Fast Simulation of Gamma-Ray Logs for Uranium Exploration in Roll-Front Deposits

Orano Mining is evaluating the potential of the CeBr 3 spectrometric gamma-ray logging probe developed by Advanced Logic Technology (ALT) for estimating uranium concentration in roll-front deposits, where decay chain disequilibrium disrupts its relationship with total gamma count rate. The Nuclear Measurement Laboratory of CEA IRESNE, in Cadarache, France, is working on predictive algorithms capable of fully exploiting the shape of the recorded gamma spectra. However, the current number of logged wells is insufficient to properly train such algorithms. This calls for the creation of a diverse database of simulated gamma-ray logs, using the Monte Carlo N-Particle (MCNP) transport code. Conventional analog simulations based on MCNP pulse-height (F8) tally provide accurate estimates but are too computationally expensive for this task. Achieving low statistical uncertainty in every energy bin necessitates simulating a large number of particle histories. Moreover, modeling borehole measurements involves simulating numerous probe locations. Designing a more efficient yet sufficiently accurate simulation procedure is therefore crucial. We propose a 2-step method, where the flux reaching the probe surface and the detector response to this flux are simulated separately. The bulk of runtime reduction is achieved in the first step, through the use of MCNP point detector (F5) tally, which acts as a variance reduction technique. However, treating the detector as a point may lead to significant bias. We conducted an experimental validation on Orano CIME calibration blocks (Bessines, France) and observed good agreement between measured and simulated spectra. These findings support the 2-step approach as a viable option for building our database.