The rapid neutron capture process, or the so-called r-process, occurs in neutron-rich environments such as neutron star mergers or certain types of supernovae. This process is thought to produce many of the chemical elements heavier than iron, but the details are poorly understood and cannot be studied in the lab. In new research, Los Alamos National Laboratory theoretical physicist Matthew Mumpower and his colleagues analyzed r-process element abundances previously observed in stars. They identified correlated excess abundances of certain elements in some stars, which is consistent with these elements being fission products of even heavier elements. These results indicate that some r-process events make elements heavier than uranium, which then decay into the elements observed in stars.
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The merger of two neutron stars is among the leading candidate sites for synthesizing the heavier elements on the periodic table through the r-process. Image credit: Matthew Mumpower, Los Alamos National Laboratory.
“People have thought fission was happening in the cosmos, but to date, no one has been able to prove it,” Dr. Mumpower said.
“Using the latest observations, we found a correlation between light precision metals like silver and rare earth nuclei like europium.”
“When one of these groups of elements goes up, the corresponding elements in the other group also increase — the correlation is positive.”
Astrophysicists have long believed heavy elements beyond iron were formed in stellar explosions called supernova or when two neutron stars merged.
As the name implies, the latter are composed largely of neutrons, which together with protons form the nuclei of all atoms.
Through the r-process, atomic nuclei grab neutrons to form heavier elements. Whether some grow too heavy to hold together and split, or fission, forming two atoms of lighter but still heavy elements (and releasing tremendous energy) has remained a mystery for a half century.
In 2020, Dr. Mumpower and co-authors first predicted the distributions of fission fragments for r-process nuclei.
A subsequent study predicted the co-production of light precision metals and rare earth nuclei.
This co-production of elements like elements ruthenium, rhodium, palladium and silver, and those like europium, gadolinium, dysprosium and holmium, can be tested by comparing the prediction with elemental abundances in a collection of stars.
In the new research, the authors combed through observational data from 42 stars and found precisely the predicted correlation.
The pattern provides a clear signature of fission creating these elements and a similar pattern of elements slightly heavier and higher on the periodic table.
The results also indicate that elements with an atomic mass of 260 — heavier than those charted at the high end of the periodic table — may exist.
“The correlation is very robust in r-process enhanced stars where we have sufficient data,” Dr. Mumpower said.
“Every time nature produces an atom of silver, it’s also producing heavier rare earth nuclei in proportion. The composition of these element groups is in lock step.”
“We have shown that only one mechanism can be responsible — fission — and people have been racking brains about this since the 1950s.”
The team’s paper appears in the journal Science.
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Ian U. Roederer et al. 2023. Element abundance patterns in stars indicate fission of nuclei heavier than uranium. Science 382 (6675): 1177-1180; doi: 10.1126/science.adf1341
