Ames Collaborative Study of Cosmic Ray Neutrons

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Abstract
Cosmic ray neutron flux densities, neutron dose equivalent rates and exposure rates for ionizing radiation were measured in a series of flights at 12.5 km (41,000 ft) in an Ames Research Center [California, USA] high-altitude aircraft. Instrumentation and analysis techniques were used which were developed to measure accelerator-produced radiation. Neutron instrumentation consisted of Bonner spheres, threshold detectors, a moderated BF3 counter and a liquid scintillation spectrometer. A dose-equivalent meter from Brookhaven National Laboratory and an Ar-filled ionization chamber were also flown. On 3 flights at average geomagnetic latitudes of 45, 38 and 48.degree. N, the neutron dose equivalent rates were 0.20, 0.14 and 0.22 mrem/h. The corresponding exposure rates from the ionizing component were 0.37, 0.31 and 0.41 mR/h. As measured by the liquid scintillation counter, the dose equivalent from 3 to 13-MeV neutrons was 30% of the total for all neutrons. The flux measured in this energy range agreed with previously calculated values. At a geomagnetic latitude of 43.degree. N, a plot of the exposure rate against the pressure altitude yielded a relaxation length of 140 g/cm2. Simultaneous measurements of the neutron and ionizing components verified that the former was more sensitive to changes in geomagnetic latitude. A neutron spectrum derived from Bonner sphere measurements indicates that the flux densities in the Hess spectrum may be 2 or 3 times too high, and that the evaporation bump at 1 MeV may not exist.

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