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

• Received 8 August 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.920