National calibration of low background gamma spectrometers. Problems and results

A national programme for a calibration of low background gamma spectrometers is started in Bulgaria. The following vessels have been chosen for standardization: cylinder with diameter 60 mm, height 20 mm and filled up to 50 ml; 'Marinelli' vessel with volume 450 ml; 1000 ml; and a vessel worked out in the Bulgarian Academy of Science (BAS). Additional geometries have been chosen - filters with diameters 32 mm and 47 mm and point source with spot diameter 3 mm. The preparation of volume radioactive sources is described. It has been performed in two stages. During the first stage a sets 33 sources have been made and tested with two sets of radioactive solutions, containing 241Am and 210Pb and Amersham cocktail. Two series of sources have been prepared. The first one consists of 33 volume sources using organic and non-organic compounds and with geometry: cylinder V=50 ml; Marinelli (IEC), V-450 ml; Marinelli (BAS), V=450 ml; Marinelli (IEC), V=1000 ml. 23 gamma spectrometers have been calibrated with these sources. After the evaluation of the first set of sources, and after some improvements of the technology, the second set for calibration of Ge detectors has been worked out - cylinder V=50 ml and Marinelli (BAS) V=450 ml and in addition 3 plane sources (filters) and 3 point sources with spot diameter 3 mm. The calibration with the new set is in process. The first results from measurements are presented in the paper