A coincidence method of thorium measurement

Literature survey showed that the majority of thorium measurements were performed using alpha or gamma spectrometry, although inductively coupled plasma mass spectrometry, X-spectroscopy and neutron activation analysis were also used for various thorium measurements. Coincidence methods developed using different spectrometers and coincidence schemes were applied for thorium measurements as well. We are developing a coincidence method for thorium and its decay products measurement by a multidetector γ-ray 4π- spectrometer. This method, based on coincidence counting of 583 keV and 2615 keV photons from cascade transitions that follow β--decay of 208Tl, is presented here. The method, developed on the six-crystal spectrometer PRIPYAT-2M, which allows measurements of large volume samples and which is practically insensitive to the shape of analyzed sample, has relatively high detection efficiency and a good sensitivity. In the case of thorium measurement by the 583 keV photopeak, a minimum detectable activity in a spectrometric double coincidences mode of counting, for 1 000 s, is estimated to be 1.7 Bq/kg, and in a radiometric double coincidences mode of counting, for 100 s - 1.3 Bq/kg. For the same measurement times, in the case of thorium measurement by the 2615 keV photopeak, in spectrometric and radiometric double coincidences mode of counting, a minimum detectable activity is estimated to be 2.2 Bq/kg and 2.4 Bq/kg, respectively. A determination of thorium activity in various samples from environment is one of possible applications of the method. Therefore, using this method, double coincidences mode of counting and the 583 keV photopeak, thorium activity was determined in samples from the Montenegrin environment (including red mud samples from the Aluminium Mill nearby Podgorica - the capital of Montenegro), and results are also presented here. (author)