Measurement of underground neutron flux

Measurements of the underground thermal neutron flux have been made in a salt mine, a lead-zinc ore mine, where the host rock is limestone, and an old uranium mine with a granitic host rock. These are compared with calculated values. Underground neutrons originate in the spontaneous fission of U238, nuclear reactions of the (α,n) type in light nuclei of the rock matrix and reactions with cosmic muons. The neutron production rate from these sources has been calculated for the three rock types and the total production rate used to calculate the neutron flux in the three cases. In salt, the thermal neutron flux is very low, although the neutron production rate is high. This is caused by the very high absorption of thermal neutrons in chlorine. The measurements failed to detect the neutron flux in the salt matrix. In the other two cases, limestone and granite, the measured values confirmed the calculated estimates within the limits of accuracy. (UK)