Density of cosmic-ray neutrons in the atmosphere

A new method of measuring cosmic-ray neutrons has been developed, yielding a precision (±2 per cent) heretofore unattainable in balloon instrumentation. Ionization chambers using a quartz fiber electrometer of the Neher design and filled with B10F3 detected the neutrons by means of the B10(n, α) Li7 reaction. Ionization chambers filled with B11F3 or argon measured the background current due to the ionizing component of cosmic radiation. Calibration uncertainties and self-shielding effects limited the absolute accuracy of the measurements to ±20 per cent. This method was employed to measure the density of cosmic-ray neutrons in the atmosphere at λm = 41°N on several balloon flights during 1962. The data from the balloon flights showed a maximum in the neutron density of 4.8 ± 1.2 × 10−7 neutron cm−3 at a pressure altitude of 100 g cm−2 (53,000 feet), 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. There is satisfactory agreement between the results of this experiment and the work of the New York University Cosmic-Ray Group. The measured neutron densities of Hess et al. are as much as 60 per cent higher than the values obtained in this experiment.