  The addition of O2 to gas mixtures in time projection chambers containing CS2
has recently been shown to produce multiple negative ions that travel at
slightly different velocities. This allows a measurement of the absolute
position of ionising events in the z (drift) direction. In this work, we apply
the z-fiducialisation technique to a directional dark matter search. In
particular, we present results from a 46.3 live-day source-free exposure of the
DRIFT-IId detector run in this completely new mode. With full-volume
fiducialisation, we have achieved the first background-free operation of a
directional detector. The resulting exclusion curve for spin-dependent
WIMP-proton interactions reaches 1.1 pb at 100 GeV/c2, a factor of 2 better
than our previous work. We describe the automated analysis used here, and argue
that detector upgrades, implemented after the acquisition of these data, will
bring an additional factor of >3 improvement in the near future.
