Scientists have for the first time measured how arsenic-73 captures a proton to grow selenium-74, a reaction long thought to occur only inside exploding stars.
The measurement was made inside a hydrogen-filled chamber at Michigan State University’s Facility for Rare Isotope Beams, where a beam of unstable arsenic-73 was directed into hydrogen gas, allowing protons to be captured and the resulting selenium-74 to emit detectable gamma rays as it stabilized.
Selenium-74 is the lightest member of the p-nuclei, a group of proton-rich isotopes that cannot be formed through the standard neutron-capture processes responsible for most heavy elements, leaving their origins a puzzle for over six decades.
For years, this specific step in the gamma process—a chain of light-driven nuclear reactions in supernovae—had remained theoretical, as the isotopes involved decay too quickly to study in conventional labs.
By producing arsenic-73 using FRIB’s ReA accelerator and observing its transformation, the team led by Artemis Tsantiri provided the first direct experimental anchor for a reaction that models had only estimated.
The gamma rays released after proton capture acted as a signal, letting researchers count reaction frequency and derive a rate that had previously relied on extrapolation.
This data cuts the uncertainty around selenium-74’s predicted abundance in stellar explosions by about half when plugged into astrophysical simulations.
Yet the measurement also reveals a mismatch: observed abundances still don’t fully align with predictions, suggesting the gamma process alone may not explain selenium-74’s presence in the universe.
Tsantiri, a postdoctoral fellow at the University of Regina, noted that access to experimental data on short-lived isotopes like arsenic-73 has been nearly nonexistent for over 60 years, making this breakthrough possible only because FRIB can isolate and accelerate such fleeting nuclei.
The study involved more than 45 researchers from 20 institutions across the U.S., Canada, and Europe, and was published in Physical Review Letters.
Understanding both the formation and destruction of selenium-74 is critical, as stellar photons in supernovae can break it apart just as quickly as it forms.
This experiment doesn’t close the case on p-nuclei origins—it sharpens the debate by showing where current models succeed and where they fall short.
Why has the origin of p-nuclei like selenium-74 been so difficult to study?
The isotopes involved are extremely short-lived and decay rapidly, making them nearly impossible to isolate and measure in conventional laboratory settings without specialized beam facilities like FRIB.
How does this fresh measurement change what scientists thought about selenium-74’s formation?
It provides the first direct experimental data on a key reaction in the gamma process, reducing uncertainty in abundance predictions by about half and revealing gaps that suggest current models are incomplete.
What role does the Facility for Rare Isotope Beams play in this kind of research?
FRIB’s ability to produce, accelerate, and study rare isotopes like arsenic-73 independently of its main accelerator enables experiments on short-lived nuclei that were previously considered inaccessible.