Realization of radioactive equilibrium in the KRISS radon chamber
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, can be calculated using the following Eq. (1).
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
References (6)
- et al.
Development of the primary measurement standard for gaseous radon-222 activity
Appl. Radiat. Isot.
(2012) - et al.
Development of radon calibration chamber at KRISS
(2004) - EML, 1997. Procedures Manual, Environmental Meaurements Laboratory, HASL-300, 28th...
Cited by (1)
A radon chamber specifically designed for environmentally relevant exposures of small animals
2020, Journal of Environmental RadioactivityCitation Excerpt :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.