Radon exhalation rates

In Table I results of Rn-226 activity measurements cn geological samples are shewn together with measurements cn Rn exhalation rates from the sanples. The exhalation rates varies considerably with the moisture content of material. The exhalation rate is lew for dry samples and when the moisture content increases, the exhalation rate increases until it reaches a plateau. When the moisture content increases further, a rapid increase in radon exhalation occur. When the saturation level of moisture is reached, the exhalation rate drops dramatically. The exhalation rates given in Table I are obtained by assuming that the most probable moisture content is whithin the plateau of exhalation rate/moisture curve. (Stranden et al, 1984, Stranden et al, 1984a). 

In order to simplify the situation, we assume that our porous sample under investigation covers the bottom of an open straight-walled can and fills it to a height d (Figure 1). Such a sample will exhibit the same areal exhalation rate as a free semi-infinite sample of thickness 2d, as long as the walls and the bottom of the can are impermeable and non-absorbant for radon. A one-dimensional analysis of the diffusion of radon from the sample is perfectly adequate under these conditions. To idealize the conditions a bit further we assume that diffusion is the only transport mechanism of radon out from the sample, and that this diffusive transport is governed by Fick s first law. Fick s law applied to a porous medium says that the areal exhalation rate is proportional to the (radon) concentration gradient in the pores at the sample-air interface... 

From Figure 3 we have that the radon concentration after 4 minutes is about 6.5 Bq m 3 in the outer volume of the can. Let us assume that the can has a horizontal cross section area of 0.2 m. The outer volume is then 20 litres and the concentration 6.5 Bq m" corresponds to a mean exhalation rate of 2.7 mBq m 2 s l, which is lower than the free exhalation rate by a factor 3.15/2.7= 1.17. 

Figure 3 The internal radon concentration of the sample in Figure 2 at time of closure (corresponding to free exhalation rate) and four minutes after closure of the can (theory).

If we choose a much larger than 1 (thin samples d<0.5L) or h pL (thick samples d>>L), the final steady-state exhalation deviates very little from the free exhalation rate and we do not need to know the reshaping time or use Equation 2 for corrections. An air grab sample taken at any time (and corrected for radioactive decay if necessary) after closure, will yield the free exhalation rate to a good approximation, provided that the can is perfectly radon-tight. 

Figure 6 displays exhalation rate curves versus time for the sample in Figure 2, with the leakage factory as the variable parameter. Large leaks make the final steady-state exhalation rate deviate less from the free exhalation rate, in accordance with Equation 4. It must be remembered, however, that the radon activity accumulating in the outer volume is dependent on y, the exhalation rate is only the strength of the radon source feeding this volume. 

Figure 5. The areal exhalation rate from the porous sample in Figure 2, enclosed in three different exhalation cans. Two of them ( a1 and 0 ) are completely radon-tight and the third Cb1) has a radon leak rate constant v, numerically equal to the radon decay rate constant (v=A= 2.1 10" s" ). The cans are closed at time zero. The radon exhalation evolution as a function of time is discussed in the text (theory).

Jonassen N., The Determination of Radon Exhalation Rates, Health Physics 45 369-376 (1983). 

Keller, G., Folkerts, K.H. Muth, H. (1982) Methods for the determination of222Rn (radon) and 220Rn (thoron) exhalation rates using alpha spectroscopy. Radiation Protection Dosimetry, 3, 83-9. 

Radon enters the atmosphere principally by crossing the soil-air interface. The transfer rate of radon across the interface between a solid phase and the atmosphere is referred to as a radon flux or exhalation rate. In SI units it is measured in Bqm s. The radon flux gives a measure of the source strength and varies strongly from soil to soil. A global average of 17mBqm s from continental soil has been estimated. 

The radionuclide Rn (Ty2 = 56 s) is an intermediate of the thorium decay series (from Th to Pb) this radionuclide (another isotope of the element radon besides its most common isotope Rn) has the special name of thoron. Due to the short half-life, the exhalation rate of Th from minerals to the atmosphere is small, and the concentration in air is about 10 times less than that of Rn, at equilibrium. Therefore, the dose contribution is negligible, except in the atmosphere where the upper soils are rich in thorium content. The actinium series (from to Pb) involves the intermediate of Rn (Ti/2 - 4 s). Due to the very short half-life of Rn, its exhalation from soil is very small, as is the dose contribution. 

Humans are exposed to natural radiation soil is a major source of external and internal exposure of radiation. The external exposure from the soil is associated with gamma radiation and internal exposure with radon inhalation. Exposures of radiation derived from soil are different in each region. The aim of this study is to evaluate radionuclides content, Rn exhalation rates, radium equivalent activity and hazard indices in soil samples around Institute de Pesquisas Energeticas e Nucleares (IPEN) facilities. 

The radon exhalation rate from soil was determined using a model proposed by UNSCEAR for the evaluation of the flux density of Rn at a surface of dry soil. The UNSCEAR model used is represented by the following equation ... 

All the results obtained for radon exhalation rate, radiiun equivalent activity and external and internal hazard indices indicate that the exposure around IPEN facilities due to the radioactive discharges is negligible. 

The radon emanation and the ventilation rate of a room can be derived from the increase of the radon concentration by the radon exhalation and from the steady state condition between exhalation and air exchange with the free atmosphere. In Fig. 2 the variation of the radon concentration as function of time is shown measured in two houses with different radon emanations and ventilation rates. 

A method has been developed to measure the rate of elimination of radon in exhaled breath (Stehney et al. 1955). Based on the assumption that 70% of the radon from fixed body radium is exhaled, this test can be used to calculate approximate levels of the body burden of radium. 

A grossly oversimplified calculation shows the first effect. If Xi> Xo are the concentrations of radon in a groundfloor room and outside, Q the rate of exhalation from the floor, assumed to be the same as from the ground outside, H the height of the room and Xv the ventilation rate, then neglecting exhalation from the walls... 

The actual release of radon from the pore space or soil-gas to ambient air is called exhalation. The rate of this process is a function of many variables including the concentration of radon in the soil-gas, the soil porosity, and meteorological factors such as precipitation and variations in atmospheric pressure (WHO 1983). 

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