The suggestions that the source abundances of cosmic ray nuclei heavier then Fe differ significantly from Solar System abundances are not well supported by the data without assuming preferential acceleration. The Solar System abundances of Pb and Bi are split into r-, standard s-, and cyclic 8-process components; the apprarent deficiency of Pb seen in the HEAO-3 Heavy Nuclei Experiment data might indicate an absence of Pb from the recycling 8-process

Blake, J. B.

Margolis, S. H.

English

NASA Technical Reports Server

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OG 7.1 - 7
SOURCES OF THE ULTRAHEAVY COSMIC RAYS
STEVEN H. MARGOLIS
McDonnell Center for the Space Sciences
Washington University
St. Louis, MO 63130 USA
and
J. B. BLAKE
Space Sciences Laboratory
The Aerospace Corporation
Los Angeles, CA 90009 USA
ABSTRACT
The suggestions that the source abundances cosmic ray nuclei heavier than Fe differ
significantly from Solar System abundances are not well supported by the data
without assuming preferential acceleration. The Solar System abundances of Pb
and Bi are split into r-, standard s-, and cyclic s-process components; the apparent
deficiency of Pb seen in the HEAO-3 HNE data might indicate an absence of Pb
from the recycling s-process.
J. Introductions. The nuclei substantially heavier than Fe have been measured only relatively recently
with electronic detectors (Fowler et al. 1983, Binns et al. (1984), and these experiments have
yielded so far only results on even-Z elements or broad abundance groups in estimated charge. The
calculations of Blake and Margolis (1981a} provided a selected set of standard propagations of Solar-
like abundances (26 < Z _<59) which could be used to evaluate the experimental results in terms of r-
and s-process contributions and the effects of preferential acceleration with a first ionization potential
dependence (hereinafter FIPD). In this paper, propagation calculations of even heavier nuclei (Z __ 60)
are discussed in light of the now-published data in this range and other published analyses (Protheroe
and Ormes, 1981; Blake and Margolis, 1981b; Brewster, Freier, and Waddington, 1983; Tsao et
al., 1983). The analysis presented here demonstrates that the simple picture of Solar abundances
propagated a£cording to models inferred from the lower-Z measurements does not account for the
structure reported in the higher-Z abundances.
2.CalculationsOf Propagated Abundances. The chemical and isotopicabundances of Anders and
Ebihara (1982)providea specificsample ofnucleosynthesisinthe Galaxy. These were propagated (a
fulldescriptionofthe calculationappearsinMargolisand Blake,1985)togeneratea setofabundances
similartowhat might be measured nearthe Earth,excludingthe effectsofSolarmodulation.Figure1
compares one suchcalculation,thatfora 7 g cm -_ leakybox (shown asa solidline)with a composite
data set (shown as isolatedcircles)assembled from the preliminaryresultsof the HEAO-3 Heavy
NucleiExperiment (Binns et al.,1981, 1984). Both setsofvalueshave been normalizedto 106 Fe
nuclei,and the calculatedvalueshave been smeared with a Gaussian profileofwidth varyingwith
chargeinordertojointhe deconvolvedlowerchargestothe reportedresolutionofthe highercharges.
For comparison,the dashed lineinthat figuredisplays,wlth the same normalizationand smearing,
the sourceabundances withoutpropagation.In general,the patternstrackreasonablywell.
The plottingrange necessaryfor displayingFigure 1 makes itan inconvenientform to use for
most comparisons with observations,but such a displayhas a significantadvantage over the more
usualelementgroup plots.The typicalgroup plotillustratesthe changingbalancebetween secondary
and primary elements resultingfrom propagation. One must remember that the comparisons to
observationmust be made not only interms ofthe ratiosofwidelyseparatedelements.The systematic
effectsofnucleosynthesisaffectboth kindsof ratios.The SolarSystem abundances,a known sample
of cosmic matter,providea definite,globalnormalizationto the valueof a singleelement. In the
absence ofany isotopicdata, thisnormalizationisallthe more important. For example, elements
near theTe-Ba peak do not change theirabundances relativetoFe with propagation,but thoseatthe
Pt-Pb peak do. This contrast implies that the group ratio (50 < Z < 56)/(Z --=26) is not a measure
of propagation but of the most basic nucleosynthetic indicator: the abundance of r- and s-process
https://ntrs.nasa.gov/search.jsp?R=19850026555 2020-03-20T16:59:31+00:00Z
22
OG 7ol - 7
' I I I
4 - Figure 1. The circles represent
a composite of preliminary re-
sults from the HEAO-3 Heavy
2- _ Nuclei Experiment (Binns et
" o al., 1981, 1984). The dashed
z _: . V o line shows the Solar abundances
"" " o of Anders and Ebihara (1982)
convoived with a Gaussian pro-
0 - "......... _ o file of varying width. The solid
line shows the Solar abundances
-2 __' "_" propagated through a 7 g cm -2
_y_ leaky box, also smeared.
-'2o iOo
Z
material relative to that of the iron peak. In this respect, the data indicate that the cosmic ray source
is consistent with the solar balance of those nucleosynthesis processes.
Although propagation does affect the observed abundances, removing the effects of a reasonable
propagation model is probably sufficient. At least, the available data do not warrant more careful
attention to such detail. It is not yet clear that any of the current experiments can recover a global
normalization to Fe with the precision reported in a calculation. However, it is clearly worth con-
siderable effort to obtain the best possible global abundance pattern from the experimental data.
Important questions can be addressed with only minimal requirements on the charge resolution of an
experiment.
3. Preferential Acceleration/Selection Effects. It is now standard procedure to consider selection ef-
fects in the cosmic ray chemical abundances based upon atomic properties of the element in question.
The parameter usually chosen is the first ionization potential of the atom. A straight-line fit is made
to a semi-log plot of the ratio of cosmic ray source abundances (derived from the data) to Solar System
abundances vs. the ionization potential of the atom in question. This fit is then used to weight the
source abundances which are input into a propagation calculation. Binns et al. (1984) question this
approach, and argue that an equally good fit to their data is obtained with a step function rather than
a straight line. They suggest that the discontinuity is an artifact caused by selecting CI carbonaceous
chondrites rather than C2 as the baseline for the Solar System abundances.
This is an interesting suggestion but there is considerable reason for regarding it with suspi-
cion. First, at the sun and in planetary magnetospheres where individual acceleration events can be
observed, preferential acceleration is ubiquitous. Second, the plot of the abundance ratio vs. first
ionization potential continues to be ordered after exchanging C2 abundances for CI abundances.
Difference in the ratio of r- to s-process abundances between cosmic rays and the Solar System can
indicate either source differences or preferential acceleration. The question of preferential acceleration
in the study of ultraheavy cosmic rays is so vexing because the r- and s-process peaks also have, on
the average, significantly different first ionization potentials. However, selection effects should depend
on the overall atomic physics of the atom and not simply the first ionization potential. Margolis and
Blake (1983) noted that the influence of the second ionization potential disperses the predictions of
preferential acceleration models in a manner consistent with the scatter of the observations about the
straight-line fit. Although not perfect, such modifications are certainly the direction for further study.
Without more knowledge of the nature of the cosmic ray source, it is difficult to make a meaningful
improvement on the presently existing work.
2_
OG 7.1 - 7
4. Nucleosynthesis Considerations. It is very well known that the nucleosynthesis of the elements
with A > 60 results largely from neutron capture processes; the p-process is responsible only for
rare isotopes. In the analysis of ultraheavy cosmic ray data it is traditional to examine the paired
abundance peaks which are due to neutron shell closure -- Se and Sr, Te and Ba, and Pt and Pb --
as an indicator of the relative abundance of the r- and s-process in the cosmic ray source(s). For (Te,
Ba) and (Pt, Pb), the lower mass peak in the pair is largely the result of the r-process, and the upper
one to the s-process nuclei. The heavier nuclei around Sr are mainly s-process, but the nuclei around
Se show substantial contributions from both processes.
Implicit in these assignments is the assumption that each peak comes from a single nucleosynthetic
site, at least once the contribution from the companion process is removed. However, in the case of the
s-process peak at Pb, this assumption is incorrect. The s-process which fits the heavier Solar System
isotopes up through 2°4pb underproduces the heavier Pb isotopes (K_ppeler et al., 1982; Ulrich 1983:
although these references are recent and refer to the latest comprehensive work, this fact has been
known for a long time.) The s-process terminates at 2°9Bi because the next heavier nucleus, 21°po,
decays by emission back to 2°6Pb. Therefore, with sufficient neutron exposure, all nuclei capture
sufficient neutrons to join the quartet consisting of 2°e,2°_,2°Spb, and 2°9Bi. The recycling proces_
was described in detail by Clayton and Rassbach (1967). The important point for present purposes
is that the site of the intense s-process exposure required to cause recycling may well not be the same
as that which creates the s-process isotopes with A <_ 204 (Truran and Iben, 1977; Ulrich, 1983).
Therefore, if the cosmic ray source were deficient in s-process material only from the recycling site,
then the r-process and s-process nuclei at (Se, Sr) and (Te, Ba) would appear Solar, but at (Pt,Pb),
Pb would be deficient.
Table 1. The nucleosynthetic
Nucleus Total Normal Radiogenic Recycling origins of the nuclei affected
Abundance s-process r-process s-process by the cyclic s-process. The
2°6Pb 0.603 0.180 0.12 0.30 Total Abundance is taken from
2°_Pb 0.650 0.176 0.10 0.37 Anders and Ebihara (1982),
2°Spb 1.838 0.402 0.06 1.38 and the Normal s-process from
_°gBi 0.144 0.016 0.12 0.01 K[£ppeler et al., (1982).
Given here is a zeroth-order decomposition of the Pb peak. The details can be found in Max-
golis and Blake (1985). It can be seen from the abundance values given in Table 1 that _ 65% of
the Pb is produced in the recycling s-process. "Pruran and Iben (1977) have suggested stars with
M > 15 M e produce many of the s-process isotopes in the mass range 25 < A < 70, that stars
with 2 M® < M < 8 M o make the bulk of the s-process isotopes for A > 70, and that stars with
M < 1 Mo are responsible for 2°e,2°v,2°SPb and 2°°Bi. Thus one possible explanation for the HEAO-3
observation of a deficiency of counts in the Pb region is that the nucleosynthetic contribution of stars
with M < 1 Mo is under-represented in the cosmic ray source.
, With the contribution of the cyclic s-process deleted, the modified Solar System source abundances
have been propagated through a 7 gcm -2 leaky box for comparison with the measured abundances.
A detailed look at the contribution of the terminal s-process is visible in Figure 2, which compares
the abundances of the nuclei above the Te-Ba peak with (solid line) and without (dashed line) the
additional material. Calculations show that about 75°A of the difference in source abundances appears
as contributions to the elements at lower Z. Such is the relative abundance of Pb that this contribution
is significant throughout the illustrated charge region. Despite this change, the measured values in
the rare-earth region appear too high. This might indicate a more extreme propagation model, but
more probable is that the source abundances in the rare-earth region need further adjustment if the
data are correct. The rare-earth elements have nearly the same first ionization potentials, all lower
than that of iron, but are refractory to differing degrees. Should that have an effect on the injection
process (see above), the variation of injection efficiency across these elements migh provide a useful
gauge of the nature of preferential acceleration.
5. Conclusions. The most important conclusion of this worL is that the overall composition of the
galactic cosmic ray source appears remarkably like that of the Solar System. The abundances above
24
OG 7.1 - 7
Z > 60: WITH AND WITHOUT CYCLIC S-PROCESS
15 .... I .... I .... [ .... I ....
WITH
_-- WITHOUT Figure 2. The net contribution to the
propagated abundances is illustrated
as the difference between the solid line,10-
._. I- showing the standard Solar System
_ !i source, and the dashed line, showing the
I r--1 modified source calculated in this work.
Approximately 75% of Pb at the source
- is incorporated into elements of lower Z.
The vertical scale is linear, but the units
are arbitrary.
I i I J I , , , , ' ' I i _"J
30 65 70 75 80 ' Ei
ATOMICNUMBER
Fe display a very Solaz-like balance between several different nucleosynthetic processes. However, if
the Pb abundance reported by Binns et al. (1984)is correct, it might signify the lack of the products
of the terminal or cyclic s-process. Such evidence could be used to rule out the appearance of stars
with masses M < 1 M o on the roster of cosmic ray sources. Until more precise determinations axe
available, however, it seems prudent not to consider this abundance as additional confirmation of ever
more complex models for selective acceleration.
6. Acknowledgements. The work reported here was supported at Washington University under NASA
grant NAG8-448 and McDonnell Center reseaxch funds and at the Aerospace Corp. under company-
sponsored research funds.
7. References.
Anders, E., and Ebihara, M., 1982. Geochim. Cosmo. Acta, 46, 2263.
Binns, W. R., Fickle, R. K., Garrard, T. L., Israel, M. H., Klarmann, J., Stone, E. C., and Waddington,
C. J., 1981. Ap. J. (Letters), 247, L115.
Binns, W. R., Fixsen, D. J., Garrazd, T. L., Israel, M. H., Klarmann, J., Stone, E. C., and Waxidington,
C. J., 1984. Adv. Space Rcs., 25, 4.
Blake, J. B., and Margolis, S. H., 1981a. Ap. J., 21il, 402.
Blake, J. B., and Margolis, S. H., 1981b. Proc. 17th Int. Cosmic Ray Conf. (Paris), 2, 41.
Brewster, N. R., Freier, P. S., and Waddington, C. J. 1983. Ap. J., 264, 324.
Clayton, D. D., and Rassba£h, M. E., 1967. Ap. J., 143, 69.
Fowler, P. H., Masheder, M. R. W., Moses, R. T., Walker, R. N. F., and Worley, A., 1983. Proc. 18th
Int. Cosmic Ray Conf. (Bangalore), 9, 110.
K_ppeler, F., Beer, H., Wisshak, K., Clayton, D. D., Macklin, R. L., and Ward, R. A., 1981. Ap. J., *
257, 821.
Margolis, S. H., 1983. Proc. 18th Int. Cosmic Ray Conf. (Bangalore), 9, 267.
Margolis, S. H., and Blake, J. B., 1983. Proc. ISth Int. Cosmic Ray Conf. (Bangalore), 9, 283.
Margolis, S. H., and Blake, J. B. 1985. Ap. J., (in press).
Protheroe, R. J., and Ormes, J. F., 1981. Proc. 17th Int. Cosmic Ray Conf. (Paris), 9, 114.
Truran, J. W., and Iben, I., Jr., 1977. Ap. J., 216, 797.
Tsao, C. H., Silberberg, R., Jr., J. H. Adams,, and Letaw, J. R., 1983. Proc. 18th Int. Cosmic Ray
Conf. (Bang,lore), 2, 225.
Ulrich, R. K. 1983. Essays in Nuclear Astrophysics, ed. Barnes, C. A., Clayton, D. D. and Schramm,
D. N. (Cambridge: Cambridge U. Press), 301.


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