Reviewed by Louis CastelFeb 3 2025
Researchers at the University of Jyväskylä’s Accelerator Laboratory have created radioactive, neutron-rich lanthanum isotopes using the Ion Guide Isotope Separation On-Line (IGISOL) facility, according to a study published in Physical Review Letters.
Doctoral Researcher Arthur Jaries use the RFQ device. He will defend his PhD thesis at the Department of Physics in June. Image Credit: University of Jyväskylä
Researchers discovered an intriguing characteristic in the nuclear binding energies of radioactive lanthanum isotopes after measuring their atomic masses with extreme precision. In addition to triggering additional study to clarify the underlying nuclear structure driving this unexpected change in nuclear binding energies, the discovery offers crucial information to understand how elements heavier than iron are generated in the Cosmos.
Calculations addressing the formation of heavy elements in the Cosmos require the nuclear binding energies of neutron-rich radioactive nuclei.
The generated isotopes are difficult to examine because of their short half-life.
Thanks to the highly sensitive phase-imaging ion cyclotron resonance technique, masses for six lanthanum isotopes could be determined with a very high precision using the JYFLTRAP Penning trap mass spectrometer. The masses for the two most exotic isotopes, lanthanum-152 and lanthanum-153 were measured for the first time.
Anu Kankainen, Professor, University of Jyväskylä
The Phenomenon Observed in Neutron Star Collisions
The neutron separation energies of the lanthanum isotopes were investigated using the high-precision mass measurements. The energy needed to extract a single neutron from an isotope's nucleus is indicated by the neutron separation energy.
Kankainen added, “It gives information on the structure of the nucleus and is an essential input to calculate astrophysical neutron-capture rates for the rapid neutron capture (r) process taking place at least in neutron-star mergers, as evidenced, e.g., by the kilonova observation from the merger GW170817.”
An Unknown “Bump” Showed Up on a Scientist’s Screen
When the number of neutrons increases from 92 to 93, researchers found a significant, local increase, or “bump,” in the two-neutron separation energies of the lanthanum isotopes. The bump that was observed is distinct and warrants more research.
After I did the mass data analysis and calculated the two-neutron separation energies, I was surprised to find this feature. None of the current nuclear mass models can explain it. There are some hints it could be caused by a sudden change in the nuclear structure of these isotopes, but it will require further investigations with complementary methods, such as laser or nuclear spectroscopy.
Arthur Jaries, PhD Researcher, University of Jyväskylä
Theoretical Models Should be Developed
In the most extreme circumstances, the new accurate mass estimates lowered the mass-related uncertainties by up to a factor of 80 and altered the projected astrophysical neutron-capture reaction rates by up to about 35%.
“These improved reaction rates are important to address the formation of the rare-earth abundance peak in the r-process. More importantly, the measurements show that the current nuclear mass models used in the astrophysical models fail to predict this feature and will require further developments in the future”, added Kankainen.
Journal Reference:
Jaries, A., et. al. (2025) Prominent Bump in the Two-Neutron Separation Energies of Neutron-Rich Lanthanum Isotopes Revealed by High-Precision Mass Spectrometry. Physical Review Letters. doi.org/10.1103/PhysRevLett.134.042501