  The use of negative ions in TPCs has several advantages for high-resolution
rare- event detection experiments. The DRIFT experiment, for example, has taken
full advantage of this technique over the past decade in a directional search
for dark matter. This paper focuses on the surprising discovery of multiple
species of ionization-created CS2 anions, called minority carriers, in gas
mixtures containing electronegative CS2 and O2, identified by their slightly
different drift velocities. Measurements of minority carriers in gas mixtures
of CS2, CF4 and O2 are reported in an effort to understand the nature of these
charge carriers. Regardless of the micro-physics however, this discovery offers
significant practical advantages for experiments such as DRIFT, where the
difference in arrival time may be used to fiducialize the original ionization
event without an external start pulse.
