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

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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.

Author information


  1. Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA

    • Xinlin Li
    • , Kun Zhang
    • , Hong Zhao
    •  & Daniel N. Baker
  2. Ann and H. J. Smead Department of Aerospace Engineering Sciences, University of Colorado, Boulder, Colorado 80309, USA

    • Xinlin Li
    •  & Kun Zhang
  3. Space Vehicles Directorate, Air Force Research Laboratory, Kirtland AFB, New Mexico 87117, USA

    • Richard Selesnick
  4. Heliophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA

    • Quintin Schiller
  5. Space Sciences Laboratory, University of California, Berkeley, California 94720, USA

    • Michael A. Temerin


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X.L. developed the project, directed the data analysis and was primarily responsible for writing the paper. R.S. was involved with the project from the beginning, gave advice on data analysis, and helped to revise the paper. Q.S. calibrated REPTile’s response and produced related data and figures. K.Z. and H.Z. performed data analysis and produced related figures. D.N.B. gave advice on data analysis and revision of the paper. M.A.T. gave advice on data analysis and helped to revise the paper.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Xinlin Li.

Reviewer Information Nature thanks M. Hudson and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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