Improved Background Rejection in Proportional Counters a Front Guard

Details are presented of a new technique for providing a front guard in multi-wire array proportional counters. It involves the use of a grid plane 2 mm below the entrance window. An electron drift region is set up between the window and the grid and rise-time discrimination is used to veto electron tracks emanating from the window. A Xe/CH4 MWA counter fitted with 3 sided anti-coincidence anQ RTD shows a reduction in residual background by a factor of 2 when the front guard is applied.