Axel: A balloon-borne X-ray astronomy experiment

Following a successful programme to investigate the physics of ultra high pressure proportional counters, a counter array has been developed for hard X-ray astronomy. A parallel investigation has evaluated the performance of a large area phoswich scintillator detector for the same purpose. The two detectors have been integrated in a balloon-borne payload, the Astrophysical X-ray Experimental Laboratory (AXEL) This paper describes the instrumentation aboard the payload.