Study on influences of radionuclide depth distributions on the FEPEs for measurements of soil activity using INSITU HPGe gamma spectrometry

In this work, the influences of the soil densities and the radionuclide depth distributions (RDD) on the FEPE (Full Energy Peak Efficiency) calculation of the in-situ gamma ray detector using the ISOCS software were studied. The data of the RDDs at the sites were estimated by using laboratory HPGe gamma spectrometry. Six different RDDs of ⁴'0K, ²²'6Ra and ²³'2Th were found at four studied cites with radionuclide deposition moving from surface to deeper positions. They are mainly the exponential, the quasi Lorentz, linear increase of radioactivity to the 30 cm depth and the quasi uniform distribution. The results show that FEPE-energy curves obtains highest values for radionuclide depositions at the ground surface and the lowest values for radionuclide depositions at the 30 cm depth. FEPE values vary strongly for the different RDDs, especially for the low gamma ray energies. Use of the uniform model for calculating FEPEs can result in noticeable errors from 29 % to 101 % for the realistic RDD of the exponential form (surface radionuclide deposition), negative variations from 14 % to 30 % for the realistic RDD of having radionuclide deposition at the 30 cm depth, and negligible variations of less than 5 % for the realistic RDD of quasi uniform form in the range of gamma ray energies of interest. (author)