Activity at Kamioka Double-Beta Decay Facility
S. Umehara for the non-accelerator groupResearch Center for Nuclear Physics (RCNP), Osaka University, Ibaraki, Osaka 567-0047, Japan

RCNP has the double beta decay facility at the Kamioka underground laboratory, Institute for Cosmic
Ray Research, the University of Tokyo, for underground science such as double beta decay search and low
background measurements.

Study for Double Beta Decay of 48 Ca with CANDLES

Events(Counts/0.1MeV)

Events(Counts/0.1MeV)

Almost all substance of the universe is made of “matter”, not “anti-matter”. This is a mystery of “the matter
dominated universe”. The key point to solve the mystery is the lepton number non-conservation that means“anti-particle” and “particle” can convert to each other. CANDLES project aims at discovering neutrino-lessdouble beta decay which does not conserve the lepton number. For the measurement of the neutrino-less double
beta decay, we have developed the CANDLES III system using calcium fluoride (CaF2 ) crystals that include
double beta decay nuclei 48 Ca, at Kamioka Observatory.
In this paper, we briefly report the first result from CANDLES III system as published in ref.[1] andadditional spectra. The CANDLES III system employs 96 CaF2 scintillation crystals (305 kg) with naturalCa isotope which corresponds 350 g of 48 Ca. External backgrounds were rejected using a 4π active shield of aliquid scintillator surrounding the CaF2 crystals. The internal backgrounds caused by the radioactive impurities
within the CaF2 crystals can be reduced eﬀectively through analysis of the signal pulse shape. We analyzed
the data obtained for a live-time of 130.4 days to evaluate the feasibility of the low background measurementwith the CANDLES III detector. The major backgrounds in the Qββ value region of CANDLES III were the208
Tl and 212 BiPo decays, which are decay in 232 Th series within the CaF2 crystals. Therefore, the amountof 232 Th series impurities was used as an index for the high-purity crystal. According to fig. 1 (a), 21 CaF2crystals were selected as high purity crystals by using radioactive impurities of the 232 Th series that were lessthan 10 µBq/kg. On the other hand, fig. 1 (b) is obtained from 93 CaF2 crystals whose radioactive impurities
were including more than 10 µBq/kg.
We observed no events in the 0νββ window 4.17 – 4.48 MeV for the selected 21 high purity CaF2 crystals,
whereas 6 events were observed for the 93 CaF2 crystals. On the other hand, using Monte Carlo (MC) simulations, we estimated the background rate from the radioactive impurities in the CaF2 crystals and the rate ofhigh energy γ-rays caused by the (n, γ) reactions induced by environmental neutrons. According to the MC

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Figure 1: Obtained energy spectra (plots) and simulated background spectra (lines) obtained using 21/93 CaF2
crystals in (a)/(b). Blue, magenta, and green lines correspond to simulated background spectra of 208 Tl and212
BiPo, 2νββ decay, and (n,γ) events, respectively. Red line represents summed up spectrum of all threesimulated spectra. The experimental energy spectrum around the Qββ value region is well reproduced by thesimulated one.

simulation, the expected background rate was estimated to be 1.0 counts in the 0νββ window for the 21 CaF2
crystals, considering the position of the crystals and the impurities contained therein.
By using experimental data and the MC simulation, We can set a lower limit and experimental sensitivity
on the half-life of 0νββ decay. The half-life limit with 90% C.L. obtained by selecting 21 high purity CaF2
crystals was 5.6 × 1022 year. This limit was comparable to the most stringent value obtained via measurementfor over two years using the ELEGANT VI detector [3]. We also obtained an experimental sensitivity of 2.7 ×
1022 year (90% C.L.). Based on the obtained half-life limit, the upper limit on the eﬀective Majorana neutrinomass ⟨mν ⟩ ≤ 2.9 – 16 eV (90% C.L.) was derived using the nuclear matrix elements obtained from ref. [4] andthe reference therein.
The present limits on the half-life and eﬀective Majorana mass were obtained using nat. Ca instead of enriched48
Ca crystals. The limit on ⟨mν ⟩ did not reach suﬃcient sensitivity compared with those in experiments using
other enriched ββ isotopes, such as 76 Ge and 136 Xe, because of the lack in 48 Ca isotope. The natural abundanceof 48 Ca is approximately 0.2%, which is the smallest among the ββ decaying nuclei used in other experiments.
On the other hand, this result exhibits a large potential of approximately 500 enhancement in case enrichment
could be achieved. The highest enrichment achieved thus far is 96.6%, which leads to approximately 540
enhancement[5]. Several approaches to 48 Ca enrichment, such as electromagnetic/optical separators, laser
isotope separation, chemical separation, and electrophoresis, each with the aim of producing large amounts of48
Ca, are under development [6].
The observed energy spectrum around the Qββ value region was well reproduced by the simulated one,estimated with the three background candidates considered. In other words, there were likely no additional highimpact backgrounds. The observed event rate in the 0νββ window was approximately 10−3 events/keV/yr/(kgof nat. Ca), which was comparable or less than those of other sensitive 0νββ experiments. The present result isuseful for the development of a more advanced detector and shows that 48 Ca is a promising target nucleus for
the 0νββ decay search using CaF2 crystals.

Detector system for measurement of radioactive contaminations(Ge detector)
The materials used for construction of the detectors which aim to detect the rare processes such as double beta
decays and dark matter, should be carefully selected by the measurement of long-lived radioactive impurities,
because the impurities might be main source of backgrounds. Especially for 0νββ decay search, the main sourcesof backgrounds are due to the radiative decays of 214 Bi, 208 Tl and 212 Bi nuclei in U- and Th-series. Thus theamounts of U and Th contaminations largely influence on the sensitivity of the detectors.
RCNP oﬀers detector systems for measurement of radioactive contaminations as the joint usage/research.
The one is the system to measure radiative decays within scintillating crystals. Details are shown in the 2019RCNP annual report. The other one is a low-background γ-ray detector system, which consists of a high purity
Ge detector. The system was used to measure radioactivities in various kind of materials. The system has
been installed in the Kamioka underground laboratory. The sensitivities of the system for U- and Th-chainsare ∼ 1 mBq/kg for CaF2 powder. In this year the system used to select low background materials for adilution refrigerator, which will be used for dark matter search. Details are shown in report of the RCNP joint
usage/research. The system will be applied for such material selection.

References[1] S. Ajimura et al., accepted by Phys. Rev. D.
[2] T. Kishimoto et al., 2003 Proc. of 4th Workshop on Neutrino Oscillations and their Origin p 338; T. Iida etal., 2016 Journal of Physics : Conference Series 718 062026; S. Umehara et al., 2016 Proceedings of Science(INPC2016) 246.
[3] S. Umehara et al., Phys. Rev. C, 78, 058501, (2008).
[4] J. Engel and J. Menéndez, IOP Publishing, 80, 046301, (2017).
[5] E. Mateosian et al., Phys. Rev., 146, 810-815, (1966).
[6] S. Nabiev et al., Phys. Atomic Nuclei, 64, 1541-1548 (2001); K. Matsuoka et al., J. Phys. Conf. Series, 1468,012199 (2020); S. Umehara et al., Prog. Theor. Exp. Phys., 053C03, 1-8 (2015); R. Hazama et al., KURRI
Progress Report, 0919-1038, 19 (2016); T. Kishimoto et al., Prog. Theor. Exp. Phys., 033D03, 1-10 (2015).