Fig.4. Pixel charge distribution for a prototype DAMIC-M CCD with skipper readout: pixels with zero, one, two electrons are counted with high resolution (0.07 e- from the width of the Gaussian peaks). The DAMIC-M CCD will feature 6k x 6k pixels over a 9 cm x 9 cm area and a thickness of 1 mm, for 20 g of mass. This will be the most massive CCD ever built, three times more massive than those at SNOLAB. A one-kg detector will consist of 50 CCDs.
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.
This readout has been successfully implemented in a 6 Mpixel DAMIC-M prototype CCD. Its pixel charge distribution is shown in Fig. 4, with well-separated peaks corresponding to zero, one, two, etc. electrons.
With a noise of only ≈ 0.1 e-, DAMIC-M will detect with high-resolution a single electron. Thus, it will be sensitive to extremely small energy transfers from a DM-electron interaction, since 1.1 eV (band gap energy) is the threshold to produce an electron-hole pair in silicon (with 3.7 eV required on average).
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, ≤ 2x10
-22 A/cm
2 at an operating temperature of 140 K.