Observation of the 'head-tail' effect in nuclear recoils of low-energy neutrons

We present an experimental method to determine the direction tag ('head-tail') of dark matter wind using a low-pressure time-projection chamber. We demonstrate the method by tagging the direction of the elastic nuclear recoils created in the scattering of low-energy neutrons with CF4 nuclei. The decreasing ionization rate along the recoil trajectory allows us to determine the direction of the incoming neutrons, and proves that the 'head-tail' effect can be measured