Feasibility study of a proton fluence monitor for LEO-Nanosatellite missions based on displacement damage induced in GaAs-LED

Abstract

High-energy protons trigger irreparable displacement damage (DD) in unbiased Gallium Arsenide (GaAs) Light-Emitting Diode GaAs-LED (GAL) . The DD is caused by non-ionizing-energy-loss (NIEL) process and results in reduction of light emission of the GAL when connected to a power source. Based on this finding a lightweight, low current consuming proton detector for space applications was developed. A commercial off the shelf (COTS) yellow GAL optically coupled to a Light-to-Frequency converter chip (LFC) made the basic assembly of the proposed proton detector. The device was calibrated using selected GAL samples irradiated with a 180 MeV proton beam from a proton-therapy medical cyclotron to 2, 10, 50, 100, 200 and 300 Gy dose levels. The light output of the GAL was measured with the detector assembly and presented as frequency (kHz). Protons of energy distribution between 100 keV to ∼500 MeV are trapped in the inner-shell of the Van Allen belt (VAB) surrounding the earth, whereas the intensity of trapped electrons (Emax ∼10 MeV) remains insignificant. The GAL light output data (frequency) was fitted with a second-degree polynomial function of imparted proton dose (∼ number of protons). The results were parameterized for widely used NASA radiation belt models AP8-Max and AP8-Min using the proton NIEL distribution in Gallium Arsenide (GaAs). Spacecrafts, in particular Nanosatellites operating in low earth orbit (LEO) environment are primarily exposed to those trapped protons in the VAB . Nanosatellites endure severe radiation exposure while passing through the South Atlantic Anomaly (SAA) region. The footprint, mass and average current consumption of the proton detector was 0.027 cm2, 0.28 g and 2.5 mA respectively. The highest detectable level proton fluence (protons⋅cm−2) was evaluated to be 1.17× 109 and 1.01× 109 for AP8-Max and AP8-Min models respectively. Hence the proposed GAL based radiation detector is only suited for proton fluence monitoring on board LEO-Nanosatellites (NanoSat) of short lifetime, usually 6–18 months.