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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.