In-flight calibration of RADEM, the JUICE mission radiation monitor

Abstract

The RADiation-hard Electron Monitor (RADEM) aboard the Jupiter Icy Moons Explorer (JUICE), launched on 14 April 2023, measures high-energy protons and electrons during the cruise phase and will continue throughout the nominal mission. Initial in-flight observations could not be explained by pre-flight ground calibration, motivating an in-flight calibration campaign. We calibrated the RADEM sensors using galactic cosmic rays by progressively increasing detector thresholds, thereby modifying their response to high-energy particles. Threshold-dependent in-flight count rates were compared with theoretical expectations derived from the Badhwar-O'Neill 2020 galactic cosmic ray model and corresponding response functions. These results were used to derive new in-flight calibration coefficients and to develop a flux reconstruction algorithm based on the bow-tie method. In several cases, the in-flight calibration slopes differ by up to an order of magnitude from ground calibration values. Proton fluxes from solar energetic particle events reconstructed with this method agree within a factor of two with independent measurements from the Solar and Heliospheric Observatory. These results demonstrate that RADEM provides accurate proton flux measurements in interplanetary space and is well suited for both single-spacecraft analyses and coordinated multi-mission studies of solar energetic particles. While electrons were clearly detected during the JUICE Lunar-Earth gravity assist, reliable reconstruction of their fluxes requires further analysis.

Figures (16)

• Figure 1: JUICE cruise phase trajectory (purple) as a function of its distance to the Sun and its four gravity assists. The average distance of Venus (green), Earth (blue), Mars (red) and Jupiter (orange) to the Sun are shown for reference. JUICE is expected to perform three gravity assists of Earth (the first combined with a lunar gravity assist), and one of Venus.
• Figure 2: JUICE spacecraft visualization with marked location of RADEM (adapted from radem_paper). Also indicated are the spacecraft and RADEM reference frames archived in the Spacecraft, Planet, Instrument, C-matrix, Events (SPICE) JUICE kernels with the names JUICE$\_$SPACECRAFT and JUICE$\_$RADEM ESA_JUICE_SPICE_2022. The location of JUICE$\_$RADEM in the JUICE$\_$SPACECRAFT reference frame is: $X_{sc}$= 1.1853 m, $Y_{sc}$ = 0.4624 m, $Z_{sc}$ = 1.7822 m.
• Figure 3: Side cut-view of the PDH, HIDH and EDH with their respective silicon sensors (green), copper collimators (beige), and aluminum (gray) and tantalum (red) absorbers. Dimensions are to scale.
• Figure 4: Example of a particle (yellow) interacting with a detector stack. The particle is able to pass through the first absorber (gray) and the first sensor (D1 - green) and stops in the second absorber. Depending on the the particle species (electron, proton or ion), and on the threshold of HGLT 1 and HGHT 1, it generates a signal (black) that may or not activate the first pattern/counter but not the second.
• Figure 5: RADEM Geant4 model. The aluminum is depicted in gray, the tantalum shielding in black, the copper collimator in blue, and the 6-wall spacecraft aluminum equivalent in red.