THE SCREENING EFFECT OF A TROPICAL FOREST ON AIRBORNE GAMMA-RAY SPECTROMETRY

This work estimates the screening effect of a tropical rain forest on airborne gamma-ray measurements. The screening effect is expressed as a multiplicative coefficient. The results were obtained using cylindri cal sources on the ground. A layered approximation was used to obtain a first approximation for the screening coefficient for the biomass of the forest. However, the layered approach, neglects the strong anisot ropy of the absorber. This is particularly conspicuous for the trunks of trees that contribute up to 80% to the total value of the screening coeffiect. Due to the anisotroy of the absorbers layered calculations may produce upward biased screening coefficient values. A Monte Carlo simulation was used to calculate the effect of anisotropy on the scre ening coefficient of the biomass. The situations where the anisotropy can be neglected are shown here. An additional source of bias, the rad iation from the biomass itself, is also estimated. With the results of this work it is possible to gamma-radiation measurements taken over a tropical forest.