A gamma–gamma coincidence/anticoincidence spectrometer for low-level cosmogenic ²²Na/⁷Be activity ratio measurement

In this study, a digital gamma–gamma coincidence/anticoincidence spectrometer was developed and examined for low-level cosmogenic ²²Na and ⁷Be in air-filter sample monitoring. The spectrometer consists of two bismuth germanate scintillators (BGO) and an XIA LLC Digital Gamma Finder (DGF)/Pixie-4 software and card package. The spectrometer design allows a more selective measurement of ²²Na with a significant background reduction by gamma–gamma coincidence events processing. Hence, the system provides a more sensitive way to quantify trace amounts of ²²Na than normal high resolution gamma spectrometry providing a critical limit of 3 mBq within a 20 h count. The use of a list-mode data acquisition technique enabled simultaneous determination of ²²Na and ⁷Be activity concentrations using a single measurement by coincidence and anticoincidence mode respectively. - Highlights: • A coin/anticoincidence spectrometer to measure ²²Na and ⁷Be by list-mode acquisition. • The method provides a more sensitive way to quantify trace amounts of ²²Na. • Compared with HPGe detector, critical limits improved by a factor 6–10. • ²²Na and ⁷Be activity can be determined simultaneously using a single measurement