The “Hoyle state,” an excited form of carbon-12, is a unique arrangement of carbon that may be measured in a novel way. Since the 1950s, researchers have hypothesized that three helium-4 nuclei, or alpha particles, may easily combine to make carbon-12 in this form in stars. The Hoyle state of this carbon-12 will then decay to simple ground-state carbon, releasing energy along the way.
Schematic showing how neutrons in stars can cause stars to burn more quickly by stealing the energy from carbon as it is freshly created in a highly excited form (called the ‘Hoyle state’) to produce stable carbon in its “ground state." Image Credit: Jack Bishop, Texas A&M University
The new experiment was created to determine whether it was possible to accurately observe the Hoyle state splitting into three alpha particles in reverse.
The scientists using this technique then tested the role of neutron upscattering in the fusion of three alpha particles to generate carbon.
When a neutron meets with a resonance (a vibration), it de-excites it, taking energy from the resonance in a process known as neutron upscattering. For many years, scientists have hypothesized that this phenomenon causes stars to burn more quickly than they should.
The Impact
This experiment studied the characteristics of the Hoyle state for the first time in history. The level of sensitivity in the detector makes it possible to conduct the hitherto impractical experiment of monitoring neutron upscattering.
According to an examination of the experiment’s findings, upscattering may not be as crucial to the production of carbon in stars as previously believed.
Summary
It is challenging to measure internal star reactions. On Earth, it is impossible to replicate the extreme temperatures and densities required. As a result, scientists rely on quantifying similar reactions that can be carried out in the lab.
This effort aimed to understand how the interaction of three alpha particles (helium nuclei) to generate the Hoyle state of carbon-12 is influenced by the existence of neutrons in stars.
Through neutron upscattering, the neutrons can accelerate the fusion of the three alpha particles. The time-reverse of this process, where neutrons split carbon into three alpha particles, is used to measure the reaction in a laboratory setting.
Scientists from the University of Birmingham, Texas A&M University, Ohio University, Washington University in St. Louis, the Université Paris-Saclay, and the Korean Institute for Basic Science participated in the project.
The scientists fired neutrons into TexAT; a detector developed and built at the Cyclotron Institute at Texas A&M, using a source of high-energy neutrons at the Edwards Accelerator Laboratory at Ohio University.
The scientists then calculated the probability that carbon-12 would divide into three alpha particles. They discovered that carbon-12 has a lower tendency to split into three alpha particles than predicted by theoretical models.
This shows that neutron upscattering has a smaller impact than initially thought on the synthesis of carbon-12. This answers the question of whether neutron upscattering can affect how a star burns to produce heavier elements, which has been debated for about 50 years.
Journal References:
Bishop, J., et al. (2022) Neutron-upscattering enhancement of the triple-alpha process. Nature Communication. doi.org/10.1038/s41467-022-29848-7.
Bishop, J. et al. (2020) Almost medium-free measurement of the Hoyle state direct-decay component with a TPC. Physical Review. abstract/10.1103/PhysRevC.102.041303.