Neutrino mass is a key issue of current physics. Super KAMIOKANDE presents non-zero mass differences, but does not give the neutrino masses themselves. The study of double beta decay might be the only probe to access neutrino mass.
Background reduction and its identification are crucial technical challenge for double beta decay measurements. In addition to good energy resolution, good position resolution is a powerful tool to identify the events.
We have developed a new method to measure the position of the event in a scintillator. Photons arrived at the scintillator surface with large angle will be reflected. Photon comes from only limited area. Therefore, the measurement of photon distribution on the scintillator surface gives the position of the scintillation.
Natural radioactivity 220Rn in a scintillator causes two successive alpha decays. They cause scintillation light at the same position. We measured energy and position of two successive events. Selecting pairs of two events at the same position, the successive decay events are clearly selected. We obtained the 220Rn concentration of about 90mBq/kg in the scintillator.
It demonstrates the position measurement is a powerful tool to measure very small amount of radioactive sources.