Superdeformed bands and ‘shears’ bands in 197Pb and 198Pb

Abstract

Excited states in 197 Pb and 198 Pb, produced in the 186 W ( 18 O,xn) reaction, were studied using the EUROGAM phase 2 γ ray spectrometer array. Two new superdeformed bands have been observed in 197 Pb, and the existence of a superdeformed band in 198 Pb has been confirmed. The increased statistics provided by the latest generation of arrays has resulted in the reordering of some of the previous known cascades of magnetic dipole transitions, with the first observation of the extremely weak E2 crossover transitions.High spin states in 197 Pb and 198 Pb nuclei [1,2] were produced in the 7n and 6n channels of the 186 W ( 18 O,xn) reaction at nominal laboratory bombarding energies of 110 and 115 MeV. Gamma rays were detected with the EUROGAM phase 2 spectrometer [3], with approximately 1.2 × 10 9 events collected in which at least four γ rays were in prompt time coincidence. The data were unpacked using the technique of Beausang et al. [4] and sorted into RADWARE format [5] cubes.Two new superdeformed (SD) bands, which were observed with an intensity of ∼ 0.2% and 0.1%, respectively, of the 197 Pb reaction channel, have been assigned to this nucleus as they were found to be in coincidence with lead x rays and several low-lying states [6]. These bands are seen more strongly in the 115 MeV than the 110 MeV data set, where the production of 198 Pb dominates. They are believed to be signature partner bands as crosstalk has been observed: lower band members from band 2 are seen in coincidence with band 1, and vice versa, with an intensity that decreases as the rotational frequency increases.The dynamic moments of inertia (J (2) = dI/dω) for the two bands in 197 Pb follow very closely those of bands 1 and 2 in 195 Pb [7], and a pair of similar bands have been reported in 193 Pb [8]. These bands are thought to be based upon the odd neutron occupying either the favoured or unfavouredsignature Present address: Department of Physics and Astronomy, McMaster University, Hamilton, ON L8S 4M1, CanadaFig. 1.Triple-gated quadruples spectra of the two SD bands in 197 Pb, with gates set on all the known transitions in the band. Insets show the relative transition intensities, corrected for E2 internal conversion, as a percentage of the largest peak intensity in the band. Also shown is the 1005 keV γ ray from 197 Pb components of an N=7 neutron intruder orbital. The expected alignment [9, 10] of j 15/2 neutrons is then prevented by Pauli blocking, producing a relatively constant J (2) . However, because there is less fission competition in the 197 Pb system, the new bands are observed to extend to higher rotational fre-

Intensity profiles of superdeformed bands in Pb isotopes in a two-level mixing model

“…However, SD bands have also been observed in other Pb isotopes, including 190 Pb [26] and 198 Pb [27]. Several different predictions of the excitation energies of SD bands in Pb have been made, but only a small number have considered all five even-even isotopes [28,29].…”

“…This means that the model predictions for 198 Pb are very similar to those for 196 Pb. Thus in order to reproduce the experimentally observed decay profile [27], the band in this nucleus must sample region II of Fig. 4 at the onset of decay and at at least one subsequent level.…”