Search for Weakly Interacting Massive Particles in Ancient Mica

For a WIMP search the collecting power of a crystal of mica with a track-retention age of 10 ⁹ years is far greater than can be attained with electronic detectors. We use an Atomic Force Microscope to measure the depths of etchpits due to recoil atoms in mica that has been calibrated with fast neutrons and low-energy ions of O, Si, K, and Fe. The background of alpha recoils is eliminated by making a range cut. The mica contains target atoms (F-19, Al-27, K-39, and K-41) with high cross sections for elastic scattering by neutralinos. Our first result with the mica technique is a WIMP flux limit comparable to the best limit attained with Ge detectors. It may be possible to increase the sensitivity by several orders of magnitude.