Measured spatial dependence of delayed neutron signals for uranium exploration probes

Abstract

We are implementing a neutron activation method to assay uranium ore in situ. The method detects delayed neutrons from the uranium fission induced by cyclic activation from a pulsed, 14 MeV, neutron generator. A series of laboratory experiments has provided spatial maps that show the contribution to the delayed neutron logging signal as a function of radial and axial coordinates in a simulated ore body. Individual maps for 235 U and 238 U were obtained by using enriched, natural and depleted uranium line sources. The response to thorium was also obtained, as was the background due to the reaction 17 O(n, p) 17 N. The 238 U (fast neutron fission) signal drops off more quickly with radius than does that for 235 U (slow neutron fission) in a fashion reminiscent of the corresponding fast and slow primary-neutron fluxes. After radial integration, the 235 U signal is consistently higher than that for 238 U, typically 60% of the total uranium signal, depending on conditions. For equal concentrations, the thorium signal is roughly 15% as large as that for uranium. The 17 O activation signal is roughly equivalent to that of 238 U for 0.1% grade ore. Typical signals with PVC borehole casing material drop to 57% of the original value. Borehold fluid effects are also presented.