  Directional detection of dark matter can provide unambiguous observation of
dark matter interactions even in the presence of background. This article
presents an experimental method to measure the direction tag ("head-tail") of
the dark matter wind by detecting the scintillation light created by the
elastic nuclear recoils in the scattering of dark matter particles with the
detector material. The technique is demonstrated by tagging the direction of
the nuclear recoils created in the scattering of low-energy neutrons with CF4
in a low-pressure time-projection chamber that is developed by the DMTPC
collaboration. The measurement of the decreasing ionization rate along the
recoil trajectory provides the direction tag of the incoming neutrons, and
proves that the "head-tail" effect can be observed.
