Double beta decay

Double beta decay is a rare nuclear process changing the nuclear charge by two units leaving atomic number unchanged. The detection of the neutrino accompanied mode (A,Z)→(A,Z + 2) + 2e^? + &;2v^? ₂ with half-lives around 10²⁰ years is among the rarest decays ever observed. Of outmost importance for particle physics and especially neutrino physics, is the neutrinoless mode (A,Z)→(A,Z + 2) + 2e^?. This process is violating lepton number by two units and requires massive Majorana neutrinos, i.e. neutrino and antineutrino are identical. The current experimental status is reviewed and an outlook towards future activities is given.     

Near-stoichiometric sodium beta alumina

The preparation of near-stoichiometric sodium beta alumina is described. The activation energy for sodium-ion conduction, 0.62 eV, is significantly larger than for the highly non-stoichiometric starting material.     

Double beta decay experiments

The present status of double beta decay experiments is reviewed. The results of the most sensitive experiments are discussed. Proposals for future double beta decay experiments with a sensitivity to the 〈m _ν〉 at the level of (0.01–0.1) eV are considered.     

Neutrinoless double beta decay

The physics potential of neutrinoless double beta decay is discussed. Furthermore, experimental considerations as well as the current status of experiments are presented. Finally, an outlook towards the future, work on nuclear matrix elements and alternative processes is given.     

Neutrino-less double beta decay experiments

The observation of neutrinoless double beta decay would unambiguosly demonstrate that neutrinos are Majorana particles and would provide unique information about the ordering and absolute scale of neutrino masses. This very rare decay is actively searched for in a number of candidate isotopes. It violates lepton-number and is predicted by many extensions of the standard model. The most recent experimental results are reviewed. The technological advances and the most compelling requirements for the new generation of experiments are discussed.     

Constraining neutrinoless double beta decay

A class of discrete flavor-symmetry-based models predicts constrained neutrino mass matrix schemes that lead to specific neutrino mass sum-rules (MSR). We show how these theories may constrain the absolute scale of neutrino mass, leading in most of the cases to a lower bound on the neutrinoless double beta decay effective amplitude.     

Bremsstrahlung spectrum in beta decay

The spectrum of the bremsstrahlung due to the complete slowing down in aluminium of beta particles belonging to the continuous spectrum, was calculated for an arrangement, in which the beta emitter is completely enclosed in an absorber. The Bethe-Heitler cross-section, corrected by an empirical factor, was used. A table is given which makes it possible to calculate bremsstrahlung spectra of beta emitters with their maximal energy between 0·6 MeV and 2·0 MeV; the necessary additional calculations are quite simple. The error in the calculation is practically determined by the uncertainty in the determination of the cross-section and is equal to about 10%.

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Nuclear responses for neutrinos and neutrino studies by double beta decays and inverse beta decays

This is a brief report on recent studies of nuclear responses for neutrinos (v) by charge exchange reactions, v masses by double beta (ββ) decays and of solar and supernova v’s by inverse β decays. Subjects discussed include (1) v studies in nuclear micro-laboratories, (2) v masses studied by ββ decays of ¹⁰⁰Mo and nuclear responses for ββ — v, (3) solar and supernova v’s by inverse β decays and v responses for ⁷¹Ga and ¹⁰⁰Mo, and (4) MOON (molybdenum observatory of neutrinos) for spectroscopic studies of Majorana v masses with sensitivity of m _v ∼ 0.03 eV by ββ decays of ¹⁰⁰Mo and real-time studies of low energy solar and supernova v’s by inverse β decays of ¹⁰⁰Mo.     

Neutrino mass constraints on beta decay

Using the general connection between the upper limit on the neutrino mass and the upper limits on certain types of non-standard-model interactions that can generate loop corrections to the neutrino mass, we derive constraints on some non-standard-model d --> ue- nu interactions. When cast into limits on n --> pe- nu coupling constants, our results yield constraints on scalar and tensor weak interactions improved by more than an order of magnitude over the current experimental limits. When combined with the existing limits, our results yield absolute value(C(S)/C(V)) approximately < 5 x 10(-3), absolute value(C'(S)/C(V)) approximately < 5 x 10(-3), absolute value(C(T)/C(A)) approximately < 1.2 x 10(-2), and absolute value(C'(T)/C(A)) approximately < 1.2 x 10(-2).     

Double beta decays and Majorana neutrinos

Neutrino-less double beta decays (0νββ) are sensitive and realistic probes for studying the Majorana nature of neutrinos (ν), the ν-mass spectrum, the absolute ν-mass scale, the Majorana CP phases and other fundamental properties of neutrinos and weak interactions. Current 0νββ experiments, which use detectors with the mass sensitivity of the order of 300 meV, study the ν-mass in that mass region. Future experiments with higher sensitivities of the orders of 100meV and 30 meV, using different nuclei and methods (calorimetric, spectroscopic), are indispensable for establishing 0νββ in the quasi degenerate and the inverted hierarchy mass regions. R&D for ultra-high sensitivity detectors are encouraged for studying the normal hierarchy mass region. Theoretical and experimental studies for evaluating nuclear matrix elements are important for extracting the sensible ν-mass from the 0νββ rate. Charge exchange reactions by means of nuclear, electromagnetic and ν probes provide valuable data which are used to evaluate the nuclear matrix elenments. International collaborations for 0νββ experiments and for the matrix elements are crucial for next generation 0νββ studies.     

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.