The subsurface neutron flux and reaction probability

RADIOGENIC, NUCLEOGENIC, AND FISSIOGENIC NOBLE GASES The subsurface neutron flux and reaction probability 

Neutron interaction with atomic nuclei is directly linked to the production of several noble gas isotope species in the crust. Before considering the production of crustal noble gases it is essential to first understand the factors controlling the subsurface neutron flux. There are three main types of reaction that produce neutrons in the crust Cosmic ray interactions spontaneous fission and alpha particle interaction with light nuclei. Cosmic ray interaction is only important within the top few meters of the crust (Niedermann 2002, this volume) and we neglect this source of neutrons here. 

Neutrons are produced by (a,n) reactions within 0-40 pm of U and Th, together with a small contribution from the spontaneous fission of to give  

Spontaneous fission neutron contributions from Th and U are negligible (Andrews 

The neutron yield from (a,n) reactions is a function of the energy dependant (a,n) reaction cross section (Feige et al. 1968) and the rate of energy loss of the a-particles and reaction channels with the excited states of residual nuclei (Yatsevich and Honda 1997). The two important parameters determining the reaction cross section are the reaction energy, Q, and the height of the Coulomb barrier, B (Feige et al. 1968). B is defined by  

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