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 ~(210)Po source. Three-dimensional range reconstruction and range spectra were used to identify alpha particles from the decay of ~(222)Rn, ~(218)Po, ~(220)Rn and ~(216)Po. This study found that (22 ± 2)% of ~(218)Po progeny (from ~(222)Rn decay) did not plate out and remained suspended in the 40 Torr CS_2 gas fill until they decayed. A likely explanation for this is that some of the polonium progeny are produced in an uncharged state. For ~(216)Po progeny (from ~(220)Rn 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 CS_2~- ions.
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