Elsevier

Applied Radiation and Isotopes

Volume 81, November 2013, Pages 226-229
Applied Radiation and Isotopes

Realization of radioactive equilibrium in the KRISS radon chamber

https://doi.org/10.1016/j.apradiso.2013.07.007Get rights and content

Highlights

  • The activity concentrations of radon decay products are evaluated by the Modified Tsivoglou Method.

  • Mosquito-repellent incense is used to produce aerosol particles in the radon chamber.

  • The radioactive equilibrium in the chamber was achieved within 2 days and confirmed by the Bateman equation.

Abstract

The maintenance of radioactive equilibrium between radon and its decay products in a radon chamber is necessary to calibrate radon decay product monitors. In this study, the activity concentrations of radon decay products have been measured, and mosquito-repellent incense has been used to produce aerosol particles in the chamber. Filter papers with 8 μm pore size were used to collect aerosol in the chamber. The activity concentrations of radon decay products have been evaluated by the Modified Tsivoglou Method. The correction factors due to the differences in counting time requirements of the Modified Tsivoglou Method and the time delay between consecutive measurements have been determined. Finally, the radioactive equilibrium has been confirmed by applying the Bateman equation.

Introduction

The radon-in-air activity concentration can be determined from the measurement of alpha particles emitted from the radon passed through a filter by using an ion chamber or a Si detector (NCRP, 1988). The radon decay products activity concentrations in air are not easy to determine due to their chemical characteristics and their short half-lives. The modified Tsivoglou Method (MTM) is widely used to measure the concentration of radon decay products and it is well known to provide accurate results if the concentration of thoron decay products is relatively small (Thomas, 1970, EPA, 1992). If the concentration of thoron decay products in air is not small, it is necessary to eliminate their contribution from the gross alpha count rate. This contribution can be estimated using the gross alpha count rate measured after the radon decay products being decayed.

The radiation counting devices usually have a function for periodic repeated measurements. Since it needs some time to store the data obtained from each measurement in a data storage device, the time delay occurs before the next measurement. Therefore, it is needed to correct the starting time of each measurement, when the MTM is applied to measure the concentration of radon decay products, in order to improve the measurement accuracy.

In this study, an alpha counting system was set to have constant time interval between sequential measurements. The concentration of radon decay products, with corresponding uncertainties, was calculated using the MTM.

Since the activity concentration of radon decay products can be known if radon and the radon decay products are in equilibrium in the radon chamber, it is very useful to evaluate the measurement methods for radon decay products. The radon chamber used in this study has 25.3 m3 in size (Lee et al., 2004). The mosquito-repellent incense was used to generate aerosol in the chamber, and the radioactive equilibrium was achieved in the chamber. In this study, the radioactive equilibrium was confirmed by evaluating the concentration of radon decay products.

Section snippets

Experimental

The KRISS radon chamber used in this study has a dimension of 3.2 m×3.3 m ×2.4 m (25.3 m3). The inner wall, ceiling, and floor of the chamber are lined up with the polished stainless steel plates in order to avoid the attachment of radon decay products. 100 kg of radium enriched soil with the nominal specific activity of 2 Bq/g were introduced in the chamber and stored for 4 weeks to maintain constant radon concentration and to achieve equilibrium between the radon and its decay products. A RAD7

Results and analysis

Assuming that the number of atoms per unit volume for radon decay products (Po-218, Pb-214, and Bi-214) is known and also the air flow rate of filter and the sampling time of grap sampling is given, the number of atoms per unit volume collected in the filter can be calculated as a function of time by applying the Bateman equation. Therefore, the count rate for alpha particles emitted from the radon decay products attached to the filter, Cα(t), can be calculated using the following Eq. (1).Cα(t)=

Conclusions

In this study, the correction factors for activity calculation of radon decay products by the MTM have been studied. The software to calculate the concentrations of radon decay products has been developed using parameters such as the starting time of alpha counting, the time delay of consecutive measurements, the counting efficiency for alpha particles, and the flow rate of grab sampling. It has been confirmed that the KRISS radon calibration chamber could achieve the equilibrium in less than 2

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    Radon chambers provide an enclosed environment where the user can control temperature, humidity and radon levels, and are typically used to test and calibrate radon monitoring instruments. These chambers range in size, from about 0.25 m3 (Moore and Kearfott, 2005) to large, walk-in chambers (Borysenko et al., 2013; Bowser Morner, 2016; Kansas State University, 2018; Kessler et al., 2017; Lee et al., 2013; Skubacz et al., 2017; Tate and Long, 2016; Tokonami et al., 2005). Many are designed to reach very high concentrations of radon.

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