Excited Kπ Research Articles

Octupole correlations in the structure of02+bands in theN=88nuclei150Sm and152Gd

Knowledge of the exact microscopic structure of the 01+ ground state and first excited 02+ state in 150Sm is required to understand the branching of double β decay to these states from 150Nd. The detailed spectroscopy of 150Sm and 152Gd has been studied using (α,xn) reactions and the γ-ray arrays AFRODITE and JUROGAM II. Consistently strong E1 transitions are observed between the excited Kπ = 02+ bands and the lowest negative parity bands in both nuclei. These results are discussed in terms of the possible permanent octupole deformation in the first excited Kπ = 02+ band and also in terms of the “tidal wave” model of Frauendorf.Received 10 December 2012DOI:https://doi.org/10.1103/PhysRevC.87.044333©2013 American Physical Society

New development of the projected shell model and description of low-lying collective states in transitional nuclei

Description of the interplay between different nuclear shapes is an interesting but challenging problem. The original projected shell model (PSM) is applicable to nuclei with fixed shapes. We extend the PSM by superimposing (angular-momentum- and particle-number-) projected product wave functions in the spirit of the generate coordinate method. With this development, the Gd isotopes across the N = 90 region are studied, and the results indicate spectroscopic features of shape phase transition with varying neutron number. In order to illustrate the shape distribution in microscopic wave functions, we introduce a deformation representation and show that the collectively excited Kπ = 0+ states in the Gd isotopes have characters of shape vibration.

High-resolution spectroscopy of decay pathways in the12C(12C,γ) reaction

The decay branchings of a resonance in the 12C(12C,γ)24Mg reaction at Ec.m. = 8.0 MeV have been studied with high resolution using the Gammasphere array. Radiative capture residues were discriminated from scattered beam and the dominant evaporation channels using the fragment mass analyzer coupled to a multistage Parallel Grid Avalanche Counter (PGAC)/ion chamber system. The clean selection of residues has allowed the population of excited states up to 10 MeV in 24Mg to be examined in detail. Strong feeding of an excited Kπ=0− band is observed. A Jπ = 4+assignment to the resonance is strongly favored.

EFFECTS OF VIBRATIONAL AND PAIRING CORRELATIONS ON ISOSPIN MIXING IN THE HIGHER TAMM-DANCOFF APPROXIMATION

In the framework of the Higher Tamm-Dancoff Approximation which allows for a consistent treatment of pairing and quadrupole vibrational correlations preserving the particle-number symmetry and the Pauli principle, we compare the isospin content of ground and excited Kπ = 0+ states including either type of the above mentioned correlations using an approximate projection-after-variation technique for isospin. The pairing correlations are described with a delta interaction using the same strength in the T = 0 and T = 1 channels, whereas the vibrational correlations are treated with an isoscalar quadrupole-quadrupole interaction. As a first study, we apply this approach in the N = Z, even-even 16 O nucleus. For the same amount of correlations measured by the diffusivity of Fermi surface or depletion of particle-hole vacuum in the ground-state solution, we find very similar isospin mixing in the ground state for both types of correlations, but different patterns for the distribution in the calculated solutions of the expectation value of the square of the isospin operator.

Nuclear levels in 228Th populated in the decay of 228Pa (II)

The electron-capture decay of 228Pa to levels in 228Th was studied using mass-separated sources and a γ-ray detection system consisting of five Compton-suppressed Ge detectors. A total of 87 levels were observed up to an excitation energy of 2 MeV which are connected by approximately 500 γ-ray transitions. The complete octupole quadruplet, three excited Kπ=0+ bands and two Kπ=2+ bands were identified below 1.4 MeV. The observed level structure is compared to calculations within the quasiparticle-phonon nuclear model. The surprisingly good agreement indicates that 228Th has less transitional character than hitherto assumed.

Collective excitations built on the 2+ γ state in 168Er

To examine the collectivity of the previously proposed K π=4+γγ-two-phonon state in 168Er at an excitation energy of 2056 keV and of the first excited K π=0+ band at 1217 keV (b-band), a Coulomb excitation experiment has been carried out at the Heidelberg-Darmstadt Crystal Ball spectrometer. The B(E2) value connecting the 4+ γγ state to the γ-band was remeasured to be B(E)2,4+ γγ→ 2+ γ)=(600± 130) e2fm4, which almost exhausts the harmonic expectation. In addition, the B(E2) values connecting the band head of the lowest lying excited 0+ b band to the 2+ g and 2+ γ states were measured to be B(E2,2+ g→ 0+ b)=(4.4±0.6) e2fm4 and B(E2, 2+ γ→ 0+ b)=(30.4±4.6) e2fm4; the latter is almost a factor of 10 smaller than the B(E2,2+ γ→ 0+ g) value, which shows that the 0+ b band head has no significant contribution of the K π= 0+γγ-two-phonon state in its wave function.

Variety of vibrational excitations in 228Th

The level structure of 228Th was studied using mass-separated 228Pa sources and a γ - γ coincidence setup of 5 Compton suppressed Ge detectors. The complete octupole quadruplet, three excited Kπ = 0+ bands and two excited Kπ = 2+ bands were identified at excitation energies elow 1.4 MeV.

Nuclear levels in228Th populated in the decay of228Pa

The electron-capture decay of228Pa to levels in228Th has been studied using mass-separated sources and high-resolutionγ-ray and conversion-electron spectroscopy. A level at 979.5 keV is assigned as 2+ member of a second excited Kπ=0+ band, with the 0+ band head at 938.6 keV. The 2+ and 3+ members of a second excited Kπ=2+ band at 1153.5 and 1200.5 keV, which decay by strongE0 transitions to the 969 keVγ-vibrational band, are confirmed. In addition we tentatively propose a Kπ=1+ band at 944 keV. The Kπ=0−, 1− and 2− members of the octupole quadruplet are confirmed, and theγ decay of these levels is analysed in an approach, in which the mixing of the octupole bands by the Coriolis interaction is taken into account. It is suggested that octupole correlations might be important for theE1 transition moments. A total of 29 levels is observed between ∼1.4 and ∼2.0 MeV, for which the nuclear structure, and the possible assignment to rotational bands, is unclear.

16O(α, γ) 20Ne capture γ-ray measurements on the excited Kπ = 0 + bands in 20Ne near 7 MeV

Radiative widths have been measured for the E α = 3074 keV ( J π = 0 +), 3363 keV ( J π = 2 +), 3872 keV ( J π = 2 +) and 5368 keV ( J π = 4 + z) resonances in the 16O(α,γ) 20Ne reaction, corresponding to level energies at 7191±3, 7420±1, 7826±3 and 9029±3 keV respectively. The radiative widths for decay to the 2 +, first excited state and the 0 +, ground state of 20Ne are: 7191 → 1633 (4.35±0.75 meV), 7420 → 1633 (29±4 meV), 7420 → 0 (≦3.0±0.6 meV), 7826 → 1633 (11.7±1.6 meV), 7826 → 0 (57±7 meV) and 9029 → 1633 (340±42 meV). The following upper limits on the decay widths to the 4 +, 4246 keV level were also obtained: 7420 → 4246 (< 2.2 meV), 7826 → 4246 (< 2 meV) and 9029 → 4246 (< 6.8 meV). The experimentally known interband electromagnetic matrix elements are compared with nuclear model predictions. The excitation energy region from 10.2 to 10.8 MeV in 20Ne has been searched for a possible E x → E(4 +, 4246 keV) transition and an upper limit of ωγ < 0.6 eV can be placed on such a transition.