An active background discrimination technique using a multiple detector event by event recording system

For rare event experiments (environmental, solar neutrino, ββ-decay and dark matter) a careful selection of the construction materials of the detectors with respect to low radioactive contamination is fundamental. Non-destructive low-level γ-ray spectrometry serves this purpose best. To improve the sensitivity of a germanium detector, background reduction is a prerequisite. This paper describes an active background discrimination technique, which in principle is a coincidence method. Instead of an active veto, an event-by-event recording technique is used to improve the signature of each event. Two detectors, one low level HPGe-detector in a lead shield and a large area plastic scintillation detector (above the Ge-detector and outside the lead shield) are operated simultaneously (the system allows up to 12 ADC inputs). For each event the detector identification number, the measured pulse-height and the time of occurrence of the event are stored. The coincidence or anti-coincidence conditions can be chosen by software after the measurement, the conditions can be optimised and altered without losing information. The technique allows to distinguish between fast muon induced events, radioimpurity events and, under special detector conditions, Compton events. These background events can then be subtracted, leading to a smaller statistical uncertainty of the net signal from the sample. From the recorded data the muon flux can be extracted as well. Experiments at ground level and in a depth of 500 m water equivalent are described.