The Quiet Time Flux of 0.16-1.6 MeV Cosmic Ray Positrons

Abstract

Measurements of the cosmic ray positron flux near ~ 1 MeV are of interest because of possible contributions in this energy region from β-decay sources. Previously we reported (Hurford et al.,1973) an upper limit for the quiet time positron flux at ~1 MeV of ~2x10^(-3) cm^(-2)•sec^(-1)•sr^(-1)•Mev^(-1), based on a preliminary analysis of data from the Caltech Electron/Isotope Spectrometer on IMP-7. The method of positron detection is discussed in Mewaldt et. al. (1975). Our IMP-8 instrument has somewhat improved positron detection efficiency and includes an analysis mode that allows a determination of the rate of background events with positron- like signatures. Subtraction of this background contribution and the accumulation of additional data with improved statistical accuracy from both instruments shows that less than 30% of the positron-like events are due to positrons. This results in the new upper limits to the 0.16 to 1.6 MeV positron flux shown in Figure 1. These new upper limits are a factor of ~ 2 lower than those we reported previously. Thus we find no evidence for ~ 1 MeV positrons which might result from the β-decay of radioactive interstellar secondaries or the decay of ^(56)Co in supernovae.