[Paper Review] The Skipper CCD for low-energy threshold particle experiments above ground

Guillermo Fernández Moroni, Fernando Chierchie+12 more|arXiv (Cornell University)|Jul 1, 2021

Particle Detector Development and Performance30 references4 citations

TL;DR

This paper demonstrates that Skipper-CCD technology achieves single-electron resolution in low-energy particle detection above ground, with a background rate that does not increase exponentially at sub-keV energies. By applying spatial and charge-based event selection criteria, the authors suppress dominant background sources—particularly from the serial register and dark current—achieving a stable, low background rate down to 5 electron-hole pairs (equivalent to ~7.5 eV), outperforming other low-threshold technologies in surface-level experiments.

ABSTRACT

We present experimental results using a single-electron resolution Skipper-CCD running above ground level to demonstrate the potential of this technology for its use in reactor neutrino observations and other low-energy particle interaction experiments. Operating conditions and event-selection criteria are provided to decouple most of the background rate at low energies. Our final results for events with energies as low as $5$ ionized electron-hole pairs show that the exponentially increasing rate of events seen in other technologies is not present in our data. This demonstrates that the Skipper CCD proves to be among the best options to measure low energy and weakly interacting particles at ground level.

Motivation & Objective

• To demonstrate the viability of Skipper-CCD for low-energy particle detection in surface-level experiments.
• To identify and mitigate dominant background sources in above-ground Skipper-CCD operation.
• To achieve stable, low-background performance at energies below 1 keV, where other technologies show exponentially increasing event rates.
• To validate the use of spatial and charge-based selection criteria for rejecting non-physical events in thick, fully depleted CCDs.
• To compare Skipper-CCD performance directly with state-of-the-art low-threshold technologies in surface-based measurements.

Proposed method

• A single-electron resolution Skipper-CCD was operated at 140 K using a Sunpower cryocooler to minimize thermal noise.
• The CCD was housed in a 2-inch lead shield with no radio-purity filtering, relying on passive shielding and event selection for background suppression.
• Spatial selection criteria were applied: events with y-size between 0.3 and 1.0 pixels were selected to reject serial-register events.
• Charge-based cuts were applied: only events with total charge below 2000 e− were retained, and events with high charge in the serial register were rejected.
• A simulation framework using a diffusion transport model was used to estimate selection efficiency across the energy spectrum.
• The final energy spectrum was normalized by active mass but not by selection efficiency, enabling direct comparison with other surface-based low-threshold experiments.

Experimental results

Research questions

• RQ1 Can Skipper-CCD achieve stable, low-background operation at sub-keV energies when operated above ground, where cosmic ray backgrounds are high?
• RQ2 What are the dominant background sources in above-ground Skipper-CCD operation, and can they be effectively suppressed using spatial and charge-based event selection?
• RQ3 Does the background rate in Skipper-CCD increase exponentially at low energies—like in other low-threshold technologies—under surface conditions?
• RQ4 How does the performance of Skipper-CCD compare quantitatively with other leading low-threshold technologies (e.g., SuperCDMS, Nucleus-1g, EDELWEISS) in surface-based measurements?
• RQ5 To what extent does the selection efficiency of spatial and charge-based cuts distort the intrinsic energy spectrum of the detector?

Key findings

• The Skipper-CCD demonstrated stable background performance down to 5 electron-hole pairs, corresponding to an equivalent energy of approximately 7.5 eV.
• The final energy spectrum showed no exponential rise in event rate at low energies, in contrast to other low-threshold technologies that exhibit such behavior above ground.
• The background rate for events with equivalent deposited energy from 5 e− to 2000 e− (7.5 keV) was measured at 12 kilo-detectable rate units (kdru), indicating a low and flat background level.
• Spatial and charge-based selection cuts effectively suppressed serial-register and dark-current events, with selection efficiency remaining nearly flat across the energy spectrum.
• When compared to other low-threshold technologies in the EXCESS workshop comparison, the Skipper-CCD showed the lowest background rate below 500 eV, confirming its superiority for surface-based low-energy experiments.
• The simulation results confirmed that the selection criteria did not distort the intrinsic energy spectrum, as the efficiency remained nearly constant across all energy bins.