Gamma-decay properties of 20Ne levels below 12 MeV

Abstract

Nine levels in 20Ne below 12 MeV excitation were studied using the 12C( 12C, αγ) 20Ne reaction. Doppler-shift attenuation measurements on de-excitation γ-rays in three Ge(Li) detectors at 0°, 90° and 125° yielded mean lifetimes (in ps) of levels at 1.633 MeV (1.15 ± 0.20), 4.246 MeV (0.093 ± 0.009), 5.618 MeV (0.20 ± 0.05), 7.004 MeV (0.47 ± 0.09), 9.950 MeV (< 0.035), 10.609 MeV (0.023 ± 0.007), 10.920 MeV (< 0.03) and 11.528 MeV (< 0.03). The mean lifetime of the 4.966 MeV level measured by the recoil-distance method was determined to be 4.8 ± 0.5 ps. New γ-decay branches of (4.8 ± 1.6) % between the 5.618 (3 −) and the 4.966 (2 −) MeV levels, and of (4.5 ± 1.2) % between the 10.609 (6 −) and 8.446 (5 −) MeV levels were observed. These branches firmly establish these levels as members of the negative parity K = 2 band based on the 4.966 MeV level. Particle-gamma angular correlation measurements, with six NaI(T1) detectors yielded the main decay modes of levels at 9.950, 10.920 and 11.528 MeV. The presently known electromagnetic matrix elements are compared with nuclear model predictions. The 5.618 → g.s. E3 transition probability is considerably larger than that predicted by a shell model with four valence particles.