Optical detection of radon decay in air.

Radon gas is released in soil as a result of radioactive decay of uranium and thoron series. As a radioactive noble gas, radon emanates easily through porous ground to housings and is responsible for 42% of the annual radiation dose of population in the world1. It is widely observed that exposure to radon leads to increased risk of lung cancer2,3. Radon and some of the daughter atoms decay by emitting alpha particles which have short range in air but high damage potential if absorbed in living cells. Radon progenies are easily adhered to surfaces and therefore, the upper respiratory tract is exposed to the highest radiation dose. Due to its carcinogenic nature, radon monitoring is required in risk areas. Radon levels are typically measured by leaving a piece of special film in a room for a fixed period of time, and the number of alpha particles incident on the film is later counted in a laboratory analysis. This approach provides a reliable and low-cost estimate of the average radon level in the premises but it is not suited to online monitoring applications. In contrast, a fast response is required in the fields of mining industry, uranium exploration, and in verification of radon repairs. Continuous radon monitoring can also be used as a warning system for earthquakes which are known to increase radon levels shortly prior to the event4,5. Currently, detectors employing ionisation chambers, semiconductor sensors, or zinc-sulphide scintillation (Lucas) cells are often used for these applications3. The absorption of alpha particles in air induces secondary radioluminescence light which can be utilized for remote detection of alpha decay6. The light is generated by radiative relaxation of nitrogen molecules, excited by secondary electrons. The conversion efficiency from kinetic energy into optical radiation is 19 photons per each MeV of energy released in air7. This corresponds to approximately 100 photons when a single 222Rn nucleus releases all of the 5.6 MeV decay energy into air. Most of the photons are observed in the near UV region between 300 nm and 400 nm8. The increased range and multiplication of signal carriers are the key benefits of an optical alpha particle detection method. This work presents the principle and first results of an optical radon measurement. The feasibility of the technique is proven using a demonstration device which is applied to a step-response test and to a longer field test to observe daily variation of radon concentration at an office property. Furthermore, the optical detector is calibrated against an established commercial detector. The technique enables direct radon detection with exceptionally large active volume and high efficiency.