The murder in 2006 of Alexander Litvinenko, a former KGB agent living in London, showed the importance of being able to detect quickly if an area has been contaminated by nuclear material. Mr Litvinenko died from radiation poisoning after, it would seem, being slipped a potion containing an isotope of polonium. Almost 50 buildings, including bars and restaurants, that he had visited before being admitted to hospital had to be checked for radiation. The operation tied up teams of experts for months and cost some £2m ($3.7m at the time). One reason the check was so difficult is that the radiation emitted by the isotope in question, 210Po, comes in the form of alpha particles. These are damaging at short range (for example, if the radioactive material has been ingested, as happened to Mr Litvinenko) but even in air that range is only about 4cm, so they cannot be detected at a distance. A new type of portable radiation detector that has been developed by Britain’s National Physical Laboratory (NPL) should, however, speed up the process of screening places that may have been contaminated with alpha-emitters such as 210Po. The most effective way of looking for alpha particles is with a scintillation counter. A sensitive material inside this device releases a photon of light when it is hit by an alpha particle (or any other form of ionising radiation), and that photon is then detected and counted. Scintillation counters, though, require trained operators. On top of this, checking all the surfaces in a room at a range of less than 4cm is time-consuming and tedious. The machine that the NPL has developed gets round that by using the air itself as the sensitive material. If an alpha particle (which is actually the nucleus of a helium atom, consisting of two protons and two neutrons bound together) hits a molecule of nitrogen, the collision produces a photon of ultraviolet light of a characteristic frequency. Such light can travel much farther than the original particle, and can also pass through transparent materials such as plastic and glass. Since air is almost 80% nitrogen, the chance of such a collision is high. All you have to do is detect the resulting photon. That is not, in itself, a new idea. But previous attempts could work only in complete darkness, making them impractical for general use. The NPL’s researchers have come up with a way of filtering out other wavelengths of light with a technique that they are, for now, keeping up their sleeve. So far, they claim, this technique allows the new detector to be used in a room illuminated by sodium lighting at a range of 20cm from the source. With more development it will, they hope, also be operable in daylight and at greater range. Source: The Economist