Angular Resolution of Electron Recoils in Gas

Directional recoil detection is highly desirable for neutron detection, neutrino detection, and dark matter searches. Traditionally, the focus has been on directional detection of low-energy nuclear recoils, which offers a method for penetrating the neutrino floor and confirming the galactic origin of a dark matter signal. More recently, there has been growing interest in directional detection of electron recoils, which enable unique neutrino physics capabilities. A particularly interesting example is the possibility of obtaining a firm measurement of the Sun's CNO neutrino flux. It is estimated that an O(10m^3) gas TPC operating at atmospheric pressure can make such a measurement via the electron recoil channel. Given the direction to the Sun and the combined measurement of recoil energy and direction, event-by-event reconstruction of the neutrino energy spectrum is possible. This requires a good understanding of the detector's energy resolution and the angular resolution of electron recoils. However, electron recoils have complex trajectories and the angular resolution that can be achieved is not well understood. We discuss a general method for approximating and optimizing the angular resolution of electron recoils in gas, including both scattering and detector effects.