the eye. The delayed neutron tag varies considerably more slowly with increasing multiplicity than the prompt tag. This results from the former being dominated by detection of multiple high energy $\gamma$-rays following de-excitation of the $^{158}$Gd nucleus.](nvg_norm.eps){width="8.6cm"}

\[gammatagging\]$\gamma$-ray tagging
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Following a $\gamma$-ray Compton scatter in the xenon target, the energy deposition from an electron recoil will trigger both ZEPLIN–III and veto detector data acquisitions. The scattered $\gamma$-ray may interact promptly within the veto detector and only a narrow coincidence window is required to identify these $\gamma$-rays. The criteria for designation of such ‘prompt tags’ are described below. These include determining the optimal prompt coincidence window and setting of an appropriate veto detector threshold (in energy and multiplicity) to maximise acceptance for genuine prompt events whilst minimising those that are accidentally coincident. This is followed by the resulting tagging efficiency for prompt $\gamma$-ray signals.

Selection criteria for prompt signals
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ZEPLIN–III discriminates between incident particle species by recording a prompt scintillation coming directly from the interaction site, [*S1*]{}, and a delayed electroluminescence signal caused by the ionisation escaping into the gas volume, [*S2*]{}. The delay between [*S1*]{} and [*S2*]{} can be up to $\sim$16 $\mu$s depending on the depth in 