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

Dark Matter Search Requires Radiation-Resistant Sensors

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.

Comparing irradiation damage with reference sensors

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.

Advanced p-Channel Skipper-CCD Technology Explained

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.

Achieving High X-ray Absorption Probability Rates

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.

Quantifying Spectral Degradation After Proton Irradiation

Quantifying X-ray spectral degradation in Skipper-CCDs

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.

Implications for space-based dark matter detection missions

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.