Search for penetrating, highly charged particles at mountain altitude

We have exposed a 16-${\mathrm{m}}^2$ detector consisting of three layers of CR-39 plastic track detector and an aluminum absorber at White Mountain (646 g/${\mathrm{cm}}^2$) for one year. We etched these detectors in such a way as to reveal tracks of penetrating particles with ionization rate greater than that of a nucleus with $\frac{Z}{\beta }\gtrsim 15$. Our negative result implies that the flux of electrically charged particles with $\frac{Z}{\beta }>15\mathrm{}$ at a depth of 646 g/${\mathrm{cm}}^2$ is less than ∼${10}^{-12\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{s}}^{-1\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$ (90% C.L.) and that the flux of monopoles with magnetic charge $g>~\frac{e}{2\alpha }$ and velocity greater than $7\times {10}^{-3\mathrm{}}c$ is less than ∼${10}^{-12\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{s}}^{-1\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$ (90% C.L.).