Double Beta Decay Of 76Ge Research Articles

GERDA: a germanium detector array to search for neutrinoless double beta decay

The GERDA, a new experiment to search for the double beta decay of 76Ge, is being installed at Laboratori Nazionali del Gran Sasso. The potentialities of this experiment as well the status of the project are reviewed.

Ultrapure gases for GERDA

The Gerda experiment searches for the neutrinoless double beta decay of 76Ge. A major reduction of background is envisaged by operating almost bare germanium diodes in ultrapure liquid nitrogen or liquid argon. The purity of the cryogenic liquid in terms of 222Rn is crucial for the success of the experiment. In this work techniques for measuring 222Rn in nitrogen and argon at the μBq/m 3 level are presented. It is shown that the required purity can be obtained for both gases, nitrogen and argon.

The neutrinoless double beta decay and the nuclear matrix elements

The standard model of the electroweak and strong interaction does not allow neutrinoless double beta decay but it is allowed in most Grand Unified Theories (GUT’s). The neutrino must be a Majorana particle (identical with its antiparticle) and must have such a mass as to allow the neutrinoless double beta decay. Apart from one claim that the neutrinoless double beta decay in 76Ge has been measured, one has only upper limits for this transition probability. But even the upper limits allow one to give upper limits for the electron Majorana neutrino mass and upper limits for parameters of GUT’s and the minimal R-parity violating supersymmetric model. One can further give lower limits for the vector boson mediating mainly the right-handed weak interaction and the heavy mainly right-handed Majorana neutrino in left–right symmetric GUT’s. For that one has to assume that the specific mechanism is the leading one for the neutrinoless double beta decay and one has to be able to calculate reliably the corresponding nuclear matrix elements. In the present contribution, we discuss the accuracy of the present status of calculating the nuclear matrix elements and the corresponding limits of GUT’s and supersymmetric parameters.

Pulse shape analysis in segmented detectors as a technique for background reduction in Ge double-beta decay experiments

The need to understand and reject backgrounds in Ge-diode detector double-beta decay experiments has given rise to the development of pulse shape analysis in such detectors to discern single-site energy deposits from multiple-site deposits. Here, we extend this analysis to segmented Ge detectors to study the effectiveness of combining segmentation with pulse shape analysis to identify the multiplicity of the energy deposits.

Double Beta Decay and Neutrino Masses Accuracy of the Nuclear Matrix Elements

The neutrinoless double beta decay is forbidden in the standard model of the electroweak and strong interaction but allowed in most Grand Unified Theories (GUT's). Only if the neutrino is a Majorana particle (identical with its antiparticle) and if it has a mass, the neutrinoless double beta decay is allowed. Apart of one claim that the neutrinoless double beta decay in 76Ge is measured, one has only upper limits for this transition probability. But even the upper limits allow to give upper limits for the electron Majorana neutrino mass and upper limits for parameters of GUT's and the minimal R-parity violating supersymmetric model. One further can give lower limits for the vector boson mediating mainly the right-handed weak interaction and the heavy mainly right-handed Majorana neutrino in left-right symmetric GUT's. For that one has to assume that the specific mechanism is the leading one for the neutrinoless double beta decay and one has to be able to calculate reliably the corresponding nuclear matrix elements. In the present contribution, one discusses the accuracy of the present status of calculating the nuclear matrix elements and the corresponding limits of GUT's and supersymmetric parameters.

Gerda: A new 76Ge Double Beta Decay Experiment at Gran Sasso

In the new 76 Ge double beta decay experiment Gerda [I. Abt et al., arXiv hep-ex/0404039 ; Gerda proposal, to be submitted to the Gran Sasso scientific committee] bare diodes of enriched 76 Ge will be operated in highly pure liquid nitrogen or argon. The goal is to reduce the background around Q β β = 2039 keV below 10 −3 counts/(kg⋅keV⋅y). With presently available diodes from the Igex and HdMs experiments the current evidence for neutrinoless double beta decay [H.-V. Klapdor-Kleingrothaus, et al., Mod. Phys. Lett. A16 (2001) 2409ff] can unambigously be checked within one year of measurement.

Two-neutrino double beta decay of 76Ge within deformed QRPA

The effect of deformation on the two-neutrino double decay (2 νββ-decay) for ground state transition 76 Ge→ 76 Se is studied in the framework of the deformed QRPA with separable Gamow–Teller residual interaction. We found that when the parent and daughter nuclei have different deformations, the 2 νββ-decay matrix element is suppressed with respect to the case of 2 νββ decay between nuclei having the same deformation, in particular, spherical going to spherical. An advantage of this suppression mechanism in comparison with that associated with ground state correlations is that it allows a simultaneous description of the single β and the 2 νββ-decay. By performing a detail calculation of the 2 νββ-decay of 76Ge, it is found that the states of intermediate nucleus lying in the region of the Gamow–Teller resonance contribute significantly to the matrix element of this process.

Improved constraints on WIMPS from the International Germanium EXperiment IGEX

One IGEX Ge double-beta decay detector is currently operating in the Canfranc Underground Laboratory in a search for dark matter WIMPs, through the Ge nuclear recoil produced by the WIMP elastic scattering. A new exclusion plot, σ(m), has been derived for WIMP-nucleon spin-independent interactions. To obtain this result, 40 days of data from the IGEX detector (energy threshold Ethr ∼ 4 keV), recently collected, have been analyzed. These data improve the exclusion limits derived from all the other ionization germanium detectors in the mass region from 20 GeV to 200 GeV, where a WIMP supposedly responsible for the annual modulation effect reported by the DAMA experiment would be located. The new IGEX exclusion contour enters, by the first time, the DAMA region by using only raw data, with no background discrimination, and excludes its upper left part. It is also shown that with a moderate improvement of the detector performances, the DAMA region could be fully explored.

Back to the line of stability

The Stockholm Penning trap has been connected to an electron beam ion source named CRYSIS located at the Manne Siegbahn Laboratory. It is combined to a high-resolution isotope separator that can provide singly charged mass selected ions of practically any element. These ions are fed into CRYSIS where it is subject to a very intense electron beam with an energy of 10-20 keV. The mass of the neutral atom is obtained by adding the masses of the missing electrons and subtracting their binding energies. The results on some 16 mass determinations made at an uncertainty from 3 to 0.3 ppb are commented on. In these measurements the mass number varies from 1 to 204 and the ion charges from 1+ to 52+. New mass values are obtained for the 3H, 3He and 4He masses. We have confirmed the Manitoba measurements of the Q-value of the double beta-decay of 76Ge and their mass measurements of the masses of 198Hg and 204Hg reaching the higher accuracy that traps offer. At present the mass uncertainty limit is about 3×10-10 which is demonstrated by comparing our results with the most accurately measured masses by other groups.

Double-Beta Decay of 150Nd and 76Ge to Excited States

New results of experimental search for double beta decay of 76Ge (2νββ) and 150Nd (0ν + 2ν) to excited states of the daughter nuclei are presented. Data from 228 days of measurements performed with four HPGe detectors in the low background laboratory of Baksan neutrino observatory yield new limits on life-times T1/2(0+→ 0+1)≥6.2×1021} yr at 90% C.L. for transition of 76Ge to 0+1 level of 76Se and T1/2(0+→ 0+1)≥1.5×1020 yr at 90% C.L. for transition of 150Nd to 0+1 level of 150Sm.

The Calculation of 2νββ Matrix Elements Within the Collective Description

The problem of particles coupled by an isovector pairing interaction is solved by means of a collective treatment. The Hamiltonian and the beta-decay operators acting within the np-subspace are constructed. The procedure reproduces exact results for a schematic model. The realistic calculation of the Fermi amplitude for the double-beta decay of 76Ge confirms the current expectation about its smallness relative to the Gamow-Teller decay.

Determination of the (76)Ge double beta decay Q value.

The Q value of the (76)Ge double beta decay has been determined by measuring the masses of (76)Ge and (76)Se in a Penning trap using neon- and fluorinelike ions. The obtained masses are 75.921 402 758(96) u and 75.919 213 795(81) u, respectively. The systematic errors of these two determinations are nearly equal, and therefore, the remaining systematic uncertainty of the Q value is drastically reduced. A Q value of 2 039.006(50) keV was obtained improving the accuracy of the accepted value by a factor of 6.

First results of the IGEX dark matter experiment at the Canfranc Underground Laboratory

The enriched 76 Ge double-beta decay detectors from the International Germanium EXperiment (IGEX), operating in the Canfranc Underground Laboratory with an overbuden of 2450 m.w.e., were recently upgraded to use them also in a search for WIMPs. This paper presents a description of the experiment and the data analysis as well as a new exclusion plot, σ( m), derived from the IGEX data for WIMP-nucleon spin-independent interaction. To obtain this result, 30 days of data from one 2-kg IGEX detector, with an energy threshold E thr ∼ 4 keV, have been considered. These results improve the exclusion limits derived from other conventional ionization germanium experiments in the ∼ 50 GeV DAMA region.

Recent results of the IGEX 76Ge double-beta decay experiment

The International Germanium Experiment (IGEX) has now analyzed 117 mol yr of data from its isotopically enriched (86% 76Ge) germanium detectors. Applying pulse shape discrimination (PSD) to the more recent data, the lower bound on the half-life for neutrinoless double-beta decay of 76Ge is deduced: T1/2(0ν)>1.57×1025 yr (90% C.L.). This corresponds to an upper bound on the Majorana neutrino mass parameter, 〈mν〉, between 0.33 eV and 1.35 eV depending on the choice of theoretical nuclear matrix elements used in the analysis.

Limits on the Majorana Neutrino Mass in the 0.1 eV Range

The Heidelberg-Moscow experiment gives the most stringent limit on the Majorana neutrino mass. After 24 kg yr of data with pulse shape measurements, we set a lower limit on the half-life of the neutrinoless double beta decay in 76Ge of T_1/2 > 5.7 * 10^{25} yr at 90% C.L., thus excluding an effective Majorana neutrino mass greater than 0.2 eV. This allows to set strong constraints on degenerate neutrino mass models.

The status of the IGEX 76Ge double-beta decay experiment in 1997

The International Germanium Experiment (IGEX) has six detectors containing ∼ 90 fiducial moles of 76Ge. Data from 74.84 fiducial mole years yield a lower bound T 1 2 0ν > 0.8 × 10 25 y for neutrinoless double-beta decay of 76Ge.

The Heidelberg-Moscow experiment: improved sensitivity for 76Ge neutrinoless double beta decay

Abstract The Heidelberg-Moscow experiment operates five enriched 76Ge detectors with 10.96 kg active mass in the Gran Sasso underground laboratory. After 28.7 kg y of measurement with a counting rate between 2000 keV and 2080 keV of 0.20 ± 0.01 cts/kg y keV we receive a lower limit on the neutrinoless double beta decay of 76Ge of T 1 2 (0 + → 0 + ) > 1.1 × 10 25 y (90% C.L.). This restricts the Majorana neutrino mass to 0.46 eV. Recently we achieved a reduction in the background counting rate by a factor of 3–5 within the energy region of the neutrinoless double beta decay by recording the differentiated preamplifier pulse shapes and doing an off-line analysis to distinguish between pointlike and multiple scattered interactions.

A large Hilbert space QRPA and RQRPA calculation of neutrinoless double beta decay

A large Hilbert space is used for the calculation of the nuclear matrix elements governing the light neutrino mass mediated mode of neutrinoless double beta decay (Ovββ-decay) of76 Ge,100 Mo,116 Cd,128 Te, and136 Xe within the proton-neutron quasiparticle random phase approximation (pn-QRPA) and the renormalized QRPA with proton-neutron pairing (full-RQRPA) methods. We have found that the nuclear matrix elements obtained with the standard pn-QRPA for several nuclear transitions are extremely sensitive to the renormalization of the particle-particle component of the residual interaction of the nuclear hamiltonian. Therefore the standard pn-QRPA does not guarantee the necessary accuracy to allow us to extract a reliable limit on the effective neutrino mass. This behavior already known from the calculation of the two-neutrino double beta decay matrix elements, manifests itself in the neutrinoless double-beta decay but only if a large model space is used. The full-RQRPA, which takes into account proton-neutron pairing and considers the Pauli principle in an approximate way, offers a stable solution in the physically acceptable region of the particle-particle strength. In this way more accurate values on the effective neutrino mass have been deduced from the experimental lower limits of the half-lifes of neutrinoless double beta decay.

Grand unification, nuclear structure and the double beta-decay

Unification of the electroweak and the strong interaction prefers that the neutrino is a Majorana particle and therefore essentially identical with its own antiparticle. In such grand unified models the neutrino has also a finite mass and a slight right-handed weak interaction, since the model is left-right symmetric. These models have also left handed and right-handed vector bosons to mediate the weak interactions. If these models are correct the neutrinoless double beta-decay is feasable. Thus if one finds the neutrinoless double beta-decay one knows that the standard model can not be correct in which the neutrino is a Dirac particle and therefore different from its antiparticle. Although the neutrinoless double beta-decay has not been seen it is possible to extract from the lower limits of the lifetime against the double neutrinoless beta-decay upper limits for the effective electron-neutrino mass and for the effective mixing angle of the right-handed and the left-handed vector bosons mediating the weak interaction. One also can obtain an effective upper limit for the mass ratio of the light and the heavy vector bosons. The extraction of this physical quantities from the data is made difficult due to the fact that the weak interaction must not be diagonal in the representation of the mass matrix of the six neutrinos requested by such left-right symmetric models. A condition for obtaining reliable limits for these fundamental quantities from the measured lower limits of the half lifes of the 0νββ decay are correct calculations of the nuclear matrix elements involved. These nuclear structure calculations can be tested by calculating the two neutrino double beta decay (2νββ) for which we have experimental data and not only lower limits as for the 0νββ decay. The 2νββ decay is dominated by the Gamow Teller (GT) transitions. The intermediate 1 + states in the odd-odd mass nucleus are usually calculated within the Quasi-particle Random Phase Approcimation (QRPA). Since the proton-proton and neutron-neutron pairing correlations are treated in the BCS approach particle numbers are only conserved in average for the initial |0 i +〉 and the final vb0 f +〉 states. Particle number projection is used to improve on this point. Since QRPA with the physical particle-particle interaction from the Bonn potential is close to breakdown, where the excitation energy goes to zero, higher up to third order RPA is included, by allowing anharmonicities up to three bosons in the physical wave functions. Third order RPA which is the lowest order to allow modifications of the ground state correlations affect the results appreciably. Since the intermediate states are supperpositions of proton- neutron two quasi-particle states one expects that proton-neutron pairing affects these states. From the masses of the nuclei one can extract a proton-neutron (pn) pairing gap. Due to the finite basis and the inclusion of only T = 1 (and not also T = 0, J = 1) pn-pairing the bare Brückner matrix elements of the Bonn potential yield no pn-pairing gap in most nuclei. If one fits the experimental pn-pairing gap by renormalizing the pn J π = 0 + particle-particle Brückner matrix elements by a factor d pn ≈ 1.3 to 1.4 one obtains strong pn-pairing correlations, which reduce the GT strength. Since now part of the pn particle-particle correlations are already included in the BCS wave functions the 2νββ strength is reduced and the dependence on the 1 + particle-particle strength parameter g pp gets weaker. In heavier nuclei like 76Ge to 76Se this dependence on g pp of the second order GT matrix elements Σ α GT(0 i + → 1 α + → 0 f +) almost disappears. The upper limits derived from the neutrinoless double beta decay of 76Ge are for the effective neutrino mass 〈 m νe 〉≤ 0.88 eV, for the left-right mixing angle 〈 tgζ 〉≤ 1.55 × 10 −8 and for the mass ratio of the light over the heavy vector boson squared 〈 M 1 2 M 2 2 〉 ≤ 1.08 × 10 −6 .

Some comments on the real limit for neutrinoless double beta decay of 76Ge

Results of the UCSB/LBL1 and ITEP/YePI2 searches for neutrinoless double beta decay of 76Ge have been revised in attempt to understand real accuracy of the claimed lifetime limits. New value of the total energy of the decay and existence of gross-structure in the collected spectra were taken into account. The limit was proved to be T12 > 1·.1024y(90%CL) from analysis of the ITEP/YePI results.