The production rates of cosmogenic nuclides were calculated for the primary protons in the galactic and solar cosmic rays. At 1 AU, the long-term average fluxes of solar protons usually produce many more atoms of cosmogenic nuclide than the primary protons in the galactic cosmic rays (GCR). Because the particle fluxes inside meteorites and other large objects in space include many secondary neutrons, the production rates and ratios inside large objects are often very different from those by just the primary GCR protons. It is possible to determine if a small object, was small in space or broken from a meteorite. Because heliospherical modulation and other interactions change the GCR particle spectrum, the production of cosmogenic nuclides by the GCR particles outside the heliosphere will be different from that by modulated GCR primaries

Reedy, R. C.

English

The production rates of cosmogenic nuclides were calculated for the primary protons in the galactic and solar cosmic rays. At 1 AU, the long-term average fluxes of solar protons usually produce many more atoms of a cosmogenic nuclide than the primary protons in the GCR, the exceptions being nuclides made only by high-energy reactions (like Be-10). Because the particle fluxes inside meteorites and other large objects in space include many secondary neutrons, the production rates and ratios inside large objects are often very different from those by just the primary GCR protons. Thus it is possible to determine, by examining its cosmogenic nuclides, if a small object, such as found among deep-sea spherules, was small in space or broken from a meteorite. Because heliospherical modulation and other interactions change the GCR particle spectrum, the production of cosmogenic nuclides by the GCR particles outside the heliosphere will be different from that by modulated GCR primaries. Production rates and ratios for cosmogenic nuclides would be able to identify small particles, possibly interstellar in origin, that were exposed to an unmodulated spectrum of GCR particles and to characterize the spectrum of particles to which they were exposed. 6 refs., 1 tab

UNT Digital Library

Cosmogenic-nuclide production by primary cosmic-ray protons

The production rates of cosmogenic nuclides in the solar system and in interstellar space were calculated for the primary protons in the galactic and solar cosmic rays. At 1 AU, the long-term average fluxes of solar protons usually produce many more atoms of a cosmogenic nuclide than the primary protons in the galactic cosmic rays (GCR), the exceptions being nuclides made only by high-energy reactions (like /sup 10/Be). Because the particle fluxes inside meteorites and other large objects in space include many secondary neutrons, the production rates are much higher and ratios inside large objects are often very different from those by just the primary GCR protons in small objects. The production rates of cosmogenic nuclides are calculated to vary by about factors of 2.5 during at typical 11-year solar cycle, in agreement with measurements of short-lived radionuclides in recently fallen meteorites. The production of cosmogenic nuclides by the GCR particles outside the heliosphere is higher than that by the modulated GCR primaries normally in the solar system. However, there is considerable uncertainty in the fluxes of interstellar protons and, therefore, in the production rates of cosmogenic nuclides in interstellar space. Production rates and ratios for cosmogenic nuclides would be able to identify particles that were small in space or that were exposed to an unmodulated spectrum of GCR particles. 25 refs., 2 figs., 2 tabs

Nuclide production by primary cosmic-ray protons

NASA Technical Reports Server

3 8 2 SR 7.1-6
COSMOGENIC-NUCLIDE PRODUCTION BY PRIMARY COSMIC-RAY PROTONS
Robert C. Reedy
Nuclear Chemistry Group, Mail Stop J514
Los Alamos National Laboratory
Los Alamos, NM 87545, USA
ABSTRACT
The production rates of cosmogenic nuclides were calculated for
the primary protons in the galactic and solar cosmic rays. At
1AU, the long-term average fluxes of solar protons usually
produce many more atoms of a cosmogenic nuclide than the
primary protons in the GCR, the exceptions being nuclides made
only by high-energy reactions (like Be-10). Because the
particle fluxes inside meteorites and other large objects in
space include many secondary neutrons, the production rates and
ratios inside large objects are often very different from those
by just the primary GCR protons. Thus it is possible to deter-
mine, by examining its cosmogenic nuclides, if a small object,
such as found among deep-sea spherules, was small in space or
broken from a meteorite. Because heliospherical modulation and
other interactions change the GCR particle spectrum, the pro-
duction of cosmogenic nuclides by the GCR particles outside the
heliosphere will be different from that by modulated GCR
primaries. Production rates and ratios for cosmogenic nuclides
would be able to identify small particles, possibly interstel-
lar in origin, that were exposed to an unmodulated spectrum of
GCR particles and to characterize the spectrum of particles to
which they were exposed.
1. Introduction. The energetic particles in the cosmic rays are about
90% protons and induce a wide variety of nuclear reactions in extra-
terrestrial matter (i). Relatively low energy (~10- to 100-MeV)
particles are emitted occasionally from the sun--the so-called solar
cosmic rays (SCR). The SCR particles are rapidly stopped in matter
(within a few centimeters) and produce a high density of product nuclei
very near the surface. The high-energy (~I-GeV) galactic cosmic-ray
(GCR) particles produce a large cascade of secondary particles,
especially neutrons, that penetrate meters into solid matter. The flux
D
of GCR particles in the solar system varies with solar activity and is
lowest at periods of maximum solar activity. Most studies of cosmogenic
nuclSdes have been for large objects (the Earth, moon, and meteorites)
and for long periods of time (averaged over many solar cycles).
Recently, there has been more interest in cosmogenic nuclide production
in very small objects (2) and also in the production variations over a
solar cycle (3), outside the solar system (4), or for periods of unusual
solar activity (5).
2. Primary Cosmic-Rays. The spectrum of solar protons is well approxi-
mated by an exponential function in rigidity (6); it has an average
spectral shape over the last million years (I) of R = I00 MV.
O
Castagnoli and Lal (5) give an equation for the spectral shape of the GCR
as a function of a solar modulation parameter (M). For the GCR protons,
spectra like those for the last two solar minima (M = 375 MeV) and maxima
https://ntrs.nasa.gov/search.jsp?R=19850026785 2020-03-20T16:50:37+00:00Z
3 8 3 Sh 7.1-6
(M = 950 MeV) (1,5) were used, as well as one that is similar to the
average over a solar cycle (M = 550 MeV). A fourth GCR spectrum was the
expression of (5) for no modulation (M = 0), possibly like that of the
GCR particles outside the solar system in the local interstellar space
(IS). The shape for the expression with M = 0 is very similar to (E +
1050) to the -2.87 po_er, with E in MeV. Such an unmodulated spectrum
could be approached in the solar system during the long periods of
essentially no solar activity that occur about every few hundred years
(1,5). One estimate of the proton spectrum at the source of GCR
particles is a power law in rigidity. At high energies, this rigidity
power law was assumed to have the same intensity and shape as that
observed in the solar system. As any source GCR-particle spectrum is
modified by passage through N5 g/cm 2 of matter, this pure rigidity
power-law spectrum overestimates the proton fluxes at lower energies,
especially below NI00 MeV (6). Production rates for seven cosmogenic
nuclides were calculated with these proton spectra and with cross
sections for proton-induced reactions (see Table I). The production
rates of these nuclides observed in meteorites (with typical radii of i0
to 30 cm) are also shown in Table I.
3. Cosmo_enic-Nuclide Production Rates. Production by solar protons
usually dominates that by primary GCR protons. Only nuclides made mainly
by high-energy protons, such as Be-10 and CI-36, have very low production
rates by the low-energy solar protons. These high rates by solar protons
only occur very near the surface, and solar-proton production rates
become relatively unimportant below depths of a few centimeters (1,6).
Only the GCR source spectrum exceeds the solar-proto_ production rates,
mainly because of its high fluxes at lower energies. In most meteorites,
production by solar protons is usually not observable because the surface
layers are removed by ablation during the meteorite's passage through the
Earth's atmosphere. The ratio of the amount of a nuclide readily made by
solar protons (for example, AI-26) to that of a high-energy product (such
as Be-10) is a good indicator of the object's size when it was irradiated
in space. Activities of AI-26 and Be-IO were measured in several groups
of small (0.3- to 0.5-mm) spherules collected from sediments on the ocean
floor (2). The AI-26/Be-IO ratio and the AI-26 activity were quite high
in several of them, which indicate that those spherules probably came
from parent bodies less than a few Centimeters in diameter. Studies of
such small objects would be interesting because they may be different
" from the forms of solar system matter found in most meteorites.
The production rates of nuclides by GCR protons in interstellar
° space are high, similar to the rates produced by both primary and second-
ary GCR particles in meteorites. The relatively low-energy protons
normally removed by solar modulation in the inner solar system have
produced about the same number of nuclides as are made by secondary
neutrons in meteorites. These high production rates, plus the relatively
low loss of product nuclides by recoil in small grains (4), mean that one
should be able to identify grains that were irradiated in interstellar
space. Such grains may have been incorporated in meteorites or could
enter the Earth's atmosphere as cosmic dust. The cross sections as a
function of energy for making secondary neutrons are similar in shape to
those for producing He-3. The He-3 production rate in interstellar space
is 4 times that for the average over a solar cycle, and the flux of
384
SH7.1-6
protons above I GeV in interstellar space is 2.4 times that for the
solar-cycle average. Thus, the production rates of cosmogenic nuclides
by GCR particles that have not been modulated are higher (by factors up
to 3 to 4) than those observed near the Earth during long periods of
typical solar activity.
During periods of normal solar variations, the extremes in the
activity of the sun and its subsequent modulation of the GCR particles
are represented by the solar minimum and maximum used in Table I. The
ratio of 2.4 for the He-3 production rates between these extremes is
about the variation that would be expected for the production of second-
ary neutrons. When Evans et al. (3) measured the activities of short-
lived radionuclides in a number of meteorites that fell from 1967 to
1978, the activities varied by about a factor of 3. Some of these
variations probably were caused by differences in the meteorites' sizes
or shapes or in the sample location. However, the calculations reported
here show that most of these radioactivity variations are caused by the
solar modulation of the GCR-particle flux. The observed radioactivity
variations correlated well with other indicators of solar activity (3).
Larger variations in nuclide production rates would be expected if the
solar activity exceeded one or both of the average extremes used here.
In Table I, the ratio of the GCR production rates typically observed
in meteorites to the solar-cycle-averaged rates for primary GCR protons
ranged from 2.3 to 8.0. Because the flux of primary protons inside a
meteorite is attenuated by nuclear interactions, the ratios of observed
activities to those made only by the primary GCR protons should be even
larger. These relatively low contributions by the primary particles
illustrate the importance of secondary particles in nuclide production in
large objects like meteorites. This big difference between nuclide
production by primaries only and by the fully developed secondary cascade
present in most meteorites indicates that small meteorites without a
fully developed cascade could have some unusual production rates or
ratios. Studies of such small meteorites also would help us understand
the production and transport of secondary particles in meteorites.
4. Conclusions. The calculations and results in Table I show that the
production of cosmogenic nuclides can vary considerably with both the
size of the extraterrestrial object and the amount of the solar modula-
tion of the primary GCR particles. Very small objects have high produc-
tion rates by solar protons. Typical meteorites are large enough that
the cascade of secondary particles dominates nuclide production.
Intermediate-size objects could have some unusual production rates and °
ratios. Temporal and spatial variations in nuclide production can also
result from differing GCR-particle modulation. Variations by factors of
2 to 3 can occur during a normal solar cycle, and even larger deviations
can occur when solar modulation is much stronger or weaker than usual.
5. Acknowledgements. This research was supported by NASA work order
W-14,084 and done under the auspices of the US DOE, Discussions with
J. Evans, H. V_ik, R. Jokipii, and M. Forman helped in planning this
work.
3 8 5 SH 7.1-6
References
I. Reedy, R. C., et al., (1983), Science 219, 127-135.
2. Raisbeck, G. M., et al., (1983), In Lunar and Planetary Science XIV
(Lunar and Planetary Institute, Houston), pp. 622-623.
3. Evans, J. C., et al, (1982), _. Geophys. Res. 87, 5577-5591.
4. Ray, J., and H. J. V_ik, (1983), Icarus 54, 406-416.
5. Castagnoll, G., and D. Lal, (1980), Radiocarbon 2_2, 133-158.
6. Reedy, R. C., and J. R. Arnold, (1972), J__.Geophys. Res. 77,
537-555.
TABLE I. Nuclide Production Rates by Primary Cosmic-Ray Protons,
Assuming C2-Chondritic Chemistry.
106 years Solar Solar Solar Local GCR Xypical
Av SCR Fax. GCR Av GCR Min. GCR IS GCR Source Meteorite
Parameter R =100 M=950 M=550 M=375 M = 0 R-2"65
O
Integral
Flux a 70 1.21 1.80 2.39 4.33 4.33
Nuclide (atoms/minute/kg)
He-3 66. 190. 318. 459. 1258. 4095. 900.
Be-lO 4.1 5.0 8.1 11.5 28.6 60.8 22.
Ne-21 395. 11.6 20.8 31.7 117. 1603. 150.
AI-26 344. 3.9 7.5 11.9 56. 1344. 60.
CI-36 0.9 1.8 3.1 4.5 11.2 20.1 7.
Ar-38 72. 3.3 5.5 8.0 24.4 260. 20.
Mn-53 590. 7.6 13.6 20.8 90. 2177. 105.
a In protons/cm2/s: solar protons for E > I0 MeV, GCR for E > 1 GeV.


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Cosmogenic-nuclide production by primary cosmic-ray protons

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