Predictions and measurements of mass spectra of the charged nucleonic component of cosmic rays at mountain altitude

A cosmic-ray mass spectrometer utilizing a superconducting magnet, digitized wire spark chambers, and time-of-flight scintillation counters has examined the mass spectrum of low-momentum charged particles at 2750-m altitude (747 g/${\mathrm{cm}}^2$). For the first time at mountain altitude, the deuteron spectrum has been clearly resolved. The proton results confirm earlier work. The data indicate the presence of a few ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ nuclei. Upper limits are given to fluxes of antiproton and hypothetical massive particles in the 5-to-10-GeV/${\mathit{c}}^2$ range. The low-momentum deuteron spectrum, as well as the proton spectra at various atmospheric depths, can be well understood in terms of a cosmic-ray atmospheric cascade calculation. The deuterons are principally produced by $N+N\to d+\pi$ and quasielastic nucleon scattering on $n-p$ pairs within the nucleus. Calculations are also presented of the expected intensities of low-momentum antiprotons and hypothetical massive particles at mountain altitude.