Photomultiplier Tubes:

A low noise, high gain (> 10⁶), photon counting detector, with fast time response (risetimes < a few ns) matched to the intrinsic temporal width of the gamma-ray induced air shower is necessary to increase the signal-to-noise ratio and to operate VERITAS at the lowest possible energy threshold. These requirements are presently satisfied only by PMTs and 25 years of experience with these devices leads us to base the VERITAS system upon them. The focal plane will contain 499 hexagonally packed PMTs providing an effective FoV of $\sim 3.5^\circ$. The spacing between the PMTs corresponds to a focal plane angular distance of $0.15^\circ$. The active light collection diameter of the PMTs is 25mm. The collection efficiency is increased by the use of Winston light concentrators. Tests have identified the Hamamatsu R7056, with a bialkali photocathode and a UV glass window, as one PMT which satisfies our requirements.