PROOF OF THE PRODUCTION OF NEUTRAL MESONS BY PHOTONS

Steinberger et al.1 observed double coincidences in two telescopes aimed at a target of the 330-MeV synchrotron—the source of γ-rays.*)

The arrangement of the apparatus is shown in Fig. 1: β is the angle between the axes of the telescopes; α is the angle between the direction of the γ-beam and the median of the angle β.

Fig. 1.

Each telescope consists of three crystal counters I, II, III (I being the counter nearest the target) and operates according to the coincidence scheme II + III − I. A lead radiator is placed between counters I and II.

It is established that the coincidences are caused by photons produced in the target. Namely, coincidences are practically absent if the radiator is absent; the coincidence count depends only on the radiation length of the radiator; it increases in the required manner with increasing radiator thickness.

The energy of the γ-quanta under study is estimated at 100 MeV from measurements of the absorption curve in an aluminum absorber inserted between counters II and III.

When the maximum energy of the synchrotron is reduced from 330 to 175 MeV, the yield falls approximately in the same way as the yield of γ-quanta from π-mesons. The latter circumstance excludes the possibility of a Coulomb origin of the γ-quanta under investigation and makes it possible to estimate the reaction threshold as ∼150 MeV.

The principal result of the authors should be considered the angular distribution of the double coincidences obtained by them. One of the graphs, corresponding to α = 45°, is shown in Fig. 2.

In the decay of a particle with total energy \(E\) (expressed in units of the particle rest mass) into two photons, the distribution in the angle β in the laboratory system is a function of \(E\). On the basis of the distribution in \(E\) of π+-mesons obtained on an H¹ target under analogous conditions, the authors construct the theoretical angular distribution for the two-photon decay of the neutral meson π⁰ (solid curve in Fig. 2) and compare it with that observed on a Be⁸ target.

*) Preliminary work on the study of the properties of the neutral meson was carried out on the same synchrotron by Bjorklund et al.2

Qualitative agreement of the shapes of the curves indicates that the neutral meson has an average velocity of the order of 0.8 of the speed of light and decays into two photons.

The distribution in \(\alpha\) obtained by the authors thus indicates a considerable difference between the distributions of \(\pi^0\)-mesons from a \(\mathrm{Be}^8\) target and the distributions of \(\pi^+\)-mesons from \(\mathrm{C}^{12}\) and \(\mathrm{H}^1\) targets. The authors measured the ratios of effective cross sections
\(\sigma_{\pi^0}(\mathrm{H}^1)/\sigma_{\pi^0}(\mathrm{C}^{12}) = 0.12 \pm 0.03\). For \(\pi^+\)-mesons the same ratio is 0.55. The authors explain the difference by the fact that the growth of \(\sigma_{\pi^0}\) with increasing atomic number of the nucleus is associated with the growth of the total number of nuclear particles, whereas the cross section \(\sigma_{\pi^+}\) increases in connection with the growth of the number of protons in the nucleus.

Corresponding meson energy (in MeV): 140, 112, 84, 56, 28, 14, 0
\(\alpha = 45^\circ\)
Relative counting rate
Angle between the two arms of the telescope

Fig. 2.

The effective cross sections obtained are
\(\sigma_{\pi^0}(\mathrm{Be}^8)=7.5\cdot 10^{-28}\);
\(\sigma_{\pi^0}(\mathrm{C}^{12})=10\cdot 10^{-28}\);
\(\sigma_{\pi^0}(\mathrm{H}^1)=1.3\cdot 10^{-28}\)—all in \(\mathrm{cm}^2 Q^{-1}\), where \(Q\) is the total energy of the synchrotron \(\gamma\)-spectrum divided by its maximum energy.

Yu. Khokhlov

CITED LITERATURE

1. J. Steinberger et al., Phys. Rev. 78, 802 (1950).
2. Bjorklung et al., Phys. Rev. 77, 213 (1950); see also UFN 41, 389 (1950).