Pulsed Neutron Gamma Ray Logging for Minerals Associated with Uranium

A logging tool is being developed by Sandia National Laboratories to look for minerals associated with uranium deposits. The tool uses a pulsed neutron generator to irradiate the nuclei in the formation surrounding the borehole, and an "N" type, high purity, germanium detector to observe the returning gamma rays. The presence or absence of particular elements, in conjunction with information gained from other logs, is expected to predict the location of uranium deposits away from the borehole. The "Proof of Principle" probe has been assembled and has a length of 3.66 meters and a maximum diameter of 7.6 cm. The cryostat in the probe has an operational life in the borehole exceeding 7.5 hours. The resolution of the borehole detection system is presently about 5 keV due to noise from the internal DC - DC converter in the probe. Tests have been run using an external power supply which have demonstrated a resolution better than 2.6 keV. Experiments in a simulated borehole configuration with a pulsed neutron generator have established an output level of 1 × 105 neutrons/pulse for the detection of inelastic, capture, and activation gamma rays. Gating of the ADC was shown to improve the signal-to-noise ratio for specific gamma ray lines. Several analysis techniques are currently being examined for use in the project. In addition to the conventional peak stripping analysis, a technigue using ratios between a known peak and the unknown peaks has been proposed for quantitative analysis of samples.