Measuring the $^{14}$C content in liquid scintillators


In order to detect low-energy neutrinos with a liquid scintillator detector requires
that the intrinsic $^{14}$C content in the liquid is at extremely low level. In the
Borexino detector, a 300-ton liquid scintillation detector at Gran Sasso, Italy,
the ratio of $^{14}$C to $^{12}$C of approximately $2 \times 10^{-18}$ has been
achieved. It is the lowest value ever measured.

$^{14}$C 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 $^{14}$C-to-$^{12}$C ratio should be. 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 are starting a series of measurements where the $^{14}$C-to-$^{12}$C ratio will be
measured from several liquid scintillator samples. The measurements take place in two
underground laboratories: in the Pyhäsalmi mine, Finland, at the depth of 4000 mwe
and at the Baksan Underground Laboratory, Russia at 4800 mwe, for reducing and better
understanding systematical uncertainties. The aim is to measure ratios smaller than
10$^{-18}$, if such samples exists.