The Modular X- and Gamma-Ray Sensor (MXGS) of the ASIM Payload on the International Space Station

Abstract

The Modular X-and Gamma-ray Sensor (MXGS) is an imaging and spectral X-and Gamma-ray instrument mounted on the starboard side of the Columbus module on the International Space Station. Together with the Modular Multi-Spectral Imaging Assembly (MMIA) (Chanrion et al. this issue) MXGS constitutes the instruments of the Atmosphere-Space Interactions Monitor (ASIM) (Neubert et al. this issue). The main objectives of MXGS are to image and measure the spectrum of X-and γ -rays from lightning discharges, known as Terrestrial Gamma-ray Flashes (TGFs), and for MMIA to image and perform high speed photometry of Transient Luminous Events (TLEs) and lightning discharges. With these two instruments specifically designed to explore the relation between electrical discharges, TLEs and TGFs, ASIM is the first mission of its kind. With an imaging system and a large detector area MXGS will, for the first time, allow estimation of the location of the source region and characterization of the energy spectrum of individual events. The sensors have fast readout electronics to minimize pileup effects, giving high time resolution of photon detection for comparison with measurements on µs-time scales of lightning processes measured by the MMIA and other sensors in space or on the ground. The detectors cover the large energy range of the relevant photon energies. In this paper we The ASIM mission on the International Space Station B N. Østgaard Page 2 of 28 N. Østgaard et al.describe the scientific objectives, design, performance, imaging capabilities and operational modes of the MXGS instrument.

On the High‐Energy Spectral Component and Fine Time Structure of Terrestrial Gamma Ray Flashes

“…Concerning the fine time structure of TGFs, our measurements do not show any clear evidence of a fine time structure of TGFs on microsecond time scale. Finally, valuable insight both on the TGF high‐energy spectrum and time structure will be provided by the Modular X‐ and Gamma ray Sensor (MXGS) of the recently launched ASIM mission (Østgaard et al, ), sensitive up to 40 MeV and with a readout electronics specifically designed and tailored for the first time to an accurate control of dead time and pileup effects in the TGF high‐photon flux regime.…”

The First Terrestrial Electron Beam Observed by the Atmosphere‐Space Interactions Monitor

“…We verified that the TEB spectrum does not change significantly as a function of radial distance from its center (the corresponding data and figures are presented in the associated data repository, see Acknowledgments). The results of this first simulation are then used as input for the ASIM Geant4 mass model (presented by Østgaard et al, ), and a set of simulated MXGS‐HED spectra are produced. The quality of the simulated data compared to the observation can be quantified using the reduced chi squared ( ).…”

“…The MXGS instrument consists of a Low Energy Detector (LED) and a High Energy Detector (HED). Østgaard et al () described the instrument in details. The HED is based on 12 bismuth germanium oxide (BGO) scintillator crystal bars of 15 × 5 × 3.2 cm 3 interfaced to photomultiplier tubes and is sensitive to energies of ∼300 keV to ∼40 MeV.…”

Gamma Ray Glow Observations at 20‐km Altitude

“…The three BGO bars were energy calibrated independently on ground using known gamma ray sources and the muon peak, which was modeled by Geant4 simulation to be at 31.7 MeV for a 3.2‐cm‐thick BGO bar. The UIB‐BGO detector and front‐end electronics are similar to the high‐energy detector on Atmosphere‐Space Interactions Monitor (Østgaard et al, ), but contrary to Atmosphere‐Space Interactions Monitor, which has a triggered system, a data acquisition and storage system was developed for UIB‐BGO to enable continuous data recording during the flights.…”