Isotope Shift Measurements for Superdeformed Fission Isomeric States

Optical isotope shift measurements have been performed for the 240,242Amᶠ fission isomers with low target production rates of 10s^-1 employing resonance ionization spectroscopy in a buffer gas cell. Isotope shift ratios IS240f,241/IS243,2410ex0ex=0ex0ex39.2(8) and IS242f,241/IS243,2410ex0ex=0ex0ex41.4(8) have been measured at the 500.02 nm transition. A difference in the nuclear mean charge radii δ〈r²〉ₒₚₜ242f,2410ex0ex=0ex0ex5.34(28)fm² and an intrinsic quadrupole moment Q₂₀²⁴²ᶠ0ex0ex=0ex0ex(35.5±1.0ₛₜ±1.2mod)eb have been deduced, neglecting the nuclear polarization correction. The small difference δQ₂₀242f,240f0ex0ex=0ex0ex0.63(8)eb demonstrates the stability of the deformation if two neutrons are removed.