Distribution of Slow Neutrons in Free Atmosphere up to 100,000 Feet

Two identical proportional counters filled with boron trifluoride of 96 percent B¹⁰ have been sent aloft up to an altitude of 102,000 ft at a geomagnetic latitude of 51^∘{} 46{'} N by means of free balloons. One counter was shielded with 0.030 in. of cadmium, and the other enclosed in tin of the same thickness for the compensation of possible effects caused by "stars" produced in the cadmium shield. The difference in the counts of the two counters is due only to slow neutrons ($E<~0.4$ ev). The counts and the pressure and temperature were radioed down by an FM telemetering system. Up to about 20 cm of Hg the counts increase exponentially with altitude according to an absorption depth λ=156 g/cm², in agreement with previous measurements, and roughly with the increase of "stars" in the atmosphere. The counts of both counters as well as their difference show a maximum at high altitude, as expected theoretically. The maximum for the cadmium difference counts appears at about 8.5-cm Hg pressure and drops down sharply to about one-fourth of its maximum value at 1 cm Hg. The counter sensitivity was calibrated against a standard neutron source and the absolute number of slow neutrons absorbed per gram and second in the atmosphere is computed and compared with the number of protons produced at the same altitudes.