X-ray characterization of fully-depleted p-channel Skipper-CCDs for the DarkNESS mission

TL;DR

DarkNESS aims to detect X-ray lines from decaying dark matter with Skipper-CCDs in low-Earth orbit. This study irradiates Oscura-prototype Skipper-CCDs with 217 MeV protons and uses Mn–K X-rays from a $^{55}$Fe source to quantify radiation-induced spectral degradation, including gain, energy resolution, and charge-transfer inefficiency (CTI). The results show trap-induced CTI causes spectral broadening and low-energy tailing that are strongest in the beam-exposed quadrant, enabling an empirical link between displacement damage and end-of-life performance. By mapping laboratory displacement-damage dose to AP9-IRENE trapped-proton environments, the authors project end-of-life X-ray energy resolution for representative DarkNESS orbits and find the degradation is modest enough to preserve the mission’s photometric detection capability.

Abstract

The Dark matter Nanosatellite Equipped with Skipper Sensors (DarkNESS) mission is a 6U CubeSat designed to search for X-ray lines from decaying dark matter using Skipper-CCDs. Thick, fully-depleted p-channel Skipper-CCDs provide low readout noise and high quantum efficiency for 1-10 keV X-rays, but their X-ray performance has not yet been demonstrated in the space environment. DarkNESS will operate in low-Earth orbit, where trapped protons induce displacement damage in the sensor that increases charge-transfer inefficiency and degrades the X-ray energy resolution. This work measures the X-ray line response of Skipper-CCDs before and after proton irradiation and quantifies the associated degradation. A sensor was exposed to 217 MeV protons at a fluence of 8.4 x 10^10 protons cm^-2, corresponding to a displacement-damage dose more than an order of magnitude above the three-year expectation for representative mid-inclination and Sun-synchronous low-Earth orbits. A 55Fe source was used to compare the energy resolution of the beam-exposed quadrant to adjacent unexposed quadrants and a non-irradiated reference sensor. These measurements provide a quantitative assessment of radiation-induced spectral degradation in Skipper-CCDs and enable an estimate of the end-of-life X-ray energy resolution expected for DarkNESS operation in low-Earth orbit.

Figures (5)

• Figure 1: Sample arrangement during irradiation showing four Skipper-CCDs with the beam profile center aligned at the sensor carrier. The CCD under study (red outline) contains four skipper readout amplifiers, one in each corner of the chip. The square-profile 217 MeV proton beam (rose overlay) delivers a fluence of $8.4\times10^{10}\,\mathrm{p\,cm^{-2}}$, corresponding to decades of equivalent LEO exposure (18--42 years depending on orbit assumptions). Data from the irradiated adjacent amplifier 2 (IA2) of the irradiated CCD marked orange are compared with the equivalent amplifier 2 region of a non-irradiated (NI2) reference CCD marked iris. The beam-irradiated quadrant read out through amplifier 4 (BI4) provides the direct comparison region, marked in red.
• Figure 2: Thresholded 4-neighbor connected-component clustering. (Left) A $64 \times 64$ pixel tile of the pedestal-subtracted image with values in units of readout noise ($\sigma_{\rm read}$). The scale highlights pixels above the split threshold ($3\sigma_{\rm read}$) used to collect charge and the event threshold ($9\sigma_{\rm read}$) used to seed clusters. (Right) The corresponding segmentation mask. Events are classified by topological grade, including charged-particle tracks (orange), single-pixel (red), 2-split (cyan), L-split (purple), and extended (green). The C-split grade (yellow) identifies column-wise trails from vertical CTI.
• Figure 3: Full-quadrant $^{55}$Fe X-ray spectra for the non-irradiated reference quadrant (NI2), the adjacent quadrant of the irradiated sensor (IA2), and the beam-exposed quadrant (BI4). Points with cross-mark error bars show the measured cluster-energy distributions, and solid curves give the best-fit double-Gaussian model to the Mn--K$\alpha$ and Mn--K$\beta$ lines. The vertical dashed lines mark the nominal Mn--K$\alpha$ (5.895 keV) and Mn--K$\beta$ (6.490 keV) peak energies. Relative to NI2, IA2 exhibits only modest broadening of the Mn--K$\alpha$ line, whereas BI4 shows a factor-of-two increase in FWHM and pronounced low-energy tailing associated with radiation-induced displacement damage.
• Figure 4: Pixel-value distributions as a function of vertical transfer distance (Y-position) for NI2 (top-left), IA2 (top-right), and BI4 (bottom-left), highlighting the Mn–K$\alpha$ (5.9 keV) signal band. The bottom-right panel overlays the ridge fits used to track the Mn–K$\alpha$ signal through the distributions. NI2 and IA2 show shallow, nearly parallel slopes, indicating minimal CTI, while BI4 exhibits a steeper decline consistent with increased CTI in the irradiated quadrant.
• Figure 5: Mn--K$\alpha$ spectra for vertically segmented, 100-row subsets in NI2, IA2, and BI4. The BI4 segments display broadened peaks with enhanced low-energy tailing relative to the nearly identical, symmetric line shapes in NI2 and IA2, confirming that the observed broadening is intrinsic to the irradiated segment rather than an artifact of quadrant size or sampling.