300 s and light emitted from the fibres is transmitted through the scintillator to the PMT photocathode, generating for each pulse an average of 48 photoelectrons in each module. A relatively low rate is used so as not to distort spectral shape through induction of afterpulsing or saturation effects in the PMTs. The number of photoelectrons contributing to the peak LED response is calculated from the SPE peak position and estimated from the width of the distribution for consistency. The normalised number of photoelectrons detected in each module is then tracked as a function of time throughout the course of the experiment and is plotted in Figure \[ledTrend\], showing no detectable change in optical transmission so far. A full study of the properties of UPS–923A scintillator, including results from dedicated measurements of its nuclear quenching factor for low energy recoils, will be presented elsewhere [@leaQF].

![\[ledTrend\]Evolution of the number of photoelectrons from LED exposures (normalised to the first measurement) for a single typical veto module.](ledTrend.eps){width="8.0cm"}

ZEPLIN–III - Veto event synchronisation
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The ZEPLIN–III and veto detector data acquisition systems operate at different sampling speeds and digitise waveforms of different duration with different resolutions. Accurate target-veto timing is achieved using a 