Backbending in theN=96 isotones

The backbending in the even-even,N=96 isotones can be quantitatively accounted for by the rotation-alignment of the spins of neutrons in i 13/2 orbits, as shown by comparing the aligned angular momentum and relative Routhian for thes-bands in these isotones and for the i 13/2 bands in the corresponding isotopes withN=91. The influence of protons on this backbending situation is shown to be indirect, acting through a change of the nuclear deformation, which yields a change of the moment of inertia of the g.s. band and of the non-rigid character of the rotation. The experimental data on theN=96 and 97 isotones are in reasonable agreement with cranking model calculations. Possible reasons for the inhibition of backbending in the h 9/2 proton bands in the odd-Z, N=96 isotones, all related to a change of deformation, are presented.     

Decay of 209At to levels in 209Po

The electron-capture decay of ²⁰⁹At has been investigated and present data define twenty-one levels in ²⁰⁹Po. The multipolarities of thirty-six transitions have been deduced and combined with data from recent reaction studies to assign spins and parities to levels. Evidence is presented to identify seniority one and seniority three states below 2.3 MeV which arise from couplings of the one-neutron hole with two-proton configurations. A calculation using experimental data from neighboring isotopes in the lead region to approximate residual interactions was found to give remarkable agreement with the experimental level structure below 1.6 MeV. First-forbidden electron-capture transition rates to even parity levels at 2.3 and 2.9 MeV in ²⁰⁹Po are discussed with reference to the $\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{and i}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$. seniority one neutron states.     

Neutron hole states in theN=81 nuclei

The energy spectra of theN=81 nuclei have been calculated by coupling the neutron single-hole motion to the quadrupole vibrations of the nuclear surface. The wave functions are used to calculateM1 andE2 moments. Gamma-ray branching ratios are extensively discussed and compared with the many experimental data on ₅₄ ¹³⁵ Xe₈₁, ₅₈ ¹³⁹ Ce₈₁ and ₆₀ ¹⁴¹ Nd₈₁. Also spectroscopic factors for (p,d) reactions have been calculated and compared to experimental single-neutron pick-up reactions. The excitation energy and the lifetime of the isomeric stateπ ^π, decaying by anM4 transition, are studied, in particular in their dependence on the number of extra protons. The behaviour of the centroid of the neutron single-hole energies as a function of the number of extra protons is calculated with a delta force for the neutron-proton interaction.     

Seniority-three yrast states in theN=82 nucleus147Tb

Firm spin-parity assignments for the high-spin states up to 3.5 MeV in the one-proton nucleus¹⁴⁷Tb were obtained from¹⁴⁴Sm(⁶Li, 3n) in-beamγ-ray and conversion electron measurements. The energies of these two-particle one-hole excitations were calculated from the shell model with empirical nucleon-nucleon interaction energies. The calculated energy splittings agree well with experiment, whereas the theoretical excitation energies disagree by ?1 MeV if recently measured ground state masses are used.     

On the yrast two proton-two neutron hole states in208Po

High-spin levels in²⁰⁸Po, populated in the²⁰⁸Pb(α,4n)-reaction, were studied usingα-particles in the energy region 41–51 MeV. The energies of levels above the 6⁺ level have an uncertainty of about 10 keV due to the fact that the 8⁺→6⁺ transition has not been observed so far, but this transition has previously been established to be converted neither in theK-shell nor in theL-shells. It was found that the yrast cascade ofγ-rays from a 19⁺ level at 5896+ε keV feeds levels of lower spin which all can be explained as originating from two proton-two neutron hole configurations. In the higher part of the cascade it is mainly the neutron holes which change their configuration, while the lower part of the cascade is dominated by changes in the proton configuration. The yrast levels in the angular momentum regionJ=8–19 vary practically linearly with energy in the region 1.5–6 MeV. No isomeric traps were found above the 11^? level at 2699+ε keV.     

Neutron halos in the excited states for N=127 isotones

Properties of the ground states and the excited states of N=127 isotones are investigated by using the nonlinear relativistic mean field theory with the interactions PK1. By analyzing the rms of proton and neutron, the single particle energies of valence nucleon and the density distributions of neutron, proton and the last neutron, it can be found that there exists a neutron halo in the excited states of 3d5/2, 4s1/2 and 3d3/2 in ²⁰⁹Pb. It is also predicted that there exists a neutron halo in the excited states of 3d5/2, 4s1/2 and 3d3/2 in ²⁰⁷Hg, ²⁰⁸Tl, ²¹⁰Bi and ²¹¹Po.     

Neutron halos in the excited states for N=127 isotones

Properties of the ground states and the excited states of N=127 isotones are investigated by using the nonlinear relativistic mean field theory with the interactions PK1. By analyzing the rms of proton and neutron, the single particle energies of valence nucleon and the density distributions of neutron, proton and the last neutron, it can be found that there exists a neutron halo in the excited states of 3d5/2, 4s1/2 and 3d3/2 in 209Pb. It is also predicted that there exists a neutron halo in the excited states of 3d5/2, 4s1/2 and 3d3/2 in 207Hg, 208Tl, 210Bi and 211Po.     

Inhibited transitions and isomeric states in the N = 50 isotones

Measured E2 transition rates in the N = 50 isotones (⁹⁰Zr–⁹⁴Ru) are satisfactorily reproduced by a shell model 0699 0 with proton configurations (p_1/2g_9/2)ⁿ. The inhibited 8⁺→6⁺ E2 transition in ⁹⁴Ru and its connection with seniority mixing is discussed. Isomeric states are predicted in ⁹¹Nb and ⁹³Tc. 0699     

Shell model yrast states in theN=79 nuclei141Sm and143Gd

In (α,5n) in-beam γ- and electron measurements we have established high-spin states in the 3-neutronhole nuclei¹⁴¹Sm and¹⁴³Gd. Observed energy levels below 4 MeV are interpreted in terms of shell model configurations where the odd h_11/2 neutron hole couples to the N=80 core excitations containing one h_11/2 proton particle or one h_11/2 neutron hole. Higher-lying yrast states are built on a 23/2? level at Rd 3.2 MeV probably involving the πh ₁ ^2/2 guh ₁₁ ^2/?1 configUTa-tion.     

Nuclear orientation study of the decay of208At (T 1/2=1.63 h) and209At (T 1/2=5.41 h) implanted in iron host

We have measured the angular distribution of gamma-rays in 208,209Po following the decay of the mass-separated isotopes 208,209At polarized at low temperatures in an iron host. Multipole mixing ratios were deduced for 14 and 9 gamma-ray transitions in 208Po and 209Po, respectively, and unambiguous spin-assignments were made for several levels in both nuclei.     

Shell model study of high spin states in theN=50 nucleus93Tc

High spin states in theN=50 nucleus⁹³Tc were reinvestigated by using the reaction⁶⁴Zn (³⁵Cl,4p 2n) at a beam energy of 140 MeV. This was done particularly with a view to observe anyγ rays upto 2.7 MeV which may have been missed in our earlier study where the experimental conditions were set to observeγ rays upto 2 MeV. We found four newγ rays of energy: 2484, 2164, 2130 and 69 keV. We have placed theseγ rays in the level scheme and it now gets extended to 49/2^?. Though there is no substantial change in the level scheme, placing theγ rays in the level scheme has resulted into two important conclusions: (1) We have performed shell model calculations for⁹³Tc nucleus within a model space which encompasses an enlarged proton configuration and allows for the excitation of the neutron across theN=50 core. The excitation of a single neutron across theN=50 core satisfactorily explains the new level scheme. (2) The energy of the 17/2^? isomeric state is now unambiguously placed at 2185 keV.     

Microscopic description of the collective states in N = 70 isotones on the IBM basis

Low-lying collective quadrupole states in four N = 70 isotones are analyzed within interacting boson model (IBM1) using parameters calculated on the basis of a microscopic quasiparticle theory that considers variation of the superconducting and collective quadrupole properties with increasing excitation energies and spins.     

Structure of theN=59 nucleus97Sr: coexistence of spherical and deformed states

A band with a rotational pattern based on a state at 585.1 keV has been identified in theN=59 neutron-rich nucleus⁹⁷Sr. Its properties lead to the [422] 3/2 Nilsson-orbital assignment for the band head. There is evidence for a second band with the head at 644.7 keV and the configuration [541] 3/2. Since the ground state and the lowest excited levels are spherical, shape coexistence is established for⁹⁷Sr. A deformed nature of several levels at 500–600 keV results also from QRPA-model calculations. The structure of the low-lying spherical levels has been studied in the frame of the IBF model. The results of the present investigations lead to a better understanding of theN=59 isotones which constitute the link between the spherical and deformed nuclei atA～100 as a species with shape coexistence but without any indications of particular softness.     

Yrast bands in N = 91 isotones

The in-beam spectroscopy on N = 91 isotones has been carried out using the ¹²C + ¹⁵⁰Nd reaction. The νi _13/2 and νh _11/2 bands of ¹⁵³Sm have been extended up to 33/2⁺ and 31/2^-, respectively. Two new γ-rays have been located on the top of the unfavored band with νi _13/2 configuration in ¹⁵⁷Dy. Two identical relationships have been established in the low-spin region of the yrast νi _13/2 configuration between ¹⁵³Sm and ¹⁵⁵Gd, and between unfavored bands of ¹⁵⁵Gd and ¹⁵⁷Dy. Here all these nuclei have the same neutron number N = 91. Received: 28 June 2000 / Accepted: 20 October 2000     

MO X-ray angular distributions from 1 GeV208Pb+208Pb and209Bi+209Bi collisions

The Doppler shift and the anisotropy of the MO X-rays above the separated-atomK-lines have been measured in²⁰⁸Pb+²⁰⁸Pb and²⁰⁹Bi+²⁰⁹Bi collisions at beam energies between 4.2 and 4.8 MeV/u. The Doppler shift is found to be consistent with a source moving with 90% of the center of mass velocity of the colliding system. One peak is observed in the anisotropy spectrum. This peak might be attributed to transitions from the continuum into the minimum of the 2p _1/2 σ MO or from the 3p _1/2 σ and 2p _3/2 σ MO's into the 2 _sσ MO near the united atom limit. Radiative electron capture and secondary electron bremsstrahlung could be excluded as major sources of the observed radiation. Contrary to lighter systems no anisotropy corresponding to transitions into the 1 _sσ orbital could be observed.     

New high-spin states of 147Nd and 145Ce: Octupole correlation in the N = 87 isotones

High-spin states of the N = 87 nuclei, ¹⁴⁷Nd and ¹⁴⁵Ce, have been populated in the ¹²C + ²³⁸U and ¹⁸O + ²⁰⁸Pb fusion-fission reactions at 90 MeV and 85 MeV bombarding energy, respectively. The emitted γ-radiation was detected using the Euroball III and IV arrays. High-spin states of the ¹⁴⁷Nd isotope have been identified for the first time. The high-spin yrast and near-to-yrast structures of the ¹⁴⁵Ce nucleus have been considerably extended. The newly observed structures, discussed by analogy with the neighbouring isotones, show the coupling of an h_9/2 neutron to the quadrupole and octupole excitations of the core.     

High-spin states in89Nb,88Zr and88Nb with a shell-model description of theN=48 andN=47 nuclei

The reactions⁸⁹Y(α,4nγ),⁸⁹Y(α,p4nγ) and⁸⁹Y(α,5nγ) were used to populate high-spin states in, respectively,⁸⁹Nb,⁸⁸Zr and the previously unstudied nucleus⁸⁸Nb. These states were deduced via in-beam gamma-ray spectroscopy. The results of a shell-model study ofN=48⁸⁸Zr,⁹⁰Mo,⁸⁷Y,⁸⁹Nb andN=47⁸⁷Zr,⁸⁸Nb nuclei are compared to experiment.     

High-spin states in 208Pb

High-spin states in ²⁰⁸Pb have been studied by γ-ray-spectroscopy methods in deep inelastic reactions induced by beams of ²⁰⁸Pb, ¹³⁶Xe and ⁷⁶Ge beams on a thick ²⁰⁸Pb target. The 11⁺ ₂ state and new γ-transitions between the one-particle one-hole states of highest spin have been found and electromagnetic matrix elements verified. High-spin states of two-particle two-hole structure have been detected for the first time. The results are compared to shell model calculations with realistic interactions in the complete Kuo-Herling space. Received: 31 October 2000 / Accepted: 26 March 2001     

Double-octupole states in 208Pb

The states of 208Pb are calculated in a 24 orbit model space which includes all excitations up to two particles and two holes beyond the closed shell. All of the known low-lying states are reproduced by this model. The observed distributions of electromagnetic excitation strength from the ground state to low-lying excited states are well described by the model. The double-octupole excitation strength is calculated and found to be concentrated in 0+, 2+, and 4+ states around 5.2 MeV, but it is fragmented over many 6+ states, in agreement with recent experiments.