The fused-silica Cherenkov detector system for the Qweak experiment

The Qweak experiment will measure the proton's weak charge via the parity-violating asymmetry in electron-proton scattering, measuring the asymmetry to a precision of 5~parts per billion. A continuous $\sim 180~\mu$A beam of polarized 1.1~GeV electrons interacts with a 35~cm liquid hydrogen target. Elastically scattered electrons with momentum transfer $Q^2 \approx 0.03$~GeV$^2$ are focused onto an array of eight fused silica detectors, each 2~m long. Cherenkov light emitted inside the silica is collected by photomultiplier tubes. The typical electron rate in each detector is $\sim$700~MHz; during production data-taking, the photocurrent is measured continuously by low-noise integrating electronics. The accelerator can also deliver the same beam at much lower currents ($\sim100$~pA); for these measurements, the phototubes can be connected to higher-gain fast electronics capable of resolving single-electron events. In this talk I will describe the construction, operation, and performance of these detectors.