PHOTOGRAPHS OF NARROW SHOWERS

It has already been reported in the pages of this journal that nuclei of chemical elements have been discovered in the composition of primary cosmic radiation1. Bradt and Peters, who are the coauthors of this discovery, present new photographs of nuclear collisions involving cosmic nuclei2. Thanks to the use of highly sensitive photographic plates (Ilford; TU-3) in nuclear collisions, in addition to ordinary stars, it has been possible to record the occurrence of “narrow showers.” Two photographs of such showers are given below (Figs. 1 and 2).

As can be seen from the photographs, after a collision the initial cosmic nucleus breaks up into a narrow bundle of particles. In one case the resulting shower consists of α-particles and protons; in the other case it consists of the initial cosmic nucleus with reduced charge and the α-particles and protons accompanying it. In all, at an altitude of 23 km (Kyushu coast, 29.5° N latitude), 8 such showers were recorded. Three of them were formed by nuclei of cosmic carbon, nitrogen, and oxygen and consisted of α-particles and protons. In five cases before the collision the cosmic particles had charges \(Z = 14;\ 19;\ 20;\ 26\) and \(26\), and after the collision, respectively, \(Z = 10;\ 11;\ 10;\ 10\) and \(20\).

The angles between the tracks left by the particles of the shower can be measured with great accuracy, since the tracks of these particles are visible over a great length. From the magnitudes of these angles one can estimate the energy of the cosmic particle. Indeed, one should expect the particle after the collision to be deflected by an angle of the order

Fig. 1. Narrow shower of protons and α-particles arising as a result of the collision of a nitrogen nucleus. One α-particle has a range of 5 cm in the emulsion. The projections of the angles between the tracks are respectively equal to \(0.033^\circ;\ 0.077^\circ\) and \(0.110^\circ\).

\[ \vartheta = \frac{p_{1\mathrm{cp}}}{p_0} = \left[ \frac{(p_{1\mathrm{cp}})^2}{p_0^2} \right]^{\frac12} = \left[ \frac{\frac{(p_{1\mathrm{cp}})^2}{2M}\cdot 2Mc^2}{p_0^2 c^2} \right]^{\frac12} \simeq \frac{(E_{1\mathrm{cp}}\,2Mc^3)^{\frac12}}{E_0}, \]

where \(p_{1\mathrm{cp}}\) is the average “transverse” momentum inside the cosmic nucleus; the corresponding energy \(E_{1\mathrm{cp}} \simeq 8\ \mathrm{MeV}\), \(p_0\) is the longitudinal momentum, calculated per nucleon, and \(E_0\) is the energy of the cosmic nucleus per nucleon.

For the case of Fig. 2, such an estimate of the energy of the cosmic particle gives \(5\ \mathrm{BeV}\) per nucleon.

A large fraction of all nuclear collisions is accompanied either by the formation of stars or by the occurrence of showers. The numbers of the two are comparable,

Fig. 2. Photograph a) gives the track of a nucleus with charge \(Z=19\) before the collision. Photograph b) gives the narrow shower that arose in the glass in which the collision took place. The shower consists of a heavy fragment with \(Z=11\), an \(\alpha\)-particle, and 6 protons. The \(\alpha\)-particle and 3 protons can be seen in the photograph.

If one takes into account the collisions that lead to the formation both of narrow showers and of stars, it turns out that the effective radius for collisions is equal to the geometrical nuclear radius: \(R_{\mathrm{e}} = 1.45 \cdot 10^{-13} A^{1/3}\ \mathrm{cm}\), reduced by the constant amount \(\Delta R = 0.8 \cdot 10^{-13}\ \mathrm{cm}\).

V. S. Anastasevich

CITED LITERATURE

1. A. Weisenberg, UFN 38, 456 (1949).
2. H. L. Bradt and B. Peters, Phys. Rev. 75, 1779 (1949).