Photomultiplier tubes (PMTs) turn light signals into electrical signals by means of the photoelectric effect, a quantum mechanical phenomenon in which a particle of light (a photon) hits a metal surface (the photocathode) and causes an electron to be ejected from the surface. For MiniBooNE PMTs, the photocathode is a very thin layer of material on the inside surface of the tube's glass envelope. This thin layer gives the tubes their brownish-gold color.
Knocking out an electron from the photocathode is just the first step in the light-to-electrical signal process. The photon-ejected electron is attracted to an adjacent plate (the next dynode), from which it ejects still more electrons; each of those is attracted the next dynode, ejecting more electrons, and so on. This dynode-cascade of electrons is how the tube "multiplies" the electronic signal started off by the initial photon.
Each of the 1500 tubes in MiniBooNE's tank underwent a detailed testing procedure, designed to find the tube's optimal operating voltage and other response characteristics. These data were used as part of the installation and calibration procedure, and become critical parts of the experiment's analysis and modeling programs.
|One of MiniBooNE's photomultiplier tubes from Hamamatsu|