Data extraction and analysis of the LDEF Ultra Heavy Cosmic Ray Experiment is continuing. Almost twice the pre LDEF world sample has been investigated and some details of the charge spectrum in the region from Z approximately 70 up to and including the actinides are presented. The early results indicate r process enhancement over solar system source abundances

Bosch, J.

Domingo, C.

Jansen, F.

Keegan, R.

Osullivan, D.

Thompson, A.

Wenzel, K.-P.

English

NASA Technical Reports Server

N 5-Z3 U9
EARLY RESULTS FROM THE
ULTRA HEAVY COSMIC RAY EXPERIMENT
D. O'Sullivan, A. Thompson, J. Bosch, R. Keegan
Dublin Institute for Advanced Studies (DIAS), Ireland
Phone: +353-1-6621333, Fax: +353-1-6621477
K.-P. Wenzel and F. Jansen
Space Science Dept of ESA, ESTEC, Noordwijk, The Netherlands
Phone:+31-1719-83573, Fax: +31-1719-84698
C. Domingo
Universitat AutSnoma de Barcelona, Spain
Phone:+34-3-581-1530, Fax: +34-3-581-2155
ABSTRACT
Data extraction and analysis of the LDEF Ultra Heavy Cosmic Ray Experiment
is continuing. Almost twice the pre LDEF world sample has been investigated and
some details of the charge spectrum in the region from Z _ 70 up to and including the
actinides are presented. The early results indicate r process enhancement over solar
system source abundances.
INTRODUCTION
Prior to the 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-31 along with the Bristol experiment on board Ariel-
VI 2 provided most of the data available at that time.
Two important requirements for a successful experiment in the study of charge
spectra are good charge resolution and a sufficiently large sample of data. The Dublin-
ESTEC experiment has collected almost fifteen times the combined (HEAO-3 + Ariel-
VI) data set 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 actinides (Z >_ 87) observed in these earlier experiments combined was 3, whereas
preliminary results for the Dublin-ESTEC experiment indicate a total of -,_ 70 + 15
actinides in the sample obtained.
 P_g
129
https://ntrs.nasa.gov/search.jsp?R=19950017389 2020-06-16T08:32:02+00:00Z
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. Ac-
cordingly, 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 processes
occur in quite different astrophysical environments so that the measured abundance
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.
To extract the interesting astrophysical implications from data measured in Earth
orbit they are usually compared with 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 (FIP) a 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 Ariel-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 --_ 1GeV/N. The main conclusions drawn
by the HEAO-3 and Ariel-VI groups were that (i) for the 33 _< Z _< 60 charge region
the observed abundances agree well with solar system source material with PIP frac-
tionation with 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. It may seem strange that cosmic rays should
be richer in r-process material at the upper end of the periodic table than in the region
below Nd(Z = 60) but there are r-process conditions which synthesise only nuclei with
Z>60.
Experiment Design
130
Details of the Dublin-ESTEC experiment designed to investigate cosmic ray nuclei
with Z > 65 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 designwasextremely successfuland resulted in an average
detector stack temperature below 0°C with a mean maximum spreadof +10°6 '. The
corresponding charge shifts were -l-0.Se for uranium and +0.6e 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 pre-
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 resulted in no
observable degradation of latent image 6.
Locating and Identifying Ultra Heavy Cosmic Ray Nuclei
in the Lexan Polycarbonate Stacks
Locating the ultra heavy nuclei is straightforward. Because of the orbital incli-
nation only high energy (E >_ 2GeV/N) particles reach the detectors. The ionisation
threshold for lexan is such that at these energies only particles with Z >_ 65 register
and there is no significant background from lighter nuclei (a major consideration when
you spend almost 5 years in space !) Approximately 2500 particles with Z > 65 have
been located.
Following carefully controlled etching of 20 plates from the top of each stack and a
similar number from the bottom, a set of the 40 etch rate values V_ were determined for
each ultra heavy candidate. Charge identification is based on measurement of (i) G, the
i ds where S = v_j_ V9 is the bulk etch rate and X isfractional etch rate gradient; G - s dx v_ ,
the path length and (ii) the effective reduced etch rate Self. S is related to a restricted
energy loss function through S = g(REL) h where g and h are determined from our
U(-_ 950MeV/N) and Au(_ 1150MeV/N) calibrations at the Bevalac. Typical S
versus path length plots are shown in references 7 and 8.
The Preliminary Charge Spectrum
Results and Discussion
The preliminary charge spectrum, based on approximately 15% (_ 450 events) of
the total sample, is shown in Fig 1. The presence of nuclei beyond the actinide gap
is significant and in view of the modest charge spread (_ le) expected from statisti-
cal 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 Z > 87 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.
131
O
'_" '' I I I .... I
0
r")
0
60 70 80 9O 1 O0
Z
Figure 1: A preliminary charge spectrum for nuclei with Z > 70
based on _ 450 nuclei (_ 15% of total sample) from the Dublin-
ESTEC experiment.
Selection from stacks with the smallest temperature excursions will also help and
the possibility of obtaining abundance ratios for charge groups (and even a Th/U value)
in this region looks promising. The ratio of (Z >__88)/(74 < Z ___87) from Fig 1 is
0.031 + 0.009 compared with 0.025-t-0.015 for the combined HEAO-3+Ariel-VI data. It
should be emphasised however that this early value from the Dublin-ESTEC experiment
should be taken as a lower value 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 74 < Z < 87 charge region 9. The relative significance of
fission and electromagnetic interactions at high energies for both of these groups of
nuclei will be the subject of future investigations. Our initial method of scanning,
while efficient at locating those nuclei which survive ] of their trajectory through the
stacks, does not pick up those which interact and lose sufficient charge to fall below
threshold (Z _ 65), as would occur in fission for example. We would expect this effect
to be highest for the actinide elements. We hope to tackle this problem shortly. Further
132
down the chargescale 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.
Within another year we hope to have completed measurements on a total of 1000
ultra heavy nuclei and will have had the opportunity to consider any systematic effects
which may exist before trying any serious deconvolution of the spectral peaks. The
overall situation at present certainly gives good reason for optimism.
REFERENCES
1. W.R. Binns, T.L. Garrard, P.S. Gibner, M.H. Israel, M.P. Kertzmann, J. Klar-
mann, B.J. Newport, E.C. Stone and C.J. Waddington, The Astrophysical Jour-
nal, (1989), 346, 997-1009
2. P.H. Fowler, R.N.F. Walker, M.R.W. Masheder, R.T. Moses, A. Worley and A.M.
Gay, The Astrophysical Journal, (1987), 314, 739-746
3. J.R. Letaw, R. Silberberg and C.H. Tsao, Ap. J., (1984), 279, 144
4. J.F. Ormes and R.J. Protheroe, Ap. J., (1983), 272,756
5. R. Silberberg and C.H. Tsao, Ap. J. Suppl., (1973), 25, 315
6. A. Thompson, D. O'Sullivan, K-P Wenzel, J. Bosch, R. Keegan, C. Domingo, J.
Daly and A. Smit, Proc 22nd ICRC, (1991), 2, 543, (Dublin)
7. D. O'Sullivan, A. Thompson, J. Bosch, R. Keegan, K-P Wenzel, F. Jansen, C.
Domingo, Adv. Sp. Res., (1993), to be published.
8. A. Thompson, D. O'Sullivan, J. Bosch, R. Keegan, K-P Wenzel, F. Jansen and
C. Domingo, Proc Second LDEF Post-Retrieval Symposium, (1992), 1,261, (San
Diego)
9. L.Y. Geer, J. Klarmann, B.S. Nilsen, C.J. Waddington, W.R. Binns, J.R. Cummings
and T.L. Garrard, Proc 23rd ICRC, (1993), 2, 191, (Calgary)
133



Early results from the ultra heavy cosmic ray experiment

Early results from the ultra heavy cosmic ray experiment

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server