Microscopic Study of Band Structures in98-102Sr and 100-104Zr Isotopes

The properties of the high spin states of the neutron-rich ^98-102Sr and ^100-104Zr isotopes have been studied using the projected shell model. In particular, the upbending phenomenon is investigated for these isotopes along the yrast line. The results show that the occurrence of upbending phenomenon is
attributed to the band crossing between ground state band and 2-qp neutron band having configuration 2νh_{11 / 2}[-3/2, 5/2], K=1. Furthermore, the neutron two-quasi-particle structure of side bands in ⁹⁸Sr and ¹⁰²Zr is discussed in this paper.     

Clustering in neutron-rich nuclei

Clustering in nuclei is discussed putting emphasis on the investigation of the role of nuclear clustering in neutron-rich nuclei. The subjects we discuss include clustering in neutron-rich Be, B and C isotopes, clustering in the island of inversion around N = 20, and clustering in the region with A ≈ 40. Be isotopes present us typical examples of clustering in neutron-rich nuclei not only in their ground band states but also in their excited band states, for which we show the analyses based on antisymmetrized molecular dynamics (AMD). Clustering in Be isotopes near neutron dripline is intimately related to the breaking of the neutron magic number N = 8. In this connection we report our study about the possible relation of the clustering with the breaking of the neutron magic number N = 20 in the island of inversion including ³²Mg and ³⁰Ne. Our discussion is not only about the positive parity states but also about negative parity states. Recently in the latter half of sd shell and in the pf shell many excited rotational bands with large deformation have been found to exist. Since the first excited K ^π = 0⁺ and K ^π = 0^- bands in ⁴⁰Ca have been regarded as constituting inversion doublet bands having the ³⁶Ar + α structure, and since the first excited K ^π = 0^- band in ⁴⁴Ti has been concluded to have ⁴⁰Ca + α structure through the α transfer reaction and by using the unique α optical potential on ⁴⁰Ca, it is important to investigate the role of α clustering in these newly-found rotational bands with large deformation. We will report the AMD study about this problem.     

Nuclear clusters and nuclear molecules

Clustering has long been known to be influential in the structure of ground and excited states of N=Z$N=Z$ nuclei. States close to the decay thresholds are of particular interest, as clustering becomes dominant. Recent studies of loosely bound light neutron-rich nuclei have focused attention on structures based on clusters and additional valence neutrons, which give rise to covalent molecular binding effects. These nuclear molecules appear only at the extremes of deformation, in the deformed shell model they are referred to as super- and hyper-deformed. The beryllium isotopes provide the first examples of such states in nuclear physics. Further nuclear molecules consisting of unequal cores and also with three centres can be considered. These arise in the isotopes of neon and carbon, respectively. Molecular states in intrinsically asymmetric configurations give rise to parity (inversion) doublets. Examples of recent experiments demonstrating the molecular structure of the rotational bands in beryllium isotopes are presented. Further experimental evidence for bands as parity doublets in nuclei with valence neutrons in molecular orbits is also analysed. Work on chain states (nuclear polymers) in the carbon isotopes is discussed. These are the first examples of hyper-deformed structures in nuclei with an axis ratio of 3:1. Future perspectives are outlined based on a threshold diagram for covalent nuclear molecules with clusters bound via neutrons in covalent molecular configurations.     

High spin investigation in 193Pb with EUROGAM 2: coexistence of superdeformed and dipole bands

The nucleus ¹⁹³Pb was populated via the ¹⁶⁸Er(³⁰Si, 5n) reaction at a beam energy of 159 MeV and studied with the EUROGAM II spectrometer. Five new dipole ΔI = 1 cascades have been found. The ¹⁹³Pb dipole bands are discussed in terms of microscopic HF+BCS calculations. The proposed configurations are based on a high-K two-quasiproton excitation coupled to rotation aligned quasineutrons. Parallel to the dipole bands the six superdeformed bands have been discussed in the framework of microscopic mean-field calculations. The bands are interpreted as three pairs of signature partners based on quasineutron excitations. Cross-talk transitions linking two signature partner superdeformed bands have been observed and, for the first time in lead isotopes, a mean B(M1)/B(E2) ratio value of 0.15±0.04 μ _N ² /e^2b2 has been extracted.     

Shape evolution for Sm isotopes in relativistic mean-field theory

The evolution of shape from the spherical to the axially deformed shapes in the Sm isotopes is investigated microscopically in relativistic mean-field theory. The microscopic and self-consistent quadrupole deformation constrained relativistic mean-field calculations show a clear shape change for the even-even Sm isotopes with N = 82-96. The potential surfaces for ¹⁴⁸Sm, ¹⁵⁰Sm and ¹⁵²Sm are found to be relatively flat, which may be the possible critical-point nuclei. By examining the single-particle spectra and nearest-neighbor spacing distribution of the single-particle levels, one finds that the single-particle levels in ¹⁴⁸Sm , ¹⁵⁰Sm, and ¹⁵²Sm distribute more uniformly.     

Qβ measurements of 158, 159Pm , 159, 161Sm , 160-165Eu , 163Gd and 166Tb using a total absorption BGO detector

Q _β values of the neutron-rich isotopes of ^160-165Eu and ¹⁶³Gd were measured for the first time using a total absorption bismuth germanate (BGO) detector, and previously obtained data on ^{158, 159}Pm , ^{159, 161}Sm and ¹⁶⁶Tb were re-analyzed. These radioactive sources were prepared by an on-line mass separator (Tokai-ISOL) following the ²³⁸U (p,f reaction. The deduced Q _β values are the following: 6085(80)keV for ¹⁵⁸Pm , 3805(65)keV for ¹⁵⁹Sm , 5460(140)keV for ¹⁵⁹Pm , 4705(60)keV for ¹⁶⁰Eu , 5065(130)keV for ¹⁶¹Sm , 3705(60)keV for ¹⁶¹Eu , 5575(60)keV for ¹⁶²Eu , 4690(70)keV for ¹⁶³Eu , 3170(70)keV for ¹⁶³Gd , 6430(70)keV for ¹⁶⁴Eu , 5800(120)keV for ¹⁶⁵Eu , and 4695(70)keV for ¹⁶⁶Tb . Moreover, the deduced mass excesses and two-neutron separation energies ( S _2n values) were compared with those of the atomic mass evaluations and theoretical predictions.     

Evidence for deformation in <Superscript>113-116</Superscript>Cd isotopes

High-spin states in ^113-116Cd have been investigated with the fusion-fission reaction ²⁸Si +¹⁷⁶Yb at 145 > MeV. The experiment has been performed with the Eurogam2 array. New rotational bands based on two quasi-particle states have been observed for even-even cadmium isotopes. A new level scheme based on the 11/2⁻ isomeric state is proposed for ¹¹⁵Cd and the one of ¹¹³Cd has been extended to spin (31/2⁻). The decoupled bands identified in both odd-A nuclei are interpreted as being built upon a low-Ωh_11/2 quasi-neutron configuration. Microscopic Hartree-Fock + BCS calculations confirm the prolate deformation in this mass region especially for the odd cadmium isotopes. Received: 21 December 1999 / Revised version: 16 February 2000     

Suppression of band crossing in the neutron-rich nuclei 172, 173Yb due to the absence of a static pair field

High-spin states in the neutron-rich nuclei ^172,173Yb have been populated in a ¹⁷⁰Er(⁷Li,(p,d,t)xn) incomplete-fusion reaction and the emitted γ-radiation was detected with the GASP array. The signature partners of the 7/2⁺[633] rotational band of the odd-N ¹⁷³Yb isotope have been newly established and were observed up to spin values of (45/2⁺) and (43/2⁺), respectively. The ground-state band of the even-even nucleus ¹⁷²Yb has been observed up to a spin value of (22⁺). No band crossings were found in these bands. To explain this observation, it is proposed that the static pair field is absent, considering that the neutron odd-even mass differences reach for these nuclei very small values and that the band crossing is absent in cranked shell modell calculations without pairing. The results indicate, however, that strong dynamic correlations are still present.     

Green’s function method in the problem of complex configurations in fermi systems with pairing

The Green’s function method is used to derive general equations for describing effects of pairing in Fermi systems where there are two types of interaction, two-particle and quasiparticle-phonon interaction. These equations generalize Bardeen-Cooper-Schrieffertheory to the case of complex configurations involving “strong” phonons. In the approximation of weak coupling to phonons, realistic equations that make it possible to describe excited states of nonmagic even-even nuclei with allowance for a single-particle continuum and complex configurations of the two quasiparticles ? phonon type are formulated for the first time. These equations are solved for an isovector E 1 resonance in the stable isotope ¹²⁰ Sn and in the unstable isotopes ^104,132Sn. It is shown that complex configurations must be taken into account in order to describe E1 excitations—in particular, in a broad energy region around the nucleon binding energy.

     

Nuclear structure and nuclear moments studied by the shell model and the interacting boson model

Magnetic dipole and electric quadrupole moments are discussed in terms of the interacting boson model (IBM) and the shell model. From the viewpoint of the IBM, systematic variations of magnetic moments will be discussed by analyzing data of Xe and Ba isotopes. Magnetic and quadrupole moments of various states of Sm and Os isotopes are discussed, pointing out an open problem in the magnetic moments of Os isotopes. The importance of measuring the quadrupole moment of O(6) or -unstable nuclei is emphasized by the example of¹²⁸Xe. The structure of neutron-rich unstable nuclei will be studied in terms of the shell model, by paying attention to the break-down of the closed shell structure, for instance, the collapse ofN=20 closed shell withZN=20. The magnetic moment of the anomalous ground state of¹¹Be is another topic of this discussion, and it is studied in terms of a new theoretical framework called variational shell model.     

Observation of signature inversion in odd-odd 178Ir

High spin states in ¹⁷⁸Ir have been studied via the ¹⁵²Sm(³¹P, 5nγ)¹⁷⁸Ir reaction through excitation functions, X-γ and γ-γ-t coincidence measurements. According to the band structure characteristics and the measured intraband B(M1)/B(E2) ratios, two rotational bands are identified and assigned to be associated with the πh _9/2⊗νi _13/2 and πh _11/2⊗νi _13/2 configurations, respectively. Both bands show the signature inversion feature. Received: 23 March 2000 / Accepted: 31 July 2000     

On deexcitation of β-and γ-rotational bands in nonspherical even-even nuclei

New possibilities for phonon-structure investigations in even-even nonspherical nuclei have been found. The multipole-mixing parameters δ for E2 + M1 transitions from β-and γ-rotational bands to ground-state rotational band are used. The new possibilities are based on the relationship of the negative sign of δ with negative magnetic momentum of some neutron Nilsson states, invariability of δ sign for γ transitions between rotational bands, and enlarged probabilities for M1 transitions only in the presence of certain two-quasiparticle configurations in initial and final states. The text was submitted by the authors in English.     

In-beam gamma-ray spectroscopy of 116Te

High-spin states of the even-even ¹¹⁶Te were studied by in-beam g-ray spectroscopy using the ¹⁰³Rh (¹⁶O, p2n) fusion-evaporation reaction at a beam energy of 80 MeV. γ-γ coincidence and γ-γ angular correlation analyses were employed for determining the level scheme of ¹¹⁶Te. Levels up to I^π = 27^? and several new states were established. We identified two rotational bands with negative parity, one of which was newly established in the present work. We suggest that such two bands be associated with two proton [(g_7/2) ? (h_11/2)] and two neutron [(d_5/2) ? (h_11/2)] configurations, respectively.     

Rotational bands and signature inversion phenomena in πh9/2⊗νi13/2 and πi13/2⊗νi13/2 structures in odd-odd 176Ir

The ^109,111,113Rh nuclei have been produced as fission fragments in the fusion reaction ¹⁸O + ²⁰⁸Pb at 85 MeV. Their level schemes have been built from gamma-rays detected using the Euroball IV array. High-spin states of the neutron-rich ^111,113Rh nuclei have been identified for the first time. Several rotational bands with the odd proton occupying the πg _9/2, πp _1/2 and π(g _7/2/d _5/2) sub-shells have been observed. A band of low-energy transitions has been identified at excitation energy around 2 MeV in ^109,111Rh, which can be interpreted in terms of three-quasiparticle excitation, πg _9/2νh _11/2νg _7/2/d _5/2. In addition another structure built on states located at low excitation energy (608 keV in ¹¹¹Rh, 570 keV in ¹¹³Rh) points out that, as already observed in the lighter isotopes ^107,109Rh, triaxial deformation plays a role in the neutron-rich Rh isotopes well beyond the mid-shell. Received: 15 July 2002 / Accepted: 9 October 2002 / Published online: 3 December 2002 RID="a" ID="a"e-mail: porquet@csnsm.in2p3.fr RID="b" ID="b"Present address: CSNSM IN2P3-CNRS and Université Paris-Sud 91405 Orsay, France. RID="c" ID="c"Present address: CEA/Saclay, DSM/DAPNIA/SPhN, 91191 Gif-sur-Yvette Cedex, France. Communicated by D. Schwalm     

Identification of Rotational Band in Doubly Odd 170Ta

High spin states in ¹⁷⁰Ta have been studied via ¹⁵⁹Tb(¹⁶O,5nγ)¹⁷⁰Ta reaction through excitation functions, K X-γ and γ-γ-t coincidence measurements. Three rotational bands have been identified among which one coupled band and an unfavored δI=2 E2 transition sequence are newly found in this work. The possible quasiparticle configurations of these bands are discussed. Received: 7 November 1997     

Study of Collective Rotational Bands in 179Pt

High-spin states in ¹⁷⁹Pt have been studied experimentally using the ¹⁴⁹Sm(³⁵Cl, p4n) fusion-evaporation reaction at beam energies of 164—180MeV. Measurements of γ-ray excitation function, X-γ coincidences and γ-γ-t coincidences were carried out with 13 BGO(AC) HPGe and 3 LOAX detectors. Based on the measured results, three rotational bands built on intrinsic states assigned to the 1/2^－[521], 5/2^－[512] and 7/2⁺[633] Nilsson configurations have been established for ¹⁷⁹Pt. It has been observed that the aligned angular moment in the 1/2^－[521] band increases suddenly around hω=0.27MeV. The big signature splitting has been observed in the 7/2⁺[633] band. These phenomena are discussed by referring to the systematics of the level structure in the Pt isotopes and isotones of ¹⁷⁹Pt.     

Collective bands in 104,106,108Mo

We have used our analysis of γ-γ-γ data (5.7 × 10¹¹ triples and higher folds) taken with Gammasphere from prompt γ rays emitted in the spontaneous fission of ²⁵²Cf to study the collective bands in ^104,106,108Mo. The one-phonon and two-phonon γ-vibrational bands and known two-quasiparticle bands in neutron-rich ^104,106Mo were extended to higher spins. The one-and two-phonon γ-vibrational bands have remarkably close energies for transitions from the same spin states and identical moments of inertia. Several new bands are observed and are proposed as quasiparticle bands in ^104,106Mo, along with the first β-type vibrational band in ¹⁰⁶Mo. The quasiparticle bands have essentially constant moments of inertia near the rigid-body value that indicate blocking of the pairing interaction. Candidates for chiral doublet bands in ¹⁰⁶Mo are strong. These are the first reported chiral vibrational bands in an even-even nucleus. The text was submitted by the authors in English.     

High-spin states in neutron-rich Dy isotopes populated in deep-inelastic reactions

High-spin states in neutron-rich Dy isotopes, populated in deep-inelastic processes produced by the interaction of 234 MeV ³⁷Cl ions with a ¹⁶⁰Gd target, have been studied using the highly sensitive EUROBALL IV gamma-ray detector array. The previously known level schemes for ^{159,160,161,162}Dy have been extended to significantly higher spin ( ? 30?) and the i _13/2 band crossing in ¹⁵⁹Dy has been observed for the first time. The experimental results are discussed within the framework of cranked shell model and projected shell model calculations with particular reference to the observed delayed band crossing in ¹⁶²Dy. Received: 20 October 2000 / Accepted: 16 January 2001     

Exotic cluster structure in light neutron-rich nuclei

The cluster structure of C isotopes is investigated using a microscopic α+α+α+n+n… model based on the molecular orbit (MO) model. The stability of the linear chain of 3α with respect to the breathing mode and the bending mode for various neutron configurations is investigated. The combination of the valence neutrons in the π- and σ-orbits is promising to stabilize for these modes, and the excited states of ¹⁶C with the (3/2 _π ^- )²(1/2 _σ ^- )² configuration for the four valence neutrons are one of the most promising candidates for such structure. Furthermore, the equilateral-triangular shape of 3α surrounded by valence neutrons is suggested for ¹⁴C. The 3^-, 4^-, and 5^- members of this rotational band appear around the ¹⁰Be+α threshold, and these calculated states correspond to the experimentally observed 3^- state (9.80 MeV) and 4^- state (11.67 MeV). A positive-parity rotational band (0⁺, 2⁺, 4⁺) also arises around this threshold energy, and these results suggest that the picture of inversion doublet structure works also in neutron-rich nuclei.     

Deformed shell model for T = 0, 1 and 2 bands in <Superscript>52</Superscript>Fe and <Superscript>72</Superscript>Kr

Deformed configuration mixing shell model based on Hartree-Fock states with extension to include isospin projection (DSMT) for two- and four-particle configurations (generated by particle-hole excitations) is applied to study the structure of the low-lying T = 0, 1 and 2 bands (or levels) in the even-even N = Z nuclei ⁵²Fe and ⁷²Kr. The pf-shell KB3 interaction for ⁵²Fe and a modified Kuos interaction for ⁷²Kr are employed in the calculations. In this first application of DSMT with four-particle T projection, low-spin (J 10) members of the T = 0, 1 and 2 bands in ⁵²Fe are compared with experiment including the known E2 transition strengths. The agreement between DSMT and experiment is reasonably good. Similarly, the low-spin members of the observed (prolate) yrast band in ⁷²Kr are also well described by DSMT.