OPTICAL ISOMER-SHIFT FOR THE SPONTANEOUS-FISSION ISOMER AM-240M

The optical isomer shift in the S728→P7210 atomic transition in neutral americium has been measured for the ∼1-ms spontaneously fissioning isomer, Amm240(SF). With use of a laser-excited optical pumping technique, this transition is shifted by +2.6 ± 0.2 A to 6407.2±0.2 A implying a difference in nuclear mean square radii, δar2a, of 5.1±0.2 fm2 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. © 1979 The American Physical Society.