Dark Matter Hunt Gets Boost from Space-Proof X-Ray Sensors

Researchers are meticulously characterising the performance of novel detectors crucial for the upcoming DarkNESS mission, a 6U nanosatellite designed to hunt for signals of decaying dark matter. Phoenix Alpine from the University of Illinois, Urbana-Champaign, and Ana M. Botti and Brenda A. Cervantes-Vergara from Fermi National Accelerator Laboratory, alongside colleagues including Claudio R. Chavez, Fernando Chierchie, and Alex Drlica-Wagner et al., present detailed X-ray measurements of fully-depleted p-channel Skipper-CCDs before and after exposure to proton irradiation. This work is significant because it quantifies the degradation of these sensitive detectors in conditions mimicking the harsh radiation environment of low-Earth orbit, allowing for a robust prediction of the DarkNESS mission’s ultimate ability to detect faint X-ray signatures from dark matter.

Skipper-CCD performance degradation under proton irradiation for dark matter detection is a critical concern

Scientists are meticulously evaluating the radiation resilience of a novel sensor technology crucial for detecting elusive dark matter. The Dark matter Nanosatellite Equipped with Skipper Sensors (DarkNESS) mission, a 6U CubeSat, relies on Skipper-CCDs to search for X-ray signatures of decaying dark matter, but the impact of space radiation on these sensors remained largely unknown.
This work presents a detailed assessment of Skipper-CCD performance before and after exposure to proton irradiation, quantifying the resulting degradation in X-ray spectral resolution. Researchers subjected a sensor to a proton fluence of 8.4x 10^10 protons cm^-2, exceeding the anticipated three-year radiation dose in low-Earth orbit by over an order of magnitude.

The study employed a 55Fe source to meticulously compare the energy resolution of irradiated and non-irradiated sensor quadrants, alongside a fully functional reference sensor. These measurements provide a quantitative understanding of how radiation-induced damage affects the ability of Skipper-CCDs to accurately measure the energy of X-rays.
By analysing the spectral broadening and efficiency loss, the team has established a baseline for predicting the end-of-life performance of the DarkNESS mission’s X-ray detectors. This is vital for optimising the search for decaying dark matter and ensuring the mission’s scientific objectives can be met despite the harsh space environment.

The core of this breakthrough lies in the use of thick, fully-depleted p-channel Skipper-CCDs, which offer exceptionally low readout noise and high quantum efficiency for detecting X-rays in the 1, 10 keV range. These sensors utilise a floating-gate amplifier, enabling repeated, non-destructive measurements that reduce readout noise to sub-electron levels.

The Oscura prototype devices, used in this study, are fabricated on high-resistivity n-type silicon and feature a buried p-channel architecture, enhancing their radiation tolerance. The experimental setup involved a direct comparison between the irradiated quadrant of the sensor and adjacent, unexposed quadrants, alongside a fully non-irradiated reference sensor.

Energy resolution was determined using the 55Fe source, enabling precise measurement of spectral broadening caused by radiation damage. Image processing techniques were employed to extract X-ray events and reconstruct their energies, facilitating a detailed analysis of the detector’s gain and charge transfer performance.

Specifically, the Oscura prototype sensors, fabricated on high-resistivity n-type silicon with a buried p-channel architecture, were utilized. These devices, fully depleted to a thickness of 725μm, achieve an X-ray absorption probability exceeding 99% at 10 keV. The analysis of measured spectral broadening and efficiency loss allows for modelling spectral degradation under representative low-Earth orbit trapped-proton environments, establishing a performance baseline for the DarkNESS decaying dark matter search.

Quantified X-ray spectral degradation in Skipper-CCDs following proton irradiation is presented

A sensor received a proton fluence of 8.4x 10^10 protons cm^-2, simulating displacement damage exceeding three-year expectations for low-Earth orbit. This irradiation level allowed quantification of the resulting degradation in X-ray performance of Skipper-CCDs. Subsequent measurements using a 55Fe source compared the energy resolution of the irradiated quadrant to both unexposed quadrants and a non-irradiated reference sensor.

The analysis revealed that the irradiated sensor exhibited a measurable broadening of X-ray spectral lines, indicating a loss of energy resolution. Detector gain and charge transfer inefficiency were also assessed to quantify the impact of radiation-induced traps. These measurements provide a quantitative assessment of radiation-induced spectral degradation, crucial for predicting end-of-life performance.

Specifically, the study establishes a baseline for the expected X-ray energy resolution of the Dark matter Nanosatellite Equipped with Skipper Sensors (DarkNESS) mission after operation in low-Earth orbit. The Oscura prototype sensors, used in this work, are fully depleted silicon imagers with a thickness of 725μm, achieving an X-ray absorption probability exceeding 99% at 10 keV.

DarkNESS flight sensors will utilise a 50nm aluminum entrance window and a thinned silicon bulk of 500μm. The research utilized a 217 MeV proton beam at the Northwestern Medicine Proton Center to simulate the radiation environment, providing a controlled reference for interpreting in-orbit degradation. The study focused on assessing the X-ray line response of these sensors both before and after exposure to proton beams, simulating the radiation environment of low-Earth orbit.

A sensor received a proton fluence exceeding the anticipated three-year dose for typical LEO missions, allowing for a robust evaluation of radiation-induced damage. This degradation was then used to estimate the expected end-of-life performance of the DarkNESS mission, specifically for its decaying dark matter search utilising X-ray spectral analysis.

The authors acknowledge that the proton irradiation experiment, while representative, doesn’t fully capture the complexity of the space radiation environment, which includes other particle types and a broader energy spectrum. Future work could involve correlating these laboratory measurements with in-flight data from the DarkNESS mission to refine the radiation damage models and improve the accuracy of performance predictions for future space-based Skipper-CCD instruments. These findings establish a baseline for understanding and mitigating radiation effects on Skipper-CCDs, supporting the development of more sensitive dark matter detection experiments in space.