Density of cosmic-ray neutrons in the atmosphere

Abstract

Neutrons are produced in the atmosphere by the interaction of cosmic-ray particles with the nuclei of nitrogen and oxygen. The density of these neutrons was determined experimentally by means of ionization chambers filled with B^(10), B^(11)F_3, and argon. The ion- ization current resulting from the B^(10)(n,α)Li^7 reaction was obtained as a function of pressure altitude from 760 grams cm^(-2)(8,000 ft) to 6 grams cm^(-2)(115,000 ft) in a series of balloon flights at a geomagnetic latitude of 41 °N. By comparing the current from the ionization chamber to the counting rate of a B^(10)F_3 proportional counter, when irradiating the instruments with a thermal neutron source, an absolute calibration of the ionization current in terms of thermal neutron density was made. The large volume of the ionization chambers (9 liters) resulted in a significant reduction of statistical errors relative to earlier balloon flights. The small size of the flight units (3 kilograms) avoided the problems associated with local neutron production and moderation which are present in airplane measurements. The data show a maximum in the neutron density of 4.8 ± 1.2 x 10^(-7) neutrons cm^(-3) at a pressure altitude of 100 g cm^2 (53,000 ft), a rapid decline above this altitude, and an exponential decrease below 250 g cm^(-2) with an absorption length of 165 ± 20 g cm^(-2). If the data are extrapolated to the top of the atmosphere by the use of neutron transport theory, a value of 1.6 ± 0.8 x 10^(-8) neutrons cm^(-3) is obtained. These results are in reasonable agreement with the work of the New York University Cosmic-Ray Croup (2,3).