Radon-222, a radioactive noble gas isotope, the daughter product of Radium-226 in the naturally occurring Uranium-238 Series has become a centre of attention in radiation protection. Similarly, another isotope of radon, i.e., Radon-220 (called thoron) is the daughter product of Radium-224 in Thorium-232 series. The radon isotopes, Rn-222 and Rn-220 are alpha emitters with half lives of 3.82 days and 55.6 seconds respectively. The decay products of the two isotopes are also short-lived, but they are particulate radionuclides of elements polonium, lead, bismuth and thallium. Official regulatory requirements in some countries call for the measurement of radon indoors before clearance of dwellings for human occupation.
Becquerel (Bq) is the unit used to express the quantity of radioactive material disintegrating at a rate of 1disintegration per second). The radon daughter concentration can also be expressed in terms of working level (WL) units. One WL corresponds to an equilibrium concentration of 3700 Bq / cubic meter of air. Sievert (Sv) is the unit used to express the dose (energy absorbed per kg - effective dose) received due to the exposure to radiation.
Being gaseous, the radon isotopes generated in the underground rocks (about 4 ppm-parts per million), in soil and from the building materials diffuses out and gets mixed up with the air in the environment. The radon gas also comes out from the cracks in the rocks and in hot spring water. Healing property of spring waters is attributed to the presence of radon. Once out in the open, the outside air provides enough dilution. Typical average concentration of radon outdoors is 10 Bq per cubic meter. It varies from place to place. Seasonal variations are seen. In the uranium mines, the concentration can be much higher and it is a potential occupational hazard for the mine workers. However, the radon concentration is much higher indoors due to limited dilution provided for the radon diffusing inside the dwellings from the building materials and the underlying soil (particularly in basement areas). The radon concentration varies with the building material used, the type of the house, quality of construction and ventilation provided. Thus, depending upon the time spent indoors, the exposure of the population can be significant, particularly in cold countries where the ventilation rate provided is low.
On inhalation of the contaminated air, the particulate activity gets deposited in the lung and gives dose to the lung tissues. As compared to the radiation dose resulting from inhalation of unit concentration of the particulate radon daughters, the dose from inhalation of unit concentration of the inert gas is about 50 times lower. Lung cancer is the radon induced malignancy known amongst the mine workers.
Major contribution (70 to 80%) to the average dose of 2.4 milliSv per year to the public from natural sources comes from these radon isotopes and their short-lived daughter products. In some of the high background areas (HBRA), the radon concentration in dwellings may be much higher, which may call for intervention measures, such as relocation to reduce the human exposure.
The International Commission on Radiological Protection (ICRP) recommends Action Levels for regulatory intervention of 200-600 Bq per cubic meter of air with an indoor occupancy of 7000 hours in a year and an equilibrium factor of 0.4 (concentration of radon daughters divided by the concentration of radon gas). This concentration corresponds to an effective dose of 3-10 mSv.
There are various radon measurement techniques / portable monitors are available commercially for the measurement of radon accurately for the estimation of radiation dose and for regulatory compliance. Portable Alpha spectrometry based detection systems, electrets, direct scintillation based radon gas counting systems are employed for the measurement of radon in water, air and for radon exhalation measurements from soil and construction surfaces. Calibration of the systems is one of the important requirements for validation of the system operation and reliability of the data.
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