Isotope Shift Measurements for Superdeformed Fission Isomeric States

Abstract

Optical isotope shift measurements have been performed for the ${}^{240,242}{\mathrm{Am}}^{f}$ fission isomers with low target production rates of $10{\mathrm{s}}^{\ensuremath{-}1}$ employing resonance ionization spectroscopy in a buffer gas cell. Isotope shift ratios ${\mathrm{IS}}^{240f,241}/{\mathrm{IS}}^{243,241}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}39.2(8)$ and ${\mathrm{IS}}^{242f,241}/{\mathrm{IS}}^{243,241}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}41.4(8)$ have been measured at the 500.02 nm transition. A difference in the nuclear mean charge radii $\ensuremath{\delta}〈{r}^{2}{〉}_{\mathrm{opt}}^{242f,241}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}5.34(28){\mathrm{fm}}^{2}$ and an intrinsic quadrupole moment ${Q}_{20}^{242f}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(35.5\ifmmode\pm\else\textpm\fi{}{1.0}_{\mathrm{st}}\ifmmode\pm\else\textpm\fi{}{1.2}_{\mathrm{mod}})e\mathrm{b}$ have been deduced, neglecting the nuclear polarization correction. The small difference $\ensuremath{\delta}{Q}_{20}^{242f,240f}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.63(8)e\mathrm{b}$ demonstrates the stability of the deformation if two neutrons are removed.