Surface and Underground Ultra Low-Level Liquid Scintillation Spectrometry

Abstract

Cosmic background and its variation have been removed in the Gran Sasso National Laboratory (National Institute of Nuclear Physics) 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 (guard for short) that allows monitoring of gamma radiation in the background. The guard detector efficiency in radiocarbon background reduction is 8% in the Gran Sasso National Laboratory, while 80% is observed in surface laboratories. Thus, atmospheric pressure variations in surface laboratories cause variation in cosmic radiation flux. The Quantulus anti-Compton detector is highly efficient in detecting cosmic radiation, and the sample count rate remains stable in long-term counting. Also, correlation of sample backgrounds with environmental gamma radiation in various laboratories is examined.

Development of a low background liquid scintillation counter for a shallow underground laboratory

“…One method of double containment is to use a 6 mL vial to contain the sample and seal the 6 mL vial within a larger 20 mL vial. Using small sample vials was evaluated in measurements of 14 C with the benefit of producing a lower background as the smaller cocktail volume presents a smaller target for interaction from ubiquitous radiations (Plastino and Kaihola, (2004)). A laboratory measurement comparison of the scintillation emission from the 6 mL vials to the 20 mL vials was performed.…”

“…The ultra-low-background instrument suggested above is only achievable by both siting the instrument in a location shielded from cosmic-ray products and incorporating background reduction methods in the system design. For example, a report shows the performance of commercial Quantulus systems, comparatively, in surface and underground locations (Plastino and Kaihola, (2004)). Roughly a factor of three background reduction was obtained in the Quantulus system operated underground.…”

Tritium in water electrolytic enrichment and liquid scintillation counting

“…For the tritium measurements, an ultra low-level liquid scintillation spectrometer Quantulus TM was used in underground conditions at Gran Sasso National Laboratory, where a 3800 m water equivalent shielding almost fully removes the cosmic contribution to the background (Arpesella, 1992;Plastino et al, 2001;Plastino and Kaihola, 2004).…”

“…A great part of the remaining background signals below channel 300 rising towards lower channels in the wide tritium window is due to the presence of the radon gas in the laboratory air (Arpesella, 1992;Plastino and Kaihola, 2004). The nitrogen luminescence by radon alpha particles in the counting chamber produces an identifiable spectral peak in the tritium window.…”

Background Components of a Liquid Scintillation Counter in the 14C Window

“…The figure shows that the increase in the background count rate varies linearly with volume, but it is not proportional to the volume, because the mass of the vial is a part of the sensing material, as some of the Compton electrons released in the glass may penetrate into the benzene. In Quantulus™ machines, the relation between benzene volume and 14 C background with Teflon vials is different; the background is nearly proportional to the benzene volume (Piastino and Kaihola 2004). The most plausible explanation for the difference between the 2 systems is that pulse-shape analysis is applied in Quantulus, which detects ionization in the glass, which produces fluorescence with longer decay time than fluorescence in benzene, causing a tail in these pulses that marks them as external background.…”