New Chinese accelerator detects hafnium-153 isotope unseen for 40 years

The High Intensity heavy-ion Accelerator Facility (HIAF) in China has recorded the first reliable detection of hafnium-153, an isotope predicted but not observed since the 1980s. During 10 days of commissioning runs, the facility's SRing spectrometer ring captured 10 nuclei of the isotope across 8,240 beam pulses. A single previous claim from 2000 was never confirmed by peer-reviewed publication.

The experiment used a bismuth-209 beam accelerated to 836.66 MeV per nucleon in the BRing accelerator, striking a graphite target. The reaction cross-section for producing hafnium-153 is estimated at roughly one picobarn, meaning only about one successful event occurs per trillion collisions. Fragments were separated by the HIRIBL separator and injected into the SRing storage ring operating in isochronous mode.

In isochronous mode, an ion's revolution time depends only on its mass, not its velocity, allowing separation of neighboring isotopes with a precision of one ten-thousandth of a percent. The key innovation is the zero-background principle: each ion circles the 277-meter ring dozens of times, and a valid event must produce a periodic signal sequence lasting at least 100 microseconds — over 80 complete revolutions. Noise or random overlaps cannot mimic this pattern.

A single peak at 1,159.348 nanoseconds matched the calculated position for a fully stripped hafnium-153 ion (72 protons, 81 neutrons), enabling unambiguous identification without statistical caveats. Simultaneously, the RIKEN Radioactive Isotope Beam Factory (RIBF) in Japan independently registered the same isotope, removing any remaining doubt.

Hafnium-153 sits on the neutron-deficient side of the nuclear chart, exactly one neutron short of the magic number 82 — a particularly stable neutron shell closure. This makes it an ideal probe for testing how robust that shell remains in heavy, proton-rich nuclei. Theoretical models from macroscopic to relativistic frameworks predict the nucleus should be either stable against proton emission or a very weak emitter, consistent with the survival of 10 nuclei to the detector.

Precise mass measurement of hafnium-153 on the SRing will provide the first experimental data point to directly calculate the N=82 shell gap on the proton-rich frontier, a parameter critical for understanding the binding of heavy nuclei. The HIAF facility operated at only about 10% of its design beam intensity, 0.3 Hz pulse rate versus a planned 3 Hz, and the separator acceptance exceeded the ring aperture by more than an order of magnitude.

At full power, sensitivity to rare isotopes is expected to increase by hundreds of times, potentially adding dozens of new nuclides in the difficult-to-reach region near the proton drip line — the boundary beyond which nuclear forces can no longer hold protons and neutrons together. The result appears in Science Bulletin.