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.
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This work was supported in part by the National Science Foundation (NSF) CubeSat Program, NSF grant AGS 1443749, and NASA/Radiation Belt Storm Probes (RBSP)-Energetic particle, Composition and Thermal plasma (ECT) funding through Johns Hopkins University (JHU)/Applied Physics Laboratory (APL) contract 967399 under prime NASA contract NAS5-01072.
The authors declare no competing financial interests.
Reviewer Information Nature thanks M. Hudson and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
Electron flux measured by the DEMTER satellite20, which has an altitude of 710 km and inclination of 98.3°, plotted versus geographic longitude for different energies (E) and different L bins, and averaged over every 10° for the time period 20–30 April 2010. The magnitude of flux of quasi-trapped electrons at 0.5 MeV is the same as that measured by REPTile.
Extended Data Figure 2 Flux conversion on the basis of the β-decay spectrum of albedo neutrons and the detector’s energy response.
The ambient electron flux (J; solid line) is normalized and calculated from equation (1). The dashed line, representing the known β-decay spectrum (φ(E)), is normalized to a maximum value of 1 (but the value of φ = 1.2 used in the text is normalized with the area under the curve that is equal to 1) and calculated using equation (4) of ref. 21). The response of REPTile channel 1 (Ch1) to normally incident electrons is shown by the dotted line. The y axis shows J, the normalized β-decay spectrum (maximum normalized to 1), and the channel 1 efficiency (maximum is 1).
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Li, X., Selesnick, R., Schiller, Q. et al. Measurement of electrons from albedo neutron decay and neutron density in near-Earth space. Nature 552, 382–385 (2017). https://doi.org/10.1038/nature24642
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