Beta Decay Of Nuclei Research Articles

Coulomb corrections to Fermi beta decay in nuclei

We study the influence of the Coulomb force on the Fermi beta-decays in nuclei. This work is composed of two main parts. In the first part, we calculate the Coulomb corrections to super-allowed beta decay. We use the notion of the isovector monopole state and the self-consistent charge-exchange Random Phase Approximation to compute the correction. In the second part of this work, we examine the influence of the anti-analog state on isospin mixing in the isobaric analog state and the correction to the beta-decay Fermi transition.

Development of the laser isotope separation method to study for the neutrino-less double beta decay of 48Ca

Search for the ultra-rare process, neutrino-less double beta decay, is a powerful tool to test the Majorana nature of neutrinos. Among the potential double beta decay nuclei, 48Ca has the largest Q-value, hence we can expect the measurement with least background. On the other hand, due to its low natural abundance, isotope enrichment is essential to achieve the sensitivity in the region of interest (inverted/normal mass hierarchy of neutrinos).We have been developing a laser isotope separation method using a tunable semiconductor laser that can excite only a specific isotope (48Ca). The laser is irradiated perpendicular to the collimated calcium vapor beam. Only 48Ca atoms are separated by deflecting them from the original atomic beam by momentum transfer due to multiple absorption and emission of laser photons. The isotope separation is confirmed by irradiating an ionization laser and time-of-flight measurement.Following the success of the proof-of-principle experiment, we are now developing the mass production system which include efficient generation of atomic beams, frequency-stabilized high-power laser, efficient laser irradiation method, and collection method of deflected isotope beams. In this paper, we report on the status and prospects of these developments.

Multifarious Roles of Hidden Chiral-Scale Symmetry: “Quenching” gA in Nuclei

I discuss how the axial current coupling constant gA renormalized in scale symmetric chiral EFT defined at a chiral matching scale impacts on the axial current matrix elements on beta decays in nuclei with and without neutrinos. The “quenched” gA observed in nuclear superallowed Gamow–Teller transitions, a long-standing puzzle in nuclear physics, is shown to encode the emergence of chiral-scale symmetry hidden in QCD in the vacuum. This enables one to explore how trace-anomaly-induced scale symmetry breaking enters in the renormalized gA in nuclei applicable to certain non-unique forbidden processes involved in neutrinoless double beta decays. A parallel is made between the roles of chiral-scale symmetry in quenching gA in highly dense medium and in hadron–quark continuity in the EoS of dense matter in massive compact stars. A systematic chiral-scale EFT, presently lacking in nuclear theory and potentially crucial for the future progress, is suggested as a challenge in the field.

Analysis of β+ decay of 13N nucleus using a modified one-particle approach

It is known that the calculated values of log(ft) and the half-life of beta decay are less than the measured values when a single-particle approach is used to calculate these quantities. In this article, we discuss the importance of taking into account the spectroscopic factor in the calculation of the half-life and log(ft) beta decay of nuclei containing one nucleon in the outermost shell. We also emphasize the dominant role of the asymptotic normalization coefficient, which takes into account the many-particle effect and allows us to obtain the spectroscopic factor necessary to describe the reaction 13N → 13C + β+ + νe . We find the asymptotic normalization coefficients using the experimental data of the elastic scattering phase-shift of the proton and neutron on 12C. Using overlap functions instead of single-particle functions, we obtain a better comparison with the experimental data. The overlap function is represented as the product of the single-particle function and the root of the corresponding spectroscopic factor.

The laser Isotope separation (LIS) methods for the enrichment of 48Ca.

Neutrino-less double beta decay measurement is a powerful tool to test the Majorana nature of neutrinos. Among the potential double beta decay nuclei, 48Ca has the largest Q value (4.3 MeV), we can expect the least background measurement. On the other hand, a smallness of natural abundance of 48Ca (~0.2%) requires an enrichment to increase the sensitivity to reach interesting regions (Inverted Hierarchy and Normal Hierarchy). We have been studying the laser isotope separation (LIS) methods for the enrichment of 48Ca, where a narrow band diode laser is used to excite the specific isotope in the calcium vapor beam. For the separation, two methods (deflection and ionization) are employed. In the deflection method, the 48Ca atoms are deflected by momentum transfers from laser photons by multiple absorption and emission. In the ionization method, an additional dye laser is used to ionize the excited 48Ca. In this paper, we will report the current status and future prospects of two methods of LIS for the enrichment of 48Ca.

Processes that break baryon number by two units and the Majorana nature of the neutrino

We employ the simplest possible models of scalar-fermion interactions that are consistent with the gauge symmetries of the Standard Model and permit no proton decay to analyze the connections possible among processes that break baryon number by two units. In this context we show how the observation of n-n¯ oscillations and of a pattern of particular nucleon–antinucleon conversion processes — all accessible through e-d scattering — namely, selecting from e−p→e+p¯, e−p→n¯ν¯, e−n→p¯ν¯, and e−n→e−n¯ would reveal that the decay π−π−→e−e− must occur also. This latter process is the leading contribution to neutrinoless double beta decay in nuclei mediated by new short-distance physics, in contrast to that mediated by light Majorana neutrino exchange. The inferred existence of π−π−→e−e− would also reveal the Majorana nature of the neutrino, though the absence of this inference would not preclude it.

Spin dipole nuclear matrix elements for double beta decay nuclei by charge-exchange reactions

Spin dipole (SD) strengths for double beta-decay (DBD) nuclei were studied experimentally for the first time by using measured cross sections of (3He, t) charge-exchange reactions (CERs). Then SD nuclear matrix elements (NMEs) for low-lying 2− states were derived from the experimental SD strengths by referring to the experimental α = GT (Gamow–Teller) and α = F (Fermi) strengths. They are consistent with the empirical NMEs based on the quasi-particle model with the empirical effective SD coupling constant. The CERs are used to evaluate the SD NME, which is associated with one of the major components of the neutrino-less DBD NME.

Deformed shell model results for neutrinoless double beta decay of nuclei in A = 60 - 90 region

Nuclear transition matrix elements (NTME) for the neutrinoless double beta decay (Oνββ or OνDBD) of 70 Zn , 80 Se and 82 Se nuclei are calculated within the framework of the deformed shell model (DSM) based on Hartree–Fock (HF) states. For 70 Zn , jj44b interaction in 2p3/2, 1f5/2, 2p1/2 and 1g9/2 space with 56 Ni as the core is employed. However, for 80 Se and 82 Se nuclei, a modified Kuo interaction with the above core and model space are employed. Most of our calculations in this region were performed with this effective interaction. However, jj44b interaction has been found to be better for 70 Zn . The above model space was used in many recent shell model (SM) and interacting boson model (IBM) calculations for nuclei in this region. After ensuring that DSM gives good description of the spectroscopic properties of low-lying levels in these three nuclei considered, the NTME are calculated. The deduced half-lives with these NTME, assuming neutrino mass is 1 eV, are 1.1 × 1026, 2.3 × 1027 and 2.2 × 1024 yr for 70 Zn , 80 Se and 82 Se , respectively.

Isospin corrections to super-allowed beta decays in nuclei

Isospin corrections to the super-allowed beta decay matrix elements are evaluated in perturbation theory using the notion of the giant isovector monopole state. The calculation avoids the separation into different contributions and therefore presents a consistent, systematic and more transparent approach. Explicit expressions for δc as a function of the mass number A are derived. These corrections affect the values of the Vud matrix element in the Cabbibo-Maskawa-Kobayashi matrix. Also, it is pointed out that in some nuclei with a low number of excess neutrons (protons) Coulomb mixing with the anti-analog state can introduce significant isospin impurities in the isobaric analog state.

Beta-decay of nuclei near the neutron shell N = 126

A self-consistent approach to the weak interaction rates is presented. It is based on the generalized energy-density functional method and continuum QRPA. The study has been made of the β-decay total energy releases, half-lives and β-delayed neutron emission branchings for recently identified near-spherical nuclei with charge numbers Z = 76–79 approaching the closed neutron shell at N = 126. Together with our previous calculations near N = 28, 50, 82 this provides an important update to the standard set of weak rates for the r-process modeling, radioactive beam experiments and advanced reactor applications. Within the fully microscopic framework a significant competition is found of the Gamow-Teller and first-forbidden decay contributions to the total half-lives.

Momentum analyzers DCBA for neutrinoless double beta decay experiments

Momentum analyzers called Drift Chamber Beta-ray Analyzer (DCBA) are being developed at KEK in order to search for neutrinoless double beta decays of nuclei. A test prototype, DCBA-T2, has been constructed to confirm the principle detecting electron tracks in a uniform magnetic field. Another prototype, DCBA-T3, is now under construction to improve the energy resolution. The test results and the present statuses of these prototypes are presented.

Beta-decay and exotic nuclei

Selected examples are given of how beta-decay can provide information on nuclei far from the line of beta-stability. Emphasis is put on beta-delayed particle emission, in particular processes with emission of several particles. The physics questions addressed include halo nuclei and multi-particle emission mechanisms. Muon capture on radioactive nuclei is discussed as an alternative way of accessing the region of high excitation energy.

A measurement of the double beta decay half-life of 48Ca

48 Ca is the lightest of the many double beta decay nuclei, and the only one simple enough to be treated exactly in the shell model without truncation. Thus a ββ 2 v measurement of this isotope provides a unique test of the nuclear physics involved in ββ matrix element calculations. Enriched 48 Ca sources of two different thicknesses have been exposed in a time projection chamber, and yield a preliminary T 1/2 2 v = (5.5 −1.5 +3.5 ) × 10 19 y, in agreement with shell model calculations.

New vistas in the nuclear shell model

The general problems and goals of modern shell-model calculations are briefly reviewed. They are illustrated by the application to three questions current interest. (1) How is the angular momentum in a nucleus distributed between the orbital and the intrinsic spin degrees of freedom? (2) What are the orbital occupations and spacial extent of the neutrons in nuclei near the neutron drip line? and (3) Can we understand two-neutrino double beta decay in nuclei?

Mechanisms of neutrinoless double beta-decay and the mass of heavy neutrino

Abstract In this paper we have analysed mechanisms of the neutrinoless double beta-decay in nuclei, the mass of heavy Majorana neutrino and the influence of the nuclear short-range correlation.

Meson-Exchange Currents in Time-Like Axial-Charge Transitions

The role of axial-vector currents in nuclear forces is reviewed. The beta decay of nuclei with A = 11, 12, 16, and 18 is discussed. (AIP)

Alignment of inner atomic shells in nuclear α decay

It is shown that in inner-shell ionisation during nuclear alpha decay the vacancy produced with J>1/2 should be aligned with the direction of alpha -particle escape. This alignment manifests itself in polarisation and angular anisotropy of the following X-rays and Auger electrons. The estimation of these effects made in the semiclassical approximation shows that the alignment is large and strongly model-dependent. Analogous effects may be expected in atomic ionisation during beta decay of nuclei.

Radiative corrections to the electron polarization in the beta-decay of a free neutron

An expression for the electromagnetic radiative corrections of the order α(= 1 137 ) to the longitudinal polarization rate of the electron in the beta-decay of a free neutron is derived in the approximation Δ/ M⪡1, where Δ is the neutron-proton mass difference and M the mass of the neutron. The corrections are not larger than 0.2% and it seems that they cannot explain the deviation of the experimental value for the electron polarization from the value − ν/ c (measured in the beta-decay of nuclei).

Theory of Bound-State Beta Decay

The theory of beta-decay processes in which an electron is created in a bound atomic state is developed in the allowed approximation. The correlations and total decay rate are calculated with the renormalized $V\ensuremath{-}A$ theory and the results are valid for atoms of arbitrary electronic configuration. The relative probability of bound-state to continuum-state decay is shown to be independent of nuclear matrix elements; some bound-state decay rates are presented that were calculated by making use of this fact. The possibility of experimentally detecting bound-state decay is also briefly examined. The beta decay of nuclei in stellar interiors is discussed and a number of examples are presented for which bound-state decay is more likely than continuum-state decay under the conditions that obtain in stellar interiors.

Emission of gamma radiation during the beta decay of nuclei

Summary Experimental investigations have shown that the disintegrateion of RaE is accompanied by a fairly weak γ-radiation. Theoretical calculations have been made which show that a large part, if not all of this radiation arises from the creation of the electron and its loss of energy as it leaves the nucleus. This process is an “inner” Bremsstrahlung to be contrasted with the “oute” Bremsstrahlung due to the impingement on the nucleus of electron coming from the outside. It has the order of magnitude α = 1/137 per outcoming electron. Two methods of treating the problem have been employed and it is found that in the Born approximation they give identical results. In the first the electron is represented by the free, outgoing, not-allowed, spherical solutions of the Dirac equation and radiation transitions are calculated to standing waves. In the second method the calculations are based on the Fermi theory of β-decay. The effect of a quantized electromagnetic radiation field is introduced by means of a coupling term between the electrons and the radiation field. Radiation intensity curves are obtained which are very similar to those for ordinary Bremsstrahlung.