Detection of scattered X-rays from an electron linac in a borehole

An electron linac has been used as a borehole X-ray generator in determining geological density. A detection system has been developed for the linac and measures the amount of Compton scattering over a variety of paths in the formation. Since the linac is pulsed and has a low duty factor, the detectors are designed to respond to the instantaneous radiation energy rather than to the count-rate. In some cases, more than a GeV of energy can enter a detector during a linac pulse. Both NaI detectors and xenon-filled ion chambers are used. Energy discrimination is accomplished by X-ray filtering. Since all detectors receive signals simultaneously, a sample-hold and multiplexing system is employed. Gain drifts of the NaI detectors are monitored with weak 137Cs sources. NaI afterglow may influence the gain monitoring. Borehole compatibility requires high-temperature operation and ruggedization. This detection system has been used downhole to measure formation density. (orig.)