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

Abstract

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 Ris\o{}, Denmark. The medium- and high-energy $\ensuremath{\gamma}$ rays, coincidence spectra, and $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ 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 \ensuremath{\sim}520 observed transitions, \ensuremath{\sim}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 ${I}^{\ensuremath{\pi}}$ assignments was made. In particular ${I}^{\ensuremath{\pi}}={0}^{+}$ was assigned to a level at 1515 keV. Four ${1}^{\ensuremath{-}}$ 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.[NUCLEAR REACTIONS $^{199}\mathrm{Hg}(n,\ensuremath{\gamma})$, $E=\mathrm{thermal},33.5,129.7,\mathrm{and} 175.1$ eV measured ${I}_{\ensuremath{\gamma}}$, $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma} \mathrm{coin}(\ensuremath{\theta})$ in $^{200}\mathrm{Hg}$. $^{200}\mathrm{Hg}$ deduced levels, transitions, $J$, $\ensuremath{\pi}$, ICC, multipolarities, $\ensuremath{\delta}$.]