PHOTO MULTIPLIER TUBE (PMT)

 

 A photo multiplier tubes(PMTs), useful for light detection of weak signals a photoemissive device in which absorption of a photon results in the emission of an electron. These detectors work by amplifying the electrons generated by a photocathode exposed to a photon flux.

PRINCIPLE: A Photo Multipler converts light into an electrical signal, then amplifies that signals to a useful level by emission of secondary electrons.

 Hence it work on two principles:

1. Photoelectric Effect

2.. Secondary Emission

Photoelectric Effect:- The photoelectric effect is the emission of electrons or other free carriers, when light is incident on a material. Electron emitted in this manner are called photoelectrons and the phenomenon is called Photoelectric Effect.

secondary Emission:- It is the emission of free electrons from the metal surface, which occurs when the high speed electrons hit the free electrons in the metal.

CONSTRUCTION OF PHOTO MULTIPLIER TUBE(PMTs):-

Photo Multipliers are contained within a glass tube that maintains a vacuum within the device.

There are three main electrodes within a photomultipliers:

1. Photocathode

2. Dynode

3. Anode 

• PHOTOCATHODE:- Photocathode is a thin deposit of metal on the entry window to which the photon been strikes and photoemission takes place.

• ANODE:- Anode is a metallic rod which finally collects the electrons and constitute the electric current in the circuit detected by output meter.

• DYNODES:- Dynodes is an electrode which serves as an electron multiplier through secondary emission. Dynodes in a photo multiplier Tube acts as both anode as well as cathode.

The arrangement is done such that all the dynodes are put in between the cathode and anode connected across the terminal of a high voltage battery such that the potential increase in going from cathode to anode.

Materials used for making Photocathode:

Generally have low work function and are therefore prone to thermionic emission.

Most common used materials are-

• Cs:Sb -        Cesium antimony have 12% efficiency and cut off frequency in about 400nm. 
• Ag-o-Cs -    Also called Si sensitive from 300-1200nm.
• GaAsCs -    Cesium activated Gallium Arsenide sensitive from 300 to 850nm fading towards UV.
• InGaAs -   Cesium activated Indium Gallium Arsenide. Higher  IR sensitivity between 900-1000nm
• Cs-Te, Cs-I -  Solar blind sensitive to UV insensitive to IR and visible light.

WINDOW MATERIAL:-

The windows of Photomultipliers act as wavelength filters.

• Borosilicate glass is commonly used for near IR to about 300nm.
• Magnesium Fluoride transmits UV to 115nm.
• Ultraviolet Glass transmits visible and UV to 185nm.
• synthetic silica.

WORKING OF PHOTO MULTIPLIER TUBE(PMTs):

A Photo multiplier tube consists of electrodes which are maintained of electrodes which are maintained at increasing potentials in a sequence from photocathode at lowest potential to anode at the highest potential. These typically utilizes 1000 to 2000V to accelerate electrons within the chain of dynodes.

when light falls on the cathode, photoelectrones are emitted which are accelerated towards dynode 1st. Since it is at higher potential than electrons liberate secondary electrons which are attracted by dynode 2nd because it is also at relatively high potential. These electrons release further secondary electrons from it which are attracted by dynode 3rd. This process continues till the electrones are finally connected by the anode. At each dynode the number of secondary electrons keeps on multiplying. Photo Multiplier Tube operate at voltage 1 to 5KV.

APPLICATIONS:

1. It is used in research laboratories to detect the radiations of very low intensity.

2. It is used to detect nuclear and particles radiations.

3. PMTs are used in numerous medical equipment design. For examples , in blood analysis devices, PMTs used to find the relations concentration of various components in vials of blood drawn in Doctor's offices, in combination with optical filters and incandesent lamps .

4. It is very highly sensitive to radiations.

5. It is the first electric eye device, being used to measure interruption in beams of light.

Because of these advantages they are extensively employed in scintillators, radiation detector and for space exploration.

PMTs also exhibit allow noise level, high frequency response(less than 1 nanosecond), having ultrahigh sensitivity, small dark current and a large collection area.