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The Monument experiment measures ordinary muon capture (OMC) on isotopes relevant for neutrinoless double-beta ( [Formula omitted]) decay and nuclear astrophysics. OMC is a particularly attractive tool for improving the theoretical description of [Formula omitted] decay. It involves similar momentum transfers and allows testing the virtual transitions involved in [Formula omitted] decay against experimental data. During the 2021 campaign, Monument studied OMC on [Formula omitted]Se and [Formula omitted]Ba, the isotopes relevant for next-generation [Formula omitted] decay searches, like Legend and nEXO. The experimental setup has been designed to accurately extract the total and partial muon capture rates, which requires precise reconstruction of energies and time-dependent intensities of the OMC-related [Formula omitted] rays. The setup also includes a veto counter system to allow selecting a clean sample of OMC events. This work provides a detailed description of the Monument setup operated during the 2021 campaign, its two DAQ systems, calibration and analysis approaches, and summarises the achieved detector performance. Future improvements are also discussed.

Abstract Highly radiopure NaI(Tl) was developed to search for particle candidates of dark matter. Optimized methods were combined to reduce various radioactive impurities. $^{40}$K was effectively reduced by the recrystallization method. The progenies of the decay chains of uranium and thorium were reduced by appropriate resins. The concentration of natural potassium in NaI(Tl) crystal was reduced to 20 ppb. Concentrations of alpha-ray emitters were successfully reduced by appropriate resin selection. The present concentrations of the thorium series and $^{226}$Ra were $1.2\\pm1.4$$\\mu$Bq/kg and $13\\pm4$$\\mu$Bq/kg, respectively. No significant excess in the concentration of $^{210}$Pb was obtained, and the upper limit was 5.7 $\\mu$Bq/kg at 90$\\#$ CL. The achieved level of radiopurity of NaI(Tl) crystals makes the construction of a dark matter detector possible.

The PICO-LON project aims at search for cold dark matter by means of highly radio-pure and large volume NaI(Tl) scintillator. The NaI powder was purified by chemical processing to remove lead isotopes and selecting a high purity graphite crucible. The concentrations of radioactive impurities of 226Ra and 228Th were effectively reduced to 58 ± 4 µBq/kg and 1.5 ± 1.9 µBq/kg, respectively. It should be remarked that the concentration of 210Pb, which is crucial for the sensitivity to dark matter, was reduced to 24 ± 2 µBq/kg. The total background rate at 10 keVee was as low as 8 keV−1kg−1day−1, which was sufficiently low to search for dark matter. Further purification of NaI(Tl) ingot and future prospect of PICO-LON project is discussed.

Ejiri H, Uchida H, Tsuchiya K, Fujiwara K, Kikuchi S, Hirao K. Neuropsychiatr Dis Treat. 2021;17:2739-2748. Page 2739, Abstract, Trial Registration, the text \"NCT03864484\" should read \"NCT04707495\". The authors apologize for this error. Read the original article

Abstract The dark matter observation claimed by the DAMA/LIBRA experiment has been a long-standing puzzle within the particle physics community. NaI(Tl) crystals with radiopurity comparable to DAMA/LIBRA’s are essential for adequate verification. Existing experiments using a NaI(Tl) target have been hampered by the high radioactivity concentration of NaI(Tl) crystals. The Pure Inorganic Crystal Observatory for LOw-energy Neutr(al)ino (PICOLON) experiment conducts an independent search for Weakly Interacting Massive Particles using the highest-purity NaI(Tl) crystals. In 2020, a PICOLON NaI(Tl) crystal (Ingot#85) reached the same purity level as DAMA/LIBRA crystals. The DAMA/LIBRA group has stressed that verifying their signal requires high-purity NaI(Tl) crystals with long-term stability. Based on a six-month measurement, we have confirmed the long-term stability of its radiopurity. This stability provides a significant advantage for future efforts to adequately verify the DAMA/LIBRA result using NaI(Tl) crystals. In this paper, we present the background stability of purity in the Ingot#94 NaI(Tl) detector, which was produced using the Ingot#85 purification method, along with the first annual modulation search conducted by the PICOLON experiment.

Recent data from the DAMA/LIBRA phase-2 confirmed detection of a signal with independent Dark Matter (DM) annual modulation signature at a 12.9 σ CL. Our attempts to verify the DAMA/LIBRA DM observation claim led to construction of underground clean rooms at the KamLAND site and specialized laboratory for production of NaI(Tl) detectors. Current status of these facilities, methods used to grow ultra-low background NaI(Tl) crystals, and radio-purity of the latest NaI(Tl) DM detector prototype are discussed.

In the present work we examine the possibility of detecting electrons in light dark matter searches. These detectors are considered to be the most appropriate for detecting dark matter particles with a mass in the MeV region. We analyze theoretically some key issues involved in such detection. More specifically we consider a particle model involving WIMPs interacting with fermions via Z-exchange. We find that for WIMPs with mass in the electron mass range the cross section for WIMP-atomic electron scattering is affected by the electron binding. For WIMPs more than 20 times heavier than the electron, the binding affects the kinematics very little. As a result, many electrons can be ejected with energy which increases linearly with the WIMP mass, but the cross section is somewhat reduced depending on the bound state wave function employed. On the other hand for lighter WIMPs, the effect of binding is dramatic. More specifically at most 10 electrons, namely, those with binding energy below 10 eV, become available even in the case of WIMPs with a mass as large as 20 times the electron mass. Even fewer electrons contribute if the WIMPs are lighter. The cross section is, however, substantially enhanced by the Fermi function corrections, which become more important at low energies of the outgoing electrons. Thus events of 0.5–2.5 per kg-y become possible.

This is a brief report on experimental studies of double beta decays (DBD) in Japan, the MOON project for spectroscopic studies of neutrino-less DBD (0vββ) and on experimental studies of DBD nuclear matrix elements. Experimental DBD studies in Japan were made by geochemical methods on 130Te, 128Te and 96Zr and by a series of ELEGANT(EL) counting methods, EL III on 76Ge, EL IV, V on 100Mo, 116Cd, and EL VI on 48Ca. Future counter experiments are MOON, CANDLES, XMASS and DCBA. The MOON project, which is based on EL V, aims at studies of the Majorana nature of the neutrino (v) and the v-mass spectrum by spectroscopic 0vββ experiments with the v-mass sensitivity of < mmv > 100-30 meV. The MOON detector is a super ensemble of multi-layer modules, each being composed by PL scintillator plates and position-sensitive detector planes. DBD nuclear matrix elements have been studied experimentally by using charge exchange reactions. The 2-neutrino DBD matrix elements are expressed by successive single-β matrix elements through low-lying intermediate states.

Direct search for dark matter is one of the most important problems in astrophysics. Significant signal for dark matter will be a hint to clarify the origin of the universe. Only DAMA/LIBRA experiment with NaI(Tl) detector has ever suggested the presence of dark matter signal. Verifying the DAMA/LIBRA result by a NaI(Tl) detector is urgent and important task. We have tried to purify NaI(Tl) crystal to search for dark matter. In this presentation, the present status of purification will be discussed. The concentration of potassium is successfully reduced to desired sensitivity. The 210Pb, which is difficult to reduce, has been reduced effectively. Present status of low background measurement in Kamioka observatory will be shown.

The studies of neutrino fundamental properties are widely investigated by double beta decay and inverse beta decay. Recent muon capture experiment provides promising ways to directly evaluate the neutrino nuclear responses. This study provides theoretical explanation for neutrino nuclear responses by muon capture experiment. The effects of muon binding energy towards nuclear excitation region and pre-equilibrium (PEQ) and equilibrium (EQ) neutron decay mode will be discussed. The interpretation of muon strength model by comparison of calculator output and recent experimental data may provide relevant information towards determination of nuclear matrix element (NME) and its missing parameter.