Superdeformed States Research Articles

Yrast superdeformed band in194Pb:JπandEx

The yrast superdeformed band in ${}^{194}$Pb has been populated using the ${}^{174}$Yb${(}^{25}$Mg,5n)${}^{194}$Pb reaction at ${E}_{\mathrm{beam}}=130$ MeV. Decay $\ensuremath{\gamma}$ rays were detected using the GAMMASPHERE array at the 88-Inch Cyclotron. Twelve $\ensuremath{\gamma}$-ray transitions have been observed directly linking three members of the ${}^{194}$Pb yrast superdeformed band to low-lying normal deformed levels. Anisotropy measurements indicate that these linking decays include $E$1, $M$1, and mixed $M1/E2$ multipolarities. The radiative widths deduced are very inhibited, typically $B(E1)\ensuremath{\sim}{10}^{\ensuremath{-}8}$ Weisskopf units (W.u.) and $B(M1)\ensuremath{\sim}{10}^{\ensuremath{-}5}$ W.u. Without recourse to a priori assumptions ${J}^{\ensuremath{\pi}}{=6}^{+}$ and ${E}_{x}=4878.4(3)$ keV have been unambiguously assigned to the lowest-lying observed superdeformed state (the state populated by the 170-keV intraband transition). The intensity of the observed primaries accounts for 21(2)% of the superdeformed band population.

Collective and quasiparticle excitations in superdeformed 190Hg.

Superdeformed (SD) states of $^{190}\mathrm{Hg}$ have been studied with the Eurogam Phase 2 \ensuremath{\gamma}-ray spectrometer using the $^{160}\mathrm{Gd}$${(}^{34}$S,4n) reaction. Two new excited SD bands have been found and identified as belonging to this nucleus, bringing the total number of SD bands in $^{190}\mathrm{Hg}$ to 4. One of the new bands has a dynamic moment of inertia that is very similar to that of the yrast SD band of $^{190}\mathrm{Hg}$ and most other SD bands in the A\ensuremath{\sim}190 region. In contrast, the other band has a dynamic moment of inertia which is mainly constant as a function of rotational frequency and exhibits a dramatic increase at the lowest frequencies. The observed dynamic moments of inertia are compared with the results of random phase approximation calculations based on the cranked shell model. Finally, the known excited SD band has been extended towards lower frequencies and new transitions have been found linking this band to the yrast SD band. The extracted B(E1) values of the new linking transitions give further support for the possible octupole vibrational character of this band. \textcopyright{} 1996 The American Physical Society.

Single step links of the superdeformed band in 194Pb: a measure of the absolute excitation energy, spin and parity of the superdeformed states

The EUROGAM array has been used to investigate the decay out of the yrast superdeformed (SD) band in 194Pb. Six single step decays from the lowest observed SD states to low-lying states at normal deformation (ND) have been identified. From this observation, the excitation energy of the SD band in 194Pb is established at 4877 ± 1.5 keV for the 6 + SD state. The most probable spins and parities of all members of the SD band are also deduced assuming that the SD states have even spin and positive parity.

Coupling of the giant dipole resonance to low lying octupole modes - generator coordinate method study

The effect on the properties of low lying octupole modes of the coupling with the giant dipole resonance is studied within the Generator Coordinate Method. Results are presented for 152Sm which is deformed in its ground state and for the superdeformed state of 190Hg. A basis is first generated by Hartree-Fock+BCS calculations with constraints on the octupole and dipole moments. The same Skyrme SkM ∗ effective interaction used in the mean field is then diagonalised by the GCM. For the octupole K = 0 mode, the effect of the coupling is marginal and the dipole properties of low lying states are satisfactorily described by pure octupole calculations. For the K = 1 mode, the dipole-octupole coupling slightly reduces the E1 transition strength.

In-beam γ-ray spectroscopy with GASP

The performance of the multi-detector array GASP is described in detail with emphasis on the improvement due to utilization of ancillary detectors. GASP has been running for three years producing interesting new data on different nuclear structure topics. The study of the decay-out of superdeformed bands in the A=130 mass region is presented. The sudden disappearance of the superdeformed bands in the odd133, 135, 137Nd isotopes is explained by the Total Routhian Surface calculations through a change of the nuclear shape which is microscopically related to the transfer of the valence neutron from a N=6 to a N=4 Nilsson orbital. A transient fieldg-factor measurement in the superdeformed band of133Nd will also be presented. A meang-factor of\(\bar g = 0.31(8)\) has been determined at the I*=41/2+ superdeformed state. The experimental value is compared with theoretical predictions and supports the assignment of the superdeformed band to thev[660]1/2+ Nilsson intruder orbital.

Superdeformed band g-factor in 133Nd

Abstract A g-factor measurement in the superdeformed band of 133Nd has been performed by applying the transient field method in coincidence mode with the multidetector array GASP. The 133Nd nuclei were populated by the 104Pd(32S,2pn) reaction at a bombarding energy of 135 MeV. Polarized Gd has been used as ferromagnetic host. A mean g-factor of g = 0.31(8) has been determined at the I π = 41 2 + superdeformed state. The experimental value is compared with theoretical predictions and supports the assignment of the superdeformed band to the ν[660] 1 2 + Nilsson intruder orbital.

Band Staggering in Some Superdeformed States and Intrinsic Vortical Motion.

The staggering recently observed in some superdeformed rotational bands could be explained by a collective model including two quantized quantities. One of the latter being the angular momentum $I$, we propose for the other the Kelvin circulation $J$ associated with an intrinsic uniform vortical motion. This explanation is consistent with the observed strong $E2$ intraband transitions. Some preliminary qualitative assessment of the relevant mass parameters is made.

Superdeformation and high spin states

The current experimental status of the study of nuclei at high spin is reviewed with an emphasis on superdeformed states.

Feeding of superdeformed bands: The mechanism and constraints on band energies and the well depth.

Entry distributions leading to normal and superdeformed (SD) states in $^{192}\mathrm{Hg}$ have been measured. A model, based on Monte Carlo simulations of \ensuremath{\gamma} cascades, successfully reproduces the entry distribution for SD states, as well as all other known observables connected with the population of SD states. Comparison of experimental and model results, together with the measured SD entry distribution, suggest that the SD band lies 3.3--4.3 MeV above the normal yrast line when it decays around spin 10 and that the SD well depth is 3.5--4.5 MeV at spin 40.

Radiation originating from unresolved superdeformed states in 149Gd

Using γ-γ correlation techniques, unresolved γ radiation associated with superdeformed (SD) excited states has been investigated for the nucleus 149Gd populated via the 124Sn( 30Si, xn) reaction at 155 MeV bombarding energy. the intensity distrib ution has been measured as a function of rotational frequency. A “quasi-damped” region of excitation energy where the identity of individual SD bands melts is suggested by the relatively small value of the damping width Γ rot required by the fit to the experimental data.

Pairing vibrations and stability of superdeformed states

Within the generator coordinate method, we study the influence of pairing vibrations on the stability of the superdeformed state in 194Hg. We find an increase of the density of collective states at small deformation above 5 MeV and a large mixing of the superdeformed state with states of the first well. Compared with calculations without pair vibrations, our results predict an earlier depopulation of the superdeformed band.

A model for the decay out of superdeformed bands

The decay out of superdeformed, rotational bands is studied in terms of the spreading of the superdeformed strength on states corresponding to normal configurations. The mixing of the superdeformed state depends on the tunneling through the potential energy barrier separating the two shapes. Simple, yet accurate estimates of the decay probability are presented in terms of the transmission coefficient of the barrier, and the electromagnetic width and level density of the normal states.

Collective structure of Iπ=0 + shape isomers in the 190,192,194Hg isotopes

Structure studies based on either the Strutinsky method or self-consistent Hartree-Fock (HF) and Hartree-Fock-Bogoliubov (HFB) mean field theories suggest that superdeformed (SD) states might well develop at large quadrupole deformation (i.e. β⩾0.5) in mercury isotopes. In this work, we predict the existence of I π =0 + shape isomers which might take place at excitation energies of 4.4, 5.4 and 6.9 MeV in 190Hg, 192Hg and 194Hg, respectively. These SD levels are obtained by solving the Griffin-Hill-Wheeler equation in the gaussian overlap approximation. Inputs for that collective hamiltonian are tensors of inertia as well as potential energy surfaces which are deduced from constrained HFB calculations based on the finite-range, density-dependent effective force of Gogny.

Superdeformation and other phases at very high spin

Recent advances in the physics of nuclei at high angular momentum are reviewed. In particular the new phenomena observed using the latest generation of γ-ray Escape Suppressed Spectrometer (ESS) arrays are discussed. The breath of current high spin nuclear physics is illustrated by an overview of recent data on quasiparticle alignments, band crossings and band terminations; high spin shell structure; lifetime measurements; magnetic properties; decay of high K isomers. The depth of the subject is illustrated by a more detailed discussion of both discrete superdeformed states and also the decline of static pairing effects at high spin. Plans for new γ-ray spectrometers are mentioned and brief speculations made on the future directions of high spin physics.

Escape suppressed spectrometer arrays: A revolution in γ-ray spectroscopy

The design and performance of Escape Suppressed γ-ray Spectrometer Arrays are described. These arrays have revolutionised the field of γ-ray spectroscopy with their significant improvement in signal to background ratios enabling very weak photon lines to be detected in the presence of many stronger γ-rays. This enables the properties of nuclei to be studied at the highest spins and for very weak channels to be detected. The highlights of the new physics achieved with these arrays are described. These include: the observation of discrete superdeformed states up to spin 60 kh: the systematic studies of aligned many quasi-particle states in rotating nuclei; band terminations and phase changes at high spins; classical rotations and the disappearance of static pairing; correlations in continuum radiation; the spectroscopy of proton rich nuclei at and near the N=Z line; fission-fragment spectroscopy; the measurement of sub-Coulomb reaction cross-sections and the associated input angular momentum.