Optical Isomer Shift for the Spontaneous-Fission IsomerAmm240

The optical isomer shift in the ${}_{}{}^8{}_{}{}^{}S_{\frac{7}{2}}^{}{}_{}{}^{}\to {}_{}{}^{10}{}_{}{}^{}P_{\frac{7}{2}}^{}{}_{}{}^{}$ atomic transition in neutral americium has been measured for the ∼1-ms spontaneously fissioning isomer, ${}_{}{}^{240}{}_{}{}^{}\mathrm{Am}_{}^m{}_{}{}^{}\left(\mathrm{S}\mathrm{F}\right)$. With use of a laser-excited optical pumping technique, this transition is shifted by +2.6 ± 0.2 Å to 6407.2±0.2 Å implying a difference in nuclear mean square radii, $\delta 〈r^2〉$, of 5.1±0.2 ${\mathrm{fm}}^2$ between the fission isomer and the normal ground state. These experiments provide the first direct experimental proof for the large deformations expected for fission isomers.