SPATIAL REsolUTION AND CALIBRATION CHARACTERISTICS OF TWO NARROW PROBE GAMMA‐RAY TRANSMISSION SYSTEMS FOR THE MEASUREMENT OF SOIL BULK DENSITY IN SITU

Summary Two dual-probe gamma-ray transmission systems using caesium-137 sources were examined. The performance of a small Geiger-Muller detector was compared in respect to spatial resolution and calibration characteristics with an NaI(TI) crystal detector fitted to a thermally stabilised narrow photo multiplier with variable photon energy discrimination. The NaI(TI) detector had progressively better depth resolution with increasing discrimination threshold and more linear calibration relationships than the GM detector. The calibration linearity and sensitivity with the GM detector were strongly dependent on the mass thickness of the probe wall tubing. Count rate was not dependent on soil composition with either system. The GM detector system provided a satisfactory instrument of restricted performance. For good spatial resolution near the surface a narrow photo multiplier with thermal stabilisation overcame some of the difficulties previously found with NaI(Tl) detectors.