Magnetic monopoles, if they exist, should be trapped and accumulated in ferromagnetic materials. Using the high field of a pulsed magnet, we have sought to extract monopoles from a magnetite outcrop on the earth's surface and from fragments of a stony-iron meteorite. In the nuclear emulsions used for detection, no tracks were found satisfying our geometric criteria and having an energy-loss rate compatible with the theoretical expectation for monopoles. The area-time product of the magnetite cosmic-ray exposure is estimated to be about ${10}^{13}$ ${\mathrm{cm}}^2$ sec. From the negative results, upper-limit monopole production cross sections in the atmosphere are estimated as a function of assumed monopole mass.

• Received 21 May 1963

DOI:https://doi.org/10.1103/PhysRev.132.387