Abstract
An Engineering Radiation Monitor (ERM) has been developed as a supplementary spacecraft subsystem for NASA’s Radiation Belt Storm Probes (RBSP) mission. The ERM will monitor total dose and deep dielectric charging at each RBSP spacecraft in real time. Configured to take the place of spacecraft balance mass, the ERM contains an array of eight dosimeters and two buried conductive plates. The dosimeters are mounted under covers of varying shielding thickness to obtain a dose-depth curve and characterize the electron and proton contributions to total dose. A 3-min readout cadence coupled with an initial sensitivity of ∼0.01 krad should enable dynamic measurements of dose rate throughout the 9-hr RBSP orbit. The dosimeters are Radiation-sensing Field Effect Transistors (RadFETs) and operate at zero bias to preserve their response even when powered off. The range of the RadFETs extends above 1000 krad to avoid saturation over the expected duration of the mission. Two large-area (∼10 cm2) charge monitor plates set behind different thickness covers will measure the dynamic currents of weakly-penetrating electrons that can be potentially hazardous to sensitive electronic components within the spacecraft. The charge monitors can handle large events without saturating (∼3000 fA/cm2) and provide sufficient sensitivity (∼0.1 fA/cm2) to gauge quiescent conditions. High time-resolution (5 s) monitoring allows detection of rapid changes in flux and enables correlation of spacecraft anomalies with local space weather conditions. Although primarily intended as an engineering subsystem to monitor spacecraft radiation levels, real-time data from the ERM may also prove useful or interesting to a larger community.
Highlights
The Radiation Belt Storm Probes (RBSP) spacecraft must operate in a high-radiation environment consisting primarily of energetic electrons and protons (Mauk et al 2012, this issue)
We established four challenging objectives for the Engineering Radiation Monitor (ERM) experiment: (1) Measure all relevant electrical parameters of the Radiation-sensing Field Effect Transistors (RadFETs) devices in flight to understand better their response to dose in a mixed-particle environment and develop confidence in their long-term application to spacecraft dosimetry; (2) Characterize the actual on-orbit dosedepth curve using an array of dosimeters at different effective shielding depths and compare this result with predictions; (3) Profile the dose and dose rate of the RBSP orbit over a period of two years or longer; (4) Measure the internal charging conditions on each spacecraft ranging from quiescent to the largest, worst-case storm with sufficient sensitivity and time resolution to enable correlation of spacecraft anomalies with local space weather conditions
Each ERM contains an array of RadFET dosimeters with carefully-designed absorbers ranging in thickness from 50 microns to 9 mm
Summary
The RBSP spacecraft must operate in a high-radiation environment consisting primarily of energetic electrons and protons (Mauk et al 2012, this issue). Real-time monitoring of the dynamic radiation dose and dose rate near the location of sensitive electronics can help eliminate inaccuracies associated with static particle environment models and the transport of radiation through simplified spacecraft shielding models. Analytical estimates from the existing models show long-term radiation rates averaged over six years that are approximately 4X to 10X higher than the measured on-orbit dose for the same time period This large discrepancy appears to be due primarily to the dynamics of the electron fluence during the period from 1997 to 2003, but could be due to the use of simplified spacecraft geometry models or calculation methods. We were able to develop and build the ERM on an accelerated schedule and deliver the qualified flight units to the spacecraft at the same time as other payload elements
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