## LSU Historical Dissertations and Theses

1994

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Department

Physics and Astronomy

John P. Wefel

#### Abstract

The balloon-borne Superconducting Magnet Instrument for Light Isotopes flew for the second time on July 24-25, 1991 at the altitude of 36 km during conditions of extremely high solar modulation. The detector was designed to measure the charge, rigidity, and velocity of cosmic rays heavier than hydrogen within the kinetic energy range 100-2000 MeV/nucleon. The helium spectrum measured over this wide energy range is consistent with an interstellar helium spectrum expressed as a power law in the rigidity with spectral index of $-$2.68 and modulated to a level given by the solar modulation parameter $\phi$ = 1.5 GV. The $\sp2$H/$\sp4$He ratio measured in the kinetic energy range 100-200 MeV/nucleon at 1 A.U. is consistent with the previous measurements made at similar solar modulation levels, but the uncertainties related to the production of secondary deuterons by the primary cosmic ray protons in the medium above the detector make these results upper limits. The derived corrections for the secondary fluxes of protons and deuterons at low kinetic energies are compared with the published data. This comparison results in the conclusion that the kinematics of the nuclear interactions is very important in determining accurate fluxes of secondary protons and deuterons. The low kinetic energy $\sp3$He/$\sp4$He ratio at 1 A.U. is determined more accurately due to a smaller contribution from secondary production to the $\sp3$He flux measured at the atmospheric depth of 5 g/cm$\sp2$, but the energy dependence of this ratio depends on the $\sp3$He production spectrum in emulsion used to derive the fluxes of secondary $\sp3$He in this energy range. The $\sp3$He/$\sp4$He ratio energy range 100-500 MeV/nucleon is consistent with the previous measurements at similar levels of solar activity. Comparison with previous data allows us to conclude that, at these energies, the $\sp3$He/$\sp4$He ratio is not sensitive to the level of the solar modulation in agreement with the predictions from the cosmic ray propagation models. Moreover, these measurements demonstrate that the propagation of the helium cosmic ray component through the interstellar medium is the same as the propagation history of the medium elements, carbon, nitrogen, and oxygen.

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