How do beryllium-10 measurements help estimate the travel time of galactic cosmic rays?

Beryllium-10 is a radioactive isotope that serves as a "clock" for estimating the travel time of galactic cosmic rays (GCRs) through the Milky Way Galaxy.1

These cosmic rays are primarily atomic nuclei that produce light elements—such as lithium, beryllium, and boron—when they collide with interstellar gas during their journey. Among these fragments, beryllium-10 is particularly significant because it is radioactive, with a half-life of approximately 1.5 million years.1

Because the number of beryllium-10 particles that survive to be detected near Earth depends on their total travel time, scientists can compare the observed abundance of this isotope against the expected production rates from interstellar collisions. By analyzing how many of these radioactive fragments have decayed during their transit, researchers can estimate that galactic cosmic rays have been traveling for roughly 15 million years before reaching our solar system.1