A. Siivola

The Isomeric 9/2+ and the Lowest 7/2- State in the Weakly Deformed N = 89 Nuclei

The decays of the isomeric 9/2+ states which have been produced by (d, pγ) and (3He, xnγ) reactions, have been investigated in three N = 89 nuclei. The half-lives have been determined to be 5.1 ± 0.3 ns in 149Nd and 3.5 ± 0.4 μs in 153Gd. In 155Dy a 5.5 ns half-life was found, but we were unable to assign it to the 9/2+ state. The calculated E1 transition probability from the 9/2+ state has been used as a probe in suggesting the following main components for the lowest-lying 7/2- state: 7/2 5/2-[523] in 149Nd, 7/2 3/2-[521] in 153Gd and 7/2 3/2-[521] in 155Dy. The corresponding 7/2- state in 151Sm is proposed to be 7/2{1}3/2-[532] + 5/2-[523]{1}. All calculations have been performed using a particle-rotor model with a nonspheroidal Woods-Saxon potential.

Low-spin states in151Gd

The level structure of the 87-neutron nucleus151Gd has been investigated by studying the EC- andβ+-decay of151Tb. Gamma-ray and conversion electron spectra as well as gamma-gamma coincidence spectra were measured by using semiconductor spectrometers and a high-capacity two-parameter recording system. The proposed decay scheme contains several new energy levels in151Gd, among them a 5−/2 state at 427 keV. To explain the low-energy level structure, a small stable deformation is assumed for151Gd. Using a single-particle-plus-rotor model based on a generalized Woods-Saxon potential, complete mixing of the shells 1h9/2, 2f7/2 and separately 1i13/2 is observed. By these mixings the correct level order and approximately correct excitation energies up to 1 MeV are reproduced.

Low-spin states of153Gd observed in the decay of153Tb

Level structure of the 89-neutron nucleus153Gd has been investigated by studying theEC-decay of isotope-separated153Tb sources with several semiconductor detectors. In addition to singles gamma and electron spectra,γ-γ ande−-γ coincidences were investigated by using Ge(Li) and Si(Li) spectrometers and a high-capacity two-parameter recording system. In all, 191γ-rays are assigned to the decay of the 2.4d153Tb. The proposed level scheme of153Gd containing most of the observed transitions shows a very high density of low-spin states (45 states below 1.5MeV). Spin and/or parity assignments based mainly on coincidence data and measured transition multipolarities are proposed for a majority of these states. Nilsson model classifications of some levels are discussed. On the basis of logf t values the spin and parity of the ground state of153Tb are suggested to be 3/2+ or 5/2+.

In-beam study of excited states in the nucleus150Nd

Energy levels and decay scheme of the nucleus150Nd produced by using 10 MeV deuterons have been investigated with efficientγ-γ coincidence equipment during an in-beam150Nd bombardment. In addition to the ground band, aKπ=0− octupole band is strongly populated allowing the identification of its three lowest members. The assignments of the levels are based on coincidence data and on systematic comparison with the (d, d′) cross sections known from three other doubly-evenN=90 nuclei. The decay properties of the 1/2+ levels in149Nd populated in this study through the (d, tγ) reaction are also discussed.

Low-energy rotational bands in the nucleus155Eu

Excited states in the nucleus155Eu have been produced during in-beam bombardments of a154Sm target with3He beams at 22 and 27 MeV. Decay gamma rays were detected using coincidence equipment optimized for low-energy photons. The level scheme is based on the observedγγ-coincidence relationships combined with the information on relative intensities. Tentatively suggested spin assignments follow from the apparent rotational character of the nucleus. Experimental observations are compared with predictions calculated from a particle-rotor model with a nonspheroidal Woods-Saxon potential.

Low-lying levels in the nuclei151Nd and155Sm

Decay properties of excited states in the151Nd and155Sm nuclei produced by 10 MeV douterons have been investigated with in-beam gamma-gamma-coincidence equipment during bombardment of the150Nd and154Sm targets. The results largely confirm the data obtained earlier for these nuclei. The (d, p γ) reaction channel favors population of odd-parity low-spin levels, whose energies follow the rule of regular band structure for well-deformed nuclei. The level schemes are interpreted with calculations using an axial particle-rotor model with a Woods-Saxon potential.

Remeasurement of the Lifetime of the Isomeric 9/2+ State in 155Dy

The isomeric 9/2+ state at 132.2 keV in the nucleus 155Dy has been populated through 3He bombardment (E = 27 MeV) of a 155Gd target. The half-life of the the 9/2+ level has been determined as 51 ± 3 ns. The decay modes of the 11/2-, 234.2 keV level have been confirmed. For both levels the hindrance factors calculated with the particle-rotor model have been compared with other predictions available.

Low-lying excitations in the odd-odd nucleus154Eu

The doubly odd nucleus154Eu was produced during in-beam bombardments of a154Sm target with3He and deuteron beams at 27 and 10 MeV. The resulting gamma-rays were investigated using prompt and delayed gamma-gamma-coincidence equipment. The half-life of the isomeric 2+ level was determined as 2.2±0.1 μs. The partial level scheme, including numerous previously unidentified excitations, can be divided into two separate groups of levels. The results provide evidence for the existence of a very regular ground band and two rather regularK=3 level structures, whose configurations are closely related to it. In addition, several rotational sequences built on the band heads with otherK values have been deduced. Our interpretation of the level scheme disagrees with the previous conclusion that theN=91 nucleus154Eu might have a stable deformation only in its ground state configuration.