EP0445444A1

EP0445444A1 - Photographic emulsions

Info

Publication number: EP0445444A1
Application number: EP90302312A
Authority: EP
Inventors: Karen Nicola Harvey; Trevor James Maternaghan; David John Turner; Douglas James Edwards; Amanda Mary Jackson
Original assignee: Ilford Ltd
Current assignee: Ilford Imaging UK Ltd
Priority date: 1988-09-13
Filing date: 1990-03-05
Publication date: 1991-09-11
Also published as: GB8821433D0; GB8920532D0; GB2222694A; GB2222694B

Abstract

There is described a silver halide emulsion which comprises a colloid medium having dispersed therein silver halide emulsion crystals which comprise at least two layers of different silver halide composition, there being sub-developable silver centres present at the boundary between two of the layers which have been formed by a reduction process.

By sub-developable centres is meant small clusters of silver atoms which increase the inherent light sensitivity of the silver halide grains but which do not contribute substantially to the formation of fog when unexposed silver halide crystals are developed.

Description

This invention relates to the production of silver halide emulsions with improved sensitivity.
It is known to surface sensitise silver halide grains after they have reached their full size by acting on the surface with a chemical sensitiser such as a compound having a labile sulphur atom or a noble metal compound such as gold. Sensitivity centres can be formed on the surface of the silver halide grain which comprise either silver sulphide or noble metal atoms or ions. It is also known to form so-called sensitivity nuclei in the interior of silver halide grains which after exposure form internal latent image centres. Silver halide grains with interior sensitivity nuclei also have improved sensitivity but a so-called solvent developer is required to develop such silver halide grains to cause these grains to exhibit their improved sensitivity.
Chemical sensitisation of this type both of the surface and of the internal type leads to the formation of so-called sensitivity nuclei. Thus by using an appropriate developing agent and no light exposure spontaneously developable fog is observed on development.
However it is known to increase the sensitivity of silver halide grains by forming internal reduction sensitisation centres. These are usually formed by the presence of a reducing agent during preciptitation of the silver halide grains. These are believed to be small clusters of silver atoms.
These centres are so-called sub-developable centres as they do not contribute substantially to the formation of fog when unexposed emulsions are developed. It is thought that such sub-developable centres may act as hole-trapping centres therefore increasing the speed of the emulsion without contributing directly to latent image formation.
Thus these sub-developable centres do not act in the same way as the sensitivity specks formed during chemical sensitisation since they do not promote the formation of latent image nuclei in the interior of the grain on exposure to light. Emulsions of this type are described in British Patent Specification 1426012. In BP 1426012 the sub-developable centres are formed by the action of a reducing agent during the formation of the silver halide crystals and before they have reached their final size. A modification to the method of preparing such silver halide emulsions as described in BP 1426012 is described in the British patent 1445192. Another method of forming sub-developable centres before the silver halide crystals have reached their full size is described in British Patent Specification 1588943. In this specification the crystals are acted on by ionising radiation. In US patent specification 4623612 there is described a silver halide emulsion layer on a support wherein said silver halide emulsion layer contains core/shell type silver halide crystals having a shell thickness of 25 to 150 Å wherein the surface of the core is subjected to chemical sensitisation treatment and the surface of the shell is not subjected to chemical sensitisation, and a developing solution contains a solvent for silver halide.
It is possible to carry out spectral sensitisation without causing a reduction in intrinsic sensitivity. However in USP 4623612 the core is chemically sensitised and thus sensitivity specks are formed and not sub-developable centres.
We have found a silver halide emulsion which contains internal sub-developable centres which exhibits superior properties to the emulsions described in the prior art as set forth.
Therefore according to the present invention there is provided a silver halide emulsion which comprises a colloid medium having dispersed therein silver halide emulsion crystals which comprise at least two layers of different silver halide composition, there being sub-developable silver centres which have been formed at the boundary between two of the layers by a reduction sensitisation step.
By sub-developable centres is meant small clusters of silver atoms which increase the inherent light sensitivity of the silver halide grains but which do not contribute substantially to the formation of fog when unexposed silver halide crystals are developed. One test for this is the test described in BP's 1426012, 1445192 and 1588943.
Preferably all the silver halide crystals in the emulsion have been both chemically and spectrally sensitised at the surface.
A useful size of crystal in the emulsion is from 0.5 µ to 1.5 µ. Preferably the population of crystals in the emulsion is monodisperse having a size coefficient of variation of 30% and preferably less than 20%.
In one preferred embodiment the inner layer of the boundary in the silver halide crystals at which the sub-developable centres are present, is silver iodobromide and the outer layer is silver bromide. Preferably the amount of iodide in the silver iodobromide layer is from 2 to 35 mole % iodide.
However both the inner and the outer layers of the silver halide crystals can comprise the same mixed halides as long as there is a marked difference in the halide ratios resulting in a abrupt change in halide composition at the boundary i.e. at least 5% difference in the proportion of a major component such as silver bromide across the boundary.
The clusters of sub-developable silver atoms may be formed at the silver halide layers by a reduction process. However an excessively long reduction treatment can lead to increased fog or formation of prelatent image specks.
Therefore according to another aspect of the present invention there is provided a method of preparing a silver halide emulsion as just defined which comprises preparing in a hydrophilic colloid dispersing medium a silver halide emulsion using water-soluble silver salt solutions and water-soluble halide solutions to form silver halide. crystals of a requisite size and having at their surface silver halide of a first composition then changing the halide/halides to form silver halide having a second composition which is laid down on the already formed silver halide crystals, then carrying out a reduction sensitisation step and continuing to add the silver and halides of the second composition until the silver halide crystals reach a final requisite size. In one method after the reduction step a period is allowed before resumption of addition of silver and halides.
The reduction sensitisation step may be carried out by adding a reducing agent to the dispersing medium, or by use of an ionising radiation or by altering the pAg and/or pH conditions to produce the silver digestion conditions as described by H W Wood J. Phot. Sci. 1 (1953) 163 in which the pAg is held relatively low from 7 to 0, preferably at 3 at a relatively high temperature of about 50^oC, the pH preferably being from 7 to 10.
Thus in order to obtain conditions favouring reduction sensitisation by naturally occurring substances in the gelatin the pAg should be less than 8 and the pH greater than 5.5. These conditions produce reduction sensitisation without any great increase in fog.
The reduction sensitisation step is preferably carried out after at least 1 minute of the change of halide being added to ensure that a boundary between halides of different composition has formed on the silver halide crystals.
The preferred reducing agents are stannous chloride and glutaraldehyde. Other useful reducing agents are hydrazine and other aldehydes such as formaldehyde. Sulphur containing reducing agents such as thiourea are not preferred.
The use of ionising radiation to produce the sub-developable centres may be carried out as described in BP 1588943.
Preferably after the silver halide crystals have reached their requisite size they are then chemically sensitised using both gold and a sulphur sensitiser.
Most preferably the silver halide crystals are then spectrally sensitised.
Preferably the soluble silver salt solutions and the ammonium or alkali metal halide solutions are added to the dispersing medium by the well known double-jet method of preparing silver halide emulsions.
The outer halide layer may be formed by an Ostwald ripening process in the presence of small crystals of different halide composition from the previously formed silver halide crystals but this is not preferred as the presence of a solvent obviates the abrupt change of halide composition.
The silver halide crystals of the present invention may be of any of the well known crystal habits employed in photographic silver halide emulsion. Useful habits are cubic, twinned octahedral and tabular twinned octahedral.
The crystals may be formed using any of the well-known techniques for forming such silver halide crystals. For example British Patent Specification 2110831 described the formation of tabular silver halide crystals which may be twinned.
Similarly within the scope of the present invention twinned silver halide photographic emulsions of the intermediate tetradecahedral habit may be produced by selection of the appropriate solution conditions.
The process of the present invention is particularly suitable for the production of twinned silver halide emulsions of the monodisperse type.
To produce such emulsions the outer layer of the silver halide crystals are formed by adding further silver and halide solutions by a double jetting method and at a controlled pAg. Preferably the additional halide added during this stage is such that the iodide content of the final crystals is about 5 - 15 mol % which is the amount of iodide which has been found to be most beneficial, yielding high-speed emulsions for negative working photographic material.
The pAg can be varied during the formation of the outer halide layer to modify the habit of the final twinned emulsion crystals. For example by selection of a fixed pAg in the range 6 to 9, (100) external faces are favoured leading to cubic crystals, whereas with iodide contents greater than 5 M % and pAg values greater than 7, external octahedral faces are favoured.
The water-soluble salts formed or the ripening agents added during the process of the present invention may be removed by any of the well-known methods. Such methods often involve flocculating the silver halide and colloid dispersing agent, removing this flocculate from the then aqueous medium, washing it and redispersing it in water. One other common method is ultrafiltration, in which the emulsion is passed over a membrane under pressure. The pore size of the membrane is such that the silver halide crystals and most of the colloid dispersing medium is retained, whilst water and solutes permeate through. Most of the well-known methods allow the emulsion to be concentrated as well as washed. This is important when weak reagent solutions are employed, particularly those with concentrations below 3M.
As hereinbefore stated the silver halide crystals are preferably chemically sensitised on the surface by any of the well known means, for example by use of sulphur or selenium compounds or salts of the noble metals such as gold, iridium rhodium, osmium, palladium or platinum. Chemical sensitisation is preferably carried out in the presence of sulphur-containing ripening agents such as thioethers or thiocyanate compounds. The fully grown crystals are sensitised in this manner, so that the products of chemical sensitisation are formed on the surface of the crystal so that such sensitised crystals become developable in a surface developer after exposure to light.
The emulsions of the present invention are preferably spectrally sensitised by the addition of spectral sensitisers for example carbocyanine and merocyanine dyes to the emulsions. Suitable dyes are described in James, The Theory of the Photographic Process. 4th Edition. McMillan. Chapter 8.
The emulsions may contain any of the additives commonly used in photographic emulsions for example wetting agents, such as polyalkene oxides, stabilising agents, such as tetraazaindenes, metal sequestering agents, growth or crystal habit modifying agents commonly used for silver halide such as adenine and plasticisers such as glycerol to reduce the effect of mechanical stress.
Preferably the colloid dispersing medium is gelatin or a mixture of gelatin and a water-soluble latex for example or latex vinyl acrylate-containing polymer. Most preferably if such a latex is present in the final emulsion it is added after all crystal growth has occurred. However the water-soluble colloids for example casein, polyvinyl pyrrolidone or polyvinyl alcohol may be used alone or together with gelatin.
The silver halide emulsions of the present invention exhibit high speed and low granularity, thus they are of particular use in high speed camera film material.
The invention includes the novel silver halide emulsions, methods for preparing these emulsions and coated photographic silver halide material containing at least one such emulsion.
The following Examples will serve to illustrate the invention.

EXAMPLE 1

Emulsion 1

Preparation of twinned octahedral silver iodobromide emulsion.

Step A

Preparation of monosized silver iodide emulsion. 2.4 litres of 10% inert gelatin was stirred at 40^oC with 0.2 cm³ of tri-n-butyl phosphate as an antifoam. Aqueous 4.7 solutions of silver nitrate and potassium iodide were double jetted as follows:
The pI of the emulsion was maintained throughout at 1.0 (monitored by the potential being maintained throughout at - 280 mV by addition of KI, measured by a silver ion electrode with standard calomel reference). The median crystal size produced was 0.15 um.

Step B - Recrystallisation

1 litre of 20% inert gelatin solution was added to the emulsion prepared in Step A and the temperature raised to 65^oC with constant agitation. 4.7 solutions of silver nitrate and sodium bromide were added by double jetting at 25cm³ min⁻¹ until 2500 cm³ of silver nitrate had been added. The potential was maintained at -10mV at 65^oC. To this emulsion was added 2.5 litres of a 20% solution of inert gelatin.

Step C - Growth

4.7 M silver nitrate and sodium bromide were added by double jetting to the emulsion produced in step B, whilst stirring at 65^oC. The solutions were added at 247cm³ min⁻¹ until 11844 cm³ of silver nitrate had been added. The potential was maintained throughout at -110 mV at 65^oC. 4.99 litres of a 24.6 % w/v solution of inert gel were added and stirred thoroughly. 4.7 M silver nitrate and sodium bromide were added by double jetting at 602 cm³ min⁻¹ until 15906 cm³ of silver nitrate had been added. The potential was maintained at -110 mV at 65^oC.

Step D - Further growth

One-tenth of the emulsion produced in step C was taken and 10 litres of a 5% w/v inert gel solution added with thorough agitation at 65^oC. The volume of each crystal was then increased 5-fold by monosize growth as follows; whilst the potential was maintained at -120mV at 65^oC, 4.7m solutions of silver nitrate and sodium bromide were added by double jetting. The initial flow rate was 100 cm³/min, this was progressibely increased linearly with respect to time to a final flow rate of 645 cm³/min, by which time 12720 cm³ of silver nitrate had been added.
The overall iodide content of the final emulsion was 1%.

Emulsion 2

Preparation of twinned octahedral silver iodobromide emulsion.

Step A - As for emulsion 1

Step B - As for emulsion 1

Step C - As for emulsion 1

Step D

One tenth of the emulsion produced in step C was taken and 10 litres of 5% w/v inert gel solution added with thorough agitation at 65^oC. The volume of each crystal was then increased 5-fold by monosize growth as follows; whilst the potential was maintained at -120 mV at 65^oC, 4.7 M solutions of silver nitrate and sodium bromide were added by double jetting. The initial flow rate was 100 cm³/min, this was linearly increased to 335cm³/min until 3180 cm³ of silver nitrate had been added. Double jetting was then continued with solutions of 4.7 M silver nitrate and 4.7 M 90% sodium bromide/10% potassium iodide. The initial rate of addition was 335 cm³/min, this was linearly increased to 561 cm³/min until 6360 cm³ of silver nitrate had been added.
The final stage of growth was to add 4.7 M silver nitrate and 4.7 M sodium bromide by double jetting. The initial rate of addition was 335 cm³/min, this was linearly increased as before to 645 cm³/min until 3180 cm³ of silver nitrate had been added.
The overall iodide of this emulsion was 5%.

Emulsion 3 (according to the present invention)

Preparation of twinned octahedral silver iodobromide emulsion, incorporating sub-surface reduction sensitization with a halide

Step A - As for emulsion 1

Step B - As for emulsion 1

Step C - As for emulsion 1

Step D

As for emulsion 2, except that after one minute had elapsed in the final stage of growth, 10 cm³ of 0.0025 M SnCl₂ was added to the precipitation vessel.

Emulsion 4

Preparation of twinned octahedral silver iodobromide emulsion incorporating reduction sensitization with no halide boundary.

Step A - As for emulsion 1

Step B - As for emulsion 1

Step C - As for emulsion 1

Step D

As for emulsion 3, except 4.7 M silver nitrate and 4.7 M sodium bromide used throughout the whole step. The SnCl₂ addition was made as for Emulsion 3.
The overall iodide content of the emulsion 3 was 5%.
The overall iodide content of the emulsion 4 was 1%.
The accompanying figures show diagramatically the various layers formed during steps B to E in emulsions 1 to 4.
In all the figures the same letters and figures have the same signification.
Figure 1 is a depiction of a silver halide crystal made according to the prior art.
Figure 2 is a depiction of a silver halide crystal with a halide boundary.
Figure 3 is a depiction of a silver halide crystal made according to the present invention.
Figure 4 is a depiction of a silver halide crystal having sub-developable silver centres but not at a halide boundary.
In all the figures A represents the silver iodobromide core formed in steps A and B, that is to say the preparation of monosized silver iodide and the recrystallisation step. In all the figures B represents the silver bromide layer formed in step C, the growth step.
In all the figures C represents the silver bromide shell formed in the further growth step D.
In figure 1 which represents emulsion 1 layers A, B and C are all that are formed.
In figure 2 which represents emulsion 2 the further growth step D takes place in two stages and a silver iodobromide layer D is laid down on the silver bromide layer B. After the silver iodobromide layer D has formed the shell C of silver bromide is laid down on it.
In figure 3 which represents emulsion 3 the further growth step takes place in three stages. A silver iodobromide layer D is laid down on the silver bromide layer B. Then a ring of silver clusters E is formed on layer D by adding a dilute solution of stannous chloride one minute after the silver bromide layer C is starting to form on layer D. This figure represents the emulsion of the present invention. In figure 4 which represents emulsion 4 the further growth step takes place in two stages. The formation of the shell C of silver bromide is interupted to form a ring of silver cluster E in the shell and then the remainder of shell C is formed. This formation of silver clusters is carried out by adding stannous chloride to the emulsification vessel before all the silver bromide has been added in step D.
Thus in emulsion 1 there is no halide boundary in the shell and no ring of silver clusters. In emulsion 2 there is a halide boundary in the shell but no ring of silver clusters. However in emulsion 3 which is the emulsion according to the present invention there is a halide boundary and at the halide boundary a ring of silver clusters. Whilst in emulsion 4 there is a ring of silver clusters in the shell but no halide boundary
Step E the desalination and chemical sensitisation step took place after the completion of crystal growth in step D. The emulsions 1 - 4 were desalinated by ultrafiltration and redispersed with a solution of limed ossein gelatin. They were then adjusted at 40^oC to pH 6 and pAg 8.2.
They were then chemically sensitized at 52^oC for one hour and optimum photographic sensitivity was found when approximately 13 mg sodium thiosulphate pentahydrate and 4 mg sodium tetrachloroaurate dihydrate per mole of silver halide were used. The emulsions were stabilised using 0.41 g 4-hydroxy-6-methyl-1,3,3a,7tetraazaindene per mole of silver halide. All the emulsions were then coated on a cellulose triacetate base at a coating weight of 45 mg Ag/dm².
Samples of the coating were then exposed through a step-wedge for 0.03 s by a light source of 1000 Lux, developed in a solution of composition A, fixed, and the characteristic curves were then plotted as developed density versus log exposure. From these curves the speed at 0.1 density units above fog S0.1 the average contrasts between density values 0.5 and 2.0 and the fog values were tabulated.
The development time in developer A in each case was selected to give a contrast value close to a contrast value of 0.55, corresponding to a reasonable comparison for partially developed monochrome camera films. The characteristic curves for more complete development, that is 10 minutes in a developer of composition B were also measured.

Developer A

2 g Metol
5 g HQ (Hydroquinone)
100 g Na₂SO₃
3 g sod. tetraborate 10H₂O
3.5 g sod. tripolyphosphate
water to 1 litre

Developer B

2 g Metol
90 g Na₂SO₃
8 g Hydroquinone
45 g Na₂CO₃
5 g KBr
water to 1 litre
All the samples were then fixed in an acid ammonium thiosulphate fixing bath and after washing the following sensitometric results were obtained. In the following table the results obtained from the four emulsions using developer A and Developer B are shown.
The higher the figure for the fog shown the worse the result.
S0.1 is the foot speed in log Exposure (E) units. The higher the figure the better the result. A 0.3 log E increase in speed corresponds to a doubling of the arithmetic speed of the film coating.
G1.5 is the contrast obtained. In general the higher the figure the better the result. This figure is obtained by measuring the slope of the curve between points on the photographic curve corresponding to densities of 0.1 and 1.5.
It can be seen by comparison of the results presented in the table, that the emulsion of the invention has the highest foot speed and favourable fog.
A comparison of the results for emulsion 3 (invention) with both a halide boundary and deliberate reduction sensitisation, and emulsion 2 (same crystal structure as emulsion 3, with halide boundary but no reduction sensitisation) shows that the application of the stannous chloride treatment has increased foot speed. Hence the presence of reduction sensitisation at a sub-surface boundary gives increased speed.
A comparison of the results for emulsion 4 with no sub-surface halide boundary but with a stannous chloride treatment and emulsion 1 (control, without subsurface halide boundary or reduction treatment) shows that the addition of stannous chloride alone produces no appreciable benefit, as it should be noted that the fog has been considerably increased for emulsion 4, rendering the emulsion unusable for monochrome applications. Thus the presence of the sub-surface boundary is required for full exploitation of the internal reduction sensitisation, preventing the excessive growth of fog seen in the prior art methods.

EXAMPLE 2

In this Example the reduction sensitisation step is carried out by varying the pH from the norm which is 5.
Each of the emulsions in this Example were grown as described in Example 1 of British Patent Specification No 1596602 except that the silver iodide seed emulsion had an average size of 0.6 µm, and the addition rates of silver and bromide salt solutions during step (ii) and step (iii) were lowered to approximately 10% of those stated. The emulsions had an average crystal size of 1.2 µm with a coefficient of size variation of 30% and an iodide content of 7.5 mole %. The pH and pAg growth conditions were as set out below.
These variations in the experimental conditions were carried to provide a minimal, optimal intercrystal spread of size and iodide content. Addition rates during step (ii) and step (iii) were adjusted for different pAg values using the procedures described in BP 1,596,602.
Thus these emulsions compared with the emulsions prepared in Example 1 comprised a silver iodobromide core obtained by the recrystallisation of the silver iodide seed crystals and a silver bromide shell formed during the further growth stage.
All the emulsions were desalinated by ultrafiltration and chemically sensitised using sodium thiosulphate and sodium chloroaurate as in Example 1. The emulsion were then spectrally sensitised to the red region of the spectrum using the dye A set forth below. All the emulsions were then coated on a triacetate base.
The pAg and pH conditions during growth for the emulsions prepared were as follows :-
Recrystallisation step (ii)

Further growth step (iii)

Dye A had the formula :-
All the coated emulsion samples were then exposed for 0.02 seconds to a step wedge. One set of samples were then developed in a black and white developing agent of the following formula for 4 minutes :-
The other set of samples were developed for 3.25 minutes in a colour developing agent of the formula :-

CD3 is β-methanesulphonamidoethyl ethyl amino toluidine sesquisulphate.
Both sets of samples were then fixed in an ammonium thiosulphate fixing bath and then washed and dried.
The foot speed (S0.1) and the fog of all the samples was then noted.
These results show that increases in foot speed of about 0.1 log E were in both black and white and in chromogenic development obtained as the pH during the further growth step was raised above standard value of 5.0.

EXAMPLE 3

Preparation of tabular silver iodobromide emulsion, with a halide boundary between regions of 12% iodide and pure bromide.

Step A - Growth

2.68 litres of 0.8% inert gelatin solution was stirred at 65^oC with 0.2 cm³ of tri-n-butyl phosphate as an antifoam. Aqueous 4.7 M sodium bromide was added so that the pBr of the solution was 1.0 (the potential was -125 mV, measured by a silver ion electrode with standard calomel reference).
Aqueous 1.2M solutions of silver nitrate and sodium bromide were added by double jetting at 7.2 cm³ min ⁻¹ until 36 cm³ of silver nitrate had been added. The potential was maintained at -125 mV at 65^oC with stirring.

Step B - Gelatin Addition

0.22 litres of 20% inert gelatin solution was added to the emulsion and then the emulsion was stirred for 1 minute at 65^oC.

Step C - Growth

1.2 M silver nitrate was added by a single jetting at 18 cm³ min ⁻¹ until -101 mV was reached. 1.2 M silver nitrate and a 1.2 M solution of 88% sodium bromide / 12% potassium iodide were added by double jetting. The initial flow rate was 18 cm³ min⁻¹, this was linearly increased to 41cm³ min⁻¹ until 3312 cm³ of silver nitrate had been added. The silver ion potential was maintained at -101 mV at 65^oC.
The emulsion was desalinated and redispersed with a solution of limed ossein gelatin. It was then re-adjusted to 65^oC, pH6 and pAg 8.

Step E - Growth

40% by weight of the emulsion produced in Step D was taken and 1.2 M silver nitrate was added at 15 cm³ min⁻¹ until a potential of -101 mV was reached with stirring. 1.2 M silver nitrate and 1.2 M sodium bromide were added by double jetting at 15 cm³ min⁻¹ until 120 mls of silver nitrate had been added whilst maintaining the potential at -101mV at 65^oC. 1.2 M silver nitrate was added at 15 cm³ min ⁻¹ until a potential of -10 mV was reached. This is emulsion M.
A further 40% by weight of the emulsion produced in Step D was taken and growth proceeded as above except that after 1 minute had elapsed into the double jetting stage, 20 cm³ of 0.0025 M SnCl₂ was added to the precipitation vessel. This is emulsion N.

Step F - Sensitisation

Samples of the emulsion M and N were then digested at 52^oC for a range of times and with a range of chemical sensitiser quantities. Optimum photographic sensitivity was found when approximately 7.11 mg sodium thiosulphate pentahydrate and 1.07 mg sodium tetrachloroaurate dihydrate per mole of silver halide were used. Then half of the emulsion samples of M and N were spectrally sensitised by addition of 0.15 g mole⁻¹ of a carbocyanine dye of the following formula :-

Then all the emulsions M and N were stabilised using 0.41 g of 4-hydroxy-6-methyl-1,3,3,7tetraazaindene per mole of silver halide. The optimally sensitised emulsions were then coated on a triacetate base at 50 mg Ag dm⁻².
Samples of the coatings were then exposed through a continuous wedge for either 0.03 secs by a light source of 250 lux or for 1 second by a light source of 1 lux. These were then developed in a solution of developer A used in Example 1, fixed, and the characteristic curves were plotted as developed density versus log exposure. From these curves the fog S0.1 the speed at 0.1 density units above fog, the average contrast over a 1.5 log E range starting at 0.1 density units above fog were all determined.
The development time in each case was fixed at 8 minutes. The characteristic curves for a more complete development 10 minutes in a developer B as used in Example 1 were also measured.
The accompanying drawings figures 5 and 6 are diagrammatic views of tabular Emulsions M and N as just prepared. Figure 5 shows Emulsion M and Figure 6 shows Emulsion N. In both figures the same numbers have the same signification. The aspect ratios of both Emulsions M and N as produced were 20 : 1. In figure 5 the silver bromide core 1 as produced in step A is surrounded by a silver iodobromide layer 2 having 12% iodide as produced in the growth step C. The silver iodobromide layer 2 is surrounded by a silver bromide shell 3 as produced by the growth step E. Separating the layer 2 from the shell 3 is a halide boundary 4.
In figure 6 core 1, layer 2 and shell 3 are as in figure 5 but around the halide boundary 4 are silver halide clusters 5 forming sub-developable centres. These clusters have been formed at the boundary by the addition of Sn Cl₂ after 1 minute into growth step E.
Samples of the emulsion M and N were digested at 50°C for 1 hour optimum photographic sensitivity was found when 7.11 mg sodium thiosulphate pentahydrate and 1.07 mg sodium tetrachloroaurate dihydrate per mole of silver halide were used. The half of the emulsion samples of M and N were spectrally sensitised by addition of 0.15 g mole ⁻¹ of a carbocyanine dye of the following formula :-

Then all the emulsions M and N were stabilised using 0.41g of 4-hydroxy-6-methyl-1,3,3,7tetraazaindene per mole of silver halide. The optimally sensitised emulsions were then coated on a traicetate base at 50 mg Ag dm⁻².
The results were as follows :-
It can be seen by comparison of the results presented in the above table that the emulsion of the invention has the highest foot speed and favourable fog.
A comparison of the results for Emulsion N with Emulsion M a tabular emulsion with a halide boundary but no reduction sensitisation shows that the stannous chloride treatment has increased foot speed for both the dyed and undyed emulsion. Hence the presence of reduction sensitisation at a sub-surface halide boundary gives increased speed.
A comparison of the long exposure (1 second at 1 LUX) results and the short exposure (1/30 sec, 250 LUX) for Emulsion N (invention) and Emulsion M (tabular emulsion with bromide shell but no reduction sensitisation) shows that the low intensity reciprocity failure (LIRF) is much improved for the Emulsion N. Hence the presence of reduction sensitisation at a sub-surface halide boundary gives improved low intensity reciprocity failure.

EXAMPLE 4

Emulsion Growth Scheme

EMULSION 1 (control)

Step A - Preparation of monosized silver iodide emulsion

The silver iodide seed emulsion was grown as described in Example 1 of British Patent Specification No. 1596602 except that the crystals had an average size of 0.65 µm.

Step B - Recrystallisation

16.9 litres of 4.1 % gel solution was added to a quantity of the emulsion prepared in step A containing 35 moles of silver at a temperature of 70°C and with constant stirring. 1.5m solutions of silver nitrate and sodium bromide were added by double jetting until 35 litres of silver nitrate had been added. The initial jetting rate was 199 cm³/minute at the end of addition. The potential was maintained at -58mV at 70°C.

Step C - Growth

3.6 % of the emulsion produced in step B was taken, and .35 litres of a 29% w/v inert gel solution added with thorough agitation at 65°C. 0.2 cm³ of tri-n-butyl phosphate was added as an antifoam. The volume of each crystal was then increased by double jetting 1.5 m solutions of silver nitrate and sodium bromide while maintaining the potential at -33 mV at 65°C. The first 355 cm³ was added as a jetting rate of 10 cm³ minute and finally 1070 cm³ at 40 cm³/minute.
At this stage the emulsion was composed of AgIBr in which 15 mole % of the halide was iodide and the median crystal size was 1.0 um (based on equivalent circular diameter).

EMULSION 2 (Invention)

A further 3.6 % of the emulsion from Step B was taken and growth proceeded as for Emulsion 1 with the following difference : after one minute of the first double jetting stage had elapsed, 104 cm³ of 20 g/litre glutaraldehyde bisulphite solution (made up in freshly boiled deionised water) was added dropwise.

EMULSION 3 (Invention)

A further 3.6 % of the emulsion from step B was taken and growth proceeded as for Emulsion 1 with the following difference : after one minute of the first double jetting stage had elapsed, 104 cm³ of 2 g/litre glutaraldehyde bisulphite solution (made up in freshly boiled deionised water) was added dropwise.
After the completion of crystal growth, the emulsions were desalinated and redispersed with a solution of lined ossein gelatin.

Chemical Sensitisation

The emulsions were adjusted at 40°C to pH6 and pAg 8.2. They were then digested at 52°C for a range of times and with various quantities of chemical sensitisers. Sulphur was added as sodium thiosulphate pentahydrate at levels of 0, 7 and 13 mg per mole of silver. Gold was added as sodium tetrachloroaurate dihydrate at levels of 0, 2 and 4 mg per mole of silver. The emulsions were stabilized using 0.41 g of 4-hydroxy-6-methyl-1,3,3a,7tetrazaindene per mole of silver.
Each emulsion was coated on a cellulose triacetate base at a coating weight of 50 mg Ag/dm².
Samples of the coatings were then exposed through a step-wedge for 1/30 second by a light source of 250 lux, developed for eight minutes in a solution of composition A, fixed in an acid ammonium thiosulphate bath and washed. The characteristic curves were then plotted as developed density versus log exposure from these curves the speed at 0.1 density units above fog, S_0.1, the average contrast between density values 0.5 and 2.0, G_1.5, and the fog values were calculated. The characteritic curves for more complete development, 10 minutes in a solution of composition B, were also measured.

Developer A

2 g Metol
5 g Hydroquinone
100 g Na₂SO₃
3 g Sodium tetraborate 10 H₂ O
3.5 g Sodium tripolyphosphate
Water to 1 litre

Developer B (more active developing agent)

2 g Metol
8 g Hydroquinone
90 g Na₂SO₃
45 g Na₂CO₃
5 g KBr
Water to 1 litre
In the following table the results obtained from the three emulsions using developers A and B are shown.
S_0.1 is the foot speed in Log Exposure (E) units. The higher the figure the better the result. A 0.3 Log E increase in S_0.1 corresponds to a doubling of the arithmetic speed of the film coating.
G_1.5 is the contrast obtained. In general, the higher the figure the better the result.
The higher the figure for fog, the worse the result.
The first three lines of results compare emulsions 1,2 and 3 with no chemical sensitisation.
The next pair of results compares emulsion 1 (the control) with emulsion 2 after digestion with 7mg/mole Ag of sodium thiosulphate pentahydrate and 4 mg/mole Ag of sodium tetrachloroaurate dihydrate for 60'.
These results are followed by a comparison of emulsion 1 and emulsion 3 after digestion with 3.5 mg/mole Ag of sodium thiosulphate pentahydrate and 2 mg/mole Ag of sodium thetrachloroaurate dihydrate for 30'.
Comparison of the results presented above shows that the emulsions of the invention have higher foot speeds and constraints than the control. Emulsions 2 and 3 (invention) contain a halide boundary and deliberate reduction sensitisations while emulsion 1 is of identical crystal structure but with no reduction sensitisation. Hence the presence of reduction sensitisation at the sub-surface boundary gives substantially increased foot speed an contrast while fog levels remain acceptable.

Etching followed by gold latentisifaction

It was possible to reveal the sub-developable centres formed by reduction sensitisation at the internal halide compositional boundary by a process of etching the crystals down to the boundary, and then latensifying the sub-developable centres with a gold thiocyanate complex solution, thus converting them into fog centres.
The results below compare Emulsion 1 (control) with Emulsion 3 (invention). The etchant used was sodium thiosulphate solution.
The high fog level shown by Emulsion 3 after etching and latensification demonstrates the presence of sub-developable latent image centres as a result of reduction sensitisation at the compositional boundary.
The gold latensification method used was that described by T H James, W Vanselow and R F Quirk. Photographic Science and Engineering 5, 219 (1961).

Claims

A silver halide emulsion which comprises a colloid medium having dispersed therein silver halide emulsion crystals which are characterised in that they comprise at least two layers of different silver halide composition, there being sub-developable silver centres which have been formed at the boundary between two of the layers by a reduction sensitisation step.
A silver halide emulsion according to claim 1 characterised in that all the silver halide crystals in the emulsion have been both chemically and spectrally sensitised at the surface.
A silver halide emulsion according claim 1 characterised in that the inner layer of the boundary in the silver halide crystals at which sub-developable centres are present, is silver iodobromide and the outer layer is silver bromide.
A silver halide emulsion according to claim 3 characterised in that the amount of iodide in the silver iodobromide layer is from 2 to 35 mole % iodide.
A method of preparing a silver halide emulsion as claimed in claim 1 which is then characterised in that it comprises preparing in a hydrophilic colloid dispersing medium a silver halide emulsion using a water soluble silver salt solution and water-soluble halide solutions to form silver halide crystals of a requisite size and having at their surface silver halide of a first composition then changing the halide/halides to form silver halide having a second composition which is laid down on the already formed silver halide crystals, then carrying out a reduction sensitisation step and continuing to add the silver and halides of the second composition until the silver halide crystals reach a final requisite size.
A method according to claim 5 characterised in that after the reduction sensitisation step an interval of time occurs before resumption of the addition of silver and halides.
A method according to claim 5 characterised in that the reduction sensitisation step is carried out by adding a reducing agent to the dispersing medium.
A method according to claim 7 characterised in that the reducing agent is stannous chloride or glutaraldehyde bisulphite.
A method according to claim 5 characterised in that the reduction sensitisation step is carried out by use of ionising radiation.
A method according to claim 5 characterised in that the reduction sensitisation step is carried out by lowering the pAg or raising the pH.