A Threshold Gas Čerenkov Detector for the Spin Asymmetries of the Nucleon Experiment

Abstract

We report on the design, construction, commissioning, and performance of a threshold gas Čerenkov counter in an open configuration, which operates in a high luminosity environment and produces a high photo-electron yield. Part of a unique open geometry detector package known as the Big Electron Telescope Array, this Čerenkov counter served to identify scattered electrons and reject produced pions in an inclusive scattering experiment known as the Spin Asymmetries of the Nucleon Experiment E07-003 at the Thomas Jefferson National Accelerator Facility (TJNAF) also known as Jefferson Lab. The experiment consisted of a measurement of double spin asymmetries $A_{\parallel}$ and $A_{\perp}$ of a polarized electron beam impinging on a polarized ammonia target. The Čerenkov counter's performance is characterised by a yield of about 20 photoelectrons per electron or positron track. Thanks to this large number of photoelectrons per track, the Čerenkov counter had enough resolution to identify electron-positron pairs from the conversion of photons resulting mainly from $π^0$ decays.

24 pages, 18 figures. In preparation for submission to Nuclear Instruments and Methods