The radon diffusion length in building materials: Measurement and impact on concentration indoors

Full Text:The radon (Rn222) gas, progeny of the natural uranium (238U) decay chain, is continuously produced in soil and in building materials derived from soil. Their porous structure allows radon penetration into the dwellings, from soil via the material or by emanation from the material itself. Radon progeny inhalation into the lungs leads to strong alpha particle damage of the tissue. The prolonged irradiation may result in lung cancer. The diffusion length is the essential parameter determining the radon exhalation from the building material. This work presents a simple technique for determining the radon diffusion length in different building materials. A mathematical model describes the process of radon diffusion in a hermetically closed chamber, separated in two sections by a material sample. The derived relations allow evaluating the radon diffusion length in the material from measurements of the radon concentrations in the two sections. The radon concentration is measured by either passive (electret and activated charcoal) standard detectors or by a continuous radon monitor. Experimental results are presented for the diffusion lengths in ordinary building materials (concrete and gypsum) and in materials used for insulating the building from the soil (polyethylene and bitumen foils). The measurement of the radon diffusion length in concrete and in the insulating material located underneath the basement slab allows optimizing the material thickness (about three times the diffusion length) to suppress radon penetration from the soil. Reliable measurements of the radon diffusion length in ordinary building materials (massive concrete and building blocks) allows estimating, already at design stage, the expected exhalation rate from the walls, the radon concentration in the air and finally the annual radiation dose to the population, as required by a recent Israeli standard