Question

A Photomultiplier tube can detect a single photon, but a photodiode cannot. Why is this? As part of your answer sketch the main components of both devices and describe how they work.

Answer 1

Ans: Photodiode :- It is a pr-junction diode that converts - light energy into electrical energy. It is designed to work in reverse-bias condition ( p-side is connected to ove terminal of battery and vice-versal so that the thickness of junction barrier is large. When light falls on this barrier, electrons absorbs energy and election- hole pairs are created. This are separated by E-F. produced within the barries and further collected by anode Chole) and cathode (election). As a result a photo current flows and we can detect lights Isole o electron o hole b-type | Scanned with il CamScanner a

Photo multiplier tubes ; - These are extremely sensitive to light, as - photo electron multiplication takes place at various points in the tube. It may have multiplication factor of lo8, so it can easily detect even a single photon. Initial photon creates a electron 3 photon from photo cathode (primary é ) photocathode pirn the focus which is focussed on a dynodo by brimary a focussing electrode. These t dynodes are at relatively increasing potential (~100u) to the secondary previous. At each stage, more e and more slow energy es (secondary) are produced and further accelerated by next dy node. Thus no. of secondary és becomes lange Anodo connector bing dynoch et م و MO and can be detected. so photo multiplier tube is Scavery efficient for low incident flux. CamScanner