Electrochemical etching of fast neutron induced recoil tracks in cellulose triacetate

Nucl. Tracks Vol. 4, pp. 41-47 Pergamon Press Ltd. 1980. Printed in Great Britain

ELECTROCHEMICAL E T C H I N G OF FAST N E U T R O N I N D U C E D RECOIL TRACKS IN CELLULOSE TRIACETATE* R. B. GAMMAGEand A. CHOWDHURY'~ Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, U.S.A. (Received 26 July 1979; in revised form 19 December 1979) Abstract---Optimum parameters for electrochemical etching (ECE) of fast-neutron irradiated Kodak cellulose triacetate have been established. When etching in KOH solution at 25 ~C for 4 h, the optimum frequency is 3.4 kHz, the optimum voltage gradient is 13 kVcm-t, and the optimum concentration is 8N. The clearly visible etched tracks have a mean diameter of 180#m under these conditions. The optimum frequency and voltage gradient were obtained from differential leakage-current measurements made between unirradiated and fast-neutron irradiated foils in which the maximum differential leakagecurrent was sought. The lower limit of detectability is 170 mrad of fast neutrons. This is an order of magnitude too insensitive for practical personnel dosimetry. One of the limiting factors is a background track density of ~20 tracks cm-z. The threshold energy for track registration in cellulose triacetate has yet to be established experimentally but is expected to be less than that for polycarbonate.

that are chemically more reactive than polycarbonate (Gammage and Cotter, 1977). The intention was to produce an efficient ECE dosimeter able to detect fast neutrons with energies of 1 MeV or less. This communication outlines the continuing research carried out in this area. It also describes a methodology for pin-pointing, in a relatively easy manner, some important ECE parameters that have partial dependence upon each other. These parameters are frequency and voltage-gradient, and optimization is aided by making differential leakagecurrent measurements between undamaged and fastneutron irradiated foils.

1. I N T R O D U C T I O N FAST-NEUTRON dosimetry advanced significantly when it was demonstrated (Sohrabi, 1974; Tommasino and Armellini, 1973) that electrochemical etching (ECE) could amplify the size of etch pits associated with recoil-particle tracks in polycarbonate. This development eliminated the need for fissile radiators with their associated radioactivity. The technique showed sufficient promise so that there is now a commercial personnel dosimetry service available that utilizes this technique (Oswald and Wheeler, 1977; Wheeler, 1977). Although the sensitivity of electrochemically etched polycarbonate is sufficient to detect a dose equivalent of a few tens of millirem (Wheeler, 1977), there is a major drawback. The polycarbonate type of ECE dosimeter is too insensitive to record neutrons with energies much below about 1.5MeV (Griffith et al., 1976). Above 3 MeV, however, the response follows the dose-equivalent response quite closely (Wheeler, 1977). It was for the purpose of lowering the fast-neutron threshold energy that investigations were initiated on other types of plastic

2. EXPERIMENTAL P R O C E D U R E S An unmasked, clear cellulose triacetate (supplied by Eastman Kodak Chemical Products, Kingsport, Tennessee) had a nominal thickness of 380 #m. This plastic was irradiated with fission neutrons from the Health Physics Research Reactor at Oak Ridge National Laboratory. A description of the apparatus for electrochemical etching has been pub-

*Research sponsored by the Office of Health and Environmental Researcla, U.S. Department of Energy under contract W-7405-eng-26 with the Union Carbide Corporation. tlAEA fellow, on leave from the Bangladesh Atomic Energy Commission. 41

42

R . B . GAMMAGE and A. CHOWDHURY

lished previously (Thorngate et al., 1976). The maximum voltage-gradient and frequency that were applied were 26 k V c m - I and 3.7 kHz, respectively. The normality of the KOH solution at 25 °C was varied between 4 and 9.5. The differential leakage-current measurements employed two identical cells in parallel. One cell contained neutron-irradiated (50 rad), and the other cell unirradiated, circular foils of cellulose triacetate. In this experiment the KOH solution was replaced by tap-water, since the aim was to avoid etching during the conductance measurements. 3. EXPERIMENTAL RESULTS AND DISCUSSION Factors such as voltage gradient, frequency, concentration of the etchant, temperature, and etching time are major interdependent variables that can affect the size, shape, clarity, and number of etched, neutron-induced recoil-particle tracks. The number of background tracks, and hence the signal-to-noise characteristics, are also related to these parameters. It is a very tedious and frustrating exercise to attempt optimization by varying one parameter while keeping each of the others at fixed, initially arbitrary values. It was surmised by us that at the electric field and frequency for maximized conductance, the rapid "electric tree growth" (Somogyi, I 0,08

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1977), and the rate of etching within these zones should be maximized. Differential leakage-current measurements between undamaged and fast-neutron damaged foils of plastic should aid in locating this optimum voltage gradient and frequency. Experimentally, it was found that as the voltage gradient is reduced, a more distinct peak is obtained in the net leakage current at increasing frequency, as shown in Fig. 1. A maximum differential of 0.3V was obtained at 13kVcm 1 and 3.4kHz. Electrochemical etchings of irradiated (1.25 rad) foils were conducted at different voltage gradients and frequencies that produced the maximum shown in Fig. 1. Plotted in Fig. 2 are signalto-noise (S/N) values that represent figures of merit. The best S/N value was obtained at 13kVcm and 3.4kHz. This result vindicates the method of making differential leakage-current measurements to pinpoint the optimum voltage-gradient and frequency. Etching for 4 h with 8 N KOH at room temperature at 13kVcm 1 and 3.4kHz generally produced about 20 background tracks cm-2. The variation in the densities of background and recoil particle tracks with the voltage-gradient and the frequency is shown in Fig. 3. The third parameter to be optimized was the concentration of the KOH solution. The normality of the solution was increased in steps between 4 and

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The mean track diameter is shown in Fig. 4, as a function of the normality of the etchant; and in Fig. 5 the sensitivity is plotted in terms of track density per unit dose of fast neutrons. The track diameter increases monotonically with increase in concentration of the K O H , from 1 0 0 # m to more than 2 0 0 # m . In contrast, the sensitivity drops nearly three-lold between 4 and 7 N. Between 7 and 9.5 N, the sensitivity changes only slightly with a small m a x i m u m at 8 N . With respect to sensitivity, cellulose triacetate behaves differently from polycarbonate, the latter showing a general increase in sensitivity at increasing concentration of K O H (Gammage and Chowdhury, 1979).

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44

R . B . GAMMAGE and A. CHOWDHURY

The etch pits that develop electrochemically at low concentrations of KOH are imperfectly shaped and lack clarity of definition (Fig. 6(a)). The practical difficulty of identifying and counting these imprecise pits negates the advantage of the high sensitivity. At 7 N and above the etch pits are much more distinctive, visible to the naked eye, and easily counted at low magnification. Examples of the appearance of the etch pits are shown in Figs 6(b), (c) and (d). For practical purposes, 8N is recommended as the optimum concentration of KOH, since the tracks have excellent definition (Fig. 6(c)), adequate size for ease of counting (180/~m), and the sensitivity is at a maximum within the range of normality 7 to 9.5 N. The surface and internal structure of ECE pits in polycarbonate have been delineated by electronmicroscopic examination (Griffith et al., 1977). Similar structures are suggested by the optical micrographs of electrochemically-etched cellulose triacetate. The large, distinct calderas are indicative of cavitation having occurred below the surface, with survival of a thin surface-membrane. The basin-like depression at the surface is contained within a clearly defined rim. Two other parameters expected to affect the rate of development and size of pits significantly are temperature and time of ECE. Our studies were limited to fixed, but practically-convenient, values of room temperature (~25 °C) and 4h development time. The general conclusions of this study, however, should not alter significantly if other times and temperatures were selected. It is appropriate to mention two difficulties encountered with the Kodak cellulose triacetate, since both difficulties are detrimental to its use in fastneutron personnel dosimetry. According to the U.S. Nuclear Regulatory Commission guide (N8.14) issued in August 1977, a fast-neutron personnel dosimeter should measure dose-equivalents as low as 30mrem. Currently-used personnel dosimeters are inadequate for this and other requisite tasks. The dosimeter should, however, be reasonably accurate at the dose-equivalents of several hundred mrem used in intercomparison studies (Gilley and Dickson, 1979). The currently-used Kodak cellulose triacetate would produce a S / N ratio of 2 at a dose of fission-spectrum neutrons of 170mrad or dose*Bayer AG of Leverkusen, W. Germany.

equivalent of 1600mrem. The sensitivity to fast neutrons is, therefore, about an order of magnitude too low. The easiest way to obtain such a ten-fold improvement would be to identify a cellulose triacetate with 2 rather than 20 background tracks cm -2. The same type of inadequacy existed several years ago with polycar'bonate. The solution of the background problem came with the availability and use of Transilwrap polycarbonate (masked polycarbonate from Rohm and Haas) that gave background densities of less than 5 tracks cm -2 (Wheeler, 1977; Griffith et al., 1976). Perhaps there exists a commercially available cellulose triacetate with similar low background characteristics, although the authors are not presently aware of such a source. The other difficulty lies with significant variations in background track-density observed in samples cut from the same sheet of cellulose triacetate over a period of several months. Maximum and minimum densities of 37 and 13 tracks cm -2, respectively, were observed. Annealing for 16 h in air or nitrogen at 100:C, however, appeared to prevent the appearance of more than 20 tracks cm-2. The variability in background and the contributing factors should be closely scrutinized in any future studies. 4. SUMMARY Recoil-particle tracks caused by fast neutrons can be etched electrochemically in Kodak cellulose triacetate. Optimum parameters are a frequency of 3.4kHz, a voltage gradient of 13kVcm -1, and a concentration of KOH solution of 8 N. Electrochemical etching at room temperature for 4h produces clearly visible pits with a mean diameter of 180#m. Establishment of these parameters was aided by differential leakage-current measurements made with unirradiated and irradiated foils. In order to be useful in personnel fast-neutron dosimetry, the sensitivity needs to be increased by an order of magnitude. Finding a cellulose triacetate with a background of 2 rather than 20 tracks cm -2 would be a solution. Other cellulose triacetates, such as Triafol TN manufactured by Bayer,* need to be examined, not only to find a low-background material but to establish whether

ECE OF

FIG. 6.

FAST-NEUTRON

RECOIL

TRACKS

45

Electrochemically etched recoil particle tracks developed after 4 h at 3.4 kHz, 13.4 kV cm ~ and'25 C, using (a) 4 N, (b) 7 N, (c) 8 N, and (d) 9.5 N solutions of KOH.

ECE OF FAST:NEUTRON the findings presented in this p a p e r have general significance for all cellulose triacetates. The next step in the d e v e l o p m e n t of the electrochemical etching of cellulose triacet~te is to establish its fastn e u t r o n energy-response characteristics, a n d particularly the threshold energy. If the threshold is significantly lower t h a n the value for polycarbonate, then further d e v e l o p m e n t will be warranted.

REFERENCES Gammage R. B. and Cotter S. J. (1977) Evaluation of different polymers for fast neutron personnel dosimetry using electrochemical etching. Sixth ERDA Workshop on Personnel Neutron Dosimetry. Oak Ridge, Tennessee, Rep. No. PNL-2449, pp. 118-122. Gammage R. B. and Chowdhury A. (1979) Effect of etching solution normality on electrochemical etching of recoil particle tracks in polycarbonate. Hlth Phys. 36, 529 533. Gilley L. W. and Dickson H. W. (1979) Third Personnel Dosimetry lntercomparison Study. Oak Ridge National Laboratory Rep. TM-6114.

RECOIL TRACKS

47

Griffith R. V., Fischer J. C. and Harder C. A. (1976) Light particle track registration for fast neutron personnel dosimetry. Hazard Control Prog. Rep. UCRL-5000776-1, No. 52, pp. 40~44. Griffith R. V., Fischer J. C. and Harder C. A. (1977) Developments in a combined albedo electrochemical track etch personnel neutron dosimeter. Hazards Control Prog. Rep. UCRL-50007-76-2, No. 53, pp. 6-9. Oswald R. A. and Wheeler R. V. (1977) Energy dependence of electrochemically processed track etch dosimeters. Paper presented at the Health Physics Society 22nd A. Mtg, Atlanta, GA. Sohrabi M. (1974) The amplification of recoil particle tracks in polymers and its application in fast neutron personnel dosimetry. HIth Phys. 27, 598 600. Somogyi G. (t977) Processing of plastic track detectors. Nucl. Track Detection 1, 3-18. Thorngate J. H., Christian D. J. and Littleton C. P. (1976) An apparatus for electrochemical etching of recoil particle tracks in plastic foils. Nucl. Instrum. Meth. 138, 561 563. Tommasino L. and Armellini C. (1973) A new etching technique for damage track detectors. Radiat. Effects 20, 253-255. Wheeler R. V. (1977) Commercial implementation of track etch neutron dosimetry. Sixth ERDA Workshop on Personnel Neutron Dosimetry, Oak Ridge, Tennessee, Rep. No. PNL-2449, pp. 116-117.