Download A Possible Effect of Cosmic Rays on Celestial Chemical Composition

464
Letters
A Possible Effect of Cosmic Rays
on Celestial Chemical Composition
Satio Hayakawa
Research Institute for Fundamental Physics,
Kyoto University, Kyoto
April 22, 1955
The purpose of this note is to point out a
possibility that the production of cosmic rays at stars
may give rise to an appreciable nuclear transformation,
so as to make influences on the chemical composition
of stars. Our attention is paid to the production of
light nuclei, such as Li and Be, which are hardly
produced by thermo-nuclear reactions in the interior
of stars.ll .
As a typical star, we consider the sun, at which
the production of cosmic rays is well established.
The cosmic rays are associated with solar flares and
their average production rate is estimated as about
2 x 10~3 particles/sec-GV at a rigidity of 4 GV.2>
Taking account of the fact that the energy of the
solar cosmic rays ranges up to nearly 10 GeV, the
average intensity at the solar surface is no smaller
than
]=10 sec1 cm-2.
(1)
If the energy spectrum of such nuclear particles is
continuous between cosmic ray and aurora energies,
the intenstty of parttcles with energies larger than
1 MeV may be as large as 104 sect em-~. These
particles have energies high enough to cause nuclear
transformations.
Due to the bombardment of nuclei heavier than
B by the cosmic rays of GeV energies, a light
nucleus is expected to be produced with the cross
section of the order of <T-10 mb, as observed in
Cosmotron experiments.") As the yield of the light
nuclei decreases with decreasing energy, the contribution of particles with energies below 1 GeV is at
most that of cosmic ray part1cles. If the irradiation
continues as the age of the sun, T -1017 sec, the
heavier nuclei of fractional abundance F are partly
transformed into a light nucleus by an amount
J-]<TTF-10--BF.
(2)
Measuring F and f in the unit of the hydrogen
content, it is observed that F-10-3 and f-10-11 to
10-10• Thus relation (2) is roughly satisfied,
to
the Editor
The following remarks may be necessary in connection with our problem.
(i) The mtensity of
general cosmic rays near the sun is about one order
smaller than that gtven in (1), so that the general
cosmic rays impinging on the solar surface give a
contribution to such nuclear transformations less than
the solar ones do. (ii) The same reason applies to
the nuclear transformations of interstellar elements ;
those are quite small, though not negligible, unless
the cosmic ray ir.tensity is much higher in a regton
of high gas concentration than near the sun. (iii)
The above mechanism is responsible for such an
element whose abundance is far smaller than that of
heavier elements. Hence the production of Tc by
this mechanism may hardly be accounted for, because
the abundance of Tc is observed as large as that of'
neighbouring elements. (iv) All of the abut!dance
of such light nuclei is not necessarily to be explamed
by the transformations due to cosmic rays. In this
respect it should be noticed that the relative
abundances of Be and Li in meteorites are as great
as or even greater than in the sun. If both the sun
and planetary bodies were originated from the condensation of a common interstellar matter, the
similarity in the relative abundances should be expected.
Then the cosmic rays would only be
responsible to alter the fine structure of relative
abundances.
If there are such stars that produce cosmic rays
more efficiently than the sun (remember that the
sun is a very poor cosmic ray producer, provided
that the main part of the cosmic rays are ortginated
at the stars m our galaxy), sizeable nuclear transformations may take place within relatively short
time. As emphasized by Hoyle4 l and ter Haar5l,
supernovae may possibly produce a large amount of
cosmic rays (ten or more order higher than at the
sun, according to ter Haar). During their explosion
considerable nuclear transformations are expected, for
example, a factor 10-4 instead of 10-s in (2). If
the explosion of supernovae plays an important role
for the origin of celestial elements and if it is
associated with the copious production of cosmic rays,
the following speculation is born out on the chemical
compositions of celestial elements as well as cosmic
rays.
Accepting the general belief that heavy elements
are relatively rich in supernovae, the chemical composition of cosmic rays, if accelerated there, is expected to be richer in heavy elements than that ot
the average celestial elements, m accordance with
observations. These cosmic rays may h;1ve given
Letters to the Editor
rise to nuclear transformations in such a way as to
produce those elements which were hardly produced
by thermo-nuclear reactions.
It is my pleasure to mention that this work was
born from stimulating discussions of those who attended at the seminar of nuclear astrophysics held
at Research Institute for Fundamental Physics ;
among them Professors T. Hatanaka and C. Hayashi
helped me by their kind comments.
2)
3)
4)
1)
The possible production of deuterium by nuclear
particles of MeV energies was mentioned by P.
5)
465
Morrison, S. Olbert ,, ' B. Rossi, Phys. Rev.
94 (1954), 440. The author is grateful to
Professor Morrison who pointed out a significant
role of protons with energies lower than GeV.
]. W. Firor, J. A. Simpson and S. P. Treiman, Phys. Rev. 93(1054), 1015.
J- Hudis et al., Phys. Rev. 94 (1954), 775;
A. Turkevtch and N. Sugerman Phys. Rev.
94 (1954), 728.
F. Hoyle, M.N.R.A.S. 106 (1947), 384.
D. ter Haar, Rev. Mod. Phys. 22 (1950), 119.