Researchers from the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences and their collaborators have recently found an isomeric state in ²¹⁸Pa for the first time. The result was published in Physics Letters B. 

An isomeric state generally refers to a long-lived exited state. Isomeric transitions in nuclei near closed proton or neutron shells are of special interest, for they provide relatively pure and detailed information on the single-particle structure of these nuclei. 

In previous studies of odd-proton N=127 isotones, a low-lying α-decaying isomeric state has been found in ²¹⁰Bi, ²¹²At, ²¹⁴Fr and ²¹⁶Ac. However, it was not clear whether there was an isomeric state in ²¹⁸Pa. 

By means of α and γ spectroscopy, the experiment of ²¹⁸Pa was performed at the Heavy Ion Research Facility in Lanzhou (HIRFL). The nucleus was produced in the fusion reaction ⁴⁰Ar+¹⁸²W and separated in flight by the gas-filled recoil separator.  

An α-decaying isomeric state was firstly identified in ²¹⁸Pa. From the N=127 systematics and shell model calculations, it was inferred that the ground state and the isomeric state in ²¹⁸Pa were expected to be 8^- and 1^- states, respectively.  

This indicates that the spins and parities for ²¹⁸Pa do not follow the trend of the 1^- ground state and the 9^- or 8^- isomeric state as observed in lighter odd-proton N=127 isotones. Researchers thought that this phenomenon might be caused by the change of particle-particle interaction to hole-particle interaction with increasing proton number. 

Besides, the previously known α-lines of ²¹⁸Pa as well as some α-γ coincidence events were measured with improved precision. The two known α-lines were newly assigned to the decay of the ground state in ²¹⁸Pa. The result removed the disagreement on the assignments of the 9524 keV α-line in previous studies. 

Link to the paper: https://doi.org/10.1016/j.physletb.2019.135102 

Fig 1. Alpha-decay schemes of the odd-proton N=127 isotones from At to Pa. The α-branch marked by gray is tentative. Energies are given in keV. (Image by YANG Huabin) 

Fig 2. Results from a correlation analysis of the type ER-α-γ: (a) The spectrum of α particles correlated to recoil events within 15 ms; (b) The spectrum of α particles correlated to recoil events within 1.2 ms in the ²¹⁸Pa region. Red lines are the results from fit; (c) α-γ coincidences associated with the α decay of ²¹⁸Pa. (Image by YANG Huabin) 

Contact :

LIU Fang

Institute of Modern Physics

Email: fangliu@impcas.ac.cn