Constraints on the electron-hole pair creation energy and Fano factor below 150 eV from Compton scattering in a skipper CCD

Fully depleted thick silicon skipper-charge-coupled devices (skipper CCDs) are an important technology to probe neutrino and light-dark-matter interactions due to their subelectron read-out noise. However, the successful search for rare neutrino or dark-matter events requires the signal and all backgrounds to be fully characterized. In particular, a measurement of the electron-hole pair creation energy below 150 eV and the Fano factor are necessary for characterizing the dark matter and neutrino signals. Moreover, photons from background radiation may Compton scatter in the silicon bulk, producing events that can mimic a dark matter or neutrino signal. We present a measurement of the Compton spectrum using a skipper CCD and a Am241 source. With these data, we estimate the electron-hole pair-creation energy to be (3.71±0.08) eV at 130 K in the energy range between 99.3 eV and 150 eV. By measuring the widths of the steps at 99.3 eV and 150 eV in the Compton spectrum, we introduce a novel technique to measure the Fano factor, setting an upper limit of 0.31 at 90% C.L. These results prove the potential of skipper CCDs to characterize the Compton spectrum and to measure precisely the Fano factor and electron-hole pair creation energy below 150 eV.