Calibration of HPGe detector for in situ measurements of 137Cs in soil by “peak to valley” method

INTRODUCTION The contamination of soil with gamma-ray emitters can be measured in two ways: soil sampling method and in situ spectrometry of the ambient gamma-ray radiation. The conventional soil sampling method has two disadvantages: samples may not be representative for a large areas and determination of the depth distribution of radionuclide requires the measurement of several samples taken from different depths. In situ measurement of a radionuclide activity in soil is more sensitive and provides more representative data than data obtained by soil sample collection and subsequent laboratory analysis. In emergency situations time to assess the contamination is critical. For rapid assessment of the deposited activity direct measurement of ambient gamma-ray radiation are used. In order to obtain accurate measurements of radionuclides in the soil, the detector should be placed on relatively even and open terrain It is our customary practice to place the detector 1 m above the soil surface. At this height, a tripodmounted detector can be handled easily and still provide a radius of view for gamma emitting sources out to about 10 m. The “field of view “ actually varies, being somewhat larger for higher sources. Depending upon source energy, the detector effectively sees down to a depth of 15-30 cm. Commonly used method for field gamma spectrometry is method by Beck (1). The most important disadvantages of in situ spectrometry by Beck are that the accuracy of the analysis depends on a separate knowledge of the radioactivity distribution with soil depth. This information can be obtained by calculations using data from in situ measurements and energy dependence of absorption and scattering of photons in soil and track length distribution of photons in soil (2). A method of in situ measurements of Cs in soil where radionuclide distribution in soil profile is calculated by unfolding of detector responses in the full energy peak net area at 0.662 MeV and in the valley under the peak (peak to valley method) has been developed at IPCM (3). The detector is used with and without collimator in order to achieve more independent responses for unfolding, which result in better resolution in radionuclide distribution.