Low-spin states inHg200studied with the (n,γ) reaction

The level scheme of ${}_{}{}^{200}{}_{}{}^{}\mathrm{Hg}$ has been studied by thermal and resonance neutron capture. The low-energy region has been measured with the high-resolution bent-crystal spectrometer at Risø, Denmark. The medium- and high-energy $\gamma$ rays, coincidence spectra, and $\gamma -\gamma$ angular correlations were measured at the Brookhaven National Laboratory (BNL) high flux beam reactor. Primary transitions from resonance capture in ${}_{}{}^{199}{}_{}{}^{}\mathrm{Hg}$ were studied with the fast chopper facility at BNL. Out of ∼520 observed transitions, ∼330 were placed in a level scheme containing 60 levels below 3.3 MeV. Many of these states are new and a great number of new $I^{\pi }$ assignments was made. In particular $I^{\pi }=0^{+}$ was assigned to a level at 1515 keV. Four $1^{-}$ states between 2.4 and 3 MeV are suggested. Possible explanations of their low excitation energy are offered. The simplest require and therefore suggest oblate shapes in ${}_{}{}^{200}{}_{}{}^{}\mathrm{Hg}$, at least for excitations involving the unique parity orbitals. A very systematic behavior of the level feeding by nonprimary transitions was found in this nucleus and was compared to simple statistical predictions. A detailed comparison was made with model calculations for core-coupled proton states. The most likely candidates for these states are: $2_1^{+}$ 368 keV, $2_2^{+}$ 1254 keV, $2_3^{+}$ 1573 keV, $0_2^{+}$ 1515 keV, $1_1^{+}$ 1570 keV, $3_1^{+}$ 1659 keV. The possibility of a reduced central density of ${}_{}{}^{200}{}_{}{}^{}\mathrm{Hg}$ is discussed.