Development of a neutron probe to perform a combined measurement of uranium concentration and hydrogen porosity for mining applications

This work reports the development of a new neutron probe for mining prospection and exploitation. It allows a combined measurement of hydrogen porosity, based on the detection of neutrons backscattered by the geological formation, and of uranium concentration based on induced-fission prompt neutrons, with the Differential Die-away Technique. The probe includes a pulsed neutron generator and a single neutron counter embedded in a polyethylene block wrapped with cadmium, saving the place for a gamma-ray detector compared to usual porosity probes with two 3He counters. We first report design and feasibility studies using MCNP 6.2 simulations, showing the significant effect of hydrogen porosity on the uranium signal and the need for a combined interpretation. Then, we report laboratory experiments that validate the simulation methods of both the porosity and uranium useful signals, but also the active background due to 17O activation. Taking into account neutron absorbers in the sand of our testing system, like boron, lithium and gadolinium measured by ICP-MS, the simulated and experimental signals of prompt fission neutrons agree within +20 %. The relative effect of water, introduced in the central hole of our experimental system, is about 30 % on the backscattered neutron signal, in both simulation and experiment. Finally, the spatial sensitivity of the tool, which is maximal in front of the detector and extends to approximately 20 cm (FWHM) in the vertical axis, is well reproduced with MCNP.