and/or the radium content of water samples can be determined from two ... concerns about our natural radiation environment, the interest in radon and radium ...
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PROCEEDINGS
XA05C0074
MEASUREMENT OF DISSOLVED RADON AND RADIUM CONTENT OF WATER SAMPLES BY TRACK ETCH TECHNIQUE J. Hakl ", I. Hunyadi ]) , I. Csige", A. Vásárhelyi °, J. Somlai 2) , G. Faludi 2) , 1
2
Institute of Nuclear Research of the Hungarian Academy of Sciences, H-4001 Debrecen, POB 51, Hungary Department of Radiochemistry, University of Veszprém, H-8200 Veszprém, Egyetem út 10, Hungary
ABSTRACT We have developed a method to determine the dissolved radon and radium content of water samples using track etch type radon monitors. The device is an immersed, small volume radon monitor with CR-39 track etch detector. The monitor is sealed from the water by a thin radon permeable rubber foil. The dissolved radon and/or the radium content of water samples can be determinedfrom two independent radon exposures by using the solutions of a non steady state differential equation which describes the temporal variation of radon content of water and by using an experimantally determined calibration coefficient. KEYWORDS Radon, radium, water, CR-39 track etch detector.
INTRODUCTION During the last decades several types of track etch detector based radon monitors were developed and utilized in large scale radon measuring programmes. In these 'track etch linked' environmental studies the radon concentration was examined mostly in gases (in air or soil gas). On the other hand, with increasing concerns about our natural radiation environment, the interest in radon and radium concentrations of waters is also rising. Numerous methods based on different procedures to separate radon and radium from water have been developed to fulfil this task. Direct application of a bare track etch detector for this purpose is rarely considered, because of the sensitivity decrease by three orders of magnitude of these detectors in water. In field circumstances, however, the application of procedures based on chemical or other separation methods is often difficult or expensive when large number of samples has to be measured. Therefore, we have developed a method using sealed track etch radon monitors to measure radon and radium content of water samples. GENERAL CONSIDERATIONS Let us consider an air bubble in water. The radon concentration of the air inside the bubble (CA) is governed by the following differential equation: dC
*
-
•- c
-c
I
m
where X is the decay constant of radon, y is a factor characteristic for the time necassary for radon to cross the water-air interface, T| is the partition coefficient of radon between water and air phases and C w is the radon concentration of water at the water-air interface. The solution of eq. (1) depends on C w , which is, in general a function of time. The radon concentration inside the bubble also depends on the time necessary for a radon atom to pass the water-air interface («1/y). For constant C w and CA(0) = 0,
(2) y
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which shows the effect of both factors. As the time necessary for passing through the interface is usually short (eg. \ly«\IX) (Nazaroff 1988), the value of CA will be equal to Cw/r). However, because of diffusion a concentration gradient of radon around the bubble in the water is formed, so there may be considerable difference between the Cwand the mean radon concentration of water. This phenomenon may significantly affect the sensitivity of the method (track density rate / mean radon concentration in water). For spherical symmetry the delay (T) attributed to this effect is (Boeker 1994): T =
(3)
,
W AD where R is the radius of the bubble and D is the diffusion constant of radon in water. Substituting value for the diffusion coefficient (10"9mV, Nazaroff 1988), the obtained delay time is of the order of few days (depending on the bubble radius). Taking into account this effect the radon concentration in water at the water-air interface can formally be written as:
Cw a
w
j~m
—
Cw
'
