Abstract:Much improved data on the superdeformed band in rSgHg have been obtained in the study of the la~ 5n) reaction at 165 MeV. Energies of ten intraband transitions have now been determined and the decay into the yrast line has been delineated. A discussion of probable spins for the 1891-Ig superdeformed band raises questions about proposed procedures for assigning spins to superdeformed states. PACS: 23.20.Lv, 27.80.w The e:dstence of an "island" of superdeformed (SD) nuclei in the vicinity of A = 190 is now well established [1]. Among the many unanswered questions arising from the available results, those regarding the limits of the SD island in proton and neutron number and those regarding the possibility of assigning spins to the SD states have received considerable attention [1,2]. In an experiment designed for the study of superdeformation in 19~ Drigert et al. [3] found a weak SD band which was assigned to 189Hg on the basis of the available coincidence information. However, this information was rather fragmentary since, at the selected beam energy, the intensities of the measured transitions were very weak. We report here on a subsequent experiment designed to optimize the yield in the lS9Hg SD band. Additional transitions have been observed and the decay towards the yrast line has been delineated.The experiment was performed at the ATLAS superconducting linear accelerator at Argonne National Laboratory with the Argonne-Notre Dame BGO v-ray facility consisting of i2 Compton-suppressed germanium detectors (CSG) surrounding an inner array of 50 BGO ele-* Present address: GSI, W-6100 Darmstadt, Federal Republic of Germany ** Permanent address: Physics Department, Australian National University, Canberra, Australia *** Present address: Physics Department, University of Washington, Seattle, WA98195, USA ments. The 1S~ reaction at 165 MeV was selected to populate the highest spin states in lSgHg. Two stacked thin targets, totaling lmg/cm 2, of enriched (98%) 16~ were used. Only events where at least 4 array elements fired in prompt coincidence with a minimum of 2 CSG's were recorded. In the analysis, transitions between the highest-spin states were relatively enhanced by selecting V -V coincidence matrices with the condition that at least 10 of the array elements had fired. The final coincidence matrix contained approximately 10 s events.All the lines reported in ref. 3 have been observed in the present data as can be seen from fig. 1, where the sum of the spectra in coincidence with the three cleanest transitions is presented. The improved statistics results in a more precise determination of the transition energies in the SD band as well as in its extension by at least one, and more likely two, additional transitions. Furthermore, the SD v-rays are clearly in coincidence with known [4] yrast transitions in lS9Hg, some of which are indicated in fig. 1. Thus, there is no remaining uncertainty concerning the assignment of this band to lSgHg. This nucleus is the lightest Hg isotope for which superdeformati...