Speaker
Dr
Kyohei Nakajima
(Osaka University)
Description
CANDLES is a double beta decay experiment using 48Ca in CaF2 crystals. Thanks to the highest Q value among all isotope candidates (4.27 MeV), we can measure signals in small background contribution. A remarkable distinction in the measurement is the active veto by liquid scintillator. Because the time constant is largely different in pulse shape between CaF2 and liquid scintillator, large amount of external backgrounds can be removed by choosing only CaF2 scintillation signals.
One of the main background around Q value comes from radioactive impurities in CaF2 crystals, especially 232Th decay series. We developed highly pure crystals, and those with least impurities can be chosen in the analysis. Pile-up events of 212Bi and 212Po decays are background, and they can be reduced by pulse shape discrimination. 208Tl is also dominant background, and we apply veto time after tagging parent nuclei 212Bi. Impurities of each crystals and analytical method of background reduction is presented in this talk.
The other main background is high energy gamma rays (< 10 MeV) from neutron captures on the material surrounding detector, such as rock. Observed spectrum using neutrons sources are compared with MC, and we discuss the effect of elemental rock composition to the spectrum. Since this is the most serious background in CANDLES, we plan to install the shield for external gamma rays and neutrons.
Primary author
Dr
Kyohei Nakajima
(Osaka University)
