Researchers at the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS) and their collaborators have recently synthesized two new isotopes osmium-160 and tungsten-156. Their findings shed new light on nuclear structure, and hint that lead-164 could be a doubly magic nucleus with increased stability.

The results were published in Physical Review Letters on February 15 and highlighted as an Editors’ Suggestion.

"Magic numbers" of protons and neutrons can make an atomic nucleus particularly stable. The traditional magic numbers include 8, 20, 28, 50, 82 and 126. In previous studies, researchers discovered the vanishing of traditional magic numbers and the emergence of new magic numbers on the neutron-rich side of the chart of nuclides.

Will other traditional magic numbers be disappeared in extremely neutron-deficient nuclear region? Further study of this basic question is of great significance to enrich and develop nuclear theories, and to deepen our understanding of nuclear forces.

The researchers carried out the experiment at the gas-filled recoil separator, Spectrometer for Heavy Atoms and Nuclear Structure (SHANS), at the Heavy Ion Research Facility in Lanzhou (HIRFL), China.

Using the fusion evaporation reaction, the researchers synthesized osmium-160 and tungsten-156 for the first time. They measured the α-particle energy and the half-life of osmium-160, which is an α-emitting isotope. Meanwhile, they determined that the daughter nucleus, tungsten-156, is a β⁺ emitter with a half-life of 291 ms.

With the newly measured α-decay data, the researchers derived α-decay reduced width for osmium-160 and compared it with those of other nuclei with 84 neutrons but fewer protons, and they found a surprising trend: the higher the proton number, the lower the decay rate.

"This trend is interpreted as evidence for the strengthening of 82-neutron shell closure towards the proton drip line, which is supported by the increase of the neutron-shell gaps predicted in theoretical models," said Dr. YANG Huabin from IMP, the first author of the paper.

Moreover, the researchers suggest that the enhanced stability of 82-neutron shell closure can be attributed to the increasing closeness to the doubly magic nucleus lead-164, which might be a stable nucleus with 82 protons and 82 neutrons. Although lead-164 is predicted beyond the proton-drip line, the enhanced shell effect has the potential to make it a bound or quasi-bound nucleus.

The study was conducted in collaboration with University of CAS, Advanced Energy Science and Technology Guangdong Laboratory, Shandong University, Sun Yat-sen University, Guangxi Normal University, Tongji University, and the Institute of Theoretical Physics of CAS.

Doi: https://doi.org/10.1103/PhysRevLett.132.072502 

Figure. Position of the new isotopes, osmium-160 and tungsten-156, on the chart of nuclides. (Image by YANG Huabin)