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
ð
22

2
Þ
%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
ð
100
þ
0

35
Þ
% 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.