The effect of the chemical composition of a ferrallitic soil on neutron probe calibration

Abstract

In a ferrallitic soil in French Guiana, the neutron probe calibration appeared to be problematic: considerable variations in the neutron count rate were observed at very short range and with an almost constant volumetric water content. This local variability of the count rate was explained by the mineralogical heterogeneity of the schist weathering horizon where subverticaly oriented layers are especially rich in boron, element with large thermal neutron absorption cross section. Various calibration methods were carried out and their limits were pointed out. The field gravimetric calibration without taking into account the soil physical and chemical spatial variations appeared to be risky, even if different pedological horizons are considered separately. A calibration based on the neutron absorption Σa and diffusion Σd cross sections calculated from chemical analysis led to overestimates of the volumetric water content. This could be explained by the concentration of boron atoms in sand-size tourmaline crystals which reduces their neutron absorption properties. The direct measurement of thermal neutron absorption and diffusion cross sections on soil samples in a graphite pile seems to be the best calibration procedure, but it has to be repeated as often as the spatial variability required.