Process for the preparation of a hybrid direct positive emulsion and photographic material containing such an emulsion

A process is disclosed for the preparation of a hybrid direct positive silver halide emulsion comprising the steps of (1) forming an essentially cubic host grain emulsion consisting of silver bromide or silver iodobromide, with a iodide content between 0 and 10 mole %, by a balanced double jet, (2) depositing epitaxially on the corners of said formed essentially cubic host grains a silver iodide crystallographic phase wherein said silver iodide phase contains at most 5 % of the total crystal silver halide, either by precipitating a silver chloride epitaxial phase and converting it to silver iodide, or by adding silver ions and an organic iodide releaser. The organic iodide releaser is preferably mono-iodoacetic acid.

DESCRIPTION 
1. Field of the Invention 
The present invention relates to a process for the preparation of a new 
type of direct positive photographic emulsion and to a photographic 
material containing such an emulsion. 
2. Background of the Invention 
Hybrid silver halide emulsions comprising a first crystal phase, usually 
called "host grain", and a second crystal phase of different halide 
composition epitaxially deposited on selected sites of the host grain are 
known for quite some time in the art of photographic emulsion making. For 
instance, epitaxial deposition of a silver chloride or bromide phase on 
silver iodide host grains was described by Maskasky in U.S. Pat. Nos. 
4,094,684, 4,124,900 and 4,158,565. U.S. Pat. No. 4,496,652 discloses 
negative or direct positive tetradecahedral emulsions on whose (111) faces 
a second phase is deposited by a second double jet of silver and halide 
salts. Further teachings on host grains rich in iodide to which another 
halide phase is epitaxially junctered include JP-B-88/043733 and 
JP-B-88/042768 (JP-B- meaning Examined Japanese Patent Publication) and EP 
0 019 917. Internal latent image type silver halide particles to wich a 
surface sensitive epitaxial phase is adjoined are disclosed in 
JP-B-88/43734. A photothermographic material containing an epitaxial 
emulsion is described in JP-A-58-046339 (JP-A- meaning Unexamined Japanes 
Patent Publication). Tabular grains having epitaxial deposition are for 
the first time disclosed by Maskasky in U.S. Pat. Nos. 4,435,501. Maskasky 
4,463,087 reveals silver halide grains predominantly bounded by (111) 
crystal faces to which a so-called "site director" is adsorbed before the 
epitaxial deposition takes place. Further Maskasky disclosures include 
U.S. Pat. Nos. 4,459,353 and 4,471,050. Other additions and improvements 
on the teachings on epitaxial emulsions can be found a.o. in U.S. Pat. No. 
4,735,894, JP-A-61-088252, JP-A-61-279848, U.S. Pat. No. 4,888,272, 
JP-A-63-264739, JP-A-63-274943, JP-A-63-316048, JP-A-01-077045, 
JP-A-01-113745, JP-A-01-179140, JP-A-01-213638, JP-A-01-205151 and 
JP-A-01-213636. 
Almost all of the teachings on epitaxial emulsions deal with negative 
working emulsions or with crystallographic aspects only. As far as we are 
aware, only U.S. Pat. No. 4,496,652, cited above, discloses either a 
chemically ripened negative host grain emulsion or an externally fogged 
direct positive host grain emulsion. 
With externally fogged direct positive emulsions it is not easy to reach 
the high sensitivity level which can be obtained with internal latent 
image type direct positive emulsions, which are developed by fogging 
development, e.g. by using hydrazines. Probably this is due at least 
partially to the heterogeneous character of the external chemical fogging. 
However direct positive emulsions show the advantage of not requiring a 
rather unecological processing as it is the case when fogging developers 
are used. So there is a permanent need for externally fogged direct 
positive emulsions with improved sensitivity and improved other 
sensitometric properties, e.g. exposure latitude in case of recording 
purposes. 
The present invention is an extension of the teachings of U.S. Pat. No. 
4,496,652 as far as direct positive emulsions are concerned. 
It is an object of the present invention to provide a process for the 
preparation of an externally fogged direct positive emulsion with improved 
sensitivity and exposure latitude. 
It is a further object of the present invention to provide a photographic 
material containing such an improved direct positive emulsion. 
SUMMARY OF THE INVENTION 
The objects of the present invention are realized by providing a process 
for the preparation of a hybrid direct positive silver halide emulsion 
comprising the following steps: 
(1) forming an essentially cubic host grain emulsion consisting of silver 
bromide or silver iodobromide, with a iodide content between 0 and 10 mole 
%, by a balanced double jet precipitation of a silver ion and a halide ion 
solution at a pAg between 7 and 9; 
(2) depositing epitaxially on the corners of said formed essentially cubic 
host grains a silver iodide crystallographic phase wherein said silver 
iodide phase contains at most 5% of the total crystal silver, either by, 
(2.1) performing a second balanced double jet precipitation of an organic 
silver ion and chloride ion solutions at a pAg between 7 and 9, and then 
adding an amount of iodide ions at least equivalent to the chloride ions, 
in this way converting the formed epitaxial silver chloride phase into an 
epitaxial silver iodide phase; or, 
(2.2) adding a silver ion solution and an organic iodide releasing compound 
represented by formula (R) to the emulsion grains formed according to step 
(1); 
EQU A-L-I (R) 
wherein A represents a group with a positive .sigma..sub.p value, and L is 
a divalent linking group; 
(3) externally fogging the obtained hybrid emulsion crystals; 
(4) adding an electron-accepting spectral dye which adsorbs to the fogged 
surface of said hybrid emulsion crystals. 
Unexpectedly direct positive emulsions with higher sensitivity and extended 
exposure latitude were obtained. 
DETAILED DESCRIPTION OF THE INVENTION 
The choice of the pAg range for the precipitation--between 7 and 9--of the 
host grains is such that an essentially cubic emulsion grain is obtained. 
By essentially cubic is meant a grain which either is (a) perfectly cubic, 
or (b) cubic with rounded corners, or (c) cubic with small (111) faces on 
the corners so that in fact a tetradecahedrical emulsion is obtained, the 
total area of these (111) faces however being small compared to the total 
area of the (100) faces. 
The precipitation in connection with the present invention forming the 
silver bromide or silver iodobromide host emulsion can be principally 
performed by one double jet step; alternatively it can consist of a 
sequence of a nucleation step and at least one growth step. In the latter 
case, of the total silver precipitated preferably 0.5% to 5.0% is added 
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 then 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) the flow rate of the 
silver salt and halide solutions can be kept constant; alternatively an 
increasing flow rate of silver salt and halide ion solutions can be 
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. 
However in a preferred embodiment of the present invention the essentially 
cubic host emulsion is formed simply by one double jet step at a pAg 
maintained at a constant value between 7 and 9 without separate nucleation 
step and at a constant flow rate. The constant pAg is realized by the use 
of a so-called "bypass solution" the addition of which is alternatingly 
switched on and off. The concentrations of the main silver salt and halide 
solutions typically range between 0.5 and 3 molar, and most preferably 
between 1 and 2 molar. 
In one embodiment of the present invention the host emulsion is freed from 
excess of soluble inorganic salts by a conventional wash technique, e.g. 
flocculation by ammonium sulphate or polystyrene sulphonate, followed by 
several washing steps and redispersion. Another well-known wash technique 
is ultrafiltration. Finally extra gelatin can be added to the emulsion in 
order to obtain the desired gelatin/silver ratio. In another embodiment of 
the present invention the epitaxial deposition of silver iodide, to be 
described in detail hereafter, is performed immediately after the grain 
formation of the host emulsion, and subsequently the washing procedure, 
redispersion if needed and adjustment of the final gelatin / silver halide 
ratio are performed. 
In the prior art teachings on epitaxial emulsion the epitaxial phase is 
usually grown by a second balanced double jet of silver salt and halide 
ions. However it was established experimentally by us that if one tries to 
deposit a silver iodide phase on an essentially cubic host emulsion by a 
direct double jet of silver salt and iodide ions unreproducible and 
worthless results are obtained due to aspecific conversion and secondary 
nucleation phenomena. It is the merit of the present invention to have 
overcome this problem by either, in a first embodiment, epitaxially 
depositing a silver chloride phase and then converting it quantitatively 
to a silver iodide phase, or, in a second embodiment, by performing the 
silver iodide epitaxial growth with the aid of an organic iodide releasing 
compound. 
In the first embodiment the temporary deposition of the silver chloride is 
achieved by a balanced double jet precipitation of silver salt and 
chloride ion solutions at a pAg between 7 and 9 at a temperature 
preferably between 40.degree. and 70.degree. C. Preferably equimolecular 
amounts of silver ions and chloride ions are added preferably at a 
constant flow rate. Then a dilute solution of iodide ions, e.g. potassium 
ions, is added to the reaction vessel in an amount at least equimolar, and 
preferably exact equimolar to the silver chloride amount, and the mixture 
is allowed to digest for at least 5 minutes. The concentration of the 
iodide solution is preferably comprised between 0.5 and 3 molar. Due to 
the great difference in solubility product the epitaxial silver chloride 
phase is quantitatively converted to a silver iodide phase without the 
occurence of aspecific conversion on unwanted sites of the emulsion grain. 
In a second embodiment of the present invention the epitaxial deposition of 
silver iodide is realized by the addition of a silver salt solution and a 
solution of an organic iodide releasing compound. Preferably the two 
solutions are added simultaneouly and in equimolar amounts. Due to the 
slow release of iodide from the organic releaser the pAg is first lowered 
but increases again lateron. After completion of the addition the reaction 
mixture is stirred for at least 15 minutes, before it is allowed to cool. 
The epitaxial deposition of silver iodide can be confirmed by well-known 
analytical techniques such as X-ray fluorescence and X-ray diffraction. 
The use of organic halide releasing compounds is not new in the 
photographic art. In Zh. Nauch. Prikl. Fot. Kine 35, 142, (191), 
Poloznikov and Shapiro describe new forms of silver bromide microcrystals 
as a result of the interaction of silver nitrate and bromoacetic acid. 
JP-A-02-68538 describes the use of organic chloride, bromide and iodide 
slow releasers to avoid microscopic heterogeneities inherently associated 
with non-homogeneous concentrations of silver ions and halide ions in the 
conventional circumstances of nucleation and growth. Examples of such 
heterogeneities include heterogeneous particle size, crystal 
irregularities and heterogeneous halide distribution between and within 
particles. Further extensions of this teaching can be found in EP 0 341 
728 and EP 0 531 799. 
The iodide ion releasers for use in the present invention are represented 
by the following general (R): 
EQU A--L--I (R) 
wherein A represents a group with a positive Hammett .sigma..sub.p value, 
and L is a divalent linking group. 
A group with a positive Hammett .sigma..sub.p value (A) is commonly known 
as an electron-accepting or electon-withdrawing group. Hammett 
.sigma..sub.p values have been defined e.g. on p. 96 of 
"Structure/Activity Correlations for Drugs" published by Nankodo (1979). 
Most preferred A groups include a carboxylic acid group, a cyano group, a 
carbamoyl group, an acyl group, a sulphonyl group, an oxycarbonyl group, a 
sulphamoyl group. 
The organic iodide releasing compounds are preferably corresponding to the 
above formula wherein the divalent linking group is chosen from 
unsubstituted or substituted alkylene, unsubstituted or substituted 
oxyalkylene, unsubstituted or substituted aralkylene, or combinations of 
two or more thereof, wherein several atoms of L, or of A and L, can 
combine to form a ring, and wherein the iodine atom is not bound to an 
aromatic moiety or to an atom bearing a double bond, or to a hetero-atom. 
Useful examples of such iodide ion slow releasers include following 
compounds: 
##STR1## 
In a particular preferred embodiment the iodide ion slow releaser is simply 
mono-iodoacetic acid (R-1). 
The iodide releaser is preferably used in aqueous solution in a 
concentration between 0.1 and 1 molar. 
In both embodiments of applying the epitaxial silver iodide phase the 
amounts of reagents necessary for the deposition of that phase are chosen 
in such a way that the epitaxial phase comprises at most 5 mole % of the 
total crystal silver halide. 
At an appropriate time the prepared hybrid silver halide emulsion is melted 
again and surface fogged in order to obtain a direct positive working 
emulsion. External fogging can be accomplished according to the teachings 
of U.S. Pat. Nos. 3,367,77.8, 3,501,305, 3,501,306, 3,501,307 and 
3,637,392. In a preferred embodiment the fogging is performed by the 
combined use of a reductor and a noble metal salt. In a most preferred 
embodiment of the practice of the present invention the fogging is 
accomplished by using a combination of thioureadioxide and gold(III) 
chloride. The fogging is continued with periodical monitoring until the 
desired maximum density/sensitivity relationship is reached and then 
terminated by cooling. 
In the final stage of the preparation of an externally fogged direct 
positive emulsion an electron-accepting spectral dye is added and adsorbed 
to the emulsion grains. Dyes which desensitize negative working emulsions 
are usually suitable as electron-accepting spectral sensitizers for fogged 
direct positive emulsions. Typical heterocyclic nuclei featured in cyanine 
and merocyanine dyes well-suited for this purpose are derived from 
nitrobenzothiazol, 2-aryl-1-alkylindole, pyrrolo[2,3-b]pyridine, 
imidazo[4,5-b]quinoxaline, carbazole, pyrazol, 5-nitro-3H-indole, 
2-arylbenzindole, 2-aryl-1,8-trimethyleneindole, 2-heterocyclylindole, 
pyrylium, benzopyrylium, thiapyrylium, 2-amino-4-aryl-5-thiazole, 
2-pyrrole, 2-(nitroaryl)indole, imidazo[1,2-a]pyridine, imidazo[2, 
1-b]thiazole, imidazo[2,1-b]-1,3,4-thiadiazole, imidazo[1,2,-b]pyridazine, 
imidazo[4,5-b]quinoxaline, pyrrolo [2,3-b]quinoxaline, 
pyrrolo[2,3-b]pyrazine, 1,1-diarylindole, 1-cyclohexylpyrrole and 
nitrobenzoselenazole. Such nuclei can further be enhanced as desensitizers 
by electron-withdrawing groups such as nitro, acetyl, benzoyl, sulphonyl, 
benzosulphonyl and cyano groups. 
Specific examples of useful electron-accepting spectral dyes will be 
mentioned in the examples lateron. 
To the thus finished direct positive emulsion can be added conventional 
useful photographic ingredients and coating aids and the thus prepared 
coating solution can be applied to a support giving rise to a direct 
positive photographic material by any of the known coating techniques, 
e.g. air knife, slide hopper and curtain coating. It is specifically 
contemplated that such a photographic material containing a hybrid direct 
positive emulsion prepared by the process described above forms part of 
the scope of the present invention. 
The use of such a direct positive photograhic material of the present 
invention is not limited to any particular field or application. However 
it will be clear that the full benifit of the present invention will 
become most perspicuous in those applications were a high sensitivity is 
required, e.g. recording purposes. 
The emulsion layer of the photographic element according to the invention 
can be simply just one single layer or it can be splitted into a double 
layer or even into a multilayer pack. Beside the light-sensitive emulsion 
layer(s) the photographic material can contain several non-light-sensitive 
layers, e.g. a protective top layer, one or more backing layers, and one 
or more intermediate layers. 
Besides the silver halide another essential component of a light-sensitive 
emulsion layer is the binder. The binder is a hydrophilic colloid, 
preferably gelatin. Gelatin can, however, be replaced in part or 
integrally by synthetic, semi-synthetic, or natural polymers. Synthetic 
substitutes for gelatin are e.g. polyvinyl alcohol, poly-N-vinyl 
pyrrolidone, polyvinyl imidazole, polyvinyl pyrazole, polyacrylamide, 
polyacrylic acid, and derivatives thereof, in particular copolymers 
thereof. Natural substitutes for gelatin are e.g. other proteins such as 
zein, albumin and casein, cellulose, saccharides, starch, and alginates. 
In general, the semi-synthetic substitutes for gelatin are modified 
natural products e.g. gelatin derivatives obtained by conversion of 
gelatin with alkylating or acylating agents or by grafting of 
polymerizable monomers on gelatin, and cellulose derivatives such as 
hydroxyalkyl cellulose, carboxymethyl cellulose, phthaloyl cellulose, and 
cellulose sulphates. 
The binders of the photographic element, especially when the binder used is 
gelatin, 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 binders can also 
be hardened with fast-reacting hardeners such as carbamoylpyridinium salts 
as disclosed in U.S. Pat. No. 4,063,952. 
The silver halide emulsion layer(s) may further comprise compounds 
preventing the formation of fog or stabilizing the photographic 
characteristics during the production or storage of 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, nitrobenzotriazoles, mercaptotetrazoles, 
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 U.S. Pat. No. 
4,727,017, and other compounds such as benzenethiosulphonic 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.degree. 17643 (1978), Chapter VI. 
The photographic material of the present invention may further comprise 
various kinds of surface-active agents in the photographic emulsion layer 
or in another 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, as compounds preventing or reducing adhesion. 
The photographic elements according to 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 elements 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. 
Spacing agents can be present, preferably in the top protective layer; in 
general the average particle size of such spacing agents is comprised 
between 0.2 and 10 micron. They 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 
U.S. Pat. No. 4,614,708. 
The support of the photographic materials in connection with the present 
invention can be transparent base, preferably 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. On the other hand the support of 
the photographic material can be a paper base preferably a polyethylene or 
polypropylene coated paper base. 
The photographic materials according to the invention have to be exposed by 
a convenient light source according to their application. They can be 
processed by any means or any chemicals known in the art depending on 
their composition and particular application. For producing a 
black-and-white image they are preferably processed in a conventional 
Phenidone/hydroquinone or substituted Phenidone/hydroquinone developing 
solution and a conventional sodium or ammonium thiosulphate containing 
fixing solution. The development time is usually between 10 and 30 seconds 
at a temperature of about 35 .degree. C.

The following examples illustrate the present invention without however 
limiting it thereto. 
EXAMPLES 
EXAMPLE 1 
Preparation of control emulsion C-1 
The following solutions were prepared: 
a dispersion medium (C) in a reaction vessel containing 1500 ml of 
demineralized water, 50 g of inert gelatin and 15 g of methionine; 
2000 ml of a 1.47 molar silver nitrate solution (A); 
2165 ml of a solution 1.3 molar in potassium bromide and 0.0135 molar in 
potassium iodide (B1); 
216,5 ml of the same latter solution (B2; "bypass" solution). 
Solutions (A) and (B1) were added during 31 minutes at 57 .degree. C. in a 
balanced double jet to the reaction vessel containing dispersion medium 
(C) in such a way that, by means of interrupted addition of bypass 
solution (B2), the silver potential, measured by a silver electrode versus 
a saturated calomel electrode (SCE), was maintained at a constant value of 
+20 mV, corresponding to a pAg of 8.3. The precipitated emulsion was 
physically ripened at 57.degree. C. during 23 minutes. Then the pH was 
adjusted to 3.5 with sulphuric acid and the emulsion was flocculated by 
the addition of sufficient low-molecular weight polystyrene sulphonate 
solution. The flocculated emulsion was washed thoroughly for several times 
with water. Finally the emulsion was redispersed and gelatin and water 
were added to obtain a final emulsion of about 3.2 kg, having a 
gelatin/silver ratio (gesi), the latter expressed as silver nitrate, of 
0.6, and an average grain size of 0.24 .mu.m. The crystal morphology was 
essentially cubic as confirmed by electron microscopy. 
This control emulsion was set aside to serve as future host emulsion for 
the epitaxial depositions described hereafter. 
preparation of invention emulsion I-1 
7 kg of the cubic host emulsion described above, containing 165,7 g of 
Ag/kg, were melted at 40.degree. C. The pH was adjusted to 7.5 and the pAg 
to a value corresponding to +100 mV. Then an epitaxial deposition of a 
silver chloride phase was effected on the corners of the cubic grains by 
the simultaneous addition over a period of 70 seconds of 49.8 ml of a 2.94 
molar solution of silver nitrate and 49.8 ml a 2.94 molar solution of 
sodium chloride. 
Then 580 ml of a 0.294 molar solution of potassium iodide was added over a 
period of 70 seconds and the mixture was stirred for 30 minutes. Due to 
the much lower solubility product of silver iodide compared to silver 
chloride the epitaxial silver chloride phase was completely converted to a 
silver iodide epitaxial phase (as was confirmed by X-ray fluorescence and 
X-ray diffraction. 
preparation of invention emulsion I-2 
980 g of a similar cubic host emulsion containing 169.2 g of Ag/kg was 
melted at 40 .degree. C. and the pH was adjusted to 7.5 and the pAg was 
adjusted to a value corresponding to +100 mV. In a period of 100 seconds 
49.8 ml of a 0.3 molar silver nitrate solution and 49.8 ml of a 0.3 molar 
mono-iodoacetic acid solution were added, and the reaction mixture was 
stirred for another 30 minutes and then allowed to cool. In this way a 
silver iodide epitaxial phase was applied to the corners of the original 
basic grains as was confirmed by X-ray fluorescence and X-ray diffraction. 
preparation of control emulsion C-2 
This control emulsion was identical to emulsion I-1 with the exception that 
no final iodide conversion was performed. So a silver chloride epitaxial 
phase remained on the corners of the emulsion crystal. 
further treatment and photographic evaluation 
The four emulsions described above were externally fogged at 55.degree. C., 
pH 7.5 and +100 mV using a 0.005 % thioureadioxide solution (fogging 
solution A) and a 0.06% gold(III) trichloride solution (fogging solution 
B) in amounts given in table 1. The fogging was terminated by cooling when 
an optimal maximum density--sensitivity relationship was reached. Then 
four different coating solutions were prepared each containing one of the 
fogged emulsions and 51 mg/100 g Ag of an electron-accepting spectral dye, 
actif in the red spectral region, and corresponding to following formula 
EA-1: 
##STR2## 
Further to the coating solution were added suitable amounts of the 
stabilizer 5-nitrobenzimidazol and various coating aid solutions. These 
photosensitive coating solutions were applied at a silver coverage of 2.5 
g/m.sup.2, expressed as AgNO.sub.3, on top of a transparent support and an 
"undercoat" layer, the latter having a gelatin coverage of 1.2 g/m.sup.2 
and containing a conventional anti-halation dye. On top of each coated 
photosensitive layer a protective layer was applied. 
The four samples were exposed to flash (??) light through a continuous tone 
wedge and developed in a conventional hydroquinone/methyl-Phenidone 
developer for 14 seconds at 30 .degree. C. They were fixed in a 
conventional fixer, rinsed with water and dried. The sensitometric results 
are represented in table 1. The sensitivity (S) was measured at density 
0.2 + Dmin and expressed as relative log H, higher figure meaning higher 
sensitivity. 
TABLE 1 
______________________________________ 
fogging sol. dir. pos. sensitometry 
emul- B (/50 fogging gra- 
sion A g Ag) time Dmin S dation 
Dmax 
______________________________________ 
C-1 1 ml 0.5 ml 4.00 h 
0.11 69 3.27 2.38 
I-1 0.5 ml 0.25 ml 3.00 h 
0.40 82 1.03 2.05 
I-2 " " 2.50 h 
0.15 95 2.23 2.20 
C-2 1 ml 0.5 ml no dir. Pos. sensitometry 
______________________________________ 
The results of table 1 illustrate several advantages of the invention 
emulsions. The fogging time is reduced, even using lower amounts of 
fogging agents, the sensitivity is increased, and, as can be seen from the 
lower gradation and the only slightly lower maximum density, the exposure 
latitude is increased. Especially the results with emulsion I-2 are 
favourable. 
EXAMPLE 2 
In this test series preparation of some of the emulsions of the previous 
example was repeated and extended to new emulsions with various other 
percentages of the epitaxial siver iodide phase. The series included 
following emulsions: 
C-3: preparation identical to C-1; 
I-3: similar to I-2 (use of mono-iodoacetic acid) but the silver iodide 
phase comprises only 1% of the total silver halide; 
I-4: identical to I-2 (use of iodoacetic acid, and the silver iodide phase 
comprise 2.5% of the total silver halide); 
I-5: identical to I-1 (iodide conversion of a AgCl phase, the silver iodide 
phase comprises 2.5% of total silver halide); 
I-6: similar to I-5, but the silver iodide phase comprises 5% of the total 
silver halide. 
The emulsions were fogged in a similar way as in example 1. Then an 
electron-accepting spectral dye, actif in the green+spectral region, and 
corresponding to following formula EA-2 was added (instead of compound 
EA-1 of ex. 1): 
##STR3## 
Furtheron conventional coating aids were added and the various coating 
solutions were applied on a transparent support at a silver coverage of ? 
g/m.sup.2. The samples were exposed with tungsten light through a 
continuous tone wedge and developed in a conventional 
methyl-Phenidone/hydroquinone developer, fixed, rinsed and dried. The 
direct positive sensitometric results are summarized in table 2. 
TABLE 2 
______________________________________ 
conc. fogging agent 
B (/50 dir. pos. sensitometry 
emulsion 
A g Ag) Dmin S Dmax 
______________________________________ 
C-3 1 ml 0.5 ml 0.030 0.89 2.25 
I-3 0.5 ml 0.25 ml 0.004 1.00 3.15 
I-4 " " 0.004 1.13 3.67 
I-5 " " 0.010 1.62 2.57 
" 0.25 ml 0.125 
ml 0.003 1.23 2.70 
I-6 " " 0.085 0.95 2.10 
______________________________________ 
Table 2 again illustrates the better sensitometic properties, especially 
sensitivity, of the invention emulsions.