Shape coexistence in nuclei appears to be unique in the realm of finite many-body quantum systems It differs from the various geometrical arrangements that sometimes occur in a molecule in that in a molecule the various arrangements are of the widely separated atomic nuclei In nuclei the various ''arrangements'' of nucleons involve (sets of) energy eigenstates with different electric quadrupole properties such as moments and transition rates, and different distributions of proton pairs and neutron pairs with respect to their Fermi energies Sometimes two such structures will ''invert'' as a function of the nucleon number, resulting in a sudden and dramatic change in ground-state properties in neighboring isotopes and isotones In the first part of this review the theoretical status of coexistence in nuclei is summarized Two approaches, namely, microscopic shell-model descriptions and mean-field descriptions, are emphasized The second part of this review presents systematic data, for both even- and odd-mass nuclei, selected to illustrate the various ways in which coexistence is observed in nuclei The last part of this review looks to future developments and the issue of the universality of coexistence in nuclei Surprises continue to be discovered With the major advances in reaching to extremes of proton-neutronmore » number, and the anticipated new ''rare isotope beam'' facilities, guidelines for search and discovery are discussed« less

Shape coexistence in atomic nuclei

Neutrinoless double-beta decay, which is a very old and yet elusive process, is reviewed. Its observation will signal that the lepton number is not conserved and that the neutrinos are Majorana particles. More importantly it is our best hope for determining the absolute neutrino-mass scale at the level of a few tens of meV. To achieve the last goal certain hurdles must be overcome involving particle, nuclear and experimental physics. Nuclear physics is important for extracting useful information from the data. One must accurately evaluate the relevant nuclear matrix elements--a formidable task. To this end, we review the sophisticated nuclear structure approaches which have recently been developed, and which give confidence that the required nuclear matrix elements can be reliably calculated employing different methods: (a) the various versions of the quasiparticle random phase approximations, (b) the interacting boson model, (c) the energy density functional method and (d) the large basis interacting shell model. It is encouraging that, for the light neutrino-mass term at least, these vastly different approaches now give comparable results. From an experimental point of view it is challenging, since the life times are long and one has to fight against formidable backgrounds. One needs large isotopically enriched sources and detectors with high-energy resolution, low thresholds and very low background. If a signal is found, it will be a tremendous accomplishment. The real task then, of course, will be the extraction of the neutrino mass from the observations. This is not trivial, since current particle models predict the presence of many mechanisms other than the neutrino mass, which may contribute to or even dominate this process. In particular, we will consider the following processes: The neutrino induced, but neutrino-mass independent contribution. Heavy left and/or right-handed neutrino-mass contributions. Intermediate scalars (doubly charged, etc). Supersymmetric (SUSY) contributions. We will show that it is possible to disentangle the various mechanisms and unambiguously extract the important neutrino-mass scale, if all the signatures of the reaction are searched for in a sufficient number of nuclear isotopes.

/pdf/theory-of-neutrinoless-double-beta-decay-2c6bzk0uf6.pdf

Theory of neutrinoless double-beta decay.

We review the particle physics aspects of neutrino-less double beta decay. This process can be mediated by light massive Majorana neutrinos (standard interpretation) or by something else (non-standard interpretations). The physics potential of both interpretations is summarized and the consequences of future measurements or improved limits on the half-life of neutrino-less double beta decay are discussed. We try to cover all proposed alternative realizations of the decay, including light sterile neutrinos, supersymmetric or left-right symmetric theories, Majorons, and other exotic possibilities. Ways to distinguish the mechanisms from one another are discussed. Experimental and nuclear physics aspects are also briefly touched, alternative processes to double beta decay are discussed, and an extensive list of references is provided.

/pdf/neutrino-less-double-beta-decay-and-particle-physics-4fssdtwcty.pdf

Neutrino-less double beta decay and particle physics

Background: Direct determination of the neutrino mass through double-$\ensuremath{\beta}$ decay is at the present time one of the most important areas of experimental and theoretical research in nuclear and particle physicsPurpose: We calculate nuclear matrix elements for the extraction of the average neutrino mass in neutrinoless double-$\ensuremath{\beta}$ decayMethods: The microscopic interacting boson model (IBM-2) is usedResults: Nuclear matrix elements in the closure approximation are calculated for ${}^{48}$Ca, ${}^{76}$Ge, ${}^{82}$Se, ${}^{96}$Zr, ${}^{100}$Mo, ${}^{110}$Pd, ${}^{116}$Cd, ${}^{124}$Sn, ${}^{128}$Te, ${}^{130}$Te, ${}^{148}$Nd, ${}^{150}$Nd, ${}^{154}$Sm, ${}^{160}$Gd, and ${}^{198}$Pt decayConclusions: Realistic predictions for the expected half-lives in neutrinoless double-$\ensuremath{\beta}$ decay with light and heavy neutrino exchange in terms of neutrino masses are made and limits are set from current experiments

Nuclear matrix elements for double- β decay

A self-consistent calculation of nuclear matrix elements of the neutrinoless double-beta decays ($0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$) of $^{76}\mathrm{Ge}$, $^{82}\mathrm{Se}$, $^{96}\mathrm{Zr}$, $^{100}\mathrm{Mo}$, $^{116}\mathrm{Cd}$, $^{128}\mathrm{Te}$, $^{130}\mathrm{Te}$, and $^{136}\mathrm{Xe}$ is presented in the framework of the renormalized quasiparticle random phase approximation (RQRPA) and the standard QRPA. The pairing and residual interactions as well as the two-nucleon short-range correlations are for the first time derived from the same modern realistic nucleon-nucleon potentials, namely, from the charge-dependent Bonn potential (CD-Bonn) and the Argonne V18 potential. In a comparison with the traditional approach of using the Miller-Spencer Jastrow correlations, matrix elements for the $0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$ decay are obtained that are larger in magnitude. We analyze the differences among various two-nucleon correlations including those of the unitary correlation operator method (UCOM) and quantify the uncertainties in the calculated $0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$-decay matrix elements.

0νββ-decay nuclear matrix elements with self-consistent short-range correlations

A gas target system has been developed for nuclear scattering experiments with a highly dispersive spectrometer at small scattering angles including 0°. The system is equipped with a gas cell that has a wide beam window of 44 mm in width and 14 mm in height. The size is sufficient to pass a dispersed beam, which is essential for high energy-resolution measurement. A gas handling system allows target gas that is often limited in quantity, to be collected for reuse. Typical areal density is 1 mg/cm 2 for 20 Ne or 36 Ar gas. The target can be operated either at room temperature or at the liquid nitrogen temperature. The gas target has been employed in a proton inelastic-scattering experiment at 295 MeV with the Grand Raiden spectrometer at the Research Center for Nuclear Physics. A good energy resolution of 20 keV (FWHM) has been achieved at scattering angles of 0–4.5°.

Wide-window gas target system for high resolution experiment with magnetic spectrometer

A facility for studying (n,p) reactions at neutron kinetic energies of 140 ≲ T n ≲ 300 MeV has been constructed by using the existing Large Acceptance Spectrometer at the Research Center for Nuclear Physics, Osaka University. A neutron beam produced by the 7Li(p,n) reaction hits the (n,p) targets located in a segmented target system, which allows one to use a relatively thick target in total ( ∼ 0.5 g / cm 2 ) with a moderate energy resolution of ∼ 1 MeV . The facility enables one to obtain spectra at 300 MeV up to an excitation energy of 70 MeV with a single setting of the spectrometer. This (n,p) facility has a large effective solid angle of 11 msr for an effective target size of 30 W × 20 H mm 2 .

The RCNP (n,p) facility

The double differential cross sections at 0°–12° were measured for the 90Zr(n,p) reaction at 293 MeV in a wide excitation energy region of 0–70 MeV. The experiment was performed by using the (n,p) facility at the Research Center for Nuclear Physics. The multipole decomposition (MD) technique was applied to the measured cross sections to extract the GT component in the continuum. After subtracting the contribution of the isovector spin‐monopole excitation we obtained the GT strength of Sβ+ = 3.0 ± 0.3 ± 0.8 ± 0.5 up to 31.4 MeV excitation. The quenching factor Q was deduced by using the present result and the Sβ− value obtained from the MD analysis of the 90Zr(p,n) spectra. The result is Q = 0.83 ± 0.06 in regards to Ikeda’s sum rule value of 3(N − Z) = 30.

Determination of the Gamow‐Teller Quenching Factor via the90Zr(n, p) Reaction at 293 MeV

Determination of the Gamow-Teller Quenching Factor via the 90 Zr(n,p) Reaction at 293 Mev

Determination of the Gamow-Teller Quenching Factor via the 90 Zr(n, p) Reaction at 293 Mev

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