Performance evaluation of a silicon-based 6U CubeSat detector for soft γ-ray astronomy

γ-ray astronomy has matured significantly over the course of the last decade and a half, leading to the exploration of the high-energy universe with unprecedented sensitivity. The observation of the low-energy γ-ray (0.1 to 30 MeV) sky has been significantly limited since the Imaging Compton Telescope (COMPTEL) instrument aboard the Compton Gamma Ray Observatory (CGRO) satellite was decommissioned in 2000. The exploration of γ-ray photons within this energy band, often referred to as the MeV gap, is crucial to address numerous unresolved mysteries in high-energy and multi-messenger astrophysics. Although several large MeV γ-ray missions have been proposed (e.g., e-ASTROGAM, AMEGO, and COSI), most of these are in the planning phase, with launches not expected until the next decade, at the earliest. Recently, there has been a surge in proposed CubeSat missions as cost-effective and rapidly implementable pathfinder alternatives. An MeV CubeSat dedicated to γ-ray astronomy could serve as a valuable demonstrator for large-scale future MeV payloads. In a research article recently published in Space: Science & Technology, researchers from The University of Hong Kong, Institute of Astronomy Space and Earth Science, and University of California together proposes a γ-ray payload design with a silicon-based tracker and a cesium iodide-based calorimeter, by which a sensitivity better than IBIS for energies between 0.1 and 10 MeV and that comparable to COMPTEL for energies up to around 1 MeV can be achieved, therefore opening up a window toward cost-effective observational astronomy with comparable performance to past missions.

First, the detector design is illustrated. The MeV detector design is based on stacking a series of CubeSat micro-satellites. A CubeSat is a class of nanosatellite, adhering to standardized size and weight requirements (i.e. 10 × 10 × 10 cm³ and 1.33 kg). The proposed MeV payload is extended to 3U (10 × 10 × 30 cm³) and 2U (10 × 10 × 20 cm³). Figure 1 shows the side and top views of the 3U detector assembly, comprising 3 components: (a) silicon tracker, (b) CsI calorimeter, and (c) anti-coincidence detector (ACD). The entire CubeSat is expected to be roughly 6U in size (10 × 20 × 30 cm³), with the non-payload volume of the satellite reserved for electronics and other commercial-off-the-shelf (COTS) components. Specifically, a double-sided cross-strip silicon detector (DSSD) is chosen to provide the tracking information for γ-rays, electrons, and positrons generated during Compton scattering and pair production. The payload uses 90 (60) layers of silicon distributed in 90 (60) layers of DSSD in the 3U (2U) tracker, with a 2-mm tracker pitch. The silicon tracker constitutes around 90% of the total payload budget. The calorimeter consists of 2 sets of cuboid-shaped CsI scintillation crystals measuring 5 cm × 0.5 cm × 0.5 cm and 22.9 cm (15.2 cm) × 0.5 cm × 0.5 cm for the 3U and 2U configurations, respectively. The ACD is made from plastic scintillation material, shielding the silicon tracker and the calorimeter.

Then, simulations and performance calculation are carried out. The detector design and its performance are simulated using MEGAlib, a software package based on Geant4 and ROOT. The detector design described in the “Detector design” section is shown in Fig. 1, which is simulated using the Geomega module in MEGAlib. The simulation focuses only on detector components responsible for interactions with γ-ray photons (i.e., silicon tracker, CsI calorimeter, and ACD), excluding the electronic components and noise for this study. The effective area, energy resolution, and angular resolution for the simulations were performed using monochromatic point sources with energies 0.1 ≤ E ≤ 6 MeV, simultaneously considering albedo photon- and cosmic-ray background radiation. This background model was obtained by fitting background spectrum data provided from the MEGAlib GitHub repository. The continuum sensitivity is simulated with a power law model E^−Γ with spectral index Γ = 1 and the energy range for the continuum source energy is taken from 0.1 MeV up to 10 MeV. All simulations are performed for an effective exposure time Teff = 10⁶ s, using the Cosima module in MEGAlib. The physical interactions of simulated γ-ray photons within the payload are recorded for a certain angular acceptance window α_acc concerning the energy peak (for point sources) or energy bin (for continuum sources) of the source’s energy spectrum. Moreover, the simulated sources are studied at the zenith, i.e., (θ, ϕ) = (0, 0). Subsequently, simulated interactions are reconstructed and classified as tracked/untracked Compton or pair-production events based on the precision and details from the reconstruction algorithm using the Revan module of MEGAlib and then analyzed with the Mimrec module of MEGAlib, leading to the detector performance estimations.

Last, a comparison of various detector performance parameters is present and the conclusion is drawn. In order to assess the detector performance, the effective area, angular resolution measure (ARM), energy resolution, and continuum sensitivity for the different simulated detector designs at 1 MeV are calculated. In Fig. 2, authors present a comparison of the CubeSat’s effective area and ARM, varying the default widths of the calorimeter and tracker for both the 2U and 3U payloads. Figure 2 A illustrates that increasing the tracker width leads to a larger effective area, as the increased Compton cross-section enhances particle detection and reconstruction. Meanwhile, Fig. 2 B demonstrates that increasing the calorimeter thickness similarly results in increased effective area. As shown in Fig. 2 C, increasing the tracker thickness enhances the ARM, reflecting the more precise tracking of Compton events. In contrast, increasing the calorimeter thickness decreases the ARM as shown in Fig. 2 D, given the inverse relationship ARM ∝ 1/E. Taking into account the simulation study and the constraints imposed by electronics and cost, we have determined that a tracker thickness of 500 μm and a calorimeter thickness of 0.5 cm constitute the most suitable configuration for our payload. A more complete comparison of the CubeSat detector’s sensitivity with other instruments is shown in Fig. 8. A sensitivity better than IBIS for energies between 0.1 and 10 mev and that comparable to COMPTEL for energies up to around 1 mev can be achieved.