Spectroscopy of Pb and Tl via evaporation residue detection

The competition between nuclear shapes in the lead region below mass 190 has been topical for many years. A recent example is the triple shape coexistence found in Pb (Z = 82) — i.e. the presence of spin 0 states associated with spherical, oblate, and prolate shapes [1]. More recently, such coexistence has been also found in neighboring nuclei with Z 82 viz. Au and Hg (see e.g. Ref. [2]) and in Pb [3]. The continuing interest in this topic is also documented by two very recent theoretical papers [4, 5]. The prolate shape with a calculated quadrupole deformation ∼ 0.25 dominates with increasing spin, as evidenced by the “decoupled” nature of the rotationallike yrast band and the size of its moment of inertia. However, as far as the 182−188Pb nuclei are concerned, not until very recently has the electric quadrupole ( 2) character of any yrast band been established through a measurement of -ray angular distributions or correlations ( -ray angular distributions for Pb are reported in Ref. [3]). As implied in the paragraphs above, a prolate yrast band is also observed in several proton-rich Tl nuclei, namely in those with odd mass. However, there is lack of information on the yrast structures of the neigboring even-mass Tl nuclei. Specifically, one would like to learn about the evolution of the “semi-decoupled” bands in these nuclei, based on h9 2 and i13 2 quasi-particles with oblate ( 2 ∼ 0.15) character [6] as one depletes the neutron shell. The present paper reports new spectroscopic information for the yrast states in both Pb and Tl. The two nuclei were populated in the Ti + Nd reaction via the fusion-evaporation exit channels 4n and p3n, respectively. The experiment was carried out as a commissioning run for the evaporation residue detector system HERCULES [7], an auxiliary device for Gammasphere [8]. The nucleus of main interest was Pb, the in-beam study of which requires the use of a “filter” device to separate the rays of the evaporation residues from those of the amply produced fission products and other sources of background [9, 10]. Under the experimental conditions chosen, the yield of Pb was found to be low relative to other evaporation residue yields. Nevertheless, it was possible during a rather short time period of running to consolidate and extend the level scheme for Pb compared to a recent experiment [10]. The present result includes the measurement of -ray angular distributions for most of the yrast transitions, establishing their 2 multipolarity, and the observation of transitions from non-yrast to yrast states. Another side channel of interest in the present reaction led to Tl for which the onset of a band structure seen in both signatures is reported.