Terrestrial and solar neutrons

Abstract

The study of neutrons in the solar system is reviewed, and the theories and experiments that relate to this study are described. Neutron capture in the atmosphere produces among other isotopes carbon 14 and tritium. The earth's neutron albedo is also a source of Van Allen radiation. The importance of the source is, however, in doubt; therefore, some studies of geomagnetically trapped charged radiation are also reviewed to emphasize the areas of greatest uncertainty in the theory of injection by the neutron albedo. There are conflicting data on the absolute value of the albedo itself, although there is now better agreement on neutron fluxes within the atmosphere. Hence a critical review of the techniques employed in neutron experiments is undertaken, and some possible ways of resolving some of the difficulties are suggested. The search for solar neutrons is also described, and some possible solar‐neutron experiments are suggested. In the study of both Van Allen and solar problems, high‐energy neutrons are most interesting, and yet this energy region is the least explored.

Neutron measurements in space

“…Some of the earlier measurements reported by Haymes (1965) Table III, when corrected for solar modulation is 0.24 + 0.05 cm-2 s-2, and agrees with the other values at 40 ~ latitude. The measurements of Greenhill et al (1965), although in essential agreement, are not included on Figure 15 or in Table III because they were made at --;15 g cm -2, requiring a large extrapolation to 0 g cm -2.…”

“…First, an extrapolation of the 1-10 MeV spectral shapes which are given by E-1.1 _+ 0.2 ( Figure 17) for E > 10 MeV yield too large a flux near 15 MeV, except for the measurements of Haymes (1964). However, allthough the differential neutron energy spectrum deduced from the latter experimnt is somewhate steeper (E-1.3) than the others, the essential difference may simply lie in the absolute magnitude.…”

“…Haymes (1965) reviewed slow neutron measurements in the atmosphere and since then no new methods have been developed nor have any significant new measurements been made. We will emphasize here the difficulties of applying the known laboratory techniques for measuring fast neutrons to experiments conducted in the atmosphere and in space.…”

“…Many different fast neutron detectors using the proton recoil detection mechanism have been designed and flown on balloons and rockets to measure atmospheric and solar neutron intensities Haymes, 1964;Baird and Wilson, 1966;Albernhe and Talon, 1969;and St. Onge and Lockwood, 1969). There are some differences in the pulse shape discriminating techniques used in these detectors.…”

“…The emphasis will be upon measurements since 1964 made outside the Earth's atmosphere, but considerable reference will be made to high energy neutron experiments conducted within the Earth's atmosphere at <10 gcm -2 altitude. A review of earlier neutron measurements of terrestrial and solar neutrons has been made by Haymes (1965).…”