The High Intensity heavy-ion Accelerator Facility (HIAF), which began trial operations on July 21, has enabled Chinese scientists to observe an extremely rare isotope, hafnium-153. The observation marks the first physics result achieved during HIAF's commissioning phase, highlighting the facility's capability to explore the boundaries of the nuclear landscape.

Published in Science Bulletin, the research was led by the State Key Laboratory of Heavy Ion Science and Technology affiliated with the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS), in collaboration with the University of CAS, GSI Helmholtz Centre for Heavy Ion Research, the University of Cologne, East China University of Technology, and the Advanced Energy Science and Technology Guangdong Laboratory.

Exploring unknown isotopes and determining the limits of nuclear existence are key frontiers of nuclear physics. Located in the neutron-deficient heavy nuclear region, hafnium-153 offers a unique window into the evolution of nuclear structure and the limits of nuclear stability. Its observation and precise measurement provide critical experimental information for testing nuclear models and advancing our understanding of nuclear structure.

Location of hafnium-153 on the nuclear chart. Blue lines show the boundaries of the known nuclides around it. (Image by HU Houyu and LI Hongfu)

Located in Huizhou in South China's Guangdong Province, HIAF is a world-class next-generation heavy-ion accelerator. In this experiment, a bismuth-209 primary beam, provided by HIAF's Booster Ring (BRing), was delivered onto a graphite target to produce radioactive nuclei through projectile fragmentation reactions. The cocktail beam was purified and transported by the HIgh rigidity Radioactive Ion Beam Line (HIRIBL) before being injected into the Spectrometer Ring (SRing), where isochronous mass spectrometry was employed for precision measurements.

Despite an extremely low production cross section, the researchers observed a total of ten hafnium-153 ions. The results indicate that hafnium-153 is a bound or weakly bound isotope, consistent with predictions from various nuclear mass models.

Comparison of experimental and simulated revolution-time spectrum of the stored ions in SRing. The single additional peak located at a revolution time of 1159.348 ns, marked by the red label, is assigned to hafnium-153. (Image by IMP)

Concurrently, the Radioactive Isotope Beam Factory (RIBF) at RIKEN in Japan also independently reported the observation of hafnium-153. These two independent measurements provide mutual confirmation and further highlight the significance of this isotope in nuclear physics research.

According to the researchers, the detection of hafnium-153 showcases the integrated capabilities of HIAF. The high-intensity heavy-ion beams enhance the production of rare isotopes; high-performance HIRIBL enables efficient separation of the nuclei of interest; and SRing's isochronous mass spectrometry provides single-ion sensitivity for precise identification and measurement of rare nuclei.

This new achievement not only marks an important advance in probing the limits of nuclear existence, but also opens the door to broader scientific exploration at HIAF, the researchers said.

With further improvements in beam intensity and experimental efficiency, HIAF is expected to play an increasingly important role in the discovery of new isotopes and the investigation of nuclear properties under extreme conditions, they added.

The research was supported by the National Key R&D Program of China, the HIAF project, the Youth Innovation Promotion Association of CAS, the National Natural Science Foundation of China, the Talent Support Project of Guangdong Program, and the Science and Technology Research Foundation of Gansu Province.