Low-Energy Level Structure ofHg198

The ${\mathrm{Hg}}^{198}$ level scheme has been investigated from the ${\mathrm{Tl}}^{198}$ and ${\mathrm{Tl}}^{198m}$ ($\epsilon +{\beta }^{+}$) decays. A level scheme is proposed in which the following spins and parities have been determined by means of $\gamma$ and conversion-electron spectra and $\gamma -\gamma$ angular correlations: 1401.4 keV, $I^{\pi }=0^{+}$; 1612.7 keV, $I^{\pi }=2^{+}$; 1635.6 keV, $I^{\pi }=5^{-}$; 1832.8 keV, $I^{\pi }=2^{+}$; 1846.8 keV, $I^{\pi }=3^{+}$; 1858.9 keV, $I^{\pi }=2^{+}$; 1900.7 keV, $I^{\pi }=2^{+}$; 2360.6 keV, $I^{\pi }=3^{+}$. We have measured the isomeric cross-section ratio $\frac{\sigma {(\alpha ,3n)}^m}{\sigma {(\alpha ,3n)}^{\mathrm{g}.\mathrm{s}.\mathrm{}}}=5\mathrm{}\pm 1$ for $35 \mathrm{MeV}\le E_{\alpha }\le 42 \mathrm{MeV}$. Lifetimes of two negative-parity states have been determined: $\tau (1635.6\mathrm{} \mathrm{keV},5^{-})=89\mathrm{}\pm 15$ psec and $\tau (1683.4\mathrm{} \mathrm{keV},6^{-}{7}^{-})=10.7\mathrm{}\pm 0.5$ nsec which lead to $B(E2; 6^{-},7^{-}\to 5^{-})=0.17\mathrm{}\pm 0.02 {\left(e \mathrm{b}\right)}^2$; $B(E1; 5^{-}\to 4^{+})=(3.5\mathrm{}\pm 0.6)\times {10}^{-7\mathrm{}} e^{2}b$; $B(M2; 5^{-}\to 4^{+})=0.033\mathrm{}\pm 0.008 {{\mu }_N}^2\mathrm{b}$. Experimental data are well reproduced by calculations based on the hole-core coupling model using a pairing residual force between proton holes.