The individual isotopes of galactic cosmic ray Ne, Mg, and

Si at ~100 MeV/nucleon have been clearly resolved with an

rms mass resolution of ~0.20 amu. Our results suggest that

the cosmic ray source is enriched in ^(22)Ne, ^(25)Mg, and ^(26)Mg

when compared to the solar system. In particular, we find

(^(25)Mg + ~^(6)Mg)/^(24)Mg = 0.49+/-.2314 compared to the solar system value of 0.27, suggesting t~at the cosmic ray source and

solar system material were synthesized under different

conditions

Mewaldt, R. A.

Spalding, J. D.

Stone, E. C.

Vogt, R. E.

The individual isotopes of galactic cosmic ray Ne, Mg, and Si at 100 MeV/nucleon were clearly resolved with an rms mass resolution of 0.20 amu. The results suggest the cosmic ray source is enriched in Ne-22, Mg-25, and Mg-26 when compared to the solar system. The ratio of (Mg-25)+(Mg-26) to Mg-24, which is approximately 0.49 compared to the solar system value of 0.27, suggest that the cosmic ray source and solar system material were synthesized under different conditions

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https://ntrs.nasa.gov/search.jsp?R=19800014759 2020-03-21T18:09:08+00:00Z
(NASA-CR-162923) SASELLISE MEASUREflENTS OF	 N80-23248
THE ISOTOPIC CCMPOSITICN OT GALACTIC COSAIC
RAYS (California inst. of jech.) 7 p
11C A02,0iF A01	 C$CL 03E	 Unclas
G3/93 46814
SATELLITE MEASUREMENTS OF THE ISOTOPIC
COMPOSITION OF GALACTIC COSMIC RAYS
R. A. Mewaldt, J. D. Spalding, E. C. Stone, and R. E. Vogt
California Institute of Technology
Pasadena, California 91125
SRL 79-9
Presented at
the 16th International Cosmic Ray Conference
August 6 - 18, 1979
Kyoto, Japan
Paper OG 7-2
^ I
	^ x
I
	
OG 7-2
Satellite Measurements of the Isotopic
Composition of Galactic Cosmic Rays
R. A. Mewaldt, J. D. Spalding, E. C. Stone, and R. E. Vogt
California Institute of Technology
Pasadena, California 91125
The individual isotopes of galactic cosmic ray Ne, Mg, and
Si at — 100 MeV /nucleon have been clearly resolved with an
rms mass resolution of N 0.20 amu. Our results suggest that
the cosmic ray source is enriched in 22Ne, 25Mg, and 25Mg
when compared to the solar system. In particular, we find
(25Mg + 6Mg) /2aMg = 0.49± .2 4 compared to the solar system
value of 0.27, suggesting t^iat the cosmic ray source and
solar system material were synthesized under different
conditions.
1. Introduction. Galactic cosmic rays provide a sample of matter from
outside the so ar system, which, in view of its younger age and ,possible assoc-
iation with supernovae, may have experienced a different nucleosynthetic his-
tory than solar system material. Because the elements neon, magnesium
and silicon each possess more than one relatively abundant isotope and because
they are the result of several nucleosynthetic processes, they are excellent
choices for investigating possible isotopic differences between the cosmic ray
source and the solar system.
Neon is the first element for which there is strong evidence for an anom-
alous galactic cosmic ray isotopic composition. Several recent studies
(Fisher et al. 1976; Prezler et al. 1975; Garcia-Munoz et al. 1979; Greiner et
al. 1979 find a several-fold excess of 22Ne at the cosmic y source when
compared to the Camerosr (1973) solar system compilation ( 20 Ne/22Ne = 8.2) or
to solar flare nuclei (2 Ne/ 22 Ne = 7.6 1;8, Mewaldt et al. 1979; see also
Dietrich and Simpson 1979); or to the solar wind ( 20N-e7nNe = 13.7, GEiss 1972).
In this paper we report the first cosmic ray observations with clearly
resolved individual isotopes of Ne, Mg and Si. These observations cover the
energy interval from N 30 to N 180 MeV/nucleon, where a mass resolution of
o = 0.20 amu is achieved.
2. Observations. The Caltech Heavy isotope Spectrometer Telescope (HIST)'is
carried on ISEE- , 'launched 8/12/78. This study includes quiet-time data from
launch until 12/1/78. The HIST telescope (Altho;:;e et al. 1978) consists of an
array of solid state detectors, including a pair ;)f tw- o-3&imensional position-
sensitive detectors which determine individual particle trajectories, thereby
leading to significant improvement in isotope resolution over previous cosmic
ray instruments.
The method of resolving isotopes in HIST has been discussed by Mewaldt
et al,(1979). In this study we used the outputs of the last three triggered
Te-
 ectors to make two separate determinations of the charge Z and the mass M
for each event. Those events for which the two mass determinations were con-
sistent to within ± 5% were accepted for further analysis and a best estimate
2	 OG 7-2
for M was computed from a weighted average of the two individual mass determi-
nations. Only 4 events were eliminated by this consistency test. Figure la
shows mass histograms obtained in accelerator calibrations of HIST at the Beva-
lac, while Figure lb shows the flight data from the same energy range. Both
data sets have been analyzed in exactly the same manner, using identical selec-
tion criteria and instrument 	 calibrations. The Bevalac data therefore pro-
vide absolute knowledge of the mass scale, The average mass resolution in the
calibration data is am = 0«18 amu, while in the flight data am = 0,20 amu«
We determined the relative isotopic abundances using two-dimensional max-
imum likelihood techniques that take into account both mass determinations for
each event. These abundances were ;. ,;en corrected for small energy interval
differences, assuming modulated energy spectra dJ/dT a T0•6 at 1 AU, as sug-
gested by measurements by Garcia -Munoz et al. (1977a). Table 1 summarizes our
measurements at 1 AU. In Figure 2 we compare our observations with others, and
with the expected isotopic Fractions for a cosmic ray source of solar system
isotopic composition.
Other investigations measured the mean mass of neon (Fisher et al .1976;
Dwyer 1978), or the 22Ne/2 % ratio (Prezler et al. 1975; Garcia-Munoz et al.
1979; Greiner et al. 1979), without determining tle 21 Ne abundance. To`Tnclude
these measurements in Figure 2 we assumed a 21 Ne fraction of 0.11, as is calcu-
lated assuming negligible 2 1 Ne at the source. Our observations are consistent
with those studies that f ,>nd excess 22Ne in the cosmic rays. The mean mass
measurements of Fisher of al. (1976) and of Dwyer (1978) do not yield unambigu-
ous individual isotopic abundances for Mg and Si, but their results are gen-
erally consistent with a source having solar system isotopic composition. In
TABLE 1 - FRACTIONAL ISOTOPIP_ADUNDANCES_
Cosmic Ray Solar
Source 
t
System
6Fraction Fraction
0.75	 14 0.889 q
< 0.06 0.003 2
0,25 0.10813 c
m
0.67+: 09 0.787
0,16 ,06 0.101
0	 1
0.17+
.06 0.112 I
Observed
Isoto a Fractiont
20 Ne :0.620^
2l Ne 0.07+,03
22Ne 0,31+.08
24Mg 0.60.07
25M9 0.19	 04
26M
g
0.21..04
25Mg + 26Mg 0.40+ : 0704
2851 0.86+ 11
2951 0.07+.0
3051 0.07+*03
2 9
SI 
+ 3051 0.14+.03
+ The fraction relative to the total element
abundance.
t 68% confidence intervals
$ Cameron (1973)
(a) Bevoloc'Calibration Data
00	 Neon	 Mogneswm	 Silicon( X 5)	 (x2)
00
0.
0	 / --1
(b) Flight Data
5
Neon	 Mogriesium	 Silicon
0
5
0 n
 20	 ?2	 24	 26	 28	 30	 32
Atomic Number
0,33 
,07	 0.213
0,91+ 02	 0.92,2
0,04 02	 0.047
0,05+03	
0.031
0109; 03	 0.078
Fig . 1 « Maas W tognam.6 o6 (a) Bevatac
c c tation data and (b) 6ti,ght data gon
N 30 to ^« 180 MeV/nucteon Ne, Mg and Si
aotope,5. In additi.on to ztabte .isotopes,
the eaP.i.bnation data ,incCude Aont-Zi.ved
radioactive 6 peaie^s .
ORIGINAL PAGE IS
OF POOR QUALITY
OG 7-2
0.e
4v Cal tech 0979)- Satellite
o UCB (1979)-Salellile
0 Chicago (1979)-Satellite
x Chicago (1978)-Bolloon
o GSFC (1976)-Balloon
• UNH (1975)-Bolloon
0,0	
21Ne 22Ne	 21mg No
	
29sl _ i
Isotope
0.4
G0
0
li 0.3
0N
i 0,2
aO
0,1
tM. 2. A compat"on os Shad oftat
U o opic abundances. The dotted
In order to calculate the effect	 Vn,e6 ate eaeeueated Son a zotat
of solar modulation on the relative 	 6y.6tem .6outce. Vata teSeAence,6:
abundances of isotopes with different 	 UCB (Gte.inet et ae. 9979); Chicago
charge to mass ratios we evaluated num-
	
1979 (Ganaia.-Munoz et aZ. 1979)
erical solutions of the Fokker-Planck 	 Chicago 1978 (Uwye 71W),; GSFC
equation, A convenient measure of the	 (F,iahet et at. 1976); UNH Wkeztet
degree of solar modulation is the 	 et at. 1T13T.
energy-loss parameter 4) measured in
MeV/nucleon (Gleeson and Axford 1968).For an assumed source spectrum
dJ/dT « (T + E.) -2.6 , where T is the kinetic energyy in MeV/nucleon, Garcia-
Munoz et al. R977b) find o = 220 MeV/nucleon for 1973-1976 with Eo = 400
MeV'/nucleon. We used this spectrum and o = 300 MeV/nucleon for late 1978..For
E 
2:2
400+200 
 
MeV/nucleon, and corresponding values of 0 = ( 300 ± 100)
M8V/ ucleon, we find that the 1 AU 2 Ne/ 2O Ne ratio at ^ 100 MeV/nucleon is
(19 + 6)%'greater than the interstellar value at T = (100 + o) MeV/hucleon.
Solar modulation effects on the other isotopic ratios considered here are
correspondingly smaller than for 22Ne/2ONe.
Figure 3 shows the predicted 25Mg + 26Mg fraction at 1 AU as a function
of the source fraction for various combinations of 0 and a, assuming that 26A1
decays. Also indicated are the 68% (la) and 90% (1.65x) confidence intervals,
as derived from the maximum likelihood analysis. Note that our observations
imply a 25Mg + 26Mg source fraction of 0.33+ • 07, significantly greater than
the solar system fraction of 0.21. Using similar curves for the individual Ne,
Mg and Si isotopes, we obtain the source composition in Table 1. The uncertain-
ties in Table 1 include statistical uncertainties, as well as uncertainties
associated with propagation and modulation (typically — .01 ), as derived from
the envelope of the calculated curves for a = (5.5 ± 1) g/cm2 and
o = (300 ± 100) MeV/nucleon (see, e.g., Figure 3).
,u
3
contrast our results suggest excesses
of both isMg and 26Mg at the source.
In addition, Simpson et al. (1977) have
reported both isotopic fractions and
mean mass measurements for Ne, Mg and
Si at N 3 g/cm 2 residual atmosphere
which they concluded were consistent
with a solar system uiijrce composition,
3.	 Interpretation. In order to in-
terpret our- o s`--nervations we performed
interstellar propagation and solar mod-
ulation calculations. We assumed a
standard leaky-box propagation model
with an escare mean free path of
A = 5.5 g/cm (Garcia-Munoz et al.1977b)
and used the semi-empirical cross sec-
tions of Silberberg e t al. (1976). The
element source abundances (6 s Z s 28)
were adjusted to fit measurements by
Garcia-Munoz et al. (1977a, 1977c) and
Webber and Lezniac (1978). The Source
was assumed to have solar system iso-
topic composition except for Ne, Mg and
Si, where the isotopic fractions were
varied for comparison with the observa-
tions.
4-400
f
0.5 r'
1'L .200
0,4-90'A	 6®Y.^
0.3
oo
/4.5
0.i	 90%
SS
0.0 1
 .0
	
0,1	 0.2	 913	 0,4	 0,5
Source 2pM9 + l6Mq Fraction
Catcutationz o6 the ob-
b ecvea 2 5A4g + 2 sMg 6naetion vA .
the eouhee 6,kactLon. Sotid
cunve,6: X = 5.5 g1em 2 , with
= (300±100) MeV/nucCeon.
Dabhed euhvez: X = 4.5 and 6.5
g/cm2 toi th o = 300 MeV/nucleon.
Con6,idenee inteAva z o6 68% and
90 ane indicated, as wett a.6
the 6otaA z otem (S.S. ) bna.cti.on.
system value of 0.12.
eu_
+
N
0
I , Y
4	 OG 7-2
An additional uncertainty in the
propagation calculations is associated
With cross section uncertainties. Since
the observed 21 Ne should be almost en-
tirely of secondary origin, 2r Ne obser-
vations provide a check that the propa-
gation model has not grossly underesti-
mated the secondary contributions to the
other nuclei. We find from Stone and
Wiedenbeck (1979) that systematic under-
estimates of a number of 22Ne-production
cross sections by a factor of > 3 rela-
tive to 21 Ne production would be required
to account for the observed 22Ne excess,
while 25M9 and 26Mg excesses of a factor
of N 1.8 would require systematic errors
of a factor of 2 to 3. We feel that such
large systematic errors in the relative
cross sections are very unlikely.
From Table 1 and Figure 2 we note
that 22Ne, 2 5Mg and 26Mg appear to be
more abundant in the cosmic ray source
than in the solar system. For 22Ne we
note that the direct isotopic measure-
ments (including Prezler et al. 1975;
Garcia-Munoz et al. 1979; G^ reTner et al.
1979; and this work) all indicate an ex-
CeS5 of 22Ne, while there is disagree-
ment among the indirect measurements
(Fisher et al. 1976; Dwyer 1978; Simpson
et al. 1977 . For the direct measurements
shown in Figure 2, the mean observed 22Ne
fraction is 0.33 + .02, corresponding to
a source fraction of 0.27 + .03, and a
22Ne/2ONe source ratio N 3 times the solar
For Mg, the derived 25Mg + 26Mg source fraction of 0.33±'g7 (Figure 3)
corresponds to a ( 25Mg + 26Mg)/ 24Mg source ratio that is 1.81.9.5 times the
solar system value of 0.27. Fi lom a maximum likelihood analysis we find only
a 5% probability that the cosmic ra M9 source composition is identical to that
of he solar system. For 29 Si and °Si our observations are statistically
limited and do not rule out possible enhancements of these nuclei in the source.
4.	 Discussion. Because the individual isotopes of Ne, Mg and Si differ in
mass
 `by- , --5  1.0%, mass dependent acceleration is unlikely to be significant, and
isotopic anomalies in the derived source composition more likely reflect the
composition of the material from which cosmic rays are accelerated. Garcia-
Munoz et al. (1979) argued that the interstellar medium (ISM) could not be the
dominant component of cosmic ray source material, since the expected change in
the 22Ne/2O Ne ratio in the ISM since the birth of the solar system is 4 50%.
However, there are a wide variety of neon components observed in the solar sys-
tem (see, e.g,,_Podosek 1978) and it is conceivable that the solar system
neon-A component is not representative of theISM N 4,5 x 10 9
 yearsago. We
therefore conclude only that the solar system neon-A and comic ray source com -
positions are different, independent of their origin.
5	 OG 7-2
Several mechanisms of nucleosynthesis are thought to contribute to Ne,
Mand Si isoto a production. The major contributions to solar system 20Ne,
2^, 25 , 26Mg and 2 ,3 OM and 29,30Si are thought to come from explosive carbon
burning, while 21,22Ne production is the result of He burning, and 28Si is
due mainly to explosive oxygen burning. Woosley (1979) suggested two factors
that might lead to cosmic ray 22Ne enhancements; we discuss below their im-
plications for the Mg and Si isotopes.
One factor contributing to the 22Ne enhancement might be continuing
galactic evolution, since 22Ne and other neutron-rich nuclei are second gen-
eration products of nucleosynthesis. Because the cosmic rays (age — 2 x 107
years) are much younger than the solar system, they may reflect contributions
from later stars of higher average metallicity. During helium burninthe
larger abundance of metals will result in an increased abundance of 2 Ne which
in turn is the source of an increased neutron excess n during explosive car-
bon burning.
The Mg isotopes are particularly sensitive to n, as emphasized by Casse
(1979). Based on Figure 3 of Pardo et al. (1974), our 25Mg + 26Mg excess,,
would result from an increase in n of(30 i 20)% over the solar system value
of n :% 2 x 10- 3 . On the other hand, a factor of N 3 increase in n, as sugges-
ted by 22Ne, would lead to 25Mg/24Mg/ and 26Mg/24 Mg ratios > 1. It therefore
appears unlikely that the Ne and Mg isotope measurements can be explained in
a consistent manner in terms of galactic evolution effects alone.
Since 2ONe and 22Ne are produced in different burning processes in
different zones of a massive star, Woosley (1979) suggested that a second con-
tributing factor to a non-solar 22Ne/20Ne ratio might be a variation in the
relative contribution of these zones, as might be expected if the cosmic rays
and the solar system come from stars of different average mass. We note from
Couch et al. (1974) and Lamb et al. (1977) that in stars with N 8 to N 50 Mo,
22Ne may provide a source of neutrons for a helium-burning s-process with
significant yields of 25Mg and 26Mg. A mixture of such helium-burning products
with typical carbon-burning yields of 20Ne and 24,25,26Mg (and/or with additio-
nal material of solar system composition) could produce the cosmic ray source
Ne and Mg composition (Table 1) if the helium-burning yield of 25Mg + 26Mg did
not exceed that of 22Ne, a constraint that limits the average mass of the stars
involved, We conclude that an enrichment of helium burning products in cosmic
rays can enhance both 2 2Ne/2ONe and ( ?5Mg + 26Mg)/24Mg, as observed, without
significantly affecting 21 Ne or the silicon isotopes. Observations of other
s-process nuclei (see, e.g., Wefel et al. 1977) might test this possibility.
5. Acknowledgments.	 W. E. Althouse, A. C. Cummings and T. L. Garrard
made significant contributions to the development of HIST. We are grateful to
the Heckman/Greiner group and the Bevalac staff at Lawrence Berkeley Laboratory
for making the heavy-ion calibrations possible. We have benefited from dis-
cussions with W,, A. Fowler, M. E. Wiedenbeck and S. E. Woosley. This work
was supported in part by NASA under Contract NAS5-20721 and Grant
NGR 05-002-160.
6. References.
Althouse, W. E., Cummings, A. C., Garrard, T. L., Mewaldt, R. A., Stone, E. C.,
and Vogt, R. E. 1978, Geoscience Electronics, 16, 204.
Cameron, A.G.W. 1973, Space Sci. Rev., 15, 121.
Casse, M. 1979, Ap. J., submitted.
rt
6	 OG 7-2
Couch, R. G., Schmiedekamp, A. B. and Arnett, W. D. 1974, Y. J „ 190, 95.
Dietrich, W. F., and Simpson, J. A. 1979, Y. J., 231, in press.
Dwyer, R. 1978, Ap. J., 224, 691
Fisher, A. J, Hagen, F. A., Maehl, R. C., Ormes, J. F., and Arens, J. F.
1979, Ap. J,, 205, 938.
Garcia-Munoz, M., Mason, G. M., and Simpson, J. A. 1977a,Proc. 15th Internat.
Cosmic „Ra Conf., Plovdiv, 1, 301.
Ibid, 1977b, p. 224.
Garcia-Munoz, M., Mason, G. M., Simpson, J. A., arjd Wefel, J. P. 1977,
Op. Cit., p, 230.
Garcia-Munoz, M., Simpson, J. A., and Wefel, J. P. 1979, Ate. J. Letters), 232,
in press.
Geiss, J. 1973, 13th Internat. Cosmic Ray Conf., Denver, 5, 3375.
Gleeson, L. J., and Axford, W. E. 1968, Ap. J., 134, 1011.
Greiner, D. E., Wiedenbeck, M. E., Bieser, F. S., Crawford, H. J., Heckman,
H. H., and Lindstrom, P. J. 1979, this conference,' Paper OG 7-10,
Lamb, S. A., Howard, W. M., Truran, J. W., and Iben, I., Jr. 1977,
Ap. J., 217, 213_
Lezniak, J. A., and Webber, W. R. 1978, Ap. J., 223, 676.
Mewaldt, R. A., Spalding, J. D., Stone, E. C., and Vogt, R. E. 1979, Ap. J.,
(Letters) , 231, in press.
Pardo, R. C., Couch, R. G., and Arnett, W. D. 1974, Ap. J., 191, 711.
Podosek, F. A. 1978, Ann. Rev. Astron. Astrophys., 16, 293.
Prezler, A. M., Kish, J. C., Lezniak, J. A., Simpson, G., and Webber, W. R.
1975, 14th Internat. Cosmic Ray Conf. Munich, 1^, 4096.
Silberberg, R., Tsao, C. H., and Shapiro, M. M. 1976, in Spallation Nuclear
Reactions and their Applications, ed. B.S.P. Shen and M. Merker
Dortrecht: Rei el	 p. 49
Simpson, G. A., Kish, J. C., Lezniak, J. A., and Webber, W. R. 1977,
15th Internat. Cosmic Ray Conf., Plovdiv, 1, 347.
Stone, E. C., and Wiedenbeck, M. E. 1979, Ap• J., in press.
Wefel, J. P., Schramm, D. N., and Blake, J. B. 1977, Astrophys• Sp. Sci.,
49, 47.
Woosley, S. E. 1979, personal communication.


English

Satellite measurements of the isotopic composition of galactic cosmic rays

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Satellite Measurements of the Isotopic Composition of Galactic Cosmic Rays

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