Non-yrast Levels Research Articles

Nuclear shapes studied with low-energy Coulomb excitation

Coulomb excitation is one of the rare methods available to obtain information on static electromagnetic moments of short-lived excited nuclear states, including collective non-yrast levels. It is thus an ideal tool to study shape coexistence and shape evolution throughout the nuclear chart. Historically, these experiments were limited to stable isotopes, however the advent of new facilities, providing intense beams of short-lived radioactive species, has opened the possibility to apply this powerful technique to a much wider range of nuclei. Here, we present some recent complex Coulomb-excitation studies and use the example of superdeformed states in 42Ca to demonstrate the sensitivity of the method to second-order effects such as relative signs of electromagnetic matrix elements and quadrupole moments.

Collective band structure of 166,168Hf in IBM and DPPQ models

166,168Hf are the lightest isotopes of Hf, for which the spectral information for non-yrast levels is now available from recent experiments. The algebraic Interacting Boson Model IBM-1 is employed to reproduce their level structures and to predict the E2 transition probabilities. The pairing plus quadrupole model is used to predict their spectra and E2 transition rates and the static moments in a microscopic approach. The spin assignments I π of new levels and their K-band structures are studied. The validity of the inclusion of 166,168Hf as members of a U(12) super group is studied using various empirical observables. The potential energy surfaces for the two isotopes are compared and the filling of the nucleons in Nilsson orbits is analyzed, to yield a consistent comprehensive view of the spectra of the two Z = 72 isotopes.

Candidate chiral bands in 198Tl

High-spin states in Tl-198 were studied using the Au-197(alpha, 3n) reaction. The level scheme was considerably extended including two new bands and several non-yrast levels. One of the new bands is possibly a chiral partner to the yrast pi h(9/2) circle times vi(13/2)(-1) band. Two-quasiparticle-plus-triaxial-rotor model calculations suggest an aplanar orientation of the total angular momenta for these bands, thus supporting possible chirality.

Generalization of theNpNnscheme to nonyrast levels of even-even nuclei

In this paper we present the systematics of excitation energies for even-even nuclei in two regions: the 50$<$Z$\le$66, 82$<$N$\le$104 region, and the 66$<$Z$<$82, 82$<$N$\le$104 region. Using the $N_pN_n$ scheme, we obtain compact trajectories for the ground band as well as quasi-$\beta$ and quasi-$\gamma$ bands. This suggests that the $N_pN_n$ scheme is useful even if one extends it to non-yrast levels, and thus can serve as a general tool to disclose new types of structural evolution for higher excitations, besides the yrast states which have been investigated extensively. It is highlighted that deformations in non-yrast quasibands of nuclei with $Z \sim 80$ and $N \sim 104$ are often very different from those in the ground bands.

Isomerism in 96Ag and non-yrast levels in 96Pd and 95Rh, studied in β decay

The β decay of 96Ag ( Z=47, N=49) was investigated by measuring positrons, X rays as well as β-delayed protons and γ rays. The γ radiation was studied by means of germanium detectors and a NaI total-absorption spectrometer. Two β-decaying isomers in 96Ag were established with half-lives of 4.40(6) and 6.9(6) s and tentative spin–parity assignments of (8 +) and (2 +), respectively. For both isomers, the intensities of β transitions to low-lying levels of 96Pd ( Z=46, N=50) and β-delayed proton decays to levels in 95Rh ( Z=45, N=50) were measured. Several new 96Pd levels were firmly established. The level energies, their γ decays and the Gamow–Teller decay of 96Ag are compared to shell-model predictions. A new low-lying level in 95Rh was found at 680 keV excitation energy. Through a comparison with low-lying states of N=50 isotones, this level is interpreted as the first excited 7/2 + state built on the proton 9/2 + ground state. The assignments of further excited states in 95Rh are discussed.

First observation of nonyrast levels in 103Zr and level systematics of N = 63 Sr, Zr, and Mo isotones.

The {beta} decay of the very-neutron-rich nucleus {sup 103}Y has been studied at the isotope separator IGISOL, allowing for the first time the observation of nonyrast levels in its daughter {sub 40}{sup 103}Zr{sub 63}. The level structure is similar to that of its isotones {sup 101}Sr and {sup 105}Mo, suggesting a large ground-state deformation of {beta}{approx_equal} 0.4. Level systematics allows for new level assignments in both {sup 103}Zr and {sup 105}Mo. {copyright} {ital 1996 The American Physical Society.}

Dynamic microscopic basis for IBM-2: A new approach

A dynamic approach, based on deformed Hartree-Fock solution of a nucleus, is suggested for obtaining correlated identical nucleon pair wave function for neutrons and protons. Expressions for single pair energies and two pair interaction matrix elements amongst the neutron and proton pairs in the microscopic fermion basis are presented. These matrix elements define the IBM-2 Hamiltonian through Marumori mapping. The entire procedure is illustrated by obtaining the IBM-2 spectra of20Ne,44Ti,60Zn and94Mo and comparing them with shell model (SM) and/or experimental results. The Yrast levels given by our calculations match well with those of the SM and the experimental results for all the four nuclei, while the non-Yrast levels do not barring the case of94Mo. This is due to the loss of isospin symmetry for light nuclei in IBM-2. These results are discussed in detail.

Low-spin and isomeric states in 198Pb

Low- and medium-spin states in 198Pb are studied using the reaction 198(3He, 3n) at laboratory energy 27.5 MeV. The in-beam measurements comprise γ-ray singles, γ-RF timing, γ-ray angular distributions and γ-γ coincidences. Seven new levels are located and assigned spins and parities. The previously reported lifetimes are unambiguously assigned: T1/2(5−) = 50.4 ± 0.5 ns, T1/2(7−) = 4.19 ± 0.10 μs and T1/2(9−) = 240 ± 15 ns. The corresponding transition rates B(λ) are deduced. The new non-yrast levels are compared to the existing systematics. A tentative excited (collective) band structure is proposed and compared to a similar one in 196Pb. Systematic trends in E1, E2 and E3 transition rates in light Pb isotopes are discussed.

Particle-hole multiplets in146Gd from in-beam studies of non-yrast levels

Non-yrast levels of146Gd have been investigated by in-beamγ-ray and conversion electron spectroscopy following the144Sm(α, 2n) reaction at 25.7 MeV, an energy which was determined to provide optimum population of levels above the yrast line. The detailed146Gd level scheme includes twenty-one newly identified levels, many of which can be characterized in terms of specific proton particle-hole configurations. Several twoparticle-twohole states involving the 3− phonon have also been identified.

High-Spin States in 204Bi and the Question of Many-Nucleon Configuration Mixing

The properties of high-spin levels in 204Bi have been studied using conventional in-beam γ-ray and conversion-electron spectroscopy on the products of the reactions 205Tl(α, 5n) 204Bi and 203Tl(α, 3n) 204Bi. The yrast cascade is followed to a 19+ level at 3808 keV and several non-yrast levels in the spin interval J = 9-16 are reported. The energies of the levels in 204Bi were calculated within the shell-model frame of multi-nucleon states, and the agreement is found to be good. A previously reported T1/2 = 1 ms isomeric state is shown to have a main configuration πh9/2ν(j-2)0+(i13/2-2)12+f5/2-1 and a spin-parity 17+. The configuration mixing for the neutron-hole components of the wave functions is discussed.

The decay of mass-separated 99g, mAg

The decays of neutron-deficient 15 s 99Ag and 124 s 99gAg nuclides have been investigated at the Leuven Isotope Separator on-line facility. Sources were produced by the 92Mo( 14N, 2p5n) and nat Zr( 14 N, x n) reactions. Positron, conversion electron, X- and γ-ray singles spectra together with γ-γ coincidence measurements have been performed on mass-separated samples. The 1 2 − isomeric level in 99)Ag decays with a 163.6 keV isomeric transition to a 7 2 + level at 342.6 keV. Of the 106 γ-rays observed in the β + EC decay of the 99Ag ground state, 68 γ-rays (92 % of the γ-ray intensity) have been placed in a proposed level scheme. The relative variation of the J π = 9 2 + and 1 2 − levels in odd-mass Ag isotopes has been calculated using a residual proton-neutron delta interaction. The occurrence of decoupled bands in the odd Pd isotopes is discussed. Detailed calculations for 99Pd have been carried out in the framework of the Nilsson model (strong-coupling wave functions) using a strong Coriolis band mixing. Energy spectra for high-spin states, wave functions and low-lying non-yrast levels (below E x = 1 MeV) are also discussed.

Shell-model study of the N = 49 isotones

The N = 49 isotones 88Y, 89Zr, 90Nb, 91Mo, 92Tc and 93Ru are studied in the framework of the shell model with valence nucleons occupying the 2 p 1 2 and 1 g 9 2 single-particle levels. The residual proton-neutron effective interaction is determined by a least-squares fit to the experimental energies of states in these nuclei with well established spins and parities. We examine the effects of requiring the effective interaction to conserve seniority and model-space isospin. It is found that the existing body of experimental data is consistent with conservation of T = 1 seniority but that the data call for small isospin mixing that can be traced to modelspace truncation effects. The resulting shell-model wave functions are used to calculate energy levels, single-nucleon spectroscopic factors and electromagnetic transition rates. These model wave functions also provide a basis for a qualitative discussion of the weak coupling multiplet structure that persists in the low-lying states of all of these nuclei. In general, all observed properties of yrast levels and analog states are well reproduced by our model as are the positions of many non-yrast levels. Within this model space unique 0 + anti-analog states emerge in 88Y, 90Nb and 92Tc, but the 9 2 + and 1 2 − anti-analogs are highly fragmented in the even- Z nuclei.