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Microsecond isomers in the very proton-rich TV ==82 nuclei 153 Lu and 154 Hf have been identified by yray spectroscopy following mass analysis of 102 Pd + 245 MeV 54 Fe reaction products using the Daresbury Recoil Separator. The decays of the isomers, interpreted as {nh\\ii) n states, are characterized. Examination of reduced E2 transition rates between (/rA 11/2)" states in the TV = 82 isotones indicates that the half-filling of the Khun subshell occurs just below Z=71.

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Positron production in 238 U 1 232 Th and 238 U 1 181 Ta collisions near the Coulomb barrier has been studied. Earlier experiments reported narrow lines in the spectra of positrons, accumulated without the requirement of electrons detected in coincidence. No evidence of such structure is observed in the present data. The positron energy spectra are compared with estimates from dynamic atomic processes, and from internal pair conversion of electromagnetic transitions from the excited nuclei. [S0031-9007(96)02277-6] PACS numbers: 25.70. Bc, Positron production in the collisions of very heavy nuclei at Coulomb-barrier energies has attracted a great deal of attention over the past 15 years. This process is of particular interest since the positron production mechanism is intimately linked to the very strong electromagnetic fields which occur in these collisions. These fields become "supercritical" when the united charge of the two ions (Z U ) is greater than approximately 173, and when the nuclei approach to within approximately 100 fm [1]. This phenomenon has been previously investigated in a number of experiments (e.g., [2,3]). In later experiments with Z U 180 to 188, the energy spectra of positrons displayed unexpected narrow structures at energies near 300-400 keV, with widths of 60 to 80 keV. The initial observation of such peaks in the 238 U 1 248 Cm system (Z U 188) [4] prompted the suggestion that they might be related to the spontaneous production of positrons associated with the overcritical binding of vacant electron orbitals in a relatively long-lived quasiatomic system. The analysis of data from subsequent experiments, however, showed evidence for similar structures in a number of other systems, with characteristics which were inconsistent with the expectations for the spontaneous production mechanism [5]. In particular, the expected very strong Z U dependences of the peak energy and yield (~Z N U where N ഠ 20) were not observed.Structures were also observed in a different set of measurements for the 238 U 1 238 U and 238 U 1 232 Th systems [6,7]. While the energies of the peaks in these data were lower, the general features appeared to be similar to those described previously. In particular, the narrow widths of the lines were taken to imply that the source velocities in the laboratory frame were relatively low, close to that of the center-of-mass of the colliding heavy ions. Finally, weaker evidence has been reported for peaks in subcritical (Z U , 173) systems, including 238 U 1 181 Ta (Z U 165) [8].The characteristics of these structures suggested that their source might be the decay of a light neutral particle X 0 into a positron-electron pair, which would lead to almost monoenergetic positrons of kinetic energy E e 1 ഠ ͑m X 0 2 2m e ͒c 2 ͞2. Sharp sum-energy lines consistent with this hypothesis have been reported in positron-electron coincidence spectra, from collisions in 618 0031-9007͞97͞78(4)͞618(4)$10.00