A high energy focal-plane gas scintillation proportional counter

We are developing high-pressure gas scintillation proportional counters for the focus of balloon-borne hard-X-ray telescopes as part of our HERO (High-Energy Replicated Optics) program. Each detector has an active diameter of 5 cm, and is filled with a xenon +4% helium mix at a total pressure of 10 ATM. Imaging is via a position-sensitive photomultiplier tube located immediately below a UV transmitting exit window. Two units are currently under construction and evaluation for a night in the spring of 2000. We present details of their design and construction together with data from preliminary X-ray testing.