On Nuclear Evaporation in Cosmic Rays and the Absorption of the Nucleonic Component. II.

Abstract

The nuclear evaporations produced by cosmic rays at 3500 m altitude were studied using Ilford C2 and Kodak NT4 plates. The results are discussed in comparison with the results obtained in Berkeley for stars produced by nucleons of controlled energies and with some of the data furnished by the Bristol and Brussels groups. The results are as follows. (1) The frequency of stars versus the number of prongs does not change very much in the transition between air and condensed materials. (2) The small transition effect observed in lead is concentrated in the first few cm of the lead and is restricted to the small stars. (3) For greater thicknesses (of the order of the nucleon-nuclei geometrical range) the differences between paraffin and lead seem to be negligible. (4) The differences between the Berkeley and the cosmic ray stars are concentrated in the large stars (more than six prongs), in agreement with the fact that a large fraction of cosmic ray stars (about 80 percent) are produced by secondary nucleons having energies not exceeding some hundreds of Mev. (5) The prongs of the small stars (less than six prongs) produced by cosmic rays show an angular distribution around the vertical which is, at least qualitatively, that which must be expected from the Berkeley results. (6) The large stars show an isotropic distribution in the heavy prongs but not in the "grey" ones, and a correlation between the number of grey and the number of black prongs seems to be evident. On the average, every grey prong is accompanied by about 3.5 black prongs. (7) The energy spectrum of the black proton- and alpha-prongs is given with a statistic based on about 4000 stars. The average excitation energy associated with every black prong is about 35 Mev; the average nuclear excitation corresponding to the black prongs emission is about 150 Mev. (8) A comparison of the experimental proton spectrum with the Weisskopf-Bagge theory seems to show satisfactory agreement, assuming a value for the Gamow barrier of about five Mev. (9) Similarly, the alpha-spectrum is in fairly good agreement with the evaluations of Le Couteur. (10) Some conclusions concerning the nuclear evaporations and the nucleonic cascade are discussed.