A measurement of the isotopic composition of galactic cosmic ray neon in the energy range 70 to 260 MeV/amu has been made using the U.C. Berkeley HKH instrument aboard ISEE-3. A combination of high resolution and good statistical accuracy makes possible a precise determination of the local interplanetary neon composition. We find /sup 22/Ne//sup 20/Ne = 0.64 +- 0.07 and /sup 21/Ne//sup 20/Ne < 0.30 in local interplanetary space. These ratios, when interpreted in using standard galactic propagation and solar modulation models, yield cosmic ray source abundances which are inconsistent with a solar-like source composition

Bieser, F. S.

Crawford, H. J.

Greiner, D. E.

Heckman, H. H.

Lindstrom, P. J.

Wiedenbeck, M. E.

UNT Digital Library

To be presented at the 16th International 
Cosmic Ray Conference, Kyoto, Japan, 
8-19 August 1979 
LBL-8921 
The Isotopic Composition of Neon in the Galactic 
Cosmic Rays--A High Resolution Measurement 
D.E. Greiner, M.E. Wiedenbeck, F.S. Bieser, H.J. Crawford, 
H.H. Heckman, and P.J. Lindstrom 
University of California Space Sciences Laboratory 
and Lawrence Berkeley Laboratory 
Berkeley, California 94720 
June 1979 
Prepared for the U.S. Department of Energy 
under Contract W-7405-ENG-48 
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DISTRIBUTION OV THIS TJ0CUMENT IS OTTXHTTED 
1 OG7-10 
The Isotopic Composition of Neon in the Galactic 
Cosmic Rays--A High Resolution Measurement 
D.E. Greiner, M.E. Wiedenbeck, F.S. Bieser, H.J. Crawford, 
H.H. Heckman, and P.J. Lindstrom 
University of California Space Sciences Laboratory 
and Lawrence Berkeley Laboratory 
Berkeley, California, USA 94720 
ABSTRACT 
A measurement of the isotopic composition of galactic cosmic ray neon in 
the energy range 70 to 260 MeV/amu has been made using the U.C. Berkeley HKH 
instrument aboard ISEE-3. A combination of high resolution and good statistical 
accuracy makes possible a precise determination of the local interplanetary neon 
composition. We find 2 2Ne/ 2 0Ne = 0.64±0.07 and 2 ,Ne/ 2 0Ne < 0.30 in local inter­
planetary space. These ratios, when interpreted in using standard galactic 
propagation and solar modulation models, yield cosmic ray source abundances 
which are inconsistent with a solar-like source composition. 
INTRODUCTION 
Studies of the galactic cosmic ray (GCR) source nuclei at intermediate 
energies (~ 10-1000 MeV/amu) have shown that the composition of this material 
bears a strong resemblance to the composition of solar system material.1 There 
are indications that differences between these two compositions may be related 
to charge-dependent selection mechanisms.2 It is, therefore, important to 
examine the isotopic composition of the GCRs to see whether the pattern of 
composition similiarities persists. Of the elements for which measurements of 
isotopic composition have so far been made, the strongest indication of a GCR 
source composition different from solar system material occurs in the neon 
isotopes.3L6 We present here the results of a new high-resolution measurement 
of the isotopic composition of GCR neon made using the U.C. Berkeley HKH 
instrument aboard ISEE-3. These data, interpreted using standard models for 
solar modulation and galactic propagation, imply a source in which the 2 2Ne/ 2 0Ne 
abundance ratio is significantly enhanced over that found in solar system 
material. Furthermore, we find that the observed enhancement cannot be 
explained in terms of a solar-like source composition with a large over-produc­
tion of 2 2Ne by spallation reactions during propagation. 
OBSERVATIONS 
The data used in this study were collected from launch (12 August 1978) 
through February 1979, omitting days when the count rate in the front Si(Li) 
detector indicated an enhanced flux of low energy particles. We have described 
the instrument and analysis techniques in another paper in this session and 
will only mention a few points of particular significance for our analysis of 
the neon composition. For this analysis we use particles stopping in any 
detector from D2 through D9. Furthermore, we restrict the analysis to those 
particles incident within 20° of the normal to the detector surfaces. This 
selection results in an energy interval of approximately 70 to 260 MeV/amu. 
In Figure 1 we show a histogram of the masses calculated for the 546 neon 
events in this range. Two well resolved peaks separated by 2.0 amu show the 
2 OG7-10 
significantly 
the 20N'e or 2 2Ne 
presence of substantial fluxes of 
both 20N'e and 2 2Ke. A separate 
2 1\e peak has not been obtained anu 
it is clear that the 2 1Ne flux is 
smaller than either 
... or "Ne fluxes. The 
absolute mass scale, fixed by the 
positions of the measured 1 2C and 
^ 60 peaks, yields neon masses which 
are in error by approximately 0.15 
amu. This effect, which is still 
under investigation, does not' 
affect the present analysis since 
the strong 2 0Ne and 2 2Ne peaks 
provide a correct renormalization 
of the mass scale. Also evident 
in Figure 1 is a low-level of 
background events. These are mostly 
due to particles whose incidence 
angles have been incorrectly identified. 
IIL 7H-:oou 
Fig. 1 
gi^ n uc JJM.HI.-U; M ~ „ ^ ~ ~ . "he majority of such events are 
eliminated by the requirement that the three measurements of a track's x and y 
coordinates be consistent to ±0.5 cm. This consistency criterion has not yet 
been optimized and we expect that further work will improve our rejection of 
this sort of background. 
In order to obtain the relative abundances of the three neon isotopes we 
have performed fits to the 2 0Ne and 2 2Ne peaks using a number of different 
assumptions concerning the shapes of the tails and the nature of the background. 
We are able to obtain 2 2Ne/ 2 0Ne ratios in the range 0.56 to 0.60 and *iNe/Z0Ne 
ratios less than 0.24. These ratios are obtained using equal intervals of 
particle range for each of the isotopes. In order to convert the abundance 
ratios to equal energy per nucleon it is necessary to know the spectral shapes. 
These shapes have not yet been obtained from the present investigation and 
previous measurements of the neon spectrum cannot be used due to the changing 
level of solar modulation. The correction factor should, however, be between 
1.06 obtained for a flat spectrum and 1.11 obtained for a J = AT spectrum. We 
expect that the latter value is more appropriate for the present near-solar-
maximum period and. hence, we have adopted this value. Thus, we obtain the 
following isotopic'composition for neon in local interplanetary space: 
2 2Ne/ 2 0Ne = 0.64±0.07 
2lNe/20Ne < 0.30 
The uncertainty on the 2 2Ne/ 2 0Ne ratio includes both statistical errors and our 
estimate of the uncertainty introduced by our peak fitting and background 
subtraction procedures. The upper limit on 2 1Ne/ 2 0Ne is a two standard devia­
tion limit, taking into account these same uncertainties. 
DISCUSSION 
In Figure 2 we compare our measurement of 2 2Ne/ 2 0Ne with a number of 
other measurements.5"6^ There is reasonable agreement among the more recent 
experiments (i.e. those with the best mass resolution) indicating that the 
2 2Ne abundance is a substantial fraction of the 2°Ne abundance, contrary to 
expectations based on the assumption of a source with solar-like composition. 
3 OG'7-10 
••ing 
1.0 
.9 
.0 
.7 
.6 
.3 
.4 
.3 
.2 
.J 
0 
r r r i 1 
i 
GSFC 
(31 vmm 
• /fwf}?/ (fl 
T
—
i—
i—
r- u of cm (3) \ 
T
—
i—
i—
r
en (s) 
(Ml 
Ml 
Oof 
CHI 
(B) 
- ' ' ' _ i _ , , i i i , . i_ I.J I I I _.. 
100 
E (MeV/amu) 
1000 
Fig. 2 
Boxes plotted show the ±la limits and 
energy ranges for the Z 2Ne/ 2 0Ne values 
reported by several groups. When only 
a mean mass was reported we have con­
verted it to 2 2Ne/ 2 0Ne by using 2 1Ne/ 2 0Ne 
= 0.18. 
In order to compare the observed 
abundances with the predictions of 
standard source and propagation 
models, it is necessary to first apply 
a correction for the solar modulation 
undergone by the particles observed 
at earth. We have made roungh esti­
mates of the size of this correction 
by assuming local interstellar spectra 
of the form J « (E/M+U)" 2' 5 and u t the force-field solar modulation 
approximation9 characterized by a 
modulation parameter $ (calculated 
using the mass-to-charge ratios of 
the neon isotopes and the rigidity-
change parameter, $, which results 
when a diffusion coefficient of the 
form $a f3P is assumed). We have 
adopted a correction factor of 0.8 
(corresponding to U = 400 MeV/amu 
and $ = 330 MeV/amu) with an 
uncertainty of ±0.1 (corresponding 
to the extreme values obtained 
ivhen U is varied between 0 and 931 MeV/amu and $ is varied between 75 and 400 
MeV/amu) to be applied to the local 2 2Ne/ 2 0Ne ratio. 
In order to interpret the observed abundances in terms of a 2 2Ne/ 2 0Ne 
abundance ratio at the cosmic ray source, we have performed a calculation using 
the observed 2 1Ne abvindance as a tracer of the amount of interstellar material 
traversed by the cosmic rays. 10,11 This approach, which assumes that 2 1Ne is 
absent at the cosmic ray source, will correctly take into account propagation 
through any exponential distribution of potential pathlengths. Thus our deri­
vation of a source abundance ratio does not depend on a particular choice of 
the mean interstellar pathlength. In addition, ionization energy loss effects 
are included in a rough form by assuming a local interstellar spectrum of the 
form J a (E/M+400 MeV/amu) 2" 6 for all three neon isotopes. Total destruction 
cross sections on interstellar material (assumed to have He/H = 0.1 by number 
of atoms) are calculated using the Bradt-Peters form with coefficients given 
by Westfall et al.,12 while partial cross sections are obtained from the 
Silberberg and Tsao formulas. 
The validity of any propagation calculation depends on the accuracy of the 
partial cross sections used. A claim of an over-abundance of 2 2Ne in the GCR 
sources must rest on our confidence that the partial cross sections for 
producing 2 2Ne are not badly underestimated. 2 2Ne production in the inter­
stellar medium is dominated by contributions from p -unstable 2 2Na. The cross 
sections for the production of 22Na--widely used as a beam monitor--are well 
measured^3"1.5 and are reliably predicted by the Silberberg and Tsao 
formulas.11^ A preliminary report on other reaction channels leading to 
2 0Ne, 2 1 N e , and 2 2Ne by RegnierlS indicates that the isotopic production ratios 
are also in close agreement with Silberberg and Tsao. We conclude that use 
of the Silberberg and Tsao formulas gives a reliable prediction of 2 2Ne 
production at 400 MeV/amu. 
IBL 796-HH3b 
The calculation of the secondary contribution to the observed neon fluxes 
involves computing a sum over all heavier species of the product of the 
4 OG7-10 
equilibrium interstellar flux times the appropriate production cross section. 
The isotopic composition of elements heavier than neon is largely unmeasured, 
but preliminary indications^ are that the composition of magnesium and silicon 
(the most important producers of neon) is not greatly different than expected 
from a source of solar-like composition. Therefore, we have used a local 
isotopic composition for Z > 10 species which is based on measured local ele­
mental abundances^"22 g ^ a source distribution of isotopes within each 
element as given by Cameron23 for solar system material. An exponential distri­
bution of pathlengths with a mean of 5.5 g/cm2 has been assumed and ionization 
energy loss has been neglected in this calculation. Note that this simplified 
treatment of the propagation pf the parent isotopes should only slightly affect 
our results since all the elemental abundances which we use are the result of 
direct measurements. 
Figure 3 shows the relation 
between the measurable local ratios, 
2 2Ne/ 2 0Ne and 2 1Ne/ 2 0Ne. end the 
desired source ratio, 2 2Ne/ 2 0Ne. 
Tins calculation was done assuming 
that the particles which we observe 
at ~ 70-260 MeV/amu near earth had an 
energy ~400 MeV/amu in local inter­
stellar space. The location of the 
point in Figure 3 indicates our best 
estimate of the local interstellar 
neon composition. The shaded area is 
bounded by our ±lo limits on the 
local 2 2Ne/ 2 0Ne ratio and by our 
upper limit on the local 2 1Ne/ 2 0Ne 
ratio (both ratios corrected for 
solar modulation). In addition, 
there are uncertainties in the inter­
stellar ratio due to the error in our 
correction to equal energy per nucleon 
i.o 
i 
CM 
o> 
<? 
o 
tt 
O 
« O.I 
- — I - 1 1 — 1 ! 1 , 
- ^00^-
- *^^^J/ / 
-
/ / 1 ' 
- 7 Solar Syiltm / **N»/i0Na ~ 
- 0 
1 1 
0.1 p.e 
1 i 
0.3 
1 i 
0.4 -Local 2,N»/'°Nt _ 
/ 1 i i 
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 
LOCAL ssNe/z0Ne 
Hg. 3 
XBL 796-10037 
and to errors in the solar modulation correction. The former uncertainty will 
be resolved when we obtain spectral information from these data. Some of the 
uncertainty in the modulation correction can be reduced by an improved calcu­
lation of this effect using parameters determined to be appropriate to the 
time period over which these data were taken, but a sizeable uncertainty will 
remain due to our incomplete understanding of the modulation process. 
Besides the uncertainties involved in obtaining the interstellar abundance 
ratios, there are errors in the calculation of the source abundances due^o-Q 
errors in the parameters involved in the calculation. It has been shown ' 
that the dominant source of error in the calculation is due to uncertainties in 
the fragmentation cross sections. In the present case, this should contribute 
an error of ~ 5-10% in the source 2 2Ne/ 2 0Ne ratio. 
Also shown in Figure 3 is the ratio, 2 2Ne/ 2 0Ne = 0.122, found in solar 
system material. It is clear from this figure that the observed isotopic 
composition of GCR neon is incompatible with the assumption that the GCR source 
composition is similar to solar-system composition. We again emphasize that 
this conclusion is independent of any assumption about the mean interstellar 
grammage traversed, and that the cross sections which are most important for 
calculating the secondary yield of 22N'e have been measured. 
5 OG7-10 
CONCLUSIONS 
The neon isotopic abundances in the GCR present the first clear case of a 
cosmic ray source composition significantly different from solar system compo­
sition. Since a number of different processes can contribute to the synthesis 
of the neon isotopes, it is not yet clear how this difference is to be inter­
preted. There is now a definite need to examine models of galactic and stellar 
evolution and of cosmic ray acceleration in an attempt to understand the GCR 
neon composition. Emphasis should be placed on identifying observable effects 
on other abundances which would be implied by models capable of explaining the 
2 2Ne/ 2 0Ne ratio. 
ACKNOWLEDGMENTS 
We thank Lance Wilson for providing the computer code used for cross 
section calculations. This work was supported by NASA under contract NAS 5-
20995 and grant NCR 05-003-513, and by the Nuclear Physics Division of DOE 
under contract W-7405-ENG-48. 
REFERENCES 
C.J. Waddington, Fund, of Cosmic Phys. 3, 1 (1977). 
M. Casse and P. Goret, Ap. J. 221, 703 T1978). 
A.J. Fisher, F.A. Hagen, R.C. Maehl, J.F. Ormes, and J.F. Arens, Ap. J. 
205, 938 (1976). 
W.R. Webber, J.C. Kish, and S.M. Yushak, Bull. Am. Phys. Soc. 24, 692 
(1979). 
M. Garcia-Munoz, J.A. Simpson, and J.P. Wefel, Bull. Am. Phys. Soc. 24, 
692 (1979). ~ 
J.D. Spalding, R.A. Mewaldt, E.C. Stone, and R.E. Vogt, Bull. .Am. Phys. 
Soc. 24_, 692 (1979). 
M.E. Wiedenbeck, D.E. Greiner, F.S. Bieser, H.J. Crawford, H.H. Heckman, 
and P.J. Lindstrom, paper OG7-9, this conference. 
R. Dwyer, Ap. J. 22£, 691 (1978). 
L.J. Gleason and W.I. Axford, Ap. J. 154, 1011 (19681. 
10. M.E. Wiedenbeck, unpublished Ph.D. thesis, Calif. Inst, of Tech., SRL-77-8 
(1977). 
11. E.C. Stone and M.E. Wiedenl>2ck, Calif. Inst, of Tech., SRL 78-3 (1978); Ap. 
J., in press. 
12. G.D. Westfall, Lance W. Wilson, P.J. Lindstrom, H.J. Crawford, D.E. 
Greiner, and H.H. Heckman, Phys. Rev. C19_, 1309 (1979). 
13. J. Tobailem, C.H. de Lasus St. Genis, and L. Leveque, CEA-N-1466 (1) 1971. 
14. H.R. Heydegger, .Anthony L. Turkevich, A. Van Grinneken, and P.H. Walpole, 
Phys. Rev. C14_, 1506 (1976). 
15. Ralph J. Korteling and Albert A. Caretto, Phys. Rev. CI, 1960 (1970). 
16. R. Silberberg and C.H. Tsao, Ap. J. Suppl. 25_, 315 (11573). 
17. R. Silberberg and C.H. Tsao, Proc. 15th ICRC (Plovdiv) 2, 84 (1977). 
18. *R. Silberberg and C.H. Tsao, Proc. 15th ICRC (Plovdiv) I, 89 (1977). 
19. S. Regnier, Proc. 15th ICRC (Plovdiv) 2_, 76 (1977). 
20. M. Garcia-Munoz, G.M. Mason, J.A. Simpson, and J.P. Wefel, Proc. 15th 
ICRC (Plovdiv) 1_, 224 (1977). 
21. M. Garcia-Munoz, G.M. Mason, J.A. Simpson, and J.P. Wefel, Proc. 15th 
ICRC (Plovdiv) 1_, 230 (1977). 
22. R. Silberberg, C.H. Tsao, and M.M. Shapiro, in Spallation Nuclear 
Reactions and their Applications, Shen and Marker (eds.), Reidel 
(Dordrecht), 1976 , p. 49. 
6 
23. A.G.W. Cameron, Space Sci. Rev. 1!5, 121 (1973). 
OG7-10 


Isotopic composition of neon in the galactic cosmic rays: a high resolution measurement

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Isotopic composition of neon in the galactic cosmic rays: a high resolution measurement

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