Americium-241 is a very low-activity radioactive element that emits alpha particles, a low-energy form of radiation. As air passes across the americium source within the detector, the radiation ionises any smoke particles. This ionised air is then picked up by an electronic sensor, sounding the alarm.
A range of radioactive elements are used in the fight against cancer. The key factor is to accurately target the cancer cells and destroy them, while as far as possible avoiding exposing the rest of the body to radiation. For example, samarium-153 is used to treat prostate cancer. A very high dose of radiation can be given to just the prostate gland by inserting little grains of radioactive material into it in a relatively simple operation, sometimes carried out under local anaesthetic. It exposes the rest of the body to only a very low dose of radiation.
Another common element used in cancer treatment is iodine-131, which is used to tackle cancers and other disorders of the thyroid - both the former US President George Bush and his wife were both treated for Grave's disease with radioactive iodine.
Some radioactive elements can be used as "tracers" - they are injected into the bloodstream so that its flow can be tracked and the function of various organs monitored. For example, in Parkinson's disease, technetium-99m is injected and follows the dopamine pathway in the brain, so that doctors can determine whether it is working normally.
Similar techniques can be used to monitor blood flow to the heart. If there are any blockages inhibiting the flow, these can be detected before they cause a heart-attack.
It is typically very difficult to control the pain for patients who have tumours that have spread into bone tissue. But by using a radioactive material such as samarium-153 or strontium-89, high doses of radiotherapy can be delivered just to the bone tumours and relieve the pain. In trials, the treatment has been shown to be effective.
Passing food through a beam of gamma rays given off by the element cobalt-60 can extend the shelf life of meat, poultry, vegetables and fruit by inhibiting bacteria. It cannot be used on produce that has a high water content, because the food would explode. It was first used in spices, as they were kept for a long period of time between uses.
Appliance indicator lights
When you turn on an appliance, such as a coffee-maker or kettle, an electrical charge passes through the gas krypton-85. The current causes the gas to become active, ionising the air around it to produce the glow.
Uranium is not just for bombs and power stations. Uranium-238, the most common isotope of uranium found in nature, can be mixed in with the material that is used to make dentures to increase their strength. It is also used to help dentists try to match up the colour of the dentures to the real teeth that remain. The uranium in this case is not being used for its radioactivity - it has a half-life of 4.46 billion years, so it is safe.
When there have been suspected leaks at nuclear power stations, or when authorities suspect that a dirty bomb may have been used, they need be able to verify that their instruments are working correctly. A radioactive source, typically strontium-90, which emits gamma rays, is used.
Plutonium-238 is emits radioactive alpha particles, the type of radiation that can be stopped entirely by no more than a sheet of paper. As it decays, it gives off heat, and this heat is then used to generate electricity to power the satellite.
Uranium-235 is the isotope of uranium widely used in nuclear power stations, which generate 20 per cent of the electricity used in Britain. When the atom breaks apart it gives off a huge amount of energy in the form of heat. The heat is transferred to water, and the resulting steam is used to run a turbine generator.
Thorium-229 is a gas often used in lighting. As a small electric charge passes through a tube of the gas, it triggers the thorium to react and produce the light.
Polonium has no stable isotopes, but in a sealed and controlled form that reduces radiation hazards, the element is used to eliminate static charges - particularly in textile mills and on brushes used to remove dust from photographic film. The isotope polonium-210 represents a radiation hazard if it is taken into the body, by breathing it in, taking it into the mouth, or getting it into a wound.