IAEA SAFETY GUIDES - gnssn - International Atomic Energy Agency

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This publication is no longer validPlease see http://www-ns.iaea.org/standards/The tube is preferably a metallic cylinder with a high efficiency glass fibrefilter in a holder at each end and a connection from the exit filter to an air pump.A convenient tube size is 50 cm long with a diameter of 50 mm. The diameter ofthe filters should be the same as that of the tube. The exit filter holder in particularshould be of a type that disassembles easily so that the filter may be removed andcounted promptly after sampling.The pump should have a capacity of 5-10 L -m in -1 against the resistance ofthe two filters of diameter 50 mm in series, and should be equipped with a flow meterof suitable range and a vacuum gauge to indicate the pressure drop across the firstfilter. For use in mines, it should be battery powered.The alpha scintillation counter-scaler may be of conventional design; for usein mines it should be battery powered and capable of operation in adverse environmentalconditions. If the alpha counters available accept only filters of smallerdiameter than that specified (50 mm), an exit filter smaller in diameter than thetwo filter tube can be used without introducing any significant error into themeasurement.ProcedureA five minute sample is collected through the two filter tube at a flow rate of5 or 10 L • min ~~1. Promptly after sampling, the exit filter is removed and placed inan alpha counter. A five minute count is begun one minute after the termination ofthe sampling. The radon concentration is calculated from the following equation:'- R n12JXpEZVwhere:Cr„XEZVFis the radon concentration in B q -L ~ ';is the number of alpha counts;is the counting efficiency;is a factor that is a function of the sampling and counting times:Z = 1.672 for a 5 min sample and a 5 min count (see Tables Vand VI);is the tube volume in litres;is the fraction of RaA atoms formed in the tube that deposits on theexit filter (see Table VII).Longer sampling and/or counting periods may be used to improve the sensitivityof the method. Appropriate values of Z may be found in Tables V and VI.The inlet filter may be used for measuring the radon daughter concentrationsif desired. Otherwise, it need be replaced only occasionally when the dust loadingbecomes excessive.2 8
This publication is no longer validPlease see http://www-ns.iaea.org/standards/TABLE V. VALUES OF Z FOR USE IN THE TWO FILTER METHOD FORMEASURING RADONt T, t 2 Z5 1 6 1.6725 1 15 2.5975 1 30 3.4115 1 100 6.31410 1 6 2.31210 1 15 3.80310 1 30 5.42510 1 100 11.06815 1 6 2.65615 1 15 4.63415 1 30 7.07015 1 100 15.281Note: Z is the decay correction factor (min);t is the sampling time (min);T, is the start of the counting period (min);T2 is the end of the counting period (min) (T = 0 at the end of the sampling period).LimitationsFor the tube size discussed earlier, the sensitivities achievable by this methodare not better than those achieved with scintillation cells. The reproducibility of thismethod is rather poor (results could differ from day to day by as much as 25%,although the precision is very high), probably because the flow and plateout characteristicschange. For example, if the pressure drop across the first filter changes bya factor of two (as a result of using a different filter, for instance), the count changesby 25% [11]. The presence of trace gases (N 02) and humidity affect the diffusioncoefficient, resulting in counts that differ even more [34],Leaks on the first filter, indicated by an insufficient pressure drop across thefilter, can cause significant errors by allowing outside radon daughters to deposit onthe exit filter. The exit filter is vulnerable to contamination by contact with almostany surface or even with mine air. Membrane filters are especially prone to contaminationwith mine air because of inherent electrostatic forces, and they should thereforenot be used. To prevent contamination, filters should be sealed until used and29
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IAEA SAFETY GUIDES - gnssn - International Atomic Energy Agency

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