Mysterious Nuclear Bump Challenges Current Physics Models

Researchers at the Accelerator Laboratory of the University of Jyväskylä, Finland, have precisely measured the atomic masses of radioactive lanthanum isotopes, revealing an unexpected feature in their nuclear binding energies, the journal Physical Review Letters reported.

This discovery provides crucial data for understanding the formation of elements heavier than iron in the universe and prompts further investigation into the underlying nuclear structure responsible for this anomaly.

Nuclear binding energies of neutron-rich radioactive nuclei play a key role in modeling the origin of heavy elements in the cosmos. Using the Ion Guide Isotope Separation On-Line (IGISOL) facility, the researchers successfully produced short-lived, neutron-rich lanthanum isotopes. Due to their fleeting existence, these isotopes are particularly challenging to study, making the precise mass measurements a significant achievement.

“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,” says Professor Anu Kankainen from University of Jyväskylä, who led the research as a part of her ERC CoG project MAIDEN.

The high-precision mass measurements were utilized to study neutron separation energies of the lanthanum isotopes. The neutron separation energy tells how much energy is required to remove one neutron from the nucleus of a given isotope.

“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,” explains Kankainen.

In this work, researchers determined two-neutron separation energies of the lanthanum isotopes and discovered a strong, local increase, a “bump”, in the values, when the number of neutrons increases from 92 to 93. The observed bump is unique and calls for further studies.

“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,” says a PhD researcher Arthur Jaries from the University of Jyväskylä, who will defend his PhD thesis at the Department of Physics in June.

The new precise mass values changed the calculated astrophysical neutron-capture reaction rates up to around 35% and reduced the mass-related uncertainties by up to a factor of 80 in the most extreme cases.

“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,” says Kankainen.

4155/v