide is disclosed. Further it is the first time that their surprising effect on the homogenisation of the size distribution of said tabular silver (iodo)bromide grains is recognized.
The new method of the present invention is particularly suited for the preparation of tabular silver (iodo)bromide grains with an average aspect ratio comprised between 2 and 12.

DETAILED DESCRIPTION OF THE INVENTION

The aminoazaindenes used in connection with the present invention can be added at any stage of the silver halide precipitation procedure e.g. before the start of the precipitation, during the nucleation step, during a physical ripening stage or during one or more of the growth steps with the proviso that the total amount of aminoazaindene is added before 50 % of the total silver amount is precipitated. The substance can be added to the dispersion medium itself as a solid or as a predissolved aqueous solution; alternatively the substance can be dissolved in one or more of the solutions which are entered into the dispersion medium, e.g. in one or more of the halide or silver salt solutions.
The aminoazaindenes used in connection with the present invention are preferably present in a concentration ranging from 10⁻⁴ to 10⁻³ moles per mole silver halide.
As stated above, in a preferred embodiment of the invention the aminoazaindene is an adenine derivative or a 4-amino-pyrazolo-pyrimidine derivative. More preferably the adenine derivative is adenine itself (formula Ia-1); the 4-amino-pyrazolo-pyrimidine derivative preferably is 4-aminopyrazolo[3,4]pyrimidine (formula Ib-1) :
Although the emulsion precipitation can be principally performed by one double jet step it is preferred to perform a sequence of a nucleation step and at least one growth step. Of the total silver halide preferably 0.5 % to 5.0 % is precipitated during said nucleation step which consists preferably of an approximately equimolecular addition of silver and halide salts. The rest of the silver and halide salts is added during one or more consecutive double jet growth steps. The different steps of the precipitation can be alternated by physical ripening steps. During the growth step(s) an increasing flow rate of silver and halide solutions is preferably established, e.g. a linearly increasing flow rate. Typically the flow rate at the end is about 3 to 5 times greater then at the start of the growth step. These flow rates can be monitored by e.g. magnetic valves. During the growth step(s) the pAg is preferably maintained at a constant value corresponding to a silver potential preferably at least -30 milli-Volt measured by a silver versus calomel electrode. The pH is preferably established at a value of at least 4.0.
Before and during the formation of the silver halide grains it is common practice to establish a gelatin concentration of from about 0.05 % to 5.0 % by weight in the dispersion medium. Additional gelatin is added in a later stage of the emulsion preparation, e.g. after washing, to establish optimal coating conditions and/or to establish the required thickness of the coated emulsion layer. Preferably a gelatin / silver halide ratio ranging from 0.3 to 1.0 is then obtained, wherein silver halide is expressed as silver nitrate. Conventional lime-treated or acid treated gelatin can be used, preferably of the inert type. The preparation of such gelatin types has been described in e.g. "The Science and Technology of Gelatin", edited by A.G. Ward and A. Courts, Academic Press 1977, page 295 and next pages. The gelatin can also be an enzyme-treated gelatin as described in Bull. Soc. Sci. Phot. Japan, N° 16, page 30 (1966).
After completion of the precipitation a wash technique in order to remove the excess of soluble salts is applied. Any conventional wash technique can be used e.g. washing with several water portions after flocculation by an inorganic salt or by a polymeric flocculating agent like polystyrene sulphonic acid.
The emulsions containing tabular grains prepared according to the method of the present invention can be used in various types of photographic elements. However because of their (iodo)bromide composition they are preferably used in those applications for which high sensitivity is required. Preferred embodiments include black-and-white or colour negative recording materials for still photography or for cinematographic application, black-and-white or colour reversal materials, graphic arts camera sensitive films and materials for radiographic recording.
The photographic element containing one or more emulsions prepared in accordance with the present invention can be composed of one single emulsion layer, as is the case for many applications, or it can be built up by two or even more emulsion layers. In the case of colour photography the material contains blue, green and red sensitive layers each of which can be single or multiple. Beside the light sensitive emulsion layer(s) the photographic material can contain several non-light sensitive layers, e.g. a protective layer, one or more backing layers, one or more subbing layers, and one or more intermediate layers e.g. filter layers.
The emulsions containing tabular silver (iodo)bromide grains prepared in accordance with the present invention can be chemically sensitized as described e.g. in "Chimie et Physique Photographique" by P. Glafkides, in "Photographic Emulsion Chemistry" by G.F. Duffin, in "Making and Coating Photographic Emulsion" by V.L. Zelikman et al , and in "Die Grundlagen der Photographischen Prozesse mit Silberhalogeniden" edited by H. Frieser and published by Akademische Verlagsgesellschaft (1968). As described in said literature chemical sensitization can be carried out by effecting the ripening in the presence of small amounts of compounds containing sulphur e.g. thiosulphate, thiocyanate, thioureas, sulphites, mercapto compounds, and rhodamines. The emulsions can be sensitized also by means of gold-sulphur ripeners or by means of reductors e.g. tin compounds as described in GB 789,823, amines, hydrazine derivatives, formamidine-sulphinic acids, and silane compounds.
The silver halide emulsions under consideration can be spectrally sensitized with methine dyes such as those described by F.M. Hamer in "The Cyanine Dyes and Related Compounds", 1964, John Wiley & Sons. Dyes that can be used for the purpose of spectral sensitization include cyanine dyes, merocyanine dyes, complex cyanine dyes, complex merocyanine dyes, hemicyanine dyes, styryl dyes and hemioxonol dyes. Particularly valuable dyes are those belonging to the cyanine dyes, merocyanine dyes and complex merocyanine dyes. A survey of useful chemical classes of spectral sensitizing dyes and specific useful examples in connection with tabular grains is given in Research Disclosure Item 22534. In classical emulsion preparation spectral sensitization traditionally follows the completion of chemical sensitization. However, in connection with tabular grains, it is specifically considered that spectral sensitization can occur simultaneously with or even precede completely the chemical sensitization step. For example, Maskasky US Ser. No 431,855, titled CONTROLLED SITE EPITAXIAL SENSITIZATION discloses the chemical sensitization after spectral sensitization at one or more ordered discrete edge sites of tabular grains. This can be done with the tabular grains containing emulsions of the present invention.
The silver halide emulsion layer(s) in accordance with the present invention or the non-light-sensitive layers may comprise compounds preventing the formation of fog or stabilizing the photographic characteristics during the production or storage of the photographic elements or during the photographic treatment thereof. Many known compounds can be added as fog-inhibiting agent or stabilizer to the silver halide emulsion. Suitable examples are e.g. the heterocyclic nitrogen-containing compounds such as benzothiazolium salts, nitroimidazoles, nitrobenzimidazoles, chlorobenzimidazoles, bromobenzimidazoles, mercaptothiazoles, mercaptobenzothiazoles, mercaptobenzimidazoles, mercaptothiadiazoles, aminotriazoles, benzotriazoles (preferably 5-methyl-benzotriazole), nitrobenzotriazoles, mercaptotetrazoles, in particular 1-phenyl-5-mercapto-tetrazole,
mercaptopyrimidines, mercaptotriazines, benzothiazoline-2-thione, oxazoline-thione, triazaindenes, tetrazaindenes and pentazaindenes, especially those described by Birr in Z. Wiss. Phot. 47 (1952), pages 2-58, triazolopyrimidines such as those described in GB 1,203,757, GB 1,209,146, JA-Appl. 75-39537, and GB 1,500,278, and 7-hydroxy-s-triazolo-[1,5-a]-pyrimidines as described in US 4,727,017, and other compounds such as benzenethiosulphonic acid, toluenethiosulphonic acid, benzenethiosulphinic acid and benzenethiosulphonic acid amide. Other compounds that can be used as fog-inhibiting compounds are metal salts such as e.g. mercury or cadmium salts and the compounds described in Research Disclosure N° 17643 (1978), Chapter VI.
In the specific embodiment of a colour negative or colour reversal photographic material, the usual ingredients specific for colour materials can be present e.g. colour couplers, couplers bearing a releasable photographic useful group and scavengers for oxidized developer. These typical ingredients for colour materials can be soluble or added in dispersed form, e.g. with the aid of so-called oilformers or they can be added in polymeric latex form.
The gelatin binder of the photographic elements can be hardened with appropriate hardening agents such as those of the epoxide type, those of the ethylenimine type, those of the vinylsulfone type e.g. 1,3-vinylsulphonyl-2-propanol, chromium salts e.g. chromium acetate and chromium alum, aldehydes e.g. formaldehyde, glyoxal, and glutaraldehyde, N-methylol compounds e.g. dimethylolurea and methyloldimethylhydantoin, dioxan derivatives e.g. 2,3-dihydroxy-dioxan, active vinyl compounds e.g. 1,3,5-triacryloyl-hexahydro-s-triazine, active halogen compounds e.g. 2,4-dichloro-6-hydroxy-s-triazine, and mucohalogenic acids e.g. mucochloric acid and mucophenoxychloric acid. These hardeners can be used alone or in combination. The binder can also be hardened with fast-reacting hardeners such as carbamoylpyridinium salts as disclosed in US 4,063,952 and with the onium compounds as disclosed in European Patent Application No 90.201850.6
The photographic element used in connection with the present invention may further comprise various kinds of surface-active agents in the photographic emulsion layer or in at least one other hydrophilic colloid layer. Suitable surface-active agents include non-ionic agents such as saponins, alkylene oxides e.g. polyethylene glycol, polyethylene glycol/polypropylene glycol condensation products, polyethylene glycol alkyl ethers or polyethylene glycol alkylaryl ethers, polyethylene glycol esters, polyethylene glycol sorbitan esters, polyalkylene glycol alkylamines or alkylamides, silicone-polyethylene oxide adducts, glycidol derivatives, fatty acid esters of polyhydric alcohols and alkyl esters of saccharides; anionic agents comprising an acid group such as a carboxy, sulpho, phospho, sulphuric or phosphoric ester group; ampholytic agents such as aminoacids, aminoalkyl sulphonic acids, aminoalkyl sulphates or phosphates, alkyl betaines, and amine-N-oxides; and cationic agents such as alkylamine salts, aliphatic, aromatic, or heterocyclic quaternary ammonium salts, aliphatic or heterocyclic ring-containing phosphonium or sulphonium salts. Such surface-active agents can be used for various purposes e.g. as coating aids, as compounds preventing electric charges, as compounds improving slidability, as compounds facilitating dispersive emulsification and as compounds preventing or reducing adhesion. Preferred surface-active coating agents are compounds containing perfluorinated alkyl groups.
Development acceleration can be accomplished with the aid of various compounds, preferably polyalkylene derivatives having a molecular weight of at least 400 such as those described in e.g. US 3,038,805 - 4,038,075 - 4,292,400.
The photographic element in connection with the present invention may further comprise various other additives such as e.g. compounds improving the dimensional stability of the photographic element, UV-absorbers, spacing agents and plasticizers.
Suitable additives for improving the dimensional stability of the photographic element are e.g. dispersions of a water-soluble or hardly soluble synthetic polymer e.g. polymers of alkyl(meth)acrylates, alkoxy(meth)acrylates, glycidyl (meth)acrylates, (meth)acrylamides, vinyl esters, acrylonitriles, olefins, and styrenes, or copolymers of the above with acrylic acids, methacrylic acids, Alpha-Beta-unsaturated dicarboxylic acids, hydroxyalkyl (meth)acrylates, sulphoalkyl (meth)acrylates, and styrene sulphonic acids.
Suitable UV-absorbers are e.g. aryl-substituted benzotriazole compounds as described in US 3,533,794, 4-thiazolidone compounds as described in US 3,314,794 and 3,352,681, benzophenone compounds as described in JP-A 2784/71, cinnamic ester compounds as described in US 3,705,805 and 3,707,375, butadiene compounds as described in US 4,045,229, and benzoxazole compounds as described in US 3,700,455. UV-absorbers are especially useful in colour recording materials where they prevent the fading by light of the colour images formed after processing.
In the protective layer spacing agents can be present of which, in general, the average particle size is comprised between 0.2 and 10 micron. Spacing agents can be soluble or insoluble in alkali. Alkali-insoluble spacing agents usually remain permanently in the photographic element, whereas alkali-soluble spacing agents usually are removed therefrom in an alkaline processing bath. Suitable spacing agents can be made e.g. of polymethyl methacrylate, of copolymers of acrylic acid and methyl methacrylate, and of hydroxypropylmethyl cellulose hexahydrophthalate. Other suitable spacing agents have been described in US 4,614,708.
As stated above the photographic material can contain several non light sensitive layers, e.g. an anti-stress top layer, one or more backing layers, and one or more intermediate layers eventually containing filter- or or antihalation dyes that absorb scattering light and thus promote the image sharpness. Suitable light-absorbing dyes are described in e.g. US 4,092,168, US 4,311,787, DE 2,453,217, and GB 7 907 440. One or more backing layers can be provided at the non-light sensitive side of the support. These layers which can serve as anti-curl layer can contain e.g. matting agents like silica particles, lubricants, antistatic agents, light absorbing dyes, opacifying agents, e.g. titanium oxide and the usual ingredients like hardeners and wetting agents.
The support of the photographic material may be opaque or transparent, e.g. a paper support or resin support. When a paper support is used preference is given to one coated at one or both sides with an Alpha-olefin polymer, e.g. a polyethylene layer which optionally contains an anti-halation dye or pigment. It is also possible to use an organic resin support e.g. cellulose nitrate film, cellulose acetate film, polyvinylacetal) film, polystyrene film, polyethylene terephthalate film, polycarbonate film, polyvinylchloride film or poly-Alpha-olefin films such as polyethylene or polypropylene film. The thickness of such organic resin film is preferably comprised between 0.07 and 0.35 mm. These organic resin supports are preferably coated with a subbing layer which can contain water insoluble particles such as silica or titanium dioxide.
The photographic material containing tabular grains prepared according to the present invention can be image-wise exposed by any convenient radiation source in accordance with its specific application. Of course processing conditions and composition of processing solutions are dependent from the specific type of photographic material in which the tabular grains containing emulsions prepared according to the present invention are applied. Preferably an automatically operating processing apparatus is used provided with a system for automatic regeneration of the processing solutions.
The following examples illustrate the invention without however limiting it thereto.

EXAMPLE 1

A pure silver bromide emulsion (control emulsion 1) was prepared as described hereafter.
Following solutions were prepared :

3173 ml of a dispersion medium (C) containing 0.156 moles of sodium bromide and 12.5 g of inert gelatin ; temperature was established at 45 °C and pH was adjusted to 4.5;
a 2.94 molar silver nitrate solution (A);
a 2.94 molar sodium bromide solution (B).

A nucleation step was performed by introducing solution A and solution B simultaneously into dispersion medium C both at a flow rate of 25 ml/min during 28 seconds. During the following physical ripening stage of 15 min the temperature was raised to 70 °C and 532 ml of water and 48 g of inert gelatin were added; the mixture was stirred for an additional 5 minutes. Then a first growth step was performed by introducing by a double jet during 564 seconds solution A at a constant flow rate of 5.0 ml/min and solution B as to maintain a constant silver potential, measured by a silver electrode versus calomel , of -3 milli-Volt. Then a second growth step was performed by introducing by a double jet during 3764 seconds solution A starting at a flow rate of 5.0 ml/min and linearly increasing the flow rate to an end value of 25 ml/min and solution B at an increasing flow rate as to maintain a constant silver potential of -3 mV.
Finally the emulsion was concentrated and washed by an ultrafiltration technique.
A second pure silver bromide emulsion in accordance with the present invention (emulsion 2) was prepared in a way similar to control emulsion 1 with the following modification : during the physical ripening stage 0.18 g of adenine was added simultaneously with the water and the gelatin.
A third emulsion containing 99 % of bromide and 1 % of iodide (control emulsion 3) was prepared by a method similar to control emulsion 1 with following modifications :

solution B was splitted in a solution B1, 2.94 molar on silver bromide, and a solution B2, 2.87 molar on silver bromide and 0.07 molar on silver iodide ;
the growth steps were executed as follows : a first growth step was performed by introducing by a double jet during 564 seconds solution A at a constant flow rate of 5.0 ml/min and solution B as to maintain a constant silver potential, measured by a silver electrode versus calomel, of -3 milli-Volt. Then a second growth step was performed by introducing by a double jet during 2674 seconds solution A starting at a flow rate of 5.0 ml/min and linearly increasing the flow rate to an end value of 19.2 ml/min and solution B1 at an increasing flow rate as to maintain a constant silver potential of -3 mV. Then a third growth step was performed by introducing by a double jet during 1088 seconds solution A starting at a flow rate of 19.2 ml/min and linearly increasing the flow rate to an end value of 25 ml/min and solution B2 at an increasing flow rate as to maintain a constant silver potential of -3 mV.

A fourth emulsion containing 99 % of bromide and 1 % of iodide in accordance with the present invention (emulsion 4) was prepared in a way similar to control emulsion 3 with the following modification : during the physical ripening stage 0.18 g of adenine was added simultaneously with the water and the gelatin.
The grain size characteristics of emulsion samples 1 to 4 are summarized in table 1.
The results of Table 1 clearly illustrate the gain in monodispersity of the tabular grain fraction. As a supplemental result the monodispersity of the total grain population is increased. Another supplemental favourable result is the increase in the percentual share of the tabular grain fraction.
The emulsion samples were chemically ripened to an optimal fog-sensitivity relationship using conventional sulfur-gold sensitization. Then they were sensitized to the green spectral region by the addition of a green sensitizer with following chemical formula :
As a stabilizer 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene was added. Finally conventional coating aids were added and then the emulsion samples were coated on both sides of a transparent polyethylene terephtalate support at a coverage of 3 g expressed as AgNO₃/m² on both sides. A protective top layer was coated on each side.
The coated emulsion samples were image-wise exposed through a continuous tone wedge using a conventional white light source. Then they were developed on one hand in a active radiographic developer containing per liter 40 g of hydroquinone, 1.5 g of Phenidone and a polyoxyalkylene development accelerator at pH 10.3, and on the other hand in a rather weak developer containing per liter a.o. 26.7 g of hydroquinone and 1.6 g of Phenidone at pH 10.4. Finally the coated samples were fixed in a conventional ammonium thiosulphate containing fixing solution and dried.
Table 2 presents the sensitometric results in the active developer and the difference in speed and in gradation between development in the weak and in the strong developer; a low difference is considered to be an indication of a good developability of the emulsion sample under consideration.
Table 2 illustrates the better developability of the emulsions in connection with the invention.

EXAMPLE 2

A pure silver bromide emulsion (emulsion 5) was prepared in a way similar to emulsion 2 with the modification that 0.09 g of 4-aminopyrazolo[3,4d]pyrimidine was added during physical ripening instead of 0.18 g of adenine;
Table 3 compares the grain size characteristics of emulsion 5 with those of control emulsion 1; the sensitometric characteristics are given in table 4. TABLE 3

sample d_S v d_EM V_EM th AR d_S ^# % t. gr.

em. 1 0.74 0.28 1.46 0.12 0.25 6 0.93 60.1

em. 5 0.53 0.21 1.05 0.07 0.15 7 0.63 60.1.
The results of table 3 again illustrate the greater monodispersity and the greater procentual share in tabular grains of the emulsion in connection with the invention compared to the control emulsion. TABLE 4

sample sensitometry strong dev. △S △grad.

fog S grad. Dmax

em. 1 0.005 170 301 3.73 -12 -9

em. 5 0.006 183 302 3.34 -5 +5.3
The results of table 4 show the better developability of the emulsion prepared according to the invention.

Claims

Method for the preparation of a silver bromide or iodobromide emulsion containing monodisperse tabular grains accounting for at least 50 % of the total projected area of all grains, wherein the coëfficiënt of variation of the grain size distribution of said tabular grains is at most 15 %, characterized in that an aminoazaindene is added at any stage of the emulsion preparation before 50 % of the total silver halide is precipitated.
Method according to claim 1 wherein the aspect ratio of said tabular grains is comprised between 2 and 12.
Method according to claim 1 or 2 wherein said aminoazaindene is an adenine derivative according to formula Ia or a 4-amino-pyrazolo-pyrimidine derivative according to formula Ib :
wherein each of R¹ and R² which may be the same or different represents hydrogen, alkyl, alkenyl, aryl, alkoxy, hydroxy, mercapto, carboxy, amino or halogen, and
each of R³ and R⁴ which may be the same or different represents hydrogen or alkyl;
Method according to claim 3 wherein said adenine derivative according to formula Ia is adenine itself.
Method according to claim 3 wherein said 4-amino-pyrazolo-pyrimidine derivative according to formula Ib is 4-aminopyrazolo[3,4d]pyrimidine.
Method according to any of claims 1 to 5 wherein said aminoazaindene is present in a concentration ranging from 10⁻⁴ mole to 10⁻³ per mole silver halide.
Method according to any of claims 1 to 6 wherein said aminoazaindene is added during a physical ripening stage.
Method according to any of claims 1 to 7 wherein said tabular grains account for at least 70 % of the total projected area of all grains.
Method according to any of claims 1 to 8 wherein the coëfficiënt of variation of the grain size distribution of said tabular grains is at most 10 %.
Photographic material comprising a support and at least one emulsion layer containing an emulsion prepared according to the method of any of claims 1 to 9.