Development of the Penning Gas Imager

The prototype Penning Gas Imager (PGI)[l] and its performance are briefly described. We have built a new laboratory version of the PGI whose active diameter of 180mm can cover the entire focal plane of the telescope to be flown on NASA´s Advanced X-ray Astrophysics Facility (AXAF). The PGI has good quantum efficiency over the AXAF energy range which extends from 0.1 keV to 8 keV. The energy resolution is 20% FWHM at 1.5 keV and the position resolution over the full field of view is 200¿m FWHM at 1.5 keV. This major improvement in energy resolution over Multi-Wire Proportional Counters (MWPC) (36% FWHM) and in position resolution over imaging Gas Scintillation Proportional Counters (GSPC) (> lmm FWHE) is achieved by using a Penning gas [2], two stage avalanching and a wedge and strip anode (WSA) [3]. We show that the particle background encountered in any space application can be reduced by an estimated 99% by an anticoincidence technique.