Status of 48Ca double beta decay search and its future prospect in CANDLES

CANDLES(CAlcium fluoride for the study of Neutrinos and Dark matters by Low Energy Spectrometer) is the experiment to search for the neutrino-less double beta decay(0vββ) of 48Ca with CaF2 scintillator. 48Ca has the highest Qββ-value (4.3 MeV) among all isotope candidates for 0vββ. It enables us to measure signals with very low background condition. After rejection analysis with 131 days × 86 kg data for background events from radioactive contaminations in the CaF2 scintillators, no events are observed in the Qββ-value region. As a result, the 0vββ half-life of 48Ca is greater than 6.2 × 1022 yr (90% confidence level). For further high sensitive measurement of 48Ca 0vββ search, we have been developing the 48Ca enrichment and CaF2 scintillating bolometer techniques. In this paper, the latest result for CANDLES and the status of scintillating bolometer development are described

Development of CANDLES Low Background HPGe Detector and Half-life Measurement of 180Tam

A low background HPGe detector system was developed at CANDLES Experimental Hall for multipurpose use. Various low background techniques were employed, including hermatic shield design, radon gas suppression, and background reduction analysis. A new pulse shape discrimination (PSD) method was specially created for coaxial Ge detector. Using this PSD method, microphonics noise and background event at low energy region less than 200 keV can be rejected effectively. Monte Carlo simulation by GEANT4 was performed to acquire the detection efficiency and study the interaction of gamma-rays with detector system. For rare decay measurement, the detector was utilized to detect the nature’s most stable isomer tantalum-180m (180Tam) decay. Two phases of tantalum physics run were completed with total livetime of 358.2 days, which Phase II has upgraded shield configuration. The world most stringent half-life limit of 180Tam has been successfully achieved

Status and future prospect of 48Ca double beta decay search in CANDLES

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

Status and future prospect of 48Ca double beta decay search in CANDLES

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

Upgrading of shielding for rare decay search in CANDLES

In the CANDLES experiment aiming to search for the very rare neutrino-less double beta decays (0νββ) using 48Ca, we introduced a new shielding system for high energy γ-rays from neutron captures in massive materials near the detector, in addition to the background reduction for 232Th decays in the 0νββ target of CaF2 crystals. The method of background reduction and the performance of newly installed shielding system are described