RADIOLOGICAL SURVEY OF HUNGARIAN CLAYS; RADON EMANATION AND EXHALATION INFLUENTIAL EFFECT

OF SAMPLE AND INTERNAL STRUCTURE CONDITIONS*

Z. SAS1, J. SOMLAI1, J. JÓNÁS1, G. SZEILER1, T. KOVÁCS1, CS. GYÖNGYÖSI1, T. SYDÓ2 1 University of Pannonia, Institute of Radiochemistry and Radioecology, H-8201 Veszprém, POB. 158. Hungary, E-mail: ilozas@almos.vein.hu, somlai@almos.vein.hu, jacint.jonas@gmail.com,

szega@almos.vein.hu, kt@almos.vein.hu csaba.gyongyosi@rhk.hu 2 Social Organization for Radioecological Cleanliness, H-8200 Veszprém, Egyetem stu. 10, Hungary,

Email: sydo@infornax.hu

Received November 15, 2012

In case of indoor conditions the probability of the radon accumulation is high, greatly depends on the features of the surrounding materials. In order to prevent the elevated indoor radon levels in buildings the influencing parameters of the migration and origin of the radon – generated in the building materials – should be investigated beyond the Ra-226 content. The natural radioactive content of 27 clay samples, which are used as raw material in brick factories were investigated in radiological point of view. The radon emanation factors were determined as well. The optimal sample thickness of the powdered and wet clay samples were determined in order to measure the free exhalation. The effect of the moisture content modifying the radon emanation was determined as well. The obtained results clearly prove that the measurement of emanation in dry state is not enough to characterize the radon emanation features and via that the radon exhalation capacity of the material.

Key words: Radon emanation, clay, sample condition.

1. INTRODUCTION

Owing to the modified habits of the mankind the spent time in indoor conditions on a par attains the 80 %. This is the reason why the composition of the surrounding materials gets into the focal point of interest.

The radon is a radioactive noble gas with relatively long half-life (3.82 d) and this time can be enough to get out of the matrix into the pore space and into the air as well. While the alpha particle ejected as a result of the alpha decay the daughter element is recoiled and it can be released into the pore space or it can be embedded in adjacent particles [1, 2].

* Paper presented at the First East European Radon Symposium – FERAS 2012, September 2–5, 2012, Cluj-Napoca, Romania.

Rom. Journ. Phys., Vol. 58, Supplement, P. S243–S250, Bucharest, 2013

Z. Sas et al. 2 S244

Only the emanated radon has a chance to leave the matrix. This is the reason why that parameter has to be determined in case of building materials.

So many factors determine the amount of the emanated radon such as, variation of the radium concentration in particles, density, grain size, volume of pore space, and last but not least the moisture content.

The moisture content of the building materials owing to the conditions of the surrounding area is not constant. These are the reasons why the radon emanation characteristic of the building materials should be determined in function of the moisture content [3-5], since the elevated radon emanation coefficient can cause higher radon level.

Due to the diffusion and convection the radon can be exhaled from the pore space into the air. The radon exhalation greatly depends on the features of the investigated material, such as the thickness, the internal structure and the moisture content of the solid phase as well.

Due to the listed facts the radon exhalation dependency should be investigated in function of the sample thickness and the moisture content as well in order to obtain comprehensive view from the main exhalation influential factors.

If the samples has extremely lower thickness against the diffusion length that case the free exhalation or mass standardized specific exhalation can be determined. Due to that fact the maximal radon exhaling capacity of the investigated sample can be obtained in function of the mass, which is independent of the sample features [6-10].

In this study the radon emanation and exhalation modifying effect were investigated in function of sample thickness (powdered and wet clay) and the moisture content. Beside them several clay samples were measured in order to get comprehensive radiological classification of Hungarian clays (used in brick factories). Altogether 27 different clay raw materials were investigated. The radionuclide content was determined by gamma spectrometry and the I-indexes were calculated.

2. MATERIALS AND METHODS

2.1. SAMPLING AND SAMPLE PREPARATION

The clay samples that used in brick factories as building material were taken from different clay mines in Hungary (Figure 1).

2.2. DETERMINATION OF Ra-226, Th-232, K-40 ACTIVITY CONCENTRATION BY GAMMA-SPECTROMETRY

The samples were dried to constant mass at the temperature of 105 ± 3 °C. After drying the samples were ground, sieved under 0.63 mm and stored for 30 d in

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air-tight aluminium Marinelli beakers with volume of 600 cm3 to reach the secular equilibrium between the Ra-226 and the Rn-222. The determination of the Ra-226 activity concentration was carried out via the radon progenies Pb-214 (295 keV) and Bi-214 (609 keV) by high resolution gamma ray spectrometry, using an ORTEC GMX40-76 HPGe detector with efficiency of 40 %, and an energy resolution of 1.95 keV at 1332.5 keV. The activity of K-40 was measured by the 1461 keV gamma ray, Th-232 by the 911 keV gamma ray of Ac-228, and the 2614 keV gamma ray of Tl-208 [8].

The data and spectra recorded by a Tennelec PCA-MR 8196 MCA.

Fig. 1 – Provenience of the clay samples.

2.3. DETERMINATION OF THE EXHALATION RATE

The clay samples were enclosed in an accumulation chamber. Radon free N2 gas was used to ensure the initial radon free atmosphere. During sampling the chamber was connected in a closed loop system wherein the radon enriched air content was flowed by a radon proof pump into a Lucas-cell. The progenies and the solid grains in the airflow were filtered and the moisture content was removed by CaCl2 desiccant. Then the cells were stored for 3 hours to ensure the secular equilibrium between the Rn-222 and its short live progenies.

The sampled radon was measured with the help of EMI photomultiplier, and NP420P single channel analyzer system was used for 1000 s measurement time three times in case of every cells. The Lucas cells were calibrated with a PYLON RN 2000A passive radon source in a volume of 210.5 dm3 Genitron EV 03209 type calibration chamber.

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The accumulation time ranged between 24-48 h. The radon exhalation was calculated in function of the elapsed time with equitation 1.

0 1 eff

efft

C VE

e −λ ⋅

⋅ λ ⋅=

− (1)

whereas: E0 is the radon exhalation, C is the measured activity concentration, λeff is the effective decay constant, V is the volume (accumulation chamber, Lucas cell, and the remainder part of the sampling system). The radon emanation can be obtained via the free exhalation, since the secular equilibrium radon concentration can be calculated. Due to that fact relatively short time can be enough to determine the emanation factor.

2.4. DETERMINATION OF THE INHIBITION EFFECT OF THE SAMPLE THICKNESS

In the case of the powdered clay the free exhalation was measured gradually. The height of the sample was increased from 1 to 15 cm in a plastic cylindrical sample holder. The powdered samples were massed before enclosure to avoid the incidental inhomogeneity in the sample holder (Figure 2).

2.5. EFFECT OF THE MOISTURE CONTENT

The radon emanation modifying effect of the moisture content was observed via cylindrical clay sticks with 0.4 cm diameter and 5 cm (Figure 2). Previously carried out exhalation measurements proved that less than 1.0 cm thickness the diffusion inhibition effect of the wet clay sample is negligent and the free exhalation can be measured. The initial moisture content of the samples was 25 % which was reduced gradually to zero.

Fig. 2 – Examined geometries.

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3. RESULTS AND DISCUSSION

3.1. DIFFUSION INHIBITION EFFECT OF THE SAMPLE THICKNESS

The results of the exhalation measurement in case of the dry, powdered and massed clay sample can be seen in in Figure 3. The obtained exhalation results were in direct proportionality with the amount of the sample. Reduced specific exhalation was not observed up to 15 cm thickness.

Due to the porous matrix the diffusion inhibiting effect is negligible and that case the total amount of the emanated radon has a chance to exhale without loss.

Fig. 3 – Free and surface exhalation dependency of powdered clay in function of thickness.

The exhalation characteristic in function of the diameter of the wet spherical clay samples can be seen in the Figure 4.

The exhalation rate was decreased in function of the diameter. Significant diffusion inhibition effect appeared above 1 cm diameter. In order to investigate the free exhalation the required sample thickness should be thinner than 1 cm to avoid the inhibition effect of the sample thickness.

By reason of the results it can be clearly stated that the free exhalation can be determined in the case of wet samples as well. Although the diffusion coefficient greatly depends on the internal water content of the materials with the adaption of the applied method the applicable range of the sample thickness where the inhibition effect is negligent can be determined easily.

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Fig. 4 – The exhalation dependency of wet clay in function of sample thickness.

3.2. EFFECT OF THE MOISTURE CONTENT

The radon emanation dependency in function of the moisture content was determined as a result of the gradually drying. The obtained results are illustrated in Figure 5. Owing to the obtained results the moisture content dependency can be clearly observed. Between 0–5 % moisture content the initial radon exhalation duplicated as a reason of the recoil energy absorbing effect of the internal moisture content. Above the 5 % a quasi linear increasing tendency can be observed up to 25 % where the emanation factor reached 45 ± 0.9 %. The emanation factor increment was approximately three times higher than the initial value (17 ± 0.5 %). Above 25 % moisture the extrusion of the clay sticks was not possible.

Fig. 5 – Emanation coefficient and free exhalation dependency in function of moisture content.

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3.3. GAMMA-SPECTROMETRY, RADON EXHALATION AND EMANATION

The mean result of the Ra-226 activity concentration was 36.1 ± 6.6 Bq/kg (16.1 to 104.7 Bq/kg) as a result of gamma-spectrometry measurements.

The radon exhalation features of the collected, dried and powdered clays were determined. The mean value was 83.3 ± 12.9 mBqkg-1h-1 and the results varied between 31.0 ± 8.2 – 271.4 ± 27.7 mBqkg-1h-1. The exhalation features are very different because of the different radium content and the diverse internal structures of the investigated materials.

The radon emanation coefficient values were calculated in comparison with known radium content and the measured free exhalation values. The obtained result ranged between 8.4 ± 2.0 % – 34.0 ± 5.9 % with mean of 18.1 ± 3.9 %.

The result of the gamma-spectrometry, radon exhalation and emanation measurements are illustrated together in Figure 6.

Fig. 6 – The result of the gamma-spectrometry, radon exhalation and emanation measurements.

4. CONCLUSION

Due to the modifying effect of the sample features the free exhalation is the recommended method for characterization of porous materials, since only the sample thickness has influencing effect during the measurement. Furthermore the free exhalation measurement is suitable for determination of radon emanation.

In the case of emanation or exhalation measurement the applied dry sample is not qualified for providing enough information about the examined materials, since the moisture content has strong modifying effect on the radon emanation. Furthermore in practical point of view the chance of totally dry state of building

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materials is negligent. Due to the obtained relatively strong radon emanation increasing effect of slight moisture content variation it can be expressly stated that the moisture content dependency measurement is required in case of the investigated materials.

Clays that used as base materials in Hungarian brick factories were collected from several regions of the country. 27 different type of clay were selected and the natural radionuclide content was determined with gamma spectrometry. The emanation coefficient ranged between 8.4 ± 2.0 % – 34.0 ± 5.9 % with mean of 18.1 ± 3.9 %.

The emanation and free exhalation rate results are ordinary results if we compare them with other measurements which can be found in the literature. The inbuilt of the examined clay or the produced brick products doesn’t spell danger in radiological aspect.

On the basis of the measured values it can be stated that the collective application of the performed measurements, such as determination of natural radionuclide content, the radon emanation factor and the free exhalation of the samples can characterize the building materials more precisely in radiological point of view.

Acknowledgements. We acknowledge the financial support of the Hungarian State and the

European Union under the TÁMOP-4.2.2/B-10/1-2010-0025 and TÁMOP-4.2.2.A-11/1/KONV-2012-0071).

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