Tomsk Polytechnic University

Tomsk Polytechnic UniversitySARAD GmbH

Dr. Valentina S. Yakovleva, Prof., Dr. Thomas Streil

Features of simultaneous measurements of radon and thoron

flux density from the surface of porous materials

The work was fulfilled with financial support of RF Analytical departmental target program “Development of higher school scientific potential” № 2.1.1/544.

Application of THORON FLUX DENSITY (TFD) value

1. Radioecology – assessment of doses and risks of exposure.

2. Atmosphere physics/Climatology/Nuclear Meteorology –

1) to study air mass transport and to validate the climate models;

2) to study ion generation rate function in atmosphere and atmosphere electricity dynamics.

FEATURES OF SIMULTANEOUS MEASUREMENTSOF RADON AND THORON FLUX DENSITY

Means of Tn separation in RFD measurementsAll means for Tn separation are based on

short lifetime of thoron (T1/2 = 55,6 s) and include the following technical and theoretical means:

1. Time delay (Chalupnik and Wysocka, 2003);

2. Delay volume (Szegvary, 2007);

3. Diffusion filters (electret and track methods) (Kotrappa et al., 1988, Sorimachi et al., 2009);

4. Linearity of radon activity accumulation function (slope angle)is usually used in ionization methods with Alpha-GUARD (Mazur and Kozak, 2009).


Means of thoron separation in RFD measurements

Time delay

Thoron separation from radon-thoron mixture using time-delay method is mainly applied in RFD measurement methods using scintillation alpha-radiometry.

After the transport of air mixture out of accumulation chamber through a filter, detaching aerosol decay products of radon and thoron, into the measurement chamber of radiometer (or Lucas cell), one performs a delay until thoron is fully decayed.

AChAccumulation time ~ 2,5 h

Lucas cell

Delay time ~ 3 h

Aerosol filter

Delay volume

This method consists in the use of accumulation chamber joined with thoron delay volume and with radon radiometer (for instance, Alpha GUARD).

Air-gas mixture coming out from the soil surface into accumulation chamber was pumped trough delay volume (1,5 L) with speed of 0,5 L/min for thoron decay. Then the mixture got inside radiometer through a filter, detaching radon and thoron decay products. Radon activity accumulated inside the accumulation chamber, which is proportional to RFD was measured by radiometer.

AChRn (Tn)

Radiometer

Aerosol filter

Tn delay volume Pump


Diffusion filter

A chamber base is shut by a filter. Thoron, coming out from the soil surface is decayed during its diffusion through a filter.

According to detailed research, carried out in work [11] with electret chambers E-PERM, diffusion filter infiltrates 99% of radon and only 2,5% of thoron.


ACh Diffusion filter

SSNTDfan

Linearity of radon activity accumulation function(Slope angle)

Numerous theoretical and experimental studies showed that radon activity inside accumulation chamber increases linearly. Therefore to measure RFD one uses linear section of accumulation function measured by radiometer, and exactly slope angle which is proportional to the flux density. Use of slope angle of linear section allows to separate thoron contribution into measurement result. RFD, Bq m-2 s-1, can be calculated by relation

SVq

where α – coefficient is determined from equation A(t)= αt,A(t)– radon volumetric activity accumulated inside the ACh in time t;S – square of ACh bottom; V – volume of ACh.


Means for simultaneous measurementsof Rn and Tn flux densities

For discrimination of Rn from Tn: 1. alpha- and gamma-spectrometry and appropriate algorithms of spectra

processing;

2. measurement of sum device signal before and after thoron separation, in combination with different means of thoron separation (Zahorowski and Whittlestone, 1996; Whittlestone et al., 1998; Schery et al, 1989; Lehmann, 2000, 2004);

3. analysis of futures of radon isotope and their decay products accumulation inside accumulation chamber (Saegusa, 1996; Hosoda et al, 2007;

Yakovleva and Vukolov, 2010; Ujic et al., 2008).

The second and third groups of means were mainly designed for devices operating in counting regime (radiometry).


The second groupis based on use of one accumulation chamber and two measuring devices (radiometers), and also, in some cases, Tn delay volume.

By the reading difference of the first (radon+thoron) and second (only radon) devices one determines the number of alpha-radiation impulses, caused by thoron, which is converted into volumetric activity~TFD.Examples:with Lucas cells - Zahorowski and Whittlestone, 1996; Whittlestone et al., 1998; Schery et al, 1989with AlphaGUARDs – Lehmann, 2000, 2004


ACh(only Rn)

α-radiometer

Tn delay volume

if needed

Pump

(Rn+Tn)

α-radiometer

In other variants of the second group of means

of RFD and TFD simultaneous measurement one can use two passive radiometers with SSNTDs or electret detectors placed

inside accumulation chamber, at that the entrance window of one of the detectors is shut by a diffusion thoron arresting filter.


AChDiffusion

filter

SSNTDfan

SSNTD

3. The third group of means

is based on features analysis of accumulation of radon, thoron and their decay products inside accumulation chamber and the changes of ionizing radiation yield having their own laws.

Variant 3.1. consists in separation of radon and thoron by the growth curve form of alpha-radiation impulses counting rate, measured inside exposed accumulation chamber.

Within this way one uses a measuring device (alpha-detector), operating in counting regime and accumulation chamber.

A simple solution of the problem was found after analysis of the growth curve.


One can see, that a growth curve of yield of thoron and its decay products alpha-particles has a specific form and is determined by physical properties of radionuclides, and exactly by their half-lives relation.

In 6-7 min after the beginning of accumulation the curve (α Tn) reaches saturation (equilibrium state) and further it doesn’t practically change.

The yield growth of alpha-particles from radon and its decay products (α Rn) has a linear form.

0 10 20 30 40 50 60 70 80 90 100 110 1200

2

4

6

8

10

12

14α Rn+Tnα Tnα Rn

Accumulation time, min.

Yiel

d of

α-p

artic

les,

1/s


Variant 3.2

The other means is based on the analysis of radon and thoron dynamics inside accumulation chamber, when measurement of accumulated activity is carried out by track detectors.Here it is taken into account that length of thoron diffusion in the air is small (2,9 cm).

If air mixing absent - it is impossible to obtain homogeneous thoron concentration C(z) inside accumulation chamber.Hence, dynamics equation, used for radon inside accumulation chamber is not applicable for thoron. For thoron one should use the following equation (stationary case when advection is absent)

where z is a distance from the surface of porous sample. Here SSNTDs are placed inside ACh at different height from a sample surface, so

that upper detector can measure only radon, and a lower one – radon and thoron.


Upper detector - determines RFD.Lower detector - determines TFD using next expression, obtained at solution of thoron transport equation.

This method is rather laborious and has a number of limitations for the thickness of a test specimen layer, heights of detectors installation, etc., that makes it unsuitable for routine measurements.


Fig. from Ujic et al., 2008

Within the method designed by Sisigina, 1970,the RFD measurement was carried out without storage for thoron decay.

Here the activity of radon coming out of the soil surface was measured by scintillation alpha-radiometer at the initial moment of accumulation and after the accumulation time (15-60 min). In this case, the overall uncertainty of measurement result can exceed 100%.

Neglect the thoron influence on the resultof RFD measurement - RADIOMETRY

ACh

Radiometer with α-scintillation detector

ACh

Even with equal specific activity of 226Ra and 232Th in the soil, for example 25 Bq/kg, and with only diffusion transport of soil gases, the number of alpha-particles formed in the result of decay of thoron and its decay products is greater than in the case of decay of radon and its decay products during the first 60 minutes of counting.


Thoron contribution to RFD measurement result is also neglected in methods using activated carbon, where accumulated radon activity is measured by radiometric method. For instance, in measurement systems “Kamera” (Russia) the activity of radon accumulated on activated carbon is measured by beta- (and sometimes gamma-) radiometer. At that, beta- (gamma-) radiation of thoron decay products is also detected.

The method, using containers PICO-RAD with activated carbon and alpha-/beta-counting system based on liquid scintillator Tri-Carb 3100TR, also neglects thoron contribution (Iimoto et al., 2008).


Contribution of Tn into the total signal:alpha-radiating 212Bi, 212Po and beta-radiating 212Pb, 212Bi and 208Tl

Accumulation in air and alpha counting only:

Specific activity rate 226Ra/232Th =1 --------Tn contribution to ∑α ≈ 0.1%Specific activity rate 226Ra/232Th =0.1 ------Tn contribution to ∑α ≈ 1%

Accumulation in air and beta counting only:

Specific activity rate 226Ra/232Th =1 --------Tn contribution to ∑β ≈ 0.24%Specific activity rate 226Ra/232Th =0.1 ------Tn contribution to ∑β ≈ 2.4%

Accumulation in activated charcoal and alpha+beta counting:

Specific activity rate 226Ra/232Th =1 --------Tn contribution to ∑α ≈ 3% --------Tn contribution to ∑β ≈ 9%


ACh Scintillator

Contribution of Tn into the total signal:


End of accumulation

Start of measurement

222Rn, 218Po, 214Pb, 214Bi and 214Po 220Rn, 216Po, 212Pb, 212Bi, 212Po and 208Tl

Bq/m3 Bq/m3

Conclusion Analysis of measuring methods of radon and thoron flux densities from the surface of porous materials allowed to suggest classification of means:1. of thoron separation in radon flux density measurements;2. of simultaneous measurement of radon and thoron flux densities.

Measuring methods of radon flux density where the thoron influence on the measurement result is neglected were detected.

The above mentioned examples of neglecting thoron influence on RFD measurement results, lead to warning – a critical approach to comparing and analysis of literary data, obtained by different methods.


Thank you very much for your attention!

Tomsk, Siberia, Russian Federation

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