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A series of measurements has been started where the ¹⁴C concentration is determined from several liquid scintillator samples. A dedicated setup has been designed and constructed with the aim of measuring concentrations smaller than 10−18. Measurements take place in two underground laboratories: in the Baksan Neutrino Observatory, Russia, and in the new Callio Lab in the Pyhäsalmi mine, Finland. Low-energy neutrino detection with a liquid scintillator requires that the intrinsic ¹⁴C concentration in the liquid is extremely low. In the Borexino CTF detector the concentration of 2 × 10−18 has been achieved being the lowest value ever measured. In principle, the older the oil or gas source that the liquid scintillator is derived from and the deeper it situates, the smaller the ¹⁴C concentration is supposed to be. This, however, is not generally the case and the concentration is probably due to the U and Th content of the local environment.

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.

In order to detect low-energy neutrinos, for example the solar neutrinos from the ppchain (with the maximum neutrino energy of approximately 400 keV) requires that the intrinsic ¹⁴C content in a liquid scintillator is at extremely low level. In the Borexino detector, a 300-ton liquid scintillation detector at Gran Sasso, Italy, the ratio of ¹⁴C to ¹²C of approximately 2 × 10⁻¹⁸ has been achieved. It is the lowest value ever measured. The detector situates underground at the depth of 3200 mwe (1200 m). 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 should be the ¹⁴C-to-¹²C ratio. This, however, is not generally the case, and the ratio depends on the activity (U and Th content) in the environment of the source. We have started a series of measurements where the ¹⁴C-to-¹²C ratio will be measured from liquid scintillator samples. The measurements take place in two underground laboratories: in the Pyhäsalmi mine, Finland, at the depth of 4000 mwe (1400 meters) and at the Baksan Underground Laboratory, Russia at 4800 mwe, for reducing and better understanding systematical uncertainties. There will be about ten samples with the known origin, each of them 2 litres. The liquid scintillator vessel, light quides and low-active PMTs will be shielded with thick layers copper and lead. Nitrogen flow is used to reduce the radon background. The aim is to measure ratios smaller than 10⁻¹⁸, if such samples exists. One measurement takes several weeks.