Measurements of the Low-Energy Cosmic Radiation during the Summer of 1966

Abstract

The results obtained during two balloon flights of a counter telescope designed to measure the composition of low-energy cosmic rays are presented. The telescope consists of two ¼-in.-thick plastic scintillators used to measure $\frac{\mathrm{dE}}{\mathrm{dx}}$, followed by a 5-in.-thick crystal of NaI (Tl) to measure the residual energy $E$ of stopping particles. From these measurements the charge and the mass of each particle are calculated. The accuracy of the measurements is sufficient to separate the isotopes of hydrogen and helium and to identify the nuclei of elements up to oxygen. The detector system was flown from Fort Churchill, Canada to a residual atmospheric depth of 2.3 g/${\mathrm{cm}}^2$ for a total of 25 hours in July and August 1966. The differential energy spectra from 60 to 220 MeV/nucleon for protons, deuterons, ${\mathrm{He}}^3$, and ${\mathrm{He}}^4$ are presented after correcting for the effects of the overlying atmosphere. From these spectra the proton/${\mathrm{He}}^4$ ratio is 4.3±1.0 over the full energy range, while an upper limit on the deuteron/${\mathrm{He}}^4$ ratio of 0.25 has been set at 150 MeV/nucleon. The latter ratio corresponds to a traversal of less than 6 g/${\mathrm{cm}}^2$ of interstellar hydrogen by the cosmic rays. The ${\mathrm{He}}^3$/(${\mathrm{He}}^3$+${\mathrm{He}}^4$) ratio obtained is 0.094±0.017 in the energy interval from 78-220 MeV/nucleon. The effects of the solar modulation on the proton and helium spectra have been considered by comparing the present data with previous measurements. This comparison is consistent with a modulation function proportional to either $R\beta$ or $R$. The demodulated ${\mathrm{He}}^3$/(${\mathrm{He}}^3$+${\mathrm{He}}^4$) ratio is consistent with a traversal of 2.7±0.5 g/${\mathrm{cm}}^2$ of interstellar gas, as calculated using a simple "slab" model, and is also consistent with a model in which a distribution of path lengths is considered.