RN-222 RELEASE TO THE ENVIRONMENT: COMPARISON BETWEEN DIFFERENT GRANITE SOURCES

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1 EG Proceedings of the Environmental Physics Conference, Feb. ~, -...,., - OJr - RN-222 RELEASE TO THE ENVIRONMENT: COMPARISON BETWEEN DIFFERENT GRANITE SOURCES A. Mamoon and Salah M. Kamal Radiation Protection Department, Nuclear Research Center (NRC), Atomic Energy Authority (AEA), Cairo, P.O. Box 13759,Cairo, Egypt E- mail: egykamal@yahoo.com ABSTRACT In this work three different types of granites were studied, namely: pure granite, alkali granite and altered (hydrated) alkali granite. General radioactivity of the granites was studied along with the potential for 222 Rn emanation. The study indicated that altered alkali granite releases, relatively, the highest 222 Rn emanation to the surrounding air while alkali granite emits the more intense gamma radiation of the three granites. Hence, altered alkali granite can be used as a laboratory source fox 222 Rn. INTRODUCTION Granites- of different types are known to contain traces of uranium ^]' 2 \ Among the decay products of uranium is Ra which in turn decays to 222R rij which being a gas, leaks out from the mother rock and can escape to the atmosphere or dissolve in surrounding surface or ground water ' 3 " 4 l Hence granites can serve as convenient laboratory scale source for studies 222^ on Rn. The present work conducts a comparative study of the general radioactivity characteristics and 222^ emanation capability of three different types of granite, namely: pure granite, alkali granite and altered (hydrated) alkali granite [5]. The aim of the study is to select the granite source with the greatest potential for emanating Rn [6]. MATERIALS AND METHODS Samples from the three granite rocks were crushed to 2mm particle diameter. Equal masses (250g each) of the crushed granites were placed in two containers; in either Marinelli beakers for gamma spectroscopy using HPGe detector, or in emptied charcoal canisters for measuring the surface dose to TLD-200. The dosimeters were positioned on top of the crushed source as shown in (Fig. 1). 53

2 5,4k i-*- {;'.?;* Proceedings of the Environmental Physics Conference, Feb. 2004, Minya, Egypt TLD chips 3 layers of Vinyl tape Fig. 1. Diagrammatic representation of TLD chips placed on surface of granite rock filling an emptied canister The two containers were closed tight for four weeks before making the measurements. 222 Rn emanation assay was carried out using standard charcoal canister technique. The granite rocks along with an open regular charcoal canister were placed, one by one, in a confined exposure desiccator (regular laboratory system) for three days. Measurements of the 222 emanated Rn air concentrations (for the three granite rocks) in the desiccator air were carried out using standard charcoal counting technique. RESULTS AND DISCUSSION Equal masses of the crushed granites, having the same mesh number, were used in all the experiments. While gamma spectra of the granites showed similarities of the many gamma energies present yet there was variation in the relative intensities of some gamma lines as shown in (Fig. 2). For example, in the case of alkali granite, this has an increased level of potassium, the 40K gamma line being more intense. While, 214 Pb and Bi gamma energies were present in all granites at about the same intensities. The gross gamma count rates (table 1) of the granites show as expected, that alkali granite gave the highest count rate. The dose rate at the surface of the crushed granites, as measured by TLD-200, was highest for alkali granite (Fig. 3) namely about 460DR/hour compared to about 28DR/hour for pure granite. For altered alkali granite, the mass used, i.e. 250 grams, has water as part of the used mass. This is reflected in the less surface dose rate compared to alkali granite. Increasing the mass of altered alkali granite used (Fig. 4) showed, at first, increase in the surface dose but this tapers off gradually, perhaps due to self shielding by the increased mass. Regarding 222j^n emanation from the different granite sources, altered alkali granite (which is hydrated alkali granite) released more 222^. (pig 5) illustrated that it released 1.73 Bq/L as compared with 0.3 Bq/L for pure granite (Fig. 2) illustrated that more 222Rn release as measured by charcoal canister technique is perhaps due to more porosity or more permeability to 222^ exhalation in the altered alkali granite. 54

3 Proceedings of the Environmental Physics Conference, Feb. 2004, Minya, Egypt.214 Bl K - Fig. 2. Gamma-ray spectra detected by HPGe detector for different types of granite collected: (a) alkali granite (b) altered alkali granite and (c) pure granite. Notice presence of 40K in all granites but at different relative concentrations. 214Pb and 214Bi (222Rn daughters) main energy peaks are present in all granites. The counting time was sec. The results indicate that, as a laboratory source for 222R 11J altered alkali granite seems to be the better granite to use in such situations. Table 1. Comparison between the three different granite types in terms of gross counts of selected region of interest around energy peaks of radon daughters ( 214 Pb and 214 Bi) to be analyzed IL No Granite Type* Alkali Altered alkali Pure Region of Interest Gross Count 10,105,028 3,036, ,414 * Granite Mass = 1000 g and ** Counting Time = 1000 sec. 55

4 sdose Proceedings of the Environmental Physics Conference, Feb. 2004, Minya, Egypt OSeriesi / control 2.68 Pure Granite slj Altered Alkali I 1 r AlJkali Gr anite Fig. 3. The dose-rate as measured by TLD chips placed on top of 250 grams of different granite rocks having 2.0 mm particle diameter, the control is an empty canister. I o Q M r. - -ri- -j I 0.0 Mass of Altered Alkali Granite Rocks ( g) Fig. 4. Dose-rate measured by TLD chips Placed On top of different masses of granite rock having 2.0 mm particle diameter. u 3 o ' <u u u o Iz +- «- Alkali Pure Granite Type Altered Alkali Fig. 5. Variation of ambient radon concentration in the exposure system (i.e. in desicator 2.0 mm particle diameter. Difference between pure and alkali granite is negligible at this particle diameter. 56

5 Proceedings of the Environmental Physics Conference, Feb. 2004, Minya, Egypt REFERENCES [1] Crameri, R.; Brunner, H. H.; Buchli, R.; Wemli, C. and Brkart, W., "Indoor Rn Levels in Different Geological Areas in Switzerland," Health Physics Vol. 57, No. 1, pp , July, [2] Buch, R. and Burkart, W., "Influence of Subsoil Geology and Construction Technique on Indoor Air Rn-222 Levels in 80 Houses of the Central Swiss Alps," Health Physics Vol. 56, No. 4, pp , April, [3] Saito, K.and Jacob, P.: "Gamma ray fields in the air due to sources in the ground". Radiation Protection Dosimetry Vol. 58, No. 1, pp (1995) [4] Luetzelschwals, J.W.; heleeick, K. L. and Hurst, K. A., "Radon Concentration in Five Pennsylvania Soils." Health Physics Vol. 56, No. 2, pp , Feb., [5] Sextro, R.G.; Moed, B.A.; Nazaroff, W.W.; Revzan, K.K. and Nero, A.V., "Investigations of Soil as a Source of Indoor Radon", American Chemecal Socity Symposium Series No. 331, cited by Philip K. Hopke, [6] BEIR, "Health effects of exposure to Radon", National Academy Press, Washington D.C., BEJR VI, Committee on Health Risks of Exposure to Radon, (1999). [7] Anjos, R.M.; Veiga, R.; Soares, T.; Santos, A.M.A.; Gguiar, J.G; Frasca, J.A. P.; Uzeda, D.; Mangia, L.; Facure, A.; Nosquera, B.; Carvalho, C. and Gomes, P.R.S.; " Natural Radiohuclide Distribution in Brazilian Commercial Granites ", Radiation Measurements, Vol. 05, No. 002, pp. 1-14,14 July,

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7 Session IV The Eco System

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