Scientists get better understanding of how rare earth metal in lightbulbs forms
Researchers at CERN’s Neutron Time-of-Flight facility (n_TOF) collaboration are investigating how cerium is produced in stars.
Cerium is a rare earth metal that is used in some types of lightbulbs, flat-screen TVs and biomedical applications. While the element is rare in our planet’s crust, it is slightly more abundant in the universe.
“The measurement we carried out enabled us to identify nuclear resonances never observed before in the energy range involved in the production of cerium in stars,” Simone Amaducci, first author of the paper that describes the findings, said in a media statement. “This is thanks to the very-high-energy resolution of the experimental apparatus at CERN and the availability of a very pure sample of cerium 140.”
The abundance of elements heavier than iron observed in stars -such as tin, silver, gold, and lead- can be reproduced mathematically by hypothesizing the existence of two neutron capture processes: the slow (s) process and the rapid (r) process.
The s process corresponds to a neutron flux of 10 million neutrons per cubic centimetre. The r process has a flux of more than one million billion billion neutrons per cubic centimetre. The s process is theorized to produce about half of the elements heavier than iron in the universe, including cerium.
CERN’s n_TOF is designed to study neutron interactions, such as those that occur in stars. In this study, the scientists used the facility to measure the nuclear reaction of the cerium 140 isotope with a neutron to produce isotope 141.
According to sophisticated theoretical models, this particular reaction plays a crucial role in the synthesis of heavy elements in stars. Specifically, the scientists looked at the reaction’s cross-section: the physical quantity that expresses the probability that a reaction occurs. They measured the cross section at a wide range of energies with 5% more accuracy than previous measurements.
The results open up new questions about the chemical composition of the universe.
Sagittarius has some clues
“What intrigued us at the beginning was a discrepancy between theoretical star models and observational data of cerium in the stars of the M22 globular cluster in the Sagittarius constellation,” Sergio Cristallo, who proposed the experiment, said. “The new nuclear data differs significantly, up to 40%, from the data present in the nuclear databases currently used, definitely beyond the estimated uncertainty.”
These results have notable astrophysical implications, suggesting a 20% reduction in the contribution of the s process to the abundance of cerium in the universe. This means a paradigm shift is required in the theory of cerium nucleosynthesis: other physical processes that are not currently included would need to be considered in calculations of stellar evolution.
Furthermore, the new data have a significant impact on scientists’ understanding of the chemical evolution of galaxies, which also affects the production of heavier elements in the universe.