the absence of accidental coincidences. The difference between this value and the measured tagging efficiency for any given acceptance window is then the accidental coincidence rate for that window. At 0.4 $\mu$s window width the accidental rate is predicted to be less than 0.4%, in agreement with measurements.

The prompt tag threshold requires a minimum of 2 photoelectrons equivalent pulse signal distributed in any pattern across the veto detector, provided that any coincident pulses across multiple modules are themselves within $\pm$0.2 $\mu$s of one another. Lowering the threshold any further would result in a prohibitive accidental rate due to PMT dark noise and light leakage at the single photoelectron level. This is illustrated in Figure \[gammaaccidental\], showing the fraction of events accidentally tagged as $\gamma$-rays in ZEPLIN–III, as a function of signal threshold in the veto detector. Here, the veto detector channel timelines are searched for accidental coincident pulses outside of the prompt coincidence window and the pre-trigger region. This ensures genuine coincidences with [*S1*]{} signals are ignored.

![\[gammaaccidental\]Accidentally tagged events in the 0.4 $\mu$s prompt window as a function of veto detector threshold. The band represents 1$\sigma$ errors. For the prompt tag threshold of 2 phototelectrons equivalent signal in 