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Adv. Space Rcr. Vol. 15, No. 6, pp. (6)25-(6)2X4 1995 Ccpynght 0 1995 COSPAR

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EARLY RESULTS FROM THE ULTRA HEAVY COSMIC RAY EXPERIMENT

D. O’Sullivan,* A. Thompson, * K.-P. Wenzel** and F. Jansen**

* Dublin Institute for Advanced Studies (DIAS), Dublin, Ireland ** Space Science Department of ESA, ESTEC, Noordwijk The Netherlands

ABSTRACT

Analysis of the Dublin-ESTEC ultra heavy cosmic ray experiment which was exposed for 69 months in Earth orbit aboard the LDEF spacecraft is continuing. To date, approximately three times the prt+LDEF world sample has undergone most of the processing and measurement required to estimate the relative abundances of nuclei with 2270. A preliminary charge spectrum has been completed for -450 UH nuclei and the measurement status of another 350 UH nuclei which are under investigation will be described. Early results, which do not yet include detailed analysis of fragmentation in the detector stacks, indicate r-process enhancement over solar system abundances.

INTRODUCTION

Prior to launch of the LDEF, a number of major experiments designed to study ultra heavy nuclei in Earth orbit had already been completed and the Heavy Nucleus Experiment on HEAO-3 /l/ along with the Bristol experiment on board Ariel-VI /2/ provided most of the data available at that time.

Two important requirements for a successll experiment on the study of charge spectra are good charge resolution and a sufiiciently large sample of data. The Dublin-ESTEC experiment has collected more than fifteen times the combined (HEAO-3 + Ariel VJ) data set for 2274 and there are indications that the charge resolution is at least as good, if not better than planned. To put things further in perspective, the total number of a&rides (a88) observed in these earlier experiments combined, was 3, whereas preliminary results for the Dublin-ES’T’EC experiment indicate a total of - 70f15 actinides in the sample obtained.

ASTROPHYSICAL SIGNIFICANCE OF ULTRA HEAVY NUCLEI

Ultra heavy nuclei are synthesised by the neutron capture process. This reaction is characterised by two distinct types which depend on the rate at which capture takes place, namely the slow (s) and rapid (r) processes. The contribution of each process to the abundance of nuclei with Z>30 varies widely from element to element. Accordingly, a detailed knowledge of relative abundances can provide evidence for either s or r process domination in a given charge region. The key to solving some problems associated with cosmic ray origin and acceleration is the fact that the s and r process occur in quite different astrophysical environments so that the measured abundances should reflect different nucleosynthesis histories and provide information on the source of ultra heavy nuclei. The r-process is usually associated with supernova explosions, whereas the s-process is believed to take place in normal stars at an advanced stage of evolution.

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(6)= I). O’Sullivan et al

To extract the interesting astrophysical implications from data measured in Earth orbit they are usually compared with the values derived from models based on particular types of source abundances which have been corrected for fractionation and propagation through the Galaxy. The effect of the first ionisation potential o;Ip)/3/ of each element in determining its likelihood of being ionised and accelerated as part of the galactic cosmic ray flux is now well established and is also taken into account in these models. The HEAO-3 and Ariei-VI groups used the standard leaky box model of propagation along with the rigidity dependent path length distribution of Ormes and Protheroe /4/ and the semiempirical fragmentation cross sections of Silberberg and Tsao /5/ which are assumed to be independent of energy above -lGeV/nucleon. The main conclusions drawn by the HEAG-3 and Ariel VI groups were that (i) for the 3312<60 charge region the observed abundances agree well with solar system source material with FIP fractionation and a slight enrichment of r-process material compared to the accepted solar system mixture, (ii) Pt(Z=78) and its secondaries are best fitted by a predominantly r-process source with FIP fractionation.

EXPERIMENT DESIGN

Details of the Dublin-ESTEC experiment designed to investigate cosmic ray nuclei with 2x5 are given in several papers. A total of 192 detector stacks, comprised mainly of lexan polycarbonate were mounted in sets of four within cylindrical eccofoam moulds which were then inserted into 48 separate aluminium pressure vessels (1 atm of dry air). Following recovery of the LDEF after 69 months in space the gas pressure in all cylinders was checked and it was found that no leakage had occurred.

The experiment thermal design was extremely successful and resulted in an average detector stack temperature below O°C with a mean maximum spread of flO°C. The corresponding charge shifts, were &0.8e for Uranium and M.6e for the Pt-Pb region.

A very fortuitous aspect of the life time of the Dublin-ESTEC experiment was the close parallel with the life time of the facility at the Berkeley Bevalac for accelerating ultra heavy nuclei such as uranium and gold. The facility started in time for pm-flight calibration before launch in 1984 and survived long enough to allow post flight calibration of the detectors before it closed down in February 1993, much to the regret of the cosmic ray community. Analysis of the pre and post flight data has shown that the strict control of detector environment throughout the mission has resuhed in no observable degradation of the latent image /6/.

LOCATING AND IDENTIFYING ULTRA HEAVY COSMIC RAY NUCLEI IN TEIE LEXAN POLYCARBONATE STACKS

Locating the ultra heavy nuclei is straightforward. Because of the orbital inclination particles reaching the detectors are predominantly of high energy @Qz2 GeV/nucleon). The ionisation threshold for lexan is such that at these energies only particles with 2265 register and there is no significant background from lighter nuclei. Approximately 2500 particles with Z>70 have been located.

Following carefully controlled etching of 10 plates from the top of each stack and a similar number from the bottom, a set of 40 etch rate values Vt were determined for each ultra heavy candidate. Charge identification is based on measurement of(i) G, the fractional etch rate gradient;

G =i g where S =F, Ys is the bulk etch rate and X is the path length and (ii) the effective 8

reduced etch rate Sqr. S is related to a restricted energy loss function through S = g(l?13L)~

Early Rdts from UH Cosmic Ray Experiments (6P7

where g and h are determined from our U(-950 MeV/N) and Au(-I 150 MeVIN) calibrations at the Bevalac. Typical S versus path length plots are shown in references /7/ and /8/.

PRELIMINARY CHARGE SPECTRUM: RESULTS AND DISCUSSION

Approximately 25% of the data (4300 UH Nuclei) have so far been processed and final measurements have been completed for -450 of these. A preliminary charge spectrum based on selected events /9/ is shown in Fig. 1. A further 350 events are at an advanced stage of measurement but are not included in the spectrum yet.

60

Figure 1: A preliminary charge spectrum for nuclei with 2 > 70 based on x 450 nuclei (X 15% of total sample) from the Dublin- ESTEC experiment.

The presence of nuclei beyond the a&ride gap is significant and in view of the modest charge spread (-le) expected from statistical errors and temperature excursions we are confident even at this early stage that the Dublin-ESTEC experiment is capable of providing the first statistically significant sample of cosmic ray nuclei with 2288 with good charge resolution. How good this resolution will eventually be remains to be seen, but at present we have no reason to believe that it will not be well within the design target of <1.5e, and probably better for a sample of events with appropriate path lengths and geometry. Selection from stacks with the smallest temperature excursions will also help and the possibility of obtaining abundance ratios for charge groups (and even a ThAJ value) in this region looks promising.

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D. o?hllivaa et al.

The ratio of (Z XI8)/(74aa7) from Fig 1 is 0.03lfo.009 compared with 0.025fo.015 for the combined HBAG-3 + Ariel VI data. It should be emphasised however that this early value from the Dublin-ESTBC experiment should be taken as very preliminary since no correction has yet been applied for any differences that may exist in the fragmentation cross-section of actinides as opposed to those for nuclei in the 7412187 charge region /lo/. The relative significance of nuclear and electromagnetic interactions at high energies for both of these groups of nuclei is the subj& of investigation at present Ill/. Our initial method of scanning, while efl%zient at locating

those nuclei which survive X of their trajectory through the stacks, does not pick up those which interact and lose suflicient charge to fall below threshold (2-65), as would occur in fission for example. Further down the charge scale we note that the spectrum peaks around Z=77.5, indicating the presence of predominantly r-process nuclei such as osmium and platinum, while Pb (predominantly s-process) is not very dominant.

ACKNOWLEDGEMENTS

The authors wish to thank G. Broderick, E. Flood, A. Grace, S. Ledwidge and H. Sullivan for their continued support and J. Bosch for providing Fig. 1.

REFERENCES

1, W.R. Binns, T.L. Garrard, P.S. Gibner, M.H. Israel, M.P. Kertzmann, J. Klarmann, B.J. Newport, E.C. Stone and C.J. Waddington, The Astrophysical Journal, 346,997, (1989)

2. P.H. Fowler, R.N.F. Walker, M.R.W. Masheder, R.T. Moses, A. Morley and A.M. Gay, The Astrophysical Journal, 3& 739, (1987)

3. J.R. Letaw, R. Silberberg and C.H. Tsao, Ap. J., 279, 144, (1984)

4. J.F. Ormes and R.J. Protheroe, Ap. J., 272,756, (1983)

5. R. Silberberg and C.H. Tsao, Ap. J. Suppl.,25, 3 15, (1973)

6. A. Thompson, D. O’Sullivan, K.-P. Wenzel, J. Bosch, R. Keegan, C. Domingo, J. Daly and A. Smit, Proc 22ndZCRC, 12, 543, (Dublin), (1991)

7. D. O’SullivaqA. Thompson, J. Bosch, R. Keegan, K.-P. Wenzel, F. Jansen and C. Domingo, Adv. Sp. Res., in press, (1993)

8. A. Thompson, D. O’Sullivan, J. Bosch, R. Keegan, K.-P. Wenzel, F. Jansen and C. Domingo, Proc Second LDEF Post-Retrieval SJymposium, L, 26 1, (San Diego), ( 1992)

9. A. Thompson, D. O’Sullivan, K.-P. Wenzel, J. Bosch, R. Keegan, C. Domingo, and F. Jansen, Proc 23rdZCRC, 1,603-60, (Calgary), (1993)

10. L.Y. Geer, J. Klannann, B.S. Nilsen, C.J. Waddington, W.R. Binns, J.R. Cummings and T.L. Garrard, Proc 23rd ZCRC,2_ 19 1, (Calgary), (1993)

11. A.J. Keane, D. O’Sullivan, A. Thompson, (this conference, COSPAR 1994)