Measurements of the ion fraction and mobility of $\alpha \text{−}$ and $\beta$-decay products in liquid xenon using the EXO-200 detector

Alpha decays in the EXO-200 detector are used to measure the fraction of charged ${}^{218}\text{Po}$ and ${}^{214}\text{Bi}$ daughters created from $\alpha$ and $\beta$ decays, respectively. ${}^{222}\text{Rn} \alpha$ decays in liquid xenon (LXe) are found to produce ${}^{218}\text{Po}^{+}$ ions $50.3\pm 3.0\%$ of the time, while the remainder of the ${}^{218}\text{Po}$ atoms are neutral. The fraction of ${}^{214}\text{Bi}^{+}$ from ${}^{214}\text{Pb} \beta$ decays in LXe is found to be $76.4\pm 5.7\%$, inferred from the relative rates of ${}^{218}\text{Po}$ and ${}^{214}\text{Po} \alpha$ decays in the LXe. The average velocity of ${}^{218}\text{Po}$ ions is observed to decrease for longer drift times. Initially the ions have a mobility of $0.390\pm 0.006{\text{cm}}^2/\left(\text{kV}\text{s}\right)$, and at long drift times the mobility is $0.219\pm 0.004{\text{cm}}^2/\left(\text{kV}\text{s}\right)$. Time constants associated with the change in mobility during drift of the ${}^{218}\text{Po}^{+}$ ions are found to be proportional to the electron lifetime in the LXe.

Figure 1

Partial diagram of the uranium series. The $\alpha \text{-particle}$ energies are shown along with their corresponding relative intensities. The $\beta$ decays are shown with their branching ratios.

Figure 2

Cutaway view of the EXO-200 detector highlighting several important components of the TPC.

Figure 3

Raw scintillation versus electron equivalent charge energy $\left(\text{keVee}\right)$ of Bi-Po events. Different colors distinguish between Bi and Po on surfaces and in the bulk LXe. Surface events are defined to be within $10\text{mm}$ of the cathode or anodes. The inset shows a histogram of the time delay between Bi-Po pairs.

Figure 4

(Upper left) Raw scintillation counts versus charge energy histogram. The highlighted region (A) is used to monitor activity of the ${}^{222}\text{Rn}$ chain in the LXe. (Upper right) Fully reconstructed and light map corrected data. (Lower left) Positions of events in region (B). (Lower right) Positions of events in regions (C) (dark blue) and (D) (red).

Figure 5

(Top) Rate of $\alpha$ decays in the region defined by Fig. 4, primarily from ${}^{222}\text{Rn}$ and ${}^{218}\text{Po}$ decays. (Middle) Average electron lifetime measured in the TPCs. (Bottom) Xenon recirculation flow rate. Red lines indicate when xenon feed events occurred; wider lines correspond to larger feeds.

Figure 6

(Top) Raw scintillation signal $z$ position dependence for ${}^{222}\text{Rn},{}^{218}\text{Po}$, and ${}^{214}\text{Po} \alpha$ decays. Vertical lines designate the range of the data used $(30<|z|<150\text{mm})$. (Bottom) Histogram of the $z\text{-corrected}$ scintillation signals in TPC1 with fits to each decay.

Figure 7

Raw scintillation counts versus $z$ position of coincident pairs assigned as ${}^{222}\text{Rn}$ (red) and ${}^{218}\text{Po}$ (black) in the region $20<\left|z\right|<172\text{mm}$.

Figure 8

Scatter plot of ${}^{218}\text{Po}$ drift distance versus time between the ${}^{222}\text{Rn}$ and ${}^{218}\text{Po}$ decays. Displacement $\left(\mathrm{\Delta }z\right)$ is defined as positive when movement is towards the cathode.

Figure 9

Histogram of the mean velocity of ${}^{218}\text{Po}$ ions and atoms extracted from ${}^{222}\text{Rn}\text{−}{}^{218}\text{Po}$ coincidences.

Figure 10

(Top sketch) The “upper” and “lower” volumes in each TPC are defined using the U wires as shown in the hexagonal active volume. Black circle shows the internal diameter of the detector, and red circle shows the position of the teflon reflector. Green arrows show where the LXe flows into and out of the detector at the end caps, near the anodes. (Bottom) The mean velocity of neutral ${}^{218}\text{Po}$, measured in the different volumes of the EXO-200 detector. The green band shows the average velocity of all events in each TPC, and the red line shows the expected velocity from the LXe recirculation. Note that the vertical axis on TPC1 is reversed to account for the convention on the sign of the velocity in which velocities are defined as positive for motion towards the cathode.

Figure 11

The velocity versus time between Rn-Po coincident events (positive is towards the cathode). The TPC in which the events occur is indicated by the color of the points.

Figure 12

Histograms of the average velocity of positive ions separated into four ranges of electron lifetimes. A model (solid lines) is used to fit the data (points with error bars). The dashed lines indicate the velocities $v_1$ and $v_2$ of the fit.

Figure 13

(Left) Velocity histogram after applying the $\mathrm{\Delta }t$ cut on all of the Rn-Po coincidences. The shaded region (velocity $>$ 0.5 mm/s) is integrated to determine the number of ${}^{218}\text{Po}$ ions. (Right) The ion fraction found for different electron lifetime ranges.