Precision lifetime measurements of the first 2(+) and 4(+) states in (56)Cr at the N=32 subshell closure

Excited states in 56Cr have been populated after 48Ca(11B,p2n) reactions at a beam energy of 32 MeV. The Cologne plunger device surrounded by a γ-detector array of one EUROBALL cluster detector and five Ge detectors was employed to determine lifetimes with the recoil distance Doppler shift method. γγ-coincidence data were analyzed using the differential decay curve method and precise lifetimes for the first 2+ and 4+ state of τ=5.49(14) ps and τ=3.15(11) ps, respectively, were extracted. The corresponding B(E2; 2+→0+)=11.33(31) W.u. quantifies with a high accuracy the puzzling discrepancy between experimental B(E2) values in N=32 isotones and theoretical results from large-scale shell model calculations employing modern effective nucleon-nucleon interactions.