Characterization of the Cherenkov Photon Background for Low-Noise Silicon Detectors in Space

TL;DR

This work identifies Cherenkov photon backgrounds from energetic cosmic rays in silicon as a non-negligible source of background for space-based, photon-starved observations and develops a laboratory-calibrated model of Cherenkov production and absorption. Using Geant4 simulations validated against low-noise skipper-CCD data, the authors quantify the residual background in thick silicon detectors and assess its dependence on detector thickness, solar activity, and heavy nuclei contributions. They apply the model to exoplanet spectroscopy scenarios at L2, finding that thick detectors retain advantages at long wavelengths despite higher Cherenkov backgrounds, and that preserving extended Cherenkov halos by minimal masking can maximize SNR for very faint signals.

Abstract

Future space observatories that seek to perform imaging and spectroscopy of faint astronomical sources will require ultra-low-noise detectors that are sensitive over a broad wavelength range. Silicon charge-coupled devices (CCDs), such as EMCCDs, skipper CCDs, multi-amplifier sensing (MAS) CCDs, and single-electron sensitive read out (SiSeRO) CCDs have demonstrated the ability to detect and measure single photons from X-ray energies to near the silicon band gap (~1.1 $μ$m), making them candidate technologies for this application. In this context, we study a relatively unexplored source of low-energy background coming from Cherenkov radiation produced by energetic cosmic rays traversing a silicon detector. We present a model for Cherenkov photon production and absorption that is calibrated to laboratory data, and we use this model to characterize the residual background rate for ultra-low-noise silicon detectors in space. We study how the Cherenkov background rate depends on detector thickness, variations in solar activity, and the contribution of heavy cosmic ray species (Z > 2). We find that for thick silicon detectors, such as those required to achieve high quantum efficiency at long wavelengths, the rate of cosmic-ray-induced Cherenkov photon production is comparable to other detector and astrophysical backgrounds. We apply our Cherenkov background model to simulated spectroscopic observations of extra-solar planets, and we find that thick detectors continue to outperform their thinner counterparts at longer wavelengths despite a larger Cherenkov background rate. Furthermore, we find that minimal masking of cosmic-ray tracks continues to maximize the signal-to-noise ratio of very faint sources despite the existence of extended halos of Cherenkov photons.

Figures (5)

• Figure 1: (Left) Schematic representation of Cherenkov emission in silicon. (Right) Measured muon track with best fit track line (red line) and distances to Cherenkov photons measured orthogonal to the track (black lines).
• Figure 2: Distribution of perpendicular distances between muon tracks and single-electron events for images taken by Giardino:2022 (black points) and simulations from our model (teal band). Simulations include an additional dark current rate estimated from the dataset in order to match the operational conditions. The dip centered at zero comes from the masking applied to energetic particle tracks, which is applied to both the data and simulations.
• Figure 3: Expected flux of particles as a function of solar activity. Hydrogen nuclei make up the majority of the expected events, followed by helium nuclei. The contribution of heavier nuclei is barely visible, comprising $\sim0.6\%$ of the total event rate. Rates are obtained from the Cosmic-Ray Data Base Maurin_2023.
• Figure 4: Illustrative simulation of 30 s exposure for a 250-$\mu$m-thick silicon detector composed of 2000$\times$2000, $15\,\mu$m pixels. This image includes cosmic-ray proton tracks and secondary Cherenkov photons. The inset shows a small region of the image so that cosmic-ray tracks and Cherenkov photons (circled in blue) can be seen clearly. The cosmic-ray rate roughly corresponds to maximum solar activity.
• Figure 5: (Left) Examples of 2 pixel (top) and 8 pixel (bottom) masking radii (light blue) around simulated proton tracks (green/yellow pixels) and secondary Cherenkov photons (red pixels) for a 250-$\mu$m-thick silicon detector. (Middle) Residual Cherenkov photon rate after masking and combining 120$\times$30 s exposures, binned in 2$\times$2 blocks and convolved by a Gaussian kernel with a width of 1 pix. Binning and blurring are performed to better visualize the charge distribution. (Right) Effective exposure time for the combination of 120$\times$30 s exposures after masking.