Measurement of the drift velocity and transverse diffusion of electrons in liquid xenon with the EXO-200 detector

The EXO-200 Collaboration is searching for neutrinoless double $\beta$ decay using a liquid xenon (LXe) time projection chamber. This measurement relies on modeling the transport of charge deposits produced by interactions in the LXe to allow discrimination between signal and background events. Here we present measurements of the transverse diffusion constant and drift velocity of electrons at drift fields between 20 V/cm and 615 V/cm using EXO-200 data. At the operating field of 380 V/cm EXO-200 measures a drift velocity of $1.{705}_{-0.010}^{+0.014}\mathrm{mm}/\mu \mathrm{s}$ and a transverse diffusion coefficient of $55\pm 4{\mathrm{cm}}^2$/s.

Figure 1

Cut out schematic of the EXO-200 LXe TPC. Shown in the center is the shared cathode plane (blue) and on either side the APD (black), V-wire (red), and U-wire (green) planes are shown. Also included on either side of the cathode are the copper field shaping rings and their support structure. The top inset shows a zoomed view of the wire and APD planes while the bottom inset shows the same view in the $x/y$ plane to illustrate the relative angle between the wire planes.

Figure 2

Magnitude of the electric field in the $U-Z$ plane for (top) one entire drift space of the TPC (cathode is at $Z=0$ mm) and (bottom) a small segment near the anode. Also shown in black are the electric field lines that electrons would follow to the collection plane assuming no diffusion.

Figure 3

Combined distribution of drift times for $\alpha$ events from both drift spaces. The inset shows events near the cathode separately for TPC1 (blue) and TPC2 (red). Also included are the best Gaussian fits used for determining the maximum drift time in both TPCs. The offset between the TPCs corresponds to a $\sim 0.5$ mm uncertainty in the cathode position as explained in the text.

Figure 4

Distribution of drift times for events with the longest drift times for MC (red line) and data (black points) for a ${}^{228}\mathrm{Th}$ source positioned near the cathode.

Figure 5

Drift velocity measured using EXO-200 for $\gamma \mathrm{s}$ in ${}^{228}\mathrm{Th}$ source data and $\alpha \mathrm{s}$ in low background data (167 K, red). Also included are results from Miller et al. (163 K, cyan) (Miller 1968: [18]), Gushchin et al. (165 K, grey) (Gushchin 1981: [19]), and the XENON10 (177 K, green) (Sorensen 2008: [20]), XENON100 (182 K, purple) (Aprile 2014: [21]), and LUX (175 K, blue) (Akerib 2014: [22]) collaborations. LXe operating temperatures are included to account for possible variations due to temperature [23]. The inset shows a comparison of the two methods of measurement at 380 V/cm and 567 V/cm in EXO-200.

Figure 6

EXO-200 comparison of one-wire fraction between data (black points) and MC without diffusion (blue line) and with diffusion (red line) for operation at 380 V/cm. The sources shown are (a) ${}^{228}\mathrm{Th}$, (b) ${}^{60}\mathrm{Co}$, and (c) ${}^{226}\mathrm{Ra}$. For MC with diffusion a coefficient at the best fit of 55 ${\mathrm{cm}}^2$/s was used for all sources.

Figure 7

Measured diffusion coefficient versus electric field in EXO-200 (red points). Solid error bars are statistical and dashed errors include the systematic errors discussed in the text. Also plotted are previous measurements made by Doke and collaborators (black points) [4, 5, 28, 29]. Error bars for the previous measurements from [29] are shown, where errors were provided only for two of the seven points.