Low-Momentum End of the Spectra of Heavy Primary Cosmic Rays

Abstract

Measurements of the intensity of heavy primary cosmic-ray nuclei have been made above the atmosphere by means of the new Iowa balloon-launched rocket ("rockoon") technique at geomagnetic latitudes $\ensuremath{\lambda}=56\ifmmode^\circ\else\textdegree\fi{}, 76\ifmmode^\circ\else\textdegree\fi{}, \mathrm{and} 86\ifmmode^\circ\else\textdegree\fi{}$. The measuring instrument was a thin-walled, pulse-ionization chamber of 15-cm diameter. The observed data, in conjunction with geomagnetic theory, demonstrate a complete or nearly complete absence of primary heavy nuclei of $Z\ensuremath{\ge}6$ having a magnetic rigidity less than 1.5\ifmmode\times\else\texttimes\fi{}${10}^{9}$ volts ($\frac{p}{\mathrm{mc}}<0.8$), the result being the most significant for the C, N, O group. It is noted that this spectral cutoff occurs at closely the same magnetic rigidity, and distinctly not at the same velocity, as the previously reported cutoff in the spectra of primary protons and $\ensuremath{\alpha}$ particles.