Integrated Prompt Fission Neutron and Gamma-Ray Logging for Direct Uranium Quantification in Exploration and In Situ Recovery Monitoring

Conventional gamma-ray logging estimates uranium indirectly from daughter-product activity and can therefore produce substantial bias in formations affected by uranium–radium disequilibrium. This study develops and field-validates an integrated prompt fission neutron and gamma-ray (PFN–γ) logging method using the self-developed UNL4 system (version V 1.19.7) for direct uranium quantification and simultaneous radiometric characterization. The PFN unit employs a 14 MeV pulsed D–T neutron source together with thermal and epithermal neutron time-spectrum detectors to determine the direct uranium concentration (pU), whereas a LaBr3(Ce) gamma-ray spectrometer provides total gamma response and K, eU, and eTh estimates. Field trials were conducted in a sandstone-type uranium exploration area in Gansu Province and an in situ recovery mining area in Inner Mongolia. Data from 11 exploration boreholes (10,865 m in total) and repeated measurements in 10 monitoring boreholes (10,396 m in total) were processed and interpreted. Comparison with laboratory core analysis showed that the uranium content derived from pU measurements differed by −6.3% to 14.4%, while the corresponding gamma-spectrometric eU results showed deviations of −28.8% to −11.0%. The integrated measurements also distinguished uranium-enriched intervals from radium-dominant anomalies and tracked uranium depletion during mining, where conventional gamma-derived indicators did not reflect the same changes. The results demonstrate that PFN–γ logging provides a practical single-pass approach for direct uranium determination, disequilibrium discrimination, and dynamic monitoring in strategic uranium exploration and recovery.