Search

We have studied the neutrino-less double beta decay(0νββ) of 48Ca by using CaF2(pure) scintillators. Analysis for rejection of backgrounds(212Bi→ 212Po events and 208Tl events) was effective to reduce backgrounds in Qββ-value region. No events are observed in the Qββ-value region for the data of 131 days × 86 kg. It gives a lower limit (90% confidence level) of (math) > 6.2 × 1022 year (preliminary) for the half-life of 0νββ of 48Ca.

The observation of neutrino-less double beta decay (0νββ) would be the most practical way to prove the Majorana nature of the neutrino and lepton number violation. CANDLES studies 48Ca double beta decay using CaF2 scintillator. The main advantage of 48Ca is that it has the highest Q-value (4.27 MeV) among all the isotope candidates for 0νββ. The CANDLES III detector is currently operating with 300kg CaF2 crystals in the Kamioka underground observatory, Japan. In 2014, a detector cooling system and a magnetic cancellation coil was installed with the aim to increase light emission of CaF2 scintillator and photo-electron collection efficiency of the photo-multipliers. After this upgrade, light yield was increased to 1000 p.e./MeV which is 1.6 times larger than before. According to data analysis and simulation, main background source in CANDLES is turned out to be high energy external gamma-ray originating neutron capture on the surrounding materials, so called (n,γ). Upgrading the detector by installing neutron and gamma-ray shield can reduce the remaining main backgrounds by two order magnitude. In this report, we discuss the detail of (n,γ) and background reduction by additional shielding.

To visualize and measure quantitatively flow velocity distributions, a luminescence method which uses a pulse light was investigated. A method by which flow velocity can be measured regardless of the widths of excitation time and photograph exposure time was developed. Some optical and fluid characteristics of glycerin aqueous solutions in which luminescence particles are well dispersed are reported. The spatial resolving power of this method was found to be 100-300 mu m. The apparatus which has mechanical shutters with rotating discs was used to measure the velocity distribution of two-dimensional flows.

To improve the resolution of the authors' real-time signal processor using an acousto-optic light deflector and a position-sensitive device, another acousto-optic light deflector and a prism have been added in series. The 6-8 fold resolution is obtained by keeping the linear relation between the deflection angle and the frequency within the width of the frequency variation about 360 KHz.

The conventional laser Doppler velocimeter (LDV) can only furnish the instantaneous velocity at a single point in a fluid. To obtain the complete flow pattern, the measurement has to be repeated for different regions of the flow field. In the authors' laboratory new optical systems for the simultaneous measurement of flow velocity distribution have been developed (Nakatani et al. 1979). The laser beams were arranged to intersect in a line or plane in the flow region, and optical fibres connected the interfering beams at measured points to the photodetectors. By using unsteady flow in a branch tube, it was demonstrated that the velocimeter could not only be applied to obtain the flow pattern but could also be used as a flowmeter by integrating the velocities. In these systems, however, it was required to use the same number of detectors as measured points.