A research team at the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS), together with their collaborators, has recently synthesized a new isotope thorium-207, and discovered a regular and distinct odd-even staggering (OES) in α-decay energies for nuclei with Z>82 and N<126. The study has been published as a Letter in Physical Review C on May 19. 

The experiment to produce thorium-207 was performed at the Heavy Ion Research Facility in Lanzhou (HIRFL), China. The isotope was separated in flight by the gas-filled recoil separator Spectrometer for Heavy Atoms and Nuclear Structure, and identified on the basis of a correlated α-decay chain. The α decay of thorium-207, measured with an α-particle energy of 8167(21) keV and a half-life of 9.7(+46.6-4.4) ms, was assigned to originate from ground state. The new isotope thorium-207 is the 34th nuclide synthesized at IMP. 

Combining the new measurements with existing data, the researchers obtained direct evidence that there is a regular and distinct OES in α-decay energies for nuclei with Z>82 and N<126 along both isotopic and isotonic chains. They found that the magnitude of the OES is about an order of magnitude larger than the values deduced from the classical Bethe-Weizsacker formula, which clearly indicated that the OES in α-decay energies is a regular and distinct effect rather than a negligible one as commonly supposed. 

In order to probe the underlying mechanism of the OES in α-decay energies, they performed the relativistic Hartree-Fock-Bogoliubo and large-scale shell-model calculations. They found that the OES originates from both pairing correlations and blocking of particular orbitals by unpaired nucleons.  

This study reveals a novel mechanism of odd-even staggering induced by pairing scattering and presents a challenge to nuclear-mass models.  

This work was partially supported by the National Key R&D Program of China, the Strategic Priority Research Program of CAS, and the National Natural Science Foundation of China, etc. 

  Figure 1. 2D spectrum showing the correlation between energies of mother and daughter α decays in the reaction of ³⁶Ar+¹⁷⁶Hf. (Image by YANG Huabin)

  Figure 2. The OES in α-decay energies. (Image by YANG Huabin)