Over one hundred years have passed since the nuclear isomer was first introduced,in analogy with chemical isomers to describe long-lived excited nuclear states.In 1921,Otto Hahn discovered the first nuclear isomerPa.After that,step by step,it was realized that different types of nuclear isomers exist,including spin isomer,K isomer,seniority isomers,and"shape and fission"isomer[1,2].
Isomeric I^(π)=8^(-) states have been observed in the even-even N=74 nuclei ^(138)Gd[1],^(136)Sm[2],^(134)Nd[3],^(132)Ce[4],^(130)Ba[5],^(128)Xe[6]with half-lives ranging from nanoseconds(Xe)to milliseconds(Ba,Ce).Rotational bands built on the K^(π)=8^(-) isomer were identified in all these isotones,with the exception of 130Ba.The singleparticle configuration of the isomers have been deduced from theΔI=2 toΔI=1γ-ray intensity branching ratios,which allowed to extract the(gKgR)/Q0 values,and therefore the quasi-particle configuration of the state.
