Production rates of neutrons in soils due to natural radioactivity

The relative neutron yield of several targets of light elements exposed to monoenergetic ionized helium beams from the Argonne tandem accelerator was determined for alpha energies ranging from 4.8 to 8.8 Mev. Our ratio of the neutron yields for 5.3 Mev are in agreement with published data on thick target yields obtained with Po210 α particles. The average (α, n) yield per α particle of the natural radioactive series for some elements and for different soil compositions was calculated from our results assuming an (α, n) yield of 75 neutrons per 108 α particles of Po210 α for aluminum. If 3 ppm U238 and 11 ppm Th232 are assumed as representative of the earth's upper crust, a production rate of 6.7 ± 0.7 n/g/yr in sand and 13.5 ±1.3 n/g/yr in granite is obtained. Additional 1.4 n/g/yr will be generated because of the spontaneous fission of U238. As the production rate of neutrons through interactions of cosmic rays with the earth's surface at sea level is of the order of 800 n/g/yr, these (α, n) reactions cannot be of any significance to exposure rates of human populations. They may be of some interest to geologists, however, and may be the main contributors to neutron fluxes in tunnels, deep caves, or mines, where cosmic-ray background is very low.