Speaker
Description
Authors (affiliation): 1. Tomislav Ilievski (Ruđer Bošković Institute, Croatia), 2. Luka Bakrač (Ruđer Bošković Institute, Croatia), 3. dr.sc Nikola Marković (Sahlgrenska Academy at University of Gothenburg, Sweden), 4. dr.sc. Damir Bosnar (Faculty of Science, Universtiy of Zagreb, Croatia), 5. dr.sc. Ivana Tucaković (Ruđer Bošković Institute, Croatia).
As a part of RiChFALL project, analysis of radioactivity in children food in Croatia and raw materials for its production (mainly fruits and vegetables) is carried out by HPGe spectrometry. Since low level activities of naturally occurring and anthropogenic radionuclides in such samples are expected (except K-40), the aim of this work is to upgrade the existing setup. Existing setup consists of Canberra Broad Energy HPGe enclosed in the original lead shield with nitrogen flushing.
Detectors are normally placed in shields (usually made of lead) to reduce the background radiation from the environment. Radon is removed from the detector environment under pure nitrogen overpressure, pushing out of the shielded area the air that contains it. Large portion of the remaining background signals are produced by muons coming from cosmic radiation, by interacting with the detector and surrounding materials. To reduce the contribution, an active scintillator shield was introduced together with the existing lead shield. The scintillator is a plastic (polyvinyltoluene) plate (70x70x5 cm) from Saint-Gobain. It is used as a veto guard to reject events produced by muons. To achieve this, a new digital MCA (Caen DT5781 Quad Digital Multi Channel Analyzer) with time-stamping capabilities was used. Both scintillator and existing HPGe (Canberra BE5030P) were connected to separate channels of the MCA.
By using the time stamping feature of the MCA, time difference between muon detection and HPGe signal was known with sub-microsecond precision. By varying the anti-coincidance rejection window length and delay, it was found that most coincidence events are produced within 2 μs after muon detection, while coincidence was observable up to 15 μs. While scintillator position on top of the shield is obviously the most efficient one, different positions and distances from the shield were tested as well. By using the correct scintillator signal threshold, no increase in back-scattered photon signals were observed when placing the scintillator closer to the shield.
With optimized settings, when the scintillator plate is placed directly on top of lead shield, 30% to 50% of total counts have been removed from the background spectrum. Test results with shielding from the sides were also promising, so additional plates are planned to further enclose the lead shield and achieve the lowest background possible. Described cosmic veto system has proven to be quite modular and easily introduced into existing gamma spectrometry systems.
