BE894966A - RADIOGRAPHIC PRODUCT - Google Patents

Description

       

  X-ray product.

  
The present invention relates to an X-ray product comprising a first and a second layer of silver halide emulsion constituted by a dispersion medium and photosensitive silver halide grains, and a support interposed between these layers of silver halide emulsion and capable of transmitting the radiation to which the second layer of silver halide emulsion is sensitive.

  
In photography using silver halides, one or more layers of silver halide emulsion are usually applied on one side of a support. It is not the same in medical radiography. To minimize the dose of X-rays received by a patient, silver halide emulsion layers are usually applied to both sides of the support. Since silver halide emulsions absorb X-rays relatively little, the X-ray product is placed between intensifying screens which absorb X-rays

  
and emit light. The exposure through the support, which causes a reduction in image sharpness, takes place when the light emitted by a reinforcing screen passes through the adjacent emulsion layer and the support to photographically expose the emulsion layer located on the opposite side of the support. Loss of image clarity results

  
of the light scattering when crossing the support. When the level of radiographic exposure can be increased without damage to the subject, for example during the non-destructive control of materials, this disadvantage can be avoided by using only one side of the support for the application. layers.

  
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 ripening effects and, in the presence of strong ripening agents such as ammonia, the grains can even be spherical or have the form of thick, almost spherical tablets, as described for example in United States patent 3,894,871 and by Zelikman and Levi in "Making and Coating Photographic Emulsions", Focal Press, 1964, pp. 221-223.

   We have frequently observed in variable proportions rod-shaped or tabular-shaped grains associated with grains of other shape, especially when the pAg (i.e. the negative logarithm of the silver ion concentration) of the emulsions were modified during precipitation as is the case for example in single jet precipitation processes.

  
The tabular silver bromide grains have been the subject of numerous studies, but the grains thus studied were often large grains without photographic utility. What is understood in the present description to be a tabular grain is a grain delimited by two practically parallel crystalline faces which each have a surface which is considerably larger than any other face of the crystal constituting the grain. The shape index, i.e. the ratio of the diameter to the thickness of a tabular grain, is therefore clearly greater than 1: 1. Tabular-grain silver bromide emulsions of high shape index have been described by de Cugnac and Chateau in "Evolution of the Morphology of Silver Bromide Crystals During Physical Ripening", Science and Photographic Industries vol. 33, N [deg.] 2 (1962), pp. <1> 21-125.

  
From 1937 until around the 1950s, the Eastman Kodak Company sold a film for radiography called "Duplitized" and whose reference was "No screen X-ray Code 5133". This product included, on each side of a film support, a sulfur-sensitized silver bromide emulsion. Since the emulsions were intended for direct exposure to X-rays, they were not sensitive to

  
were

  
  <EMI ID = 1.1>

  
average shape index of 5 to 7: 1 and these tabular grains represented more than 50% of the projected surface, while the non-tabular grains represented more than 25% of the projected surface. By reproducing these emulsions several times, it is found that the emulsion having the highest shape index has an average tabular grain diameter of

  
  <EMI ID = 2.1>

  
and an average form index of 7: 1. Other reproductions of these emulsions have produced thicker, smaller diameter grains which have a lower average shape index.

  
However, none of the tabular grain emulsions of silver bromoiodide described in the prior art actually have a high average shape index. The question of tabular grains of silver bromoiodide is discussed by Duffin, Photographie Emulsion Chemistry, Focal Press, 1966, pp. 66-72 and by Trivelli and Smith, in "The Effect of Silver Iodide Upon The Structure of Bromo-Iodide Precipitation Series", in The Photography Journal, Vol. LXXX, July 1940, pp. 285-288. According to Trivelli and Smith,

  
there is a marked decrease in grain size

  
and the shape index as iodide is introduced. Gutoff, in "Nucleation and Growth Rates During the Precipitation of Silver Halide Photographie Emulsions",
(Photographic Sciences and Engineering Vol. 14, N [deg.] 4, July-August 1970, pp. 248-257, describes the preparation of bromide and silver bromoiodide emulsions by a single jet process using '' a continuous precipitation device.

  
Processes for preparing emulsions consisting mainly of silver halides in the form of tabular grains have recently been described in publications. U.S. Patent 4,063,951 describes the formation of tabular silver halide crystals

  
  <EMI ID = 3.1>

  
shape (calculated with respect to the edge length) is between 1.5 and 7: 1. The tabular grains have a square and rectangular shape characteristic of the faces

  
  <EMI ID = 4.1>

  
4,067,739 describes the preparation of silver halide emulsions consisting mainly of twin crystals

  
octahedral type; these crystals are formed by first preparing crystalline seed seeds which are then grown by Ostwald maturation in the presence of a silver halide solvent and the growth of the grains is completed without renucleation or maturation of Ostwald by controlling the pBr (negative logarithm of the bromide ion concentration). U.S. Patents 4,150,994, 4,184,877 and 4,184,878; as well as the English patent 1 570 581 and the German patent application publications 2 905 655 and 2 921 077 relate to the formation of twinned silver halide grains of octahedral tabular form from crystalline seed seeds the content of which iodide is at least 90% by mole.

   Unless otherwise indicated, all of these references to the percentages of halides are calculated relative to the silver contained in the emulsion, the grain or the region of the grain concerned. According to the patent of the United States of America

  
4,063,951 the upper limit of the shape index of these tabular grains is 7: 1; but, according to the very low shape indices obtained in the examples (2: 1), this upper limit of 7: 1 appears to be overestimated. It is clear, as

  
As shown in the reproduction of the examples and the examination of the published photomicrographs, the grain shape indices obtained according to the above references are also less than 7: 1. Japanese patent application 142,329, published on November 6, 1980 apparently contains the same elements

  
as those of U.S. Patent 4,150,994, although not limited to the use of seed crystals of silver iodide.

  
The subject of the present invention is a radiographic product comprising

  
first and second silver halide emulsion layers consisting of a photosensitive silver halide dispersion medium and grains, and a support interposed between these emulsion layers

  
to silver halides and capable of transmitting the radiation to which the said second emulsion layer is sensitive.

  
This radiographic product is characterized in that

  
at least said first layer of silver halide emulsion contains tabular silver halide grains

  
  <EMI ID = 5.1>

  
shape index being defined as the ratio of diameter

  
of the grain to the thickness thereof, these grains representing at least 50% of the total projected surface of the said silver halide grains present in the said silver halide emulsion, the diameter of a grain being defined as the diameter of a circle having an area equal to the projected area of the grain as it appears on a photomicrograph or on an electron micrograph of an emulsion sample; in addition, a spectral sensitizing dye is adsorbed on the surface of the tabular grains of silver halides.

  
The radiographic products according to the invention have a determined title in silver (mass of silver per unit area) and, with comparable speed, they allow

  
reduce the effects of exposure through the medium;

  
  <EMI ID = 6.1>

  
identifies the relationship between speed and granularity and sharpness, regardless of exposure through the medium. These improvements are not dependent on the silver 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 best speed-granularity relationships obtained so far with emulsions

  
silver bromoiodide in general. The sensitivity in blue of silver bromide and bromoiodide emulsions is considerably increased compared to their natural sensitivity by using spectral blue sensitizers.

  
Figure 1 is an elevational view of a test device. Figure 2 is a density layer as a function of the logarithm of the exposure.

  
The present invention applies generally

  
any radiographic product comprising separate image-forming elements, at least one of which consists of a silver halide emulsion, the elements being separated by a support which is capable of transmitting to one of the image-forming elements the radiation passing through the other element's silver halide emulsion. According to a preferred structure, the radiographic product comprises image-forming elements applied to each of the two opposite main faces of a light-transmitting support, such as a film support. Other arrangements are possible.

   Instead of applying the image-forming elements to the opposite faces of the same support, they can be applied to separate supports, the structures obtained being superimposed so that one support or the two supports separate the forming elements from one another. 'picture.

  
The imaging elements may take the form of any layer or combination of conventional radiographic image-forming type layers, provided that at least one layer consists of tabular grains of silver halide with shape index high, as more particularly described below. In an advantageous embodiment, the two image forming elements comprise a layer or layers of silver halide emulsion. While it is possible to envisage that the image-forming elements may each use different layers of silver halide photosensitive emulsion, the two image-forming elements preferably consist of halide emulsions silver grain

  
  <EMI ID = 7.1>

  
use two identical imaging elements separated by a support. The emulsions other than the prescribed emulsion with tabular grains of high shape index can be all the emulsions of conventional type such as those described in the journal Research Disclosure, Vol. 176, December 1978, publication 17643, paragraph I, Emulsion preparation and types. Research Disclosure and its predecessor Product Licensing Index are publications of International Opportunities Ltd; Homewell, Havant; Hampshire, P09 1EF, United Kingdom.

  
at. Shape index tabular grain emulsions

  
high and their preparation

  
Silver halide emulsions with tabular grains of high shape index consist of a dispersion medium and spectrally sensitized tabular grains of silver halides. In the present description, the expression "high shape index" applied to silver halide emulsions has the following meaning: it applies to grains of silver halides

  
  <EMI ID = 8.1>

  
these grains have an average shape index greater than 8: 1 and they represent at least 50% of the total projected surface of the silver halide grains.

  
The silver tabular grain halide emulsions of high shape index which are preferably used in the radiographic products of the present invention are those in which the silver halide grains

  
  <EMI ID = 9.1>

  
male less than 0.2 m) and a diameter of at least 0.6 m m have an average shape index of at least 12: 1 and, optimally, at least 20: 1. 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 main crystal faces of the tabular grains of silver halide are parallel.

  
The characteristics described above of the silver halide emulsions used in the radiographic products according to the invention can be easily demonstrated by methods well known in the art.

  
From the shadows cast on an electron micrograph of an emulsion, it is possible to determine the thickness and diameter of each grain and to identify those of the tabular grains whose thickness is less than 0.5 &#65533; m

  
  <EMI ID = 10.1>

  
the shape index of each of these tabular grains and we can then average the shape indices of all the tabular grains in the sample that satisfy these

  
  <EMI ID = 11.1>

  
constitutes the average form index. According to this definition, the average shape index is the average of the shape indices of each tabular grain. In practice, it is generally simpler to obtain an average thickness and an average diameter of tabular grains having a thickness of less than

  
  <EMI ID = 12.1>

  
which is the ratio of these two means. Whatever the evaluation method chosen, and taking into account the tolerances of the particle size measurements, the values obtained for the average shape index do not differ significantly.

  
One can add up the projected surfaces of the tabular grains of silver halide which satisfy the conditions of thickness and diameter, then separately one can add up the surfaces of the other grains of silver halide from photomicrography; from these two sums, one can obtain the percentage of the total projected surface occupied by the tabular grains of silver halides whose thickness and diameter satisfy the conditions expressed above.

  
For the above assessments, a reference tabular grain was chosen; this grain is less than

  
  <EMI ID = 13.1>

  
thin tabular grains of thicker tabular grains with inferior photographic characteristics

  
  <EMI ID = 14.1>

  
reference given that, for smaller diameters, it is not always possible to distinguish the grains which are tabular from those which are not. The term "projected area" 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,

  
  <EMI ID = 15.1>

  
p. 15.

  
The tabular grains can be of any composition of silver halide crystals the use of which in photography is known. In a preferred form offering a wide range of observed advantages, the present invention uses silver bromoiodide emulsions with high form index.

  
The silver halide emulsions consisting of tabular grains with a high shape index can be prepared by the precipitation process below. 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 <EMI ID = 16.1>

  
of money at the end of the grain precipitation. It is known from French patent 2,471,620 (Belgian patent n [deg.] 886,645) that it is possible to remove the dispersing medium from 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 a peptizer is used, it preferably represents at least 10% (more particularly at least 20%) 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.

  
Before the precipitation, the reactor contains a small part, usually less than 10% by mass, of the 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 the precipitation of 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 silver salt and bromides promotes the formation of thick grains. and not tabular.

  
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) so that there is precipitation of separate silver iodide grains. Before the introduction of silver salt, it is better to maintain

  
the iodide concentration in the reactor 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 proposed to initially maintain the pBr of the dispersing medium in the reactor at a value of 1.6 or less, preferably at a value less than 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 proposed 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, we define

  
pBr as 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, and simultaneously the bromide and iodide salts are introduced into the reactor. 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 ammonium halides,

  
alkali metal (sodium or potassium, for example) or alkaline earth metal (eg 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. This is why, during the growth phase, it is 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 -

  
  <EMI ID = 17.1>

  
It is preferable to keep the pBr inside the reactor, throughout the introduction of the silver and 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 results in an increase in the thickness of the grains, which can however be tolerated when the average shape indices obtained are still greater than 8: 1.

  
Instead of introducing the silver salt, bromide and iodide in the form of aqueous solutions, it is possible to consider adding the salts, at the beginning of the precipitation or during growth, in the form of fine grains. silver halides suspended in a dispersing medium. The grain size 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 the conditions in the reactor such as the temperature and the presence of solubilizing and maturing agents. Grains of silver bromide, silver iodide and / or bromoiodide can be introduced. 'silver. 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 diameter

  
  <EMI ID = 18.1>

  
Taking into account the conditions relating to pBr indicated above, the addition of the silver salt, of the bromide and of the iodide can be made at any speed and at all appropriate conventional concentrations. 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, from German patent application 2,107,118, from European patent application 80102242, and according to an article by Wey "Growth Mechanism of AgBr Crystals in Çelatin

  
  <EMI ID = 19.1>

  
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 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, grain or region of the grain in question; for example, a grain consisting of silver bromoiodide containing 40 mol% of iodide also contains 60 mol% of bromide. According to 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, iodide concentrations can reach up to about 40 mol% in tabular silver bromoiodide grains. In practice, precipitation temperatures can vary until they are close to room temperature,

  
  <EMI ID = 20.1>

  
graphs, it is preferred to limit the maximum iodide concentrations to approximately 20% by moles, the optimal 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 to the central regions of the tabular grains. The iodide concentrations in the central regions of the grains can vary from 0% to 5% by moles, with at least 1% by moles of iodide in the lateral annular regions, 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%. Alternatively, the addition of iodide and bromide to the developer is stopped before stopping the addition of the silver salt, so

  
that the excess bromide ion in solution reacts with the

  
silver salt. This results in the formation of an outer shell on the tabular grains of silver bromoiodide. It therefore appears that the tabular silver bromoiodide grains used in the present invention can have practically uniform or gradual iodide concentrations and that it is possible to control the variation of the concentration at will in order to obtain stronger iodide concentrations inside or on the surface of the tabular grains of silver bromoiodide.

  
The preparation of tabular grain silver bromoiodide emulsions of high shape index has been described above by a process which gives neutral or non-ammoniacal emulsions, but the emulsions used in the present invention can be prepared by other processes.

  
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 reduced to less than 0.05 mol / liter and the size of the grains of silver iodide initially present

  
  <EMI ID = 21.1>

  
High bromide tabular grain silver bromide emulsions which do not contain iodide can be prepared by the method described in detail previously, modified to exclude iodide. Alternatively, silver bromide emulsions with tabular grains of high shape index can be prepared by following a procedure based on that described by Cugnac and Chateau in the abovementioned publication. High-index silver bromide emulsions containing square and rectangular grains can be prepared using cubic seed seeds

  
  <EMI ID = 22.1>

  
maintaining the pAg of the seed emulsion at a value of 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, in order to produce tabular grains with a shape index of at least 8: 1. The examples also illustrate other methods of preparing silver bromide emulsions without the addition of iodide consisting of tabular grains of high shape index.

  
Other silver halide emulsions with tabular grains of high shape index which can be used in the implementation of the invention are prepared, using tabular grains of silver chloride whose interior is practically free from silver bromide and silver iodide. They are prepared by simultaneously introducing by double jet the chloride and silver salt solutions into a reactor containing a dispersion medium in the presence of ammonia. During the introduction of the chloride, the pAg inside the dispersion medium is between 6.5 and
10 and the pH is between 8 and 10. The presence of ammonia and higher temperatures tend to cause

  
  <EMI ID = 23.1>

  
precipitation temperatures to produce tabular grains of high-index silver chloride.

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

  
  <EMI ID = 24.1>

  
a peripheral edge parallel to a lographic crystal 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 morphology of the crystal, 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. The regions of the tabular grains containing chloride and silver bromide are formed by maintaining a molar ratio of chloride ions to bromide ions from 1.6: 1 to about 260: 1 and the total concentration of halide ions in the reactor from 0 , 10 N to 0.90 N during the introduction of the 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.

  
High form index tabular grain emulsions useful in the present invention can have extremely high average form indices. The average shape index of 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 given photographic application. You can also increase the average shape index of tabular grains by reducing the average grain thickness. Usually the tabular grains have an average thickness of at least

  
0.03 (one, preferably at least 0.05 m m, although in principle smaller tabular grains can be used, for example up to 0.01 m m. When the silver titers 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 emulsions with useful tabular grains in the present invention are a function of the maximum mean diameters of the grains acceptable for the specific photographic application and of the minimum thicknesses of achievable tabular grains.

  
that the maximum average shape indices vary according to the precipitation technique used and the halide composition of the tabular grains. The average form index

  
the highest observed, 500: 1, for tabular grains with a grain diameter useful photographically, was obtained by preparing grains of silver bromide, and it is possible to obtain shape indices of
100: 1, 200: 1 or even higher by double jet precipitation methods. In general, the presence of iodide reduces the maximum average shape indices achieved, but

  
it is possible to prepare silver bromoiodide emulsions with tabular grains having an average shape index of 50: 1, or 100: 1 or even 200: 1 or more. Tabular silver chloride grains having a shape index up to 50: 1 or even 100: 1 can be prepared, these grains possibly containing bromide and / or iodide.

  
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 can be compounds of copper, thallium,

  
lead, bismuth, cadmium, zinc, medium chalcogen (i.e. sulfur, selenium and tellurium), gold and noble metals of group VIII, according to the indications given in the patents of 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; IEF; United Kingdom: Tabular grain emulsions can be sensitized by reduction inside the grains during precipitation, as described by Moisar et al, Journal of Photography Science, Volume 25, 1977, pages 19 to
27.

  
Silver salts and halides can be added to the reactor using surface feed tubes or

  
below the surface, by gravity feed or using devices which allow the rate of addition as well as the pH, pBr and / or pAg of the reactor contents to be regulated, according to the indications given in the patents of United States of America 3,821,002 and 3,031,304 and by Claes

  
in the journal Photographische Korrespondenz, volume 102,

  
n [deg.] 10, 1967, page 162. For rapid distribution

  
reagents in the reactor, it is possible to use specially constructed mixing devices such as those described in the patents of the United States of America 2 996 287,

  
3,342,605, 3,415,650, 3,785,777, 4,147,551 and 4,171,224, British patent 2,022,431, German patent applications 2,555,364 and 2,556,885 and in the journal Research Disclosure, volume 166, February 1978, publication 16662.

  
To precipitate tabular grain emulsions,

  
a dispersing medium is initially present in the reactor. Advantageously, the dispersing medium is formed of an aqueous suspension of peptizer. Concentration

  
  <EMI ID = 25.1>

  
mass relative to the total mass of the constituents of the emulsion in the reactor. It is common to keep the peptizer concentration in the reactor below about 6% of the total mass, before and during the formation of

  
and

  
the silver halide, / Adjust later the vehicle concentration of the emulsion (the term vehicle including the binder and the peptizer) to higher values, by additional vehicle additions, to obtain the characteristics of optimal sleeping. The initial emulsion-

  
  <EMI ID = 26.1>

  
per mole of silver halide, preferably about 10 to 30 g per mole of silver halide. An additional diluent can be added later to bring the concentration up to 1000 mg / 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 30 to 70% by mass of the emulsion layer.

  
The vehicles can be chosen from the substances usually used in silver halide emulsions for this purpose. The preferred peptizers are hydrophilic colloids which can be used alone or in combination with hydrophobic substances. Suitable hydrophilic vehicles include substances such as proteins, protein derivatives, cellulose derivatives, for example cellulose esters, gelatin, for example gelatin treated with an alkaline agent (skin or bone gelatin ) or gelatin treated with an acid agent (pigskin gelatin), gelatin derivatives, for example acetylated gelatin and phthalylated gelatin. These and other vehicles are described in Research Disclosure, vol. 176, December 1978, publication
17643, section IX.

  
The vehicle, in particular hydrophilic colloids, as well as useful hydrophobic substances combined with them, can be used not only in the emulsion layers of the photographic product elements of the invention, but also in other layers, such as overlays, interlayers and layers placed below the emulsion layers.

  
The preparation of the silver halide emulsions according to the invention may comprise a step of maturation of 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 halide 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 dispersing 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 of the silver salt or the peptizer.

   According to another embodiment, the curing agent can be introduced independently during the addition of the halide and the silver salt. Although ammonia is a known curing agent, it is not a preferred curing medium for emulsions according to <EMI ID = 27.1>

  
high. Preferably, the emulsions according to the invention are not ammoniacal, would have been neutral.

  
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 thiocyanate can be used, but the advantageous concentrations are generally between
- 0.1 and 20 g of thiocyanate per mole of silver halide. The use of thiocyanate as a ripening agent is described in United States patents 2,222,264, 2,448,534 and 3,320,069. Thioethers such as those described in United States patents can also be used in a conventional manner. - United States of America 3,271,157, 3,574,628 and 3,737,313.

  
The tabular grain emulsions with a high shape index are preferably washed to remove soluble salts, by known techniques such as decantation, filtration and / or by freezing and washing 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 in order 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 and / or the excessive increase in their diameter.

  
The methods for preparing tabular grains of silver halide described above make it possible to obtain emulsions in which the tabular grains satisfying the criteria of thickness and diameter required

  
for the shape index, represent at least 50% of the total projected area of the total population of silver halide grains; but additional benefits can be obtained by increasing the proportion of tabular grains. It is advantageous that at least 70% (and optimally at least 90%) of the total projected surface is represented by tabular grains of silver halide meeting the necessary thickness and diameter criteria. Although the presence of small amounts of non-tabular grains is fully compatible with most photographic applications, the proportion of tabular grains can be increased to obtain the full benefits of tabular grains.

   The larger tabular silver halide grains can be mechanically separated from the smaller non-tabular grains in a mixed grain population using conventional separation means, for example a centrifuge or a hydrocyclone. Separation by hydroxyclone is illustrated in US Patent 3,326,641.

  
b. Sensitization

  
Although it is not necessary to obtain the advantages according to the invention concerning the exposure through the support, the silver halide emulsions with tabular grains as well as the other silver halide emulsions present in the following radiographic products the invention are preferably chemically sensitized. They can be sensitized chemically with active gelatin, as described by T.H. James, in The Theory of the Photography Process, 4th Ed. Macmillan,
1977, pages 67-76, or with sulfur sensitizers,

  
selenium, tellurium, gold, platinum, palladium,

  
with iridium, osmium, rhodium, rhenium or phosphorus, or with associations of these sensitizers, the pAg being between 5 and 10, the pH between 5 and 8, and at

  
  <EMI ID = 28.1>

  
Research Disclosure, vol. 120, April 1974, publication 12008, in the journal Research Disclosure, vol. 134, June 1975, publication 13452, in U.S. Patents 1,623,499, 1,673,522, 2,399,083, 2,642,361, 3,297,447, 3,297,446, 3,772,031, 3,761,267, 3,857 711,

  
3,565,633, 3,901,714 and 3,904,415, and in British patents 1,396,696 and 1,315,755. Chemical sensitization is optionally carried out in the presence of thiocyanates, as described in the United States patent 2 642,361, of sulfur-containing compounds of the type described in US Patents 2,521,926, 3,021,215 and 4,054,457. In particular, it is proposed to sensitize chemically in the presence of chemical sensitization modifiers, that is i.e. compounds known to remove haze and increase speed when present during chemical sensitization, e.g. azaindenes, azapyridines, azapyrimidines, benzothiazolium salts and sensitizers having one or more heterocyclic nuclei.

   Examples of chemical sensitizing modifier compounds are described in U.S. Patents 2,131,038, 3,411,914,

  
3,554,757, 3,565,631 and 3,901,714, to Canadian patent 778,723 and by Duffin in Photographie Emulsion Chemistry, Focal Press (1966), New York, pages 138-143. In addition, or alternatively, the emulsions can be sensitized by reduction, for example with hydrogen, as described in US Pat. Nos. 3,891,446 and

  
3,984,249, by treatment with low pAg (e.g. less than

  
5) and / or at high pH (for example greater than 8) or using reducing agents such as stannous chloride, thiourea dioxide, polyamines and amineboranes, as described in the journal Research Disclosure, vol. 136, August
1975, publication 13654 and to the U.S. Patents 2,518,698, 2,983,609, 2,739,060, 2,743,182,

  
2,743,183, 3,026,203 and 3,361,564. Specifically, chemical sensitization is provided on the surface or in the grain, near the surface, as described in U.S. Patents 3,917,485 and 3 966,476.

  
The high form index tabular grain emulsions of the present invention are in all cases spectrally sensitized. With the tabular grain emulsions of high shape index according to the invention and with other emulsions also mentioned in the present description, provision is made to use spectral sensitizing dyes which exhibit maximum absorption in blue and minus blue, that is to say in the red and in the green 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. One can for example use absorbent spectral sensitizers in the infrared region.

  
Silver halide emulsions with a high form index grain 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, merostyrylic dyes 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 nuclei,

  
  <EMI ID = 29.1>

  
oxazolium, oxazolinium, thiazolium, thiazolinium, selenazolium, selenazolinium, imidazolium, imidazolinium, benzoxazolium, benzothiazolium, benzoselenazolium, benzimidazolium, naphtoxazolium, naphtothiazolium, naphtoselenazidroniazide, naphtoselenazolium, naphtoselenazolium, naphtoselenazolryniazide

  
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,

  
  <EMI ID = 30.1>

  
hydantoin, 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, malononitrile, isoquinolin-4-one and chroman-2,4-dione.

  
One or more spectral sensitizing dyes can be used. Dyes are known with sensitization maxima for wavelengths distributed over the entire extent of the 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 obtained with selected combinations of spectral sensitizing dyes and other additives such as stabilizers, anti-veil, development accelerators or inhibitors, coating aids, optical brightening agents and anti-static agents.

  
Mechanisms and compounds for explaining or obtaining over-sensitization are described by Gilman in "Review of the Mechanisms of Supersensitization", Photography Science and Engineering, vol. 18, 1974, pp. 418-430. Spectral sensitizing dyes can still

  
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, a spectral sensitizing dye which exhibits a modification

  
of color according to the adsorption is adsorbed on the surface of the tabular grains of silver halides. For the implementation of the invention, it is possible to use any conventional spectral sensitizing dye known to exhibit a bathochromic or hypsochromic increase in the absorption of light as a function of the adsorption at

  
silver halide grain surfaces. Dyes that meet these criteria are well known in the art, as illustrated by T.H. James, in The Theory

  
of the Photography 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 FM 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 displacement) are known in the art; however, J aggregates (bathochrome shift) are not common for dyes in 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 naphtoselenazolium.

  
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. Even when the emulsions according to the invention are silver bromide and bromoiodide emulsions with tabular grains of index

  
of high form, very significant increases in speed are obtained thanks to the use of spectral sensitizing dyes for blue. When it is intended to expose the emulsions according to the present invention in their region of natural sensitivity, advantages can be obtained with regard to the sensitivity as well as the exposure through the support by increasing the thickness.

  
  <EMI ID = 31.1>

  
With the advantageous invention, the silver bromide and bromide emulsions are sensitized for blue and consist of tabular grains having a thickness

  
  <EMI ID = 32.1>

  
average in shape greater than 8: 1, preferably at least 12: 1

  
and which represent 50% of the total projected surface of the silver halide grains present in the emulsion, preferably 70% and optimally 90%. In the description

  
  <EMI ID = 33.1>

  
without going beyond the limits of the invention. In all cases, the maximum average diameter of the grains which it is proposed to use in the radiographic products according to the invention is

  
  <EMI ID = 34.1>

  
The spectral blue dyes for blue bromide and silver bromoiodide emulsions with tabular grains of high shape index can be selected from the classes of dyes known to provide spectral sensitizers. Dyes from the polymethine class, such as cyanines, merocyanines, hemicyanins, hemioxonols and merostyrylic dyes are advantageous spectral blue sensitizers. In general, the useful blue spectral sensitizers can be chosen from the classes of dyes according to their absorption characteristics, that is to say according to the shade. 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, publication 17643, Section III.

  
Usual amounts can be used to spectrally sensitize the emulsion layers containing non-tabular silver halide grains or tabular grains of low shape index. To obtain all the advantages offered by emulsions with tabular grains of high shape index, it is preferable to adsorb the spectral sensitizing dye on the surface of the grains in an optimal quantity, that is to say in an amount sufficient to obtain at least 60 % of the maximum photographic sensitivity that can be achieved with grains under the expected exposure conditions. The amount of dye 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 awareness is obtained with organic dyes.

  
  <EMI ID = 35.1>

  
100% or more of the total area of the silver halide grains, as described, for example, by West et al. In "The Adsorption of Sensitizing Dyes in Photographie Emulsions", Journal of Phys. Chem., Vol. 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 United States patent 3,979,213. Optimal dye concentrations can be chosen according to the methods described by Mees, Theory of the Photography 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 to be 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. U.S. Patent 4,225,666 specifically provides for the stepwise introduction of the spectral sensitizer dye into the emulsion, so that part of this dye is present before chemical sensitization while another part is introduced after 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 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.

  
High-index tabular-grain silver halide emulsions chemically sensitized

  
and spectrally have better speed / grain relationships than those obtained hitherto with conventional silver halide emulsions containing tabular grains of low shape index and with emulsions having the best known speed / grain relationships * We obtain the best results using sensitizing dyes in the blue minus.

  
According to an advantageous embodiment, the spectral sensitizers are incorporated into the emulsions before chemical sensitization. We also got in some

  
Similar results have been achieved by introducing other adsorbable substances, such as maturation modifiers into emulsions before chemical sensitization.

  
Apart from the prior incorporation of absorbable substances it is advantageous to use thiocyanates during chemical sensitization at concentrations

  
  <EMI ID = 36.1>

  
silver, as described in US Patent 2,642,361. Other curing agents can 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, ie bromide, iodide and / or silver chloride capable of undergoing Ostwald maturation, 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 tabular grain emulsion of high index form can be carried out at one or more sites distinct from the grains. It is believed that the preferential adsorption of spectral sensitizing dyes on the crystallographic surfaces forming the main faces of the flat grains allows chemical sensitization to occur selectively on

  
different crystallographic surfaces from flat grains.

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

  
selenium, and awareness of gold, sulfur and selenium. In an advantageous embodiment of the invention, the silver bromoiodide emulsions with tabular grains of high form index contain a medium chacolgene such as sulfur and / or selenium, which cannot be detected, and l which 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 may comprise another silver salt such as a silver thiocyanate, or another different silver halide (for example silver chloride or silver bromide, although this other silver salt may be present below detectable amounts.

  
Although it is not necessary to obtain all of their advantages, the emulsions defined according to the invention are advantageously chemically and spectrally sensitized in an optimal manner, 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 sensitization and under expected conditions of use and treatment. The logarithm of rapidity is defined as 100 (1-log E), where E is measured in lux. Seconds at a density of 0.1 above the veil.

  
Once tabular grain emulsions of high shape index have been produced by the precipitation processes, washed and sensitized as described above, the preparation can be completed by incorporating additives classic photographic.

  
The radiographic products according to the present invention, intended to form silver images, can be tanned to a sufficient degree to avoid the need to add an additional tanning agent during processing. This provides an increased silver hiding power compared to tanned and similarly treated radiographic products which use non-tabular grain emulsions or tabular grain emulsions of low shape index. In particular, it is possible to tan the layers of tabular grain emulsion with a high shape index and the other layers of hydrophilic colloid.

  
to a degree sufficient to reduce swelling of the diapers

  
less than 200%, the percentage of swelling being determined

  
  <EMI ID = 37.1>

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

  
  <EMI ID = 38.1>

  
the change in layer thickness. Although it is particularly preferred to tan the radiographic products intended to form silver images to a sufficient degree so that it is not necessary to incorporate tanning agents into the treatment solutions, it is possible to conventionally tan the emulsions used. according to the present invention. In addition, tanning agents can be incorporated into the processing solutions as described, for example, in Research Disclosure, vol. 184,

  
August 1979, publication 18431, paragraph K, which particularly concerns the treatment of products for radiography.

  
Usual incorporated tanning agents (pre-

  
  <EMI ID = 39.1>

  
December 1978, publication 17643, section X.

  
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,

  
flight. 184, August 1979, publication 18431. For example, emulsions may contain stabilizers, anti-veil agents and agents that inhibit the action of mechanical stress.

  
  <EMI ID = 40.1> radiographic product may contain antistatic agents and / or layers, as described in the paragraph

  
II. X-ray products may include overlays, as described in section IV. The overlays may contain matting agents such as those described in Research Disclosure, publication 17643 cited above, paragraph VI. The overcoat and the other layers of radiographic products may contain plasticizers and lubricants such as those described in publication 17643, paragraph XII. Although, in most applications, the radiographic products according to the invention are used to form silver images, they can contain colored substances as described in publication 17643, paragraph VII, to allow the formation of dye or argentic images reinforced with a dye.

   Developers and development modifiers may be incorporated therein, such as those described in publication 17643, paragraphs XX and XXI. The advantages of exposure through the support can be further improved by using conventional means to maintain exposure through the support, as described

  
in publication 18431, paragraph V.

  
According to a usual technique, it is proposed to mix emulsions with tabular grains of high shape index.

  
with each other or with conventional emulsions

  
to meet the requirements of a particular emulsion layer. For example, it is known to mix emulsions to adjust the characteristic curve of a photographic product for a predetermined result. The mixture can be used to increase or reduce the maximum densities obtained during exposure and treatment, to reduce or increase the minimum density and to adjust the characteristic curves in the part located between the foot and the shoulder of these curves. .

  
To do this, it is possible to mix the tabular grain emulsions of high shape index with conventional silver halide emulsions, such as those described in Research Disclosure, vol. 176, December 1978, publication
17643 cited above, paragraph I. In particular, consideration is given to

  
  <EMI ID = 41.1>

  
When mixing a relatively fine aroma silver chloride emulsion with the emulsions used in the present invention, especially the silver bromoiodide emulsions, this can lead to an additional increase

  
  <EMI ID = 42.1>

  
The supports can be of any conventional type known to allow exposure through the support. Advantageous supports are polyester film supports. Particularly preferred are film supports made of polyethylene terephthalate. These supports and their preparation are described in the patents of the United States of America 2,823,421,

  
2,779,684 and 3,939,000. Products for medical radiography are usually tinted blue; we usually add the dyes to get this shade directly

  
to the polyester melt before extrusion and they must therefore be heat stable. Advantageous dyes are those of the anthraquinone class, such as those described in United States of America patents 3,488,195,

  
3,849,139, 3,918,976, 3,933,502 and 3,948,664, and to British patents 1,250,983 and 1,372,668.

  
The spectral sensitizing dyes are preferably chosen so that they exhibit a modification of the absorption maximum in their adsorbed state, usually in the form of aggregates, in the H or J band, in a region of the spectrum corresponding to the electromagnetic radiation at which the product is intended to be exhibited 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 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 the 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 intensifying screens may themselves be part of the radiographic products, but they are usually separate elements which can be reused to exhibit successive radiographic products. Reinforcing screens are well known and are described in Research Disclosure, publication 18431 cited above, paragraph IX, and in United States patent 3,737,313.

  
The exposed radiographic products can be treated by any suitable conventional method. These processing methods are described in Research Disclosure, publication 17643 cited above, paragraph XIX. Particularly preferred is the advance treatment of the film by driving rollers, as described in the patents of the United States of America.

  
3,025,779, 3,515,556, 3,545,971 and 3,647,459, and British patent 1,269,268. Tanning development can be carried out, as described in United States patent 3,232,761. or the radiographic products may contain additives of the thioamine and glutaraldehyde or acrylic aldehyde type, as described in the patents of the United States of America 3,869,289

  
and 3,708,302.

  
Examples

  
The following examples illustrate the invention.

  
In each example 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 3

  
In order to compare the exposure through the support as a function of the tabular grain shape index, three tabular grain depletion of silver bromide according to the present invention and a tabular grain emulsion of silver bromide having a lower form index. The emulsions and the average shape index of the tabular grains are presented in Table I.

  
  <EMI ID = 43.1>

  

  <EMI ID = 44.1>


  
The emulsions of Examples 1 to 3 are tabular grain emulsions with a high shape index within the preferred definition limits of the present patent application.

  
  <EMI ID = 45.1>

  
are included in the calculation of the average grain diameter and the percentage of projected surface in the emulsions of these examples and other examples, except when their exclusion is indicated, the grains of small diameter are in insufficient number to modify significantly the values indicated.

  
To obtain a classification of emulsions in terms

  
performance with regard to the exposure through the support, the emulsions are applied in an identical manner on distinct and identical supports of transparent film of polyethylene terephthalate. Each emulsion is applied at the rate, per square decimetre, of 21.6 mg of silver and 28.8 mg of gelatin. Before the coating operation, each emulsion is sensitized to green with 600 mg of sodium salt of hydro-

  
  <EMI ID = 46.1>

  
oxacarbocyanine per mole of silver and 400 mg of potassium iodide per mole of silver. The pre-tanning of the emulsions is carried out with 1.5% by mass, relative to the mass of gelatin, of

  
  <EMI ID = 47.1>

  
Referring to Figure 1, one can best describe the manner in which the results of the tests relating to exposure through the support were obtained. The sample 100 is constituted by the emulsion layer 102 and by the support 104. The sample is placed on a product 106

  
  <EMI ID = 48.1>

  
commercial mining Kodak Ortho M film, consisting of an emulsion layer 108 and a support 110 of transparent film. A sheet 112 of opaque black paper is placed in the immediate vicinity of the face of the support opposite the emulsion layer. A second sheet 114 of opaque black paper is arranged so as to cover the emulsion layer 108 in a position offset laterally relative to the sample 100. On the sheet of paper 114 is arranged a separate sample 106a, identical to the radiographic product 106, its emulsion layer
108a being the furthest from the sheet of paper. A conventional screen 116 for x-rays, emitting in the green, covers the samples 100 and 106a. A test pattern 118 is placed between the screen and the X-ray source shown diagrammatically by the arrows 120.

   The target is a density scale in laminated aluminum containing 24 areas, two adjacent areas providing, on average, a difference in transmitted radiation
(E) approximately 0.10 "log E and the radiopacity of the ranges gradually increasing from practically zero for the least dense range.

  
The assembly shown in Figure 1 is exposed to the radiation emitted by a single-phase X-ray generator fitted with an X-ray tube Machlett Dymax Type 59 B. The exposure times are 1 s using a current of 100 mA and a potential of 70 kV. After exposure, the radiographic products 106 and 106 a are treated in a conventional radiographic processing machine, sold under the trade name Kodak RP X-Omat M6A-N Processing Machine, using the standard developer for this machine sold under the trade name Developer MX-810. The duration of development is from 21s to 35 [deg.] C.

  
We are more aware of the results of the test by

  
referring to FIG. 2 where two characteristic curves 201 and 203. are represented. The curves can be divided into three distinct parts. The feet 201a and 203a of the curves indicate a slight increase in density as a function of the increase in exposure. Intermediate parts
201b and 203b show a perfectly linear relationship between the increase in exposure and the increase in density. In practice, the intermediate parts of the characteristic curves are not always linear, but usually they are close to linearity. Shoulders
201 c and 203 c of the curves show, like the feet, a slight increase in density when the exposure increases.

  
  <EMI ID = 49.1>

  
comprising two emulsion layers identical to silver halides, the exposure being made using a screen such as screen 118 which is activated to emit fluorescent radiation during exposure to X-rays, the identical treatment of the layers of emulsion does not produce two identical characteristic curves. On the contrary, two characteristic curves offset laterally are obtained, such as curves 201 and 203. The emulsion layer furthest from the screen has been entirely exposed by the radiation which

  
has passed through the nearest emulsion layer and the film support. Thus, the emulsion layer furthest from the screen is fully exposed by radiation which has passed through the support. The average offset 204. (expressed in

  
  <EMI ID = 50.1>

  
Characteristic curves can be used to calculate the percentage of exposure through the support for the radiographic product using the following equation (A):

  

  <EMI ID = 51.1>


  
To obtain a qualitative classification of the exposure through the support as a function of the shape index, a characteristic curve corresponding to curve 201 is drawn for each sample 106a and compared with a second characteristic curve corresponding to curve 203 obtained from the part of the radiographic product 106 underlying the sample 100. By measuring the average offset of the intermediate parts of the characteristic curves and using equation (A), the results relating to the exposure through the medium.

  
These results appear in Table I and show that the silver bromide emulsions with tabular grains of high shape index used in the radiographic products according to the invention are capable of reducing the percentage of exposure through the support.

Example 4

  
An analogous emulsion is applied to both sides of a polyethylene terephthalate film support

  
to that of Examples 1 to 3, but with tabular grains of average shape index between 12: 1 and 15: 1 and having a

  
  <EMI ID = 52.1>

  
are tabular. The total silver titer is 54 mg / dm <2> on both sides of the support. The percentage of absorption of a sample of this radiographic product is measured as a function of the wavelength and it is found that it has an absorption peak at 545 nm.

  
The radiographic product is exposed for 1/50 s using a Macbeth sensitometer having a light source

  
  <EMI ID = 53.1>

  
the illumination of a screen emitting in the green. A density scale is interposed between the product and the exposure light source. The density scale is standard with 21 density ranges from 0 to 3.0 in increments of 0.15. The exposed samples are treated as described in Examples 1 to 3, with the exception of the treatment times and temperatures indicated below in Tables II and III.

TABLE II

  
Effect of development time

  
on speed and contrast

  

  <EMI ID = 54.1>

TABLE III

  
Developmental temperature effect

  
on speed and contrast

  

  <EMI ID = 55.1>


  
'The logarithm of the speed is defined by 100 (1-log E), log E being measured at 1.0 above the veil.

  
 <3> Contrast is the slope of the straight line between two points on the sensitometric curve at densities of 0.2 and 2.0 above the veil.

  
It can be seen from Tables II and III that the radiographic product according to the invention exhibits few variations in speed and contrast as a function of variations in the durations and the temperatures of development.

  
The present invention therefore offers significant advantages

  
of processing latitude that a conventional radiographic product does not allow.

Example 5

  
Apply on both sides of a support, at a total film strength of 43 mg / dm <2>, two samples of

  
  <EMI ID = 56.1>

  
For all the other points, the procedure of example 4 is followed.

  
The sensitometric results are summarized in Table IV below.

TABLE IV

Sensitometric comparison

  

  <EMI ID = 57.1>


  
See tables II and III

  
  <EMI ID = 58.1>

  
From the results of Table IV, it can be seen that an acceptable response is obtained from the radiographic products according to the invention, even if these have a silver titer reduced by 20% compared to that of Example 4. In addition, the radiographic products according to the invention have acceptable characteristics with regard to exposure through the support. In this regard, this example demonstrates that the invention is not limited to obtaining radiographic products characterized by low exposure through the support. The radiographic products according to the invention also make it possible to use reduced silver titers without increasing the exposure through the support to unacceptable levels.

   In addition, the advantages of the invention can be obtained by combining in any desired manner the advantages relating to the reduction in the silver content and to the reduction in exposure through the support.

Appendix 1

  
The following experimental details relate to the preparation of the emulsions used in the previous examples.

  
Preparation A. (Witness 1)

  
A 1.066 liter of an aqueous solution of bone gelatin
(1.9% gelatin) containing 1.21 M potassium bromide

  
  <EMI ID = 59.1>

  
in 11 min, by stirring, with a single jet at constant rate, an aqueous 1.11 M solution of silver nitrate (solution B). The emulsion is matured while keeping it 15 min at

  
  <EMI ID = 60.1>

  
prepare this emulsion.

  
The emulsion is chemically sensitized by adding it

  
  <EMI ID = 61.1>

  
phthalylated and by heating the emulsion for 30 min at 52 [deg.] C, the pH being 6.0.

  
At the end of the 30 min chemical maturation phase, the pH of the emulsion is adjusted to 6.0, the pAg being from 10.6 to

  
  <EMI ID = 62.1>

  
Preparation B. (For example 1)

  
A 17.5 liters of an aqueous solution of bone gelatin

  
  <EMI ID = 63.1>

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

  
Aqueous solutions containing 2.29 M of potassium bromide (solution B-2) and 2.0 M of silver nitrate (solution C-2) are then added to the reactor, by double jet and with accelerated flow rates ( these being multiplied by 4.2 between the start and the end of the addition), up to

  
  <EMI ID = 64.1>

  
14.1% of the total amount of silver nitrate used. The addition of solution B-2 is stopped.

  
A 2.0 M aqueous solution of silver nitrate (solution C-3) is added to the reactor for approximately 12.3 min until a pBr of 2.39 at 55 [deg.] C is obtained; we consume like this
10.4% of the total amount of silver nitrate used. We

  
  <EMI ID = 65.1>

  
15 mins.

  
Solution C-3 and a 2.0 M aqueous solution of potassium bromide (solution B-3) are then added, maintaining the pBr at 2.39 at 55 [deg.] C, by double jet with a

  
  <EMI ID = 66.1>

  
of the total amount of silver nitrate used. The addition of solutions B-3 and C-3 is stopped. 41.1 moles of silver were used to prepare this emulsion.

  
  <EMI ID = 67.1>

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

  
Preparation C (for example 2)

  
To an aqueous solution of gelatin of bone (1.5% of gelatin) containing 0.14 M of potassium bromide (solution A), to a pBr of 0.85 and at 55 [deg.] C, the following is added. stirring by double jet and at constant flow rate in 8 min, (3.22% of the total amount of silver nitrate used is thus 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.

  
Aqueous solutions containing 3.95 M potassium bromide (solution B-2) and 2.0 M silver nitrate (solution C-2 solution) are then added by double jet to a pBr of 0.85

  
and at 53 'Ci with accelerated flow rates (these 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 thus consumed. The addition of solution B-2 is stopped.

  
A 2.0 M aqueous solution of silver nitrate (solution C-3) is added over a constant flow rate of about 2.5 minutes until a pBr of 2.43 at 55 [deg.] C is obtained; 4.18% of the total amount of silver nitrate used is thus consumed. We main-

  
  <EMI ID = 68.1>

  
Solution C-3 and a 2.0 M aqueous potassium bromide solution (solution B-3) are then added over 31.1 min with accelerated flow rates (these being multiplied by 1.4 between the start and the end of addition); 64.4% of the total amount of silver nitrate used is thus consumed. The addition of solutions B-3 and C-3 is stopped. 29.5 moles of silver were used to prepare this emulsion.

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

  
Preparation D (for Example 3)

  
To an aqueous solution of gelatin of bone (1.5% of gelatin) containing 0.14 M of potassium bromide (solution A), to a pBr of 0.85 and at 55 [deg.] C, the following is added, while stirring, by double jet 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.

  
Aqueous solutions containing 2.29 M of potassium bromide (solution B-2) and 2.0 M of silver nitrate (solution C-2) are then added by double jet to a pBr of 0.85 and to 55 [deg.] C, with accelerated flow rates (these being multiplied by 4.2 between the start and the end of the addition), until solution C-2 is used up, in approximately 20 min; 59.5% of the total amount of silver nitrate used is thus consumed. The addition of solution B-2 is stopped. In the procedure described, 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.

  
  <EMI ID = 69.1>

  
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 nitrate were used.

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

SENSITIZATION

  
E. Optimal chemical sensitization of each of the emulsions of Examples 1, 2 and 3 prepared as described above is carried out with 5 mg of potassium tetrachloroaurate per mole of silver, 150 mg of potassium thiocyanate per mole of silver and 10 mg of sodium thiosulfate per mole

  
  <EMI ID = 70.1>

  
nant the exposure through the support are independent of chemical sensitization.

  
  <EMI ID = 71.1>

  
To further illustrate the decrease in exposure

  
through the support highlighted by the radiographic products of the present invention, three radiographic products are prepared according to the invention and a control radiographic product.

  
The emulsion of Example 6 is applied to the two faces of a polyethylene terephthalate film support. Each face comprises, per square decimetre, 22.9 mg of silver and

  
28.6 mg of gelatin, as well as an overcoat of gelatin at a rate of 8.8 mg / dm <2>. The emulsion is pre-tanned with 1.5% by mass, relative to the mass of gelatin, of bis (vinylsulfonylmethyl) ether.

  
The emulsion of example 7 is applied in a similar manner to the emulsion of example 6, except that on each side of the support the emulsion contains 28.45 mg of silver per square decimetre and is pre-tanned with 0, 75% by mass of bis-
(vinylsulfonylmethyl) ether.

  
To obtain the emulsion of Example 8, apply

  
an emulsion as in Example 7, except that on each side

  
of the support, the film-title of the emulsion is

  
  <EMI ID = 72.1>

  
A control emulsion 2 is applied in a similar manner

  
with the emulsion of Example 6, except that on each side of the support, the silver content of the emulsion is 28.4 mg / dm <2>, the emulsion being pretanned with 0.75% by mass of bis (vinylsulfonylmethyl) ether.

  
A control emulsion 3 is applied like the control emulsion 2 described above, but on each side of the support the silver content of the emulsion is 28.25 mg / dm <2>.

  
  <EMI ID = 73.1>

  

  <EMI ID = 74.1>


  
The layers are exposed to radiation emitted by a single-phase X-ray generator Picker Corp. fitted with a Machlett Dymax Type 59 B x-ray tube. The exposure times are 1 s using a current of 100 mA and a potential of 70 kV. After exposure, the radiographic products are treated in a conventional radiographic processing machine, sold under the trade name machine.

  
  <EMI ID = 75.1>

  
lator intended for this machine, sold under the trade name Developer MX-810. The development time is

  
  <EMI ID = 76.1>

  
Comparative data concerning the exposure of the layers through the support are obtained by sensitometric exposure using a reinforcing screen placed in the immediate vicinity of the film. The radiation emitted by the screen alone produces a primary sensitometric curve in the nearest layer and a slower secondary curve in the most distant layer. The curves giving the density as a function of Log E are plotted and the percentage of exposure through the support is calculated using the formula A above.

  
From the results of Table V, it can be seen that the silver halide emulsions with tabular grains of high shape index make it possible to reduce the exposure through the support when they are applied to the two faces of a support and tested in an x-ray product. Example 7 shows a significant reduction in exposure through the support compared to control sample 3 (16% and 39% respectively), even with a 20% reduction in the silver titer.

  
Furthermore, the emulsion of Example 6 with tabular grains of silver bromide is applied in colctia and the exposure is evaluated through the support as indicated for Examples 1-3 above; we find as a result 19%. This value is compatible with the results relating to the exposure through the support as a function of the shape index and indicated in Table I. The difference in the percentages of exposure through the support (27 against 19) between the sample having a layer on each of its two faces and that having a layer on only one side can be attributed to the loss of illumination which occurs when the emulsion layers are not in optical contact, which is the case when separate films are used to measure exposure across the media.

  
Is exposed and treated according to the procedure described in Example 4, except that the temperature (unless otherwise indicated) is 33.3 [deg.] C, samples bearing on their two faces layers of emulsions from examples

  
6, 7 and 8 described above. To carry out comparative tests, a control emulsion 4 of silver bromoiodide (the Br: I molar ratio being 99: 1) is applied to each side of a polyethylene terephthalate film support. shape index of 1: 1, rightly, per square decimetre, of 58.1 mg of silver and 58.1 mg of gelatin, as well as an overlay of gelatin at a rate of 8.8 mg / dm <2>. The emulsion is pretanned with 0.75% by mass, relative to the total mass of gelatin, of bis (vinylsulfonylmethy) ether. The higher silver titer of the control emulsion 4, relative to the emulsion layers of Examples 6, 7 and 8, is necessary to increase the contrast to levels comparable to those obtained with the emulsions of Examples 6, 7 and 8.

   When the control emulsion 4 is applied to silver titers similar to those of the emulsions of Examples 6, 7 and 8, the contrast obtained is much lower than that which it is customary to obtain in commercial radiographic products.

  
The effects of variations in development time on photographic speed and contrast are summarized in Table VI; the effects of variations in development temperature on photographic speed and contrast are summarized in Table VII.

  

  <EMI ID = 77.1>


  

  <EMI ID = 78.1>
 

  

  <EMI ID = 79.1>


  

  <EMI ID = 80.1>
 

  
According to Table VI, it is obvious that the control emulsion 4, with a shape index of 1: 1, exhibits a much greater modification of the speed and of the contrast, as a function of the duration of development, than the emulsions. with tabular grains of high shape index. When the emulsion of Example 6, which is a silver bromide emulsion, is compared with the emulsions of Examples 7 and 8, which are silver bromoiodide emulsions, it is found that the presence or the absence of iodide in this case does not significantly affect the results observed.

  
Table VII shows that the performance of the emulsion of Example 6 is higher and that the performance of the emulsions of Examples 7 and 8 and of the control emulsion 4 are more or less comparable. In this case,

  
it appears that the absence of iodide explains the relatively lower speed and contrast changes observed for the emulsion 6. However, when comparing products containing emulsions of shape index

  
and non-tabular emulsions each having iodide concentrations greater than about 2 mole%, the products containing the tabular grain emulsions of high shape index show relatively less changes in speed and contrast as a function of differences in processing temperature.

  
Appendix 2

  
The following preparation details are not part of the invention.

  
F. Example 6

  
  <EMI ID = 81.1>

  
bone gelatin (1.5% by mass) containing 0.142 M of potassium bromide, a 1.15 M solution of potassium bromide and a 1.0 M solution of silver nitrate are added, by double jet with flow rates constant in 2 min, at a pBr of 0.85 and consuming 1.75% of the total amount of silver used. The emulsion is maintained for 30 s, then the pBr is adjusted <EMI ID = 82.1>

  
money at constant flow in 7.33 min, consuming 6.42%

  
of the total amount of money used. A 2.0 M solution of silver nitrate is then added, by double jet and with accelerated flow rates (these being multiplied by 4.2 between the start and the end of the addition), in 26 min at a pBr of 1.22 and at 65 [deg.] C, consuming 37.57% of the total amount of silver used. The pBr of the emulsion is then adjusted to

  
2.32 to 65 [deg.] C by addition of a 2.0 M solution of silver nitrate, at a constant flow rate in 6.25 min, consuming 6.85%

  
of the total amount of money used. A 2.29 M solution of potassium bromide and a 2.0 M solution of silver nitrate are added by double jet at constant flow rates.

  
54.1 min at a pBr of 2.32 and at 65 [deg.] C, consuming 47.4% of

  
the total amount of money used. To prepare this emulsion, a total of approximately 9.13 moles of silver are used. After precipitation, the emulsion is cooled to
40 [deg.] C, 1.45 liters of a phthalylated gelatin solution (15.3% by mass) are added, then the emulsion is washed twice by the coagulation process described in the United States patent from America 2,614,929. 1.55 liters of bone gelatin solution (13.3% by mass) are then added and

  
adjusts the pH to 5.5 and the pAg to 8.3 to 40 [deg.] C.

  
The tabular grain silver bromide emulsion thus

  
  <EMI ID = 83.1>

  
11.2: 1.

  
To 2.5 liters of a well stirred 0.4 M aqueous solution of potassium nitrate containing 1479 g (1.5 moles) of the above emulsion, a 1.7 M solution of potassium bromide and a 1.5 M solution of silver nitrate, by double jet, at constant flow rates, in 135 min, at a pAg of 8.2 to
65 [deg.] C, consuming 5.06 moles of silver. Following the

  
  <EMI ID = 84.1>

  
1.0 liter of a phthalylated gelatin solution (19.0% by mass), then the emulsion is washed three times by the coagulation process described in US Patent 2,614,929. then 1.0 liter of a gelatin solution of bone (14.5% by mass) and the

  
  <EMI ID = 85.1>

  
the tabular grains represent more than 80% of the projected surface.

  
The emulsion is chemically sensitized with, by

  
mole of silver, 5 mg of potassium tetrachloroaurate, 10 mg of sodium thiosulfate pentahydrate and 150 mg of sodium thiocyanate, then it is spectrally sensitized with, per mole of silver, 150 mg of sodium salt of hydroxide d 'anhydro-5,5'-dichloro-9-ethyl-3,3-di (sulfopropyl) oxacarbocyanine and 400 mg of potassium iodide.

  
G. Example 7

  
To 9.0 liters of a well stirred aqueous solution of bone gelatin (1.5% by mass) containing 0.142 M of potassium bromide, a 1.0 M solution of silver nitrate is added by double jet at constant flow rate in 2 min, at a pBr of

  
  <EMI ID = 86.1>

  
2.0 M silver nitrate with an accelerated flow (this being multiplied by 4.55 between the start and the end of the addition), in 2 min and consuming 9.7% of the total amount of money used. Then add a 2.25 M solution

  
of potassium bromide containing 0.04 M of potassium iodide and a 2.0 M solution of silver nitrate, by double jet and with accelerated flow rates (these being multiplied by 5.6 between the start and the end of addition), in 30 min at a pBr close to 1.4 and at 70 [deg.] C, consuming 86.8% of the total amount of silver used. To prepare this emulsion, a total of approximately 6.85 moles of silver is used. Following the precipitation, the emulsion is cooled to 40 [deg.] C, 1.27 liters of phthalylated gelatin are added
(15.9% by mass), then the emulsion is washed three times according to the method described in US Patent 2,614,929. 1.2 liters of bone gelatin are then added
(13.75% by mass), and the pH is adjusted to 5.5 and the pAg to 8.3

  
at 40 [deg.] C. The silver bromoiodide (98.5: 1.5) emulsion with tabular grains thus obtained has grains with a diameter

  
  <EMI ID = 87.1>

  
the shape index being 16.8: 1, and the tabular grains represent more than 80% of the projected area of all the grains.

  
The emulsion is sensitized in a similar manner to that

  
  <EMI ID = 88.1>

  
following the addition of chemical sensitizers.

  
H. Control emulsions 2, 3 and 4

  
Control emulsion 2 is an ammoniacal emulsion prepared according to a procedure analogous to that described by G. Duffin, in Photographic Emulsion Chemistry, The Focal Press, London and New York, p. 72, 1966.

  
  <EMI ID = 89.1>

  
11.67 liters of a 0.856 M solution of silver nitrate containing 2.08 M of ammonium hydroxide are then added in a single jet in 1 min. The emulsion is maintained by stirring it for 1 h 30 min at 50 [deg.] C. Then we cool the emulsion

  
at 30 ° C, a solution of phthalylated gelatin (12%) is added

  
en masse) and the emulsion is washed by coagulation three times. 1.5 liters of bone gelatin solution (19.3% by mass) are added and the pH is adjusted to 6.0 and the pAg

  
at 8.2 to 40 [deg.] C.

  
The silver bromoiodide emulsion (99: 1) thus obtained contains spherical grains with a diameter close to <EMI ID = 90.1>

  
The control emulsion 2 is chemically sensitized with, per mole of silver, 3.0 mg of sodium thiosulfate pentahydrate, 1.5 mg of potassium tetrachloroaurate and 50 mg of 3-methyl-1,3-benzothiazolium iodide and we keep it
25 min at 60 [deg.] C. The control emulsion 2 is divided into two fractions. The first fraction, still called control emulsion 2, is spectrally sensitized with, per mole of silver,
100 mg of the sodium salt of anhydro-5,5'-dichloro9-ethyl-3,3'-di (3-sulfopropyl) oxacarbocyanine hydroxide. The other fraction, identified as the control emulsion 3, is spectrally sensitized, in the same way as the tabular grain emulsion of Example 6.

  
The control emulsion 4 is prepared in an identical manner

  
to control emulsion 2, except that it does not contain sodium thiocyanate and that 0.75 mg of potassium tetrachloroaurate is used per mole of silver, then it is maintained for 35 min.

CLAIMS

  
1. Radiographic product comprising a first and a

  
second layer of silver halide emulsion consisting of a dispersion medium and of photosensitive silver halide grains and a support interposed between these layers of emulsion and capable of transmitting

  
the radiation to which the said second emulsion layer is sensitive, characterized in that at least the said first emulsion layer contains tabular silver halide grains having a thickness less than

  
  <EMI ID = 91.1>

  
average shape greater than 8: 1, this shape index being defined as the ratio of the diameter of the grain to the thickness thereof, these tabular grains representing at least 50% of the total projected surface of all the halide grains of silver present in said silver halide emulsion, the diameter of a grain being defined as the diameter of a circle having an area equal to

  
the projected area of said grain, further characterized

  
in that a spectral sensitizing dye is adsorbed

  
on the surface of the tabular grains of silver halide.


    

Claims (1)

2. X-ray product according to claim 1, characterized in that the said tabular silver halide grains representing at least 50% of the surface <EMI ID = 92.1> 3. X-ray product according to claims 1 or 2, characterized in that the support is a film support. 4. Radiographic product in accordance with any of the resale cations 1 to 3, characterized in that the support is a transparent film holder tinted in blue. 5. Radiographic product in accordance with any of Claims 1 to 4, characterized in that the tabular silver halide grains have an average shape index of at least 12: 1. 6. Radiographic product in accordance with any of Claims 1 to 5, characterized in that the tabular grains of silver halide represent at least 70% of the total projected surface of the silver halide grains. 7. Radiographic product in accordance with any of Claims 1 to 6, characterized in that the dispersion medium consists of a hydrophilic colloid capable of being tanned. 8. Radiographic product in accordance with any of the Claims 1 to 7, characterized in that the silver halide is silver bromide or silver bromoiodide. 9. X-ray product according to claim 1, characterized in that the spectral sensitizing dye exhibits a color change as a function of the adsorption. 10. X-ray product according to claim 9, characterized in that the spectral sensitizing dye is a dye of the cyanine type. 11. Radiographic product in accordance with any of the Claims 8 to 10, characterized in that the spectral sensitizing dye changes in color as a function of adsorption and is adsorbed on the surface of the tabular grains in an amount sufficient to sensitize the tabular grains optimally. 12. Radiographic product in accordance with any of Claims 1 to 11, characterized in that the tabular grains have an average shape index of between 20: 1 and 100: 1. 13. Radiographic product in accordance with any of the Claims 1 to 12, characterized in that the sensitizing dye is a dye of the type exhibiting a change in bathochromic color as a function of adsorption. 14. X-ray product according to claim 13, characterized in that the cyanine type dye comprises at least one nucleus chosen from the quinolinium, benzoxazolium, benzothiazolium, benzoselenazolium, benzimidazolium, naphtooxazolium, naphthothiazolium and naphtoselenazolium nuclei. 15. X-ray product according to claim 14, characterized in that the cyanine type dye is a carbocyanine. 16. Radiographic product in accordance with any of Claims 11 to 15, characterized in that the sensitizing dye is present in a monolayer on 25% to 100% of the surface of the grains of bromide or silver bromoiodide. 17. Radiographic product in accordance with any of Claims 1 to 16, characterized in that the spectral sensitizing dye is a spectral sensitizing dye for green.