CCDs have been used for many years in digital cameras and in the focal plane of astronomical telescopes for the digital imaging of faint astrophysical objects. DAMIC-M will make an unconventional use of CCDs to detect dark matter particles. It will feature the most massive CCDs ever built and a novel concept for signal readout resulting in the high-resolution detection of a single electron.
The DAMIC CCDs currently installed at SNOLAB were developed by Lawrence Berkeley National Laboratory (LBNL). They have a record thickness of 675 μm and active area of 6 cm x 6 cm, for a mass of 6 g each. The detectors were fabricated from n-type, high-resistivity (>10,000 Ω cm) silicon wafers, and are fully depleted (i.e., active over their full volume).
The new DAMICM CCDs will be 36 million pixels (9 cm x 9 cm). Thanks to a new readout technology (`` skipper-CCD ''), it was possible to reach resolutions of the electronic fraction.
The principle of DM detection with a CCD is illustrated in the Figure (top). The charge produced in the DM particle interaction, through absorption or a nuclear/electronic recoil, drifts towards the pixel gates, where it is held in place until the readout.
While drifting toward the pixel gate, the ionized charge diffuses transversely, with a spatial variance that is proportional to the transit time (bottom). Hence, there is a positive correlation between the lateral diffusion of the collected charge on the pixel array and the depth of the interaction, which can be used for three-dimensional position reconstruction.
The DAMIC-M detector will consist of 50 CCDs, for a total mass of the order of kg with internal and external shielding. The design of the detector is in progress.
To reduce the background noise induced by cosmic rays, DAMIC-M will be installed in the underground laboratory of Modane in France, 1700 m underground, in the Fréjus tunnel. To limit background noise of radioactive origins, the detector is protected by archaeological lead (for the absorption of photons), polyethylene (to shield against neutrons) and special care is taken in the selection and handling of materials in order to ensure radio-chemical purity.
The 3D reconstruction and the unique spatial resolution are crucial to separate the events appearing on the surface of the CCD (which may be due to external contamination) from those deeper. In addition, the excellent spatial resolution of the pixels makes it possible to identify and reject the residual radioactive background which produces a time sequence of events appearing in the same place in the CCD.
The crucial innovation in DAMIC-M is the integration on these large devices of a novel on-chip readout scheme, where sub-electron noise is achieved by a non-destructive, multiple measurement of the pixel charge. In the so-called “skipper” readout, the charge of a pixel is measured N times, and by taking the average of these measurements the white noise is reduced by √N. Most importantly, the effect of low-frequency 1/f noise is drastically reduced, since the integration time of each measurement is much shorter than in the conventional readout. In a recent R&D at Fermilab, this readout has been successfully implemented in a DAMIC-like CCD. A resolution of a fraction of electron has been confirmed by our tests with DAMIC-M skipper-CCDs.
With a noise of only 0.1 e-, DAMIC-M will detect with high-resolution a single electron and will be sensitive to energy as small as 2-3 eV. Even with single-charge resolution, a signal may remain hidden in the fluctuations of the detector’s dark current. DAMIC CCDs have the lowest dark current ever measured in a silicon detector, ≤ 5x10-22 A/cm2 at an operating temperature of 140 K.
DAMIC-M test CCD (6000x1000 pixels)
Single Electron resolution achieved with a DAMIC-M skipper-CCD