X-Gamma-ray Imaging Spectrometer (XGIS)

We propose to develop the Compton imaging and polarization capabilities of a novel X-Gamma ray Imaging Spectrometer (XGIS) being designed for the proposed European Transient High Energy Sky and Early Universe Surveyor  (THESEUS) mission. The use of Caesium iodide (CsI) scintillators combined with Silicon Drift Detector Photodiodes (SDD-PDs) can provide the unprecedented means to detect both hard X-rays and gamma rays with a single instrument, covering an energy range of 2 keV to 20 MeV.  The Silicon Drift Detectors can be used both as direct X-ray detectors and to detect the scintillation light of gamma rays that interact in the CsI bars. An enhancement of this baseline design would provide source localization via Compton imaging and sensitivity to photon polarization, both of which are not possible at these energies with the current generation of operating gamma-ray telescopes.  Polarization measurements at these energies, in particular, have long been sought in gamma-ray astrophysics and would provide a new window into the high-energy Universe. To implement the Compton imaging and polarization capabilities, we propose to develop the ability to determine gamma-ray interaction position along the CsI bars by comparing the light signals measured at each end of the instrument. We will develop electronics for this purpose, and conduct simulations to optimize the design for polarization measurements, without degrading other operating modes.  This augmentation to the XGIS harware will be developed into a NASA Mission of Opportunity contribution to the European THESEUS gamma-ray mission, providing MSFC a role in a future flagship-class mission

The X-Gamma ray Imaging Spectrometer (XGIS) is a novel detector concept currently being developed by Italian collaborators at the University of Bologna for the proposed THESEUS mission. THESEUS is a medium-size (M5) mission concept proposed to the European Space Agency by a large international collaboration in response to the call for the Cosmic Vision Programme.  It was one of three missions selected for an assessment phase study (Phase-A equivalent) in May 2018, with a downselect scheduled for 2021, and launch in 2030. THESEUS aims to explore the early Universe by detecting the X-ray and gamma-ray signatures of the most distant gamma-ray bursts, which went supernova only a few hundred million years after the big bang. This is achieved via a unique payload providing an unprecedented combination of 1) an all-sky monitor in a very broad energy band; 2) focusing capabilities in the soft X-ray band providing large grasp and high angular resolution; and 3) on board near-IR capabilities for immediate transient identification and distance determination. Through these measurements, THESEUS aims to trace the cosmic star formation rate and chemical evolution of the Universe. Because of its all-sky monitor capabilities, THESEUS will also discover many new transients of several types: thermonuclear bursts, magnetars, binary star supernovae shock breakouts, tidal disruption events, and flares from active galactic nuclei. THESEUS will also provide valuable observations in the multimessenger era, in which gravitational wave and neutrino detectors open a new window into the transient Universe.

The XGIS design was developed to help achieve these goals by providing the ability to detect both hard X-rays and gamma-rays over a unprecedentedly broad energy band with a single instrument. The design combines Caesium iodide (CsI) scintillator bars with Silicon Drift Detectors (SDDs) on either end to achieve this capability. The top SDD can measure X-rays that directly interact in the SDD, and both SDDs can be used to detect the scintillation light of gamma rays that interact within the CsI bars. The direct interactions in the SDDs can be distinguished from the scintillation signals though pulse shape discrimination. A sensitivity to a very broad energy range can be achieved through this method, ranging from 2 keV to 20 MeV. This represents a significant increases in the sensitivity to X-ray photons compared to existing gamma-ray detectors. A single XGIS module consists of 32x32 scintillator bars, for a total of 12,288 bars. The baseline design includes a light weight 500 μm thick stainless steel mask 70 cm above the detector plane, providing a fully-coded field of view of ~10x10 deg². The coded mask casts a “shadow” onto the detector plane, which can be used to provide source localization to within 5 arcminutes in the hard X-ray and soft gamma ray regimes, roughly 2-30 keV.  The coded mask becomes transparent to gamma-rays above 30 keV, though, limiting the energy range over which the detector can localize high-energy sources.

We propose to take advantage of the unique XGIS design to develop the Compton imaging capabilities of the detector to provide localizations for sources that emit most of their light above 30 keV.  Compton imaging would rely on the use of Compton scattering of incident gamma-rays within the CsI scintillator bars.  As gamma-rays enter the scintillator, they interact and scatter off electrons bound to the CsI atoms.  During this interaction, the incoming gamma-ray transfers some of its energy to a scattered electron, which is measured by the amount of scintillation light that is emitted during the process. The scattered photon then travels and interacts at a different location within the detector, again imparting some of its remaining energy to another electron until the energy of the original gamma-ray has been entirely captured by the detector and converted to scintillated light. If the position and energy deposited in each interaction is measured, then it is possible to determine the deflection angle to the origin vector of the incident gamma-ray using the Compton equation.  With a single photon from a source, the localization can only be determined to an annulus on the sky because the rotation of the reconstructed origin vector is unknown. The width of the annulus is set by the uncertainty in the energy measurement. With many photons, the intersection of the annuli can constrain the localization to form a Compton image. The total light collected by the two SDDs at either end of the scintillator crystal yields the total energy of a single interaction, whereas the ratio of the light collected by each SDD reveals the vertical position along the scintillator bar. The horizontal position in the instrument plane will be determined by the bar in which the interaction occurred. Using this information, in principle, the energy and direction of each incident gamma-ray can be reconstructed.

            In addition to the Compton Imaging capabilities, we also seek to investigate the ability of the XGIS design to constrain the polarization of the incident gamma-rays. Linearly-polarized gamma-rays have a higher scattering cross section perpendicular to their polarization vector, which induces an azimuthal modulation to their scattering pattern. By measuring if a preferred direction of scattering exists within the XGIS module, the polarization fraction and direction can be inferred.  The degree to which we will be able to discern this modulation in the scattering angle will depend on several factors, including the accuracy with which we can reconstruct the interaction positions, which will be determined through a combination of simulations and laboratory testing.  All aspects of this idea have been demonstrated in other systems, the issue is whether these methods will work in this particular system with its particular scintillator material, geometry, light collection and SDDs.  The proposed simulations and lab measurements will advance the technology to TRL 4.

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