Evidence for shape coexistence and superdeformation in 24Mg

The E0 transition depopulating the first-excited 0 + state in 24Mg has been observed for the first time, and the E0 transition strength determined by electron-positron pair and γ-ray spectroscopy measurements performed using the Super-e pair spectrometer. The E0 transition strength is ρ 2 × 10 3 = 380 ( 70 ) . A two-state mixing model implies a deformation of the first-excited 0 + state of β 2 ≈ 1 and a change in the mean-square charge radius of Δ 〈 r 2 〉 ≈ 1.9 fm 2 , which suggests a significant shape change between the ground state and first-excited 0 + state in 24Mg. The observed E0 strength gives direct evidence of shape coexistence and superdeformation in 24Mg, bringing this nucleus into line with similar behaviour in nearby N = Z nuclei. This result agrees with recent theoretical work on the cluster nature of 24Mg and has potential ramifications for nuclear reactions of astrophysical importance.