Isospin mixing in the 4 + doublet of 54 Co

The T = 1 admixture into the T = 0 member of a recently discovered \(J^\pi = 4^{ + }\) isospin-doublet in 54Co is obtained from the measured electromagnetic E2/M1 multipole mixing ratio, \(\delta = 0.12(4)\) of the \(T = 0, 4^{ + } \rightarrow T = 0, 3^{ + }\) transition. Combining these data with shell model calculations for strong isovector M1 and isoscalar E2 electromagnetic matrix elements one obtains a value for the T = 1 admixture into the T = 0, 4 + state of \(0.23_{-0.10}^{ + 0.29}\%\). The corresponding mixing matrix element in the 4 + doublet is \(V_{\rm mix} = 10\) keV.