Neutrons are produced in the atmosphere by the interaction ofcosmic-ray particles with the nuclei of nitrogen and oxygen. Thedensity of these neutrons was determined experimentally by means ofionization 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 obtainedas a function of pressure altitude from 760 grams cm^(-2)(8,000 ft) to6 grams cm^(-2)(115,000 ft) in a series of balloon flights at a geomagneticlatitude of 41 °N. By comparing the current from the ionizationchamber to the counting rate of a B^(10)F_3 proportional counter, whenirradiating the instruments with a thermal neutron source, an absolutecalibration of the ionization current in terms of thermal neutron densitywas made.
The large volume of the ionization chambers (9 liters) resultedin a significant reduction of statistical errors relative to earlier balloonflights. The small size of the flight units (3 kilograms) avoidedthe problems associated with local neutron production and moderationwhich are present in airplane measurements.
The data show a maximum in the neutron density of4.8 ± 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 exponentialdecrease 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 ofneutron 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 workof the New York University Cosmic-Ray Croup (2,3).
