Abstract

The main background hindering low-energy (≲ 200 keV) neutrino measurements with liquid scintillators comes from the minute remanence of the cosmogenic ¹⁴C (T₁_/₂ ≃ 5700 a) present in the organic oil constituting the bulk of the scintillator. The β-decay endpoint energy of ¹⁴C is quite low, Q = 156 keV, and the counting rate from ¹⁴C is often reduced by threshold settings. However, too high concentration of ¹⁴C may results in pile-up pulses. For example, in the Borexino detector at Gran Sasso, Italy, being the most sensitive neutrino detector, the trigger rate is largely dominated by the ¹⁴C isotope [1] with the concentration of 2 × 10⁻¹⁸ [2]

It is the lowest ¹⁴C concentration value ever measured. There are only a few results available on the ¹⁴C concentration. In addition to the one in Ref. [2] there are three other measurements reported in Refs. [3, 4, 5].

Obviously ¹⁴C cannot be removed from liquid scintillators by chemical methods, or by other methods in large quantities (liters). In principle, the older is the oil or gas source that the liquid scintillator is made of and the deeper it situates, the smaller the ¹⁴C concentration should be. This, however, is not generally the case and it is believed that the ratio depends on the activity (U and Th content) in the environment of the source.

We are performing a series of measurements where the ¹⁴C concentration will be measured from several liquid scintillator samples. They need low-background environment and are taking place in two deep underground laboratories: in the new CallioLab laboratory in the Pyhäsalmi mine, Finland, and at the Baksan Neutrino Observatory, Russia, in order to reduce and better understand the systematical uncertainties. Preliminary results will be presented.