NRE VII, International Symposium May 20-24, 2002, Rhodes, Greece
REDUCING THE NATURAL RADIOACTIVITY BACKGROUND IN Ge DETECTOR SHIELDS
Gamma spectroscopy is a widely used technique for the qualitative and quantitative determination of gamma emitting radionuclides in a variety of samples. There are cases where the total activity of the radionuclides of interest in the analysed samples is extremely low. Typical such radionuclides are those of natural radioactivity in water samples, foodstuff, or even air-filters through which large volumes of air have been pumped for the determination of the radionuclides content in the air. Though Ge detectors used for such measurements of natural radionuclides are almost always equipped with very efficient detector shields, the existence of background radiation, due to the photons emitted from the environment outside the shield, the shield itself and the detector cryostat cannot be totally avoided or neglected. This background poses limitations to the lower limit of detection and the accuracy of the measurements performed, and should be minimized as possible. One important background contributor is the radon decay products, in and around the detector chamber, which is described as the interior of the detector shield. Among the radon daughters, the most important background contributors are 214Pb and 214Bi, which emit a total of five important photons with energies 295.22keV, 351.99keV, 609.32keV, 1120.28keV and 1764.51keV, which are often used for the 226Ra indirect determination. Besides the background fluctuations due to pure statistics, radon daughters concentration imply an additional background fluctuation. Gamma spectroscopy measurements in the Nuclear Engineering Section of the National Technical University of Athens (NES-NTUA) are performed in the gamma spectroscopy laboratory, which is equipped with a wide variety of Ge detectors, the oldest being a 20 years old GeLi detector with 24% efficiency and the newest an XtRa detector with 107% efficiency. The laboratory has recently moved to the basement of a new building. From repeated and extensive background measurements in the new building, it was concluded that the detectors background was increased probably due to the building aging; furthermore unreasonable background fluctuations were experienced. Because of its high efficiency, the XtRa detector was subject to the most pronounced background changes. For this reason, a detailed investigation of the factors affecting the radon concentration inside the room and the detectors background was undertaken. The radon activity inside the room was monitored as a function of parameters such as ventilation and air conditioning inside the room, and it was found to vary from 40 Bqm-3 to as much as ~120Bqm-3, over a two years period. The best conditions for as low as possible radon concentration indoors were: no air-conditioning inside the room, natural ventilation through the open doors and the increase of the air circulation by using a shaft especially constructed for this purpose. Furthermore, an effort was undertaken to reduce the background due to the radon daughters inside the detectors chambers, using various techniques. Among them the most effective it proved to be: nitrogen feed inside the chamber from the detector tank exhaust pipe, and limiting the indoor air volume inside the chamber. The latter was accomplished with fitting inside the chamber a light aluminium construction, made from a very thin aluminium foil of high purity, which was filled with nitrogen. This construction almost completely fills the XtRa detector chamber, leaving space only for the samples to be analysed, thus significantly reducing radon environment inside the chamber. From the results obtained so far, it was concluded that this was the most efficient way to reduce the background, with a reduction factor of about 0.7, and almost vanishing the background fluctuations.