A fully automated gamma-ray spectrometer for NORMs characterization

The large production of naturally-occurring radioactive material (NORM) represent a severe radiological hazard for health and for this reason has to be subject to regulation. We designed, built-up and tested an ad-hoc laboratory instrument, the MCA_Rad, dedicated to the quantification of the specific activity of 40K and several radioisotope of 238U and 232Th decay chains. MCA_Rad is a high-resolution gamma-ray spectrometer system made of two 60% relative efficiency coaxial p-type HPGe detectors, with an energy resolution of ~1.9 keV at 1332.5 keV (60Co). The detectors are lead and copper shielded for reaching a low background, positioned facing each other and cooled at approximately -190 °C by electromechanical stirling cryocoolers. The instrument is equipped with an automatic sample changer, which allows managing independently the measurement of 24 samples without any human intervention. The material is contained in a small cylindrical polycarbonate box of 180 cm3 of useful volume and labeled by a barcode that uniquely entifies the sample. A dedicated software has been developed in order to automatically manage the acquisition and the spectral analysis: sum of the spectra, background subtraction, energy calibration and abundances estimation. The fully energy calibration of the detectors of the MCA_Rad was performed using certified reference materials of natural origin. The chemical composition and density were consered in order to account for different photon attenuation within the source material itself. In our approach the correction factor for self-absorption effect was determined via Monte Carlo simulation. The overall uncertainty about the absolute efficiency of the MCA_Rad system is estimated to be <5 %. The MCA_Rad allows for checking the secular equilibrium among the U and Th decay chains segments. This makes this instrument particularly suitable for the characterization of the NORMs which are, respect to a typical rock or soil sample, more subject to several chemical and physical processes that disturb the secular equilibrium. Moreover the overall uncertainty about the absolute efficiency of estimated to be <5 % and the full automation of the experimental setup, represent adding values for the accurate and of the systematic characterization of a large number of samples and in turn for the regulation of the NORMs.