Liquid scintillation spectrometry at Gran Sasso National Laboratory: radiocarbon measurements

Radiocarbon measurements using a Quantulus™ ultra low-level liquid scintillation spectrometer were performed at the Gran Sasso National Laboratory (National Institute for Nuclear Physics) to study the efficiency and background related to the measurement site (Plastino et al. 2001). Cosmic background and its variation have been removed in the Gran Sasso laboratory by its 1400-m rock overburden. Stable, high-performance liquid scintillation counting conditions are obtained when any remaining variable components of the environmental background, such as radon, are eliminated. The ultra low-level liquid scintillation spectrometer Quantulus has an anti-Compton guard detector, which allows for the monitoring of gamma radiation in the background. Guard detector efficiency in 14C background reduction is 8% at Gran Sasso, while 80% is observed on the surface (Plastino and Kaihola 2004). The big difference in the guard detector efficiency between surface laboratories and Gran Sasso is due to the absence of cosmic and associated lower energy Compton radiation (Plastino and Kaihola 2006). The cosmic noise reduction observed at the Gran Sasso laboratory makes it possible to perform high-precision 14C measurements and to extend for these ideal samples the present maximum dating limit from 58,000 to 62,000 BP (5 mL, 3 days of counting) (Plastino et al. 2001).