A new feature discovered in radioactive lanthanum isotopes

Researchers at the Accelerator Laboratory of the University of Jyväskylä, Finland, precisely measured atomic masses of radioactive lanthanum isotopes and found an interesting feature in their nuclear binding energies. The discovery provides essential data for understanding how elements heavier than iron are produced in the cosmos and triggers new research to elucidate the underlying nuclear structure causing this unexpected change in nuclear binding energies.

Nuclear binding energies of neutron-rich radioactive nuclei are essential inputs for calculations addressing the origin of heavy elements in the cosmos. Recently, the researchers produced radioactive, neutron-rich lanthanum isotopes using the Ion Guide Isotope Separation On-Line (IGISOL) facility. The produced isotopes are short-lived and therefore challenging to study. The research is published in the journal Physical Review Letters.

“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.

A phenomenon observed in neutron star collisions

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.

An unknown ‘bump’ on a scientist’s screen

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 Ph.D. researcher Arthur Jaries from University of Jyväskylä, who will defend his Ph.D. thesis at the Department of Physics in June.

Theoretical models should be developed

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.