Uranium Logging by the Prompt Fission Neutron Technique

This paper briefly describes the Sandia prompt fission neutron uranium logging probe and outlines progress on it since 1976. The probe uses a small D-T accelerator to send out a burst of 14 MeV neutrons into the formation around a borehole, and it then detects prompt epithermal neutrons returning from thermal fissioning of 235U in the formation. A NaI detector monitors gamma rays above a 3 MeV threshold produced by capture of thermal neutrons in the formation. Signals from both detectors go to uphole multiscalers which record the number of counts occurring in each 50 μs interval for 3.2 ms after the neutron burst. Analysis of the epithermal and thermal data can provide assays of uranium concentrations as low as 200 ppm. Various models of the probe have logged hundreds of boreholes in Texas, New Mexico, and Wyoming with improvements being made in reliability, sensitivity, and data analysis. The present models produce about 2-3×108 neutrons per second at 100 pulses per second and can assay at 1 meter per minute.