PAPER EE

COMPARISON OF SCINTILLATION DETECTORS FOR BOREHOLE GAMMA-RAY LOGGING

David C. Stromswold

Bendix Field Engineering Corporation

Grand Junction, Colorado

ABSTRACT

Three types of scintillation detectors have been tested for application to gamma-ray borehole logging. NaI(Tl), CsI(Na), and Bi₄Ge₃0₁₂ 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 40_K, ²¹⁴Bi (a decay product of uranium) and ²⁰⁸Tl (a decay product of thorium). Gross count data were also obtained with small NaI(Tl) and Bi₄Ge₃0₁₂ detectors.

Both CsI(Na) and Bi₄Ge₃0₁₂ offer enhanced gamma-ray counting efficiency over that provided by NaI(Tl) which is the most common scintillator used in uranium exploration. Tests in the calibration boreholes showed that CsI(Na) averaged 45 percent higher spectral counting efficiency than did the same size Na(Tl) detector. Bi₄Ge₃0₁₂ gave an average spectral counting efficiency 310 percent higher than did the same size NaI(Tl) detector. The greater efficiency is accompanied by poorer energy resolution than is obtained from NaI(Tl). However, the poorer energy resolution did not adversely affect the stripping ratios calculated for CsI(Na) and Bi₄Ge30₁₂. On the contrary,

the stripping ratios to remove downward scattering were lower for these detectors than they were for NaI(Tl).

Finally, for gross count applications Bi₄Ge₃0₁₂ 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 Bi₄Ge₃0₁₂ was 90 percent greater than from NaI(Tl).