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The underground Soudan-1 nucleon decay experiment can measure the velocities of slowly moving particles if they can be detected by ionization at a level 0.5 times that of a relativistic muon. Magnetic monopoles predicted by grand unified theories should satisfy this condition if their velocities are greater than 10^-3c. During a live time of 0.41 yr, no candidate has been observed, implying a 90%-confidence-level limit on the flux of such objects of 4.1×{}10^-13 cm^-2 sr^-1 s^-1 for velocities between 10^-3c and 10^-2c.
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Bartelt et al. (1983) studied this question.
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