Comparison of Sodium Iodide, Cesium Iodide, and Bismuth Germanate Scintillation Detectors for Borehole Gamma-Ray Logging

Abstract

NaI(Tl), CsI(Na), and Bi4Ge3O12 detectors were installed in borehole probes and used to collect gamma-ray data at the U.S. Department of Energy's calibration facility at Grand Junction, Colorado. Spectral data were collected with all three detectors to determine their ability to assay 40K, 214Bi (a decay product of uranium) and 208Tl (a decay product of thorium). Gross count data were also obtained with small NaI(Tl) and Bi4Ge3O12 detectors. Both CsI(Na) and Bi4Ge3O12M/(bismuth germanate abbreviated BGO) offer enhanced gamma-ray counting efficiency over that provided by NaI(Tl) which is the most common scintillator used in uranium exploration. Tests in calibration boreholes showed that CsI(Na) averaged 45 percent higher spectral counting efficiency than did the same size NaI(Tl) detector. BGO gave an average spectral counting efficiency 310 percent higher than did NaI(Tl). In addition, both BGO and CsI(Na) displayed smaller stripping ratios than did NaI(Tl) due to increased photopeak efficiencies. Consequently CsI(Na) and BGO provided precisions in spectral assays which were better by 20 percent and 55 percent, respectively, than the precisions obtained from NaI(Tl) detectors of the same size with the same counting times. For gross count applications BGO produced a count rate 50 percent greater than did the same size NaI(Tl) detector in an unfiltered gamma-ray probe. When a graded filter was placed around the detectors to reduce the number of low energy gamma rays detected, the count rate obtained from BGO was 90 percent greater than from Nal(Tl).