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The X-Gamma ray Imaging Spectrometer (XGIS)

The X-Gamma-ray Imaging Spectrometer (XGIS) comprises two units (telescopes). The two units are pointed at offset directions in such a way that their FOV partially overlap. Each unit (Figure 6) has imaging capabilities in the low energy band (2-150 keV) thanks to the combination of an opaque mask superimposed to a position sensitive detector and the usage of a passive shield placed on the mechanical structure between the mask and the detector plane. Furthermore the detector plane energy range is extended up to 20 MeV without imaging capabilities. The main performances of one XGIS unit are reported in Table 2.


Energy band

2 keV – 20 MeV

# detection plane modules


# of detector pixel /module


pixel size (= mask element size)

4.5x4.5 mm2

Low-energy detector (2-30 keV)

Silicon Drift Detector

450 μm thick

High energy detector (> 30 keV)

CsI(Tl) (3 cm thick)

Discrimination Si/CsI(Tl) detection

Pulse shape analysis

Dimension [cm]


Power [W]


Mass [kg]




2 - 150 keV

> 150 keV

Fully coded FOV (1 camera)

10.5 x 10.5 deg2


Partially coded FOV (1 camera)

77 x 77 deg2


Total FOV (2 cameras)

117 x 77 deg2

>4 sr

Ang. res

120 arcminutes


Source location accuracy

~10 arcmin (for >6σ source )


Energy res

<1200 eV FWHM @ 6 keV

6 % FWHM @ 500 keV

Timing res.

10 μsec

10 μsec

On axis useful area

~500 cm2

~1000 cm2

Table 2. Top: XGS specifications. Bottom: XGIS unit characteristics vs energy range    


The three elements composing each XGIS camera are:

  • the detector assembly
  • the mask assembly
  • the collimator assembly

The detector assembly, or detection plane, of each unit (490x490 mm2) is made of 100 detector modules each one being a matrix of 8x8 detection elements capable of detecting photons in the 2 keV – 20 MeV energy range. For each energy loss in the module, whatever procured by EM radiation or ionizing particle, the energy released, the 3 spatial coordinates and the of the interaction and time of occurrence will be recorded. The detection elements (Figure 5 and 6) are made of a scintillating crystal bar 5x5x30 mm3 in size, covered at the top and bottom extremes with a Silicon Drift Detector (SDD) for the read-out of the scintillation light, while the other sides of the bar are wrapped with a light reflecting material convoying the scintillation light towards the PDs.The size of each SDD is 4.5x4.5x0.45 mm3.

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Figure 5. Left: Principle of operation of the XGS detection units: low-energy X-rays in teract in Silicon, higher energy photons interact in the scintillator, providing an energy range extended to three orders of magnitude. Right: sketch of one XGIS module. A module is made of an array of 8x8 scintillator bars with SDDs at both ends. Both the SDDs and scintillators are used as active detectors. The PDs readout electronics consist of an ASIC pre-amp mounted near each PD’s anode while the rest of the processing chain is placed at the module sides and bottom.


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Figure 6. Left: detection plane of each XGIS camera. Right: mechanical design of one XGIS camera.


The operating principle (see Figure 5 left) is the following. The top SDD-PD, facing the X-/gamma-ray entrance window, is operated both as X-ray detector for low energy X-ray photons interacting in Silicon and as a read-out system of the scintillation light resulting from X-/gamma-ray interactions in the scintillator. The bottom SDD-PD at the other extreme of the crystal bar operates only as a read-out system for the scintillations. The discrimination between energy losses in Si and CsI is based on the different shape of charge pulses.

The XGIS coded mask (mask assembly) will spatially modulate with transparent and non-transparent pixel elements the incoming X-ray radiation. The detection plane will detect this mask-modulated signal. The mask pattern (up to now, a random pattern is considered) shadows on the detector plane for a given X-ray source located within the XGIS FOV. The image reconstruction is based on a correlation procedure between the detected image and a decoding array from the mask pattern. The coded mask assembly envelope is 600x600 mm2 and will have a pattern allowing self-support in order to guarantee the maximum transparency of the open elements. The coded mask assembly includes the following parts:

  • Mask code
  • Mask support structure
    • Upper grid
    • Lower grid
    • Frame

The mask code of each XGIS unit is made of tungsten of 1.0 mm thickness for the non-transparent pixels; the code is placed 630 mm above the detector plane. The mask detector plane distance is intended as the distance from the center of the mask to the top surface of the detector (detector reference plane). The ‘detector reference plane’ is 3.2 mm below the collimator assembly/detector assembly mechanical interface. The coded area of the mask is 561x561mm2 with a 10x10mm square pixel size. A random pattern and 50% open fraction has been considered for this preliminary design. The mask support structure provides mechanical support to the code as well as mechanical interface with the collimator assembly. This support structure has an Al upper grid and an Al lower grid that encapsulate the Tungsten pixels of the coded mask.

The collimator assembly has two main objectives: to mechanically connect the coded mask assembly with the detector assembly and to act as a lateral passive shield for the imaging system. The collimator assembly has two elements:

  • Collimator
  • Passive shielding

The collimator is made of Al alloy 1 mm thickness and the necessary stiffeners in order to provide enough strength and stiffness to the Al sheet. The collimator also accommodates the passive shielding (0.25mm W) of XGIS imaging system. The passive shielding provides the required opacity to shield the diffuse cosmic ray background. The passive shielding is made of four tungsten slabs all along the collimator height. The combination of the coded area with the collimator aperture in this geometry leads to a 77.0x77.0 deg2 (partially coded) FOV up to 150 keV.

XGIS sensitivity

The following plots show to the imaging sensitivity in the nominal energy ranges for the SDD (2-30 keV) and for the CsI (30-150 keV) detectors. All plots refer to a single XGIS unit. They are for an on-axis source, but in practice the sensitivity is nearly the same in the whole Fully Coded Field of View (i.e. the central region of about 10x10 degrees). For sources at the THESEUS boresight direction (aligned with the IRT) the data of the two units can be combined. This direction is at  20 deg off axis for each unit (where their effective area is 60% of the on-axis value). The combined sensitivity is similar to the on-axis sensitivity of a single unit. Therefore the following plots can be used, as a first approximation, also for the combined sensitivity at the boresight direction. For reference: 1 Crab = 10-8 erg cm-2 s-1 (2-30 keV) = 1.6 10-8 erg cm-2 s-1  (30-150 keV).

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Figure 7. Sensitivity plots for the XGIS.


The integrated sensitivity depends on the spectral shape of the source. The following figure shows the sensitivity as a function of the peak energy for GRBs described by a Band spectral function with representative slopes of long and short GRBs.

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Figure 8. XGIS sensitivity as a function of the peak energy for GRBs