mode, timelines from all modules are recorded when an external trigger from ZEPLIN–III is received. In [*master*]{} mode, when certain conditions are met, the veto system triggers independently. In the present configuration this happens when three or more modules each register at least 10 photoelectrons in the same event. This threshold results in an event rate of approximately 2 Hz and allows for an independent measure of background from the detector surroundings, especially ambient neutrons from the laboratory rock.

Finally, a muon trigger is derived from the roof modules with no condition on multiplicity ([*i.e.*]{}, the number of modules with simulataneously occurring pulses). This requires an external triggering unit in order to avoid the large data volumes otherwise generated by a software trigger testing individual modules. In addition to providing a direct measure of the atmospheric muon flux through the laboratory, the purpose of the unit is to help determine the muon-induced neutron production rate in Pb, a measurement of interest to the rare-event search community. Using the Pb castle as a target and taking advantage of the low energy threshold of the veto detector as well as its segmented design to provide angular information, it is hoped that an 