Light-tight skipper-CCDs for X-ray detection in space

TL;DR

This work tackles the problem of optical backgrounds saturating skipper-CCDs in space-based X-ray detection by introducing a thin aluminum shield deposited directly on the CCD surface. The authors demonstrate that 50 nm and 100 nm Al coatings suppress optical light by more than 99.6% and 99.9% across 650–1000 nm while preserving X-ray detection efficiency at 5.9 and 6.4 keV, and they validate these findings with Geant4 simulations over a broader energy range. Front-illuminated geometries show that the shield does not degrade X-ray efficiency at the tested energies, whereas back-illuminated, thinned sensors could markedly improve low-energy X-ray detection (e.g., at 3.5 keV) to ~85–90% efficiency. The work suggests a low-cost, scalable approach for optical background suppression in space instrumentation and outlines directions for further optimization, backside processing, and shield integration.

Abstract

Skipper Charge-Coupled Devices (skipper-CCDs) are pixelated silicon detectors with deep sub-electron resolution. Their radiation hardness and capability to reconstruct energy deposits with unprecedented precision make them a promising technology for space-based X-ray astronomy. In this scenario, optical and near-infrared photons may saturate the sensor, distorting the reconstructed signal. We present a light-tight shield for skipper-CCDs to suppress optical backgrounds while preserving X-ray detection efficiency. We deposited thin aluminum layers on the CCD surface using an e-beam evaporator and evaluated their blinding performance across wavelengths from 650 to 1000 nm using a monochromator, as well as the X-ray transmission using an $^{55}$Fe source. We find that 50 and 100\,nm layers provide >99.6\% light suppression, with no efficiency loss for 5.9 and 6.4\,keV X-rays. In addition, we used Geant4 simulations to extend these results to a broader energy range and quantify the efficiency loss for different aluminum thicknesses. Results show that thin aluminum coatings are an effective, low-cost solution for optical suppression in skipper-CCDs intended for X-ray detection and space instrumentation.

Figures (11)

• Figure 1: Experimental configuration for testing the skipper-CCD with the aluminum shield. With this setup, we first determined the blinding factor for different wavelengths using a monochromator, and then estimated the X-ray transfer efficiency using a $^{55}$Fe source inside the vacuum vessel.
• Figure 2: Image obtained with one quadrant of the skipper-CCD. We illuminate the sensor with 950 nm photons and the $^{55}$Fe radioactive source. The x(y)-axis in the image correspond to the columns (rows), the blue rectangle represents the position of the serial register, and the red square the readout amplifier. Gray arrows indicate the direction of the read-out. We show the projections on the rows and columns after removing the high-energy events from the radioactive source and environmental radiation. Dark regions correspond to the overscan and aluminum shields of 20, 50, and 100 nm.
• Figure 3: (Top) uncalibrated charge spectrum for images with different exposures obtained with one quadrant and 200 samples. (Bottom) Zoom-in of the orange region on top, showing photoelectron peaks between 428 and 439 electrons.
• Figure 4: $^{55}$Fe X-ray event spectrum obtained using one quadrant images with 10 samples. The photoabsorption peak for 5.9 (K$_\alpha$) and 6.4 keV (K$_\beta$) X-ray appears along with the K$_\alpha$ escape and silicon fluorescence peaks.
• Figure 5: Light transmission factor through the aluminum shield as a function of wavelength for different aluminum thicknesses. We calculate the transmission as the ratio of the signal in unshielded and shielded pixels.