Measurement of electrons from albedo neutron decay and neutron density in near-Earth space

Electrons derived from cosmic rays become trapped in the radiation belts that surround Earth, but how the electrons are generated has been uncertain; new measurements confirm the involvement of neutron decay. When cosmic rays hit the upper atmosphere, the collisions with atoms create 'albedo neutrons' which subsequently decay to protons and electrons (and electron antineutrinos) with a half-life of about 10 minutes. This is the main source of the protons trapped in Earth's Van Allen radiation belts. In principle, this process could also be the source of the trapped electrons, but it has been difficult to clarify because the electron intensity varies greatly whereas the neutron-decay rate should be fairly constant. Xinlin Li and collaborators report measurements of the electron density near the inner edge of the inner radiation belt and demonstrate that this population of electrons indeed comes from neutron decay. The Galaxy is filled with cosmic-ray particles, mostly protons with kinetic energies greater than hundreds of megaelectronvolts. Around Earth, trapped energetic protons, electrons and other particles circulate at altitudes from about 500 to 40,000 kilometres in the Van Allen radiation belts. Soon after these radiation belts were discovered six decades ago, it was recognized that the main source of inner-belt protons (with kinetic energies of tens to hundreds of megaelectronvolts) is cosmic-ray albedo neutron decay (CRAND)1. In this process, cosmic rays that reach the upper atmosphere interact with neutral atoms to produce albedo neutrons, which, being prone to β-decay, are a possible source of geomagnetically trapped protons and electrons. These protons would retain most of the kinetic energy of the neutrons, while the electrons would have lower energies, mostly less than one megaelectronvolt. The viability of CRAND as an electron source has, however, been uncertain, because measurements have shown that the electron intensity in the inner Van Allen belt can vary greatly, while the neutron-decay rate should be almost constant2,3. Here we report measurements of relativistic electrons near the inner edge of the inner radiation belt. We demonstrate that the main source of these electrons is indeed CRAND, and that this process also contributes to electrons in the inner belt elsewhere. Furthermore, measurement of the intensity of electrons generated by CRAND provides an experimental determination of the neutron density in near-Earth space—2 × 10−9 per cubic centimetre—confirming theoretical estimates4.