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Register a new userChallenge on Ca-48 enrichment for CANDLES double beta decay experiment
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Chemical isotope effects of calcium were studied by liquid-liquid extraction using a crown ether of dicyclohexano-18-crown-6 for the purpose of finding a cost-effective and efficient way of enrichment of Ca-48 towards the study of the neutrinoless double beta decay of Ca-48. We evaluated each contribution ratio of the field shift effect and the hyperfine splitting shift effect to the mass effect of the calcium isotopes for the first time. The present preliminary result suggests the contribution of the field shift effect is small, especially for Ca-40-Ca-48 case, compared with the case of Chromium trichloride-crown in which the isotope enrichment factors are strongly affected by the field shifts. These indications are promising towards the mass producion of enriched Ca-48 by the chemical separation method.
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$^{48}$Ca, the lightest double beta decay candidate, is the only one simple enough to be treated exactly in the nuclear shell model. Thus, the $betabeta(2 u)$ half-life measurement, reported here, provides a unique test of the nuclear physics involved in the $betabeta$ matrix element calculation. Enriched $^{48}$Ca sources of two different thicknesses have been exposed in a time projection chamber, and yield T$_{1/2}^{2 u} = (4.3^{+2.4}_{-1.1} [{rm stat.}] pm 1.4 [{rm syst.}]) times 10^{19}$ years, compatible with the shell model calculations.
We developed a CANDLES-III system to study the neutrino-less double beta (0$ ubetabeta$) decay of $^{48}$Ca. The proposed system employs 96 CaF$_{2}$ scintillation crystals (305 kg) with natural Ca ($^{rm nat.}$Ca) isotope which corresponds 350,g of $^{48}$Ca. External backgrounds were rejected using a 4$pi$ active shield of a liquid scintillator surrounding the CaF$_2$ crystals. The internal backgrounds caused by the radioactive impurities within the CaF$_2$ crystals can be reduced effectively through analysis of the signal pulse shape. We analyzed the data obtained in the Kamioka underground for a live-time of 130.4,days to evaluate the feasibility of the low background measurement with the CANDLES-III detector. Using Monte Carlo simulations, we estimated the background rate from the radioactive impurities in the CaF$_{2}$ crystals and the rate of high energy $gamma$-rays caused by the (n, $gamma$) reactions induced by environmental neutrons. The expected background rate was in a good agreement with the measured rate, i.e., approximately 10$^{-3}$ events/keV/yr/(kg of $^{rm nat.}$Ca), in the 0$ ubetabeta$ window. In conclusion, the background candidates were estimated properly by comparing the measured energy spectrum with the background simulations. With this measurement method, we performed the first search for 0$ ubetabeta$ decay in a low background condition using a detector with a Ca isotope, in which the Ca present was not enriched, in a scale of hundreds of kg. The $^{48}$Ca isotope has a high potential for use in 0$ ubetabeta$ decay search, and is expected to be useful for the development of a next-generation detector for highly sensitive measurements.
We use coupled-cluster theory and nuclear interactions from chiral effective field theory to compute the nuclear matrix element for the neutrinoless double-beta decay of $^{48}$Ca. Benchmarks with the no-core shell model in several light nuclei inform us about the accuracy of our approach. For $^{48}$Ca we find a relatively small matrix element. We also compute the nuclear matrix element for the two-neutrino double-beta decay of $^{48}$Ca with a quenching factor deduced from two-body currents in recent ab-initio calculation of the Ikeda sum-rule in $^{48}$Ca [Gysbers et al., Nature Physics 15, 428-431 (2019)].
The NEMO-3 experiment at the Modane Underground Laboratory has investigated the double-$beta$ decay of $^{48}{rm Ca}$. Using $5.25$ yr of data recorded with a $6.99,{rm g}$ sample of $^{48}{rm Ca}$, approximately $150$ double-$beta$ decay candidate events have been selected with a signal-to-background ratio greater than $3$. The half-life for the two-neutrino double-$beta$ decay of $^{48}{rm Ca}$ has been measured to be $T^{2 u}_{1/2},=,[6.4, ^{+0.7}_{-0.6}{rm (stat.)} , ^{+1.2}_{-0.9}{rm (syst.)}] times 10^{19},{rm yr}$. A search for neutrinoless double-$beta$ decay of $^{48}{rm Ca}$ yields a null result and a corresponding lower limit on the half-life is found to be $T^{0 u}_{1/2} > 2.0 times 10^{22},{rm yr}$ at $90%$ confidence level, translating into an upper limit on the effective Majorana neutrino mass of $< m_{betabeta} > < 6.0 - 26$ ${rm eV}$, with the range reflecting different nuclear matrix element calculations. Limits are also set on models involving Majoron emission and right-handed currents.
Working with Hamiltonians from chiral effective field theory, we develop a novel framework for describing arbitrary deformed medium-mass nuclei by combining the in-medium similarity renormalization group with the generator coordinate method. The approach leverages the ability of the first method to capture dynamic correlations and the second to include collective correlations without violating symmetries. We use our scheme to compute the matrix element that governs the neutrinoless double beta decay of $^{48}$Ca to $^{48}$Ti, and find it to have the value $0.61$, near or below the predictions of most phenomenological methods. The result opens the door to ab initio calculations of the matrix elements for the decay of heavier nuclei such as $^{76}$Ge, $^{130}$Te, and $^{136}$Xe.
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