[Paper Review] DAMIC: a novel dark matter experiment

The DAMIC Collaboration, Alexis A. Aguilar-Arevalo+18 more|arXiv (Cornell University)|Oct 24, 2013

Dark Matter and Cosmic Phenomena17 references8 citations

TL;DR

DAMIC is a novel dark matter experiment using thick, fully depleted CCDs with ultra-low electronic readout noise (RMS ~3 e⁻) to detect low-mass dark matter particles below 10 GeV. Operating at SNOLAB since 2012, it achieved a sub-0.5 keV nuclear recoil energy threshold, enabling world-leading sensitivity in the low-mass parameter space, with future plans for DAMIC-100 (100 g active mass) to further explore uncharted regions of dark matter parameter space.

ABSTRACT

DAMIC (Dark Matter in CCDs) is a novel dark matter experiment that has unique sensitivity to dark matter particles with masses below 10 GeV. Due to its low electronic readout noise (R.M.S. ~3 e-) this instrument is able to reach a detection threshold below 0.5 keV nuclear recoil energy, making the search for dark matter particles with low masses possible. We report on early results and experience gained from a detector that has been running at SNOLAB from Dec 2012. We also discuss the measured and expected backgrounds and present the plan for future detectors to be installed in 2014.

Motivation & Objective

• To directly detect low-mass dark matter particles below 10 GeV, a region largely unexplored by conventional experiments optimized for WIMPs.
• To overcome the detection threshold limitation of existing experiments by using CCDs with ultra-low electronic readout noise (RMS ~3 e⁻).
• To reduce background levels at SNOLAB through advanced shielding and material screening, achieving sub-1 cpd kg⁻¹ keVee⁻¹.
• To develop and deploy a scalable, high-mass dark matter detector (DAMIC-100) with 100 g of active mass for improved sensitivity.

Proposed method

• Utilizes thick, fully depleted CCDs (250 µm) with high resistivity silicon to increase active mass and enable detection of low-energy nuclear recoils.
• Employs a cryogenic system to cool CCDs to 145 K, minimizing thermal noise and enabling low-energy threshold operation.
• Deploys a multi-layer shield: a high-purity copper box (5 mm thick), lead shielding (21 cm thick, from Doe Run mine), and polyethylene (42 cm thick) to suppress gamma and neutron backgrounds.
• Uses X-ray calibration (e.g., ⁵⁵Fe) to characterize detector response and energy resolution, with energy spectra used to identify and quantify background sources.
• Implements a modular detector package with external connectors and reduced mass substrates (e.g., V2 package with AlN frame) to minimize radioactive contamination.
• Conducts extensive material screening and simulations (MCNPX, GEANT4) to model and predict background contributions from internal and external sources.

Experimental results

Research questions

• RQ1 Can CCDs with sub-0.5 keV nuclear recoil energy threshold detect dark matter particles with masses below 10 GeV?
• RQ2 What is the dominant background source in a low-background underground environment like SNOLAB, and how can it be minimized?
• RQ3 Can the background level be reduced to below 1 cpd kg⁻¹ keVee⁻¹ through improved detector packaging and material selection?
• RQ4 How does the performance of the DAMIC prototype at SNOLAB compare to previous runs at shallow sites in terms of background suppression?
• RQ5 What is the expected sensitivity of DAMIC-100 (100 g active mass) after one year of data taking in the low-mass dark matter parameter space?

Key findings

• The DAMIC experiment achieved a detection threshold below 0.5 keV nuclear recoil energy due to its ultra-low electronic readout noise (RMS ~3 e⁻), enabling searches for dark matter particles below 10 GeV.
• At SNOLAB, the background level was reduced by an order of magnitude compared to the shallow site at FNAL, reaching ~60 cpd kg⁻¹ keVee⁻¹, primarily due to improved detector packaging.
• The V1 package was found to be limited by uranium contamination in the AlN substrate (3 Bq kg⁻¹), which was significantly reduced in the V2 package by minimizing substrate mass in the active area.
• The V2 package reduced the uranium-induced background by a factor of ~10, demonstrating that material contamination in detector packaging is a critical sensitivity limiter.
• Simulations and measurements indicate that background levels below 1 cpd kg⁻¹ keVee⁻¹ are achievable with further optimization of materials and shielding.
• The expected sensitivity of DAMIC-100 after one year of data taking (35.6 kg day⁻¹) is projected to cover the entire low-mass dark matter parameter space consistent with recent experimental hints, offering the world's best sensitivity for masses below 10 GeV.