the veto detector, the probability that a tagged event is not correlated with the signal in ZEPLIN–III is 0.4%.](gamma_acc_col.eps){width="8.6cm"}

$\gamma$-ray tagging efficiency
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With the prompt tag criteria set out above, the tagging efficiency can be determined from synchronised background data triggered by ZEPLIN–III with the veto detector running in slave mode. The resulting prompt tagging efficiency is shown as a function of veto detector threshold in Figure \[gamma\_sig\_comp\_cg\]. The plot shows the measured efficiencies for all triggered events below 100 keV$_{ee}$ energy in ZEPLIN–III (where keV$_{ee}$ is electron-equivalent energy using 122 keV $\gamma$-rays from a $^{57}$Co source to set the energy calibration). Also shown is the tagging efficiency for those events that occur in the fiducialised xenon of the ZEPLIN–III instrument. As expected, the efficiency increases when considering the central fiducial target xenon. Here background events are more likely to be from Compton scatters (rather than, for example, $\beta$-induced background or $\alpha$-particles that cannot directly give signal in the veto modules) and the $\gamma$-rays have consequently a higher probability of being detected in the veto modules. The tagging efficiency for prompt events is (28.1$\pm$0.2)% for synchronised electron recoil background in the fiducial liquid xenon volume. Monte Carlo simulations of 