Linearity of Electronographic Emulsions

Linearity of Electronographic Emulsions M. J. SMYTH and P. W. J. L. BRAND Department of Astronomy, Uniuersity of Edinburgh, Royal Observatory, Edinburgh, Scotland

INTRODUCTION The major advantages of the electronographic image tube in astronomy are the approximately linear relationship between density and exposure, and the large dynamic range. An application in which these advantages deserve especially t o be exploited is in the direct photometry of star-fields and nebulae. The conventional photoelectric calibration of star-fields photographed with modern wide-field cameras is very costly in telescope time, especially a t the faint end of the magnitude scale, and indeed, adequate photoelectric standards for the full control of the photographic characteristics are frequently lacking. Thus the possibility of measuring photoelectrically faint star sequences by means of electronographic image tubes, leaving to telescopes using photomultipliers only the task of establishing as standards bright stars in each field, is of considerable economic importance. Such use of image tubes has been reported by Lallemand, Canavaggia, and Amiot,l and by Walker and Kron.2 I n a preliminary study of some of the instrumental problems involved, the uniformity and linearity of response of a Spectracon3 mica-window image tube, already in use a t the Royal Observatory, Edinburgh, have been investigated. The useful size of the mica window is 5 m m x 25mm, but it is hoped that tubes with circular windows, more convenient for star-field photometry, will become available. The nuclear emulsion used is Melinex-backed Ilford G5, thickness 15 pm, which should absorb all incident electrons; the developer is Kodak D-lgb, equivalent t o Ilford ID-] 9. The tube is normally operated a t 40 kV. CATHODE UNIFORMITY Makers and users of image tubes appear to have avoided the topic of cathode uniformity, which is clearly important in spectrophotometry, P.E.1.D.-B

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M. J . SMYTH AND P.

W. J. L.

BRAND

and especially so in star-field photometry. Typical manufacturers’ tolerances are f 20%. The antimony-caesium (S.9) cathode of the tube investigated was uniformly illuminated with light in a fairly narrow wavelength band near 5000 8. As expected, microdensitometer traces across the images of the whole cathode showed markedly non-uniform density, the extreme range being & 25%. A considerable area near the centre of 10%. An isodensity map of the image is the cathode lay within shown in Fig. 1, the contour interval representing about 10%. Responsibility for the non-uniformity may be divided between the cathode and the mica window; but the much more uniform appearance of out-of-focus electron images indicates that the cathode is chiefly responsible.

x 25-mm image of uniformly exposed Spectracon cathode. Mean density 0.9, contour interval 0.08.

FIG.1. Isodensity map of 5-mm

Correction for local cathode response will be a disagreeable feature of star-field photometry, although Lallemand, Canavaggia, and Amiotl reported its effect to be negligible in their experiments. Moreover the non-uniformity may vary with time, and also with ~ a v e l e n g t h . ~

LINEARITY OF RESPONSE There has been disagreement among various workers concerning the linearity of the relationship between density and exposure for electronographic image tubes. A simple “single-hit” theory5 predicts a relationship of the form D = D, (1 - e-AE), where E is exposure, D, is the saturation density for the thickness of emulsion that is penetrated by the incident electrons and A is the average area of a grain. For D 2 D,/4 the ( D , E ) relationship reduces t o a linear law

D

=

D,AE,

with less than 12% error (erroneously quoted as less than 5% error by Valentine5 and by Walker and Kron2). Burge, Garrard, and Brownes

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have found that the (D, E ) response for emulsions exposed in an electron microscope falls below the “single-hit’’ relationship in a way that depends on the electron energy. They have satisfactorily represented the experimental results by a model in which the number of grains made developable per unit, electron track length increases towards the end of the track. The (D, E ) relationship for the Spectracon must be complicated by the fact that the electron stream after penetrating the mica window is no longer monoenergetic, as was pointed out by Duchesne.7 Experimental findings for the linearity of the (D, E ) relationship for electronographic image tubes range from linearity to D 2 4.9 on Ilford L4, D 2 5.1 on Ilford K5 and D 2 4.1 on Jlford G5, quoted by Walker and Kron,2 to marked non-linearity for D 5 1 on Ilford G5 with certain developers, quoted by Duchesne.’ A possible source of disagreement is the method of microphotometry, through its influence on the Callier coefficient, but in fact most workers have used the same instrument, namely the Joyce Loebl microdensitometer. I n view of these disagreements, a careful study of the (D, E ) relationship for our Spectracon under working conditions was undertaken. As explained, the entire cathode was uniformly exposed to light from a highly stabilized tungsten lamp behind filters restricting the wavelength to a fairly narrow band near 5000 A. Up to eight exposures on a single strip of G5 emulsion could be made, only the exposure duration being varied, with a minimum of 10 sec. The order of exposures was occasionally altered, t o remove any effect of lamp drift or of cathode fatigue, but there was no evidence of such effects. The films were developed in Kodak D-19b, normally for 5 min a t 2OoC, and were fixed for twice the clearing time. The films were measured on a Joyce Loebl Mark IIICS microdensitometer, using an optical wedge with a working density range of 0 to 3, so that the zero had to be offset only once when measuring high densities. The reading precision was 0.01 in density. It proved necessary to calibrate the wedge, since departures from a linear relationship between pen displacement and density appeared t o be higher than the manufacturer’s calibration graph suggested. A preliminary calibration was produced by varying the width of the “uniformly” illuminated microdensitometer slit. The variation of gradient along the length of the wedge was next found by scanning across a narrow filter strip of density about one-tenth of the working range, and using the zero control to shift the base-line of this density increment to successive positions along the wedge. Combination of these calibrations yielded a smooth correction curve, with a cumulative amplitude of nearly 0-04in density, to be applied to the linear pen-displacement/density

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M. J. SMYTH AND P. W. J. L. BRAND

relationship. Other optical wedges were found to depart from linearity both by greater and by lesser amounts. Users should therefore take account of possible non-linearity of optical wedges, especially when low-gradient wedges are used and the zero has to be offset several times on measuring high densities. Films were measured emulsion-up, i.e. towards the scanning slit, with an aperture of 1 mm x 0.1 mm, in order to suppress somewhat the local cathode variations. Great care was taken that scans across each exposed strip referred to identical locations on the cathode. It might be argued that such a large scanning aperture would include undetected pinholes; but in fact the scans across the entire cathode allowed the

FIG.2. Density-exposure relationship for a Spectracon. Ilford G5 emulsion, developed 6 min in Kodak D-19b at 2OOC; mean curve for four batches.

“continuum” density to be easily recognized. As a check, scans with

0.1 mm x 0.1 mm aperture, and also scans emulsion-down, showed no

significant differences from the normal scans. Large areas were exposed so that scattered light from nearby clear areas could not reach the measuring slit and influence the readings for high densities; this had been a source of considerable difficulty when we were measuring smalI density patches such as used by other workers. Five batches of Ilford G5 emulsion were tested, ranging in age from a few months to two years, and were developed together. Allowing for a slightly higher fog-level on the older films, the ( D , E ) curves for four of these batches were astonishingly consistent. Figure 2 shows the mean curve for these four batches. The deviations of the four individual curves from the mean were within f 30/,, while the sensitivity of the

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fifth batch, of intermediate age, was 26% higher. Thus batch-to-batch variations of the nuclear emulsions tested were not large, and smaller than might be found with photographic emulsions; indeed, the sensitivity of the three 1968 emulsions tested was within f 2.5%. These variations are considerably smaller tfhan those found by Burge, Garrard, and Browlies within a single batch, which may be attributable to differences in development. Furthermore, we found no evidence of non-uniformity on individual pieces of G5 emulsion; identical adjacent exposures, separated by 7.5 mm, agrecd within 1%. By cont,rast, Duchesne7 has reported variations of 10% over distances of 5 mm.

E kec)

FIG.3. Deiisity-cxposuro relationship for a Spectracon. Ilford G5 emulsion, developed 3 inill ill Kodtlk D-19b at 25"C, with 40 kV axid 32 kV tube potentials.

The shape of the (D, E ) curve shown in Fig. 2 was unexpected and unwelcome. Instead of an init,ial linear portion followed by an approach to saturation, we found an initial curvature that decreased with density. On some films the nearly linear portion a t higher densities extended well beyond D = 5 . There appeared to be no range of low density over which a linear characteristic might be assumed, and special lowdensity exposures measured with a wedge of low gradient confirmed this; the proportional error arising from a linear approximation would indeed be greater a t low densities. Further exposures on one battch of G5 were developed for 5 min at 25°C instead of 20°C. The mean (D, E ) curve for two such sets of exposures is shown in Fig. 3, together with that for a single set,

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M. J. SMYTH AND P. W. J. L. BRAND

similarly developed, with the image tube potential a t 32 kV instead of 40 kV. The 32 kV curve can be made to coincide almost exactly with the 40 kV curve by compressing the exposure axis by one-third. Furthermore, the curve of Fig. 2, for development a t 20°C, can also be made to coincide by compression of the exposure axis. Thus, within E ) curve has a characteristic the limits of experimental error, the (D, shape that is unaffected by dropping the tube potential from 40 kV t o 32 kV or by increasing the development temperature from 20 to 25”C, and does not vary from batch to batch of emulsion. If further tests with other developera and with other Spectracons show that this characteristic remains unaffected, we shall a t least be in the position of having a repeatable, though non-linear, (D, E ) calibration. The non-linearity coefficients as defined by Duchesne7 are approximately a, = 0.1 a t D = 1, and a2 = 0.15 a t D = 1.5, for the G5 emulsion in our tests. The characteristic curve found is curved a t low densities and becomes nearly linear for densities from 3 to beyond 5. It can be represented as the sum of a linear term extending to D > 5 and a “single-hit’’ term corresponding to D,-0.6; these terms having similar coefficients. Explanations can be invented that involve a thin surface layer saturated by the “tail” of low-energy electrons, but these do not bear quantitative examination. Detailed models for the (D, E) response of the Spectracon should await further investigation of the parameters that can influence the response curve.

CONCLUSION I n spectrophotometry, and especially in star-field photometry, using E) the mica-window image tube, both cathode non-uniformity and (D, non-linearity a t low densities must be taken into account. It is not known whether the cathode variation of f 25% found is typical; and we do not a t present have a detailed physical model for the (D, E) response curve. ACKNOWLEDUMENTS Wo are indebted to Professor H. A. Briick for his continuing intorest and support: to Professor J. D. McGee and his staff at Imperial College for supplying tubos and for advice and assistancc; and to Professor D. W. N. Stibbs for tho use of the Joyce Loobl Jsodensitracer a t the University Observatory, St. Andrews.

REFERENCES 1. Lallemand, A., Canuvwggia, R. and Amiot, F., C.R. A d . Sri. 262B,838 (1966). 2. Walker, M. F. and Kron, G. E., Publ. Aetron. SOC.Pacij. 79, 551 (1967). 3. McGee, J. D., Khogali, A., Ganson, A. and Baum, W. A., I n “Advances in

Electronics and Electron Physics”, ed. by J. D. McGee, D. McMullan, and E. Kahan, Vol. 22A, p. 11. Academic Press, London (1966).

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4. Vreux, J . M. and Maretto, G., J. Sci. Insfrztrn. 1, 668 (1968). 5 . Valentine, R. C., I n “Advances in Opt!ical a.nd Electron Microscopy”, ed. by R’. Bttror and V. E. Cosslet,t,,Vol. 1, p. 180. =\cntlemic Piwss, London (1966). 6. Burge, R.E., C:n,rrard, D. F. ant1 Hrowitt, M . ‘l’., J . Sci. f ) / , y t r t o n . 1, 7 0 7 (1968). 7. Duchcsno, M., J . Obserowtei~rs,60, 123 (l!IM). L)ISCIJSSION W. A. HAUM: WC would doubtless d l agrw t,hnt>a ciepart.ure from linoarity is related t o tho depletioii of available grains a,s t,he exposuro builds LIP, ant1 t,hrLt this depletion may havc a dependence upon t1ctpt)h in t8hecrmiilsion, as yoii have proposod. Hou7evor, on a simpler (and probably intzccrir;tto) iissrimpt,ion of 7 4 n i j o ~ r ndeplat’ionwith depth, can you sn,y what, form t’hectnparturc from linearity would t>henbe expected to take? The slim of scvcral such curves wit,h differing abscissa scales should then describe t8hesunirrietl cffcct of clifferont rates of grain tlcplot,ion a t diffuront depths in the emulsion. I n t,hese trims, t,htt considerii.l)lo length of tho straight segment in the upper half of your curve seems puzzling. M. J. SMYTH: One can mathomat.ically fit, t,ho charact,oristic curve found, witjh a combination of two “single-hit” terms representing a low and a high saturation clonsity, but t,he values of the parameters required are not. physically plitusi ble. It, is evident from work on exposure to electJrons(Ref. 6 ) that the response even to monoenergetic electrons implies a more complex effect than the “single-hit” law. w. L. WILCOCK: Is it your opinion that R calibrat.ion curvo needs to be determined for each sample of emulsion? M. J. SMYTH: As a precaution, yes. The present, liiriited experience does suggest that the (D, E ) relationship is constant apart, from scaling of t,he E-axis; but i t would be unsafe t,o depend on this for future samples. J. Y. MCGEE: Comment on question by Professor Wilcock: tJhere is evidence that different batchos of G5 do vary in chiiractJerist)ics. 1suggost, t’hat t,he onorgy dist,ribution of electrons passing through the mica helps t.0 give a morc uniform activation of the emulsion layer. A. A . MILSOM: Work don0 a t the Royal Greenwich Observatory confirms the shape of the characteristic curve shown. s. JEFFERS: Have you looked for any correlation betwwan the shape of tho charact,eristic curve and the onorgy of t,he incident electrons? M. J. SMPTH: The one chango of tubo pot,ential reported did not appear to change the shapo of the charact,eristic curve, hut further experiments are needed. w. N. CHARMAN: Havo you looked at cross-scctions of the emulsion to determine the distribution of developed grains through tho thickness of the emulsion as a test of your explanation of the lack of emulsion linearity? M. J. SMYTH: We are not equipped to do this at’present.