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STARS FORMED BY HIGH-ENERGY PHOTONS
In a large number of works the formation of stars under the action of particles of high energy (in cosmic radiation or artificially accelerated) has been studied. However, the formation of stars under the action of high-energy photons had not, until recently, been studied. On the basis of work on photonuclear reactions one could only conclude that the cross section for the formation of stars under the action of photons must be considerably smaller than under the action of high-energy particles.
In recent months three works have appeared devoted to the formation, under the action of high-energy γ-quanta, of stars in photographic emulsions (Ilford C-2). The distribution of stars according to the number of prongs, the energy distribution of the prongs, and also the average cross section for star formation for the total composition of the emulsion (excluding hydrogen atoms) were studied.
In Table I I give the prong distribution of stars formed by bremsstrahlung with maximum energy 300 MeV,^1 and also the data in article^1 for comparison with the prong distribution of stars formed by π-mesons.
Table I
| 2 | 3 | 4 | 5 | 6 | 7 | |
|---|---|---|---|---|---|---|
| Number of prongs | 2 | 3 | 4 | 5 | 6 | 7 |
| Number of stars | 103 | 64 | 57 | 21 | 4 | 1 |
| Relative number of stars, on photons | 160 | 100 | 89 | 38 | 6.4 | 1.6 |
| Relative number of stars, on mesons | 180 | 100 | 51 | 12 | — | — |
The energy distribution of the prongs is illustrated by Table II.^1
Table II
| <0.2 | 0.2–2 | 2–10 | 10–30 | 30–60 | |
|---|---|---|---|---|---|
| Prong energy in MeV | <0.2 | 0.2–2 | 2–10 | 10–30 | 30–60 |
| Number of prongs | 67 | 181 | 427 | 109 | 39 |
The star-formation cross section is several units of \(10^{-27}\ \text{cm}^2\) and increases rather rapidly with the growth of the maximum energy of the bremsstrahlung spectrum, as can be seen^2 from Table III.
Table III
| \(E_{\gamma\max}\) in MeV | 150 | 200 | 250 | 300 |
|---|---|---|---|---|
| \(\sigma_{\text{stars}}\), \(10^{27}\ \text{cm}^2\) | \(1.95 \pm 0.18\) | \(2.55 \pm 0.19\) | \(5.63 \pm 0.56\) | \(6.04 \pm 0.41\) |
A comparison of the yield of star formation at maximum photon energies of 161, 242, and 322 MeV leads to the conclusion\(^3\) that the mean cross sections, over the spectrum, for the formation of stars by photons are \((7 \pm )10^{-27}\ \mathrm{cm}^2\) between 161 and 242 MeV and \((8 \pm 1)10^{-27}\ \mathrm{cm}^2\) between 242 and 322 MeV. These figures agree with the data given in Table III.
Analysis of the angular and energy distribution in stars formed by photons and by \(\pi\)-mesons, as well as comparison of the obtained data with the results of experiments on the production of \(\pi\)-mesons by high-energy photons, led the authors\({}^{2,3}\) to the conclusion that a considerable part of the observed stars is formed by mesons arising in the interaction of \(\gamma\)-quanta with nuclei. The cross section for the formation of mesons by bremsstrahlung radiation with a maximum energy of 322 MeV is estimated for silver nuclei\({}^3\) as follows:
\[ \sigma_{\pi^+} \approx 0.7\cdot 10^{-27}\ \mathrm{cm}^2,\quad \sigma_{\pi^-} \approx 1.1\cdot 10^{-27}\ \mathrm{cm}^2 \quad \text{and} \quad \sigma_{\pi^0} \approx 4\cdot 10^{-27}\ \mathrm{cm}^2, \]
whence \(\Sigma\sigma_{\pi} \approx 5.8\cdot 10^{-27}\ \mathrm{cm}^2\). At the same time the author\({}^3\) points out that the cross section of the process in which the meson is absorbed (with star formation) in the same nucleus in which it is produced may be several times larger than the observed meson-production cross section.
G. I.
CITED LITERATURE
- S. Kikuchi, Phys. Rev. 80, 492 (1950).
- S. Kikuchi, Phys. Rev. 81, 1060 (1951).
- R. Miller, Phys. Rev. 82, 260 (1951).