Low-level counting systems in deep underground laboratories

Measurements with the same equipment in laboratories with different muon fluxes and wall materials help to disentangle the relative contribution of the different background components. Gas proportional counters, a Ge(Li) detector and a liquid-scintillation counter were tested in a normal laboratory, two underground laboratories (70 m water equivalent), one of which was shielded with special concrete, and a laboratory in the Gotthard-Strassentunnel (3000 m water equivalent underground). When using low radioactive materials for detectors and shielding in a normal laboratory μ-induced events were the main background contribution. The best background values for all systems were obtained in the shielded underground laboratory (γ- and μ-fluxes each reduced by a factor of 10). There, two-thirds of the background events are attributed to the radioactivity of detectors and shields. In the Gotthard laboratory the γ-flux is higher by a factor of three than in a normal laboratory requiring a special heavy shielding. Examples of backgrounds in the shielded underground laboratory are for: (i) 14C-dating - 1000 cm3 copper counter with 4.9 bar CH4: 0.51 counts/min background compared with 33.05 counts/min modern net effect; (ii) 39Ar-dating - 100 cm3 copper counter with 19 bar Ar (+CH4): 0.10 counts/min background compared with 0.18 counts/min modern net effect; (iii) 3H-measurements - internal anticoincidence counter with 1300 cm3 (active volume); 1.3 bar CH4: 0.06 counts/min background compared to 75 TU/(counts/min); commercial liquid-scintillation counter with 10 cm3 water plus 12 cm3 scintillator: 0.60 counts/min background compared with 60 TU/(counts/min). (author)