Applicability of an X-ray and gamma-ray portable spectrometer for activity measurement of environmental soil samples

Abstract

This paper presents a study on the applicability of a commercial X-ray and gamma-ray portable spectrometer (Rover) for in situ gamma-ray spectrometry. This portable spectrometer consists of a 3 × 3 × 1 mm (CdTe) Cadmium Telluride detector and a 30 × 30 mm [NaI (Tl)] Sodium Iodide detector. The radioactive sources used were 241Am, 133Ba, 152Eu and 137Cs tablet type, sealed with aluminum and polyethylene, as well as soil samples contaminated with 137Cs. With the aid of these radioactive sources, this study determined the efficiency curve for both detectors. In addition, measurements were carried out to identify the minimum detectable activity (MDA) for both detectors, using time acquisition change ranging from 900 s to 138 h, depending on which detector was utilized and the sample distance. The results for the tablet-type sources were as follows: The MDA for the CdTe detector, while positioned 4.15 cm from the 137Cs source, was 15 kBq and 6 kBq for energy lines 32 keV and 661.65 keV, respectively. However, when the distance between the source and detector was 100 cm, the 661.65 keV line presented a MDA of 68 kBq. Results for the soil samples were as follows: Using the CdTe detector, positioned 4.15 cm from the source, the MDA was 73 Bq for the 137Cs 32 keV line. In relation to the 7Be soil samples, the MDA was 301 Bq when the detector was 4.15 cm from the source. Using the NaI (Tl) detector to analyze the 137Cs 661.65 keV line, the MDA was 8 kBq when the detector was 100 cm from the tablet-type source. For the soil sample containing 137Cs, the MDA was 7.4 Bq when the source was 2.8 cm from the NaI (Tl) detector. For those samples containing 7Be, and measured at the same distance (2.8 cm), the MDA was of 9.6 Bq. Based on the minimum detected activities obtained for both detectors, it is concluded that the in situ gamma-ray spectrometric system used to quantify soil sample activities with 137Cs and 7Be is only appropriate when those activities are around one or more orders of magnitude larger than the usual radioactivity levels found in the environment. Even though it is a commercialized machine, the Rover system’s manufacturer does not provide the information cited in this abstract.