A tracking rangefinder for muons from kaon decay

A muon rangefinder with tracking capabilities has been constructed as part of a search for the rare decay K_L°→ μe in experiment 791 at the Brookhaven National Laboratory Alternating Gradient Synchrotron. The rangefinder consisted of two identical arms, symmetric about the beamline, and was the final detector element in a spectrometer system. Each side of the rangefinder was comprised of 75 slabs of marble and 25 slabs of aluminum, each 7.62-cm thick, covering an acceptance area 225 cm wide by 301 cm high, with a total mass of 160 tons. There were 13 pairs of x- and y-measuring proportional tube planes, providing a nominal ±10% accuracy measurement of muon momentum. During 850 hours of data collection, efficiency averaged 94%, with 160-ns drift time at 1.5-μA threshold. For well-identified muon tracks, rangefinder muon identification was 99% efficient when penetration to at least 60% of the depth expected from spectrometer-derived momentum was required