Q values of the Ge and Mo double-beta decays

Penning trap measurements using mixed beams of ⁷⁶Ge–⁷⁶Se and ¹⁰⁰Mo–¹⁰⁰Ru have been utilized to determine the double-beta decay Q-values of ⁷⁶Ge and ¹⁰⁰Mo with uncertainties less than 200 eV. The value for ⁷⁶Ge, 2039.04(16) keV is in agreement with the published SMILETRAP value, 2039.006(50) keV. The new value for ¹⁰⁰Mo, 3034.40(17) keV is 30 times more precise than the previous literature value, sufficient for the ongoing neutrinoless double-beta decay searches in ¹⁰⁰Mo. Moreover, the precise Q-value is used to calculate the phase-space integrals and the experimental nuclear matrix element of double-beta decay.     

Double-beta decay: Recent advances in experimental studies

The current situation in experiments studying double-beta decay is surveyed. The amount of experimental information about the two-neutrino mode of the process has grown considerably over the last decade. The two-neutrino double-beta decay of ten nuclei (⁴⁸Ca, ⁷⁶Ge, ⁸²Se, ⁹⁶Zr, ¹⁰⁰Mo, ¹¹⁶Cd, ¹²⁸Te, ¹³⁰Te, ¹⁵⁰Nd, and ²³⁸U) was observed in direct and geochemical experiments. However, the main fundamental question—that of neutrinoless double-beta decay, which has not yet been recorded, although the sensitivity of present-day facilities featuring germanium detectors is higher than 10²⁵ yr—remains open. The constraint on the effective Majorana mass on the basis of these results is 〈m _v〉<(0.4–1.1) eV. Further advancements in searches for neutrinoless double-beta decays must rely on developing fundamentally new experimental facilities, since the potential of those that already exist has been exhausted to a considerable extent.     

Energy density functional study of nuclear matrix elements for neutrinoless ββ decay

We present an extensive study of nuclear matrix elements (NME) for the neutrinoless double-beta decay of the nuclei 48Ca, 76Ge, 82Se, 96Zr, 100Mo, 116Cd, 124Sn, 128Te, 130Te, 136Xe, and 150Nd based on state-of-the-art energy density functional methods using the Gogny D1S functional. Beyond-mean-field effects are included within the generating coordinate method with particle number and angular momentum projection for both initial and final ground states. We obtain a rather constant value for the NMEs around 4.7 with the exception of 48Ca and 150Nd, where smaller values are found. We analyze the role of deformation and pairing in the evaluation of the NME and present detailed results for the decay of 150Nd.     

Average (Recommended) Half-Life Values for Two-Neutrino Double-Beta Decay

All existing "positive" results on two-neutrino double-beta decay in different nuclei were analyzed. Using procedure recommended by Particle Data Group weighted average values for half-lives of ⁴⁸Ca, ⁷⁶Ge, ⁸²Se, ⁹⁶Zr, ¹⁰⁰Mo, ¹⁰⁰Mo-¹⁰⁰Ru (0⁺ ₁), ¹¹⁶Cd, ¹⁵⁰Nd and ²³⁸U were obtained. Existing geochemical data were analyzed and recommended values for half-lives of ¹²⁸Te and ¹³⁰Te are proposed. We recommend to use these results as the most precise and reliable values for half-lives at this moment.     

The extrapolation of NEMO techniques to future generation 2<Emphasis Type="Italic">β</Emphasis>-decay experiments

The possibilities of using NEMO techniques for future neutrinoless double-beta decay experiments are discussed. The main idea is to have a realistic program with planned sensitivity for half-life measurement on the level of ～(1.5–2)×10²⁶ yr (sensitivity to neutrino mass ～0.04–0.1 eV). It is argued that this can be achieved using the improved NEMO technique to study 100 kg of ⁸²Se. A possible scheme for a future SUPERNEMO detector and its main characteristics are presented. Such a detector can also be used to investigate 0νββ decay in ¹⁰⁰Mo, ¹³⁰Te, and ¹¹⁶Cd with a sensitivity of up to ～(2–5)×10²⁵ yr or with a sensitivity to neutrino mass of ～0.04–0.26 eV.     

Double-β decay studies with JYFLTRAP

We have applied JYFLTRAP double Penning trap mass spectrometer to study several double-β decay Q values that could be useful for the search of neutrinoless double-β decay and, hence for the determination of neutrino properties.     

On the resonant neutrinoless double-electron-capture decay of Ce

The double-electron-capture Q value for the ¹³⁶Ce decay to ¹³⁶Ba has been determined at JYFLTRAP. The measured value 2378.53(27) keV excludes the energy degeneracy with the ⁰⁺$0^{+}$ excited state of the decay daughter ¹³⁶Ba at 2315.32(7) keV in a resonant 0νECEC$0\nu \mathrm{ECEC}$ decay by 11.67 keV. The new Q value differs from the old adopted value 2419(13) keV (Atomic Mass Evaluation 2003) by 40 keV and is 50 times more precise. Our calculations show that the precise Q   value renders the resonant 0νECEC$0\nu \mathrm{ECEC}$ decay of ¹³⁶Ce undetectable by the future underground detectors. We measured also the double-β decay Q value of ¹³⁶Xe to be 2457.86(48) keV which agrees well with the value 2457.83(37) keV measured at the Florida State University.     

Average and recommended half-life values for two-neutrino double-beta decay: Upgrade ’05

All existing “positive” results on two-neutrino double-beta decay in different nuclei were analyzed. Using the procedure recommended by the Particle Data Group, weighted average values for half-lives of ⁴⁸Ca, ⁷⁶Ge, ⁸²Se, ⁹⁶Zr, ¹⁰⁰Mo, ¹⁰⁰Mo−¹⁰⁰Ru(0 ₁ ⁺ ), ¹¹⁶Cd, ¹⁵⁰Nd, ¹⁵⁰Nd− ¹⁵⁰Sm(0 ₁ ⁺ ) and ²³⁸U were obtained. Existing geochemical data were analyzed and recommended values for half-lives of ¹²⁸Te, ¹³⁰Te, and ¹³⁰Ba are proposed. We recommend the use of these results as presently the most precise and reliable values for half-lives. Presented by A.S. Barabash at the Workshop on calculation of double-beta-decay matrix elements (MEDEX’05), Corfu, Greece, September 26–29, 2005.     

Q value related mass determinations using a Penning trap

We report here about measurements of reaction and decay Q values by precise determination of pairs of atomic masses. These were performed with the Penning trap mass spectrometer SMILETRAP. Measurements with Penning traps give reliable and accurate masses, in particular Q values, due to the fact that certain systematic errors to a great deal cancel in the mass difference between the two atoms defining the Q value. Some Q values that are of fundamental interest will be discussed here, for example, a new Q value for the ⁶Li (n,γ) ⁷Li reaction, for the β-decay of tritium, related to properties of the electron neutrino mass, and for the neutrino-less double β-decay of ⁷⁶Ge, related to the question of whether the neutrino is a Majorana particle or not. In case of the latter two we report the most accurate Q values, namely 18,589.8(12) eV for the tritium decay and 2,038.997(46) keV for the neutrino-less double β-decay of ⁷⁶Ge.     

Mass and double-beta-decay Q value of 136Xe

The atomic mass of 136Xe has been measured by comparing cyclotron frequencies of single ions in a Penning trap. The result, with 1 standard deviation uncertainty, is M(136Xe)=135.907 214 484 (11) u. Combined with previous results for the mass of 136Ba [Audi, Wapstra, and Thibault, Nucl. Phys. A 729, 337 (2003)10.1016/j.nuclphysa.2003.11.003], this gives a Q value (M[136Xe]-M[136Ba])c(2)=2457.83(37) keV, sufficiently precise for ongoing searches for the neutrinoless double-beta decay of 136Xe.     

Accurate Q value for the Se double-electron-capture decay

The Q   value of the neutrinoless double-electron-capture (0νECEC$0\nu \text{ECEC}$) decay of ⁷⁴Se was measured by using the JYFLTRAP Penning trap. The determined value is 1209.169(49) keV, which practically excludes the possibility of a complete energy degeneracy with the second ²⁺$2^{+}$ state (1204.205(7) keV) of ⁷⁴Ge in a resonant 0νECEC$0\nu \text{ECEC}$ decay. We have also computed the associated nuclear matrix element by using a microscopic nuclear model with realistic two-nucleon interactions. The computed matrix element is found to be quite small. The failure of the resonant condition, combined with the small nuclear matrix element and needed p-wave capture, suppresses the decay rate strongly and thus excludes ⁷⁴Se as a possible candidate to search for resonant 0νECEC$0\nu \text{ECEC}$ processes.     

New CUORICINO results and status of CUORE

CUORICINO is an array of 62 TeO₂ bolometers with a total mass of 40.7 kg (11.2 kg of ¹³⁰Te), operated at about 10 mK to search for ββ(0ν) of ¹³⁰Te. The detectors are organized as a 14-story tower and intended as a slightly modified version of one of the 19 towers of the CUORE project, a proposed tightly packed array of 988 TeO₂ bolometers (741 kg of total mass of TeO₂) for ultralow-background searches on neutrinoless double-beta decay, cold dark matter, solar axions, and rare nuclear decays. Started in April 2003 at the Laboratori Nazionali del Gran Sasso (LNGS), CUORICINO data taking was stopped in November 2003 to repair the readout wiring system of the 62 bolometers. Restarted in spring 2004, CUORICINO is presently the most sensitive running experiment on neutrinoless double-beta decay. No evidence for ββ(0ν) decay has been found so far and a new lower limit, T ₁ ^2/0ν ≥ 1.8 × 10²⁴ yr (90% C.L.), is set, corresponding to 〈m _ν〉 ≤ 0.2–1.1 eV, depending on the theoretical nuclear matrix elements used in the analysis. Detector performance, operational procedures, and background analysis results are reviewed. The expected performance and sensitivity of CUORE is also discussed. CUORE Collaboration The text was submitted by the authors in English.     

Beta-delayed proton emission from heavy nuclei: (I). The observation of coincidences between protons and γ-rays

Coincidences have been detected between γ-rays and β-delayed protons emitted after the decay of very neutron-deficient nuclei. In the three cases studied the decay proceeds to the ground state with branches to the first excited (2⁺) state of the even daughter nucleus of (58±7) % (¹¹⁵Xe to 709 keV in ¹¹⁴Te), (14±3) % (¹¹⁷Xe to 679 keV in ¹¹⁶Te), and (50±10) % (¹⁸¹Hg to 158 keV in ¹⁸⁰Pt). Upper limits of 2 and 6 % can be placed on the branches to the 4⁺ states of ¹¹⁴Te and ¹⁸⁰Pt. For the cases of ^115,117Xe, coincidences with annihilation radiation were also observed; the measured positon/proton ratio determines the difference Q—B_p, between the energy available for electron capture and the proton separation energy for the daughter. For ¹¹⁵Xe the result is 6.20±0.13 MeV, for ¹¹⁷Xe 4.10±0.20 MeV.     

Double-electron capture of Se and the search for neutrinoless decay

Experimental results on the measurement of the double-electron capture (εε) decay process of ⁷⁴Se are described. This decay is particularly interesting, because ⁷⁴Se is almost degenerate to the second excited state at 1204 keV in the daughter nucleus ⁷⁴Ge. Such a degeneracy favors a neutrinoless decay because of the large phase-space suppression for a decay with two neutrinos. Subsequent de-excitation would generate a 2γ-ray cascade of 608 and 596 keV, whose coincident detection provides a unique signature for this particular εε process. The 2γ-ray cascade has been searched for by a HPGe γ-ray detector in coincidence with a NaI detector using 3 kg of natural selenium.     

The CUORE/CUORICINO project: Preliminary studies

A 1-t bolometer detector, aiming at the search for neutrinoless double-beta decay, cold dark matter, and solar axions with extremely high sensitivity, has been proposed (CUORE project). At the moment, the construction of a 42-kg array of 56 TeO₂ bolometers (CUORICINO project) has been approved and is financed. CUORICINO will be a feasibility test for CUORE, while increasing the present sensitivity on 0νββ half-life of ¹³⁰Te (1×10²³ yr, 90% C.L.). The state of the art of CUORICINO, together with the physical motivations and the technical feasibility of the CUORE project, is briefly discussed.     

Neutrinoless double-beta decay and double-electron capture

The fundamental importance of searching for neutrinoless double-beta decay is widely recognized. Observation of the decay would tell us that the total lepton number is not conserved and that, consequently, neutrinos are massive Majorana fermions. The same statement could be made in the case of observing neutrinoless double-electron capture. We address the question of the sensitivity of the 0νεε decay to the effective mass of the Majorana neutrino. According to our estimates, in the case of ¹⁵²Gd and ¹⁶⁴Er the sensitivity can be comparable to the favored 0νββ decays of nuclei. The main uncertainty in the prediction of half-lives of the 0νεε decay stems from the lack of sufficient precision in measuring the mass difference between the parent and daughter atoms. More accurate measurements can be accomplished using the modern high-precision ion traps.     

Geometrical CP violation in the N-Higgs-doublet model

Many different experiments are being developed to explore the existence of the neutrinoless double beta decay (0νββ) since it would imply fundamental consequences for particle physics. In this work we present results on the evaluation of Timepix detectors with cadmium-telluride sensor material to search for 0νββ in ¹¹⁶Cd. This work was carried out with the COBRA collaboration and the Medipix collaboration. Due to the relatively small pixel dimension of 110×110×1000 μm³ the energy deposited by particles typically extends over several detector pixels leading to a track in the pixel matrix. We investigated the separation power regarding different event-types like α-particles, atmospheric muons, single electrons and electron-positron pairs produced at a single vertex. We achieved excellent classification power for α-particles and muons. In addition, we achieved good separation power between single electron and electron-positron pair production events. These separation abilities indicate a very good background reduction for the 0νββ search. Further, in order to distinguish between 2νββ and 0νββ, the energy resolution is of particular importance. We carried out simulations which demonstrate that an energy resolution of 0.43 % is achievable at the Q-value for 0νββ of ¹¹⁶Cd at 2.814 MeV. We measured an energy resolution of 1.6 % at a nominal energy of 1589 keV for electron-positron tracks which is about two times worse that predicted by our simulations. This deviation is probably due to the problem of detector calibration at energies above 122 keV which is discussed in this paper as well.     

Decay study of neutron-rich zirconium isotopes employing a Penning trap as a spectroscopy tool

A new technique to produce isobarically pure ion beams for decay spectroscopy by using a gas-filled Penning trap was commissioned at the ion guide isotope separator on-line facility, IGISOL. β-decays of neutron-rich ¹⁰⁰Zr, ¹⁰²Zr and ¹⁰⁴Zr isotopes were studied with this technique. In addition, the values of ^100,102,104Zr β-decays were determined from the direct mass measurements of zirconium and niobium isotopes performed with a high-precision Penning trap. The mass of ¹⁰⁴Nb was directly measured for the first time and the obtained mass excess value for the longer-living (1⁺) state is -71823±10 keV. For the ground states of ¹⁰⁰Nb and ¹⁰²Nb the obtained mass excess values were -79802±20 keV and -76309±10 keV, respectively. The observed distribution of the β strength supports a prolate deformation assignment for ^100,102,104Zr isotopes.