JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Reciprocity Law for X-Rays.* Part II: Failure in the Reversal Region

MARGARETE EiILICHRadiation Physics Laboratory, National Bureau of Standards, Washington, D. C.

(Received May 4, 1956)

Experimental data are presented that show the reversal behavior of the characteristic curves for one

commercial x-ray film as a function of x-ray exposure rate and establish the presence of reciprocity-lawfailure and intermittency effect in the reversal region. Under the present experimental conditions, lower

exposure rates of x-radiation are seen to favor reversal while the opposite is shown to be the case for visible

light. Earlier work with different types of emulsions and under different developing conditions mentionsan effect with visible light similar to the one here obtained with x-rays; however, a satisfactory interpretationof the basic mechanisms involved is still lacking and further experiments are required to establish the

connection between the effects of x-rays and visible light.

INTRODUCTION

RECIPROCITY-LAW failure in the reversal regionusually manifests itself in a dependence of film

sensitivity, maximum attainable density, and shape ofthe characteristic curves on the intensity of the radia-tion employed for exposure. Webb and Evans investi-gated the phenomenon with visible light.' They foundthat for their particular film type, and for their ex-posure and processing conditions, the total exposurerequired for reversal decreased with increasing lightintensity, and that the peaks of the characteristiccurves became sharper. They did not observe a changein maximum film density. On the other hand, Maurerand Yule, who investigated the reversal region of adifferent film type, using higher light intensities and adifferent processing system, found that the totalexposure required for reversal as well as the maximumdensity attainable increased with increasing light in-tensity.2 These findings were in agreement with theresults obtained earlier by Sauvenier.3 Maurer andYule also found some evidence that with an emulsionsimilar to that previously employed by Webb andEvans, they could obtain either their own results orthose of Webb and Evans, according to the intensityrange employed for the exposures and the type ofdeveloper used. However, since their primary interestlay at the time elsewhere, they did not investigate whenand how the one type of behavior changed over into theother one.

As Maurer and Yule pointed out, it seems that in thepresent state of our knowledge, a generalization aboutthe reciprocity-law failure of emulsions showing reversalwould not be in order. Nevertheless, such a generaliza-tion may have very well been the cause for assumingstrict validity of the reciprocity law for x-rays, althoughthe reversal region of the characteristic curves obtainedwith x-ray exposures had never been studied as a

* This work was supported by the U. S. Atomic Energy Com-mission and the U. S. Army Signal Corps, Fort Monmouth, NewJersey.

I J. H. Webb and C. H. Evans, J. Opt. Soc. Am. 30, 445 (1940).2 R. E. Maurer and J. A. C. Yule, J. Opt. Soc. Am. 42, 402

(1952).3 H. Sauvenier, Bull. Soc. Roy. Sci. Lifge 15, 418 (1946).

function of x-ray exposure rate. It is the purpose ofthis paper to present experimental proof for thepresence of reciprocity-law failure and intermittencyeffect in the reversal region of a representative com-mercial x-ray film (Du Pont film type 502) and todiscuss briefly the theoretical implications of thesefindings.

THEORIES OF THE REVERSAL EFFECT'

In 1930, Trivellil published a review of the theoriesof the reversal effect and grouped them into regressionand progression (coagulation) theories. While newevidence has been collected since then in favor andagainst either group, it has not been possible, as yet,to decide between them, and none of the theories hasso far been generally accepted.

The most popular regression theory seems to be therebromination theory: The bromine atoms formed inthe silver halide crystal by photoelectric effect movethrough the crystal lattice as positive holes and,eventually, are released on the crystal surface. If, dueto high total exposure or due to high dose rates, thebromine accumulates in the vicinity of a crystal grain,it recombines with the photolytically freed silver onthe grain's surface. The result is a protective silverhalide layer, preventing the development of the grain;within the grain, the latent image remains essentiallyintact.

There is a considerable amount of evidence in favorof the rebromination theory, such as the experimentalfindings that reversal is most of the time confined tothe surface-latent image,' and is removed by bromineacceptors.' Additional evidence in favor of this theorywas given by the findings of Webb and Evans thathigher intensities of visible light favor reversal.'Furthermore, May7 interpreted his results of a com-

4For a comprehensive discussion and bibliography see G.Kornfeld and G. W. W. Stevens, The Theory of the PhotographicProcess (C. E. K. Mees, editor) (The Macmillan Company, 1954),revised edition p. 243 ff.

6 A. P. H. Trivelli, J. Franklin Inst. 209, 373 (1930).6 Berg, Marriage, and Stevens, J. Opt. Soc. Am. 31, 385 (1941).7 A. May, J. Opt. Soc. Am. 33, 81 (1943).

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VOLUME 46, NUMBER 10 OCTOBER. 1956

MARGARETE EHRLICH

o 2 INTERMITTENT

° ' Id II ! d Il || | | III Aill IL. III 1 1 1. I II 1 11EXPOSURE,

FIG. 1. Characteristic curves, x-ray exposures.

parison of the peak width for visible light and x-raysas evidence in favor of this theory.

According to the coagulation theory on the otherhand, the photolytic silver on the surface of the grain,originally produced in a finely dispersed ("active")form, is assumed to coagulate upon further exposureand to lose its ability to catalyze the process of graindevelopment. Usually quoted as in agreement with thisidea are the results of Eggert and Noddack8 and ofseveral later investigators, who found that, in thereversal region, the total amount of photolytic silverstill increases with exposure, while the amount oflatent image decreases. Kornfeld and Stevens state4

that, according to the coagulation theory, low-intensityexposures should favor reversal. If such a conclusion ispermissible, then the results of Sauvenier' and Maurerand Yule2 are in agreement with the coagulationtheory.

EXPERIMENTAL PROCEDURE AND RESULTS

The present experiment was performed on theDu Pont x-ray film type 502 exposed both to x-radiationfrom a beryllium window tube produced at 50 kv

4

RELATIVE EXPOSURE

FiG. 2. Characteristic curves, exposures to visible light. Thenumbers at the individual curves are the light intensities relativeto roughly 1.5 meter candles which was arbitrarily set equalto unity.

8 J. Eggert and W. Noddack, Z. Physik 20, 299 (1923).

constant potential and to the visible light of a 500-wattphotoflood lamp. The films were processed in a surfacedeveloper similar to that described by Stevens,9 butcontaining still less sodium sulfite (0.05% rather than0.1%). For both x-rays and visible light, severalsets of exposures were given on a time scale.The x-ray exposure rates varied roughly from 0.033r/sec to 1100 r/sec. All but one set of x-ray exposuresand certain exposures on the ascending branch of thecharacteristic curve obtained with the next-to-the-highest exposure rate, were administered continuously.At 1100 r/sec, both continuous and intermittent ex-posures were given in the region of the descendingbranch, the intermittent ones consisting of one burst-about ten microseconds in duration-repeated every0.6 seconds. Details of the setup and of the procedurehave been described earlier. 0 All light exposures wereadministered continuously at intensities roughly be-tween 1.7 and 1700 meter candles. The intensity wasvaried by means of a calibrated neutral-density steptablet.

4

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1- E 10 R R . 10/EXPOSURE RATE. ta...

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FIG. 3. Maximum density as a function of x-ray exposure rate.

Figures 1 and 2 show the families of characteristiccurves obtained in this way on the DuPont film type502 exposed to x-rays and to visible light, respectively,and processed in similar developing solutions. Thefollowing differences in behavior are apparent.

1. Reciprocity Law in Ascending Branches

While the ascending branches of the individual curvesobtained with x-rays are identical within the limitsof the experimental accuracy, the ascending branchesof the individual curves obtained with visible lightshift toward higher total exposure as the light intensityis increased. This confirms the validity of the reciprocitylaw in the ascending branch for x-radiation over theentire tested exposure rate range of 1:30 000, and itsfailure for visible light. (The shift toward smaller filmsensitivity for higher rates indicates that the ratesused for this study were higher than the optimum ratefor the particular film type.)

I G. W. W. Stevens, J. Phot. Sci. 1, 122 (1953).' 0 M. Ehrlich and W. L. McLaughlin, J. Opt. Soc. Am. 46,

797 (1956).

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RECIPROCITY LAW FOR X-RAYS. PART II

2. Maximum Density

The maximum density obtained with visible lightdepends very little on light intensity. The maximumdensity obtained with x-rays increases strongly withexposure rate up to what probably corresponds to thehighest density achievable under the particular pro-cessing conditions. This density is the same for bothvisible light and for x-rays. An intermittency effect isobserved, the intermittent exposure at the highestexposure rate behaving like an exposure administered ata rate corresponding to the one prevailing during theindividual bursts, averaged over the exposure time.Figure 3 shows the maximum density as a function ofexposure rate.t Here, as well as in Fig. 4, the informa-tion obtained from the intermittent exposures is plottedtwice (black dots): once at the peak exposure rate andonce at the average exposure rate, where it is seen tofit in well with the data obtained from continuousexposures.

3. Peak Width

As was observed before by Webb and Evans,' thewidth of the peak of the characteristic curves decreaseswith increasing intensity of the visible light. However,in the case of x-rays, the width increases with increasingexposure rate, in agreement with the findings ofMaurer and Yule.2

4. Reversal Exposure

Figure 2 shows that, for visible light, the "reversalexposure," here simply defined as the exposure corre-sponding to the onset of reversal, is independent ofintensity.t On the other hand, Fig. 1 indicates that, inthe case of x-rays, the reversal exposure increaseswith increasing exposure rate. Figure 4 is a plot ofreversal exposure as a function of x-ray exposure rate.t

5. Reciprocity Law Failure and IntermittencyEffect in Descending Branches

While the descending branches of the individualcurves obtained with visible light are superimposedwithin the limit of the accuracy of the experiment,$ thedescending branches of the family of curves obtainedwith x-rays are seen to shift toward higher exposures

t The information presented in Figs. 3 and 4 is extracted notonly from the set of characteristic curves of Fig. 1 and Fig. 2,respectively, but also from other sets obtained under similarexposure conditions but processed at different times. For instance,the points to which arrows are attached form a set that wasprocessed at slightly higher temperature than the rest, whichexplains their shift to somewhat higher densities.

t Unfortunately, the DuPont film type 502 is coated on bothsides, which increases the influence of spurious effects due tohalation. For this reason, the constancy of film sensitivity andtherefore of reversal exposure with intensity of visible light maybe due to a composite effect and cannot be interpreted as beingin disagreement with the findings of either Webb and Evans orMaurer and Yule.

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FIG. 4. Reversal exposure as a function of x-ray exposure rate.

as the exposure rate is increased (i.e., the reversal effectdecreases with increasing exposure rate) as was ob-served earlier by Maurer and Yule for visible light.2

The position of the curve obtained with intermittentexposure again corresponds to that of a curve obtainedat an average exposure rate. In Fig. 5, the exposurerequired for a convenient density (here chosen as 2.5)is plotted as a function of exposure rate, on a log-logscale. This type of representation furnishes the con-ventional reciprocity-law failure plot. A loop orientedsimilarly to that shown by Maurer and Yule forvisible light2 is obtained, with the only difference thatthe branch corresponding to the negative characteristiccurve is a straight line. The point at which the twobranches meet corresponds to the exposure rate atwhich the maximum density happens to be 2.5, andthus depends on the choice of the density for which theloop is plotted.

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MARGARETE EHRLICH

DISCUSSION OF RESULTS

The experimental data presented in this paper showfor the first time the reversal behavior of characteristiccurves obtained with x-rays as a function of x-rayexposure rate and establish the presence of reciprocity-law failure and intermittency effect in this region. Whilethe course of the change of reversal as a function oflight intensity agrees with the findings of Webb andEvans, and therefore with the rebromination theory, thecourse as a function of x-ray exposure rate (and thusof x-ray intensity) agrees with the findings of Maurerand Yule for visible light, and may be considered asevidence in favor of the coagulation theory. It is notpossible to decide from the present results whether bothtypes of behavior found with visible light are present for

x-rays, as well. The course of the positive reciprocity-law failure branch of Fig. 5 suggests a decrease offailure toward higher x-ray exposure rates; however, noexperiments have as yet been undertaken to determinewhether for very high exposure rates the failure simplydisappears (line parallel to exposure-rate axis in Fig. 5)or whether the slope of the line only changes sign.

The present findings in conjunction with those ofMaurer and Yule suggest that a careful study ofreciprocity-law failure in the reversal region could beinstrumental in solving the puzzle of the theory of thereversal effect. The studies should be carried out bothwith visible light and with x-rays or high-energyelectrons, and the rate intervals covered should bewider than those used before.

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