be several pulses per event, the latter being determined by a single data acquisition trigger. As such, Figure \[veto\_rate\_full\] indicates the actual background pulse rate as measured by the veto detector. The plot extends to the point of pulse saturation, which is due to the full range of the digitiser ADCs and the gain settings of the PMTs, and occurs at approximately 65 photoelectrons (equivalent to several MeV of energy deposition across the array). The pulse rate from background data is well described by a combined fit of three components. The first is a semi-Gaussian fit to the SPE peak up to 2 photoelectrons, characterising thermionic emission from the photocathodes and ambient light leakage into the scintillators. As described in Section \[gammatagging\], such single pulses are considered below threshold for valid coincident events. The second component is an exponential fit to background from radiological contamination from within the veto detector PMTs. In particular, the $^{40}$K content dominates between 2-15 photoelectrons. $^{40}$K has an 89% $\beta^{-}$ decay branching ratio. For a refractive index of 1.49 at 400 nm, $\beta^{-}$ radiation emitted from the potassium generators in the PMT behind the photocathode will produce Cherenkov photons in the window when the electron 