First results from a 1.1-m-diameter superconducting monopole detector

We present the design and performance of a superconducting induction magnetic-monopole detector with 1.1-m-diameter gradiometer loops. The detector demonstrates that gradiometers can be overlapped with no mutual inductance to yield high redundancy without increasing shield volume. One of two detector units was sensitive to the passage of monopoles through two overlapped gradiometers with 98% coincidence efficiency for 161 hours. The unit has a coincident sensitive area averaged over 4π sr of 4400 ${\mathrm{cm}}^2$. No candidate events were observed, setting an upper limit on the flux of cosmic-ray magnetic monopoles of f≲7.1×${10}^{\mathrm{-}11}$ ${\mathrm{cm}}^{\mathrm{-}2}$ ${\mathrm{sr}}^{\mathrm{-}1}$ ${\sec }^{\mathrm{-}1}$ (90% C.L.). The detector was operated in ambient magnetic fields of ∼5–125 mG.