This study determined the lifetime of the first excited state ( \(5/2^+_1\) ) in \(^{139}\) La via \(\beta\) – \(\gamma\) time-difference measurement using a \(\hbox {LaBr}_{3}\) + plastic scintillator array. This state is populated following the decay of \(^{139}\) Ba produced in the \(^{138}\) Ba(n, \(\gamma\) ) reaction. Compared with previous experiments using only stilbene/plastic crystals, this experiment separates the background contribution in the \(\gamma\) -ray spectrum owing to the high energy resolution of \(\hbox {LaBr}_{3}\) . The L-forbidden M1 transition strength, B(M1, \(5/2^+_1\) \(\rightarrow\) \(7/2^+_1\) ), in \(^{139}\) La was measured and compared with detailed large-scale shell model calculations, with a special focus on the core-excitation effect. The results showed the importance of both proton and neutron core-excitations in explaining the M1 transition strength. Meanwhile, the effective g-factor for the tensor term of the M1 operator was smaller than the previously reported value in this region or around \(^{208}\) Pb.
