Lifetime of first excited state in 139La and the role of core-excitation on L-forbidden M1 transition

This study determined the lifetime of the first excited state (5/21+) in 139La via β–γ time-difference measurement using a LaBr3 + plastic scintillator array. This state is populated following the decay of 139Ba produced in the 138Ba(n,γ) reaction. Compared with previous experiments using only stilbene/plastic crystals, this experiment separates the background contribution in the γ-ray spectrum owing to the high energy resolution of LaBr3. The L-forbidden M1 transition strength, B(M1, 5/21+→7/21+), in 139La 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 208Pb.