SEARCH FOR A FLUX OF COSMIC-RAY MAGNETIC MONOPOLES WITH AN 8-CHANNEL SUPERCONDUCTING DETECTOR

A superconducting detector for cosmic-ray magnetic monopoles originally designed with a cross section of 1.5 m2 (averaged over 4-pi-solid angle) for double-coincident events was active from 5 May 1987 to 5 August 1988. The detector consists of eight independent inductive gradiometers located on the surface of an octagonal prism. The conductor is niobium-titanium foil and the current sensors are rf SQUID's. The signal-to-noise ratio for a single Dirac charge is greater than 50 in a 0.05-Hz bandwidth, and low-noise data was obtained over 547 days of continuous cryogenic operation. Of this time, we obtained approximately 50% live time. Open circuits in portions of two gradiometers and occasional coupling of disturbances in adjacent gradiometers reduce the active sensing area to 1.1 m2. A closed-cycle helium liquefier eliminates helium transfers and increases the stability of the data. Anticoincidence instrumentation includes strain gauges, a flux-gate magnetometer, an ultrasonic motion detector, and a wideband rms rf voltmeter. The exposure to date represents a limit on the flux of cosmic-ray magnetic monopoles of 7.2 X 10(-13) cm-2 s-1 sr-1 at a 90% confidence level, eliminating most of the phase space for monopole plasma oscillation theories.