![]() A HEPA (high efficiency particulate) filter was placed between the pump and the sphere to remove radon progenies and dust from the inlet air. It has an estimated reflectance of 97% for the nitrogen emission.Ī pump was used to continuously circulate air through the sphere with a measured flow rate of 1.4 l/min. The sphere selected for this work (SPH-8-3 AdaptaSphere, Labsphere) has a diameter of 20 cm and it is coated with Spectraflect, which is a BaSO 4 -based diffusive reflector. For these reasons, an integrating sphere was used in the measurement setup. The ideal shape of the volume is a sphere since it has the greatest volume-to-surface-area ratio, which minimizes absorption of alpha particles into walls. This enhances detection probability of a single photon by allowing multiple reflections before absorption. Since the photons are generated along the alpha particle track and emitted isotropically, it is beneficial to a have a measurement volume with highly reflective walls. The optical radon measurement is based on simultaneous detection of multiple secondary photons from the same decay event. The technique enables direct radon detection with exceptionally large active volume and high efficiency. Furthermore, the optical detector is calibrated against an established commercial detector. ![]() 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. This work presents the principle and first results of an optical radon measurement. The increased range and multiplication of signal carriers are the key benefits of an optical alpha particle detection method. ![]() Most of the photons are observed in the near UV region between 300 nm and 400 nm 8. ![]() This corresponds to approximately 100 photons when a single 222Rn nucleus releases all of the 5.6 MeV decay energy into air. The conversion efficiency from kinetic energy into optical radiation is 19 photons per each MeV of energy released in air 7. The light is generated by radiative relaxation of nitrogen molecules, excited by secondary electrons. The absorption of alpha particles in air induces secondary radioluminescence light which can be utilized for remote detection of alpha decay 6. Currently, detectors employing ionisation chambers, semiconductor sensors, or zinc-sulphide scintillation (Lucas) cells are often used for these applications 3. Continuous radon monitoring can also be used as a warning system for earthquakes which are known to increase radon levels shortly prior to the event 4, 5. In contrast, a fast response is required in the fields of mining industry, uranium exploration, and in verification of radon repairs. 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. 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. Due to its carcinogenic nature, radon monitoring is required in risk areas. Radon progenies are easily adhered to surfaces and therefore, the upper respiratory tract is exposed to the highest radiation dose. 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. It is widely observed that exposure to radon leads to increased risk of lung cancer 2, 3. 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 world 1. Radon gas is released in soil as a result of radioactive decay of uranium and thoron series.
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