Mica can store (for $>1\mathrm{}\mathrm{Gyr}$) etchable tracks caused by atoms recoiling from weakly interacting massive particles (WIMPs). Because a background from fission neutrons will eventually limit this technique, a unique signature for WIMPs in ancient mica is needed. Our motion around the center of the Galaxy causes WIMPs, unlike neutrons, to enter the mica from a preferred direction on the sky. Mica is a directional detector and despite the complex rotations that natural mica crystals make with respect to this WIMP “wind,” there is a substantial dependence of etch pit density on present day mica orientation.

• Received 18 March 1996

DOI:https://doi.org/10.1103/PhysRevLett.78.1628