Cosmic ray spectrometry with a superconducting magnet.

Abstract

The SMILI (Superconducting Magnet Instrument for Light Isotopes) balloon borne cosmic ray instrument is described, including its design, testing and flight. The helium isotope results from 100-500 MeV/nucleon are presented and discussed. A flux calculation indicates that the solar modulation parameter during the summer of 1989 was given by $\Phi$ = 1600 $\pm$ 75 MV. This large value of $\Phi$ allowed SMILI to sample a much higher energy region in interstellar space than similar experiments flown during solar minimum. The ratio of $\sp3$He/$\sp4$He measured by SMILI from 100-1100 MeV/nucleon is rising slightly at earth's orbit. Corrections to interstellar space are made assuming an interstellar momentum spectrum of $\sp4$He which varies as $AP\sp\gamma$ with $\gamma$ = $-$2.2, P being the momentum, and A a normalization factor. A momentum spectral index for $\sp3$He of approximately $\gamma$ = $-$2.4 fits our data the best. The SMILI data, after modulation to 500 MV, is in agreement with the predictions of an energy dependent pathlength distribution derived from the B/C abundance ratio. SMILI is the first experiment to confirm the predictions for the rise in the isotopic ratio of $\sp3$He/$\sp4$He in the GeV/n region. There is no evidence in the SMILI data that the propagation history of helium is different from CNO. A re-flight of SMILI is planned for the summer of 1991 to verify and extend these results.