Perspectives of a method for measuring soil-gas radon by an opened counting vial

Transcription

1 Perspectives of a method for measuring soil-gas radon by an opened counting vial Hugo López del Río Juan Carlos Quiroga Cifuentes Unidad Académica de Estudios Nucleares Universidad Autónoma de Zacatecas E hlopez@uaz.edu.mx Abstract The aim of this paper is to present some preliminary result and future perspectives of soil-gas radon measurement by an open vial and liquid scintillation counting under the capabilities and experience in liquid scintillation counting at our Laboratory. Soil-gas radon was collected into an opened counting vial containing a mixture of water and commercial liquid scintillation cocktail. The open vial was exposed to soil-gas inside a hole of the ground. The results shown that dissolution of radon gas into liquid phase takes place under the particular experimental conditions applied. The method could be routinely applied to measure soil-gas radon at our Laboratory. Keywords: soil-gas radon; opened vial; liquid scintillation counting. Introduction Radon is a naturally occurring radioactive gas that is formed from the normal radioactive decay of radium. Because radium is in turn derived from uranium which is present in small amounts in most rocks and soil, gas radon is continuously generated in soil surface. As an inert gas, radon freely moves and some of the radon diffuse to the soil surface and enters the air, while some remains below the soil surface and enters the groundwater. 1

2 It is well known that radon contributes a large parte of natural radiation exposure (UNSCEAR, 1992). In fact, indoor radon and its decay products can significantly increase lung cancer risk in the general population (Jacobi, 1988). Many studies are being carried out currently to measure both indoor and groundwater radon concentrations while others are focused on determining the soil-gas radon concentration and its exhalation rate. Furthermore, temporal changes of radon concentration in soil-gas and groundwater have practical uses in geology for predicting earthquakes (Ramola, 2008). Ulomov and Mavashev (1967) first found the relationship between anomalous radon concentrations in well water and the Tashkent earthquake. Since then, numerous papers are being published describing the radon concentration along active faults and the mechanism for radon release (Planinic et al., 2004) Along with this, methods and instruments for measuring and detecting radon in air, soil, and water have been developed (Papastefanou, 2002; Ruckerbauer and Winkler, 2001). Particularly, methods for determining radon flux from the soil are based on either active sampling by pumping soil gas into continuously operating radon monitors or grab sampling. Devices to detect radon include scintillation cells internally coated with silver activated ZnS powder (e.g. Lucas cells), alpha track-etch detectors or solid-sate nuclear track-etch detectors (e.g. LR-115 or CR-39 films), and alpha radiation spectrometers with silicon diode. A method for measuring radon in air consists of exposing a charcoal canister to adsorb radon and subsequent measurement of the activity of its decay products by gamma spectrometry with an HPGe detector (Gómez Escobar et al., 1999; Quirino et al., 2007). Due to recent advances in liquid scintillation counters, the measurement of radon in water has been extensively performed by using liquid scintillation technique (Schoenhofer, 1989; Gómez Escobar et al., 1996). Liquid scintillation 2

3 counting is an effective technique for measuring radionuclides due to its high counting efficiencies for alpha and beta detection and easy sample preparation. Vera Tomé et al. (1999) extracted efficiently 222 Rn in air samples by pumping air through a cell containing a commercial organic liquid scintillator and measuring alpha radiation with a liquid scintillation counter. Horiuchi and Murakami (1983) developed the opened counting vial method for determining soil-gas radon. They found that a definite fraction of radon is dissolved into the liquid scintillator. The aim of the present work is to present some preliminary result and future perspectives of soil-gas radon measurement by an open vial and liquid scintillation counting under the capabilities and experience in liquid scintillation counting at our Laboratory. Experimental Site description Field exposures were performed at three sites in Zacatecas city: Cerro del Padre ground (22 45'26.66"N, '46.69"O), Nuclear Center ground (22 46'11.49"N, '52.52"O), and Law School garden (22 46'6.82"N, '54.73"O). Preparation of counting vials The opened counting vials were prepared by mixing 12 ml of liquid scintillation cocktail OptiPhase HiSafe 3 (Wallac) and 8 ml of low radioactive water in a 20-mL volume polyethylene vial. Sampling and measurement Holes of 20 cm and 40 cm in deep were drilled. The open vials were fixed at the bottom of the hole and covered with a 1-L volume Erlenmeyer flask, which ensured a close and stable system for adsorption of radon into the counting mixture. The holes were mainly covered at top with local forest and soil. An 3

4 exposure time of about two days was applied. After the exposure time, vials were carefully removed and immediately transferred to laboratory. Adsorbed radon and its decay products were measured in a Liquid Scintillation Counter (LSC) Wallac 1411 with pulse shape analysis feature by s. A previously optimized PSA level of 49 was used to ensure the best alpha/beta pulse separation (Dávila Rangel et al., 2001). Results One open vial was exposed inside a 20-cm deep hole at Cerro del Padre ground, two vials were placed in a 20-cm and 40-cm deep hole drilled in Nuclear Center ground, and finally another vial was placed inside a 20-cm deep hole at Engineering School garden. Unfortunately, the last hole was discovered and uncovered by someone, consequently soil gas equilibrium was disturbed. The results proved that radon gas was successfully adsorbed onto the liquid scintillation mixture in spite of soil type or depth. Alpha particle radiation from 222 Rn, 218 Po, and 214 Po were registered on the alpha spectrum. Despite of poor resolution of liquid scintillation counting technique, two alpha peaks were perfectly registered: one from 222 Rn+ 218 Po radiation and another from 214 Po radiation (Fig. 1). 4

5 222 Rn+ 218 Po 214 Po Fig. 1. A typical alpha spectrum of adsorbed soil-gas r adon into an open vial Quenching is an important factor to be considered in liquid scintillation counting because it can reduce counting efficiency and displacement of peaks toward lower channels. The External Standard Quench Parameter, or SQP(E) of Wallac liquid scintillation counters, gives a direct measure of the sample quench. Normally, we have observed SQP(E) values ranging from 740 to 720 when gross activities are measured in water and scintillation mixture are prepared as described above. SQP(E) values for radon samples varied between 732 and 729. Thus, there will be no distinctions made between both samples, and therefore this allows us to apply similar considerations in activity calculations. Future investigations will be needed to study major factors that influence radon adsorption into liquid scintillation mixture, to determine soil-gas radon adsorption efficiency, and to find out the best in situ experimental arrangement. 5

6 Conclusions We have successfully applied the open vial method to measure soil-gas radon. The method described is rapid and simple. It consists of exposing an opened counting vial with liquid scintillation- water solution to soil gas. Radon measurement was carried out the well characterized liquid scintillation procedure for alpha and beta radiation detection. Although further studies are necessaries, we have proved the capability of the method to measure soil-gas radon. 6

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