First lifetime measurement of dipole collective bands in neutron-deficient lead nuclei

Abstract

Lifetimes of states in two A/ = l bands in ^^*Pb have been measured using the Doppler-shift attenuation method. The in-band reduced transition probabilities are ~l-2 Weisskopf units, assuming magnetic dipole transitions. The measured lifetimes in conjunction with the partial level scheme support an oblate collective interpretation for these structures.Neutron-deficient Pb isotopes with a closed proton shell have long been good candidates for studies of excitations involving only a few active particles. In fact, low-lying structures in these isotopes can be described reasonably well in terms of two-and four-quasiparticle excitations [ll. One long-standing question is the possible onset of collective structures at high angular momentum in these isotopes. Collective states in the second minimum in the potential energy surface are known in >92j94,i96,i98p|^^ ^^^ they have been identified as prolate superdeformed bands [2][3][4][5]. Very recently, bandlike [6-10] Ml structures have been observed in I94j97,i98j99,200pb ^^d '^^Hg. These structures show similar properties [i.e., B(M\)/ ^(£2)'-20(/iyv/^b)^, little signature splitting, and small dynamic moments of inertia] to the A/1 bands [11] in the mass-130 region. Three of these papers [7][8][9] argue that the M1 structures in Pb are collective oblate, although no lifetime information was given to confirm this. In this Letter, we report for the first time lifetime measurements on two such high-spin first-well bands in a neutrondeficient Pb isotope. The lifetimes and partial level scheme obtained in this measurement do point to the interpretation of these structures as collective oblate.Experiments were carried out at the High Energy Resolution Array of the Lawrence Berkeley Laboratory 88-Inch Cyclotron. For the present measurements, 20 Compton-suppressed Ge detectors and a 40-element bismuth germanate central ball were used. The number(angle) of the Ge detectors with respect to the beam direction were the following: 4(37°), 2(51°), 4(79°), 2(103°), 4(123°), 2(152°), and 2(154°). Each detector was 15.5 cm from the target and subtended 82 msr. High-spin states in '^^Pb were populated via the reaction •^^Sm(^^Ca,4rt) at £'(^^Ca)=205 MeV. A thin selfsupporting target and two backed targets were bombard-ed. The thin target was a stack of three -0.5-mg/cm^thick ^^"^Sm foils; the backed targets were -l-mg/cm^thick ^^"^Sm, evaporated onto -12-mg/cm^ backings of Pb and Au. A total of -200x10^ doubly coincident events were collected for each target. The results from the thin-target measurement were used to identify new bands in '^^Pb and determine the feeding patterns of these bands. Backed-target results were used to establish the connection between the new levels and the known low-lying isomeric states [l] in ^^^Pb, to suggest the possible location of previously unobserved isomeric states, and to determine lifetimes using the Doppler-shiftattenuation method (DSAM).The partial level scheme of *^^Pb deduced from the present study is shown in Fig. 1. The transition ordering is b...

Two proton high-spin excitations and dipole bands in192Hg

“…Several collective structures have been observed and are interpreted in terms of an oblate core and different numbers of rotation-aligned quasi-neutrons. On the other hand, number of rotational dipole bands have been reported in several Pb [3][4][5][6][7][8][9][10][11][12][13], Bi [14] nuclei, and recently in two Hg isotopes [15,16]. Proton n (h11/2) 2 excitations have been invoked for irregular sequence in 192Hg [1], and also in 194'191Hg [1,2] nuclei where the alignment of proton-hole pairs drives the nucleus towards prolate shape and non-collective excitations.…”