Isospin dependence of electromagnetic transition strengths among an isobaric triplet

Electric quadrupole matrix elements, M_p, for the J^π=2⁺→0⁺, ΔT=0, T=1 transitions across the A=46 isobaric multiplet ⁴⁶Cr-⁴⁶V-⁴⁶Ti have been measured at GSI with the FRS-LYCCA-AGATA setup. This allows direct insight into the isospin purity of the states of interest by testing the linearity of M_p with respect to T_z. Pairs of nuclei in the T=1 triplet were studied using identical reaction mechanisms in order to control systematic errors. The M_p values were obtained with two different methodologies: (i) a relativistic Coulomb excitation experiment was performed for ⁴⁶Cr and ⁴⁶Ti; (ii) a “stretched target” technique was adopted here, for the first time, for lifetime measurements in ⁴⁶V and ⁴⁶Ti. A constant value of M_p across the triplet has been observed. Shell-model calculations performed within the fp shell fail to reproduce this unexpected trend, pointing towards the need of a wider valence space. This result is confirmed by the good agreement with experimental data achieved with an interaction which allows excitations from the underlying sd shell. A test of the linearity rule for all published data on complete T=1 isospin triplets is presented.