These are the widely used smoke detectors . It is best at work for the clean burning fire that produces small particle during combustion .

 

These alarms contains a small amount of radioactive material ( Alpha particle producing source ) in them . The Alpha Particle make the chamber ionized and hence conductive . The radiation passes through this ionization chamber which is an Air Filled space between two electrodes and allows small current to flow between electrodes . If smoke enters the chamber, it absorves the Alpha Particle consequently reducing the ionization and interrupting this current and setting off the alarm .
This type of Alarm detect invisible fire particle ( associated with fast flaming fire ) sooner than photoelectric alarms .And hence , these alarms respond best to the fast raging fire .

Advantage of Ionization Smoke Detectors –

Detectes  invisible product of combustion -It can detect fire that are in incipient stage.

Fast Acting – Provides for earlier detection than other types of smoke detectors or thermal detectors

 

Dis-advantage of Ionization Smoke Detectors –
May provide false detection if used where volatile solvents, conductive material dusts, or high humidity are present
Detects the presence of smoke only, not toxicity
Has a potential for high false alarm rate

Typical locations or hazards for ionization detection:
Clean rooms
Computer rooms
Mechanical air ducts
Locations where sensitive detection methods are needed