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Smoke Detectors

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Helpful guide on how radiation is used in a smoke detector.
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  Georgia Wowk 11xSc31 Smoke Detectors They are often advertised as your best chance of noticing and, therefore, surviving a fire. Every household is constantly urged to install one, through countless campaigns by the fire service. But how does something so small become an intricate, life-saving piece of equipment? Radioactive isotopes that emit alpha radiation are the basis for a smoke detector. Alpha particles also have ionisation properties that help them to detect smoke, and alert us of its presence. Ionising radiation is radiation that has enough energy to cause other atoms to lose electrons and form ions. This is why smoke alarms are built with an ‘ ionisation chamber ’ . This chamber consists of open channels allowing air form the room to flow through it. The alpha particles emitted from the source inside collide with the oxygen and nitrogen molecules in the air. This causes them to ionise. This loss of electrons results in negatively charged electrons  and positively charged atoms . Also inside this chamber, there is a positive and a negative electrode. The negatively charge electrons are attracted to the positive electrode and the positively charged atoms to the negative electrode. Every time this is done, it results in a very small current being generated, which is detected by the electrical circuit in the smoke detector. However, if a fire has been started, something else happens. When the smoke enters the ionisation chamber, the alpha particles collide with the smoke particles instead of the air particles. The collisions with smoke particles cannot   result in ionisation so the  Georgia Wowk 11xSc31 current drops. The electric circuit registers the drop in electric current. This triggers the alarm to sound, and alert anyone in the building to evacuate. But is it safe? Fortunately, as alpha radiation is absorbed by a few centimetres of air, it is not harmful to us, even in close proximity. As well as this, the source has a long half-life and never needs to be replaced. Perhaps the most commonly used isotope is Americium-241. In fact, most of the several kilograms of americium made each year is used in these detectors. This particular radioactive isotope has a half-life of 432 years. This can be seen on a graph of its decay over time. This is why alpha radiation (only harmful inside the body) from these isotopes (with such long half-lives) is perfect for use in vital equipment, such as the smoke detector. Though the battery may need replacing, the source will continue emitting alpha radiation for hundreds of years, constantly protecting families, schools and work places from potentially life-threatening situations.
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