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Adv. Space Res. Vol. 27, No. 4, pp. 785-789,200l 0 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved

Printed in Great Britain 0273-I 177/01 $20.00 + 0.00

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EI.3-0025

THE RELATIVE ABUNDANCE OF ACTINIDES IN THE COSMIC RADIATION

D. O’Sullivan’, A. Thompson’, J. Donnelly’, L. 022. Drury’, K.-P. Wenzel’

’ Dublin Institute for Advanced Studies (DLAS), Dublin 2, Ireland ’ Space Science Dept. of ESA, ESTEC, Noordwijk, The Netherlands

ABSTRACT

The DIAS-ESTEC Ultra Heavy Cosmic Ray Experiment (UHCRE) on the Long Duration Exposure Facility (LDEF) collected approximately 3000 cosmic-ray nuclei with 2>65 in the energy region E>l .5 GeV/nucleon during a six year exposure in Earth orbit. The entire accessible collecting area of the solid-state nuclear-track detector (SSNTD) array

has been scanned for actinides, yielding a sample of thirty from an exposure of ml50 m2 sr yr. The observed charge

spectrum is presented. The current best value for the cosmic-ray actinide relative abundance, (2>88)/(7412187), is given. This value is considered in the context of the predictions of source/propagation models.

0 200 1 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

BACKGROUND

The ultra-heavy region of the cosmic-ray composition spectrum (i.e. Z > =60) is an important source of clues to the origin, means of acceleration and subsequent propagation of cosmic rays.

Study of certain abundance ratios, for example that of the Pt peak (7612181, mainly r-process) with the Pb peak

(8112183, mainly s-process), may show enhancement of either r- or s-process nucleosynthesis in the constituents of cosmic rays. This may help locate a possible site of origin of the source material. At higher charges, since actinide cosmic rays have similar cross sections and the nuclear interactions they undergo during their traversal of the galaxy mainly result in fission, the actinides will largely maintain their source relative abundances. Anomalies may be revealed by comparing the actinide/subactinide ratio of the cosmic rays to those of the current and primordial solar system (obtained from analysis of the solar photosphere and meteorites respectively - Grevesse and Sauval, 1998).

The relative abundance of cosmochronological pairs such as Th and U can provide an estimate of the time between the nucleosynthesis of the material and its subsequent acceleration. Most 232Th/238U production ratios from r-process

calculations vary from approximately 1.4 to 1.9 depending on the mass law used and the P-delayed fission assumptions made (Thielemann et al, 1983). Subsequent decay can be simulated by chemical evolution models, which predict that the U/Th ratio tends to remain over unity until approximately lo’-10’ yr after nucleosynthesis (Meyer and Goriely, 1999). This is partly because of uranium’s five relatively long-lived isotopes in comparison to thorium’s two and partly because 244Pu and 236U take a relatively long time to decay into 232Th. The transuranics can also be used as excellent cosmic-ray ‘clocks’ since the relative abundances of Np, Pu and Cm fall drastically 10’ yr after nucleosynthesis (Blake and Schramm, 1974).

Information on the charge spectrum of this region is scarce (partly due to the drop of seven orders of magnitude between the Fe and actinide abundances). The orbiting electronic detectors aboard HEAO-3 (Binns et al, 1989) and

ARIEL VI (Fowler et al, 1987), though unable to resolve individual elements of 2260, provided statistically significant data on the ultra-heavy abundances, including three possible actinides. TREK (Westphal et al, 1998), an

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array of glass track-etch detectors aboard the Mir space station, yielded a charge spectrum of superior charge resolution while also detecting six actinides. The experiment with the largest exposure factor in the ultra-heavy region was the UHCRE. During its 69 months aboard the earth-orbiting LDEF it collected ~3000 cosmic-ray events in its solid-state nuclear-track detector (SSNTD) array. Details of experiment design, calibration and processing may be found in Thompson et al, 1993; O’Sullivan et al, 1995; and Keane et al, 1997.

RESULTS

All events collected by the UHCRE exceeded a Z/8 > ~65 detector registration threshold. A randomly selected 36% of the experiment’s SSNTD stacks were fully scanned and measured. After filtering, this yielded 760 events - including 13 actinides. The events in the remainder of the accessible detector area were then subjected to a further selection process to obtain the remaining higher charge events. In this process, the events in each SSNTD stack were measured in three detector plates equidistant throughout the stack. Those exceeding a threshold rate of ionisation were then fully measured. This method gathered a subset of cosmic-ray events from approximately Z=SO upwards, including 17 actinides. The total number of actinides found was 30.

Analysis of the events employed the REL ionisation model (Benton and Nix, 1969) with 00 set to 1 keV. The resulting cosmic-ray charge spectrum (normalised to the entire sample) can be seen in Figure 1, with an inset providing detail in the actinide region. In order to produce a unified frequency distribution, the subactinide events in the fully scanned 36% of the accessible detector area were normalised to the remainder. This distribution has been corrected for the UHCRE geometry factor, which biases against detecting lower-Z events.

0.16

0.14

0.12

co.10 s g 0.08

; 0.06

Actinides

8485B8783899J~92%94%%9798

Charge, 2

70 75 80 85 90 95 100

Charge, 2

Fig. 1. Frequency distribution charge spectrum of Z>70 cosmic rays (normalised to the entire sample) with an inset providing detail in the ZM4 region. Statistical error bars correspond to 84% conjIdence limits (Gehrels, 1986).

Charge resolution was determined by measuring the full width at half maximum of the distribution of a large sample of events of known charge in both the Pt-Pb and actinide regions. This resulted in a charge dispersion of + 1.1 e in the Pt-Pb region and f2.le in the actinide region, a value which includes intrinsic dispersion in the detector as well as etching and measurement dispersions. Other sources of error are the registration temperature effect (wherein signal strength varies with the temperature of the detector at the time of registration; Thompson et al, 1991) and energy dispersion (error in estimating the energy of the incident ion). Folding these errors together yields a maximum overall error of between +1.4e (Pt-Pb) and +2Se (actinides).

Relative Abundance of Actinides in the Cosmic Radiation 787

DISCUSSION

The actinide relative abundance [i.e. the actinide/subactinide ratio, (2288)/(7412187)] was found to be 0.0144 + 0.003 l/0.0026. As can be seen in Figure 2, this value is significantly greater than that of propagated present day solar system material (0.0077), but fully consistent with propagated primordial - i.e. dating from the formation of the solar system - material (0.013), as given by Brewster et al (1983).

0.030 -

0.025 - ~- -

0.020 -

0.015 -

UHCkE Primordial Solar System .

0.005 t

P.resent Day. Solar System

0.000 ’

Fig. 2. Observed and derived actinide relative abundances (2>88)/(7412_<87). The propagated primordial and propagatedpresent day solar system values are from Brewster et al (1983). Statistical error bars correspond to 84% confidence limits (Gehrels, 1986).

The proposal that the cosmic-ray source material predominantly originates from normal interstellar gas and dust (Meyer et al, 1997; Ellison et al, 1997; Meyer et al, 1998; Drury et al, 1999) is strengthened by the agreement between the UHCRE actinide/subactinide ratio with that of the primordial solar system. The ultra-heavy cosmic-ray composition in the actinide region appears essentially “normal” with no striking anomalies.

Charge resolution in this region will improve with the processing of further calibration ultra-heavy nuclei beams coexisting in the same detector plates as the actinide events. In addition, the processing of track etch-rate monitors should reduce any charge spectrum distortion due to small variations in etching conditions. Normalisation to these reference beams should facilitate a meaningful calculation of pertinent abundance ratios, such as ThAJ, helping to assess the cogency of the various current models of cosmic-ray origin.

ACKNOWLEDGEMENTS

The authors wish to thank the DIAS technical assistants E. Flood, A. Grace, S. Ledwidge and H. O’Donnell for their work in etching detector stacks and measuring events. They are grateful to J. Daly (DIAS) and to ESTEC technical personnel for the assembly, testing and disassembly of the hardware. They also wish to express their gratitude for all assistance received from heavy ion accelerator staff at Berkeley, Brookhaven, CERN and Darmstadt. Finally, they would like to thank J. Jones, W. Kinard and the LDEF staff at the NASA Langley Research Centre for their support and encouragement.

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