Production of 3He in terrestrial rocks

Abstract

I consider possible nuclear sources for in situ production of 3He in rocks. The in situ cosmogenic production of 3He and 3H (which decays to 3He with a half-life of 12.34 a) occurs due to both “cosmogenic” and “radiogenic” nuclear reactions as has been discussed in the literature. The subject is in a state of flux because of new experimental data on the distribution of 3He in terrestrial samples. In situ cosmogenic production of 3He( 3H) in rocks has been documented recently. Evidence for the presence of in situ cosmogenic 3He in an alluvial diamond sample has also been found. Some diamonds have been found to contain 3He at concentration levels of ∼ 50· 10 −2 cm 3/g diamond, and 3 He 4 He ratios of (70–300) ·10 −6. I discuss the propagation of cosmic-ray secondaries in the atmosphere and in rocks, and present estimates for the rates of nuclear disintegrations due to neutrons, fast μ-mesons and capture of negative muons up to equivalent depths underground of 104 g cm −2 (0- ∼ 30 m rock). For the same rock depths, I also present estimates for the production rates of sub-MeV radiogenic neutrons from granite, clays and shales and basalt. Cosmogenic spallation and thermal neutron cosmogenic and radiogenic production rates of 3He can be obtained from the production rates given.