STUDY ON THE EFFECT OF NATURAL BACKGROUND FOR GAMMA SPECTROMETER SYSTEM

In the analysis of environmental radioactive isotopes using gamma spectrometry, natural background radiation is an important parameter related to the analytical results directly. Therefore, in this work, the influence of natural background radiation was studied for two models: with and without shielding of gamma spectrometer system. The initial results showed that the minimum detectable activity (MDA) of radionuclides such as 234 Th, 226 Ra, 212 Pb, 208 Tl, 40 K, 214 Pb, 214 Bi, 228 Ac have the difference of two models from 10% to 503%. This is the basis for researches to improve the lead shielding chamber in the future. Keyword: background, lead shielding, gamma spectrometer system INTRODUCTION Environmental background radiation affects to critical limit (LC) of detectors, the lower background radiation the higher sensitivity of detector. For making low background radiation, it’s necessary to test the origin of background radiation and then find out methods for decreasing an undesired contribution of background radiation. Besides, to measure the radioactivity of environmental sample exactly, the activity of radionuclides of environmental background radiation is as low as possible. Environmental background radiation has the natural origin and they emit mainly from U, U, Th (and their daughters) and K radionuclides. When they get the radioactivity equilibrium, radioactivity of U and Th radionuclides can be calculated through the radioactivity of their daughters. Decay diagrams of U and Th chains are showed in Fig. 1 (1a and 1b). Therefore, to decrease the influence of environmental background radiation on counting sample, both of measured sample and detector are put inside the lead shielding chamber. In this work, we will study the influence of natural background radiation in two experimental setups: with and without shielding of the gamma spectrometer system. TAÏP CHÍ PHAÙT TRIEÅN KH&CN, TAÄP 14, SOÁ T5 2011 Trang 17 Fig. 1a.Decay diagram of U chain Fig. 1b. Decay diagram of Th chain The limits of gamma spectrometer system are: Critical limit (LC) Limit level in which it helps us to make decision that whether there are peaks in samples or not is as follows: C n L 1.645 B 1 2m     = +         (1) Detection limit (LD) LC minimum limit is only for the boundary between the background count or the effects of radiation. When the total number of channels used by an estimated width of the background peak of interest (n = 2m): D n L 2.71 4.65 B 1 2m     = + +         (2) In equation (1) & (2), m, n and B is the number of channels of background spectrum at both of side of peak, number of channels in range of interested peaks and background, respectively. Minimum Detectable Activity (MDA) MDA is the minimum value of activity of radionuclides in which the hyper germanium (HPGe) spectrometer system can measure the gamma rays emitting from sample with given reliability, given by the following formula:


INTRODUCTION
Environmental background radiation affects to critical limit (L C ) of detectors, the lower background radiation the higher sensitivity of detector. For making low background radiation, it's necessary to test the origin of background radiation and then find out methods for decreasing an undesired contribution of background radiation. Besides, to measure the radioactivity of environmental sample exactly, the activity of radionuclides of environmental background radiation is as low as possible.
Environmental background radiation has the natural origin and they emit mainly from 235 U, 238 U, 232 Th (and their daughters) and 40 K radionuclides. When they get the radioactivity equilibrium, radioactivity of 238 U and 232 Th radionuclides can be calculated through the radioactivity of their daughters. Decay diagrams of 238 U and 232 Th chains are showed in Fig. 1 (1a and 1b). Therefore, to decrease the influence of environmental background radiation on counting sample, both of measured sample and detector are put inside the lead shielding chamber. In this work, we will study the influence of natural background radiation in two experimental setups: with and without shielding of the gamma spectrometer system. The limits of gamma spectrometer system are: Limit level in which it helps us to make decision that whether there are peaks in samples or not is as follows: In equation (1) Where MDA, L D , ε P , I γ , t and m are minimum detectable activity (Bq/kg), detection limit, full energy peak efficiency, gamma-ray emission probability, acquisition time (s) and sample mass (kg), respectively.

Germanium Spectrometer System
Low background spectrometer system includes HPGe detector that it is put inside lead

Trang 18
shielding chamber and connected with electrical instruments. Gamma-rays emitting from radionuclides of sample will be measured by HPGe detector. Signals are amplified preliminarily when they go through preamplifier and then will be amplified at amplifier. Finally, they will be showed on display as gamma energy spectrum.

Detector
The gamma-ray spectra were measured with a spectrometer based on a p-type coaxial HPGe

Lead shielding
Lead is the kind of material with high Z, so it can reduce the background radiation inside the lead shielding chamber. However, the interaction between gamma-rays and lead material is origin of the appearance X-rays in low energy range. Therefore, we used 1 mm layer of tin and 1.5mm layer of copper and put them inside lead shielding chamber in order to absorb these X-rays.     Table 3 and Fig. 5. From the data in Table 3, it is showed that MDA of the radionuclides in the environmental background with shielding is lower from 1.09 to 5.03 times than that of without shielding, significantly daughter radionuclides of radon in two 238 U and 232 Th chains. Therefore, decreasing the contribution of the environmental background radiation (with shielding) helps us to determine easily the radionuclides in the measured samples.
Besides, with shielding, it will also decrease the environmental background radiation in low energy range. This also helps us to determine easily the radionuclides emitting gamma rays in low energy range. T khóa: phông, bu ng chì che ch!n, h ph k gamma.