Slow-Neutron Intensity at High Balloon Altitudes

In order to extend measurements of cosmic-ray neutrons to top balloon altitudes, a flight was conducted on August 24, 1959, in which our system of identical counters filled with enriched and natural B¹⁰{F}₃$ was used. An altitude of 122 000 feet at a geomagnetic latitude of 55^∘{}N was attained, corresponding to 4-mb pressure, which is considerably higher than previous measurements. The counting rate dropped to about 1% of the maximum rate, which occurs at 60 000 feet. If one extrapolates this curve to the top of the atmosphere, one obtains an upper bound for the slow-neutron albedo of 0.03 neutron per cm²-sec. The background due to highly ionizing events in the counters increased exponentially to about 63 000 feet, with a derived mean free path of 152 g/cm². The background continued an exponential increase up to 122 000 feet, but with a mean free path of 15 g/cm².