Scientists from the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS), in collaboration with international partners, have synthesized two new isotopes—berkelium-235 and americium-231—at the Heavy Ion Research Facility in Lanzhou. Their findings, recently published in Physics Letters B, provide critical experimental data for studies on neutron-deficient actinide nuclei.

The synthesis and study of new isotopes represent a key frontier in nuclear physics, offering key insights into the limits of nuclear existence, the validation of nuclear mass models, and the exploration of exotic decay modes. In the neutron-deficient berkelium region, however, the experimental synthesis and identification of new isotopes remain highly challenging, due to low fission barriers, extremely small production cross-sections, and competition between multiple decay modes.

To overcome these hurdles, the researchers conducted experiments at the China Accelerator Facility for Superheavy Elements (CAFE2) at IMP. They bombarded a gold‑197 target with a high‑intensity argon‑40 beam. Using fusion‑evaporation reactions combined with separation via a gas‑filled recoil separator, they successfully observed berkelium‑235 and its α‑decay daughter nucleus, americium‑231, for the first time.

Using an advanced atom‑at‑a‑time detection technique, the researchers identified three correlated α‑decay chains. They measured the α‑particle energy of berkelium‑235 at 7632 keV. For americium‑231, they determined its α‑particle energy to be 7109 keV and its half‑life to be 75 seconds. From the observed decay chains, the team also estimated the α‑decay branching ratio of americium‑231 to be 17%. These results greatly expand the known α‑decay systematics for neutron‑deficient actinide nuclei.

Notably, the study presents a systematic comparison between experimental α‑decay energies and predictions from theoretical mass models for the actinide region. The results show that for neutron‑deficient berkelium and americium isotopes, theoretical values are consistently higher than experimental ones, with marked discrepancies in the predicted trends for berkelium isotopes. These findings provide essential experimental constraints for refining relevant theoretical models.

This work was supported by the National Key R&D Program of China, the CAS Strategic Priority Research Program, and other funding sources.