Calibration of NaI(TI) spectrometer for radiocesium volume-samples. Some results from Transylvania region

Abstract

A four-channel NP-424 type gamma analyzer coupled with a large NaI(T1) scintillator was calibrated using a 137Cs point source. The volume-samples were plexiglas cylindrical boxes (D = 76mm, H = 40mm) directly placed on the NaI(T1) detector. Measurements for 64 indoor positions at full-662 keV-peak were made. Sample selfabsorbtion was determined considering a linear attenuation for gamma rays and measuring the absorption factor for maximum thickness of the volume sample (ho = 40mm). The obtained results for two types of environmental samples (soil and milk-powder) are compared with the data acquired from other Romanian laboratories, in the frame of the Environmental Radioactivity Action (ERA) which standardized such types of measurements. The same samples were measured with a calibrated GeLi detector (INAN -Louvain la Neuve) and the results were in good agreement with our measurements. Using this NaI(T1) calibrated spectrometer some radiocesium deposits (56 localities) from Transylvanian region were measured. The results show that a large deposit exists in the direction NE-SW in connection with the radioactive cloud passing. For Alba region were measured more than 40 kBq/m 2 of cesium deposit. IntroductionAfter the Chernobyl accident, a diversity of 137Cs deposits were found in different countries [1]. In Romania the biggest deposits were measured for the country center (Transylvania) [2, 3], some of these being higher than 20 kBq/m 2 [3]. The cesium deposits have been measured during 1986 and some years after the disaster, for undisturbed soils, using high resolution gamma spectrometry and cesium standards for volume samples [4,5]. The absence of volume standards in the immediate period following the Chernobyl accident, imposed the necessity to utilize in some laboratories the point sources for the calibration of the measuring equipment [6,7]. The use of large NaI(T1) detectors for radiocesium deposits measurements is advantageous, due to their high efficiency, but in some situations because of the their low resolution a special attention is necessary to remove other interferences in the 662 keV energy peak. In our case, the radon from the soil (610 keV), leads to the growth of the peak width, especially for the left hand side of the photopeak. The heating of the samples before measuring removes this contribution [8]. The energy peak of 134Cs (605 keV) can also contribute to the peak width. In this work a large NaI(T1) detector ((I) = 76 mm, Ho = 45 mm) using a 137Cs point standard was calibrated in order to simulate a cylindrical volume sample (D = 76 mm, H = 40 mm).The results of calibration for two different samples (soil and milk-powder) were compared with the results from other laboratories in Romania and Belgium (INAN-