Crustal He and Ne - Nucleogenic Components

4He and 3He 4He, not only in the crust but also in the Earth, is essentially radiogenic, and has been produced from radioactive decay of U, Th series elements. Only a significant source for nucleogenic 3He in the crust is a reaction 6Li(n, Cf) H( 7 , 2 = 12.3 a) — 3He, where neutrons are derived from a spontaneous fission of 23SU and from reactions of light elements such as Na, Mg, Al, and Si with a particles emitted from U, Th decays. However, in a very shallow surface region (less than a few meters), the secondary cosmic ray neutrons would be more important. 

A neutron production rate (not including the cosmogenic components) was calculated by Andrews et al. (1986) for a granitic rock as a function of U, Th contents, 

3He production rate (cm3 STP s ) in rocks is expressed as (Morrison Pine, 1955) 

4He production per gram of a specimen can be expressed as a function of U content (Craig Lupton, 1976), 

As given in Equation (5.7), the neutron production rate can be expressed as a function of U content. From the production rate, we can calculate the neutron flux (X) with the relation fn = v x n, where v denotes the mean velocity of neutrons and n is an equilibrium concentration of neutrons. The latter quantity is related to the neutron production rate (pn) as n = (pjt) where T denotes the time constant for the neutron absorption in the medium (-2500 s1). Andrew et al. (1986) estimated the average neutron flux in the Stripa granite to be 5.5 x 10 4 neutrons cm 2s, which is in good agreement with the measured flux of 4.7 x 10 4 neutrons cm 2s 1 in the borehole in the granite. 

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