The Earth’s magnetosphere offers a wealth of information on particle dynamics, acceleration, and trapping. Fast neutrons, produced in the Earth’s atmosphere by the impact of galactic cosmic rays (GCRs) and solar energetic particles (SEPs), are an important but poorly measured component of the radiation environment in the inner magnetosphere. Cosmic ray albedo neutron decay (CRAND), whereby atmospheric neutrons beta-decay into protons and electrons, is a significant source of energetic protons in the inner radiation belt. Current models of the inner proton belt rely heavily on Monte Carlo simulations for the CRAND component, validated primarily by a handful of single-point balloon measurements from the 1970s.

A neutral-particle instrument (IRAD FY14) is being built for CubeSat platforms to address several critical science goals of solar and heliospheric physics, as well as the radiation environment in low Earth orbit. A key asset of the instrument is its ability to measure neutral and charged radiation. The instrument relies on modern scintillators and silicon photomultiplier (SiPM) readout, and is thus inherently robust, cost-effective, compact, and modular.

The instrument is a neutron spectrometer with the primary objective of measuring the inner zone equilibrium injection flux by directly detecting the dominant source of high-energy proton albedo neutron decay in the energy range of –10 to 100 MeV. These observations provide a critical input to the CRAND model, enabling a better understanding of dynamics of the inner belts and, consequently, the potential radiation hazards to space-borne assets and local space weather.

Because volume, mass, and power are severely constrained on small satellites, taking full advantage of these platforms will require flexible, compact, lightweight, and available for rapid integration into payloads. The instrumentation proposed will be based on scintillators with advanced, compact readout and electronics. Scintillator detector materials have a long history in the measurement of gamma rays and neutrons, and provide a variety of advantages for space instrumentation including low cost, high stopping power, straightforward implementation that is readily scalable, room-temperature operation, and good energy and timing resolution.

This work was done by Georgia De Zolfo and David Suhl of Goddard Space Flight Center. GSC-16991-1