Silver halide emulsion comprising grains having a crater or hopper structure and method for the preparation thereof

Abstract: A process for preparing a silver halide photographic emulsion comprising chemically and spectrally sensitized silver halide grains, said silver halide photographic emulsion comprising chemically and spectrally sensitized silver halide grains having a cubic crystal lattice structure, and comprising at least one [111] and/or at least one [100] crystal face having at least one cracked or hollow zone, around which silver halide protrusions form at least one crater and/or at least one hopper structure on at least one crystal face of said silver halide grains.

Description: FIELD OF THE INVENTION The present invention relates to silver halide photographic emulsions, their preparation and their use in photographic films. In particular, the present invention relates to spectrally and chemically sensitized silver halide grains having a different halide composition in their structure and principally bounded by [100] and/or [111] crystal faces, on which silver halide protrusions are formed, said protrusions forming at least one crater or hopper structure on at least one crystal face of said silver halide grains. BACKGROUND OF THE ART The concept of combining halides to achieve the advantages of separate silver halides within a single silver halide grain structure has been recognized in the art and may have been used even earlier in the art without recognition of its use or benefits. Numerous references to core-shell structures for silver halide grains exist in the literature. Typically the core is coated over its entire surface with one or more layers or "shells" of a different silver halide. References to the core-shell structure can be found in US Patents Nos. 3,505,068, 3,695,888, 4,444,877, 4,477,564, and GB Patent No.1,027,146. Investigations of composite silver halide grains in which a second silver halide does not form a shell surrounding a first core silver halide can be found in US Patent No. 3,804,629, 4,094,684, 4,142,900, 4,435,501, and 4,463,087, all these patents referring to an epitaxial deposition of silver halide on silver halide host grains, i.e., said deposition having its orientation controlled by the silver halide grains forming the crystal structure on which the deposition is grown, and said deposition is substantially confined to a selected surface site. US 4,496,652 refers to the growth of silver halide corners on the [111] crystal faces of tetradecahedral silver halide grains to form regular cubic grains, and US 4,710,455 refers to silver halide grains having a depression and/or a cavity in each face thereof, obtained by precipitating, on host silver halide grains, a second silver halide salt having a lower solubility than the silver halide salt of the host grains. Other references can be found in Berry and Skillman, Journal of Applied Physics, Vol. 35, No. 7 "Surface Structures and Epitaxial Growths on AgBr Microcrystals", July 1964, in Karasawa, Journal of the Society of Photographic Science and Technology of Japan Vol. 47 No. 4, "Silver Halide Crystals in New Type Photographic Emulsion"; and Suzuky and Ueda "The Active Sites for Chemical Sensitization of Monodisperse AgBr Emulsion", 1973, SPSE Tokyo Symposium. EP 232,160 discloses silver halide emulsions comprising silver halide grains having 8 or 12 concavity on the surface thereof, resulting from the dissolution of the whole (111) and (110) or (nn1) faces, respectively and the consequent deposition of dissolved silver halide on other crystal faces. EP 367,248 discloses silver halide grains having calyxes formed by protrusions grown in parallel to plane faces (or ridge faces) formed along the ridge lines of a host silver halide grain. No mention is made of formation of depressed zone by dissolution of existing surface. It is known in the art to vary halide ratios of the silver halide grains and to make use of core-shell emulsions, but the particular spectrally and chemically sensitized silver halide grains of the present invention are not believed to have been described yet. SUMMARY OF THE INVENTION A process for preparing a silver halide photographic emulsion comprising chemically and spectrally sensitized hollow or cracked silver halide grains having a crater or hopper structure, said process comprising the following steps: (a) forming in a reaction vessel a host silver halide grain emulsion, which emulsion comprises, in a hydrophilic colloid dispersing medium, silver halide grains containing at least 50%M silver bromide and having a cubic crystal lattice structure comprising at least one [111] and/or at least one [100] crystal face; (b) performing, at pH of from 5 to 7 and at a temperature between 30° and 50°C, a first addition of silver and/or halide salt solutions until a pAg of from 7 to 9 is reached; (c) performing, at pAg of from 7 to 9, a second addition of silver and halide salt solutions, until from 0.01 to 0.10 moles of silver per mole of silver of the host silver halide emulsion has been added, concurrently running in the reaction vessel a solution comprising a isothiouronium compound having the following formula: wherein R 1 and R 2 are straight or branched alkyl group, alkylene group, aryl group, or allyl group, with at least one of R 1 and R 2 being an allyl group, A is a divalent organic linking group and R 3 is a moiety selected from the class of carboxyl moiety, sulfo moiety, hydroxyl moiety, or amino moiety, B is an anion, and n is 0 (when R 3 is a negatively charged group) or 1 (when R 3 is a uncharged group), and (d) ripening, at pH of from 6 to 8, pAg of from 7 to 9 and at a temperature between 30° and 50°C, the obtained silver halide emulsion. In another aspect the present invention relates to silver halide photographic emulsions obtained with the process of the present invention. DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a process for preparing a silver halide photographic emulsion comprising chemically and spectrally sensitized hollow or cracked grains having a crater or hopper structure, said process comprising the following steps: (a) forming in a reaction vessel a host silver halide grain emulsion, which emulsion comprises, in a hydrophilic colloid dispersing medium, silver halide grains containing at least 50%M silver bromide and having a cubic crystal lattice structure comprising at least one [111] and/or at least one [100] crystal face; (b) performing, at pH of from 5 to 7 and at a temperature between 30° and 50°C, a first addition of silver and/or halide salt solutions until a pAg of from 7 to 9 is reached; (c) performing, at pAg of from 7 to 9, a second addition of silver and halide salt solutions, until from 0.01 to 0.10 moles of silver per mole of silver of the host silver halide emulsion has been added, concurrently running in the reaction vessel a solution comprising a isothiouronium compound having the following formula: wherein R 1 and R 2 are straight or branched alkyl group, alkylene group, aryl group, or allyl group, with at least one of R 1 and R 2 being an allyl group, A is a divalent organic linking group and R 3 is a moiety selected in the class of carboxyl moiety, sulfo moiety, hydroxyl moiety, or amino moiety, B is an anion, and n is 0 (when R 3 is a negatively charged group) or 1 (when R 3 is a uncharged group), and (d) ripening, at pH of from 6 to 8, pAg of from 7 to 9 and at a temperature between 30° and 50°C, the obtained silver halide emulsion. The host silver halide grains can he provided by any conventional silver halide emulsion the grains of which have a cubic crystal lattice structure and are bounded by at least one [111] and/or [100] crystal face and comprising at least 50%M of silver bromide, in a preferred embodiment at least 80%M of silver bromide, the remaining part comprising less than 20%M of silver iodide, more preferably less than 8% silver iodide, and less than 10%M of silver chloride, more preferably less than 5%M silver chloride. In a more preferred embodiment silver chloride is absent. All silver halide grains, except high iodide silver halide grains, show cubic crystal lattice structures, but said grains can differ markedly in appearance. The most common aspects of silver halide grains are the cubic and the octahedral form. The cubic grains are bounded by six identical crystal faces, usually referred to as [100] crystal faces, according to the well known Miller index employed for describing crystal faces. Although the cubic crystal shape is readily visually identified in regular grains, in grains of more complex shapes the presence of [100] crystal faces can best be verified by a combination of visual inspection and the 90° angle of intersection formed by adjacent [100] crystal faces. The octahedral grains are bounded by eight identical crystal faces, usually referred to as [111] crystal faces, according to the well known Miller index employed for describing crystal faces. Although the octahedral crystal shape is readily visually identified in regular grains, in grains of more complex shapes, the presence of [111] crystal faces can be verified by a combination of visual inspection and the 109.5° angle of intersection formed by adjacent [111] crystal faces. It is important to bear in mind that both the cubic and octahedral grains have exactly the same cubic crystal lattice structure and thus the same internal relationship of silver and halide ions. The two kinds of grains differ only in their surface crystal faces. In the [100] crystal faces, each surface silver ion lies immediately adjacent five halide ions, whereas the surface silver ions at the [111] crystal faces each lies immediately adjacent only three halide ions. Although these are the most common regular forms of silver halide grains, it is well known that in practice many other varied crystal shapes are also observed. For example, silver halide grains can be tetradecahedral, i.e., formed of a combination of [111] and [100] crystal faces. A regular tetradecahedral grain has fourteen crystal faces, six [100] crystal faces and eight [111] crystal faces. In most instances, some corner or edge rounding due to silver halide solvent action is observed, and in some instances rounding is so pronounced that the grains are described as spherical. Other grain shapes, such as tabular grains and rods, can be attributed to internal crystal irregularities, such as twin planes and screw dislocations. Tabular grains are predominantly bounded by two [111] crystal faces parallel to multiply stacked parallel twin planes, which affect the relative number of edge reentrant angles and edge non-reentrant angles on alternate edges. The number of said twin planes affects the form of the tabular grain, i.e., a tabular grain having an odd number of twin planes normally has a triangular main surface, a tabular grain having an even number of twin planes normally has a hexagonal main surface. The host silver halide emulsion used in this invention may be a dispersion of silver bromide, silver chloro-bromide, silver iodo-bromide and silver chloro-iodobromide in a hydrophilic binder. Preferred silver halides are silver bromide and silver iodo-bromide containing 1 to 20 %M, preferably 1 to 8 %M silver iodide. The emulsions can be composed of coarse, medium and fine grains and can be monodispersed or polydispersed. The silver halide grains may have any crystal form such as cubical, octahedral, tabular or a mixed crystal form. The silver halide can have a uniform grain size or a broad grain size distribution. The size of the silver halide ranges from about 0.1 to about 5 µm. The silver halide emulsion can be prepared using a single-jet method, a double-jet method, or a combination of these methods or can be matured using, for instance, an ammonia method, a neutralization method, an acid method, or can be performed an accelerated or constant flow rate precipitation, interrupted precipitation, ultrafiltration during precipitation, etc. References can be found in Trivelli and Smith, The Photographic Journal, Vol. LXXIX, May 1939, pp. 330-338, T.H. James, The Theory of The Photographic Process, 4th Edition, Chapter 3, US Patent Nos. 2,222,264, 3,650,757, 3,917,485, 3,790,387, 3,716,276, 3,979,213, Research Disclosure, Dec. 1989, Item 308119 "Photographic Silver Halide Emulsions, Preparations, Addenda, Processing and Systems", and Research Disclosure, Sept. 1976, Item 14987. The single jet method in which a silver nitrate solution is added in a halide and gelatin solution has been long used for manufacturing photographic emulsion. In this method, because the varying concentration of halides in the solution determines which silver halide grains are formed, the formed silver halide grains are a mixture of different kinds of shapes and sizes. In the double jet method, in which alkaline halide solution and silver nitrate solution are concurrently added in the gelatin solution, the shape and size of the formed silver halide grains can be controlled by the kind and concentration of the solvent existing in the gelatin solution and by the addition speed. In particular, the presence of bromide in excess can promote the formation of octahedral grains, predominantly bounded by [111] crystal face, while a lower quantity of bromide promotes the formation of cubic grains having [100] crystal face. An intermediate quantity of bromide and the presence of a silver halide solvent, in particular during Ostwald ripening, promote the formation of tetradecahedral or spherical grains, having both [100] and [111] crystal faces. The composition of silver halide grains influences the shape of the grain. Silver bromo-iodide grains have a higher inclination to form [111] crystal faces than silver bromide. Silver chloride principally forms [100] faces in any condition and it is difficult to form [111] crystal faces. In a preferred embodiment of the present invention, it is possible to start from the manufacturing of silver bromide and/or silver bromo-iodide up to 8%M of iodide host grains having a spherical or tetradecahedral shape by means of a double jet precipitation technique at pAg between 8 and 9 and in presence of a solvent of silver halide such as thiourea, ammonia or thioether. Within this pAg range, increasing pAg promotes the formation of a higher ratio of [111] faces to [100] faces. To minimize this ratio the pAg is maintained at the lowest value within said range to form almost perfect cubes. In this condition rounded cube grains predominantly bounded by [100] crystal face and having at least one, and at most eight [111] crystal faces are normally formed. In another preferred embodiment, it is possible to start from the manufacturing of silver bromide and/or silver bromo-iodide up to 8%M of iodide host grains having a tabular shape by means of a double jet precipitation of silver and halide salts at pAg higher than 9. The dispersing medium in the reaction vessel is initially substantially free of iodide ions, i.e., lower than 0.5%M, since the presence of iodide prior to concurrent introduction of silver and halide salts favors the formation of thick non-tabular grains. Moreover, the pBr of the reaction vessel is initially maintained between 0.6 and 1.6. Reference to well known methods to obtain tabular grains predominantly bounded by two [111] main face can be found in Karasawa, Journal of the Society of Photographic Science and Technology of Japan Vol. 47 No. 4, "Silver Halide Crystals in New Type Photographic Emulsion", in US Patents Nos. 4,386,156, 4,434,226, 4,411,986, 4,414,304, 4,433,048, 4,435,501, 4,439,520, 4,414,310, and 4,413,053. Tabular grain emulsions can be defined as intermediate and high aspect ratio tabular grain emulsions. As applied to tabular grain emulsions, the term "high aspect ratio" is defined as requiring that the silver halide grains having a thickness of 0.3 µm or less and a diameter of at least 0.6 µm have an average aspect ratio of at least 8:1 and account for at least 50% of the total projected area of the silver halide grains present in the emulsion. The term is thus defined in conformity with the term used in the patents relating to tabular grain emulsions cited above. The term "intermediate aspect ratio" as applied to tabular grain emulsions is defined as requiring that the tabular silver halide grains having a thickness of 0.3 µm or less and a diameter of at least 0.6 µm have an average aspect ratio in the range of from 5:1 to 8:1 and account for at least 50% of the total projected area of the silver halide grains present in the emulsion. The term "thin intermediate aspect ratio" is similarly defined, except that the reference thickness of 0.3 µm is replaced by a reference thickness of 0.2 µm. The grain characteristics described above of the tabular silver halide grains can be readily ascertained by procedures well known to those skilled in the art. The term "diameter" is defined as the diameter of a circle having an area equal to the projected area of the grain. The term "thickness" means the distance between the two substantially parallel main planes constituting the tabular silver halide grains. From the measure of diameter and thickness of each grain the diameter:thickness of each grain can be calculated, and the diameter to thickness ratios of all tabular grains can be averaged to obtain their average diameter:thickness ratio. By this definition the average diameter to thickness ratio is the average of individual tabular grain diameter:thickness ratios. In practice it is simpler to obtain an average diameter and an average thickness of the tabular grains and to calculate the average diameter:thickness ratio as the ratio of these two averages. Whatever the used method may be, the average diameter:thickness ratios obtained do not significantly differ. The host tabular silver halide grains contained in the silver halide emulsion of this invention have preferably an average aspect ratio of at least 3:1, preferably at least 5:1, more preferably of from 5:1 to 8:1. Average diameters of the tabular silver halide grains suitable for use in this invention range from about 0.3 to about 5 µm, preferably 0.5 to 3 µm, more preferably 0.8 to 1.5 µm. The tabular silver halide grains suitable for use in this invention have a thickness of less than 0.4 µm, preferably less than 0.3 µm and more preferably less than 0.2 µm. The dispersing medium for said silver halide grains can be chosen among those conventionally employed in the silver halide emulsions. Preferred dispersion media include hydrophilic colloids, such as proteins, protein derivatives, cellulose derivatives (e.g. cellulose esters), gelatin (e.g. acid or alkali treated gelatin), gelatin derivatives (e.g. acetylated gelatin, phthalated gelatin and the like), polysaccharides (e.g. dextran), gum arabic, casein and the like. It is also common to employ said hydrophilic colloids in combination with synthetic polymeric binders and peptizers such as acrylamide and methacrylamide polymers, polymers of alkyl and sulfoalkyl acrylates and methacrylates, polyvinyl alcohol and its derivatives, polyvinyl lactams, polyamides, polyamines, polyvinyl acetates, and the like. At the end of grain precipitation, water soluble salts are removed from the emulsion by procedures known in the art. Suitable cleaning arrangements are those wherein the dispersing medium and soluble salts dissolved therein can be removed from the silver halide emulsion on a continuous basis, such as, for example, a combination of dialysis or electrodialysis for the removal of soluble salts or a combination of osmosis or reverse osmosis for the removal of the dispersing medium. In a particularly preferred embodiment, among the known techniques for removing the dispersing medium and soluble salts while retaining silver halide grains in the remaining dispersion, ultrafiltration is a particularly advantageous cleaning arrangement for the practice of this process. Typically, an ultrafiltration unit comprising membranes of inert, non-ionic polymers is used as a cleaning arrangement. Since silver halide grains are large in comparison with the dispersing medium and the soluble salts or ions, silver halide grains are retained by said membranes while the dispersing medium and the soluble salts dissolved therein are removed. The action mechanism of preferred membranes is described in GB 1,307,331. The membranes used in the ultrafiltration comprise a very thin layer of extremely fine pore texture supported upon a thicker porous structure. Suitable membranes consist of polymers such as polyvinylacetate, polyvinylalcohol, polyvinylformate, polyvinylethers, polyamides, polyimides, polyvinyl chloride and polyvinylidene chloride, aromatic polymers, such as aromatic polyesters, polytetrafluoroethylene, regenerated cellulose, cellulose esters, such as cellulose acetate, or mixed cellulose esters. The membranes in question have anisotropic, semipermeable properties, show considerable mechanical, thermal and chemical stability and are photographically inert. The membranes are preferably permeable to molecules having molecular weights of up to about 300,000 and, more especially, of up to about 50,000. The novel silver halide grain emulsion of the present invention is obtained by a precipitation method by adding an aqueous solution of a halide and silver to the host silver halide grain emulsion above described, under controlled conditions of pAg, pH and temperature. The preferred embodiment of the process of production according to this invention will be explained in more detail. To the host silver halide grain emulsion, as above described, at pH between 5 and 7, preferably between 5.5 and 6.5 and at temperature between 30° and 50°C, preferably between 35° and 45°C, a first addition of silver and/or halide salt solution was performed until to reach a pAg between 7 and 9, preferably between 7.5 and 8.5. Said first addition can be performed by a single jet of silver salt solution, such as AgNO 3 , and/or a single yet of halide salt solution, such as NaCl, KI, KBr, and the like. A pause between the two additions can be performed. Alternatively, silver and halide salt solutions may be concurrently added by a double jet method. In a preferred embodiment of the present invention two different halide salts are concurrently or sequentially added. In a more preferred embodiment of the present invention chloride and bromide salts or chloride and iodide salts are concurrently or sequentially added. During this first addition, a total of Ag between 1 and 20 mmoles, preferably between 4 and 10 mmoles per mole of Ag of the starting emulsion was consumed. To the resulting emulsion, at pAg between 7 and 9, preferably between 8 and 9, a double jet of silver and halide salt solutions was performed, adding from 0.01 to 0.10, preferably from 0.03 to 0.07 moles of Ag per mole of Ag of the starting emulsion. Said addition can be performed by accelerated or constant flow rate, the latter being however preferred. In a preferred embodiment of the present invention said halide salts consist of chloride and/or bromide salts. During said double-jet addition, preferably after from 2 to 15 minutes, more preferably from 3 to 10 minutes from the start, a solution of a isothiouronium compound is concurrently run. Said isothiouronium compound is represented by the following formula: wherein R1 and R2 are straight or branched alkyl group, alkylene group, aryl group having from 1 to 10 carbon atoms, or allyl group, with at least one of R 1 and R 2 being an allyl group, A is a divalent organic linking group such as, for example, straight or branched divalent alkyl group, alkylene group, or aralkyl group having from 1 to 14 carbon atoms, and R 3 is a moiety selected in the class of carboxyl moiety, sulfo moiety, hydroxyl moiety, or amino moiety, B is an anion, and n is 0 (when R 3 is a negatively charged group) or 1 (when R 3 is a uncharged group). As is well understood in this technical area, a large degree of substitution os not only tolerated but is often advisable. As a means of simplifying the discussion and recitation of these groups, the term "group" and "moiety" are used to differentiate between chemical species that allow for substitution or which may be substituted. For example, the phrase "alkyl group" is intended to include not only pure hydrocarbon alkyl chains such as methyl, ethyl, octyl, cyclo-hexyl, isooctyl, tert-butyl and the like, but also such alkyl chains bearing such conventional substituentes in the art such as hydroxyl, alkoxy, phenyl, halo (F, Cl, Br, I), cyano, nitro, amino, etc. The phrase "alkyl moiety" on the other hand is limited to the inclusion of only pure hydrocarbon alkyl chains such as methyl, ethyl, propyl, cyclo-hexyl, isooctyl, tert-butyl, and the like. Typical examples of said isothiouronium compounds are the following, being understood that the invention is not limited thereto. In a preferred embodiment of the present invention a pause of from 1 to 60, preferably of from 5 to 40 minutes is performed between said first addition and said second double jet addition. During said pause the pAg can be adjusted in the range between 8 and 9. Concurrently or sequentially to the addition of the solution of isothiouronium salt, the silver halide grain emulsion of the present invention is chemically sensitized using the usual sensitizing agents. Sulfur containing compounds, gold and noble metal compounds, polyoxylakylene compounds are particularly suitable. In particular, the silver halide emulsions may be chemically sensitized with a sulfur sensitizer, such as allyl-thiocarbamide, thiourea, cystine, etc.; an active or inert selenium sensitizer; a reducing sensitizer such as stannous salt, a polyamine, etc.; a noble metal sensitizer, such as gold sensitizer, more specifically potassium aurithiocyanate, potassium chloroaurate, etc.; or a sensitizer of a water soluble salt such as for instance of ruthenium, rhodium, iridium and the like, more specifically, ammonium chloropalladate, potassium chloroplatinate and sodium chloropalladite, etc.; each being employed either alone or in a suitable combination. Other useful examples of chemical sensitizers are described, for example, in Research Disclosure 17643, Section III, 1978 and in Research Disclosure 308119, Section III, 1989. Moreover, the silver halide grain emulsion of the present invention may be optically sensitized to a desired region of the visible spectrum. The method for spectral sensitization of the present invention is not particularly limited. For example, optical sensitization may be possible by using an optical sensitizer, including a cyanine dye, a merocyanine dye, complex cyanine and merocyanine dyes, oxonol dyes, hemioxonol dyes, styryl dyes and streptocyanine dyes, either alone or in combination. Particularly useful optical sensitizers are the dyes of the benzoxazole-, benzimidazole- and benzothiazole-carbocyanine type. The addition of the spectral sensitizer can be performed before or after said double jet addition of silver and halide salts. In a preferred embodiment said spectral sensitizers produce J aggregates if adsorbed on the surface of the silver halide grains and a sharp absorption band (J-band) with a bathochromic shifting with respect to the absorption maximum of the free dye in aqueous solution. Spectral sensitizing dyes producing J aggregates are well known in the art, as illustrated by F. M. Hamer, Cyanine Dyes and Related Compounds , John Wiley and Sons, 1964, Chapter XVII and by T. H. James, The Theory of the Photographic Process, 4th edition, Macmillan, 1977, Chapter 8. In a preferred form, J-band exhibiting dyes are cyanine dyes. Such dyes comprise two basic heterocyclic nuclei joined by a linkage of methine groups. The heterocyclic nuclei preferably include fused benzene rings to enhance J aggregation. The heterocyclic nuclei are preferably quinolinium, benzoxazolium, benzothiazolium, benzoselenazolium, benzimidazolium, naphthoxazolium, naphthothiazolium and naphthoselenazolium quaternary salts. Other useful spectral sensitizing dyes can be found in Research Disclosure 17643, 1978 and in Research Disclosure 308119, Section IV, 1989. At the end of the double-jet precipitation the silver halide emulsion of the present invention is ripened at a temperature between 30° and 60°C, preferably between 30° and 50°C. To the above emulsion may also be added various additives conveniently used depending upon their purpose. These additives include, for example, stabilizers or antifoggants such as azaindenes, triazoles, tetrazoles, imidazolium salts, polyhydroxy compounds and others; film hardeners such as aldehyde, aziridine, isoxazole, vinylsulfone, acryloyl, triazine type, etc.; developing promoters such as benzyl alcohol, polyoxyethylene type compounds, etc.; image stabilizers such as compounds of the chromane, cumaran, bisphenol type, etc.; and lubricants such as wax, higher fatty acids glycerides, higher alcohol esters of higher fatty acids, etc. Also, coating aids, modifiers of the permeability in the processing liquids, defoaming agents, antistatic agents and matting agents may be used. Other useful additives are disclosed in Research Disclosure, Item 17643, December 1978 in Research Disclosure, Item 18431, August 1979 and in Research Disclosure 308119, Section IV, 1989. The resulting silver halide grains shown, to electron microscope, a grain structure modification if compared with the starting silver halide grains. In particular, it can be noted that an etching of grains has taken place with the formation of cracked or hollow zones, around which, silver halide protrusions form crater or hopper structures on the surface of grain. A grain analysis by X-ray difractometry showed that these protrusions are predominantly formed by a silver halide composition equal to that of the second double jet addition. The term "cracked zone" defines a zone of the grain surface on which the process of the present invention makes an irregular furrow or groove on a surface of the grain. Said furrow is due to the etching of the grain surface with attendant dissolution of part of the silver halide present on the surface of the host grain. These "cracked zones" can be then defined as narrow and long depressions, that generally appear on only a small part of one grain surface. The term "hollow zone" defines a zone of the grain surface on which the process of the present invention make a concavity such as the hollow of one's hand. In this case "hollow" does not mean a cavity passing through the grain. The term "hollow zone" defines a large and rounded depression that appears on the major part of one grain surface. This is different from the feature defined by the term "cracked zone". The term "crater structure" defines a structure comprising a cracked zone, as previously defined, having additional protrusions that form a raised area around the edges of said cracked zone. Said crater structure, and in particular said additional protrusions are due to the deposition of silver halide added during the process of the present invention. The term "hopper structure" defines a structure comprising a hollow zone, as previously defined, having additional protrusions growing on the edges or corners of the grain, in such a way that a stronger marked depression is formed. Hopper development of crystal is a well known phenomenon in the art of crystallography. Hopper development takes place when a growing gradient of supersaturation from the center to the edges of the crystal face promotes a higher rate of nucleation near the edges of the crystal face and a lower one around the center. This leads to a preferential thickening at the periphery, more pronounced starvation of the face center, and thus to a hopper structure, either in the form of shallow disc or as hollow prism. References can be found in J.J.Gilman, The Art and Science of Growing Crystals, pp. 141-143 John Wiley & Sons, Inc., New York - London, 1963. The mechanism of formation of the crater structures of the present invention is not yet completely understood. It may be explained as a particular hopper development that takes place on a limited part of the crystal face, when a little gradient of supersaturation is created from the center to the edges of said cracked zones. The preferential thickening is then limited to the edges of said cracked zone, with a more pronounced starvation of its hollow center. In order to better define the scope of the present invention, it must be understood that said grain structure modification need not necessarily be present on all the silver halide grains of the silver halide emulsion obtained with the method of the present invention. The result of better sensitometric characteristics provided by the method of the present invention can be already obtained when at least 1%, preferably at least 5%, more preferably at least 10% by number of grains of the silver halide grain emulsion of the present invention show said grain structure modification. According to the present invention, when we start from silver bromide and/or silver bromo-iodide up to 8%M of iodide host grains having a spherical or tetradecahedral shape bounded by [100] crystal faces and having at least one and at most eight [111] crystal faces, the resulting silver halide grains have at least one protrusion on at least one of said [111] crystal faces. [100] crystal faces are etched with the formation of cracked or hollow zones. When silver halide protrusions are distributed on each [111] crystal face of the grain a hopper structure is formed, showing stronger marked depressions delimited by said protrusions. According to another embodiment of the present invention, when we start from silver bromide and/or silver bromo-iodide up to 8%M of iodide host grains having a tabular shape predominantly bounded by two [111] main crystal faces and an aspect ratio between 5:1 and 8:1, the resulting silver halide grains have at least one protrusion on at least one of said [111] crystal faces, said protrusions being formed around the edges of at least one cracked zone and forming at least one crater structure on said crystal face. A light-sensitive silver halide photographic material can be prepared by coating the above described silver halide emulsion of the present invention on a photographic support. There is no limitation with respect to the support. Examples of materials suitable for the preparation of the support include glass, paper, polyethylene-coated paper, metals, cellulose nitrate, cellulose acetate, polystyrene, polyesters such as polyethylene terephthalate, polyethylene, polypropylene and other well known supports. Said light-sensitive silver halide photographic material specifically is applicable to light-sensitive photographic color materials such as color negative films, color reversal films, color papers, etc., as well as black-and-white light-sensitive photographic materials such as X-ray light-sensitve materials, lithographic light-sensitive materials, black-and-white photographic printing papers, black-and-white negative films, etc. Preferred light-sensitive silver halide photographic materials are X-ray light-sensitive materials comprising the above described silver halide emulsion coated on one surface, preferably on both surfaces of a support, preferably a polyethylene terephthalate support. Preferably, the silver halide emulsion is coated on the support at a total silver coverage comprised in the range of 3 to 6 grams per square meter. Usually, the X-ray light-sensitive materials are associated with intensifying screens so as to be exposed to radiation emitted by said screens. The screens are made of relatively thick phosphor layers which transform the X-rays into light radiation (e.g., visible light). The screens absorb a portion of X-rays much larger than the light-sensitive material and are used to reduce the X-ray dose necessary to obtain a useful image. According to their chemical composition, the phosphors can emit radiation in the blue, green or red region of the visible spectrum and the silver halide emulsions are sensitized to the wavelength region of the light emitted by the screens. Sensitization is performed by using spectral sensitizing dyes adsorbed on the surface of the silver halide grains as known in the art. The exposed light-sensitive materials of this invention can be processed by any of the conventional processing techniques. The processing can be a black-and-white photographic processing for forming a silver image or a color photographic processing for forming a dye image depending upon the purpose. Such processing techniques are illustrated for example in Research Disclosure, 17643, December 1978. Roller transport processing in an automatic processor is particularly preferred, as illustrated in US Pat. Nos. 3,025,779, 3,515,556, 3,545,971 and 3,647,459 and in UK Pat. No. 1,269,268. Hardening development can be undertaken, as illustrated in US Pat. No. 3,232,761. The present invention is now illustrated by reference to the following examples. EXAMPLE 1 Emulsion 1 To a silver halide emulsion comprising silver halide rounded grains predominantly bounded by [100] crystal faces, but having at least one [111] crystal face and comprising 97.76M% of AgBr and 2.26M% of AgI was added per mole of Ag at pH 5.7 and pAg 8.2, 3 ml of NaCl 4.27N and 55 ml of a 0.4% by weight solution of optical sensitizer having the following formula: To the resulting solution a double jet addition of NaCl and AgNO 3 solution was performed for 60 minutes to precipitate 7M% of AgCl per mole of Ag. During said double jet addition the pAg was maintained to 8.2 and pH to 6.7. 17ml of a 0.05% by weight solution of isothiouronium salt having the following formula: 7ml of a solution of gold chloride 0.2mM, 12ml of a solution of potassium thiocyanate (KCNS) 0.031N and 10ml of a solution of sodium p-toluenethiosulfonate 0.027M per mole of Ag were added starting 8 minutes after the starting of said double jet addition. Then the resulting emulsion was ripened for 110 minutes at 47°C, at pH 6.7 and pAg 8.2. Emulsion 2 To the same starting silver halide emulsion of emulsion 1 was added at pH 6.8 and pAg 8.5, 15ml of a solution of benzothiazolium iodoethylate 3.4mM, 10ml of a solution of sodium p-toluenethiosulfonate 0.027M, 7ml of a solution of gold chloride 0.2mM, and 12ml of a solution of KCNS 0.031M and then the resulting emulsion was ripened for 155 minutes at 55°C. Emulsions 1 and 2 were coated double side with conventional coating methods for a total of 4.4 g of Ag/square meter, incubated 15h at 50°C, and exposed to blue and green light and to X-rays with 3M TRIMAX™ T8 intensifying screen, and processed in a 3M TRIMATIC™ XP507 automatic processor using a 3M XAD/2 Developer and a 3M XAF/2 Fixer. In the following table the photographic characteristics of the two emulsions are reported. Fog Dmax Average Contrast Speed Blue Green Em. 1 (inv.) 0.17 4.57 3.31 2.41 2.71 Em. 2 (comp.) 0.17 4.77 3.13 2.34 2.45 At the electron microscope emulsion 1 shows a modified grain structure when compared to the starting emulsion. In particular emulsion 1 shows a sharper corner profile than the starting emulsion with the presence of marked hollow zone bounded from protrusions that form a typical hopper structure on at least one crystal face of the grain. EXAMPLE 2 Emulsion 1 To a silver bromide emulsion comprising silver bromide tabular grain having an aspect ratio of 8:1 was added per mole of Ag and at pH 5.7, 5.6 ml of NaCl 4.27N and 6 mmoles of Ag, employing a solution of AgNO 3 0.25N, to have pAg 8.0, 56ml of KI 0.06N and 77 ml of a 1.0% by weight solution of optical sensitizer having the following formula: To the resulting solution a double jet addition of NaCl and AgNO 3 solution was performed for 60 minutes to precipitate 6M% of AgCl per mole of Ag. During said double jet addition the pAg was mantained to 8.0 and pH to 6.7. 8ml of a 0.1% by weight solution of the same isothiouronium salt of example 1, 7ml of a solution of gold chloride 0.2mM, 6.8ml of a solution of KCNS 0.031N and 21.7ml of a solution of sodium p-toluenethiosulfonate 0.027M per mole of Ag were added starting 8 minutes after the starting of said double jet addition. Then the resulting emulsion was ripened for 100 minutes at 43"C, at pH 7.2 and pAg 8.2. Emulsion 2 To the same starting silver bromide emulsion of emulsion 1 was added at pH 6.5 and pAg 8.4, 9,5ml of a 1% by weight solution of Leucophor™ 60ml of a 1% by weight solution of spectral sensitizer having the following formula: 25ml of a solution of benzothiazolium iodoethylate 3.4mM, 30ml of a solution of sodium p-toluenesulfinate, 25ml of a solution of sodium p-toluenethiosulfonate 0.027M, 8.7ml of a solution of gold chloride 0.2mM, and 3.5ml of a solution of KCNS 0.31N and then the resulting emulsion was ripened for 100 minutes at 70°C. Emulsions 1 and 2 were coated and treated as for example 1. In the following table 2 the photographic characteristics of the two emulsions are reported. Fog Dmax Average Contrast Speed Blue Green Em. 1 (inv.) 0.21 2.6 1.59 3.13 2.95 Em. 2 (comp.) 0.20 2.4 1.56 2.77 2.59 At the electron microscope emulsion 1 shows a modified grain structure when compared to the starting emulsion. Protrusions that form crater structures around the edges of cracked zones are evident on main faces. EXAMPLE 3 Emulsion 1 To a silver halide emulsion comprising silver halide rounded grains having at least one [111] crystal face, a mean diameter of 0.7 µm and comprising 97.74M% of AgBr and 2.26M% of AgI was added per mole of Ag at pH 5.8, pAg 8.5, and at a temperature of 40°C, 6 ml of NaCl 4.27N, 68 ml of a 1% by weight solution of optical sensitizer having the following formula: and, after a pause of 30 minutes, 5 mmoles of Ag in 15 minutes, employing a solution of AgNO 3 0.25N until a pAg of 8.0 was obtained. To the resulting solution a double jet addition of NaCl and AgNO 3 solution was performed for 60 minutes to precipitate 5M% of AgCl per mole of Ag. During said double jet addition the pAg was increased to 8.5 and pH to 6.7. 15ml of a solution of benzothiazolium, 7ml of a solution of gold chloride 0.2mM, 12ml of a solution of KCNS 0.31N and 10ml of a solution of sodium p-toluenethiosulfonate 0.027M per mole of Ag were added starting 6 minutes after the starting of said double jet addition. Then the resulting emulsion was ripened for 105 minutes at 55°C, at pH 6.7 and pAg 8.2. Emulsion 2 The same conditions of emulsion 1 were used except the use, during the double jet addition, of 8.1ml of a 0.1% by weight solution of the same isothiouronium salt of example 1, instead of benzothiazolium and a lower quantity of the solution of sodium p-toluenethiosulfonate (6.95ml per mole of Ag). The resulting emulsion was ripened for 120 minutes at 45°C and pAg 8.2. Emulsion 3 To the same starting silver halide emulsion of emulsion 1 was added at pH 6.8, temperature of 55°C and pAg 8.5, 15ml of a solution of benzothiazolium iodoethylate 3.4mM, 10ml of a solution of sodium p-toluenethiosulfonate 0.027M, 7ml of a solution of gold chloride 0.2mM, and 12ml of a solution of KCNS 0.31N and then the resulting emulsion was ripened for 170 minutes at 55°C. Emulsions 1, 2 and 3 were coated and treated as for example 1. In the following table the photographic characteristics of the three emulsions are reported. Fog Dmax Average Contrast Blue Speed Green Speed X-ray Speed Em. 1 (comp.) 0.16 3.55 2.53 2.26 2.57 2.68 Em. 2 (inv.) 0.17 3.61 2.67 2.31 2.62 2.75 Em. 3 (comp.) 0.17 3.70 2.81 2.23 2.47 2.63 The data of table 3 show that the use of isothiouronium salt gives better results in terms of blue, green and X-ray speed and allows to ripen at lower temperature, so avoiding undesired mixed cristal formation. At the electron microscope emulsions 1 and 2 show a modified grain structure. In particular emulsions 1 and 2 show a sharper corner profile than the starting emulsion with the presence of marked hollow zone bounded from protrusions that form a tipical hopper structure on at least one crystal face of the grain. EXAMPLE 4 Emulsion 1 The same emulsion 2 of example 3 was used. At the end the emulsion was ripened for 120 minutes at 45°C. Emulsion 2 The same conditions of emulsion 2 of example 3 were used except the use, during the double jet addition, of 0.3 mg of thiosulfate, 0.5 mg of NaAuCl 4 and 9 mg of KCNS per mole of silver. At the end the emulsion was ripened for 60 minutes at 45°C, for 75 minutes at 55°C, for 35 minutes at 60°C. Emulsion 3 The same conditions of emulsion 2 were used, but the use of KCNS. At the end the emulsion was ripened for 60 minutes at 45°C, for 75 minutes at 55°C, for 60 minutes at 60°C. The three emulsions were coated and treated as for example 1. The results are summarized in the following table 4. Fog Dmax Average Contrast Blue Speed Green Speed X-ray Speed Em. 1 (inv.) 0.20 3.66 2.80 2.36 2.69 2.82 Em. 2 (comp.) 0.19 2.30 0.63 1.40 1.65 1.59 Em. 3 (comp.) 0.20 2.11 0.50 1.12 1.40 1.35 These results clearly show that the gold plus thiosulfate ripening does not give good photographic property to the emulsion of the present invention. EXAMPLE 5 Emulsion 1 The same starting emulsion and conditions of emulsion 2 of example 3 were used, but without optical sensitizer addition. At the end the emulsion was ripened for 220 minutes at 45°C. Emulsion 2 The same starting emulsion and conditions of emulsion 3 of example 3 were used. The two emulsions were coated and treated as for example 1. The results are summarized in the following table 5. Fog Dmax Average Contrast Blue Speed Green Speed X-ray Speed Em. 1 (inv.) 0.17 3.72 2.68 2.35 2.55 2.62 Em. 2 (comp.) 0.18 3.85 2.79 2.21 2.45 2.49 Emulsion 1 shows the same hopper structure, showing edge and corner protrusions bounding strong marked hollow zones, as in the previous example 3. The absence of optical sensitizer does not affect sensitometric results (+0.13 X-ray speed, +0.14 and +0.10 respectively for blue and green speed). EXAMPLE 6 To a silver bromide emulsion comprising silver bromide tabular grains having an aspect ratio of 8:1 and a mean diameter of about 1.10 µm was added per mole of Ag, at pH 5.8 and at 40°C, 5.3 ml of NaCl 4.27N and 7.7 mmoles of Ag, employing a solution of AgNO 3 0.25N, to have pAg 8.0, 53ml of KI 0.06N and 4 mmoles of Ag, employing the same AgNO 3 solution as above, and, after a pause of 5 minutes, 60 ml of a 1% by weight solution of optical sensitizer having the following formula: To the resulting solution a double jet addition of NaCl and AgNO 3 solution was performed for 60 minutes until to precipitate 5.7M% of AgCl per mole of Ag. During said double jet addition the pAg was increased to 8.5. 7.6ml of a 0.1% by weight solution of the same isothiouronium salt of example 1, 7.5ml of a solution of gold chloride 0.2mM, 7.0ml of a solution of KCNS 0.31N and 17.8ml of a solution of sodium p-toluenethiosulfonate 0.027M per mole of Ag were added starting 6 minutes after the starting of said double jet addition. Then the resulting emulsion was ripened at 43°C, at pH 7.2 and pAg 8.2 at different digestion time: Emulsion 1 91 minutes Emulsion 2 95 minutes At the end, both emulsions were stabilized in conventional manner, mantained 10 minutes at 53°C, and then cooled and stored. Emulsion 3 To the same starting silver bromide emulsion of the previous emulsions 1 and 2 was added at pH 6.5, pAg 8.4 and at 65°C, 9,4ml of a 1% by weight solution of Leucophor™, 60ml of a 1% by weight solution of the same spectral sensitizer as emulsion 1, 25ml of a solution of benzothiazolium iodoethylate 3.4mM, 30ml of a solution of sodium p-toluenesulfinate, 25ml of a solution of sodium p-toluenethiosulfonate 0.027M, 8.7ml of a solution of gold chloride 0.2mM, and 3.5ml of a solution of KCNS 0.31N and then the resulting emulsion was ripened for 146 minutes at 65°C. At the end 35 ml of a solution of KI 0.066N were added and the resulting emulsion was stabilized in a conventional manner. Emulsions 1, 2 and 3 were coated and treated as for example 1. The results are summarized in the following table 6. Fog Dmax Average Contrast Blue Speed Green Speed X-ray Speed Em. 1 (inv.) 0.21 3.52 2.32 2.35 2.74 2.95 Em. 2 (inv.) 0.21 3.54 2.31 2.36 2.74 2.95 Em. 3 (comp.) 0.23 3.57 2.50 2.04 2.46 2.61 These results clearly show the speed increase for the emulsion 1 and 2 vs. the comparison emulsion 3. Under the electron microscope, emulsion 1 and 2 show a modified grain structure when compared to the starting emulsion. Protrusions that form crater structures around the edges of cracked zones are evident on main faces. Employing a solution of benzothiazolium instead of isothiouronium salt in ripening during precipitation, it is not possible to reach the same sensitometric characteristics without increasing temperature, with a serious risk of undesired mixed crystal formation. EXAMPLE 7 Emulsion 1 The same starting emulsion and conditions of emulsion 1 of example 6 were used, but without double jet addition of silver and halide salts. At the end the emulsion was ripened for 180 minutes at 43°C, stabilized in conventional manner and then mantained at 53°C for 10 minutes. Emulsion 2 The same starting emulsion and conditions of emulsion 1 of example 6 were used, but adding the spectral sensitizer after, instead of before, double jet precipitation. The resulting emulsion was ripened for 160 minutes at 43°C. Emulsion 3 The same starting emulsion and conditions of emulsion 3 of example 6 were used. Emulsions 1, 2 and 3 were coated and treated as for example 1. The results are summarized in the following table 7. Fog Dmax Average Contrast Blue Speed Green Speed X-ray Speed Em. 1 (comp.) 0.22 3.50 1.85 1.56 1.92 1.93 Em. 2 (inv.) 0.18 3.61 2.40 2.15 2.47 2.58 Em. 3 (comp.) 0.20 3.75 2.66 1.97 2.40 2.48 These results clearly show that the use of isothiouronium salt without double jet precipitation is not working (Em. 1), while the addition of spectral sensitizer before or after double jet addition of silver and halide salts does not affect photographic characteristics of the present invention (+0.10 X-ray speed, +0.18 blue speed, +0.07 green speed). Under the electron microscope, emulsion 2 shows grains having crater structures on main faces. EXAMPLE 8 Emulsion 1 To a silver bromide emulsion comprising silver bromide tabular grain having an aspect ratio of 8:1 and a mean diameter of about 1.10 µm was added per mole of Ag, at pH 5.8 and at 40°C, a KBr solution until to have pAg 8.9, 1 mmoles of Ag, employing a solution of AgNO 3 0.25N, to have pAg 8.0. Then a double jet of 53ml of KI 0.06N and AgNO 3 0.25N until to use 4 mmoles of Ag and, after a pause to correct the pAg to 8.5, 60 ml of a 1% by weight solution of an optical sensitizer having the following formula: To the resulting solution a double jet addition of KBr and AgNO 3 solution was performed for 60 minutes until to precipitate 5.7M% of AgCl per mole of Ag. During said double jet addition the pAg was increased to 8.5. 7.6ml of a 0.1% by weight solution of the same isothiouronium salt of example 1, 7.5ml of a solution of gold chloride 0.2mM, 7.0ml of a solution of KCNS 0.31N and 17.8ml of a solution of sodium p-toluenethiosulfonate 0.027M per mole of Ag were added starting 6 minutes after the starting of said double jet addition. The resulting emulsion was ripened at 43°C, at pH 7.2 and pAg 8.2 for 120 minutes, stabilized, mantained 10 minutes at 53°C, and then cooled and stored. Emulsion 2 To the same starting silver bromide emulsion of the previous emulsions 1 and 2 was added at pH 6.5, pAg 8.4 and at 65°C, 9,4ml of a 1% by weight solution of Leucophor™, 60ml of a 1% by weight solution of the same spectral sensitizer as emulsion 1, 25ml of a solution of benzothiazolium iodoethylate 3.4mM, 30ml of a solution of sodium p-toluenesulfinate, 25ml of a solution of sodium p-toluenethiosulfonate 0.027M, 8.7ml of a solution of gold chloride 0.2mM, and 3.5ml of a solution of KCNS 0.31N and then the resulting emulsion was ripened for 146 minutes at 65°C. At the end 35 ml of a solution of KI 0.066N were added and the resulting emulsion was stabilized in conventional manner. Emulsions 1 and 2 were coated and treated as for example 1. The results are summarized in the following table 8. Fog Dmax Average Contrast Blue Speed Green Speed X-ray Speed Em. 1 (inv.) 0.20 3.80 2.55 2.29 2.69 2.85 Em. 2 (comp.) 0.21 3.90 2.64 1.98 2.41 2.50 These results clearly show the improvement in speed of the photographic emulsion of the present invention. Under the electron microscope, emulsion 2 shows a sligthly modified surface of the grains when compared with the starting emulsion. In particular only few grains show protrusions that form crater structures around the edges of cracked zones on main faces.

Claims: A process for preparing a silver halide photographic emulsion comprising chemically and spectrally sensitized hollow or cracked silver halide grains having a crater or hopper structure, said process comprising the following steps: (a) forming in a reaction vessel a host silver halide grain emulsion, which emulsion comprises, in a hydrophilic colloid dispersing medium, silver halide grains containing at least 50 mol % bromide and having a cubic crystal lattice structure comprising at least one [111] and/or at least one [100] crystal surface; (b) performing, at pH of from 5 to 7 and at a temperature between 30° and 50°C, a first addition of silver and/or halide salt solutions until a pAg of from 7 to 9 is reached; (c) performing, at pAg of from 7 to 9, a second addition of silver and halide salt solutions, until from 0.01 to 0.10 moles of silver per mole of silver of the host silver halide emulsion has been added, concurrently running in the reaction vessel a solution comprising a isothiouronium compound having the following formula: wherein R 1 and R 2 are straight or branched alkyl group, alkylene group, aryl group, or allyl group, with at least one of R 1 and R 2 being an allyl group, A is a divalent organic linking group and R 3 is a moiety selected from the class of carboxyl moiety, sulfo moiety, hydroxyl moiety, or amino moiety, B is an anion, and n is 0 (when R 3 is a negatively charged group) or 1 (when R 3 is a uncharged group), and (d) ripening, at pH of from 6 to 8, pAg of from 7 to 9 and at a temperature between 30° and 50°C, the obtained silver halide emulsion. The process of claim 1 characterized in that said host silver halide emulsion comprises silver halide grains containing at least 80 mol % bromide, the remaining part containing less than 20 mol % iodide. The process of claim 1 characterized in that said host silver halide emulsion comprises silver bromide grains. The process of claim 1 characterized in that said host silver halide emulsion comprises, in a hydrophilic colloid dispersing medium, spherical or tetradecahedral silver halide grains containing at least 50 mol % bromide and having at least one, at most eight [111] crystal faces. The process of claim 4 characterized in that said silver halide grains contain at least 80 mol % silver bromide, the remaining part containing less than 20 mol % iodide. The process of claim 1 characterized in that said host silver halide emulsion comprises, in a hydrophilic colloid dispersing medium, tabular grains containing at least 50 mol % bromide and predominantly bounded by [111] crystal faces. The process of claim 6 characterized in that said host silver halide tabular grains have an aspect ratio higher than 3:1. The process of claim 6 characterized in that said host silver halide tabular grains are silver bromide grains. The process of claim 1 characterized in that said first addition is performed with silver and chloride and/or bromide salt solutions. The process of claim 1 characterized in that said first addition is performed with silver and chloride and/or iodide salt solutions. The process of claim 1 characterized in that a pause between the first addition of the silver salt and halide salt solutions is performed. The process of claim 1 characterized in that said isothiouronium compound is added starting after 2 to 15 minutes from the starting of said second addition of silver and halide salts. The process of claim 1 characterized in that the resulting emulsion is chemically and spectrally sensitized. The process of claim 1 characterized in that a spectral sensitizing dye is added before or after said second addition of silver and halide salts. The process of claim 14 characterized in that said spectral sensitizing dye is a J-aggregate forming cyanine or merocyanine dye. The process of claim 1 characterized in that said second addition is performed with silver and chloride, bromide or chloro-bromide salt solutions. A silver halide photographic emulsion comprising chemically and spectrally sensitized silver halide grains characterized in that said silver halide grains are spherical or tetradecahedral grains bounded by six [100] crystal faces and comprising at least one, at most eight [111] crystal faces, at least one of said [100] crystal faces face having at least one cracked or hollow zone and at least one of said [111] crystal faces having silver halide protrusions forming around said hollow or cracked zone at least one crater or hopper structure on at least one of said [100] crystal face of said silver halide grains. A silver halide photographic emulsion comprising chemically and spectrally sensitized silver halide grains characterized in that said silver halide grains are tabular grains predominantly bounded by [111] crystal faces, at least one of said [111] crystal faces having at least one cracked or hollow zone and silver halide protrusions, forming around said hollow or cracked zone, at least one crater or hopper structure on at least one of said [111] crystal face of said silver halide grains. The silver halide photographic emulsion of claims 17 or 18 characterized in that said silver halide grains are composed of at least 80%M of silver bromide, the remaining part containing less than 20%M of silver iodide and said silver halide protrusions are composed of at least 80%M of silver chloride and/or silver bromide.

Inventors: AMICUCCI MARCELLO, DEBENEDETTI MILENA, AMICUCCI, MARCELLO, DEBENEDETTI, MILENA
Applicants: IMATION CORP, IMATION CORP.
Application year: 1992
Main classification: G03C1
IPC codes: G03C1