PurpleYouko, I take you haven't got to your university library yet. Here is the abstract from the first reference I gave you.
The relative neutron yield of several targets of light elements exposed to monoenergetic ionised helium beams form 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 Po-210 alpha particles. The average (alpha, n) yield per alpha particle of the natural radioactive series for some elements and for different soil compositions was calculated from our results assuming an (alpha, n) yield of 75 neutrons per 10e8 alpha particles of Po-210...If 3ppm U-238 and 11ppm Th-232 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 spontaneous fission of U-238. 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 (alpha, 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 low.
Kuhn et al in the second paper made field measurements of thermal neutrons in mines deeper than 800m which yielded counts varying from 1.1 to 33 n/cc/yr with larger counts of up to 116 from shallow subsurface. Using a conversion factor of (say) 4 g/cc these figures are in fairly good agreement with Feige and indicate that there is a pretty sizable subsurface neutron flux despite JF's denial.
This reply may be a bit OT to the topic of your post 165 - perhaps more to the point is the magic figure 3.72.10e10 - the number of alpha particles emitted each second for each gm of product of the decay series (determined by Rutherford and Geiger over 90 years ago).
Chris
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