E. T. LARSON, F. W. H. MUELLER AND H. H O E R L ~ N
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Vol. 57
SENSITIZATION OF X-RAY EMULSIONS BY VERY SMALL QUANTITIES OF A LEAD SALT1 BY E. T. LARSON,F. W. H. MUELLERAND H. HOERLIN Ansco Research Laboratories, Binghamton, New York Received August id, 1066
Divalent lead ions were coprecipitated with the silver bromide crystals of a photographic emulsion which was made with a relatively inert photographic gelatin. The average grain size and the width of the grain size distribution decreased with increasing lead concentration; 0.2 mole per cent. lead (based on silver ion) produced an extreme reduction in grain size and photogra hic sensitivity to X-rays and light, thus setting an upper limit to the concentrations which are of ractical importance. f e a d concentrations of about 0.05 mole per cent. increased the X-ray sensitivity of ripened emukons 80%; the effect was only slightly dependent on X-ray energy, being least for high energies. For unripened emulsions, lead concentrations of 0.001 to 0.04 mole per cent. increased’the X-ray sensitivity about 40% for 10- and 65-kv. X-rays and a negligible amount for radium gamma rays. The sensitivity to light of the ripened emulsions was unaffected by lead in concentrations up to 0.04 mole per cent. The unripened emulsions exhibited unusual reci rocity law characteristics which were extremely susceptible to change with age and with the addition of lead. It is postuyated that the effect of lead in increasing X-ray sensitivity is due to the increase in the number of silver ion vacancies in the crystal lattice, the vacancies acting as traps for positive holes, and/or that increased silver ion mobility results from the silver ion vacancies which are generated in number equal to the number of lead ions. Preliminary tests indicate that divalent cadmium ions produce similar increases in X-ray sensitivity.
Introduction The use of metal ions to sensitize silver halide photographic emulsions has been known for many years, and today there is a continuing use of metal sensitization, in one form or another, to achieve sensitization effects of practical importance in
processes will provide further insight into the basic Problem. of .latent image formation. With the latter view 1n mind, the present study has been undertaken of the effects of adding divalent lead ions to a silver bromide emulsion, the lead ions being added to a silver nitrate solution before pre-
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modern commercial emulsions. Although a more precise understanding of the mechanism of photographic sensitivity in general might be considered as a prerequisite to explaining the observed effects of metal sensitization, it is also logical to expect that careful studies of particular sensitization
cipitation in the presence of an excess of bromide. This method of sensitization, which was found by Mueller in 1936,2 is of rather unique interest because it was described as resulting in increased sensitivity to X-ray exposures and decreased sensitivity to light.
(1) Paper presented at “Syniposium on Impurity Phenomena” a t the Research Laboratory, General Electric Company, Schenectady, New Pork, June 16-18,1953.
(2) F. W. H. Mueller, Agfa Film Plant, Wolfen, Germany (19361, Bibliography of Scientific and Industrial Reports (U.S.Department of Commerce, Washington) 8, 873,PB-70053, fr. 841-50.
SENSITIZATION OF X-RAYEMULSIONS BY LEADS A L T
Nov., 1953
This paper is presented as a report on the progress made to date in defining the effects of lead sensitization in a simple silver bromide emulsion. Although these results provide a rather limited basis for speculation as to the mechanism involved, it is believed that they are of definite value in plotting the course for further work. Also, in view of the many recent reports in the literature on the effects of divalent cations on such properties of silver halide macro crystals as the electrolytic conductivity,8-S optical a b s o r p t i ~ n ,and ~ photoconductivity,’ it is possible that the interpretations of these phenomena might be extrapolated to explain the effects obtained with the micro crystals of photographic emulsions. Experimental Details In order to keep the system under investigation as simple as possible, a simple, boiled-type, silver bromide emulsion was used, containing no iodide or chloride. The lead ion was added as Pb(N03)~to a solution of AgNOa before precipitation. I n order to minimize the effects due to sulfur sensitization the emulsion was made with a so-called photographically inert gelatin. After precipitation in an excess of KBr, the emulsion was washed, ripened, and coated 20 microns thick on one side of X-ray film base without the addition of any chemical stabilizers. After exposure the films were developed for 4 minutes in Ansco Liquadol, a commercial X-ray developer, followed by a short stop bath, fixation, washing and drying in the normal manner.
Results
A. Grain Size.-It is well-known that the size distribution of the grains in a silver halide emulsion depends markedly on the conditions under which precipitation takes place. The effect of lead ions on grain size is demonstrated in Fig. 1. Detailed study of this effect has not been made, but these curves, based on measurements of 300 to 500 grains of each emulsion, serve to indicate that with increasing concentrations of lead both the average grain diameter and the width of the distribution curve decrease. (The curves also indicate that the relative frequency of very large grains increases with lead concentration up to about 0.04 mole per cent., but this observation is only tentative, pending more extensive grain counts to establish the curve shapes more accurately.) As expected, the decrease in grain size produces an increased slope in the characteristic curves of exposed and developed emulsions containing lead. These curves also provide information about the upper limit of concentrations of lead ion which are operable for sensitization. As might be expected, the marked decrease in grain size a t 0.2 mole per cent. of lead ion is accompanied by greatly reduced photographic sensitivity. It is therefore apparent that in order to avoid the extreme effects of reduction in grain size the concentration of lead ion should be less than 0.2 mole per cent. (3) E. Koch and C. Wagner, Z.physik. Chsm.,88B,295 (1937). (4) J. Teltow, Ann. Physak, [6] 6, 63, 71 (1949). (5) S. W. Kurnick, J . Cham. Phys., 20, 218 (1952). (6) 0. Stasiw, Ann. Physik, [6] S, 151 (1949). (7) W. West, in “Fundamental Mechanisms of Photographic Sensitivity,” Butterworth’s Scientific Pitblications, Ltd., London, 1951, pp. 99-111.
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B. Effects of Ripening.-One additional fundamental factor which must be considered is the degree to which sulfur or other chemical sensitization is present. Although the gelatin used in making the emulsions is considered relatively inert in comparison with ordinary sensitizing gelatins, the effects of heat treatment after precipitation and washing-called after-ripening-were qualitatively similar to those obtained with a sensitizing gelatin, as shown in Fig. 2. These curves are for an emulsion which contained no lead. The net fog densities for the three stages of ripening are 0.03, 0.04 and 0.30. These results agree with the well-known observation that after-ripening is more effective in increasing light, sensitivity than in increasing X-ray sensitivity. For exposuTes to X-rays of lower energy the ripening is even less effective. Although the magnitudes of the effects due to after-ripening are much smaller than those usually obtained with sensitizing gelatins, it is not possible to say, on the basis of these results, that chemical sensitization is completely absent in the ripened emulsions. If it is assumed, after Lowe, et u1.,8 that the absence of chemical sensitization is indicated only if the characteristic curve is unaltered by the after-ripening of the emulsion, then it must be concluded that chemical sensitization has taken place in the ripened emulsions to some extent. On the other hand, such a restricted definition has the disadvantage that it does not admit the possibility that the changes produced by the after-ripening may be due to an annealing process involving structural changes in the grains which are not dependent on the presence of surface sensitizers. For these reasons it was considered necessary to study both unripened and ripened emulsions. C. Sensitivity to Light.-Further information on the effects of ripening was obtained during the course of measuring the variation in sensitivity to light as a function of lead concentration. It became evident that the effect of the lead in the unripened emulsions was strongly dependent on the exposure time, as shown in Fig. 3. These curves indicate the relative exposure required to produce a net density of 0.5 as a function of exposure time and will be recognized as one of the methods used to indicate reciprocity law failure. Considering first the curve for the emulsion containing no lead, three well-defined regions are apparent. In the range from about 0.0005 to 0.1 sec., sensitivity decreases with increasing exposure time (decreasing intensity). This is, therefore, a manifestation of low intensity reciprocity law failure. For the range 0.1 to 100 sec. the curve has a reduced slope. From 100 sec. to 15 hr. the slope gradually increases. Considering next the effect of addition of lead, it is seen that 0.005 and 0.01 mole per cent. of lead produce a marked peak centering about 1 see. exposure time. At short exposure times the speed is decreased; at long times it is increased. Further additions up to 0.04 mole per cent. decrease the magnitude of the hump in the center portion of the curve and increase the speed a t very short and very long exposure times. (8) W. G. Lowe, J . E. Jones and H. E. Roberts, ibid., pp. 112-125.
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E. T. LARSON, F. W. H. MUELLER AND H. HOERLIN
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Investigation of the characteristic curves of these emulsions for the three regions of the reciprocity curves delineated above showed that in each region the characteristic curve has a distinctive shape, as shown in Fig. 4. For very short exposure times the curves exhibit a high maximum density and
gradient. At intermediate times the curves are extremely flat. At longer times the curves have a higher gradient but the maximum density is greatly reduced. I n this region, over-exposure produces a reversal effect. The curves of Fig. 3 and Fig. 4, which are drawn
SENSITIZATION OF X-RAYEMULSIONS BY LEADSALT
Nov., 1953
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E. T. LARSON,F. W. H. MUELLERAND H. HOERLIN
Vol. 57
exposures, lends further support to this view. On 'the other hand, the characteristics of the highly-ripened emulsions follow n regular pattern and permit tlie conclusion that, in the concentration range studied, lead ion has very little effect on light sensitivity in a pure bromide emulsion. The small differences in the curves of Fig. 5 are of the same magnitude as the experimental errors. D. Sensitivity to X-Rays.In contrast to the data on light sensitivity, the analyses of the data for X-ray'exposures present a more consistent pattern. The curves of Fig. 6 show how the X-ray sensitivity of the unripened emulsions varies with lead concentration, for X-rays of various energies. The speed values plotted for each X-ray energy were measured relative to the speed of the emulsion containing no lead. Although there is considerable scatter among the plotted values, which .o0 I .oI .I 4 were taken from the data for Pb." CONCENTRATION - mol X. several sets of emulsions, the Fig. 6.-X-Ray sensitivity curves for unripened emulsions with various amounts smooth curves which have been of lead. drawn are considered sufficiently representative to permit the folproduced by after-ripening treatments, although of lowing observations: In the concentration range much smaller degree, as indicated by similar studies from 0.001 to 0.05 mole per cent. of lead, the sensifor slightly- and highly-ripened emulsions. tivity to 10- and 65-kv. X-rays is increased about Figure 5 gives the reciprocity curves for the 50%. The effect is less at higher energies and is highly-ripened emulsions. The sensitivity of these practically absent for gamma rays. At 0.2 mole emulsions is about 10 times that of the unripened per cent. of lead the sensitivity for all energies is emulsions. The reciprocity curves do not exhibit markedly decreased, a result which is expected on the unusual variations found with unripened emul- the basis of the grain size data. Preliminary data sions, and the characteristic curves for all exposure taken a t lower concentrations than those presented times were essentially the same as those at very here indicate that the curves are nearly flat down mole per cent. of lead. short exposure times for the unripened emulsions. to Similar studies of the slightly-ripened emulsions Figure 7 presents the corresponding results for showed that they had characteristics intermediate emulsions which were slightly ripened (net fog of to the extremes represented by the unripened and 0.04) and highly ripened (net fog of 0.30). For highly-ripened emulsions. both sets of emulsions the speeds given are relative In analyzing these results it is immediately to the speed of an emulsion containing no lead but apparent that they indicate the necessity for a well- ripened to the same degree as those containing defined system in measuring the effects of lead on lead. The fact that the two sets of data conform light sensitivity. So far, no explanation has been to the same pattern indicates that, for exposures to found for the unusual reciprocity characteristics of X-rays, the effects of the lead are essentially equal the unripened emulsions. Tentatively, it is sug- for the 2 degrees of ripening. These curves differ gested that, because of the very low sensitivity of from those for the unripened emulsions in several these emulsions, they exhibit, even at moderately respects. First, the increase in speed due to adding high intensities, irregularities such as decreased less than 0.04 mole per cent. lead is generally engradation and reversal which have been reported hanced by the ripening process. Second, the dein some instances for more sensitive emulsions at pendence on X-ray energy is less than for the unvery low intensities. This approach would indicate ripened emulsions, although the effect of adding that more nearly reproducible results for the effect lead again decreases as the X-ray energy increases. of lead ion would be found at shorter exposure times For practical purposes the sensitization is indethan those used here. The fact, to be seen later, pendent of wave length. Third, in the range of that more consistent results are obtained with X- concentrations below 0.05 mole per cent. the senray exposures, which are essentially high-intensity sitivity increases with increasing amounts of lead.
Nov., 1953
SENSITIZATION OF X-R,AYEMULSIONS BY LEADSALT
807
of the order of one grain diameter, and a high rate of energy loss per unit path length. The passage of such a low-energy primary electron through a grain will therefore result in the formad tion of a very large number of secondary electrons which, in most cases, will form sufficient 0 latent image to render the grain Iw developable. On the other hand, z the range of the primary elecItron due to the absorption of z1 a high-energy X-ray quantum will n be much greater and the specific w W ionization along most of the path a will be relatively low. Such an VI electron will produce only a relaw tively small number of electrons Iin many of the grains through 6 which it passes. -I w If it is agreed that the overlz all effect of ripening is to increase the efficiency with which conduction electrons are trapped at the surface of the grain and converted into latent image, it is clear that ripening will be more effective for exposures to highenergy X-rays, where the average number of conduction elecFig. 7.-X-Ray sensitivity curves for ripened emulsions with various amounts of trons per grain is less than for lead. low-energy X-rays. The data for the emulsion containing lead It should be pointed out that further data in the follow very nearly the same pattern as those for range from 0.05 to 0.2 mole per cent, will be neces- no lead, except that the speed is increased about sary to establish the exact shape of the curves in 50% more when lead is present than when it is this region. The marked decrease GI sensitivity a t 0.2 mole per cent. is the same as for the unripened emulsions. Experiments to extend these results to concentrations lower than 0.001 mole per cent. have so far not yielded reliable results. Further consideration of the effect of ripening on lead sensitization is given in Fig. 8, in which the increase in sp3ed clue to ripening is plotted a t various X-ray wave lengths for emulsions containing no lead and 0.04 mole per cent. lead. Considering the data for no lead, it is immediately apparent that ripening is more effective for high-energy than for low-energy X-rays. This is the expected result, as is seen from a consideration of the exposure mechanism for Xrays. In exposures to lowi energy X-rays, the primary .oi .i 1 electron produced by the abE F F E C T I V E A I N a. sorption of an X-ray quantum Fig. &-Effect of ripening on X-ray sensitivity of emulsions with no lead and 0.04 has a relatively short range, mole per cent. lead.
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E. T. LARSON,F. W. H. MUELLER AND H. HOERLIN
not, an effect which is independent of wave length (disregarding the slight lack of parallelism of the two curves at short wave lengths). In short, the effects of lead sensitization and ripening are each enhanced by the presence of the other form of sensitization.
Discussion
VOl. 57
ing the increase in silver ion Xracancies. It is postulated that increased mobility of silver ions would result in more rapid neutralization of electrons at surface traps so that fewer electrons would be repelled from the surface traps by the presence of unneutralized electrons. Some support for the second proposal is found in the fact that small amounts of lead and cadmium ion are known to increase the electrolytic conductivity of silver bromide at elevated temperatures. Preliminary tests of emulsions made with cadmium substituted for lead indicate that the increases in X-ray sensitivity produced by lead are also found with cadmium. Acknowledgment.-The authors wish to tliank Mr. W. D. Kelly, Jr., for preparation of the emulsions and Messrs. D. P. Jones and J. F. Ruddy for their help in the sensitometric testing.
In attempting to formulate a mechanism to explain the above effects the following considerations have been made. 1. In order to explain the increase in X-ray sensitivity a mechanism must be found which is effective under the conditions peculiar to X-ray exposures, namely, the production of a large number of electron-hole pairs in a crystal in a very short interval of time. Under such conditions the utilization of the available electrons is probably inefficient for either or both of two reasons-recombination with positive holes and trapping at internal sites. DISCUSSION 2. One suggested mechanism is that of increased hole trapping by silver ion vacancies which are E. J. BIECEK (Universal Oil Products Co.).-How imporgenerated in number equal to the number of diva- tant is the greater X-ray absorption by lead compared with silver? The atomic numbers are considerably different. lent lead ions added to the crystal. 3. A second, and perhaps concurrent, mechaE. T. LARsoN.-The amounts of lead that we added here nism is that of increased ionic mobility accompany- will not affect the absorption coefficient appreciably.
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