Report on the stanford octagonal magnetic monopole detector

After 1008 active (1150 elapsed) days of operation, the 476 cm 2 three-axis monopole detector has been shut down without observing any candidate events. This sets an upper limit for a cosmic ray flux of magnetic monopoles of 4.4 × 10 -12 cm -2 s -1 sr -1 at 90% confidence level. We have constructed a 1.5 m 2 (averaged over 4π solid angle) superconducting monopole detector and are testing its performance. The sensing area includes only coincident events. The detector is an octagonal cross-section cylinder with an independent planar gradiometer loop (16.9 × 521.2 cm) on each side of the octagon. Originally constructed with copper traces covered by lead/tin solder, the loops have been converted to 2 mm wide × 50 μm thick niobium-titanium (NbTi) ribbon because of excessive noise in early tests. We expect to obtain a signal to noise ratio of about 10 with the new loops.