Double-beta-decay experiment NEMO-3: Preliminary results of phase I (2003–2004)

The NEMO-3 detector has been taking data in the Fréjus underground laboratory (LSM, France) since February 2003 and is devoted to the search for neutrinoless double-beta decay (0νββ). After 389 effective days of data collection from February 2003 until September 2004 (Phase I), no evidence for neutrinoless double-beta decay was found from ∼7 kg of ¹⁰⁰Mo and ∼1 kg of ⁸²Se. The corresponding limits for the half-lives are T _1/2(0νββ) > 4.6 × 10²³ yr for ¹⁰⁰Mo and T _1/2(0νββ) > 1.0 × 10²³ yr for ⁸²Se (90% C.L.). They lead to the following limits for the effective Majorana neutrino mass: 〈m _ν〉 < 0.7–2.8 eV for ¹⁰⁰Mo and 〈m _ν〉 < 1.8–4.9 eV for ⁸²Se. The half-lives of the two-neutrino double-beta decay (2νββ) have been measured for ¹⁰⁰Mo, ⁸²Se, ¹¹⁶Cd, ¹⁵⁰Nd, and ⁹⁶Zr and reported here as well. on behalf of NEMO Collaboration The text was submitted by the author in English.     

Results of a search for 2β decay of 136Xe with high-pressure copper proportional counters in Baksan Neutrino Observatory

A search for 2β decay of ¹³⁶Xe with two high-pressure copper proportional counters was carried out in the Baksan Neutrino Observatory. The experiment is based on comparison of spectra measured with natural and enriched xenon. No evidence was found for 2β(2ν) and 2β(0ν) decay. The decay half-life limit based on data measured for 8000 h is T _1/2 ≥ 8.5 × 10²¹ yr for 2ν mode and T _1/2 ≥ 3.1 × 10²³ yr for 0ν mode (90% C.L.). The text was submitted by the authors in English.     

Phase shift analysis and extraction of structure functions for Coulomb distortion on (e, e'p) reactions at high electron energies

New results for the double beta decay of ⁷⁶ Ge are presented. They are extracted from data obtained with the HEIDELBERG-MOSCOW experiment, which operates five enriched ⁷⁶ Ge detectors in an extreme low-level environment in the Gran Sasso underground laboratory. The two-neutrino-accompanied double beta decay is evaluated for the first time for all five detectors with a statistical significance of 47.7 kg y resulting in a half-life of T _1/2 ^2ν = [1.55±0.01(stat)^+0.19 _-0.15(syst)]×10²¹ y. The lower limit on the half-life of the 0νββ decay obtained with pulse shape analysis is T _1/2 ^0ν > 1.9×10²⁵(3.1×10²⁵) y with 90% C.L. (68% C.L.) (with 35.5 kg y). This results in an upper limit of the effective Majorana-neutrino mass of 0.35 eV (0.27 eV) using the matrix elements of A. Staudt et al.'s work (Europhys. Lett. 13, 31 (1990)). This is the most stringent limit at present from double beta decay. No evidence for a majoron-emitting decay mode is observed. Received: 22 August 2001 / Accepted: 18 October 2001     

The investigation of the 2<Emphasis Type="Italic">νββ</Emphasis> decay by Pyatov method within quasiparticle random phase approximation formalism

The violated commutation condition between the total shell model Hamiltonian and Gamow-Teller operator (GT) has been restored by Pyatov method (PM). The considered nuclear model Hamiltonian in PM includes the separable GT residual interaction in ph and pp channels and is differentiated from the traditional schematic model by h ₀(restoration term). The influence of the h ₀ effective interaction on the 2νββ decay of ⁴⁸Ca, ⁷⁶Ge, ⁸²Se, ⁹⁶Zr, ¹⁰⁰Mo, ¹¹⁶Cd, ^128,130Te and ¹³⁶Xe is investigated. All the calculations have been done within the framework of standard QRPA. The results obtained by PM are compared with those of other approaches and experimental data. The influence of the restoration term on the stability of the 2νββ decay nuclear matrix elements is analysed.      

Does the weak interaction constant depend on time?

A comparison is made of the probability of the process of two-neutrino double β decay for ⁸²Se and ⁹⁶Zr in direct (counter) and geochemical experiments. The experimental data for ¹³⁰Te are analyzed. It is shown that this probability is systematically lower in geochemical experiments, which characterize the probability of 2 β(2 v) decay 10⁹ yr ago. It is proposed that this could be due to a change in the weak-interaction constant with time. It is proposed that a series of new, precise measurements be performed with the aid of counters and geochemical experiments. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 3–8 (10 July 1998)     

First results for the measurement of double-electron capture of 106Cd in the experiment TGV II

Present status of the experiment TGV II which is devoted to the measurement of double-beta decay of ¹⁰⁶Cd is given. The low background spectrometer TGV II is installed in the Modane Underground Laboratory and has been running from February 2005 with approx 10 grams of ¹⁰⁶Cd enriched at 75%. After an analysis of 3736 hours of experimental data the new improved half-life limit for 2νEC/EC decay of ¹⁰⁶Cd (0 _g.s. ⁺ → 0 _g.s. ⁺ ) is given as T ₁ ^2/2ν > 4.8 × 10¹⁹ years (90% CL). The search for 2νEC/EC decay of ¹⁰⁶Cd to the excited states of ¹⁰⁶Pd allows to determine the limits of the half-lives T ₁ ^2/2ν (0 _g.s. ⁺ → 2 ₁ ⁺ ) > 3.9 × 10¹⁹ years (90% CL) and T ₁ ^2/2ν (0 _g.s. ⁺ → 0 ₁ ⁺ ) > 5.8 × 10¹⁹ years (90% CL). Presented by I. Štekl at the Workshop on calculation of double-beta-decay matrix elements (MEDEX’05), Corfu, Greece, September 26–29, 2005.     

Two-neutrino double β decay of96Zr to excited 2+ state of96Mo

The two-neutrino double beta decay of⁹⁶Zr isotope for 0⁺ → 2⁺ transition has been studied in the PHFB model. In our earlier work, the reliability of the intrinsic wave functions of⁹⁶Zr and⁹⁶Mo isotopes has been established by obtaining an overall agreement between a number of theoretically calculated spectroscopic properties as well as half-lives of 2vββ decay for 0⁺ → 0⁺ transition and the available experimental data. In the present work, the half-life of 2vββ decay for 0⁺ ar 2⁺ transition T ₁ ^2/2v (0⁺ →²⁺) has been calculated using the same set of intrinsic wave functions.     

MOON for neutrino-less double beta decays

The MOON (Majorana/Mo Observatory Of Neutrinos) project aims at studies of the Majorana nature of the neutrino (ν) and the ν-mass spectrum by spectroscopic experiments of neutrino-less double beta decays (0νββ) with the ν-mass sensitivity of 〈m _ν ^m 〉 = 100−30 meV. The solid scintillator option of the MOON detector is a super ensemble of multi-layer modules, each being composed by PL scintillator plates and position-sensitive detector planes with good overall energy resolution of σ ≈ 2% at the Q _ββ ≈ 3 MeV. Thin ββ source films are interleaved between the detector planes. High localization of the two β tracks enables one to select true signals and reject BG ones. The multi-layer structure of the detector makes it realistic to build a compact ton-scale detector. MOON with detector ≠ ββ source is used for studying 0νββ decays from ¹⁰⁰Mo, ⁸²Se and other ββ isotopes with large Q _ββ . Real-time exclusive measurements of low energy solar neutrinos can be made by observing inverse β rays from solar-ν captures of ¹⁰⁰Mo in delayed coincidence with the subsequent β decay of ¹⁰⁰Tc.     

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.     

Issues and status of “basic divertor and SOL physics” from papers and discussions

⁴⁸Ca, the lightest double beta decay candidate, has been until now the only one simple enough to be treated exactly in the nuclear shell model. Thus, theββ(2ν) half-life measurement, reported here, provides a unique test of the nuclear physics involved in theββ matrix element calculation. Enriched⁴⁸Ca sources of two different thicknesses have been exposed in a time projection chamber, and yieldT ₁ ^2/2ν =(4.3 _−1.1 ^+2.4 [stat.] ± 1.4[syst.]) × 10¹⁹ years, compatible with the shell model calculations. The consequences of this result for the shell model calculation of theββ(2ν) rate are briefly discussed. Presented by P. Vogel at the Workshop on calculation of double-beta-decay matrix elements (MEDEX’97), Prague, May 27–31, 1997. This work was funded by the US Department of Energy under contracts DE-FG03-01ER40679 and DE-FG03-88ER40397.     

Influence of pairing in double beta decay of <Superscript>48</Superscript>Ca

Two-neutrino ββ decay matrix elements and half-life of ⁴⁸Ca are calculated after including neutron-proton pairing correlations in projected Hartree-Fock-Bogoliubov (PHFB) formalism. The GT matrix elements in 2νββ decay are reduced due to broader smearing of Fermi surfaces. Half-life results for 2νββ decay of 48Ca with np pairing are better than without pairing.     

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.     

Double-beta decay: Present status

The present status of double-beta-decay experiments (including the search for 2β ⁺, ECβ ⁺, and ECEC processes) are reviewed. The results of the most sensitive experiments are discussed. Average and recommended half-life values for two-neutrino double-beta decay are presented. Conservative upper limits on effective Majorana neutrino mass and the coupling constant of the Majoron to the neutrino are established as 〈m _ν 〉 < 0.75 eV and 〈g _ee 〉 < 1.9 × 10⁻⁴, respectively. Proposals for future double-betadecay experiments with a sensitivity for the 〈m _ν 〉 at the level of 0.01–0.1 eV are considered.     

Nuclear deformation and the two-neutrino double- β decay in <Superscript>124, 126</Superscript>Xe , <Superscript>128, 130</Superscript>Te , <Superscript>130, 132</Superscript>Ba and <Superscript>150</Superscript>Nd isotopes

The two-neutrino double-beta decay of ^{124, 126}Xe , ^{128, 130}Te , ^{130, 132}Ba and ¹⁵⁰Nd isotopes is studied in the Projected Hartree-Fock-Bogoliubov (PHFB) model. Theoretical 2ν β^-β^- half-lives of ^{128, 130}Te , and ¹⁵⁰Nd isotopes, and 2ν β⁺β⁺ , 2ν β⁺ EC and 2ν ECEC for ^{124, 126}Xe and ^{130, 132}Ba nuclei are presented. Calculated quadrupolar transition probabilities B(E2 : 0⁺ → 2⁺) , static quadrupole moments and g -factors in the parent and daughter nuclei reproduce the experimental information, validating the reliability of the model wave functions. The anticorrelation between nuclear deformation and the nuclear transition matrix element M _2ν is confirmed.     

Status of the Germanium Detector Array (GERDA) in the search of neutrinoless ββ decays of 76Ge at LNGS

The Germanium Detector Array (GERDA) in the search for neutrinoless ββ decays of ⁷⁶Ge at LNGS will operate bare germanium diodes enriched in ⁷⁶Ge in an (optional active) cryogenic fluid shield to investigate neutrinoless ββ decay with a sensitivity of T _1/2 > 2 × 10²⁶ yr after an exposure of 100 kg yr. Recent progress includes the installation of the first underground infrastructures at Gran Sasso, the completion of the enrichment of 37.5 kg of germanium material for detector construction, prototyping of low-mass detector support and contacts, and front-end and DAQ electronics, as well as the preparation for construction of the cryogenic vessel and water tank. GERDA Collaboration     

The SuperNEMO project

Neutrinoless double-beta decay (ββ(0ν)) is the most sensitive process in the search for leptonic number violation and its discovery would prove that the neutrino is a Majorana particle. From the experience of the NEMO-3 detector construction and data analysis, the NEMO Collaboration proposes a three-year R&D program in order to design a detector (SuperNEMO) sensitive to a ββ(0ν) period of few 10²⁶ yr coupling track reconstruction and calorimeter. on behalf of the NEMO Collaboration The text was submitted by the author in English.     

A neodymium-loaded liquid scintillator

One promising approach to the search for neutrinoless double beta (0ν2β) decay is the use of liquid scintillators with double β-decay sources dissolved in them. The ¹⁵⁰Nd isotope is the one best suited to observing this process. Samples of liquid scintillators based on linear alkylbenzene with a neodymium mass fraction of 0.32% are described in this work; neodymium 4-methyloctanoate was used as a Nd-loaded additive. The spectral-luminescent and scintillation properties of the new materials were studied. The possibility of using the obtained compounds in experiments to search for 0ν2β decay is demonstrated.     

Double beta decay of 100Mo

Two-neutrino double beta decay of ¹⁰⁰Mo with half-life T _1/2=[7.2±0.9(stat)± 1.8(syst)]×10¹⁸ yr was detected using a liquid argon ionization chamber. With a C.L. of 68% (90%), the bounds on neutrinoless decay and decay with majoron emission were found to be 8.4(4.9)×10²¹ and 4.1(3.2)×10²⁰ yr, respectively. An analysis of all available results provides the average “world” value T _1/2=(8.0±0.7)×10¹⁸ yr for the two-neutrino decay of ¹⁰⁰Mo, and the corresponding nuclear matrix element is M _GT=0.118±0.005.     

Investigating 2<Emphasis Type="Italic">K</Emphasis>-capture in <Superscript>78</Superscript>Kr with a large-volume copper proportional counter

A component analysis of the background of a copper proportional counter with allowance for its input to the parametric region of events due to the expected 2K-capture in ⁷⁸Kr is presented. The technique of discriminating the simulated events on the edges of the working length of the anode wire is described by comparing the pulse amplitudes of the sought pulse and the first afterpulse in a detector filled with pure krypton without quenchers. The limit on 2K(2ν)-capture for the ground state ⁷⁸Se (0⁺ → 0⁺, g.s.) was found to be T _1/2 ≥ 2.4 × 10²¹ yr (90% confidence level).     

Uncertainties in the 0νββ-decay nuclear matrix elements

The nuclear matrix elements M ^0ν of the neutrinoless double-beta decay (0νβ β) of most nuclei with known 2νββ-decay rates are systematically evaluated using the Quasiparticle Random Phase Approximation (QRPA) and Renormalized QRPA (RQRPA). The experimental 2νβ β-decay rate is used to adjust the most relevant parameter, the strength of the particle-particle interaction. With such procedure the M ^0ν values become essentially independent of single-particle basis size, the axial vector quenching factor, etc. Theoretical arguments in favor of the adopted way of determining the interaction parameters are presented. It is suggested that most of the spread among the published M ^0ν ’s can be ascribed to the choices of implicit and explicit parameters, inherent to the QRPA method. Presented by V. Rodin at the Workshop on calculation of double-beta-decay matrix elements (MEDEX’05), Corfu, Greece, September 26–29, 2005.