The energy spectrum of nuclei with charge Z≥6 in the primary cosmic radiation

Abstract

The « knock-on electron » technique has been employed to determine the energy spectrum of nuclei with charge Z≥6 in the primary cosmic radiation. In this technique the energy per nucleon of a heavy primary cosmic ray nucleus is determined by measuring the angles of emission and the energies of electrons which the nucleus « knocks on » in elastic collisions along its path in traversing nuclear emulsion. Since the energies of the knock-on electrons are relatively small (<100 MeV in this experiment), they can be measured with precision by the method of multiple Coulomb scattering. The conditions for reliable estimation of primary energy have been determined and the reliability of the method verified by comparison with results obtained by other methods. The charges and energies of the particles have been determined in the following two ways: (i) for particles which are arrested in the emulsion, from measurements of range andd-ray density; (ii) for particles which do not stop, from measurements of δ-ray density and of the energies and the angles of emission of the fast knock-on electrons; in the few cases where the particle slowed down in traversing the stack, the variation of δ-ray density along the track was determined. Measurements were made on 291 tracks obtained in a stack flown from Iowa, (λ=53°N), at 113 000ft. for four hours on March 13, 1956. 206 of these tracks were due to particles with charge Z≥6. The exponent of the integral energy spectrum of the medium (6≤Z≤9) group of nuclei has been obtained as 1.65±0.27 in the energy range 0.23 to 9 GeV/nucleon and that of the heavy (Z≥10) group of nuclei as 1.82±0.59 in the energy interval 0.41 to 9 GeV/nucleon. The exponent for the S-group of nuclei (Z≥6) is then 1.78±0.24. The geomagnetic cut-off energy at λ=54.5° N has been estimated to be 230 MeV/nucleon. The values of the flux of M and H-nuclei were found to be 10.7±1.0 and 5.3±0.7 particles/m²s sr, respectively (on March 13, 1956). A comparison of these values with those obtained in other experiments shows that a Forbush type of decrease had taken place in the intensity of the heavy nuclei, similar to that observed by McDonald in the λ-particle flux on the same flight; neutron monitors on ground had also recorded a Forbush decrease at the same time.