Latitude and altitude dependence of the cosmic ray albedo neutron flux

Preliminary measurements of the cosmic ray albedo neutron flux above 400 km are reported for the period June 7 to 17, 1969. The measurements were made with a detector on board the Ogo 6 satellite that responds primarily to neutrons in the energy range 104 to 106 ev. The latitude variation of the counting rate was found to be 7.4/1 between 90° and 0°, similar to that predicted by Lingenfelter (1963). The neutron counting rate near the poles was found to decrease by 26±5% between 450 and 950 km altitude, corresponding to a R−4.2±0.9 dependence, where R is the distance from the center of the earth. This large altitude dependence excludes any angular distributions of neutron flux at the top of the atmosphere that are more peaked toward the vertical than cos θ (where θ is the angle from the vertical), but it is not in disagreement with an isotropic neutron flux. The total neutron leakage flux found by assuming isotropy was about 0.7 times that predicted by Lingenfelter (1963). The neutron leakage flux values obtained from the Ogo 6 experiment agree with those estimated from other experiments with a similar neutron energy response, if the same angular distribution is used. Estimates of the total neutron leakage flux, deduced from experiments responsive only to neutrons above 1 Mev by using the Lingenfelter (1963) and Newkirk (1963) energy spectra of leakage neutrons, show agreement with the lower energy results when the Newkirk spectrum is used but are up to twice as great as the lower energy results when the Lingenfelter spectrum is applied.