AN ACTIVE BACKGROUND DISCRIMINATION TECHNIQUE USING A MULTIPLE DETECTOR EVENT BY EVENT RECORDING-SYSTEM

For rare event experiments (environmental, solar neutrino, beta beta-d ecay and dark matter) a careful selection of the construction material s of the detectors with respect to low radioactive contamination is fu ndamental. Non-destructive low-level gamma-ray spectrometry serves thi s purpose best. To improve the sensitivity of a germanium detector, ba ckground reduction is a prerequisite. This paper describes an active b ackground discrimination technique, which in principle is a coincidenc e method. Instead of an active veto, an event-by-event recording techn ique is used to improve the signature of each event. Two detectors, on e low level HPGe-detector in a lead shield and a large area plastic sc intillation detector (above the Ge-detector and outside the lead shiel d) are operated simultaneously (the system allows up to 12 ADC inputs) . For each event the detector identification number, the measured puls e-height and the time of occurrence of the event are stored. The coinc idence or anti-coincidence conditions can be chosen by software after the measurement, the conditions can be optimised and altered without l osing information. The technique allows to distinguish between fast mu on induced events, radioimpurity events and, under special detector co nditions, Compton events. These background events can then be subtract ed, leading to a smaller statistical uncertainty of the net signal fro m the sample. From the recorded data the muon Aux can be extracted as well. Experiments at ground level and in a depth of 500 m water equiva lent are described. Copyright (C) 1996 Elsevier Science Ltd