Radon Dose Determination for Cave Guides inCzech Republic

摘要

According to recommended approach there are six (from total of twelve) open-to-public caves in Czech Republic, reaching near to an effective lung-dose of 6mSv/year. Aconservative approach for estimating the potential effective lung-dose in caves (or underground)is based on two season's measurements, using solid state alpha track detector (Kodak in plasticdiffusion chamber). The obtained dataset is converted into an annual effective dose, inagreement with the ICRP65 recommendation, using the "cave factor" 1.5. The value of "cavefactor" which depends on the spectrum of aerosol particles, or on the proportional representationof the unattached/attached ratio (6.5 : 93.5 for residential places, 13.6 : 86.4 for caves due tolower concentration of free aerosols) and on the equilibrium factor. Thus conversion factor is 1.5times higher in comparison with ICRP 65. Is this correct? Because a more precisely determineddose value would have a significant impact on radon remedies, or on restricting the time workersstay underground, a series of measurement was initiated in 2003 with the aim to specify inputdata, computation and errors in effective dose assessment in each one of the evaluated cavesseparately. The enhancement of personal dosimetry for underground work places includes astudy of the given questions, from the following points of view in each cave: continual radonmeasurement; regular measurements of radon and its daughters to estimate the equilibrium factorand the presence of free ~(218)Po;regular indoor air flow measurements to study the location of theradon supply and its transfer among individual areas of the cave; natural radioactive elementcontent evaluation in subsoil and in water inside/outside, a study of the radon sources in thecave; determination of the free fraction from continual unattached and attached fractionmeasurement (grid and filter): thoron measurement. Air flow measurements provide veryinteresting information about the origin of "radon pockets" with very high radon concentration,and enable study of the location of the radon supply and its transfer among individual areas ofthe cave. Most of the results show the equilibrium factor around F=0.2-0.7 and the unattachedfraction around 2%-30%. One of the most important question remains: how accurately was theunattached fraction measured? Part of this project was to verify the influence of etched trackdetector position in the cave.