Measurement of B(M1) for the πp_3/2νp_1/2 ^-1 doublet in ⁶⁸Cu

Lifetimes of excited states in ⁶⁸Cu were measured by the γ-γ-t coincidence using two BaF₂ detectors through the decay of ⁶⁸Cu^m produced by the (n,p) reaction. The half-life of the 2⁺ state at 84 keV was obtained as 7.84(8) ns, corresponding to the B(M1;2⁺ → 1⁺) value of 0.00777(8)μ_N². A shell model calculation with a minimum model space of πp_3/2νp_1/2^-1 gives a good prediction of this B(M1) value by using experimental g factors of neighboring nuclei. This small B(M1) value can also be explained by a shell model calculation in the f_7/2^-r(p_3/2f _5/2p_1/2)^n+r(r=0,1) model space.