The Van Allen Probes Engineering Radiation Monitor: Mission Radiation Environment and Effects

The engineering radiation monitor (ERM) measures dose, dose rate, and charging currents on the Van Allen Probes mission to study the dynamics of Earth’s Van Allen radiation belts. Measurements from this monitor show a variation in dose rates with time, a correlation between the dosimeter and charging current data, a map of charging current versus orbit altitude, and a comparison of measured cumulative dose to prelaunch and postlaunch modeling. The measurement results and surveys of the radiation hardness for the spacecraft and science instrument electronics enable the team to predict the length of possible mission extensions. The ERM data have proved useful in investigations of two spacecraft anomalies. 3. Provide measurements that allow correlation of anomalies with radiation environmental factors. 4. Provide data and knowledge to support potential mitigation of anomalies. 5. Acquire environmental data vital for future missions to the same region of space. 6. Provide feedback on the accuracy of the environmental models used to plan the original mission. An engineering radiation monitoring experiment was devised for integration into the overall philosophy of the Van Allen Probes mission and to specifically track the total cumulative ionizing dose and dose rates due to Earth’s trapped radiation belts and their dynamics resulting from solar events and storms. Explorer 1 discovered Earth’s radiation belts at the beginning of the space age in 1958. Figure 1 shows a sketch of the spacecraft orbits and Earth’s Van Allen radiation belts. The two spaceINTRODUCTION For more than a half century, The Johns Hopkins University Applied Physics Laboratory (APL) has designed spacecraft electronics and science instruments that are exposed to the space radiation environment and its effects. The design and fabrication of an accurate and reliable radiation monitor has become increasingly important for the unique and challenging missions now occurring in Earth orbit and interplanetary space. The space radiation environment is important for spacecraft operations, spacecraft system design, mission planning, and astronaut safety in manned missions. In August 2012, the two Van Allen Probes spacecraft launched into an Earth geosynchronous transfer orbit (GTO). The engineering radiation monitor (ERM) captures data from the spacecraft, with the following goals: 1. Provide measurements that enable the mission planning team to adapt to the radiation environment. 2. Provide information to support decisions for future missions with longer mission lifetimes. R. H. Maurer and J. O. Goldsten Johns Hopkins APL Technical Digest, Volume 33, Number 3 (2016), www.jhuapl.edu/techdigest 184 craft are positioned and phased such that one will lap the other approximately four times per year, providing coverage of many relative locations and times. THE ENGINEERING RADIATION MONITOR The ERM (see Table 1 for specifications) was developed as a supplementary spacecraft experiment for NASA’s Van Allen Probes mission. The mass is 1.5 kg, the power is 0.2 W when operating, and the dimensions are 18  18  6 cm. It was designed for the baseline ~800-day mission. See J. O. Goldsten et al. for a detailed description of the ERM and its operation.1 In this article, we present a brief overview of its capabilities. The ERM monitors total dose, dose rate, and deep dielectric charging at each spacecraft in real time. Designed 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 shield thickness (0.05 mm Al, 0.39 mm Mg, 0.78 mm Mg, 1.16 mm Mg, 1.55 mm Mg, 2.32 mm Mg, 4.66 mm Mg, 9.0 mm Al) to obtain a dose–depth curve and to characterize the electron and proton contributions to total dose. The dosimeters are REM Oxford type RFT300 (300-nm gate oxide thickness) dual radiation-sensing field effect transistors (RadFETs) and operate at zero bias (with the gate held at 0 V during exposure) to preserve their response even when powered off for extended periods. The range of the RadFETs extends above 1000 krad(Si) to avoid saturation over the expected duration of the mission, and the resolution is about 10 rad(Si). Two large-area (~10 cm2) charge monitor plates set behind 1.0and 3.8-mm-thick aluminum covers 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) with sufficient sensitivity (~0.1 fA/ cm2) to characterize quiescent conditions as well. High time-resolution (5 s) monitoring allows detection of rapid changes in flux and enables correlation of spacecraft anomalies under local space weather conditions. Figure 2 shows the location of the ERM on the spacecraft. The mounting location near the edge of the deck assures a clear field of Figure 1. Both Van Allen Probes spacecraft operate in highly elliptical GTO orbits and spend a substantial part of their mission life in the Van Allen radiation belts. The two orbits have apogee altitudes between 30,050 and 31,250 km, perigee altitudes between 500 and 675 km, a period of 9 h, and inclination of 10°. Figure 2. (Left) View of the Van Allen Probes spacecraft showing the location of the ERM near the bottom center between the two lower solar panels. The ERM is located toward the edge of the aft deck and mounts near a balance mass location. (Right) Zoom on the mounting detail (insulation blanket not shown for clarity). The Van Allen Probes Engineering Radiation Monitor Johns Hopkins APL Technical Digest, Volume 33, Number 3 (2016), www.jhuapl.edu/techdigest 185 view for the two charge monitors (circular depressions in the cover) and the dosimeter array (the rectangular aperture with the thinnest absorber at its center). The small amount of absorption or shielding due to the multilayer insulation blanket (not shown) over the aperture is only significant when compared to the thinnest part of the cover and has been included in the design phase GEANT radiation transport model. Figure 3 provides internal and external views of the ERM. The rectangular aperture in the cover spans the dosimeter array and contains a variable thickness absorber to characterize dose versus depth. The circular depressions above the charge monitor plates provide two levels of shielding thickness to gauge deep dielectric charging currents over an extended range. The ERM is sensitive to radiation penetrating these defined apertures as well as from the surrounding thick box walls, necessitating the derivation of an effective thickness for each RadFET by using the GEANT radiation transport modeling.