Implementation and optimisation of cosmic veto system using digital electronics in an environmental gamma-spectrometry laboratory

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This paper presents a detailed description of construction and introduction of an assembly for cosmic veto system at the Laboratory for radioecology of the Ruđer Bošković Institute in Zagreb. It is a typical surface laboratory for environmental radioactivity measurements using HPGe detectors. In surface level laboratories a large part of the background signal is caused by radiation produced by cosmic radiation, mostly by muons. It leads to limiting factors for reaching low detection limits, essential in environmental sample measurements, where relatively low activity concentrations are expected. Thus, reduction of cosmic component of background becomes a requirement, but also a challenge and an expense for already set routine gamma spectrometric laboratories. This paper offers a detailed description of materials and steps needed for construction and implementation of such an assembly as a guideline for other laboratories. The homebuilt veto system presented here is based on large scintillator plates covering the existing passive lead shielding. For the easy and rapid characterization of the veto system, a newly acquired digitizer was used. More specifically, the timestamping capabilities of the CAEN DT5781 MCA were used to identify the coincidences caused by muons between the scintillators and the HPGe. With the 3 plates added, a reduction factor of 2.4 was achieved, reducing the count rate between 40 keV and 2700 keV from 0.58 cps to 0.27 cps. After the full characterization of the veto system, the setup was transferred back to the previously used Canberra DSA, more suitable for routine measurements. This step and its description are lacking in the existing literature, while it is very valuable for the laboratories already set up for the environmental measurement. The additional advantages of a homebuilt system are the modularity and multi-purpose of the system which can later be used for different applications.