K. Muto

Nuclear structure calculation ofβ+β+,β+/EC and EC/EC decay matrix elements

Nuclear matrix elements for double positron emisson (β+β+), positron emission/electron capture (β+/ EC) and double electron capture (EC/EC) in the 2νββ decay mode and forβ+β+ andβ+/EC decay in the 0νββ mode are calculated for the experimentally most promising isotopes58Ni,78Kr,96Ru,106Cd,124Xe,130Ba and136Ce within pn-QRPA. We point out that the matrix element for the 2νβ+/EC decay differs from the 2νβ+β+ matrix element, an effect not considered previously. For the neutrino accompanied decays our calculation predicts for theβ+/EC and the EC/EC mode half lives which are shorter typically by 4–7 orders of magnitude than those for the double positron emission. However, even for the best candidates typical values for 2νβ+/EC (2ν EC/EC) are still in the range of ∼1022 ((some) 1021) years. For 0νββ decay we have calculated all matrix elements relevant for both, the mass mechanism and the right-handed currents for the first time complete. A detailed discussion of the differences between the 0νβ+β+, the 0νβ+/EC and 0νβ−β− decay is given.

Proton-neutron quasiparticle RPA with separable Gamow-Teller forces

A comprehensive representation is presented of a generalized form of the proton-neutron quasiparticle RPA model, originally introduced by Halbleib and Sorensen almost thirty years ago. The model uses separable Gamow-Teller forces, including, in addition to the particle-hole force of the former model, the particle-particle force, which is of decisive importance for β+ decay andββ decay. The above model has further been extended to the treatment of odd-odd nuclei. An extension is also made to transitions from nuclear excited states. This is essential for calculations of nuclear weak transition rates in the high-temperature interior of massive stars. Complementing the discussion of Halbleib and Sorensen on the particle-hole force, the structure of the RPA dispersion relation is discussed with emphasis on effects of the particle-particle force.

Double beta decay nuclear matrix element of 48Ca

Abstract The 2 v mode double beta decay of 48 Ca is studied in detail, particularly in connection with validity of the closure approximation. In an extensive shell-model calculation, nuclear matrix elements for low-lying and high-lying intermediate states have opposite signs. It is therefore indicated that a very small value has to be adopted for the average excitation energy, if one utilizes the closure approximation.

Microscopic calculation of the rates of electron captures which induce the collapse of O+Ne+Mg cores

Abstract The rates of electron captures which induce the collapse of the stellar cores composed of 16 O, 20 Ne and 24 Mg are calculated microscopically using the shell model wave functions of Wildenthal. Since these are the best wave functions available for sd-shell nuclei, the capture rates calculated in this paper are at present the most reliable ones. Our electron capture rates differ up to an order of magnitude from those previously calculated with the gross theory and FFN. The difference of the capture rates is analyzed and the effects on the evolution of the O + Ne + Mg core are discussed. In the appendix, we tabulate the capture rates as well as the associated neutrino energy loss rates and γ-ray heating rates in the form available for the calculation of the evolution of the O + Ne + Mg core.

Spin alignment of43Sc produced in the fragmentation of 500 MeV/u46Ti

In the fragmentation of 500 MeV/u46Ti the spin alignment of43mSc (I=19/2−, T1/2=473 ns) fragments was observed by the Time Dependent Perturbed Angular Distribution (TDPAD) method. The measurement was performed for different cuts in the longitudinal momentum distribution. A positive spin alignment of about 35% was observed in the center and a negative alignment of about 15% in the wing of the distribution. For the different cuts the relative production of the 19/2− state was measured. In the wing of the distribution the isomeric ratio N(19/2−)/total43Sc is about 15 times larger than in the center. The results of this pilot experiment are discussed in the frame of a shell-model.

Extended quasiparticle RPA and double-beta-decay nuclear matrix elements

Abstract The quasiparticle random-phase-approximation model is extended toward restoration of fermion anticommutation relations for quasiparticle operators, beyond the quasi-boson approximation, within the framework of equation-of-motion method. The restoration of broken symmetries avoids an unreasonable enhancement of quasiparticle correlations in the ground state, and enables to make more reliable predictions. By the extended model, somewhat larger bounds are deduced on the neutrino mass and right-handed current parameters, compared to ordinary quasiparticle calculations, for a given half-life limit of the neutrinoless double beta decay.

Nuclear matrix elements for double positron emission

Abstract Nuclear matrix elements for double positron emissions are calculated for both two-neutrino and zero-neutrino decay modes within the pnQRPA model with a realistic effective nucleon-nucleon interaction. This is the first complete microscopic calculation for all potential double positron emitters. The predicted lifetimes are several orders of magnitude larger than those for β−β− decays and difficult to reach in present days experiments.

Neutron core-excited high-spin states in N = 50 nuclei

Abstract High-spin states in the N = 50 nuclei are studied in terms of the shell model, assuming π( p 1 2 , g 9 2 ) n + π p 1 2 , g 9 2 n × ν( p 1 2 , g 9 2 ) −1 d 5 2 + π( p 1 2 , g 9 2 ) n−1 d 5 2 ) configurations on the 88Sr core. The observed high-spin states in 94Ru are satisfactorily reproduced by one-neutron excitation. It is also suggested that one of the two high-spin structures lying above 4 MeV in 91Nb corresponds to neutron excited states. The obtained wave functions of the yrast high-spin states in N = 50 isotones are composed mainly of stretched or almost stretched coupling of a d 5 2 neutron to the yrast states of N = 49 isotones.

Shell-model study on magnetic dipole excitation of N = 28 isotones

Abstract Magnetic dipole excitation of N = 28 isotones — 48 Ca, 50 Ti, 52 Cr, 54 Fe and 51 V — is studied in terms of the shell model by assuming f 7 2 n−m ( p 3 2 p 1 2 f 5 2 ) m configurations with m = 0, 1 and 2 on an inert 40 Ca core. Strength distributions observed in (e,e′) and (p,p′) experiments are fairly well reproduced. Interference of proton and neutron excitations enhances B (M1) values around E x = 10 MeV and reduces those of low-lying states. The strength for T = T 0 + 1 states substantially increases when orbital magnetization is not taken into account, whereas no appreciable consequences can be seen for T = T 0 states. In comparison with single-particle model predictions with m = 0, about 20–30% reduction of the total strength is obtained due to ground-state configuration mixing. It is shown that the isoscalar spin component is reduced as well as the isovector one. However, the theoretical total strength is still much larger than experimental values, and coupling with high-lying configurations and possible delta-hole excitation might be required in order to account for the discrepancy. In an odd-mass nucleus 51 V, the ground-state transition strength is distributed continuously over a wide energy range, being very strongly fragmented.

A comparative study of the 13C(p, p′)13C and 13C(p, n)13N reactions at Ep = 35 MeV☆

Abstract Differential cross sections were measured at Ep = 35 MeV for the 13C(p, n) and 13C(p, p′) reactions leading to the four low-lying states in the mirror nuclei 13N and 13C. In addition, the analyzing powers were measured for the 13C(p, p′) reaction. The data are generally well accounted for by DWBA calculations except for the 13 C(p, p′) 13 C (3.09 MeV , 1 2 + ) reaction, for which the calculations cannot even reproduce the qualitative features of the data. A comparison of the (p, n) and the (p, p′) results suggests that the isoscalar part of the 13 C ( g.s. , 1 2 − ) → 13 C (3.09 MeV , 1 2 + ) transition is not correctly described by currently available shell-model wave functions.