contribute less than 0.01 single neutron elastic scatters per year in the ZEPLIN–III fiducial volume. $\gamma$-ray emission from radioactive impurities in the plastic scintillator also poses a low risk since it is tagged with near unity efficiency.

![\[veto\_rate\_full\]Cumulative background signal rate in the full veto detector array as a function of photoelectron threshold. All pulses are considered irrespective of multiplicity. Statistical errors are too small to be seen on this scale. The pulses are measured from slave mode triggerd events and the rate is truncated at 65 photoelectrons where pulse height saturation takes effect. The fit to the data is composed of three well resolved components made up of SPEs, internal background from the PMTs and $\gamma$-ray background from radiological contamination within surrounding materials and the veto detector itself.](ratecum_fit_99_cg.eps){width="8.6cm"}

The background signal rate in the veto detector is also of direct consequence to its performance since this defines the probability of an event in ZEPLIN–III being accidentally vetoed. The accidentals rate is determined separately for prompt $\gamma$-ray tagging and for delayed neutron tagging since the selection criteria (such as energy threshold, pulse arrival time, multiplicity, etc) for a coincident event to be labelled as a $\gamma$-ray or neutron differ.

In 