Search for magnetic monopoles trapped in old iron ores using a superconducting detector

Magnetically trapped monopoles were sought in iron ores using a set of two SQUID (superconducting quantum-interference device) fluxmeters. We operated the apparatus under a sintering furnace in an industrial ore-processing plant for two months and tried to observe a signal of a monopole penetrating superconducting coils after its release from ore paramagnetized through a heat treatment. No candidate events were found consistent with the passage of a magnetic charge greater than 0.6 times Dirac’s unit strength $g_D$. The total weight of ore examined varies between 137 and ∼${10}^4$ tons, depending on what mechanism is assumed for the release of monopoles. By taking the most modest estimate, we obtain an upper limit of 2.2×${10}^{\mathrm{-}7}$ monopoles/g [95% confidence level (C.L.)] for the density of very massive monopoles (${10}^{14}$–${10}^{17}$ GeV/$c^2$) in iron ores which involve ferro/ferri-ferrimagnetic ingredients of age ${10}^7$–${10}^9$ yr to the extent of 10%. The free-monopole flux is also obtained as less than 1.4×${10}^{\mathrm{-}10}$/${\mathrm{cm}}^2$sec sr (90% C.L.), irrespective of its mass or velocity from a 57-d live time of two 20-cm-diam search coils.