Burst Frequency as a Function of Energy

Abstract

Size-frequency distributions of bursts were obtained from analyses of records made by Carnegie model C cosmic-ray meters, shielded by 12 cm of lead, stationed at different locations. The number of particles per burst ranged from 200 to 10,000. Assuming that the number of particles in a given burst is proportional to the energy of the incident ray, energy distribution curves for the burst-producing radiation were made and compared with the depth-ionization curve obtained underground by Wilson. Because both curves have nearly the same shape, it was concluded that at least the greater part of the burst-producing radiation at sea level consists of penetrating ionizing rays, presumably mesotrons. The value of ${10}^{\ensuremath{-}4}$ was found for the creation probability of a burst by a mesotron of about 2\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ev energy in a thickness of 12 cm of lead. This leads to a cross section per nuclear particle (proton, neutron) of 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}30}$ ${\mathrm{cm}}^{2}$, which value is independent of the energy of the incident particle. The ratio of the burst rates at Huancayo (3350 m elevation above sea level) and Cheltenham (72 m elevation) for different burst magnitudes was found to be constant, within the statistical errors, up to energies of about 1\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ev, and likewise for Teoloyucan (2285 m elevation) and Cheltenham. For energies higher than ${10}^{11}$ ev, the corresponding ratios of the burst rates at the high to those at the low altitudes increase rapidly with increasing energy of the burst. In order to account for this effect, it is assumed that some of the largest bursts are created by photons or electrons of energies greater than ${10}^{11}$ ev, a part which becomes predominant at higher elevations. This suggests a possible mechanism for the creation of mesotrons by the soft component of cosmic radiation in the earth's atmosphere.