Track reconstruction and performance of DRIFT directional dark matter detectors using alpha particles

First results are presented from an analysis of data from the DRIFT-IIa and DRIFT-IIb directional dark matter detectors at Boulby Mine in which alpha particle tracks were reconstructed and used to characterise detector performance—an important step towards optimising directional technology. The drift velocity in DRIFT-IIa was 59.3±0.2(stat)±7.5(sys)ms-1 based on an analysis of naturally occurring alpha-emitting background. The drift velocity in DRIFT-IIb was 57±1(stat)±3(sys)ms-1 determined by the analysis of alpha particle tracks from a 210Po source. Three-dimensional range reconstruction and range spectra were used to identify alpha particles from the decay of 222Rn, 218Po, 220Rn and 216Po. This study found that (22±2)% of 218Po progeny (from 222Rn decay) did not plate out and remained suspended in the 40Torr CS2 gas fill until they decayed. A likely explanation for this is that some of the polonium progeny are produced in an uncharged state. For 216Po progeny (from 220Rn decay) the undeposited fraction was apparently much higher at (100-35+0)% most likely due to a shorter lifetime, causing a larger fraction of the progeny to decay whilst drifting to the cathode plane. This explanation implies a much slower drift time for positively charged polonium progeny compared to CS2- ions.