Session KH - Nuclear Structure III: High Spin States.
ORAL session, Friday morning, October 19
Awapuhi, Outrigger Wailea Resort

[KH.001] Fusion Reactions with High-Spin Isomer Beam

High-spin isomer beams (HSIB) have been developed at RIKEN Accelerator Research Facility (RARF). The most remarkable feature of HSIB compared with usual ground state beams (GSB) is the large intrinsic nuclear spin. When a fusion reaction is induced by HSIB, therefore, the angular momentum distribution in the compound nucleus is expected to extend toward a higher region due to the sum of the isomeric spin and the orbital angular momentum relevant to the collision. In recent studies, ^145mSm, which has the nuclear spin of I=\frac492, has been mainly used as a probe of secondary fusion reactions. The ^145mSm was produced via an inverse-kinematic reaction ^16O(^136Xe,7n)^145mSm, which leads to the ^145mSm beam energy of 5.5MeV/nucleon with the ^136Xe of 7.6MeV/nucleon. Some special devices, a windowless gas target and a superconducting solenoid, were incorporated into the isomeric beam line. More than 10^5/s of ^145mSm were! ! able to be obtained on the secondary target. By using this beam, a secondary fusion reaction ^13C(^145mSm,xn)^158-xEr was performed. Although an enormous background prevented us from observing the \gamma-rays of interest in the single spectra, a few \gamma-rays emitted from the secondary fusion products ^153,154Er were able to be identified for the first time by setting optimum coincidence conditions.

[KH.002] Hish-spin Isomers in N=83 isotones.

High-spin isomers were reported systematically in N=83 isotones, ^143Nd, ^144Pm, ^145Sm, ^146Eu, ^147Gd, ^148Tb, ^149Dy,^150Ho and ^151Er. The excitation energies are close each other in a range of 8.5-9.0 MeV except for ^150Ho and ^151Er. Spins and parities are 49/2^+ and 27^+ for odd and odd-odd nuclei, respectively, except for ^150Ho and ^151Er. These isomers are considered to be of stretch coupled configurations and have an oblate shape. Experimental results were discussed comparing with a deformed independent particle model (DIPM) calculations. It is interesting to see whether this kind of isomers exist or not in both nuetron deficient side and rich side. We performed experimental reinvestigation in N=83 isotones ^149Dy(Z=66) and ^151Er(Z=68) using ^141Pr(^16O,p7n) ^149Dy and ^116Sn(^40Ar,5n) ^151Er reactions respectively. In this contribution, we report results of these experiments, and of comparison with DIPM calculations.

[KH.003] Role of the Z=64 core excitation in high-spin isomers in proton-rich N\sim82 nuclei

In the N=82 nuclei with Z>64, the high-spin isomers are observed systematically in the low-lying yrast states. The mechanism of these isomers was described at first by the \pi 0h_11/2^n configuration on top of the ^146Gd core. On the other hand, we clarified in the recent work that the proton excitation from the Z=64 core is important in the isomers. Especially the proton excitation plays a crucial role in the strong E2 hindrance in ^152Yb.

In the Z>64 region, (\pi 0h_11/2^2\nu 0h_11/2^-1)27/2^- isomers are observed in the even-Z, N=81 nuclei, while (\pi 0h_11/2^3\nu 1f_7/2)17^+ isomers are observed in the odd-Z, N=83 nuclei. The relevant B(E2) values of these isomers have a similar feature to the N=82 case, where a strong Z-dependence is observed. The strong E2 hindrance is observed in the Z\sim70 nuclei, ^151Yb and ^154Lu.

We carry out the shell model calculation for Z>64, N\sim82 nuclei, taking account of the Z=64 core excitation. We discuss the energy levels and the B(E2) values in Z>64, N=81\sim83 nuclei, taking special notice on the contribution of the Z=64 core excitation.

[KH.004] Particle-hole excited states in ^133Te

Excited states in neutron-rich ^133Te have been identified with Gammasphere array by measuring three and higher-fold prompt coincidence events following spontaneous fission of ^252Cf. Four types of particle-hole bands in ^133Te are identified. The yrast particle-hole states observed upto 6.2 MeV in ^133Te have characteristics quite similar to those in ^134Te. These states are interpreted as a result of coupling a neutron \nuh_11/2 hole to the ^134Te core. The low-lying 11/2^-, 15/2^-, 19/2^- and 23/2^- levels in ^133Te are assigned the configuration \pi(1g_7/2)^2\nu(1h_11/2)^-1 corresponding to the 0^+, 2^+, 4^+ and 6^+ states in ^134Te originating from \pi(1g_7/2)^2. The group of states observed at 5.214 MeV are the result of the neutron particle-hole excitation of the double magic core nucleus, ^132Sn.

[KH.005] Particle-hole states in ^131Sb and \pi7/2[413] band in ^115Ag

Excited states in neutron-rich ^131Sb and ^114,115,116Ag have been identified with Gammasphere by measuring high-fold prompt coincidence events following spontaneous fission of ^252 Cf. Several states discovered above 1231.8 keV in ^131Sb are proposed to have the configuration \pi1h_11/2\nu(1h_11/2)^-3\nu(2f_7/2) related to spherical shell-gaps at Z=50 and N=82. The ground rotational band with the band head spin and parity of 7/2^+ discovered in ^115Ag is proposed to have a configuration 7/2[413] originating from the \pi 1g_9/2 orbital coupled to the prolate core of ^114Pd. A side band identified in ^115Ag in the present work can have a (\pi 1 g_9/2)^-3(\nu 1h _11/2)^2 configuration related to spherical shell-gaps at Z=50 and N=64.

[KH.006] Shell model study of even-even Xe isotopes

Even-even Xe isotopes near A=130 region, which display the O(6) symmetry at low energy and backbending at high spin, are studied in the shell model framework. Our calculation uses the restricted single particle orbits 1d_5/2, 0g_7/2 for protons and 1d_3/2, 0h_11/2, 2s_1/2 for neutrons under the assumption of N=Z=64 subshell closure and the monopole- and quadrupole-pairing plus quadrupole-quadrupole interaction is employed. The energy levels for quasi-\gamma band as well as ground band and E2 transition rates are well reproduced in each nuclei. The first microscopic description of backbending at I^\pi=10^+ in this region is successfully given in this talk. It is also shown how backbending depends on the number of neutrons.

[KH.007] SBT transition strengths in ^112Te

High spin band structures in ^112Te have been studied using the ^58Ni(^58Ni,4p) reaction at 240 MeV with GAMMASPHERE (97 Ge) and the MICROBALL. A 1 mg/cm^2 ^58Ni target backed with 15 mg/cm^2 ^208Pb was used for DSAM measurements; transition probabilities will be extracted from the statistically well-defined lineshapes. Several high spin bands in ^112Te show E-E_RLD characteristics vs spin consistent with Smooth Band Termination (SBT). Two of these, a dipole band and a decoupled band, are linked to the yrast sequence, allowing definite spin assignments. The dipole band with E2 crossovers, which was observed up to spin I=38\hbar, agrees with SBT predictions for the [11,3] configuration to near termination with no apparent `shears' effects previously observed for single \pi g_9/2-hole bands near Z=50. The linked decoupled band, observed to I=46\hbar, is consistent with the [22,4] configuration. ([p_1p_2,n] refers to p_1 \pi g_9/2^-1, p_2 \pi h_11/2, and n \nu h_11/2). Transition strength confirmation of these SBT interpretations is important. Information on the favored [22,3] decoupled band in ^112Te at N=60 is possible in the complete analysis.

[KH.008] Nucleon pair approximation of the backbending phenomena

The backbending in the mass region around A=130 is investigated in terms of a new model of collective S and D nucleon pairs which couple with neutron single particle excitations coming from the h_11/2 orbit. Our purpose is to simultaneously reproduce the backbending and the low-lying collective states that display the gamma instability. The spectrum is satisfactory reproduced both at band crossing and at low spin. Good agreement of the electromagnetic moments is reached. The structure of the wave function indicates the dominance of the S and D collective pairs at low spin and the large contribution of h_11/2 neutron excitations at high spin.

[KH.009] Large-scale nuclear structure calculations in the Sn region

The Sn isotopes from nucleon number A = 100 to A = 132, with valence neutrons filling the N = 4 major shell, provide a challenging testing ground for large-scale shell-model calculations and the accompanying effective interactions. We report calculations using realistic effective interactions obtained from modern meson-exchange nucleon-nucleon potentials, using many-body perturbation theory. The general properties of the energy spectra, including the approximately constant pairing gap characteristic of generalized seniority, are well reproduced. The pairing property may be attributed to the singlet s-wave component of the nucleon-nucleon interaction. We are also probing the proton-neutron effective interaction in nuclei near mass number A = 100. This seems to be feasible where the protons and neutrons occupy the same valence shell, but far more difficult when they occupy different valence shells.

[KH.010] High-spin states in the N=50 nucleus ^87Rb

High-spin states in ^87Rb have been studied following the fission of two compound nuclei (^199Tl and ^197Pb) formed in different fusion-evaporation reactions. The Gammasphere array at LBNL was used to detect \gamma-ray coincidences. The level scheme has been extended above the previously known 1578~keV, 9/2^+ isomer by observation of many states up to \sim7.2~MeV excitation energy. Coupling of the odd g_9/2 proton to the yrast states in the ^86Kr core accounts for the first excited states observed above the 9/2^+ isomer. The level scheme of ^87Rb is also compared to excitations in ^85Kr and the ^89Y isotone. This work has been supported in part by the U.S. Department of Energy under Contracts No. W-7405-ENG-36 (LANL), FG02-91ER-40609 (Yale), W-7405-ENG-48 (LLNL) and AC03-76SF00098 (LBNL) and by the National Science Foundation (Rutgers).

[KH.011] Study of High Spin States in ^192Au

Doubly odd nuclei in the A\sim190 mass region are predicted to be good candidates to search for evidence of chiral symmetry breaking. Indeed evidence for a pair of chiral twin bands has recently been reported in ^188Ir [1]. To search for additional examples of this phenomenon we studied the high spin structures in the neighboring nuclei. ^192Au and ^190Ir were populated using the ^186W(^11B,5n) and ^186W(^11B,\alpha3n) reactions at a beam energy of 68 MeV. Gamma-ray coincidence measurements were performed using the YRAST Ball at WNSL, Yale University. Analysis is in progress and preliminary results will be presented. This work is supported by the U.S. D.O.E under Grant Numbers DE-FG02-91ER-40609, DE-FG02-88ER-40417, DE-FG02-96ER-40983 and by the DFG under Grant Number Pi 393/1-1 and by the Research Fund of The University of Istanbul under Grant Number 1582/19032001 . [1] D. Balabanski et al, to be published.

[KH.012] Low Energy E1 Transitions between \nu[642]5/2 and \nu[521]3/2 Bands in ^157Gd and ^159Dy

In N=93 odd-N rare-earth nuclei, two rotational bands of opposite parity based on \nu[521]3/2 and \nu[642]5/2 coexist near the yrast line. Since it is well known that even-even nuclei in this region have the K=0^- or 1^- octupole band, this situation gives us a good opportunity to study the low energy E1 transitions in odd-N rare-earth nuclei caused by the octupole collectivity. High spin states of ^157Gd and ^159Dy have been studied by using the multiple Coulomb excitation with Xe beam and the reaction ^150Nd(^13C, 4n) respectively. Rotational bands based on \nu[521]3/2 and \nu[642]5/2 have been extended to higher spin states in both nuclei. Several E1 transitions between them have been observed and discussed based on the Generalized Intensity Relations. Spin dependences of B(E1) values or B(E1)/B(E2) ratios are compared with those in other odd-A rare-earth nuclei. It is found that the B(E1)/B(E2) ratios for 47/2^- and 43/2^- states in ^159Dy become enhanced relative to those for the lower spin states.

[KH.013] Evidence for a discrete linking transition between the yrast superdeformed band and normal deformed states in ^151Tb

A discrete \gamma-ray transition linking the yrast superdeformed(SD) band of the ^151Tb nucleus to the normal deformed(ND) states has been found. This experiment was carried out using the EUROBALL IV \gamma-ray spectrometer combined with a BGO inner ball. The nucleus ^151Tb was produced by using the ^130Te(^27Al,6n) reaction. The ^27Al beam with energy of 155 MeV was provided by the Vivitron accelerator at IReS in Strasbourg. In the higher energy part of the triple gated spectrum on the yrast SD band, the 2815(1) keV peak was determined with a 3\sigma confidence level as a linking transition between the yrast SD and ND states. This \gamma line was confirmed by the reanalysis of the previous EUROGAM phase II data. From the sum of these two data, we have deduced the intensity of 2.1(4)% relative to that of the plateau region of the yrast SD band. In the triple gated spectrum with two gate on the yrast SD transitions and one gate on the 2815 keV one, 12 \gamma-rays of the yrast SD members in ^151Tb starting with the 854 keV one were clearly observed. The ND levels fed by the 2815 keV transition could not be determined preciously due to the lack of statistics.

The possible decay mode of this linking transition, using results of HF calculation, will be discussed.

[KH.014] Superdeformation in the doubly magic nucleus ^40_20Ca_20

The study of superdeformed rotational bands in atomic nuclei has been a major experimental and theoretical effort in the last 15 years. With the aid of the large sensitivity obtained by combining the latest generation of \gamma-ray spectrometer with charged particle detectors, study of high-spin states in light mass nuclei become available. In the spectroscopic investigation of Z\approx20 nuclei with the GAMMASPHERE spectrometer, a rotational band with 7 \gamma-ray transitions between spin 2 and 16 has been observed in the doubly magic nucleus ^40Ca. The measured transition quadrupole moment of the band indicates a superdeformed shape and more elongated than in the neighboring superdeformed nuclei. The features of this band and a second newly observed structure are described by cranked relativistic mean-field calculations.

Part K of program listing