High-spin yrast isomer in theN=83151Er nucleus

An highspin yrast isomer (T_1/2 ? 420ns, E>10.286 MeV, 67/2≤ I≤ 71/2) is observed in the¹⁵¹Er nucleus excited in the¹¹⁶ Sn+⁴⁰ Ca reaction, using the recoil shadow method.     

Identification and structure of yrast levels in theN=89 odd-odd158Tm nucleus

Excited levels have been identified in the odd-odd¹⁵⁸Tm nucleus produced in the¹⁵⁰Sm (¹⁴N,6n) reaction. The observed level structure based on an isomeric state (T_1/216±4 ns) is interpreted as a rotational aligned band.     

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.     

g-Factor of the first 2+ state in theN=82 nucleus138Ba

The 2 ₁ ⁺ level of¹³⁸Ba has been Coulomb excited with a 105 MeV³²S beam. The precession of the gamma ray angular distribution induced by the transient magnetic field when traversing a 2.14 mg/cm² iron foil, has been measured to be 15.6(1.7) mr. A value of g(¹³⁸Ba 2 ₁ ⁺ )=0.72(11) was obtained assuming for the transient field strength the Chalk River calibration. The field parametrization has been checked using the first 2⁺ state in¹⁵⁰Sm as probe. Theg value obtained is sensibly lower than predicted by large scale shell model calculations in a proton subspace. Theg-factor of the first 2⁺ state in¹⁴⁸Sm has been remeasured yieldingg(¹⁴⁸Sm 2 ₁ ⁺ )=0.246(22).     

High spin states in147Tb

The level scheme of¹⁴⁷Tb has been extended to 116 levels and 198 transitions in two experiments with the OSIRIS Compton suppressed detector array at the HMI Berlin with the reactions¹²²Sn(³¹P, 6n)¹⁴⁷Tb at 160 MeV and¹²⁰Sn(³¹P, 4n)¹⁴⁷Tb at 152 MeV. Evidence for a new high spin isomer atE=7664.1 keV with a life-time of the order of 2 ns has been found.     

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.     

Shell model yrast states of the N=83 nucleus 150Ho

Studies of delayed γ-rays from the reactions ¹²²Te(³²S, p3n) and ¹²¹Sb(³²S, 3n) have located high-spin excitations in ¹⁵⁰Ho up to a 92 ± 8 ns isomer at 2625 keV. The main yrast level sequence is found to agree closely with shell model predictions (based on an inert ¹⁴⁶Gd core) of level energies, isomeric half-life and deexcitation pathway for the $\text{(πh}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}\text{)}{}^3\text{vf}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ configuration.     

Description of high-spin isomers in theN=82 region

The high spin states and of those especially the yrast isomers are studied in the region near the rare earth nuclei. The deformation energy surface is calculated with a method modelled in principle according to a shape constrained cranked HF theory (CHF). In practice, the expectation value of the many-body Hamiltonian is calculated with cranked Nilsson functions. One finds rotation around a slightly deformed oblate or prolate symmetry axis in front or behind the rare earth region, respectively. Near the Hf isotopes strongly prolate deformed nuclei rotating around the symmetry axis are found in agreement with the knownK isomers in this region. These results are explained also qualitatively with the help of the MONA (Maximisation of theOverlap ofNucleonic wave functions byAlignment of single particle angular momenta) effect. In the second part high spin states and yrast isomers in theN=82 region are calculated in the cranked Hartree-Fock-Bogoliubov approach with four and six quasi-particle excitations. For the excitation energies in¹⁴⁶Gd and¹⁵²Dy and a measuredg factor one finds satisfactory agreement.     

Description of high-spin isomers in theN=82 region

The high spin states and of those especially the yrast isomers are studied in the region near the rare earth nuclei. The deformation energy surface is calculated with a method modelled in principle according to a shape constrained cranked HF theory (CHF). In practice, the expectation value of the many-body Hamiltonian is calculated with cranked Nilsson functions. One finds rotation around a slightly deformed oblate or prolate symmetry axis in front or behind the rare earth region, respectively. Near the Hf isotopes strongly prolate deformed nuclei rotating around the symmetry axis are found in agreement with the knownK isomers in this region. These results are explained also qualitatively with the help of the MONA (Maximisation of theOverlap ofNucleonic wave functions byAlignment of single particle angular momenta) effect. In the second part high spin states and yrast isomers in theN=82 region are caluclated in the cranked Hartree-Fock-Bogoliubov approach with four and six quasi-particle excitations. For the excitation energies in¹⁴⁶Gd and¹⁵²Dy and a measuredg factor one finds satisfactory agreement.     

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.     

Particle hole yrast states in146Gd and147Gd and the Z=64 shell closure

The level structures of¹⁴⁶Gd and¹⁴⁷Gd have been investigated by in-beamγ-ray ande ^? spectroscopy with (α, xn) reactions on enriched Sm targets. Detailed level schemes up to ～4 MeV, which differ radically from earlier schemes, are reported. The energy levels are characterized as particle-hole excitations using empirical single particle energies and two nucleon interactions. Analysis of pure 1p 1h proton excitations demonstrates that theZ=64 andN=82 energy gaps are about equally large.     

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.     

Decay energies of the low-spin states of147Tb and148Tb

Using¹¹²Cd(⁴⁰Ar,pxn) reactions andγ-ray spectroscopy, the EC/β ⁺ and EC(K)/β ⁺ probability ratios were determined for individualβ-transitions in the¹⁴⁷Tb(1.6 h)→¹⁴⁷Gd and¹⁴⁸Tb(60 min)→¹⁴⁸Gd decays. Under assumption of applicability of theoretical ratios for allowedβ-decays on the first-forbidden nonunique transitions studied here, the experimental ratios were converted into information on decay energies. Combining the results with the known masses of^147,148Gd andQ _α values of neutron-deficient holmium-to-lutetium isotopes, mass excesses could be determined for¹⁴⁷Tb,¹⁴⁸Tb, low-spin¹⁵¹Ho, low-spin¹⁵²Ho,¹⁵⁵Tm,¹⁵⁶Tm and¹⁵⁹Lu.     

New yrast excited states of the N = 84 nucleus 142Ce observed in deep inelastic reactions

Excited states of ¹⁴²Ce, populated in deep inelastic reactions of ⁸²Se projectiles bombarding ¹³⁹La target, have been studied up to medium spins using in-beam γ spectroscopy techniques. Three new levels have been identified at 2625, 2995, 3834 keV, and assigned as 8⁺, 9^(-) and 11^(-), respectively. These new yrast states follow closely the level systematics of the even mass N = 84 isotones. Their strctures have been discussed with the help of empirical shell model calculations. Received: 2 November 2001 / Accepted: 15 January 2002     

First observation of yrastγ-rays in theN=78 nuclei142Gd and141Eu

Excitation function measurements with α-particles and HI-beams clearly attribute the yrast cascade previously proposed for¹⁴²Gd to the¹⁴¹Eu neighbouring nucleus. The so far unknown yrast transitions of¹⁴²Gd have been identified. Coincidence measurements establish the high-spin level schemes of the two nuclei up to 3.2 MeV.     

High spin states in the nucleus150Tb

Excited states in the nucleus¹⁵⁰Tb have been investigated up to spin 39? and 13 MeV excitation energy using the¹³⁰Te(²⁷Al, 7n) reaction and the EUROGAM array. The theoretical interpretation of the observed states has been performed in the framework of the deformed independent particle model. The analysis of the decay out of the yrast superdeformed band indicates that normal-deformed states with spins between 24? and 28? are fed.     

Shell model yrast states in the many-particle nucleus 72 157 Hf85

In-beam- andn-measurements have for the first time identified excited states in the ₇₂ ¹⁵⁷ Hf₈₅ nucleus and have established its yrast levels up to 6.5 MeV and I=(51/2). The results of parameter-free 11-particle recoupling calculations in the framework of the shell model for the configurationsh ₁₁ ^2/8 f ₇ ^2/3 andh ₁₁ ^2/8 f ₇ ^2/2 h_9/2 are in excellent agreement with the observed levels up to 39/2 at 4.758 MeV. A weakly populated 52 ns 29/2⁺ yrast isomer at 2.876 MeV is assigned as the three-neutron configurationf_7/2h_9/2i_13/2.     

Decay of oriented149Nd and levels in theN=88149Pm nucleus

Levels in the¹⁴⁹Pm nucleus populated via¹⁴⁹Nd decay were studied using the low temperature nuclear orientation technique. The parent¹⁴⁹Nd nuclei were oriented in a gadolinium host. The normalized gamma-ray intensities were determined for twenty five transitions in the daughter¹⁴⁹Pm nucleus. The spins of the¹⁴⁹Pm levels at 288.2 and 425.3 keV were determined as 9/2 and 7/2, respectively, and for several other¹⁴⁹Pm levels the previous spin-assignments could be unambiguously confirmed. Substantial new information on multipole mixing ratios of gamma-transitions in¹⁴⁹Pm is presented. Low-energy level spacing in the¹⁴⁹Pm nucleus is found to be in reasonable agreement with recent particle-rotor model calculations including the variable moment of inertia.     

Two-proton neutron-hole yrast excitations in 66 147 Dy81

The level structure of theN=81 nucleus¹⁴⁷Dy has been studied byγ-ray spectroscopy following reactions of 230–250 MeV^58,60Ni beams on⁸⁹Y and^90,92Zr targets. Yrast and near-yrast levels in¹⁴⁷Dy above the known 59-s 11/2^? state are established up to ～3.7 MeV; they include isomeric levels at 2,681, 3,407 and 3,650 keV. Guided by the results of shell model calculations, we interpret most of the observed level as senioritythree states arising from the coupling ofs _1/2,d _3/2, andh _11/2 neutron holes withπh _11/2 ².     

High-spin yrast states in Ce

Yrast states up to spin 18 have been identified in ¹³²Ce. The energies and spins of the levels suggest the existence of two fairly well-behaved collective bands. The results are compared with those obtained in the rare-earth deformed region.