Speaker
Description
Neutrino-less double beta decay (0$\nu\beta\beta$) is acquiring great interest
after the confirmation of neutrino oscillation
which demonstrated nonzero neutrino mass.
Measurement of 0$\nu\beta\beta$ provides a test for the Majorana
nature of neutrinos
and gives an absolute scale of the effective neutrino mass.
In order to search for 0$\nu\beta\beta$ of $^{48}$Ca,
we proposed CANDLES detector by using CaF$_{2}$(pure).
The CANDLES detector aims at a high sensitive measurement
by an active shield and $^{48}$Ca enrichment.
The complete 4$\pi$ active shield is realised
by immersion of the CaF$_{2}$ scintillators in liquid scintillator.
The active shield leads to a low background condition for the measurement.
On the other hand,
$^{48}$Ca enrichment is also effective for the high sensitive measurement,
since natural abundance of $^{48}$Ca is very small (0.19\%).
Currently we have been developing the CANDLES III detector,
which contained 350 g of $^{48}$Ca without enrichment,
at the Kamioka underground laboratory.
In 2015, we installed a shielding system in the CANDLES III detector
to reduce background events by the high energy $\gamma$-rays,
which were emitted from neutron capture reaction in surrounding materials.
Using this shielding system, we reduced the background events from neutron capture
by two orders of magnitude.
After this upgrade, we started a double beta decay measurement in 2016.
In this paper,
we will report result of the double beta decay measurement
after the upgrade.
