New limit on the magnetic monopole density in old iron ore

Magnetically trapped monopoles were sought in iron ores of age 107 approx. 109 years using two superconducting detectors. Each involves one search coil of diameter 8 or 14 cm. A number of putative events were recorded, but none were of the correct magnitude to be interpreted as monopoles. From the total weight of sample ore examined, 942.9 kg, an upper limit of superheavy monopole density is 3.2 x 10-6 monopoles/gram at 95% CL, which is about 50 times smaller than that given by previous authors. In addition, from the total live time of the fluxmeters, 1805 h, an upper limit of free-monopole flux irrespective of its mass or velocity is obtained as 5.9 x 10-10 cm-2 s-1 sr-1 at 90% CL. (author)