Researchers conducted highly precise measurements of the atomic masses of radioactive lanthanum isotopes and uncovered an intriguing characteristic in their nuclear binding energies. This discovery is critical for enhancing our understanding of how elements heavier than iron are created in the universe and ignites new research aimed at revealing the fundamental nuclear structures responsible for this unexpected variation in binding energies.
Researchers at the Accelerator Laboratory of the University of Jyväskylä, Finland, conducted highly precise measurements of atomic masses of radioactive lanthanum isotopes and uncovered an intriguing characteristic in their nuclear binding energies. This discovery is critical for enhancing our understanding of how elements heavier than iron are created in the universe and ignites new research aimed at revealing the fundamental nuclear structures responsible for this unexpected variation in binding energies.
Nuclear binding energies of neutron-rich radioactive nuclei play a key role in calculations related to the origins of heavy elements in the universe. Recently, researchers at the Accelerator Laboratory of the University of Jyväskylä successfully produced radioactive, neutron-rich lanthanum isotopes using the Ion Guide Isotope Separation On-Line (IGISOL) facility. These isotopes are short-lived, making them difficult to study.
“Utilizing the highly sensitive phase-imaging ion cyclotron resonance technique, we achieved very high precision in determining the masses of six lanthanum isotopes with the JYFLTRAP Penning trap mass spectrometer. This includes the first measurements of the two most exotic isotopes, lanthanum-152 and lanthanum-153,” states Professor Anu Kankainen from the University of Jyväskylä, who led the research as part of her ERC CoG project MAIDEN.
An interesting phenomenon observed during neutron star collisions
The precise mass measurements were used to analyze the neutron separation energies of the lanthanum isotopes. The neutron separation energy indicates how much energy is needed to remove a neutron from the nucleus of a specific isotope.
“This information reveals details about the nucleus’s structure and is crucial for calculating astrophysical neutron-capture rates relevant to the rapid neutron capture (r) process, which occurs during events such as neutron star mergers, as demonstrated by the kilonova detection from the GW170817 merger,” explains Kankainen.
A mysterious “bump” appeared on a scientist’s monitor
In their research, the scientists evaluated the two-neutron separation energies of the lanthanum isotopes and identified a significant and localized increase—referred to as a “bump“—in the values when the neutron count rose from 92 to 93. This unique bump warrants additional investigation.
“Upon analyzing the mass data and calculating the two-neutron separation energies, I was taken aback by this feature. None of the existing nuclear mass models can account for it. There is some indication that it might stem from a sudden shift in the nuclear structure of these isotopes, but further exploration using complementary methods like laser or nuclear spectroscopy will be necessary,” explains Arthur Jaries, a PhD researcher from the University of Jyväskylä, who is set to defend his PhD thesis at the Department of Physics in June.
Development of theoretical models is essential
The new precise mass values significantly altered the calculated astrophysical neutron-capture reaction rates by up to approximately 35% and reduced uncertainty related to mass by a factor of 80 in extreme cases.
“These enhanced reaction rates are vital for understanding the formation of the rare-earth abundance peak in the r-process. More importantly, the findings illustrate that the nuclear mass models currently employed in astrophysical models are inadequate for predicting this feature and will require further advancements,” concludes Kankainen.
