BE894968A - PRETANNED PHOTOGRAPHIC PRODUCTS AND THEIR USE IN RADIOGRAPHY - Google Patents

Description

       

  Pre-tanned photographic products and their use in

  
x-ray.

  
The present invention relates to photographic products and their use in radiography. The products are formed of a support on which are placed one or more layers of hydrophilic colloid capable of

  
be tanned comprising at least one emulsion layer containing grains of photosensitive silver halides.

  
Black and white silver halide photography has traditionally used developed silver to produce a visible image. Although black and white photography responds to diverse imaging needs, medical radiography, described below, illustrates the diverse and in some cases competing requirements that are encountered in forming a film image.

  
In medical radiography, a large area of photosensitive product is often necessary for a single exposure. In addition, the money that remains in the product to form the image may be unavailable for recovery for many years. It is therefore very desirable to make efficient use of the money contained in radiographic products. A measure of the effectiveness of the use of money is the covering power. In the present description, the covering power is defined as being

  
100 times the ratio of maximum density to the quantity of developed silver expressed in grams of silver per square decimetre. It is recognized that a high covering power is an advantageous characteristic, not only

  
radiographic products, but also other black and white photographic products. Coverage and the conditions that affect it are discussed by James, Theory of the Photographic Process, 4th ed., Macmillan,
1977, pp. 404, 489 and 490, and by Farnell and Solman,

  
"The Covering Power of Photographic Silver Deposits I.

  
Chemical Development ", The Journal of Photography Science, Vol. 18, 1970, pp. 94-101.

  
One way to approach the problem of obtaining a high covering power is to use relatively fine silver halide grains, since it is well known that increasing the size of the grains reduces the power covering. Unfortunately, in medical radiography,

  
the need to minimize patient exposure to x-rays is even more important than achieving efficient use of money. Since the silver halide becomes more sensitive (grows faster) as a direct function of the grain size, it is not surprising then that X-ray products use larger grains. Thus, while it is important to achieve high hiding power, the relatively large grain silver halide emulsions currently used are not well suited to achieving high levels of power. covering.

  
We therefore use other methods to improve

  
covering power. It is known that large grain silver halides, usually at least

  
  <EMI ID = 1.1>

  
covering power depending on the tanning. To obtain the highest covering power compatible with speed requirements (and therefore grain size requirements), technicians are used to limiting pre-tanning
(i.e. tanning during manufacturing) to the degree just necessary to allow handling of radiographic products (although the risk of damaging these products remains relatively high).

  
The final tanning is obtained to the desired degree by

  
  <EMI ID = 2.1>

  
incorporating a tanning agent into the treatment composition, usually the developer. Particularly effective tanning agents which can be used in treatment compositions are dialdehydes and their bisbisulphite derivatives of the type described in US Pat. No. 3,232,764. Unfortunately, the tanning agent must be kept separate from the treatment composition before use. In addition, the presence of such a tanning agent imposes additional constraints on the choice of developers.

  
Usually in medical radiography we use

  
a radiographic film having, applied in layers on the two main faces, relatively large grain silver halide emulsions. The emulsion layers have a minimum of pre-tanning to allow maximum covering power to be obtained. The product is more sensitive to light than to X-rays and is therefore usually placed between a pair of intensifying screens which, when exposed to X-rays according to an image, emit fluorescence according to an image to expose the radiographic product. .

  
The radiographic product is then treated with a developer containing a tanning agent. To allow rapid access to a visible image, the radiographic product is processed at temperatures above room temperature (usually about 25 [deg.] C to about 50 [deg.] C) and

  
in less than a minute. Development is usually completed in about 20 s. A process of the type we have just

  
to describe is illustrated in the patent of the United States of America

  
3,545,971.

  
Different grain forms, regular and irregular, have been observed in photographic silver halide emulsions intended for obtaining black and white images in general and, more particularly, for obtaining radiographic images. Regular grains are often cubic or octahedral in shape. The grain edges can be rounded due to the effects of matura-apparently the same elements as those of US Patent 4,150,994, although not limited to the use of seed crystals crystalline d 'silver iodide. Thus, it is considered that the aforementioned patents in this paragraph describe the preparation of silver halide emulsions with relatively thick tabular grains of intermediate shape index.

  
The present invention relates to a photographic product formed of a support on which are arranged one or more layers of a hydrophilic colloid capable of being tanned comprising at least one layer of emulsion containing grains of photosensitive silver halides which is sufficiently pre-tanned not to need additional tanning during the treatment and yet allows high degrees of covering power to be obtained.

   The above-mentioned photographic product is characterized in that at least 50% of the total projected surface of the silver halide grains in at least the emulsion layer is provided by thin tabular grains having a thickness of less than 0.3 µm , the layers of hydrophilic colloid being pre-tanned to a degree sufficient to reduce the swelling of said layers less than 200%, the percentage of swelling being deter-

  
  <EMI ID = 3.1>

  
for 3 days at 50% relative humidity, (b) by measuring the thickness of the layer, (c) by immersing the product

  
  <EMI ID = 4.1>

  
and (d) determining the percentage change in the thickness of the layer compared to the thickness of the layer measured in step (b).

  
According to a preferred embodiment, the photographic product is a product for radiography formed of a support with a high coefficient of light transmission on the two faces of which are arranged one or more layers of hydrophilic colloid comprising at least one layer of emulsion containing photosensitive silver bromide grains which contain up to 6 mol% of iodide; this product is characterized in that at least 50% of the total surface area of the silver bromide grains in at least one emulsion layer consists of thin tabular grains of silver bromide containing up to 6% moles

  
  <EMI ID = 5.1>

  
with an average shape of at least 5: 1, this shape index being defined as the ratio of the diameter of the grain to the thickness, the diameter of a grain being defined as the diameter of a circle having an area equal to the projected surface of this grain.

  
The invention also relates to the use of the photographic products according to the invention in radiography. A high hiding image can be obtained by exposing a photographic product or, specifically, a product for radiography as described above, and developing a visible image in less than a minute.

  
The present invention allows a black and white photographic product intended to form a visible silver image to be sufficiently pre-tanned so that no additional tanning is necessary during the treatment, while allowing a high covering power to be obtained. . The invention responds to an ancient need for photographic products, more particularly for products for radiography, of relatively high speed and having a great covering power, which can be treated quickly without running the risk of being damaged by tanning. incomplete, or without requiring the use of a treatment bath containing a tanning agent.

  
With the radiographic products according to the invention, the exposure of each emulsion layer through the support is significantly reduced, which results in a lesser reduction in image sharpness due to exposure through the support , taking into account other photographic characteristics. More precisely, the radiographic products according to the invention have at least one emulsion layer with silver halides which, for a given silver titer (that is to say the mass of silver per unit

  
emulsion layer surface) and comparable photographic speed, reduces the effects of exposure through the support.

  
The emulsions used in the present invention and the photographic products according to the invention have marked advantages with regard to the speed / granularity relationship and the sharpness without any relation to the exposure through the support. These improvements are made independently of the halide content of the tabular silver halide grains. Silver bromoiodide emulsions have an improved speed / granularity relationship compared to previously known tabular grain emulsions and compared to the better speed / granularity relationship achieved so far with silver bromoiodide emulsions in general. The sensitivity in blue of emulsions is considerably increased

  
silver bromide and bromoiodide compared to their natural sensitivity using spectral blue sensitizers.

  
The present invention applies generally to black and white photographic products intended to be used for the formation of retained silver images, having at least one layer of relatively large grain silver halide emulsion containing a hydrophilic colloid suitable for being tanned or its equivalent. To obtain the advantages of the present invention, emulsions are used

  
silver halides with tabular grains having a high shape index to form at least one of the relatively large grain emulsion layers.

  
In the present description, the term "thin" applied to silver halide emulsions means that the tabular grains of silver halides have a lower thickness. <EMI ID = 6.1>

  
thin grain gent have medium grain thickness

  
  <EMI ID = 7.1>

  
The advantages of the present invention relating to the covering power are, in inverse ratio, to the average thickness of the tabular grains of the silver halide emulsions with thin tabular grains used. Usually the tabular grains have an average thickness of at least

  
  <EMI ID = 8.1>

  
it is possible, in principle, to use tabular grains p: .us thin, for example as thin as

  
  <EMI ID = 9.1>

  
Although it is possible to obtain the advantages relating to the covering power with emulsions with thin tabular grains of low shape index, to obtain the other advantages of the silver halides with tabular grains, in combination with the advantages in covering power it

  
is preferable than the thin silver tabular halide emulsions used for processing

  
of the invention have an average shape index of at least

  
5: 1. The silver halide emulsions with tabular grains which are preferably used are thin tabular grain emulsions with a high shape index. High tabular, thin tabular grain emulsions are those in which the tabular grains have a shape index greater than 8: 1 and represent at least 50% of the total projected area of the silver halide grains. Preferably, these silver halide grains represent at least 70% and, optimally, at least 90% of the total projected area of the silver halide grains.

  
The increase in covering power is particularly marked when the tabular grains of silver halides whose thickness is less than 0.3 μm have a diameter of at least 0.6 μm, optimally an average diameter of at least

  
  <EMI ID = 10.1>

  
The characteristics described above of the silver halide emulsions used according to the invention can be easily demonstrated by methods well known in the art. As used in the present description, the expression “shape index” designates the ratio of the diameter of the grain to the thickness of this grain. The term "diameter" of the grain is itself defined as the diameter of a circle having an area equal to the projected area of the grain as seen on a photomicrograph or on an electron micrograph of a sample. emulsion. From the shadows cast by an electron microscopy image of an emulsion, it is possible to determine the thickness and diameter of each grain and to identify those of the tabular grains whose

  
  <EMI ID = 11.1>

  
thin tabulars. From these data, we can calculate the shape index of each of these tabular grains and we can then average the shape indices of all the tabular grains of the sample which satisfy these conditions concerning the thickness less than 0.3 ixm: this average constitutes the average shape index. According to this definition, the average shape index is the average of the indices

  
shape of each tabular grain. In practice, it is generally simpler to obtain an average thickness and an average diameter of tabular grains and to calculate the average shape index which is the ratio of these two means. That we use the average of the shape indices of each

  
grain or thickness or diameter averages to determine shape index, taking into account tolerances

  
possible particle size measurements, the values obtained for the average shape index do not differ significantly.

  
We can sum the projected values of the thin tabular grains of silver halides, then separately

  
we can add up the areas of the other silver halide grains in the photomicrograph; from these two respective sums, one can obtain the percentage of the total projected surface occupied by the thin tabular grains of silver halides.

  
For the above assessments, a reference tabular grain was chosen; this grain has a thickness of less

  
  <EMI ID = 12.1>

  
thin tabulars of thicker tabular grains with lower photographic characteristics. For

  
for the purposes of this description, the thin tabular grains are the silver halide grains whose thickness

  
  <EMI ID = 13.1>

  
under a magnification of 10,000. The term "projected surface" is used in the same sense as the terms "projective area" or "projection area", commonly used in the art
(see, for example, James and Higgins, Fundamentals of Photography Theory, Morgan & Morgan, New York, p. 15.

  
Although a single layer of tabular grain emulsion with a high shape index is necessary in the photographic products according to the invention, these can, if desired, contain several layers of tabular grain emulsion. Emulsions other than the prescribed high grain index tabular grain emulsions may have any suitable conventional form. Different conventional emulsions are described in Research Disclosure, Vol. 176, December 1978, publication 17643, Paragraph I, Emulsion preparation and types. (Research Disclosure and Product Licensing Index are publications of Industrial Opportunities Ltd; Homewell, Havant, Hampshire, P09 1EF, United Kingdom) In addition, thin tabular grain emulsion layers can be used in combination with

  
thicker tabular grain emulsion layers with a high shape index, such as those having a thickness

  
  <EMI ID = 14.1>

  
The silver halide emulsion layers and, optionally, the other layers, such as overcoats, interlayers and substratums, photographic products, can contain various colloids capable of being tanned, alone or in combination, such as vehicles. In the present description, the term "vehicle" includes both binders and peptizers. The photographic products according to the invention are pre-tanned, that is to say that the colloids are sufficiently crosslinked so that no subsequent tanning is necessary after the manufacture of the product. The layers based on hydrophilic colloid are sufficiently pre-tanned to reduce the swelling of these layers to less than 200%.

   Although a number of similar tests have been used to provide a specific definition, in the present description the percentage of swelling is defined as the percentage determined by the procedure of Example 11 of the United States patent. America 3 841 872, but with a temperature

  
  <EMI ID = 15.1>

  
More specifically, the percentage of swelling is determined
(a) by baking the photographic product at 38 [deg.] C for 3 days at 50% relative humidity, (b) by measuring the thickness of the layer, (c) by immersing the photogra-

  
  <EMI ID = 16.1>

  
by determining the percentage change in the thickness of the layer compared to the thickness of the layer measured in step (b). The percentage of swelling is the product of the difference between the final thickness and the original thickness (after baking) of the layer, divided by the original thickness of the layer and multiplied by 100.

  
It is preferable that the photographic products according to the invention have a percentage of swelling less than
100%. As is well known in the art, the percentage of swelling can be controlled by adjusting the concentration of the tanning agent used.

  
It has been found that, surprisingly, the pre-tanning of the photographic products according to the present invention does not produce the reduction in covering power which is observed in the pre-tanned photographic products without the silver halide emulsions with tabular grains of high shape index as described above, but containing silver halide grains having an average diameter calculated on the projected surface of at least 0.6 μm. In addition, the pre-tanned photographic products according to the invention have a higher covering power than comparable photographic products containing non-tabular silver halide grains having the same diameter calculated on the projected surface.

   In addition, the photographic products according to the present invention have a higher covering power than photographic products in all respects comparable using tabular silver halide grains of greater average thickness having either the same mean diameter or

  
a higher average shape index. Although hitherto high hiding power has been achieved in the art by using smaller silver halide grains on average, such grain sizes limit photographic speed. The present invention offers for the first time the possibility of preparing photographic products having a higher speed without undergoing a significant reduction in covering power.

  
The photographic products according to the invention may contain other conventional emulsion layers in addition to the prescribed emulsions with tabular grains of high shape index, the total covering power of the photographic product
(as opposed to individual emulsion layers) can vary over a wide range. However, in preferred photographic products according to the invention, in particular those in which all the emulsion layers present contain tabular grains having a thickness <EMI ID = 17.1>

  
present description, the covering power is expressed by the maximum density divided by the mass of developed silver expressed in grams per square decimeter and multiplied by
100.

  
The thin silver tabular halide emulsion layers and the other layers of photographic products may contain various colloids suitable for being tanned, alone or in combination, as vehicles. Suitable hydrophilic colloids include substances such as proteins, protein derivatives, cellulose derivatives, for example cellulose esters, gelatin, for example gelatin treated with a basic agent (bone or skin gelatin cattle), or gelatin treated with an acid agent (pigskin gelatin), gelatin derivatives, for example acetylated gelatin, phthalylated gelatin, etc., polysaccharides such as dextran, gum arabic, arrowroot, albumin, etc.

  
Gelatin and gelatin derivatives are the preferred vehicles.

  
The emulsion layers and the other layers of photographic products, such as overlays, interlayers and substratums may also contain, alone or in combination with hydrophilic water-permeable colloids as vehicles or vehicle diluents (for example under latex form), synthetic polymeric peptizers, vehicles and / or binders such as polyvinyl lactanes, acrylamide polymers, polyvinyl alcohol and its derivatives, polyvinyl acetals, polyacetates and polymethacrylates of alkyl and sulfoalkyl, polyamides, polyvinyl pyridine, acrylic acid polymers, maleic anhydride copolymers, alkylene polyoxides, methacrylamide copolymers, polyvinyl oxazolidinones, maleic acid copolymers, copolymers of acryloyloxyalkylsulfonic acid,

   sulfoalkylacrylamide copolymers, polyalkyleneimine copolymers, polyamines, N, N-dialkylaminoalkyl polyacrylates, vinyl imidazole copolymers, vinyl sulfide copolymers, halogenated styrene polymers,

  
amineacrylamide polymers and polypeptides.

  
The layers of the photographic product containing crosslinkable colloids, for example the layers containing

  
gelatin or a derivative of gelatin, can be pretanned by various organic or mineral tanning agents, such as those described by T.H. James, The Theory of the Photographic Process, 4th ed., MacMillan, 1977, pp. 77-87.

  
The pretanning agents can be used alone or in combination and in free or blocked form.

  
Common pre-tanning agents include formaldehyde and free dialdehydes, such as succinaldehyde and glutaraldehyde, blocked dialdehydes, a-diketones, active esters, sulfonate esters, active halogenates, s-triazines and diazines , epoxides, aziridines, active olefins having two or more active vinyl groups (e.g. vinylsulfonyl groups), blocked active olefins, isoxazolium salts not substituted in position 3, 2-alkoxy-N- esters carboxydihydroquinoline, N-carbamoyl and N-carbamoyloxypyridinium salts, tanning agents with mixed function such as halogen-substituted aldehyde acids (for example mucochloric and mucobromic acids), acroleins substituted with an onium group,

   vinyl sulfones containing other tanning functional groups, polymer tanning such as starches having a dialde-

  
hyde and the copolymers of acrolein and methacrylic acid.

  
The use of pretanning agents in combination

  
is known in the art. Tanning accelerators can be used.

  
The tabular grains can be of any composition of silver halide crystals whose utility in photography is known. In a preferred form with the widest range of benefits observed, the present invention uses thin grain silver bromoiodide emulsions.

  
The thin tabular grain silver bromoiodide emulsions can be prepared by the precipitation process below. Grains of index of intermediate form, as opposed to high index of form, can be prepared simply by ending precipitation earlier. Obtaining thin grains at the start of precipitation, as described below, allows the preparation of emulsions with thin tabular grains.

  
A dispersing medium is introduced into a reactor usually used for the precipitation of silver halides and comprising an effective stirring device. Generally, the dispersing medium initially introduced into the reactor represents at least 10% (preferably

  
20 to 80%) of the total mass of the dispersing medium present in the silver bromoiodide emulsion at the end of the precipitation of the grains. We know, by the French patent

  
2,471,620 (Belgian patent n [deg.] 886,645), which can be removed from the dispersing medium of the reactor, by ultrafiltration, during the precipitation of the silver bromoiodide grains. Also, the volume of dispersing medium, initially present in the reactor, can be equal to or even greater than the volume of the silver bromoiodide emulsion present in the reactor at the end of the precipitation of the grains. The dispersing medium which is initially introduced into the reactor

  
is preferably water or a dispersion of a peptizer in water, optionally containing other adjuvants such as one or more agents for maturing silver halides and / or metal dopants, described more particularly below . When we use a peptizer, it represents <EMI ID = 18.1>

  
of the total amount of peptizer present at the end of the precipitation of the silver bromoiodide grains. An additional quantity of dispersing medium is added to the reactor together with the silver salt and the halides. This additional amount of dispersing medium can also be introduced using a separate jet. It is common practice to adjust the proportion of dispersing medium, in particular to increase the proportion of peptizer, after the end of the introduction of the salts.

  
The reactor contains, before precipitation, a small part, usually less than 10% by mass, of bromide used to form the silver bromoiodide grains, in order to adjust

  
the concentration of bromide ion in the dispersing medium at the start of precipitation of the silver bromoiodide grains. In addition, before precipitation, the dispersing medium contained in the reactor comprises practically no iodide ions. Indeed,

  
the presence of iodide ions before the simultaneous introduction of salt

  
silver and bromides promotes the formation of thick, non-tabular grains. The terms "practically does not include iodide ions" used previously, mean that the iodide ions

  
are present in the reactor in an insufficient quantity (compared to that of bromide ions) for there to be precipitation of separate silver iodide grains. Before the introduction of the silver salt, it is preferable to maintain the iodide concentration in the reactor at less than 0.5 mol% of the total concentration of halide ion present in the reactor. If the pBr of the dispersing medium is initially too high, the tabular grains of silver bromoiodide obtained will be comparatively thick and therefore have low shape indices. It is possible to initially maintain the pBr of the dispersing medium in the reactor at a value of at least 1.6. Yes

  
  <EMI ID = 19.1>

  
it is necessary to keep the pBr value below 1.5. On the other hand, if the pBr is too low, the formation of non-tabular silver bromoiodide grains is favored.

  
This is why it is possible to maintain the pBr of the dispersing medium in the reactor at a value equal to or greater than 0.6, preferably at a value greater than 1.1. In the present description, pBr is defined as being the negative logarithm of the bromide ion concentration. The pH and pAg are defined analogously for the hydrogen ion and silver ion concentrations, respectively.

  
During precipitation, the silver salt, bromide and iodide are introduced into the reactor by well known methods for the precipitation of silver bromoiodide grains. Usually, an aqueous solution of a soluble silver salt, such as silver nitrate, is introduced into the reactor, and simultaneously the bromide and iodide salts. The latter salts are generally also introduced in the form of aqueous solutions, for example in the form of aqueous solutions of one or more soluble halides of ammonium, alkali metal (sodium or potassium, for example) or alkaline earth metal (for example example magnesium or calcium). At least at the start, the introduction of the silver salt and the iodide into the reactor is done separately. The bromide and iodide solutions can be introduced separately into the reactor or else as a mixture.

  
With the introduction of the silver salt into the reactor begins the nucleation of the grains. A population of germs is formed which can serve as precipitation sites for silver bromide and silver iodide when the introduction of silver salt, bromide and iodide continues. The precipitation of silver bromide and silver iodide on existing germs constitutes the growth stage in the formation of the grain. The shape indices of the tabular grains formed according to the invention are less influenced by the iodide and bromide concentrations during growth than during nucleation. It is therefore possible to increase the latitude for fixing the pBr so as to have, during the introduction of the salts, a pBr of between approximately 0.6 and 2.2 and preferably between 0.8 and 1.5.

   Of course, it is possible and in fact preferable to keep the pBr inside the reactor, throughout the introduction of the silver and the halides, within the limits described above, before the introduction of the silver salt. This is particularly preferred when the formation of seeds takes place at a high rate during the entire introduction of the silver salt, bromide and iodide, as is the case for the preparation of highly polydispersed emulsions. Raising the pBr to values greater than 2.2 during the growth of the tabular grains leads to an increase in the thickness of the grains, but this can be tolerated when thin tabular grains of silver bromide are still obtained.

  
Instead of introducing the silver salt, bromide and iodide in the form of aqueous solutions, it is possible to consider adding these salts, at the beginning of the precipitation or during growth in the form of fine grains of halides d silver suspended in a dispersing medium. Size

  
grains is such that once introduced into the reactor, these grains easily undergo Ostwald maturation on larger seeds, if there are any.

  
The maximum grain size depends on conditions in the reactor such as temperature and the presence of solubilizing and maturing agents. Grains of silver bromide, silver iodide and / or silver bromoiodide can be introduced. It is also possible to use grains of silver chlorobromide and silver chlorobromoiodide, since the bromide and / or iodide are preferably precipitated with chloride. These silver halide grains are advantageously very fine; their average diameter is,

  
  <EMI ID = 20.1>

  
Taking into account the conditions relating to pBr indicated above, the addition of the silver salt, bromide and iodide can be made at any appropriate conventional speed and concentration. The silver salts and the halides are advantageously introduced at concentrations between 0.1 and 5 moles / liter, but a wider concentration range, for example from 0.01 mole / liter at saturation, can be envisaged, in a classic way. Particularly advantageous precipitation techniques are those which allow a reduction in the duration of precipitation by increasing the amount of salt introduced per unit of time. This amount of salt can be increased either by increasing the rate of introduction itself, or by increasing the concentration of the salt solutions which are introduced.

   It is particularly preferred to increase the rate of introduction of the salts, but by keeping it below a threshold from which the formation of new germs is favored, that is to say renucleation, according to the indications of the patents of United States of America

  
3,650,757, 3,672,900, 4,242,445, at German patent application 2,107,118, at European patent application 80102242, and according to an article by Wey "Growth Mechanism of AgBr Crystals in Gelatin Solution", Photography Science and Engineering, vol. 21, N [deg.] 1, January-February 1977, p. 14 and following.

  
By avoiding the formation of additional germs after the start of the growth stage, it is possible to obtain populations of thin tabular grains of silver bromoiodide which are relatively monodispersed. Emulsions can be prepared with coefficients of variation of less than about 30%. The coefficient of variation is defined here as the standard deviation of the grain diameter, multiplied by 100 and divided

  
by the average grain diameter. It is of course possible to prepare polydispersed emulsions having much higher coefficients of variation, by intentionally promoting renucleation during the growth stage.

  
The iodide concentration of the silver halide emulsions used in the present invention can be controlled by the introduction of iodide. Any conventional concentration of iodide can be used. It is recognized in the art that even very small amounts of iodide, for example as low as 0.05% by mole, are beneficial.

  
Unless otherwise indicated, all references to the percentages of halides are given with respect to the silver present in the corresponding emulsion, the grain or the region of the grain in question; for example, a grain consisting of silver bromoiodide containing 40 mol% of iodide

  
and we

  
also contains 60 mole% bromide. / a preferred form, the emulsions used in the present invention contain at least 0.1 mol% of iodide. Silver iodide can be incorporated into the tabular grains of silver bromoiodide up to its solubility limit in silver bromide

  
at the grain formation temperature. Thus at precipitation temperatures of the order of 90 [deg.] C the iodide concentrations can reach up to about 40 mol% in the tabular grains of silver bromoiodide. In practice, the precipitation temperatures can vary until they are close to room temperature, for example around 30 [deg.] C. Generally, it is preferred that the precipitation is carried out at temperatures between 40 [deg.] C and

  
  <EMI ID = 21.1>

  
prefers to limit the maximum iodide concentrations to about 20% by moles, the optimum concentrations being approximately limited to 15% by moles. In x-ray products, iodide is advantageously present in concentrations up to 6 mol%.

  
The relative proportion of iodide and bromide introduced into the reactor during precipitation can be maintained in a fixed ratio to form a substantially uniform iodide distribution in the tabular grains of silver bromoiodide, or this proportion can vary to achieve different photographic effects. Advantages in photographic speed and / or in granularity can result from the increase in the proportion of iodide in the tabular grains of silver bromoiodide with high shape index, in regions preferably annular and lateral compared to the central regions tabular grains.

   The iodide concentrations in the central regions of the grains can vary from 0% to 5% by mole, with at least 1% by moles of iodide in the lateral ring regimes, up to the solubility limit of iodide d silver in silver bromide, preferably up to about 20 mole% and optimally up to about 15 mole%. The tabular silver bromoiodide grains used in the present invention may have practically uniform or gradual iodide concentrations, and the variation in concentration may be controlled at will to obtain higher iodide concentrations therein. or on the surface or near the surface of the tabular grains of silver bromoiodide.

  
The preparation of silver bromoiodide emulsions with tabular grains of high shape index has been described above by a process which gives neutral or non-ammoniacal emulsions, but the emulsions according to the invention and their usefulness are not limited. to any particular preparation process. In another process, silver halide seed grains with a high iodide content are initially present in the reactor; the concentration of silver iodide in the reactor is lowered preferably to less than 0.05 mole / liter and the grain size of silver iodide in the reactor is lowered

  
within 0.05 &#65533; m. Again, thin tabular grains of silver bromoiodide with an index of intermediate shape can be obtained by simply halting precipitation earlier.

  
Silver bromide emulsions with thin tabular grains of high shape index and intermediate can not contain iodide by the method described in detail modified to exclude iodide. Silver bromide emulsions with thin tabular grains of high shape index containing square and rectangular grains can be prepared in the following manner: cubic seeds are used having an edge length of less than 0.15 &#65533; m. While maintaining the pAg of the seed emulsion at a value between 5.0 and 8.0, the maturation of the emulsion is carried out practically in the absence of agents complexing silver ions other than the halides, to produce tabular grains having the desired average shape index.

   The examples also illustrate other methods of preparing silver bromide emulsions with tabular grains of high shape index without the addition of iodide.

  
The following procedure illustrates the preparation of thin tabular grain emulsions simply by

  
halting of precipitation when the desired shape index

  
has been obtained.

  
Tabular grains can be prepared containing at least 50 mol% of chloride having crystal faces

  
  <EMI ID = 22.1>

  
minus a peripheral edge parallel to a crystallographic vector <211> in the plane of one of the main faces. These tabular grain emulsions can be prepared by reacting an aqueous solution of silver salt and an aqueous solution of halides containing chloride in the presence of an amount capable of modifying the crystal morphology of an aminoazaindene and of a peptizer having a thioether bond.

  
One can also prepare tabular grain emulsions in which the silver halide grains contain chloride and bromide in at least annular regions and preferably throughout the grain. Tabular grain regions containing chloride and silver bromide are formed by maintaining a molar ratio of chloride ions

  
  <EMI ID = 23.1>

  
total ion of halide ions in the reactor from 0.10 to 0.90 during the introduction of silver salts, chloride, bromide and optionally iodide into the reactor. The molar ratio of silver chloride to silver bromide in tabular grains can vary from 1:99 to 2: 3. Modifiers may be present during precipitation of the tabular grains, either initially in the reactor, or added along with one or more of the salts, according to conventional methods. These modifiers may be compounds of copper, tallium, lead, bismuth, cadmium, zinc, medium chalcogen (i.e. sulfur, selenium and tellurium)

  
gold and noble metals of group VIII, according to the indications given to the patents of the United States of America

  
1 195 432, 1 951 933, 2 448 060, 2 628 167, 2 950 972,

  
3,488,709, 3,737,313, 3,772,031, 4,269,927 and in the journal Research Disclosure, volume 134, June 1975, publication
13452. The journal Research Disclosure and its predecessor Product Licensing Index are published by Industrial Opportunities Limited; Homewell, Havant; Hampshire, P09; 1EF; United Kingdom. Tabular grain emulsions can be sensitized by reduction inside the grains during precipitation, as described by Moisar et al; Journal of Photographie Science, Volume 25, 1977 pages 19 to
27.

  
The silver salts and the halides can be added to the reactor by means of surface or sub-surface supply tubes, by gravity feed or by means of devices which allow the rate of addition to be regulated. as well as the pH, pBr and / or pAg of the contents of the reactor, according to the indications given to the patents of the United States of America 3 821 002 and 3 031 304 and by Claes in the journal Photographische Korrespondenz, volume

  
  <EMI ID = 24.1>

  
rapid reagents in the reactor, specially constructed mixing devices such as those described in US Patents 2,996,287 can be used,

  
3 342 605, 3 415 650, 3 785 777, 4 147 551 and 4 171 224,

  
British Patent 2,022,431 A, German Patent Applications 2,555,364 and 2,556,885 and in Research Disclosure, volume 166, February 1978, publication 16662.

  
In the preparation of tabular grain emulsions, the concentration of peptizer can be between 0.2 and approximately 10% by mass relative to the total mass of the constituents of the emulsion in the reactor. It is preferred to maintain the peptizing concentration below about 6% of the total mass before and during the formation of silver bromoiodide. It is common to keep the peptizing concentration in the reactor below about 6% of the total mass, before and during the formation of the silver halide, and to adjust later to higher values, the vehicle concentration of the emulsion (the vehicle term including the binder and the peptizer), by additional vehicle additions, to obtain the optimal coating characteristics.

   The initially formed emulsion may contain from about 5 to 50 g of peptizer per mole of silver halide, preferably from about 10 to 30 g per mole of silver halide. An additional diluent can be added later to bring the concentration up to 1000 g per mole of silver halide. Advantageously, in the finished emulsion, there are approximately 50 g of vehicle per mole of silver halide. Once coated and dried in a photographic product, the vehicle forms approximately 30 to 70% by mass of the emulsion layer.

  
The preparation of the silver halide emulsions according to the invention may comprise a step of maturing the grains and the preferred way consists in carrying out the maturation of the grains in the reactor during at least the formation of the silver bromoiodide grains. Solvents of known silver halides are used to promote maturation, such as for example an excess of bromide ion in the reactor. It is clear that the bromide solution introduced into the reactor can itself promote maturation. It is also possible to use other maturing agents, which can be entirely incorporated into the dispersion medium in the reactor before the addition of silver salt and halide or which can be introduced into the reactor at the same time as a or more of the halides, the silver salt or the peptizer.

   According to another embodiment, the independent ripening agent can be introduced.

  
  <EMI ID = 25.1>

  
nice. Although ammonia is a known maturing agent, it does not constitute a preferred maturing means for the silver bromoiodide emulsions according to the invention, the relationship of speed / granularity of which is the highest.

  
Advantageous ripening agents are those which contain sulfur. Thiocyanates can be used in the form of alkali metal salts, usually sodium and potassium, and ammonium thiocyanates. Conventional amounts of thiocyanates can be used, but the preferred concentrations are generally between about 0.1 and 20 g of thiocyanate per mole of silver halide. The use of thiocyanate as a ripening agent is described in the patents of the United States of America

  
2 222 264, 2 448 534 and 3 320 069. We can also use,

  
conventionally, thioethers such as those described

  
U.S. Patents 3,271,157, 3,574,628 and 3,737,313.

  
The emulsions with thin tabular grains are preferably washed to remove the soluble salts, by known techniques such as decantation, filtration and / or by freezing and washing or by other methods as described in Research Disclosure, Vol. 176, December
1978; publication 17643, section II. Emulsions, with or without a sensitizer, can be dried and stored before use, as described in Research Disclosure, Vol. 101, September 1972, publication 10152. Washing is particularly advantageous in the present invention to complete the maturation of the tabular grains after the end of the precipitation in order to avoid increasing their thickness and reducing their shape index.

  
High form index tabular grain emulsions useful in the present invention can have extremely high average form indices. The average shape index of the tabular grains can be increased by increasing the average grain diameter. This may result in sharpness benefits, but the maximum average grain diameter is generally limited by the granularity requirements for a particular photographic application. You can also increase the average shape index of tabular grains by reducing the average grain thickness. When the silver titles are kept constant, the reduction in the thickness of the tabular grains generally improves the granularity as a direct function of the reduction in the shape index.

   Therefore, the maximum shape indices of the tabular grain emulsions useful in the present invention are a function of the maximum average grain diameters acceptable for the particular photographic application and the minimum thicknesses of achievable tabular grains. It has been observed that the maximum average shape indices vary according to the precipitation technique used and

  
the composition of the tabular halide grains. The highest average shape index observed, 500: 1, for tabular grains with a grain diameter useful photographically, was obtained by preparing grains of silver bromide by Ostwald maturation, and it is possible to obtain shape indices of 100: 1, 200: 1 or even higher by twin-jet precipitation methods. In general, the presence of iodide reduces the maximum average shape indices produced, but it is possible to prepare emulsions of silver bromoiodide with tabular grains of average form index of 50: 1, or 100: 1 or even 200: 1 or more. You can prepare tabular silver chloride grains with a shape index up to 50: 1 or even

  
100: 1, these grains possibly containing bromide and / or iodide.

  
In all cases, the average diameter of the tabular grains

  
  <EMI ID = 26.1>

  
The present invention also applies to photographic products intended to form positive or negative images. For example, photographic products can form surface images or internal images during exposure that produce negative images during development. Or photographic products can produce direct positive images after a single stage of development. When the tabular grains and other image forming silver halide grains present in the photographic product are intended to form direct positive images, they can be surface-veiled and are used in combination with a conventional organic electron acceptor .

   The organic electron acceptor can be used in combination with a spectral sensitizing dye or it can itself be a spectral sensitizing dye. If emulsions with internal sensitivity are used, the surface haze and organic electron acceptors can be used in combination, but neither the surface haze nor the organic electron acceptors are required to produce direct positive images. Direct positive images can be formed by the development of emulsions with internal sensitivity in the presence of nucleating agents which can be contained either in the developer or in the photographic product, as described in Research Disclosure Vol. 151, November 1976, publication 15162.

   Advantageous nucleating agents are those which are adsorbed directly on the surface of the silver halide grains. The tabular grain emulsions of high shape index forming internal latent images, containing nucleating agents, are useful in the implementation of the invention.

  
In addition to the specific characteristics described above, the photographic products according to the invention can also have recourse to conventional characteristics, such as those of the paragraphs cited below from Research Disclosure, publication 17643 cited above. The emulsions can be chemically sensitized, as described in paragraph III, and / or spectrally sensitized or desensitized, as described in paragraph IV of the reference. Photographic products may contain optical brightening agents, anti-veil, stabilizers, diffusing substances

  
or absorbent, coating aids, plasticizers, lubricants and matting agents, as described in Research Disclosure, publication 17643 cited above, paragraphs V, VI, VIII, XI, XII and XVI. Addition methods and coating and drying methods can be used, as described in paragraphs XIV and XV. We can use conventional photographic supports, like this

  
is described in paragraph XVII. Other conventional characteristics are known to the technician.

  
The invention is particularly applicable to products for radiography. The preferred radiographic products according to the invention are those produced by completely pre-tanning radiographic products containing at least one emulsion with thin tabular grains of high or intermediate shape index, for example radiographic products having two image-forming layers arranged on two opposite main faces of the support. The interposed support is capable of transmitting radiation to which at least one and usually the two image-forming layers are sensitive, that is to say that the support transmits practically all of the exposure radiation. The support is practically colorless and transparent, although it can be tinted.

   Each of the two image-forming layers contains at least one photosensitive emulsion with thin tabular grains of silver halide with an index of intermediate intermediate shape.

  
To obtain both the benefits of hiding power and the benefits of exposure through the support, the tabular grains of silver halide have spectral sensitizing dyes adsorbed on their surface. In particular, it is proposed to use spectral sensitizing dyes having maximum absorption in blue and in blue minus, that is to say in green and in red of the visible spectrum. In addition, for certain specific uses, spectral sensitizing dyes can be used which improve the spectral response in the region of the spectrum situated beyond the visible part. For example, spectral sensitizers absorbing in the infrared region can be used.

  
Thin grain silver halide emulsions can be spectrally sensitized with dyes belonging to various classes, in particular polymethine dyes such as cyanines, merocyanines, cyanines and complex merocyanines (tri-, tetra-, or polynuclear) , oxonols, hemioxonols, styrylic dyes, merostyrylics and streptocyanines.

  
Spectral sensitizing dyes of the cyanine type comprise two heterocyclic rings of basic character linked by a methine bond; these heterocyclic nuclei are derived for example from quinolinium, pyri-

  
  <EMI ID = 27.1>

  
lium, oxazolinium, thiazolium, thiazolinium, selenazolium, selenazolinium, imidazolium, imidazolinium, benzoxazolinium, benzothiazolium, benzoselenazolium, benzimidazolium, naphtothiazolium, naphtoselenazolium, dihydronaphtothiazolium salts.

  
The spectral sensitizing dyes of the merocyanine type comprise, linked by a methine bond, a nucleus of basic character of the type found in the formula of cyanines and an acid nucleus derived for example from barbituric acid, acid 2 -thiobarbiturate, rhodanine, hydantoline, 2-thiohydantolne, 4-thiohydantoine, 2-pyrazoline-5-one, 2-isoxazoline-5-one, indan-1,3-dione, cyclohexane -1,3-dione, 1,3-dioxane4,6-dione, pyrazolin-3,5-dione, pentane-2,4-dione, alkylsulfonyl acetonitrile, malonitrile, isoquinolin4-one and chroman-2,4-dione.

  
One or more spectral sensitizing dyes can be used. We know dyes with maxima

  
awareness for distributed wavelengths

  
across the entire visible spectrum and providing a wide variety of spectral sensitivity curves. The choice of the relative proportions of dyes depends on the region of the spectrum to which it is desired to sensitize the grains and on the shape of the spectral sensitivity curve which it is desired to obtain. Dyes with partially overlapping spectral sensitivity curves often provide, when used in combination, a curve such that the sensitivity at each wavelength in the overlap region is approximately the sum of the sensitivities of each dyes. Thus, it is possible to use dye combinations having different maxima, to obtain a spectral sensitivity curve having an intermediate maximum between the sensitization maxima of each of the dyes individually.

  
Certain combinations of spectral sensitizing dyes produce an oversensitizing effect, that is to say provide in a region of the spectrum, a spectral sensitization greater than that resulting from the use of one of the dyes alone at any concentration, or resulting from the addition of several dyes. Oversensitization can be achieved with selected combinations of spectral sensitizing dyes and other additives such as stabilizers, antifoggants, development accelerators or inhibitors, coating aids, optical brighteners and antistatics.

  
Mechanisms and compounds for explaining or achieving over-sensitization are described by Filman in "Review of the Mechanisms of Supersensitization", Photography Science and Engineering, Vol. 18, 1974, pp. 418-430.

  
Spectral sensitizing dyes can also exert other actions on emulsions. These dyes can also play the role of anti-veil, stabilizers, development accelerators or inhibitors, halogen acceptors or electron acceptors, as described in the United States patents. America 2,131,038 and

  
3,930,860.

  
According to an advantageous embodiment of the present invention, a spectral sensitizing dye which exhibits a color change as a function of the adsorption is adsorbed on the surface of the tabular grains of silver halides. For the implementation of the invention, any conventional spectral sensitizing dye known to exhibit a bathochromic or hypsochromic increase in the absorption of light as a function of the adsorption on the surface of the halide grains can be used. silver. Dyes that meet these criteria are well known in the art, as illustrated by TH James, in The Theory of the Photographic Process, 4th ed., MacMillan, 1977, Chapter 8 (particularly F. Induced Color Shifts in Cyanine and Merocyanine Dyes ) and Chapter 9
(particularly H.

   Relations Between Dye Structure and Surface Aggregation) and by F.M. Hamer, Cyanine Dyes and Related Compounds, John Wiley and Sons, 1964, Chapter XVII
(particularly F. Polymerization and Sensitization of the Second Type). Spectral sensitizing dyes of the merocyanine, hemicyanin, styrylic and oxonol type which produce H aggregates (hypsochromic modification) are known in the art; however J aggregates (bathochrome modification) are not common for dyes of these classes. The preferred spectral sensitizing dyes are cyanine type dyes which are in the form of H or J aggregates.

  
According to a particularly advantageous embodiment, the spectral sensitizing dyes are dyes of the carbocyanine type which are in the form of J aggregates. These dyes are characterized by at least two basic heterocyclic rings linked by a bond of three methine groups. The heterocyclic rings preferably comprise benzene rings condensed to promote the formation of J aggregates. Preferred heterocyclic rings to promote the formation of J aggregates are those of the quaternary salts of quinolinium, benzoxazolium, benzothiazolium, benzosélénazolium, benzimidazolium , naphtooxazolium, naphtothiazolium and naphthselenazolium.

  
Although the natural sensitivity to blue for bromide or bromoiodide of emulsion layers intended to record exposure to blue light is usually used in the technique, the use of spectral sensitizers allows significant advantages to be obtained. , even when their main absorption is in a region of the spectrum for which the emulsions have a natural sensitivity. For example, it is recognized that benefits can be obtained by using spectral blue dyes.

  
The spectral sensitizing dyes for blue of silver bromide and bromoiodide emulsions with tabular grains of high shape index can be chosen from the classes of dyes known to provide spectral sensitizers. Dyes of the polymethine class, such as cyanines, merocyanines, hemicyanins, hemioxonols and merostyrylic dyes are spectral sensitizers for advantageous blue. In general, the useful spectral sensitizers for blue can be chosen from the classes of dyes according to their absorption characteristics, i.e.

  
depending on the shade. There are, however, general structural correlations which may guide the choice of useful blue sensitizer dyes. Generally, the shorter the methine chain, the shorter the wavelength of the maximum sensitization. Germs also influence absorption. The addition of condensed cycles to seeds tends to favor longer absorption wavelengths. Substituents can also modify the absorption characteristics.

  
Among the spectral sensitizing dyes useful for sensitizing emulsions to silver halides, mention may be made of those described in Research Disclosure, Vol. 176, December 1978, publication 17643, section III.

  
Usual amounts can be used to spectrally sensitize emulsion layers containing non-tabular silver halide grains or thick tabular grains.

  
by emulsions with thin tabular grains, it is preferable to adsorb the spectral sensitizing dye on the surface of the grains in an optimal amount, that is to say in an amount sufficient to obtain at least 60% of the maximum photographic sensitivity which can be reached with the grains under the possible exposure conditions. The amount of dye used varies according to the nature of the dye or the combination of dyes chosen and according to the size and shape index of the grains.

   It is known in the photographic technique that optimal spectral sensitization is obtained with organic dyes when they form a monolayer on approximately 25% to 100% or more of the total surface of the silver halide grains, as is described, for example, by West et al. in "The Adsorption of Sensitizing Dyes in Photographie Emulsions", Journal of Phys. Chem. Flight. 56, p. 1065, 1952; Spence et al. In "Desensitization of Sensitizing Dyes", Journal of Physical and Colloid Chemistry, Vol. 56, n [deg.] 6, June 1948, pages 1090-
1103 and in U.S. Patent 3,979,213.

  
Optimal dye concentrations can be chosen according to the methods described by Mees, Theory of the Photographic Process, 1942, Macmillan, pages 1067-1069.

  
Spectral sensitization can be carried out at any stage of the preparation of the emulsion which has been recognized

  
useful for this purpose. Usually, spectral sensitization is carried out after the end of chemical sensitization. However, spectral sensitization can be carried out either at the same time as chemical sensitization, or entirely before chemical sensitization, and even it can be started before the precipitation of the silver halide grains has stopped, according to the indications given to U.S. Patents 3,628,960 and 4,225,666. According to U.S. Patent 4,225,666, it is possible to introduce the spectral sensitizer dye into the emulsion in stages so that part of this dye is present before the chemical sensitization while another part is introduced after the chemical sensitization.

   Contrary to the teaching of US Pat. No. 4,225,666, it is specifically provided that the spectral sensitizer can be added after 80% of the silver halide has been precipitated. Sensitization can be improved by adjusting the pAg, including through cyclic variations, during chemical sensitization and / or spectral sensitization. A more specific example of pAg adjustment is given in Research Disclosure, Vol. 181, May 1979, publication 18155.

  
According to an advantageous embodiment, the spectral sensitizers are incorporated into the emulsions before chemical sensitization. Similar results have also been obtained in some cases by introducing other adsorbable substances, such as curing modifiers into the emulsions before chemical sensitization.

  
Independently of the prior incorporation of absorbable substances, it is advantageous to use thiocyanates during chemical sensitization at concentrations of between approximately 2.10 3 and 2 mole percent relative to silver, as described in the United States patent. United States of America 2,642,361. Other curing agents may be used during chemical sensitization.

  
A third means which can be used alone or in combination with only one or two of the above means, consists in adjusting the concentration of silver and / or silver halides present immediately before or during the chemical sensitization. Soluble silver salts can be introduced, such as silver acetate, silver trifluoroacetate

  
and silver nitrate, as well as salts which can precipitate on the grain surfaces, such as silver thiocyanate, silver phosphate, silver carbonate, etc. Fine silver halide grains (i.e. bromide, iodide and / or silver chloride) capable of undergoing Ostwald ripening can be introduced onto the surface of the tabular grains . For example, a Lippmann emulsion can be introduced during chemical sensitization. The chemical sensitization of spectrally sensitized thin tabular grain emulsion can be carried out at one or more sites distinct from the tabular grains.

   It is believed that the preferential adsorption of spectral sensitizing dyes on the crystallographic surfaces forming the main faces of the tabular grains allows chemical sensitization to occur selectively on crystallographic surfaces different from the flat grains.

  
Advantageous chemical sensitizations to obtain the best speed / granularity relationships are awareness of gold and sulfur, awareness of gold

  
and selenium, and awareness of gold, sulfur and

  
selenium. Thus, according to an advantageous embodiment of the invention, the silver bromoiodide emulsions with thin tabular grains of high shape index contain a medium chalcogen such as sulfur and / or selenium, which cannot be detected, and gold that is detectable. Emulsions also usually contain detectable amounts of thiocyanate, although the concentration of thiocyanate in the final emulsion can be greatly reduced by known washing techniques.

   In the various embodiments indicated above, the surface of the tabular grains of silver bromide or silver bromoiodide can comprise another silver salt such as a silver thiocyanate, or another silver halide. different money
(e.g. silver chloride or silver bromide), although this other silver salt may be present below detectable amounts.

  
Although it is not necessary to obtain

  
all their advantages, the emulsions defined according to the invention are advantageously chemically sensitized

  
and spectrally optimally, in accordance with current manufacturing practices. This means that their speed represents at least 60% of the maximum of the logarithm of the speed that can be expected from grains

  
in the spectral region of awareness and under planned conditions of use and treatment. The logarithm of speed is defined as being equal to
100 (1-log E), where E is measured in lux-seconds at a density of 0.1 above the veil. Once we have characterized the silver halide grains of an emulsion,

  
it is possible to estimate, from other analyzes

  
of the product and of the evaluation of its performance, if an emulsion layer of this product has been sensitized chemically and spectrally in an optimal manner, by comparison with other comparable commercial products.

  
In addition to the characteristics specifically described above, the radiographic products according to the invention may have other usual characteristics of the products for radiography. Examples of such characteristics are given in Research Disclosure, Vol. 184, August 1979, publication 18431. For example, emulsions may contain stabilizers, anti-tarpaulins and agents inhibiting the action of mechanical stresses, as described in paragraph II, A to K. The radiographic product may contain agents and / or antistatic layers, as described in paragraph III.

  
X-ray products may contain overlays, as described in paragraph IV. The advantages of exposure through the support can be further improved by using conventional means to control exposure through the support, as described in publication 18431, paragraph V.

  
Advantageous radiographic products are those in which at least one emulsion layer with thin tabular grains is incorporated in at least one of the two image-forming layers arranged on the opposite main faces of a support capable of transmitting practically all of the radiation forming the image. The products for medical radiography are usually tinted blue and the dyes are generally added to obtain this tint directly with the polyester melt before extrusion,

  
and so they have to be heat stable. Advantageous dyes are those of the anthraquinone class.

  
The spectral sensitizing dyes are preferably chosen so that they exhibit a modification of the absorption maximum in their adsorbed state, usually in the H or J band, at a region of the spectrum corresponding to the electromagnetic radiation to which the product is intended.

  
be exposed photographically. The electromagnetic radiation producing the photographic exposure is usually emitted by the luminescent products of intensifying screens. A separate intensifying screen exposes each of the two image-forming layers arranged on the opposite sides of the support. The intensifying screens can emit light in the ultraviolet, blue, green or red regions of the spectrum, depending on the specific luminescent products chosen. According to an advantageous embodiment of the invention, the spectral sensitizing dye is a carbocyanine having an absorption band J, when it is adsorbed

  
on tabular grains, in a region of the spectrum corresponding to the maximum emission of the intensifying screen, usually the green region of the spectrum.

  
The reinforcing screens can themselves

  
part of x-ray products, but these are usually separate items that are used again

  
to display successive radiographic products. Reinforcing screens are well known in the radiographic technique. Classic reinforcing screens and their elements are described in Research Disclosure, publication
18431 supra, paragraph IX, and U.S. Patent 3,737,313.

  
To obtain a visible silver image, the photographic products or, in preferred applications, the radiographic products, are treated in an aqueous solution of a basic developer or, when the developer is incorporated into the photographic product, in a solution 'a basic activator. We prefer the development that promotes the greatest possible covering power. As James notes in The Theory of the Photography Process, supra, pages 404, 405, 489 and 490, as well as Farnell and Solman, also supra, the higher levels of hiding power are the result of more of silver developed in the form of filaments. Direct or chemical development produces a relatively higher covering power than physical development and is therefore the one that is preferred.

   When using silver halide grains which predominantly form surface latent images, it is preferred to use developers which contain small amounts of silver halide solvents, i.e. say superficial developers. It is known that the covering power is increased by a short-term development, that is to say at a relatively high speed When developing the exposed photographic products of the present invention in less than

  
1 min and, preferably, in less than 30 s to produce a visible image, they have an improved covering power; however, the covering power is significantly reduced and

  
has little to do with the grain shape index when developing in 8 min. For rapid development, it is preferred to use relatively vigorous developers. Advantageous developers are the hydroquinones used alone or, preferably, with

  
secondary developers of the pyrazolidone class, in particular 3-pyrazolidones, and aminophenols such as p-methylaminophenol sulfate.

  
Treatment methods of the type described in Research Disclosure, cited publication 17643, paragraph XIX can be used. Particularly preferred is the treatment of radiographic products with film advance by driving rollers. The photographic products according to the invention are pre-tanned, but they can be used with conventional developers containing pre-tanning agents without any loss of hiding power. Since the products are normally pre-tanned, it is of course preferable to completely eliminate the tanning agents from the treatment solutions. After development, the photographic products can be fixed to remove the residual silver by any suitable conventional method. Examples

  
The following examples illustrate the invention. In

  
each example, the contents of the reactor are vigorously stirred throughout the introduction of the silver and the halide; the term "%" means% by mass, unless otherwise indicated, and the term "M" denotes the molar concentration, unless otherwise indicated. All solutions, unless otherwise indicated, are aqueous solutions.

  
Examples 1 to 15

  
In order to compare the covering power as a function of the shape index of the tabular grains, three tabular silver bromide emulsions according to the present invention and a tabular silver bromoiodide emulsion according to the United States patent are prepared. America 4 150 994, having

  
a lower form index. Emulsions and index

  
medium-sized tabular grains are shown in

  
Table Here-after.

  

  <EMI ID = 28.1>


  

  <EMI ID = 29.1>
 

  
The emulsions (examples A, B and C) are emulsions

  
with tabular grains of high shape index within the limits of definition of the present patent application. Although

  
  <EMI ID = 30.1>

  
in the calculation of the average grain diameter and the percentage of surface projected in the emulsions of these examples and other examples, except when their exclusion is indicated, the grains of small diameter are insufficient in number to significantly modify the values indicated. To obtain a representative average shape index for the grains of the control emulsion, the average diameter of the grains is compared to their average thickness. Although it was not measured, the projected area that could be attributed to the few tabular grains satisfying the criteria for thick-

  
  <EMI ID = 31.1>

  
in the control emulsion, is estimated by visual examination as possibly representing very little of the total projected surface of the entire population of grains of the control emulsion.

  
Each emulsion is chemically sensitized to gold and to sulfur and each emulsion is sensitized to green with, per mole of silver, 600 mg of sodium salt of anhydro-5,5'-dichloro-9-ethyl-3, 3 '- (3-sulfopropyl) -oxacarbocyanine and 400 mg of potassium iodide.

  
The emulsions are then divided into separate samples for tanning. Three samples of each emulsion receive respectively 0.5%, 1.5% and 4.5% by mass, relative to the mass of gelatin, of the tanning agent bis-
(vinylsulfonylmethyl) ether (BVSME). Three samples of each emulsion receive, respectively, 0.24%; 0.75% and 2.5% by mass, based on the mass of gelatin, of mucochloric acid (MA) as a tanning agent. Immediately after the

  
  <EMI ID = 32.1>

  
identically on separate and identical media

  
of transparent film of polyethylene terephthalate. The emulsion samples are applied at a rate, per square meter, of 2.15 g of silver and 2.87 g of gelatin. An overlay is applied to each sample at the rate of 0.88 g of gelatin per square meter.

  
The percentage of swelling of the untreated coated samples is measured, 7 days after coating, including

  
  <EMI ID = 33.1>

  
measures the initial thickness of the emulsion layer, then

  
each sample is immersed in distilled water at 21 [deg.] C for 3 min. The change in thickness of the emulsion layer is then measured.

  
Only part of each sample is needed to make the swelling measurements described above. A remaining part of each sample is exposed to obtain maximum density and treated in a conventional machine for processing radiographic products, sold under the trade name Kodak RP X-Omat M6A-N Processing Machine. Development time is 21 s

  
  <EMI ID = 34.1>

  
the intended developer which contains glutaraldehyde as pretanning agent, an analogous developer of the type described in Example 1 of the patent of the United States of America is used

  
3,545,971, but without the addition of glutaraldehyde, and the developer does not contain any tanning agent. The hiding power is determined for each sample by dividing the maximum density by the silver developed, expressed in grams per square decimeter.

  
By drawing the curve giving the covering power as a function of the percentage of swelling, using three samples tanned to different degrees by the same tanning agent, the covering power of each emulsion is determined with each tanning agent at 199% swelling. ,
(unless otherwise indicated). The results are shown below in Table II.

  

  <EMI ID = 35.1>


  

  <EMI ID = 36.1>
 

  
Table II shows that, at the same degree of tanning, the photographic products according to the invention prepared with tabular grain emulsions of high shape index have a higher covering power and that the increase in covering power is due with higher shape indices of tabular silver bromide emulsions.

  
The results of Table III are similar to those appearing in Table II, but with the difference that the hiding power is measured at 99% swelling (unless otherwise indicated).

  

  <EMI ID = 37.1>


  

  <EMI ID = 38.1>
 

  
As mucochloric acid is a weaker tanning agent, the concentrations used are insufficient to reduce the percentage of swelling below
100% and, therefore, the hiding power could not be included at this degree of swelling. There is reason to believe that swelling could have been reduced below 100% with mucochloric acid, if higher concentrations had been used.

  
Appendix

  
The following preparation details are not part of the invention.

  
A. Emulsion A (example)

  
A 17.5 liters of an aqueous solution of bone gelatin
(1.5% gelatin) containing 0.14 M potassium bromide

  
  <EMI ID = 39.1>

  
jet in 8 min (1.05% of the total amount of silver nitrate used is consumed), an aqueous solution of 1.15 M of potassium bromide (solution B-1) and an aqueous solution of 1.00 M of nitrate silver (solution C-1). After 8 min, the addition of solutions B-1 and C-1 is stopped.

  
Then added to the reactor by double jet at

  
55 [deg.] C and at a pBr of 0.85, aqueous solutions 2.29 M of potassium bromide (solution B-2) and 2.0 M of silver nitrate (solution C-2), using accelerated rates
(multiplied by 4.2 between the start and the end of the addition)

  
until the solution C-2 is used up (approximately 20 min); 14.1% of the total amount of silver nitrate used is consumed. The addition of solution B-2 is stopped.

  
A 2.0 M aqueous solution is added to the reactor

  
silver nitrate (solution C-3) for approximately
12.3 min until a pBr of 2.39 to 55 [deg.] C is obtained; 10.4% of the total amount of silver nitrate used is consumed. The emulsion is maintained at a pBr of 2.39 to 55 [deg.] C

  
stirring for 15 min.

  
Solution C-3 and a 2.0 M aqueous solution of potassium bromide (solution B-3) are then added to the reactor by double jet at a constant flow rate in approximately 88 minutes (74.5% of the amount consumed total silver nitrate used), while maintaining the pBr at 2.39 to 55 [deg.] C. The addition of solutions B-3 and C-3 is stopped. A total of 41.1 moles of silver nitrate is used to prepare this emulsion.

  
  <EMI ID = 40.1>

  
coagulation, as described in U.S. Patent 2,614,929.

  
B. Emulsion B (example)

  
To an aqueous solution of gelatin of bone (1.5% of gelatin) containing 0.14 M of potassium bromide (solution A) at a pBr of 0.85 and at 55 [deg.] C, added with stirring , by double jet at constant flow rate in 8 min (3.22% of the total amount of silver nitrate used is consumed), an aqueous solution of 1.15 M of potassium bromide (solution B-1) and 1.0 M silver nitrate (solution C-1). After 8 min, the addition of solutions B-1 and C-1 is stopped.

  
Aqueous solutions containing 3.95 M of potassium bromide (solution B-2) and 2.0 M of silver nitrate (solution C-2) are then added, at pBr 0.85 and at 55 [deg.] C, by double jet and with accelerated flow rates (the flow rates being multiplied by 4.2 between the start and the end of the addition) until solution C-2 is used up, in approximately 20 minutes;

  
28.2% of the total amount of silver nitrate used is consumed. The addition of solution B-2 is stopped.

  
A 2.0 M solution of silver nitrate is added.
(solution C-3) at constant flow for approximately

  
  <EMI ID = 41.1>

  
thus consumes 4.18% of the total amount of silver nitrate used. The emulsion is maintained at 55 [deg.] C for 15 min while stirring.

  
Solution C-3 and a 2.0 M aqueous solution of potassium bromide (solution B-3) are then added.

  
  <EMI ID = 42.1>

  
(the flow rates being multiplied by 1.4 between the start and the end of the addition) for 31.1 min; 64.4% of the total amount of silver nitrate used is thus consumed. The addition of solutions B-3 and C-3 is stopped. 2.95 moles of silver nitrate were used to prepare the emulsion.

  
Finally, the emulsions are cooled to 35 [deg.] C and washed by coagulation as described in Example 1.

  
C. Emulsion C (example)

  
To an aqueous solution of gelatin of bone (1.5% of gelatin) containing 0.14 M of potassium bromide (solution A) at a pBr of 0.85 and at 55 [deg.] C, the following is added. agitating, by double jet and at constant flow rate in 8 min (4.76% of the total amount of silver nitrate used is consumed), a 1.15 M aqueous solution of potassium bromide (solution B-1) and a 1.0 M aqueous solution of silver nitrate
(solution C-1). After 8 min, the addition of solutions B-1 and C-1 is stopped.

  
Then add 2.29 M potassium bromide aqueous solution (solution B-2) and 2.0 M nitrate

  
  <EMI ID = 43.1>

  
double jet and with accelerated flow rates (multiplied by 4.2 between the start and the end of the addition), until solution C-2 is used up in about 20 minutes; we consume like this
59.5% of the total amount of silver nitrate used. The addition of solution B-2 is stopped. In the procedure described above, solutions B-1 and B-2 of halides are added at three points on the surface of solution A.

  
A 2.0 M aqueous silver nitrate solution is added to the reactor over approximately 10 minutes at constant flow.
(solution C-3) until a pBr of 2.85 is obtained.
55 [deg.] C; 35.7% of the total amount of silver nitrate used is thus consumed. To prepare this emulsion, a total of 23.5 moles of silver are used.

  
  <EMI ID = 44.1>

  
washing by coagulation as described in Example 1.

  
D. Control emulsion

  
This emulsion is prepared according to the technique described in US Patent 4,184,877.

  
To a 5% solution of gelatin in 17.5 1 of water

  
at 65 [deg.] C, 4.7 M ammonium iodide and 4.7 M silver nitrate solutions are added with stirring; the addition is carried out in 3 min by double jet and with constant flow rates

  
  <EMI ID = 45.1>

  
about 22% of the silver nitrate used to prepare the seeds for sowing. The flow rate of the two solutions is then adjusted so as to consume, in approximately 15 minutes, 78% of the total amount of silver nitrate used for the preparation of the seed germs. The addition of the ammonium iodide solution is then stopped while continuing the addition of the silver nitrate solution until a pI of 5.0 is obtained. A total of about 56 moles of silver are used for the preparation of seed seeds. The emulsion is cooled to 30 [deg.] C. We use it as a source of

  
  <EMI ID = 46.1>

  
following the precipitation described below.

  
  <EMI ID = 47.1>

  
5% tine containing 4.1 moles of the silver iodide emulsion prepared previously. By double jet and maintaining equal and constant flow rates, a 4.7 M solution of ammonium bromide and a 4.7 M solution of silver nitrate are added in 7 minutes, maintaining the pBr at 4.7 cc. who consumes
40,?% 'Of the total amount of silver used for germination. The addition of the ammonium bromide solution alone is continued until a pBr of approximately 0.9 is obtained and the jet of ammonium bromide is then stopped. 2.7 liters of an 11.7 M ammonium hydroxide solution are then added and the mixture is left to stand for 10 minutes.

   The pH is adjusted to 5.0 with sulfuric acid and the double-jet addition of the ammonium bromide and silver nitrate solutions is resumed for 14 min, keeping the pBr at about 0.9 and with a flow consuming 56.8% of the total amount of silver nitrate used. The pBr is then adjusted to 3.3 and the emulsion is cooled to 30 [deg.] C. A total of about 87 moles of silver nitrate are used. The emulsion is washed by coagulation, as described in Example 1.

  
E. Optimal chemical sensitization of the Emulsions A, B and C prepared as described above is carried out, with

  
  <EMI ID = 48.1>

  
mol of silver of sodium thiocyanate and 10 mg / mol of silver of sodium thiosulfate, at 70 [deg.] C. The awareness described in the patent of the United States of America 4 184 877 is carried out,

  
  <EMI ID = 49.1>

  
and 4.2 mg / mol of silver of sodium thiocyanate, at 70 [deg.] C.

CLAIMS

  
1. Photographic product formed of a support on which

  
are arranged one or more layers of a hydrophilic colloid capable of being tanned comprising at least one emulsion layer containing grains of photosensitive silver halides, characterized in that at least
50% of the total projected surface of the silver halide grains in at least the said emulsion layer is provided by thin tabular grains having a

  
  <EMI ID = 50.1>

  
hydrophilic being pre-tanned to a degree sufficient to reduce the swelling of the layers to less than 200%, the percentage of swelling being determined (a) by baking the photographic product at 38 [deg.] C for 3 days at 50% relative humidity , (b) by measuring the thickness of the layer, (c) by immersing the photographic product in

  
  <EMI ID = 51.1>

  
undermining the percentage change in thickness of

  
the layer compared to the thickness of the layer measured in step (b).


    

Claims (1)

2. Photographic product according to claim 1, characterized in that the silver halide grains <EMI ID = 52.1> 3. Photographic product according to claim 1, characterized in that the silver halide grains  <EMI ID = 53.1> 4. Photographic product conforming to any of the Claims 1 to 3, characterized in that the silver halide is a silver bromide or a silver bromoiodide. 5. Photographic product conforming to any of the Claims 1 to 4, characterized in that the layers of hydrophilic colloid contain gelatin or a gelatin derivative capable of being tanned. 6. Photographic product conforming to any of the Claims 1 to 5, characterized in that the layers of hydrophilic colloid are pre-tanned with a tanning agent of the aldehyde type: 7. Photographic product conforming to any of the Claims 1 to 5, characterized in that the layers of hydrophilic colloid are pre-tanned with a tanning agent of the active olefin type containing at least two active olefin binding sites. 8. Photographic product conforming to any of the Claims 1 to 5, characterized in that the layers of hydrophilic colloid are pretanned with an agent of the aldehyde acid type substituted by a halogen. 9. Photographic product conforming to any of the Claims 1 to 5, characterized in that the layers of hydrophilic colloid are pre-tanned to a degree sufficient to reduce the swelling of the layers to less than 100%. 10. Photographic product conforming to any of the Claims 1 to 9, characterized in that the tabular silver halide grains represent at least 70% of the total projected area of the silver halide grains. 11. Photographic product conforming to any of the Claims 1 to 10, in the form of an X-ray product comprising a support transmitting practically all of the light and, arranged on both sides of the support, one or more layers of hydrophilic colloid comprising one or more layers containing bromide grains photosensitive silver containing up to 6 mole% iodide and having a  <EMI ID = 54.1> in that at least 50% of the total projected area of the silver bromide grains in at least one emulsion layer is provided by thin tabular silver bromide grains containing up to 6 mole% of iodide, having a thickness of less than 0.2 µm and an average shape index of at least 5: 1, this shape index being defined as the ratio of the diameter of the grain to the thickness of it ci, the diameter of a grain being defined as the diameter of a circle having an area equal to the projected area of this grain. 12. Photographic product according to claim 11, characterized in that the silver halide grains  <EMI ID = 55.1> 13. Photographic product according to claim 12, characterized in that the silver halide grains  <EMI ID = 56.1> 14. Photographic product conforming to any of the Claims 11 to 13, characterized in that the layers of hydrophilic colloid are pretanned with at least one tanning agent of the aldehyde type or a tanning agent having at least two vinylsulfonyl groups or vinylsulfonyl precursors. 15. Photographic product conforming to any of the Claims 11 to 13, characterized in that the layers of hydrophilic colloid are pre-tanned with mucochloric acid. 16. Use in radiography of a photographic product according to any one of claims 1 to 15.