Why are lithium, beryllium, and boron unusually common in galactic cosmic rays?

Although lithium, beryllium, and boron are rare throughout the universe and within stars, they are surprisingly abundant among galactic cosmic rays (GCRs).12

These light elements are not primary components of cosmic rays at their source. Instead, they are produced through a process called spallation, which occurs when heavier primary cosmic ray nuclei—such as carbon and oxygen—collide with the thin interstellar gas composed mostly of hydrogen.1

Because these collisions happen as the cosmic rays travel through the galaxy, the presence of these light nuclei serves as a diagnostic tool for scientists. By measuring the abundance of these fragments, researchers can reconstruct the history of the cosmic rays' journey and estimate that they have been traveling through the galaxy for approximately 15 million years.1