摘要
This study determined the lifetime of the first excited state(5∕2_(1)^(+))in ^(139)La via β-γ time-difference measurement using a LaBr_(3)+plastic scintillator array.This state is populated following the decay of ^(139)Ba produced in the^(138)Ba(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 LaBr_(3).The L-forbidden M1 transition strength,B(M1,5∕2_(1)^(+)→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.
基金
supported by the Guangdong Major Project of Basic and Applied Basic Research (No. 2021B0301030006)
Young Scientists Fund of the National Natural Science Foundation of China (No. 12405144)
the National Natural Science Foundation of China (No. 12475129)
the International Atomic Energy Agency Coordinatated Research Project F41034 (No. 28649)
the computational resources from Sun Yat-sen University the National Supercomputer Center in Guangzhou
the Natural Science Foundation of Guangdong Province,China (No. 2025A1515012112)
