Oct 10, 2018 Muhammad Qasim Abdul Wali Khan University, Department of Physics, Mardan, Pakistan by

Introduction components Structure Working principle types The end 0

introduction The photomultiplier tube (PMT) is an instrument that converts light to electrical signals. Gamma camera contains PMT It detects and amplifies the electrons that are produce by photocathode when the photocathode is strike by photons from scintillating material. 1

Components  Primary parts 1.Glass vacuum tube 2.Photocathode 3.Dynodes 4.Anode  Secondary parts 1.Input window 2.Focusing electrode 2

Glass vacuum tube are the tube which are fully vacuumed from inside. All the other parts are arranged inside this tube. Photocathode are photosensitive materials which emits electrons when (scintillation)light falls on it. Dynodes are electrodes, each held at a more positive voltage than the previous one, which accelerate the electrons. When electron strike a dynode, more low energy electrons are emitted and accelerated to the next dynode in the series. 3

Structure Input window 4 Vacuumed inside

Working principle When photons hit the entrance window Electron is released by the photocathode. Electron is then multiplied by the dynodes. At the end of the dynode chain there is an anode. All the electrons are collected by the anode. The current flowing from the anode to ground is directly proportional to the photoelectron flux generated by the photocathode. 5

Working principle (cont.) 6

Types of photomultipliers 1. Silicon photomultipliers  It contain an array of avalanche diodes.  Photomultipliers can be replace with avalanche photodiodes which also exhibits an amplification mechanism.  it occurs in solid-state(semiconductor) materials, rather than vacuum tubes.  Cheaper and much more compact and robust.  Exhibits a higher quantum efficiency, but also a higher amplification noise. 7

Types cont. 2. Hybrid photomultipliers  A vacuum tube with a photocathode and a silicon avalanche diode.  Functions similarly to a PMT but with a different mechanism of amplification.  Suitable for a verity of applications including light detection and ranging.  Where electrons from a photocathode are accelerated with several kilovolts to a semiconductor chip similar to that of an avalanche diode. 8