Scientists Reassess Source of Radioactive Beryllium in Early Solar System
A team at the Department of Energy's Oak Ridge National Laboratory (ORNL) investigated the radioactive isotope beryllium-10, present during the solar system's formation approximately 4.5 to 5 billion years ago. They explored the potential for this isotope to form in significant amounts during the explosive deaths of massive stars known as supernovae.
"It is unlikely that such a stellar explosion is the main source for this isotope, as it is observed in the early solar system," said Raphael Hix, an ORNL nuclear astrophysicist involved in the study published in the journal Physical Review C. The findings "help us to understand the history of the solar system and the galaxy as a whole."
The researchers suggest that beryllium-10 likely results from cosmic ray spallation, where high-energy protons and other isotopes, such as carbon-12, interact at nearly the speed of light across the universe.
When stars die, they release core atoms into the interstellar medium - the low-density matter filling the space between stars in a galaxy. This process, called nucleosynthesis, eventually contributes to forming new stars and planets. The atomic mix includes carbon-12, which can be struck by cosmic rays.
"When these high-energy rays collide with carbon-12 atoms, it literally breaks the nucleus apart, and what's left can include beryllium-10," Hix explained.
Around 4.5 billion years ago, the solar system emerged from the collapse of a massive cloud of gaseous molecules, forming a swirling disk known as the solar nebula. Over millions of years, gravity consolidated this material, creating the sun and planets. Beryllium-10, with a short half-life of 1.4 million years, would have decayed long ago, but boron-10, a decay product of beryllium-10, is found in some meteorites, indicating freshly made beryllium-10 was present when the solar system formed.
Hix, alongside then-postdoctoral researcher Andre Sieverding, now at Lawrence Livermore National Laboratory, utilized DOE's National Energy Research Scientific Computing Center to simulate the production of elements and isotopes by supernova explosions. These explosions occur in stars 10 to 25 times the mass of the sun. University of Tennessee undergraduate Daniel Zetterberg, and colleagues at the University of Notre Dame, also contributed.
If short-lived isotopes like beryllium-10 originated from supernovae, it would suggest the solar system's formation was directly triggered by a supernova. However, recent calculations dispute this for beryllium-10. Updated nuclear experiment data revealed properties increasing the reaction rate converting beryllium-10 into other isotopes, surpassing outdated estimates by over 50 years. Laboratory measurements with improved experiments offered a clearer picture, showing new reaction rates up to 33 times faster than previous ones.
Sieverding, Zetterberg, and Hix found the new rate swiftly destroys beryllium-10 in supernovae, making it unlikely for such events to produce sufficient amounts of the isotope to account for its presence in meteorites.
"This makes it almost certain that spallation really is the source for beryillium-10," Hix added. "Unless there are major changes in the models for the structure of stars in this mass range, these findings point towards a need for another source of beryllium-10."
Sieverding said, "As a result, it is unlikely that a supernova was the source of the beryllium-10 in the early solar system."
This study was a collaboration among various institutions. ORNL's Sieverding, Hix, and Zetterberg handled astrophysical simulations and nucleosynthesis calculations. At the University of Notre Dame, Jaspreet Randhawa, Tan Ahn, and Richard James deBoer analyzed experimental data to determine the relevant reaction rate. At Germany's Technical University of Darmstadt, Riccardo Mancino and Gabriel Martinez-Pinedo performed theoretical calculations of reaction rates. Their experiments measured nuclear properties to derive the reaction rate since it was too slow for direct measurement.
Research Report:Role of low-lying resonances for the reaction rate and implications for the formation of the Solar System