Energy spectrum of geomagnetically trapped protons

Abstract

The energy spectrum of protons in the inner Van Allen radiation belt has been measured for energies greater than 12 Mev. The spectrum was measured using nuclear-track emulsions irradiated during the flight of an Atlas missile from Cape Canaveral on October 13, 1960. Above 100 Mev the data are in agreement with the albedo-neutron hypothesis. The spectrum falls below the calculations of Freden and White at high energies. The theory of Lenchek and Singer gives an improved fit to the data when the cutoff energy parameter has a value of 650 Mev. The anomalous behavior in the spectrum below 80 Mev, observed in a previous flight on July 21, 1959, is confirmed. The spectrum in the present experiment departs significantly from the above calculations for energies less than 80 Mev. A maximum is reached at about 35 Mev, with a minimum indicated near 20 Mev. There is no evidence for a time variation of the spectrum above 80 Mev. Further investigations are necessary to decide whether the spectrum below 80 Mev represents an equilibrium condition or a transient phenomenon attributable to solar protons. Mass measurements were carried out on 783 tracks with residual ranges greater than 5 mm. Five deuterons were found.