From Current Literature

Multiple Production of Mesons and Photons in Cosmic-Ray “Stars”

The papers reviewed¹˒² contain a description of a “star” produced in a photographic emulsion by a primary $\alpha$-particle. The star consists of a narrow cone of relativistic particles (see figure), surrounded by 33 relativistic particles with a broader angular distribution. In addition, 18 strongly ionizing particles are formed in the “star,” carrying an energy of $\sim 3 \cdot 10^9$ eV. The number of particles in the narrow shower at the center of the “star” was calculated by dividing the total number of grains in the emulsion by the number of grains produced by a single relativistic particle, and proved to be equal to $23 \pm 2$. Particles belonging to this star were then traced in a neighboring photographic plate, and it was found that the number of particles in the narrow shower had become equal to 44. The increase in the number of particles apparently occurred as a result of the formation of electron pairs by photons in passing through the subsequent 0.26 radiation lengths in the emulsion and glass. In one case a pair formed inside the narrow shower then gives a secondary pair, which supports the assumption of the electronic character of the observed secondary particles. The number of photons, determined from the number of pairs produced, must be $\sim 35$. The authors believe that the photons are the result of the decay of neutral mesons, whose lifetime cannot exceed $10^{-13}$ sec.*)

The data on the angular distribution of high-energy photons obtained at the Berkeley accelerator also agree with the assumption of the formation of photons from the decay of neutral mesons.

The value obtained for the lifetime of neutral mesons makes it possible to choose between two variants of meson theory. A value of $10^{-13}$ sec. agrees much better with the pseudoscalar variant. The angular divergence of the narrow shower ($\theta \sim 2.5^\circ$) makes it possible to estimate the energy of the primary $\alpha$-particle on the assumption that, in the system of the center of inertia, the mesons formed have an angular distribution close to isotropic. In this case**)

$$ \theta = (1 - \bar{\beta}^3) = \left( \frac{2}{\gamma + 1} \right)^{1/2}, $$

where $\bar{\beta}$ is the velocity of the center of inertia, and

$$ \gamma = \frac{E_\alpha}{\mu_\alpha c^2}. $$

*) The authors consider direct emission of photons in the formation of $\pi$-mesons with spin not exceeding 1 to be improbable.

**) The formula given is not valid if, in the collision, the $\alpha$-particle transfers to the nucleus only a small part of its momentum. In this case the estimate given in the article may turn out to be overestimated, on average, by two orders of magnitude.

Thus, the energy of the \(\alpha\)-particle is \(E_{\alpha}=0.8\cdot 10^{13}\) eV. With regard to the charged particles in the narrow shower, the authors assume that these are mainly \(\pi\)-mesons. If one assumes that the number of charged

[Figure: photograph of a particle track. Visible annotations: “Primary \(\alpha\)-particle”; “300 \(\mu\)”; “Narrow meson shower.”]

and neutral mesons is approximately the same, and takes into account, from the angular divergence of the electron pairs, the energy carried away by photons, it turns out that the nucleons of the \(\alpha\)-particle transfer only part of their energy to the nucleus. The appearance of relativistic particles outside the narrow cone, if no special assumptions are made about the sharply anisotropic angular distribution of mesons in the center-of-inertia system, must be expla-

[[unclear: beginning of line]] by a cascade process inside the nucleus;³ but not by the formation of all the observed particles on a single nucleon. The described “star” may apparently be that primary act in which a broad atmospheric shower is born.

N. Birger

References

1. M. F. Kaplon, B. Peters, H. L. Bradt, Phys. Rev. 76, 1735 (1949).
2. R. E. Marshak, Phys. Rev. 76, 1736 (1949).
3. L. Leprince-Ringuet, F. Bousser, Fong. Z. Jaundeau, D. Morelex, Phys. Rev. 76, 1273 (1949).