Some background effects observed with a low-level gamma-spectrometer with muon veto detector

A low background gamma spectrometer was constructed at Krakow's Institute of Nuclear Physics. The muon detector, designed and constructed in the Department of Leptonic Interactions of Institute is a multiwire Charpak's chamber. It has two active layers, with volume of 70x70x2 cm3 each. The signals from 160 anode wires are collected in 8 channels, 4 sections in each layer and transferred to NIM standard. Several logical configurations of sections were tested to optimise the effectiveness of the shielding. As the result the logical sum of signals from all sections was chosen. Produced logical signal is then transformed to 20 μs lasting positive TTL pulse, which is used as a veto for spectrometer's ADC (by Silena). Timing and duration of TTL pulse were also optimised to achieve highest possible effectiveness. The active shield itself reduces continuous spectrometer's background (from 80 keV to 3 MeV) by factor about 2 (from 0.88 cps to 0.46 cps), increasing dead-time by less then 0.5%. The influence of each part of a shield (passive and active) on background was studied. Besides this active shield, the spectrometer has special passive shield, which consists of (from outside-to-inside) 8-12 cm of paraffin, 10 cm of standard lead, 2 mm of cadmium, 5 cm of 2500 years old lead and 1 cm copper. The HPGe detector, constructed by Department of Nuclear Spectroscopy, has an aluminium-free cryostat top with a window made of a plastic foil supported by carbon fibres . The aluminium was avoided from the construction of the detector since a presence of traces of uranium in most aluminium alloys. The inner, shielded volume of spectrometer is flushed with liquid nitrogen vapours, to reduce Rn daughters contribution to the spectra. The spectrometer is now in routine operating, it was applied for the study of 22Na in the atmosphere published recently and during the study on Antarctic and Arctic contamination (not published yet). A part of the spectrometer time is used for studying background effects caused by neutrons. The most interesting effects are perhaps (n, γ) processes which occurs in construction materials but also in matrixes of sample as well. For example, a water matrix show the 2.223 MeV hydrogen neutron capture line what means, that synthesis of deuteron occurs in water sample, therefore also in certain amount in any sea waters. Introduction of a paraffin-cadmium element to the shielded area makes the system very sensitive for neutron flux. Some of this effects will be described in details in the paper