Radioisotopes in Consumer Products

• The function of many common consumer products is dependent on the use of small amounts of radioactive material.
• Smoke detectors, watches and clocks, cookware, and photocopiers, among others, all utilise the natural properties of radioisotopes in their design.

The attributes of naturally decaying atoms, known as radioisotopes, give rise to several applications across many aspects of modern day life (see also information paper on The Many Uses of Nuclear Technology).

Smoke detectors

One of the most common uses of radioisotopes today is in household smoke detectors. Smoke detectors/alarms are important safety devices, because of their obvious potential to save lives and property. There are two types of smoke detector commonly available in many countries.

One type uses the radiation from a small amount of radioactive material to detect the presence of smoke or heat sources. These 'ionisation chamber' smoke detectors are the most popular because they are inexpensive and are more sensitive to flaming fire conditions with little smoke than the other type. They contain americium.

The other type of detector does not contain radioactive material; it uses a photoelectric sensor to detect the change in light level caused by smoke. This type is more expensive to purchase, but may detect smoke from smouldering fires earlier, so is often recommended.

Dual sensor alarms contain both types of detector.

The common type of household smoke detector contains a small amount of Am-241, which is a decay product of plutonium-241 originating in nuclear reactors. The Am-241 emits alpha particles which ionise the air and allow a current between two electrodes. If smoke enters the detector it absorbs the alpha particles and interrupts the current, setting off the alarm.

Operation of ionisation smoke detectors/alarms

The vital ingredient of the ionisation type of household smoke detector is a very small quantity of Am-241 as americium dioxide (AmO₂).

Americium-241 emits alpha particles and low energy gamma rays. The alpha particles emitted by the Am-241 collide with the oxygen and nitrogen in air within the detector's ionisation chamber to produce charged particles (ions). A low-level electric voltage applied across the chamber is used to collect these ions, causing a steady, small electric current to flow between two electrodes. When smoke enters the space between the electrodes, the smoke particles attach to the charged ions, neutralizing them. This causes the number of ions present – and therefore the electric current – to fall, which sets off an alarm.

The radiation dose to the occupants of a house from a domestic smoke detector is essentially zero, and in any case very much less than that from natural background radiation. The alpha particles are absorbed within the detector, while most of the gamma rays escape harmlessly. The small amount of radioactive material that is used in these detectors is not a health hazard and individual units can be disposed of in normal household waste.

Even swallowing the radioactive material from a smoke detector would not lead to significant internal absorption of Am-241. Americium dioxide is insoluble, so will pass through the digestive tract without delivering a significant radiation dose (Am-241 is, however, a potentially dangerous isotope if it is taken into the body in soluble form). It decays by both alpha activity and gamma emissions and it would concentrate in the skeleton).

Americium

The element americium (atomic number 95) was discovered in 1945 during the Manhattan Project in the USA. The first sample of americium was produced by bombarding plutonium with neutrons in a nuclear reactor at the University of Chicago.

Am-241, with a half-life of 432 years, was the first americium isotope to be isolated, and is the one used today in most domestic smoke detectors. Am-241 decays by emitting alpha particles and gamma radiation to become neptunium-237.

Americium dioxide, AmO₂, was first offered for sale by the US Atomic Energy Commission in 1962, and the price of US$1500 per gram has remained virtually unchanged since. One gram of AmO₂ provides enough active material for more than three million household smoke detectors.

However, with constraints on the supply of Pu-238, the European Space Agency is now planning to use Am-241 in radioisotope thermoelectric generators (RTGs) for space missions, and the National Nuclear Laboratory in the UK is gearing up to supply it in multi-kilogram quantities from aged civil plutonium, where it is a decay product of Pu-241.

In combination with beryllium, Am-241 is also used as a neutron source in non-destructive testing of machinery and equipment, and as a thickness gauge in industry. Beryllium produces neutrons as it captures the alpha particles from AmO₂. AmBe neutron sources have many applications including measuring soil moisture.

Formation of americium

Typical used fuel from a power reactor contains about 1% plutonium. Of this, about 10-14% is the plutonium-241 isotope, which has a half-life of 14 years, decaying to Am-241 through emission of beta particles. Hence old civil plutonium contains significant amounts of it. Am-241 has a half-life of 432 years, emitting alpha particles and gamma radiation to become neptunium-237.

Plutonium-241 is formed in any nuclear reactor by neutron capture from uranium-238. The detailed steps of the reaction are:

• U-238 + neutron => U-239
• U-239 by beta decay => Np-239
• Np-239 by beta decay =>Pu-239
• Pu-239 + neutron => Pu-240
• Pu-240 + neutron => Pu-241

Pu-241 will decay both in the reactor and subsequently to form Am-241. Note that military plutonium, which is produced to have little Pu-240, will have even less Pu-241 and cannot be used as a source of americium.

It is of interest (and some significance in recycling spent fuel) that if too much Am-241 builds up in plutonium separated from spent fuel, it cannot readily be used in the manufacture of mixed oxide (MOX) fuel because it is too radioactive for handling in a normal MOX fuel fabrication plant. For instance, the Sellafield MOX Plant in the UK could handle plutonium containing up to 3% Am-241, hence up to six years old (any more would need special shielding).

Watches & clocks

Tritium and promethium-147 release beta particles during decay, triggering a chemical reaction upon contact with other materials. This reaction creates a 'glow' as perceived by the human eye. These properties – known as radioluminescence – make the isotopes useful for providing 'light' in the absence of electricity.

These materials are used in clocks, watches, and gun sights so that they can be used at night, as well as in 'exit' signs in commercial buildings, aircraft and ships. Earlier watches and the dials of other instruments used radium for the same purpose.

Other: non-stick cookware, photocopiers, cosmetic sterilisation

• Non-stick pans are often treated with radiation to achieve cross-linking of the coating material (typically PTFE), ensuring adherence to the pan surface and increasing abrasion resistance.
• Some photocopiers use small amounts of radioactive polonium to prevent the build-up of static.
• Gamma radiation is widely used to sterilise consumer cosmetics, such as hair products and contact lens solutions, and to remove foreign bodies such as allergens.

Related information

The Many Uses of Nuclear Technology
Radioisotopes in Food & Agriculture
Radioisotopes in Industry
Radioisotopes in Medicine
Radioisotopes in Water Resources & the Environment
Research Reactors