Implementation of a high-efficiency muon veto system for the GeSparK alpha-beta/gamma coincidence detector

One of the main background sources of low background high purity germanium (HPGe) detectors is the cosmic muon showers produced in the interaction with the lead and copper shield surrounding the detector that can induce signals not distinguishable from radioactivity events. Plastic scintillators are widely used to implement active veto systems to reduce this contribution and increase the measurement sensitivity. In this work, we present the implementation of a high-efficiency muon veto system for the alpha-beta/gamma coincidence detector, called GeSparK. The veto system consists of six plastic scintillator detectors accurately positioned around the HPGe and liquid scintillator detectors. The final design includes two detectors above and four inside the passive shielding between the copper and lead layers. In this way, we can limit both their size and the event rate produced by external radioactivity, compared to the usual veto systems implemented outside the lead shielding. A dedicated background measurement showed that the achieved background reduction is a bit less than 93%.