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

• 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.

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.