Head-tail Signature of Nuclear Recoil Tracks in Gas

An algorithm was developed which determines the forward/backward direction of nuclear re- coil tracks using the head-tail signature as measured by a novel elementary particle detector. The detector, constructed at UH by a team led by Dr. Sven Vahsen, can detect electrically neutral particles, such as neutrons and possibly dark matter particles, via their elastic scattering off gas nuclei. These nuclei recoil and leave millimeter-long ionization trails in the gas which are recorded by the detector. Theoretically the 3-D distribution of ionization carries enough information to reconstruct both the direction of nuclear recoils and the species of the recoiling nuclei. The algorithm developed in this way removes the ambiguity in the forward/backward direction of the track by exploiting the asymmetric charge distribution along the length of the track. To improve the algorithm accuracy we selected for tracks which the algorithm handles best using the following variables: head-tail charge fraction, event energy, sum of time over threshold, number of charge deposits which make up the track, and track length. A bias towards horizontally angled recoils was observed in tracks with large head-tail asymmetry. A proposed mechanism is angled tracks will have one end drift further than the other, thus inducing a fake head-tail effect. The results demonstrate that the accuracy of the algorithm can be increased to 80% by adding restrictions on these variables alone, an estimated 90% after correcting for drift diffusion and suggest that an energy dependent algorithm be required to analyze particles with energy beyond the Bragg peak