Nucl. Tracks Radiat. Meas., Vol. 15, Nos. 1-4, pp. 673-676, 1988 lnt. J. Radiat. AppL lnstrum., Part D Printed in Great Britain

0191-278X/89 $3.00 + .00 Pergamon Press plc

THE OUTLOOK FOR ULTRA HEAVY COSMIC RAY STUDIES WITH PLASTIC TRACK

DETECTORS

D. O'SULLIVAN, A. THOI~SON, C. D~INGO

Dublin Institute for Advanced Studies, Ireland

V. DOI~INGO AND K.-P. WENZEL

Space Science Department of ESA, ESTEC, Noordwijk, The Netherlands

Abstract - Recent investigations of environmental and ageing effects on the latent signal of highly ionising particles in solid state nuclear track detectors provide improved information on the charge resolution obtainable with solid state nuclear track detectors used in the study of ultra heavy cosmic ray nuclei. The impact of recent findings on the analysis of data from long term experiments in space such as the ultra heavy cosmic ray experiment (UHCRE) on LDEF, the proposed Heavy Nucleus Collector and similar studies which will be made possible during the Space Station era, is assessed.

I. INTRODUCTION

The very low flux of ultra heavy (Z > 30) cosmic rays in our galaxy presents serious problems for experimentors who wish to determine their charge and energy spectra. For instance, experiments operating near earth, at an orbital inclination of 28.5 deg., which has been the most common trajectory for Space Shuttle missions, observe an event rate of only = IO m -2 sr. yr for particles with Z > 65. During the last decade there have been only three space missions which ~ncluded experiments dedicated to the investigation of ultra heavy nuclei, namely HEAO-3 ~ and Ariel-6 launched in 1979 and the Long Duration Exposure Facility (LDEF) on which the DIAS-ESTEC ultra heavy cosmic ray experiment (UHCRE) 3 was deployed into earth orbit by the ill-fated Shuttle Challenger in 1984 (see Fig. I).

The HEAO-3 and Ariel-6 missions were the first to employ electronic detectors of adequate geometric factor (5 m 2 sr and 2 m 2 sr respectively) for the purpose of ultra heavy nucleus studies. The combined sample of events with Z a 65 from these two experiments (= 300) comprises the total world store of data collected in this charge region during the last decade.

Fig. I LDEF being deployed

into earth orbit on April 6th

1984. Retrieval is scheduled

for November 1989.

fiTq

674 D. O'SULLIVAN et al.

Fig. 2 outlines the situation as it stands at present (1988). Here the relative abundances of the elements in the Solar System are plotted for the entire periodic table. The charge spectrum is divided into five distinct regions which are characterised by the different levels of charge resolution achieved in cosmic ray studies so far. It can be seen that beyond Barium (Z = 56) element resolution has yet to be achieved and that the total sample of events measured is remarkably small. Indeed, the combined sample of Actinides (Z ~ 88) is only 3.

lO/~ r i I I I / ELEMENTS IELEMENTS I EVEN Z ELEMENTS IELEMENTS NOT IELEMENTS NOT I~ISOTOPES :RESOLVED :RESOLVED :RESOLVED IIRESOLVED I I~RESOLVED I t I t

, o - I l l I I I I

I l l I I i I I l l i I I I

..1~1 I I ITOTAL SAMPLE ITOTAL SAMPLE l o ~' • i I I

V~ i t I(HEAO-3+ ARIEL- 6) I(HEAO- 3 +ARIEL-61 ~ J~ I I =30° EVENTS I 3 EVENTS

/1'% A" " " II ' ! ~ 1 I 91, ~ i

I , , ! !

1 0 4 ~ A . I II !

V ,A^AlVlh~ ~ ~ACTINIDE GAP i "V v v'~

I I I I I i I , , I I I 0 10 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0

ATOMIC NUMBER (Z)

Pig. 2 Relative Solar System Abundances. Five distinct regions are shown corresponding to the different levels of charge and mass resolution achieved in cosmic ray studies so far.

2. PRESENT STATUS OF THE EXTENDED LDEF IIISSION

2 The UHCRE aboard LDEF has a geometric factor of 25 m sr and employs solid state nuclear

track detectors. It was originally scheduled to be recovered from earth orbit in 1985 but due to delays in the Shuttle programme and the eventual loss of the Challenger in January 1986, its duration in space has been extended considerably. Recent increases in solar activity have resulted in an increase in the rate of orbital decay of the LDEF and it is currently (March 1988) estimated that it could re-enter the earth's atmosphere by November 1990. As a result of these developments it is now likely that the recovery of LDEF will be a major priority with NASA following the resumption of Shuttle operations in the second half of 19884 . Present estimates indicate that Hission 32 or ~lission 35 which are the 7th and lOth post Challenger flights respectively will include retrieval of the LDEF. I~ either case, the LDEF will have achieved an extended exposure totalling more than 140 m z sr yr. Thus, while increasing the world sample of ultra heavy nuclei by more than a factor of four it is worth noting that the data should include more than 15 aetinide elements (Z ~ 89) compared to only 3 in the combined HEAO-3 and Ariel-6 samples.

3. PROPOSED NEW EXPERImeNTS

Due to the high cost of developing and building very large area (= 200 m 2 sr) electronic detectors for use in space experiments it is unlikely that any extensive investigations other than those involving SSNTDs will be undertaken before the early part of the next century. Prior to the Shuttle disaster in January 1986 plans for a further extension of ultra heavy studies were at an advanced stage. It was ~lanned to re-launch the retrieved LDEF or other free flying craft with approximately 60 m ~ sr of SSNTD aboard and expose the detectors for up to 6 years in earth orbit 5. At present this experiment, called

ULTRA HEAVY COSMIC RAY STUDIES 675

the Heavy Nucleus Collector (HNC), is not included in the NASA launch manifest and is stored along with some associated hardware waiting a launch opportunity. One of the main components of this experiment is Rodyne polycarbonate which is discussed in Section 4.

The Space Station Programme will provide cosmic ray researchers with a versatile multi- purpose facility on which large arrays of SSNTDs could be deployed for long periods in space. In order to optimize the results from such exposures a detailed knowledge of ageing and environmental effects on latent tracks in SSNTDs will be required. Only then can high charge resolution be achieved, with a consequent improvement in our understanding of the nature and origin of these very rare nuclei.

4. PRESENT STATUS OF KNOWLEDGE CONCERNING POST-IRRADIATION AGEING AND ENVIRONHENTAL EFFECTS ON THE REACTIVITY OF LATENT TRACKS

OF UH NUCLEI

(a) Ageing

The feasibility of successful longterm exposures of polycarhonate SSNTDs for ultra heavy cosmic ray research was recentl[ thrown in doubt by results on the variation of the reactivity of latent tracks in Rodyne u. Rodyne is a polycarbonate formerly called Tuffak which is chemically identical to Lexan polycarbonate. Fig. 3 shows the results reported for uranium ions (440 HeV/N) in Rodyne which was stored in a post irradiation environment of air at 22°C. A dramatic growth in latent track reactivity was reported, with the (S-l) versus Time curve growing faster than logarithmically right up to 700 days after exposure. Shown also in Fig. 3 are data measured by the Dublin Group for uranium ions (300 MeV/N) in Lexan Polycarbonate which is the main detector of the Dublin-ESTEC experiment on LDEF. The Lexan data does not show any indication of a dramatic rise in sensitivity with latent track storage time. Indeed measurements made over periods extending to = 3 years in Tuffak polycarbonate show a similar response 7.

(b) Registration Temperature Effect

It is now well established that the sign~19strength in SSNTDs is a function of the detector temperature during track registration ' . Consequently large temperature swings during extended space missions will cause blurring of charge resolutiQn because particles with the same Z and ~ values which register with different time and temperature histories will not produce latent signals of equal strength. This problem can be overcome by employing active temperature control, but in the case of LDEF, no such control was provided. However, temperature measurements over an annual cycle are recorded on board and will be available after retrieval.

Temperature fluctuations on the LDEF arise mainly from variations in the elevation angle (Beta) of the sun above the orbital plane of the LDEF. Beta oscillates with a 45 day period. The UHCRE trays are thermally decoupled from the LDEF frame and have a passive thermal control system designed to minimise the stack temperatures. A recent thermal analysis carried out by the NASA Langley Research Center showed that the overall mean temperature for the UHCP~ stacks will be -6°C ± 7°C with maximum excursions to +II°C and -22°C. The impact of these temperature variations is discussed in the next section.

7

60

40

20

' I ' I ' I I V 3 0 0 M e V / N U ions in air at 25"C

\ 4 4 0 MeV//N U ions in a i rat22"C

f f , I l I t 1 10 102

( t - t o ) days

103

Fig. 3 Values of (S-l)

where S = Vt/Vg for

300 MeV/N uranium ions in

Lexan Polycarbonate for

post irradiation ageing

periods up to 400 days.

The solid line represents

the best fit to data of

reference (6), in Rodyne

Polycarbonate.

676 D. O'SULLIVAN et al.

5. DISCUSSION

The HEAO-3 and Ariel-6 experiments succeeded in separating individual charges up to Z = 56. Beyond that, charge resolution was not adequate to determine individual charges, but estimates of several ratios which are important in the field of cosmic ray astrophysics were made ~''. The constraints imposed on the UHCRE by the combination of temperature effects and ageing will reduce its capability to provide a definitive study of the Z ~ 60 charge spectrum, but a brief look at the possibilities show plenty of grounds for optimism. For instance we have estimated 9 that the maximum charge shift resulting, in Lexan polycarbonate, from the registration temperature effect is AZ ffi ± 1.4e in the actinide region and ± l.Oe in the Pt-Pb region for nuclei with energies E > 2 GeV/N. This resolution combined with the greatly enhanced sample of events should provide us with exciting new data above Z = 65 and will certainly make possible the first statistically significant estimate of the actinide abundances. The drastic rate of latent track intensification caused by ageing reported recently 6 has not been confirmed~ The latter data, which for some detectors now spans periods of more than three years of post irradiation storage at different temperatures all exhibit a gradual decrease in signal stren§th after several months, followed by a levelling out to a constant value after = 2 years-. Thus it should be possible to normalise out ageing effects by storing material for an extended period after retrieval.

ACKNOWLEDGEMENTS

We thank E. Brady, A. Casey, J. Daly, E. Flood, S. Ledwidge, G. Rooney and H. Sullivan of the Dublin Institute for Advanced Studies and H. Crawford and the staff of the Berkeley Bevalac.

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