Transient-response induction detectors for magnetic monopoles: First operation at 78 K

We have built and tested a small nonsuperconducting magnetic-monopole detector based on Faraday induction. The detector operates at liquid-nitrogen temperature (78 K) and requires only simple steel shielding. It is sensitive to the passage of a Dirac monopole and has uniform velocity response down to the escape velocity from Earth (v/c=3.8×${10}^{\mathrm{-}5}$). It consists of a highly resonant copper coil with a simple field-effect-transistor (FET) amplifier, followed by an optimal filter to extract the signal. Measurements of the signal-to-noise ratio agree with calculations for the transient response induced by a monopole on the background of thermal noise. The detector was used to derive a weak upper limit on the magnetic monopole flux. We present a general analysis of such transient response detectors, and discuss the possibility of using high-$T_c$ superconducting loops and FET amplifiers to build a very large array for magnetic-monopole searches sensitive at the Parker limit.