par- 2 février 2011
To improve the detection limits in our radioactivity measurements, the background has to be reduced as much as possible. As a consequence, all radioactive measurements are realized at the LGGE semi-buried laboratory, where analyzers are protected against all interfering ambient radioactivity. Gamma and alpha / beta detection systems are located in counting room constituted of low activity levels materials.
The background spectrum of germanium detector is due to a combination of different components as (i) environmental gamma radiation, (ii) radioactivity in the construction material of the detector, (iii) radio-impurities in the shield, (iv) cosmic rays and (v) radon.
The first three contributions can be reduced drastically by means of a suitable passive shielding made of old and very-low-activity lead and by a careful selection of material surrounding the crystal. This is the case in our semi-buried laboratory where gamma ray spectrometers are surrounded by low background shield consisting of some 10cm of low lead activity with an inner lining of some 3cm of lead of very low Pb-210 activity (e.g. < 1 Bq kg-1). A thicker lead shield would increase the background due to muons and nucleons induced reactions within the shield. Further protections have been installed for the N-type gamma spectrometer, with 25cm of borine surrounding the lead shield, in order to reduce cosmic neutrons interaction with detector, and 4 mm of Cu OFHC to reduce the background from lead X-rays. Morover, the background due to radon and daughters is reduced by continuous purging of the copper shield cavity with nitrogen gas. A reduction in the radon improves the background reduction but unfortunately, it is always present in some amount. By using this method, a reduction is seen in the gamma-ray peaks of the Rn-222 progenies, Pb-214 and Bi-214 and also in the Rn-220 daughter, i.e. Pb-212. Finally, as previously described in gamma spectrometry section, the Anti-Compton device system allows also to reduce the residual environmental background.
The alpha/beta counting system, i.e. the slider and counter tubes (comprising the guard counter) are surrounded by a 10cm thick shielding made of machined lead bricks low activity specified. This system, as well as alpha spectrometry, are less submitted to “contamination” from outside environment, compared to germanium detection system.