Schematic of the XENON100 detector, showing the production of primary (S1) and secondary (S2) light from a particle interaction. A grid separates the sensitive volume, which contains liquid xenon, from the top, which contains xenon gas. Electrons produced by an interaction in the sensitive volume drift upwards in the electric field (generated by the grid and cathode) producing secondary light in the high-field region containing xenon gas. As depicted, the secondary light signal is much larger than the primary light signal. The drift time of the electrons causes a delay between the two light signals that allows reconstruction of the vertical position of the interaction to within 2 mm; the horizontal detection accuracy is 3 mm.