Double-beta decay Q values of Cd and Te

The Q values of the ¹¹⁶Cd and ¹³⁰Te double-beta decaying nuclei were determined by using a Penning trap mass spectrometer. The new atomic mass difference between ¹¹⁶Cd and ¹¹⁶Sn of 2813.50(13) keV differs by 4.5 keV and is 30 times more precise than the previous value of 2809(4) keV. The new value for ¹³⁰Te, 2526.97(23) keV is close to the Canadian Penning trap value of 2527.01 ± 0.32 keV (Scielzo et al., 2009) [1], but differs from the Florida State University trap value of 2527.518 ± 0.013 keV (Redshaw et al., 2009) [2] by 0.55 keV (2σ). These values are sufficiently precise for ongoing neutrinoless double-beta decay searches in ¹¹⁶Cd and ¹³⁰Te. Hence, our Q values were used to compute accurate phase-space integrals for these double-beta decay nuclei. In addition, experimental two-neutrino double-beta decay nuclear matrix elements were determined and compared with the theoretical values. The neutrinoless double-beta decay half-lives for these nuclei were estimated using our precise phase-space integrals and considering the range of the best available matrix elements values.     

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.     

Effects of orbital occupancies on the neutrinoless ββ matrix element of Ge

In this work we use the recently measured neutron occupancies in the ⁷⁶Ge and ⁷⁶Se nuclei as a guideline to define the neutron quasiparticle states in the 1p0f0g shell. We define the proton quasiparticles by inspecting the odd-mass nuclei adjacent to ⁷⁶Ge and ⁷⁶Se. We insert the resulting quasiparticles in a proton–neutron quasiparticle random-phase approximation (pnQRPA) calculation of the nuclear matrix element of the neutrinoless double beta (0νββ$0\nu \beta \beta$) decay of ⁷⁶Ge. A realistic model space and effective microscopic two-nucleon interactions are used. We include the nucleon–nucleon short-range correlations and other relevant corrections at the nucleon level. It is found that the resulting 0νββ$0\nu \beta \beta$ matrix element is smaller than in the previous pnQRPA calculations, and closer to the recently reported shell-model results.     

Is the weak interaction constant really constant?

A comparison is made of the probability of the process of two neutrino double-beta decay for ⁸²Se and ⁹⁶Zr in direct (counter) and geochemical experiments. The experimental data for ¹³⁰Te are also analyzed. It is shown that the probability is systematically lower in geochemical experiments, which characterize the probability of decay a few billions years ago. In addition geochemical measurements on young minerals give lower values of T (¹³⁰Te) as compared to measurements on old minerals. It is proposed that this could be due to a change in the weak interaction constant with time. The possibilities of new precise measurements to be performed with the aid of counters and geochemical experiments are discussed. A new geochemical experiment with ¹⁰⁰Mo is proposed. Received: 24 February 2000 / Accepted: 4 March 2000     

The neutrinoless double-beta decay: A test for new physics

The neutrinoless double-beta decay is not allowed in the Standard Model (SM) but it is allowed in most Grand Unified Theories (GUTs). The neutrino must be a Majorana particle (identical with its antiparticle) and must have a mass to allow the neutrinoless double-beta decay. 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 GUTs 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 GUTs. 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 GUTs and supersymmetric parameters.     

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.     

Microscopic calculations of signals of double beta decay in a Ge detector and first application to the Heidelberg–Moscow experiment

The identification of signals of neutrinoless double beta decay is a question of extreme interest. Starting from the Monte Carlo calculated time history and spatial energy distribution of neutrinoless double beta events, for the first time the expected pulse shapes to be observed in a big ⁷⁶Ge detector have been calculated ‘microscopically  ’, by using the Poisson Superfish code for determination of the field distribution in the detector. It is shown, that for the majority of 0νββ$0\nu \beta \beta$ events it is not possible to differentiate between the contributions of different particle physics parameters entering into the 0νββ$0\nu \beta \beta$ decay process—in the mass mechanism the effective neutrino mass and the right-handed weak current parameters 〈λ〉$〈\lambda 〉$, 〈η〉$〈\eta 〉$. It is shown, that on the other hand it is possible in a ⁷⁶Ge double beta decay experiment to reject a background of larger sizes (high multiplicity) gamma events by selecting low size (low multiplicity) events. First application of the theoretical ββ   pulses to events from the line observed at Q_ββ$Q_{\beta \beta }$ [H.V. Klapdor-Kleingrothaus, I.V. Krivosheina, A. Dietz, et al., Phys. Lett. B 586 (2004) 198; H.V. Klapdor-Kleingrothaus, A. Dietz, I.V. Krivosheina, et al., Nucl. Instrum. Methods A 522 (2004) 371] shows very good agreement. It is shown further, and confirmed by measurements with a collimated source, that a rather good radial position determination of ββ events in the detector is possible. By the same type of calculation it is shown that use of the pulse shapes of the 1592 keV double escape line of the 2614 keV γ-transition from ²²⁸Th for calibrating a neuronal net for search of events of neutrinoless double beta decay can be helpful.     

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.     

Re-investigation of the166Yb→166Tm Decay

The decay of¹⁶⁶Yb has been investigated with high-resolution Ge(Li) detectors. The intensity of any γ ray other than that at 82.29 keV is less than 0.3% per decay. The total decay energy was redetermined asQ=215 _?18 ⁺³³ keV. An upper limitF _w< 150 was determined for the hindrance factor of theF-forbidden 82.29 keV transition.     

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.     

Investigation of double-beta decay at the Institute of Theoretical and Experimental Physics (ITEP, Moscow)

Investigation of neutrinoless double-beta (2β0ν) decay is presently being considered as one of the most important problems in particle physics and cosmology Interest in the problem was quickened by the observation of neutrino oscillations. The results of oscillation experiments determine the mass differences between different neutrino flavors, and the observation of neutrinoless decay may fix the absolute scale and the hierarchy of the neutrino masses. Investigation of 2β0ν decay is the most efficient method for solving the problem of whether the neutrino is a Dirae or a Majorana particle, Physicists from the Institute of Theoretical and Experimental Physics (ITEP, Moscow) have been participating actively in solving this problem. They initiated and pioneered the application of semiconductor detectors manufactured from enriched germanium to searches for the double-beta decay of ⁷⁶Ge. Investigations with ⁷⁶Ge provided the most important results. At present, ITEP physicists are taking active part in four very large projects, GERDA. Majorana, EXO, and NEMO, which are capable of recording 2β0ν decay at a Majorana neutrino mass of 〈m _ν〉 ≈ 10^?2 eV.     

Prompt <Emphasis Type="Italic">γ</Emphasis> rays in <Superscript>77</Superscript>Ge and <Superscript>75</Superscript>Ge after thermal neutron capture

The reactions ⁷⁴Ge(n, γ) and ⁷⁶Ge(n, γ) have been measured with cold neutrons generated by the research reactor FRM II near Munich, Germany. The γ rays have been detected by two Compton-suppressed HPGe systems in single and coincidence mode. The number of identified prompt transitions and levels in ⁷⁷Ge was increased significantly and the decay scheme could be reconstructed for the first time. The decay scheme was also reconstructed for 68 prompt transitions in ⁷⁵Ge. Previously reported inconsistencies of the γ-ray intensities in the ⁷⁷Ge β decay could be resolved. In particular, the region around 2039 keV was investigated carefully, since the background to the neutrinoless double-beta decay of ⁷⁶Ge must be understood well for the upcoming Gerda and Majorana experiments.     

A high-resolution spectroscopic study of the gamma radiation from the decay of96Nb

The decay of 24h⁹⁶Nb has been studied with a Ge(Li) spectrometer and with a Ge(Li)-NaI(Tl) coincidence spectrometer. A total of 27 gamma-ray transitions were observed, their energies and intensities measured. All but one of these have been incorporated into the level scheme of⁹⁶Mo. The level at 1,625.9 keV reported recently in (n _th, γ) work on⁹⁵Nb has also been observed through the decay of⁹⁶Nb. No experimental evidence has been found for previously proposed levels at 2,657 and 2,791 keV in⁹⁶Mo.     

The decay of 20 min179Re

The decay of 20 min¹⁷⁹Re has been studied by means of Ge(Li) and Si(Li) detectors and a magneticβ-ray spectrometer. From the measured positon endpoint aQ-value of 2690±50 keV has been deduced. The decay scheme given is supported by coincidence relations. Fast beta transitions (logft≈5.1) to levels at 720keV and 1680 keV in¹⁷⁹W can be explained by a three quasi-particle character for these states. Nilsson assignments to these and other levels are discussed.     

The electron capture decay of 85gsr

In order to resolve the controversy concerning the existence of an 868 keV γ-transition in the decay of 65d ^85sSr and to determine the electron capture decay energy, radiochemically separated sources, Ge(Li), Ge(Li)-NaI(Tl), and NaI(Tl)-NaI(Tl) spectrometers have been used for singles and (X-ray-γ and γ-γ coincidence measurements. A γ-ray at 868.5±0.5 keV decaying with a 65±5 d half-life was conclusively identified in ^85gSr decay. From measurements of the ratio $\text{P}{}_{\mathrm{\kappa (868.5)}}\text{P}{}_{\mathrm{\kappa (514)}}$ of K-capture probabilities by (KX-ray-γ coincidences, a value of Q_EC was determined for the EC decay feeding the 868.5 keV transition. Thus, a level in ⁸⁵Rb is established at 868.5 keV, and an upper limit of 0.45 μsec is set on its lifetime.     

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: T _1/2(0ν)>1.57×10²⁵ yr (90% C.L.). This corresponds to an upper bound on the Majorana neutrino mass parameter, 〈m _ν〉, between 0.33 eV and 1.35 eV depending on the choice of theoretical nuclear matrix elements used in the analysis.     

The Majorana Ge double-beta decay project

The MAJORANA Project is a research and development activity set up to establish the feasibility and cost of a double-beta decay experiment comprising a one-ton array of Ge detectors fabricated from germanium enriched to about 86% in ⁷⁶Ge.     

ββ(2ν) decay of 100Mo to the first excited 0+ state of 100Ru

The rate of the double-beta ββ(2ν) decay of ¹⁰⁰Mo to the first 0⁺ excited state in ¹⁰⁰Ru is measured by a γγ-coincidence technique in which two HPGe detectors are used to detect the two γ rays (E _γ1=590.76 keV and E _γ2=539.53 keV) from the daughter nucleus ¹⁰⁰Ru as it deexcites to the ground state via the transition sequence 0⁺ → 2⁺ → 0⁺. In contrast to all previous ββ-decay experiments, this technique provides data that are essentially background-free. By using a 1.05-kg isotopically enriched (98.4%) disk of ¹⁰⁰Mo, 22 coincidence events (with the background estimated at 2.5 events) are detected in 440 d of measuring time, which translates into a half-life time of [5.9 _?1.1 ^+1.7 (stat.)±0.6(syst.)]×10²⁰ yr.     

Measurement of the cross sections for the production of the isotopes <Superscript>74</Superscript>As, <Superscript>68</Superscript>Ge, <Superscript>65</Superscript>Zn,and <Superscript>60</Superscript>Co from natural and enriched germanium irradiated with 100-MeV protons

The cross sections for the production of the radioactive isotopes ⁷⁴As, ⁶⁸Ge, ⁶⁵Zn, and ⁶⁰Co in metallic germanium irradiated with 100-MeV protons were measured, the experiments being performed both with germanium of natural isotopic composition and germanium enriched in the isotope ⁷⁶Ge. The targets were irradiated with a proton beam at the facility for the production of radionuclides at the accelerator of the Institute for Nuclear Research (INR, Moscow). The data obtained will further be used to calculate the background of radioactive isotopes formed by nuclear cascades of cosmic-ray muons in new-generation experiments devoted to searches for the neutrinoless double-beta decay of ⁷⁶Ge at underground laboratories.