Nuclear Tracks and Radiation Measurements, Vol. 8, Nos. 1-4, pp. 143-146, 1984. 0191-278X/84 $3.00 + .00 Printed in Great Britain. ~c) 1984 Pergamon Press Ltd.

NEW RESULTS ON THE INVESTIGATION OF THE VARIATION OF NUCLEAR TRACK DETECTOR RESPONSE WITH TEMPERATURE

D. O'Sullivan*, A' Thompson*, J.A. Adams**, L.P. Beahm**

*Dublin Institute for Advanced Studies, Ireland **Naval Research Laboratory, Washington D.C. 20375, U.S.A.

ABSTRACT

Track response of polymers is now known to depend on the temperature of the detector during passage of the ionising particle (the registration temperature effect). This has serious consequences for cosmic ray composition studies carried out in earth orbit or on high altitude balloons. Further studies are reported for Lexan, plain CR-39 and doped CR-39 exposed to 245 MeV/N Fe ions. It was found that the relative signal strengths continue to increase down to -137°C, the minimum temperature employed in this experiment. The stack temperatures were cycled between 20°C and -137°C but no evidence of a hysteresis effect.was observed.

KEYWORDS

Nuclear particle track detectors; Lexan; CR-39; registration temperature effect; cosmic ray composition.

INTRODUCTION

The first reports of the dependence of the track response of polymers on temperature during registration were the result of investigations carried out with a view to improving charge resolution in cosmic ray experiments (Thompson and co-workers 1979, O'Sullivan and co-workers 1979, O'Sullivan and Thompson 1980). Studies on Lexan polycarbonate and CR-39 (Homalite) detectors exposed to 9.6 MeV/N Ar ions in beam vacuum at the Manchester LINAC and Lexan exposed to 122 MeV/N Fe ions at atmospheric pressure at the Berkeley BEVALAC showed that signal strength varied appreciably with detector temperature between 70°C and -78°C. Further investigations were carried out at 48, 21, O, -17, -70 and -189°C with Lexan and CR-39 (Pershore) detectors which were sealed in pressure vessels in an environment of dry air at atmospheric pressure and exposed to 595 MeV/N Fe ions at the BEVALAC (Thompson and co-workers 1981). This work confirmed the earlier results but the extension of the temperature range to -189°C manifested an anomaly which was interpreted as being probably due to liquefaction of oxygen (see Fig. I).

In this paper we report some recent results obtained from a study of polymer track response in the temperature region between -70°C and the anomalous point at -189°C. By observing the track response variation when the registration temperature of the same detector stack was cycled from one value to another it was possible to check for evidence of hysteresis associated with the effect.

EXPERImeNTAL METHOD

Four detector stacks were assembled from 250 ~m Lexan polycarbonate (grade 8070-112, General Electric, U.S.A.) and from three different types of 500 ~m CR-39 (Pershore Mouldings, England) The CR-39 was cast using a 32 hour rising temperature polymerisation cycle (Adams, 1981). The three types of CR-39were characterised by (a) 2.6% IPP initiator without dopant, (b) 5.5% CHPC initiator without dopant and (c) 5.5% CHPC initiator with 1% dioctyl phthalate (DOP) dopant. The CR-39 (IPP, plain) batch was cast in June 1980, while the CR-39 (CHPC, plain) and CR-39 (CHPC, DOP) batches were cast in May 1982. Each of the four stacks consisted of fifteen

143

144 D. O'SUL, LtVAN er al.

sheets of polymer randomly sequenced and had a collecting area of 5 cm x 5 cm.

The four detector stacks were mounted in series in a single ensemble, thermally linked with conductive mass on each side, inside a brass pressure vessel fitted with a beam window, a precision wedge beam spreader and a carbon resistance thermometer. The pressure vessel complete with stacks was evacuated, pumped at 10 -3 torr for several hours and then back-filled with dry air at one atmosphere pressure before sealing.

The detector stacks, mounted in the pressure vessel, were exposed to a 245 MeV/N Iron beam at 20°C at the Berkeley BEVALAC, in August 1982. The temperature was then rapidly dropped (~ 1 hr) to -195°C, cycled up and down several times at low temperature and brought up slowly(- 5 hrs) to -137°C, when the detector stacks were exposed to the 245 MeV/N Iron beam again. The pressure vessel and contents were then immediately allowed to return to room temperature. A particle density of ~ 5 x 102 cm -2 was employed in each case. The same dip angle (~ 55 ° ) was used for each exposure to optimise direct comparison, but the exposures could be identified by different azimuth angles. The beam energy spreader wedge provided a strip of stopping Pe 56 events in every sheet of each detector stack.

The detector sheets were etched conventionally at 40°C, the Lexan for 96 hrs in 6.25N NaOH saturated with etch products and the CR-39 for 68 hrs in unsaturated 6.25N NaOH. Twelve stopping Fe 56 events were fully measured for each registration temperature in each stack giving a total of 96 events. For each event, at least ten etch rate (VT) measurements were made at different energies along the trajectory.

RESULTS AND DISCUSSION

The best fitted curve of track etch rate versus residual range was computed for each event. In order to get a single parameter for each polymer at each registration temperature, the median intercept of the best fitted VT curves at I000 Nm residual range, V-~ (I000), was computed at each registration temperature for Lexan and similarly V~ (1500) was computed at each registration temperature for each type of CR-39. These values are shown in Table I.

Table 1 Fe 56 VT Intercepts for 20°C to -137°C Cycle

Registration Lexan CR-39 CR-39 CR-39 Temperature (IPP, Plain) (CHPC, Plain) (CHPC, DOP)

VT (i000) VT(1500) VT(1500) VT(1500) (Dm/hr) (~m/hr) (Dm/hr) (~m/hr)

20°C 1.02 ± 0.03 O.81 ± 0.04 1.01 + 0.04 0.94 ± 0.04 -137°C 1.54 + 0.03 1.40 ± 0.06 2.03 ± 0.09 2.05 ± 0.I0

For comparison with earlier results the relative values of V-T (i000) and V-~ (1500) at -137°C with respect to the values at 20°C are plotted in Fig. 1 where they are identified by the key in the upper right-hand box. It may be seen that the relative signal strength continues to increase with decrease in registration temperature down to -137°C, more than halfway across

the region in question between ~ -70°C and the anomalous points. Furthermore, there is no evidence of hysteresis with a time base more than the exposure time interval (a maximum of 6 hrs).

The fractional change in VT or signal strength per °C, for Fe 56 at the relevant residual range, averaged over the registration temperature interval from 20°C to -137°C was found to be (0.32 ± 0.03)% for Lexan, (0.46 ± 0.08)% for CR-39 (IPP, plain), (0.64 ± 0.09)% for CR-39 (CHPC, plain) and (0.75 ± 0.08)% for CR-39 (CHPC, DOP). These temperature cycle results are remarkably consistent with earlier overall averages for the non-cycled registration temperature interval 21°C to -70°C (Thompson et al, 1981), and are equivalent to shifts in the charge scale of about 0.02 and 0.04 charge units per °C for Lexan and CR-39 respectively. It should be pointed out that the shift per °C in the region above ~ 20°C appears to be considerably greater (Thompson et al, 1981, O'Sullivan and Thompson 1980).

N U C L E A R T R A C K D E T E C T O R R E S P O N S E W I T H T E M P E R A T U R E 145

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Fig. I. Relative Track Etch Rate (Relative Signal Strength) versus Registration Temperature. Data points keyed in the upper right-hand box are from the present work and data points identified in the lower left-hand box are from Thompson and Co-workers (1981) and O'Sullivan and Thompson(1980). The dashed lines(CR-39 varieties) and full lines (Lexan Polycarbonate) are visual guides only and are not fitted curves.

146 D. O ' S U [ h lVAN etal .

In a very recent series of experiments on the registration temperature effect at the Berkeley BEVALAC (May 1983) we have exposed detector stacks of Lexan, Tuffak and CR-39 to 960 MeV/N U 238 beams (Thompson et al, 1983). Preliminary results confirm that the fractional change in signal strength per °C increases significantly with ionisation, an effect suggested earlier in the stopping Fe data (O'Sullivan and Thompson, 1980). Thus both the temperature and the related ionisation dependence of the relative track response are of major significance for cosmic ray abundance studies in which solid state nuclear track detectors are employed. In particular, the proposed LDEF-2 mission to study ultra heavy cosmic ray nuclei will be designed within the constraints imposed by the registration temperature effect.

ACKNOWLEDGEMENTS

We wish to thank the staff at the Berkeley BEVALAC and in particular, Dr. Hank Crawford for providing invaluable assistance during the exposures. Thanks are also due to Miss G. Broderick, Miss S. Ledwidge, Mrs. E. Rankin-Brady and Miss E. Ryan who were responsible for the track measurements.

REFERENCES

Adams, J.H. (1981). A curing cycle for detector-quality CR-39. Proc. llth [nternat. Conf. on Solid State Nuclear Track Detectors, Bristol, 145.

O'Sullivan, D., A. Thompson, J. Daly, C. O'Ceallaigh, V. Domingo, A. Smit and K°-F. Wenzel (1979). A solid state track detector array for the study of ultra heavy cosmic ray nuclei in earth orbit. Proc. lOth Internat. Conf. on Solid State Nuclear Track Detectors, Lyon, 1033.

O'Sullivan, D., and A. Thompson (1980). The observation of a sensitivity dependence on temperature during registration in solid state nuclear track detectors. Nuclear Tracks, 4 271.

Thompson, A., D. O'Sullivan, J. Daly, C. O'Ceallaigh, V. Domingo, A. Smit and K.-P. Wenzel (1979). A high resolution study of ultra heavy cosmic ray nuclei using the Long Duration Exposure Facility (LDEF). Proc. 16th Internat. Cosmic Ray Conf., Kyoto, ii, 103.

Thompson, A., D. O'Sullivan, J.H. Adams and L.P. Beahm (1981). The dependence of track response on registration temperature in Lexan and CR-39. Proc. llth Internat. Conf. on Solid State Nuclear Track Detectors, Bristol, 171.

Thompson, A., D. O'Sullivan, J.H. Adams, L.P. Beahm (1983). The variation of track response with registration temperature in solid state nuclear track detectors and its implications for cosmic ray composition studies. Proc. 18th Internat. Cosmic Ray Conf., Bangalore, paper T4-11. To be published.