Hisashi Horie

Proton-neutron correlations in nuclei with N = 30

Abstract Structure of the nuclei with N = 30 and Z = 20–28 is investigated by the nuclear shell model within the proton-neutron configurations (1f 7 2 ) z−20 p × (2p 3 2 , 2p case1 2 , 1f case5 2 ) 2 n . Effective proton-neutron interactions determined by a least-squares fit to the observed spectra of N = 29 nuclei are adopted. Agreement of the calculated spectra with experimental spectra is satisfactory. Strong correlations between protons and neutrons break down the pairing scheme and lower the first J = 2 levels in doubly even nuclei, which is shown from the resultant wave functions. A relation between the shell model and collective rotational model is discussed concerning the calculated rotation-like spectrum of 56 Fe. Electromagnetic properties and spectroscopic factors of single-nucleon transfer reactions are calculated. They are in good agreement with experiments.

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.

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.

Non-normal parity states in A = 15, 14 and 13 nuclei

Abstract Shell-model calculations for the non-normal parity states of A = 15, 14 and 13 nuclei are carried out within the framework of the 1p n-1 (2s, 1d) and 1s 3 1p n+1 configurations where n = A − 4. The matrix elements of the two-body interactions between the 1p nucleons are fixed to be those determined by Cohen and Kurath from a least-squares fit. In addition, the two-body interactions between inequivalent nucleons are assumed to arise from a central force. The results for energy levels, transition probabilities and spectroscopic factors are in good agreement with experiment.

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.

The non-normal parity states of 209Bi and 207Pb

Abstract The low-lying non-normal parity states of 209 Bi and 207 Pb are studied by means of the shell model, assuming 1h-2p and 2h-1p configurations, respectively. The observed energy spectra are well reproduced. The wave functions obtained are analysed in comparison with those defined in the weak-coupling model and existence of the octupole multiplet and the mixing with other states are discussed in detail. The conditions for the existence of the weak-coupling states are also discussed.

Shell-model study of the Gamow-Teller giant resonance in 208Bi

Abstract The Gamov-Teller giant resonance observed in the 208 Pb(p,n) 208 Bi reaction is studied in terms of shell model. 2p−2h excitations are taken into the model space in addition to 1p-1h on the doubly closed 208 Pb core.The giant resonance is well reproduced and the Gamow-Teller strength is highly fragmented among a number of 1 + states in the resonance region.

Shell model calculation of the septuplet in 209Bi

Abstract The shell model study has been carried out about the close septuplet of 209 Bi without assuming weak particle-vibration coupling. The energies and the wave functions of these states are discussed.

Shell-model study of the low-lying states of the 1 f 1/2 nuclei

There has been a significant increase of experimental information available on the 1]~ nuclei in recent years with which theoretical studies on the structure of those nuclei can be compared. The systematic shell-model calculations for the whole 1]] nuclei were performed by McCuLL~S, BAY~AN and ZAMICK (1) and GINOCCmO (2) many years ago, assuming pure ]~ configurations. The shortcomings of the pure /~ model, however, have been discussed in several aspects (3) and it has been suggested the strongcoupling symmetric-rotor model including the Coriolis coupling between bands (4) gives better agreement with experimental data for odd-A nuclei. On the other hand, the shell-model calculations with extended spaces were carried out for the calcium isotopes (5), the N = 28 isotopes (6) and A ~< 44 nuclei (7), and better agreement with experimental data was obtained in comparison with the pure ]~ model. The purpose of the present note is to report the results of the shell-model calculation with ]~ + ]~-~p~ space performed for the low-lying states of the whole lJ~ shell nuclei. First, the effective interaction for the pure ]~ configurations was searched to see whether more adequate choice of the matrix elements could provide better agreement with experimental data. The results, however, did not show significant improvement over those of the previous calculations (1,2) and newly obtained results are adopted as reference values of the ]~ model. (Referred to as cal. I in the following). The breakdown of the cross-conjugate symmetry observed among the 1]~ nuclei and other shortcomings of the pure ]~ model lead us to the configuration mixing. However, in view of our scanty knowledge about the effective interaction for the whole ] p shell it is

Shell-model study on Gamow-Teller excitation of 90Zr

Abstract The Gamow-Teller excitation of 90Zr is studied in the ( p 1 2 g 9 2 ) 14 +( p 1 2 g 9 2 ( g 7 2 d 5 2 d 3 2 s 1 2 ) configurations. Good correspondence is obtained between the calculated strength distribution and the GT peaks observed in the 90Zr(p,n)90Nb reaction at Ep = 120 MeV. The 2p-2h states with a g 7 2 neutron, which have a large spatial overlap with the 1p-1h state excited by t-σ, are most responsible for the shift of the strength to high-lying states and also for the width of the giant GT resonance. The interaction between 2p-2h states causes strong fragmentation of strength in the giant resonance region.