  Tetraphenyl-butadiene (TPB) is an organic fluorescent chemical compound
generally used as wavelength shifter thanks to its extremely high efficiency to
convert ultra-violet photons into visible light. A common method to use TPB
with detectors sensitive to visible light, such as photomultiplier tubes
(PMTs), is to deposit thin layers on the device window. To obtain effective TPB
layers, different procedures can be used. In this work a specific evaporation
technique adopted to coat 8 in. convex windows photomultiplier tubes is
presented. It consists in evaporating TPB by means of a Knudsen cell, which
allows to strictly control the process, and in a rotating sample support, which
guarantees the uniformity of the deposition. Simulation results and
experimental tests demonstrate the effectiveness of this evaporation technique
from the point of view of deposition uniformity and light conversion
efficiency.
