Solar flare neutrons observed on the ground and in space

From RHESSI Wiki

Trickier to detect than gamma-rays, energetic neutrons nonetheless offer a valuable window on solar ion acceleration. Accelerated ions in the MeV energy range and above produce them when they collide with ambient nuclei. Some slow down and thermalise in the solar atmosphere, contributing to the observed flux in the 2.223 MeV deuterium formation line if they capture on protons. Others escape completely from the solar atmosphere, potentially to be detected in space or even, if they have high enough energy, on Earth.

The possibility of detecting energetic neutrons from flares was first aired at the very start of the 1950s by Ludwig Biermann, but this was only achieved observationally some decades later, using the Gamma-Ray Spectrometer instrument on the Solar Maximum Mission.