Several source spectra were constructed from combinations of 4- and s-process nuclei to match the observed charge spectrum of VVH particles. Their propagation was then followed, allowing for interactions and decay, and comparisons were made between the calculated near-earth spectra and those observed during high altitude balloon flights. None of the models gave good agreement with observations

Blanford, G. E., Jr.

Friedlander, M. W.

Klarmann, J.

Walker, R. M.

Wefel, J. P.

English

NASA Technical Reports Server

163
THE ORIGIN AND PROPAGATION OF VVH PRIMARY COSMIC RAY PARTICLES
G. E. Blanford Jr.,* M. W. Friedlander, J. Klarmann 0
R. M. Walker and J. P. Wefel** )"H o
Laboratory for Space Physics, Washington University
St. Louis, Missouri 63130, U.S.A.
In an attempt to match the observed charge spectrum O N
of VVH particles, several source spectra have been
constructed from combinations of r- and s-process C
nuclei. Their propagation has then been followed, o
allowing for interactions and decay, and comparisons t H
have been made between the calculated near-Earth 0
spectra and those observed during high altitude
balloon flights. None of the models yet used leads in o H
to good agreement with observation, suggesting either -  n
that more complicated models need to be examined, or
that different components (for instance the r- and
s-process nuclei) have different histories. Lq
1. Introduction. In the two preceding papers, we have described n
our observations of cosmic ray primaries having Z >35, and we have
pointed to several significant features of the charge spectrum.
In the present report, we confine our attention to the charge re- U
gion from Z >50, and we examine in more detail some inferences that
can be drawn regarding the origin and history of these particles.
2. Experimental Aspects. The data for the charge region Z >50
were those described in the two preceding papers. Attention has
been confined to those events that were clearly due to particles
that were fast primaries recorded at altitude. The distribution
of detector and absorber layers in the flight systems allows us
to be certain that the present sample of events contains no un-
recognised slow secondary particles such as those we have described
elsewhere (Blanford et al, 1972).
In Table I, we have displayed the data. It should be empha-
sised that the major features of the charge spectrum at the top
of the atmosphere are already exhibited in the raw data at flight
altitude, and are not the result of any artifact of the propaga-
tion calculations that correct for collisions in the atmosphere.
3. Discussion and Conclusion. As first suggested by Mewaldt et
al, (1970) and discussed in detail by Schramm (1972), there are
some ratios of abundances that may provide an indication of the
region of cosmic ray confinement by measuring the competition
between fragmentation and radioactive decay for nuclei with Z 86.
Thus the ratios (286)/(81-85) and (286)/(76-85) in principle might
be used, but the statistical accuracy of the present experimentally
*Now at NASA-Johnson Space Center, Houston, Texas, 77058.
**Now at Naval Research Laboratory, Washington, D. C. 20357.
https://ntrs.nasa.gov/search.jsp?R=19740006345 2020-03-23T13:06:23+00:00Z
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Table I
Relative Abundances at the Top of the Atmosphere
Model Predictions
Charge Group This Work Case 1 Case 2
r s r s
>86 6 3 6 - 2 -
81-85 6 ± 3 5 16 4 13
76-80 9 ± 4 12 4 11 5
71-75 11 ± 4 4 4 3 4
61-70 16 ± 5 11 4 8 3
56-60 29 ± 6 12 55 8 51
51-55 23 ± 5 50 17 64 24
Note: All abundances are expressed as % of the total number
of particles with Z > 50.
based ratios is not sufficient to permit any firm conclusions to
be drawn.
We have observed three particles in the Z 86 region; their
most probable charge assignments were Z = 90. These ultra-heavy
nuclei can be produced only in the r-process of nucleosynthesis.
We have found no events that can be identified as due to trans-
uranic nuclei, whose production is also expected during the r-
process. Other groups, in a total flight exposure approximately
equal to ours, have observed two probable transuranic nuclei.
(Fowler et al, 1967, 1970; Price et al, 1971). Combining all
results we obtain a ratio
(Z .92)/(90 _< Z < 92) = 0.3 ± 0.2
for cosmic rays with kinetic energies 1.0 GeV/nucleon.
Schramm and Fowler (1971) have recalculated the yields of
actinide elements in the r-process, and from their results we
expect a ratio (Z> 92)/(90s Z 92) between 0.5 and 1.0, depending
on the length of the time during which these elements have under-
gone radioactive decay. The low experimental ratio might be
taken as evidence for an underabundance of transuranic nuclei,
but this conclusion must be tempered with the realities of charge
resolution. The elements with Z = 93 and 94 are major components
of the expected transuranic nuclei, and, in order to draw conclu-
sions based on the (Z >92)/(90 5Z 592) ratio, the experimental
data require a charq resolution of at least AZ = 1. Our charge
resolution of AZ = %-6% is insufficient to separate the lighter
transuranic nuclei from the uranium and thorium nuclei. Better
resolution and greatly increased statistics will be required be-
fore the situation with regard to transuranic nuclei is clarified.
If the r-process is regarded as the sole synthesising mecha-
nism for VVH cosmic rays, we would anticipate peaks in the rela-
tive abundances in the 76-80 and 51-55 charge groups. As shown
3
163
in Table I, however, we observed approximately equal abundances
for the (71-75) and (76-80) groups and for the (51-55) and
(56-60) nuclei. We have therefore investigated the possibility
that there might also be some s-process material contributing
to the VVH particle spectrum.
The large abundance of (71-75) nuclei cannot be explained
by poor charge resolution, nor by spill-over from the (61-70)
group, since many of the (71-75) events have identified charge
values of 74 or 75. Some of the (71-75) events may more properly
belong in the (76-80) group, but it seems unlikely that this
effect can be large enough to explain the overabundance in
(71-75). Similarly, the abundances of (51-60) nuclei cannot
be explained through spillover from neighboring groups. The
problem of charge resolution affects the (51-55) and (56-60)
groups, since the main contributors are expected to be at Z = 54
and 56. Our measured charge resolution in this region is
AZ = 1-3.2 which implies that there is a greater probability of
mis-identifying a true Z = 56 particle as Z = 54 than vice versa.
Thus although we cannot exclude the possibility that a few events
may have been assigned to the wrong groups, it seems unlikely
that the majority of these (51-60) events have been wrongly
identified. We can also rule out a systematic effect that would
move the entire charge scale up or down by several units. (See
preceding paper.)
In Table I, we have listed the relative abundances to be
expected from pure r- and s-process sources, for two different
cases, and compare these with the observed abundances. The r-
and s-spectra have been computed as follows. The relative
element abundances for a given case have been propagated to the
Earth using a steady-state model. We investigated leakage lengths
between 4 and 10 gm/cm2 and found little change in the relative
abundances. The spectra listed in Table I were obtained using
a leakage length of 6.5 gm/cm 2 .
Seeger, Fowler and Clayton (1965) determined the relative
fraction of each isotope that is produced by the r or s process.
Combining these with solar system element abundances yields
relative abundance spectra from r and s material. The solar
system element abundances used by Seeger, Fowler and Clayton
differ markedly in some case from a more recent compilation
of abundances by Cameron (1968). In addition Allen, et al,
(1971) have summarized recent measurements of neutron captive
cross sections and have suggested some modifications in both
the isotopic r and s fractions and the solar system abundances.
We have therefore used two sets of r and s abundances in
the calculations. Case 1 refers to the abundances as given by
Seeger, Fowler and Clayton with the r-process abundance of Pb,
Bi, and the actinide elements computed by the method of Clayton
and Rossbach (1967) using the normalization parameters r = r'
= 0.4. The second case covers the r and s spectra derived from
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163
the Cameron abundances taking into account the modifications
suggested by Allen et al, including the normalization parameter
r = 0.6. The actinide element abundances for case 2 were de-
rived from the work of Schramm and Fowler (1971) normalized to
the r-process abundance of Pt' 95 as suggested by Schramm (1971).
These cases represent, nearly, the extremes of relative r and
s process abundances for material whose history is the same as
the material of the solar system.
There are several conclusions that can be drawn from Table
I. First, the ratio (56-60)/(51-55) provides a very sensitive
test for the presence of s-process nuclei in the VVH charge
spectrum. This ratio varies from < 0.25 for a pure r-process
source to > 2.0 for an s-process source. On the other hand
the ratio (76-80)/(71-75) varies from approximately 1 to 3
with the lower values representing the s-process. In addition,
for both cases considered the abundance of (71-75) nuclei is
expected to be quite low.
Except for the (71-75) group, the experimental abundances
for Z >60 agree reasonably well with either a pure r-process or
a combination of r and s-process source abundances. However,
the (51-60) data requires a substantial contribution of s-process
material. Within the experimental uncertainties, it is not pos-
sible to determine precisely the proportion of s-process nuclei
in the VVH spectrum. The (56-60)/(51-55) ratio is consistent
with approximately half of the material having seen formed in
an s-process, but this combination does not adequately explain
the Z > 60 abundances.
Our data thus seem to indicate the presence of both r and
s process nuclei, while also suggesting that the VVH charge
spectrum is probably the result of something more complicated
than a simple r and s process combination with identical propa-
gation histries.
4. Acknowledgements. This work was supported under NASA Grant
NGR-26-008-001 and in part under the National Science Foundation
Science Development Grant at Washington University, GU-1147.
References
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Pomeroy, R. M. Walker and J. P. Wefel (1972), J. Geophys.
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CAMERON, A. G. W. (1968), Origin and Distribution of the Elements
ed. by L. H. Ahrens (Pergamon), p. 25.
CLAYTON, D. D. and M. E. Rassbach, (1967) Ap. J. 148, 69.
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The origin and propagation of VVH primary cosmic ray particles

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