window. Extending the window beyond this only increases the number of events by introducing accidental coincidences, which lower the effective exposure of the xenon target by unduly rejecting potential WIMP events. Figure \[promptWindow\] shows the efficiency of the veto detector for tagging prompt events rising rapidly as the coincidence window is opened to 0.4 $\mu$s.

![\[promptWindow\]The efficiency for tagging prompt coincidence events between ZEPLIN–III and the veto detector, as a function of acceptance window width. The efficiency rises rapidly as the window is opened to 0.4 $\mu$s, but beyond this any increases in efficiency corresponds to the inclusion of accidental coincidence events.](promptWindow.eps){width="8.6cm"}

The rate of accidental coincidences can be calculated from the product of the event rates in both systems, and can be measured directly by applying the same window off-coincidence. This rate is 0.4% for a 0.4 $\mu$s window centred on the peak in Figure \[starttime\_merge\] using a 2 photoelectron threshold. Figure \[promptWindow\] corroborates independently the accidental coincidence rate. Since the plot shows the prompt $\gamma$-ray tagging efficiency values with the thresholds as described above, extrapolating a fit to the linear region at high acceptance window values to the y-axis results in an estimate of the tagging efficiency in 