THE SOLAR COSMIC-RAY FLUX DURING THE LAST TWO MILLION YEARS, A.
Yanivl, K. Martil, R. C. Reedy2, lChem. Dept. B-017, Univ. of Calif., San Diego
La J o l l a 92093, 2 ~ o Alamos
s
S c i e n t i f i c Lab.,Box 1663, Los Alamos, N. M. 87545.
The solar cosmic-ray (SCR) p a r t i c l e s emitted from the sun provide a useful
measure of solar a c t i v i t y . Variations in the SCR flux change the r a t e s of nuclide production by nuclear reactions. Stable SCR produced nuclides provide
information on the average flux during the time of exposubaet o SCR, while
radionucl ide abundances provide information on average fluxes during time interval s determined by t h e i r ha1 f-1 ives. The re1 ations of solar phenomena and
cosmic-ray flux variations were discussed in detai 1 by Pomerantz and Duggal [ I 1.
The averaged integral fluxes of SCR p a r t i c l e s which were inferred by d i r e c t
measurements f o r solar cycle 19 (1 954-64) and d i r e c t l y measured during solar
cycle 20 (1 965-75) were tabulated by Reedy [2].
Lunar rocks are well suited f o r studying SCR-particle fluxes in the past.
A1 though erosion of the surfaces of rocks (Q 1 mm per m.y. ) a f f e c t s the depth
dependent concentration profiles of stable and long-1 ived radioisotopes, efr
small, except f o r the very surface regions.
f e c t s on 2.1 x 105y 8 1 ~ are
Average SCR fluxes determined so f a r f o r the l a s t 1-10 million years, based on
26A1 and 53Mn measurements in lunar rocks, show l i t t l e variation and a r e cons i s t e n t with values obtained for the recent solar cycles [3]. The a c t i v i t y
profiles of 1 4 C determined in lunar rocks [4,5], however, indicate t h a t SCR
fluxes averaged over the l a s t lo4 years must have been a few times larger than
those inferred from the 26A1 and 53Mn data (Table 2 ) .
Marti and Lugmair [6] measured isotopic abundances of noble gases in samples from several depths of lunar rock 12002 and observed 1 i t t l e excesses near
the surface due t o SCR production, b u t only * l ~ showed
r
c l e a r excesses due t o
SCR products. This paper reports on an extension of t h i s study and on a simil a r investigation of samples from several depths of rock 68815,234. Many of
the l a t t e r samples a r e a1 iquots of those studied f o r 26A1 and 5 3 ~ nby Kohl e t
a1
- [3]. Since the hi story of rock 12002 on the 1unar surface has been corn:
plex [6], the stable rare gas isotopes cannot be expected t o provide much useful information on average SCR fluxes. Rock 68815, on the other hand, experienced a one-stage irradiation during the l a s t 2 million years and, therefore, i s well suited f o r such a study. Our 8 1 ~ r - r~e sru l t s confirm the 2 m.y.
exposure age reported e a r l i e r [7].
The low background level of spallation
products due t o galactic cosmic rays ( G C R ) suggests the intriguing p o s s i b i l i t y
of a detectable record of stopped solar f l a r e p a r t i c l e s in the surface layers
and of an average of the recent solar wind p a r t i c l e flux in the recent past.
The r e l a t i v e abundances of spallation Kr in the 12002 samples a r e given in
r t o SCR e f f e c t s i s v i s i b l e both in
Table 1. The sharp increase in the 8 1 ~due
the r e l a t i v e (83Kr = 1.00) abundance data and in the l a s t column, where the 81Kr
concentrations are reported. Although the work on 68815 i s s t i l l in progress,
we a r e already seeing a similar increase towards the 6881 5, 234 surface layer.
The expected contributions t o the Kr isotopes have been discussed in d e t a i l by
Regnier e t a l . [8] and are based on SCR fluxes inferred by Kohl e t a l . [31.
The measured 8 1 ~ in
r the surface regions of 12002 and 68815 exceed the calculated e f f e c t s by about a factor of two. The required SCR fluxes which best f i t
the experimental data are given in Table 2 . No fluxes are calculated f o r energies below 60 MeV because the reactions producing 8 1 ~ rin lunar samples have
threshold energies above 60 MeV. The cross sections used in these calculations
were estimated on the basis of nuclear systematics and on only few experiment a l data [8]. Clearly, more experimental cross-section measurements a r e required before the larger SCR fluxes can be accepted as final data. However,
.
0Lunar and Planetary Institute
Provided by the NASA Astrophysics Data System
The S o l a r Cosmic-Ray Flux During t h e Last Two Million Years.
A . Yaniv, K. Marti and R. C. Reedy
Page 2
a preliminary e v a l u a t i o n of t h e 68815 d a t a i s c o n s i s t e n t with t h i s i n t e r p r e t a t i o n . The r a r e gas d a t a a v a i l a b l e so f a r reveal t h e predicted i n c r e a s e i n
t h e spal l a t i o n c o n t e n t s towards t h e s u r f a c e of 68815. The observed i n c r e a s e
i n 3 ~ ien t h e topmost 0.5 mm , however, exceeds by f a r t h a t a t t r i b u t a b l e t o SCR
s p a l l a t i o n and, t h e r e f o r e , provides us with a record of d i r e c t l y implanted
s o l a r - f l a r e 3 ~ e . The concentration p r o f i l e appears t o agree w i t h expected i m p l a n t a t i o n depths f o r 3 ~ of
e s o l a r - f l a r e energies. This information on
average 3 ~ fel u x e s should be he1 pful , considering t h e many r e p o r t s of highly
v a r i a b l e He i s o t o p i c abundances i n d i r e c t l y measured s o l a r f l a r e p a r t i c l e s .
This work was supported by NASA NGL 05-009-150. We thank J . R. Arnold
and co-workers f o r a1 iquot samples of 68815,214 and f o r t h e i r i n t e r e s t i n t h i s
work.
References: [ I ] Pomerantz M. A. and Duggal S. P. (1974) Rev. Geophys.
Space Phys. 12, 343-361. [2] Reedy R. C. (1977) Proc. Lunar S c i . Conf. 8 t h ,
825-839. [ 3 T ~ o h lC. P . , Murrell M. T . , Russ I11 G . P. and Arnold J . R. (1 978)
Proc. Lunar Planet. S c i . Conf. 9 t h , 2299-2310. [4] Begemann F., Born W . ,
Palme H . , Vilcsek E. and Wilnke H. (1972) Proc. Lunar S c i . Conf. 3 r d , 1693-1702
[5] Boeckl R . S. (1972) Earth Planet. S c i . L e t t . 16, 269-272. [6] Marti K.
and Lugmair G . W. (1971 ) Proc. Lunar S c i . Conf. 2 3 , 1591-1605. [7] Drozd R.
J . , Hohenberg C. M . , Morgan C . J . and Ralston C . E. (1 974) Geochim. Cosmochim.
Acta 38, 1625-1642. [8] Regnier S . , Hohenberg C . M . , Marti K. and Reedy R. C .
(1979)~roc. Lunar Planet. Sci. Conf. X, 1565-1586. [9] Hoyt H . P . , Walker R.
M. and Zimmerman D. W. (1973) Proc. Lunar S c i . Conf. 4 t h , 2489-2502.
Table 1:
Spallation Kr data i n rock 12002
7
depth
78
80
81
82
83
84
86
concentration
xi 0-1 2 c c ~ ~ ~ / g
8 3 ~81Kr
~
(mm)
12002,D-1
"spal 1I'
0-2
16.91
49.52
.287
75.35
100
50.0
=1.6
58
0.167
,D-1
2-4
16.92
48.51
.243
76.14
100
51.3
=1.6
66
0.160
,D-1
4-8
16.75
49.17
.217
75.16
100
49.10
~1.6
68
0.149
,A
9-20
16.72
48.85
.I90
75.60
100
49.37
~1.6
71
0.135
% 5 5 17.00
48.78
0.207
76.52
100
38.10
=1.6
48
0.100
"spal 1"
"spal 1"
"spa1 1 "
A#2
"spall"
0Lunar and Planetary Institute
,
Provided by the NASA Astrophysics Data System
The S o l a r Cosmic-Ray F l u x D u r i n g t h e L a s t Two M i l l i o n Years.
A. Yaniv, K. M a r t i and R. C . Reedy
Page 3
Table 2
--
-
Fluxes (protons/cm2 s)
-
E>10 MeV
E>30 MeV
E > G C MeV
1965-1975 ( S P M E ) ~
89
28
E. 0
1965-7/7zb ( S P M E ) ~
25
P e r i o d (Data)
-
1954-1964 ( 2 2 ~ a , 5 5 ~ e ) a
(TL)~
-5
14
lo4
(
3 x
lo5
c )d
y (
81 e
Kr)
lo6 y ( 2 6 ~ i ) f
5 x
lo6
y (53~n)f
4.2
E>100 MeV
0.9
378
136
59
26
-60
-14
-6
-3
~200
72
26
9
--
--
118
-6
70
25
9
3
70
25
9
3
a ~ e e d y(1977), SPME i s t h e S o l a r P r o t o n M o n i t o r Experiment (Bostrom
1967-1973)
b ~ v e r a g e d over
.
11 years.
---
g
c.,
'
2. (1973)
'tloyt
d ~ o e c k l (1972)
e ~ h i sw o r k u s i n g K r data from 12002 and 65815.
f ~ o h lg
g.
(1978)
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Provided by the NASA Astrophysics Data System