Study of factors affecting the measurements of concentrations of radon gas, thoron gas and radon progeny using solid state nuclear track detectors

Abstract

Solid state nuclear track detector (SSNTD) has been commonly and widely used in longterm measurement of radon and thoron gas concentrations with the diffusion chambers. It is well known that the radon dose delivered to the lungs is not attributed to the radon gas itself, but instead to its shortlived progeny. Therefore, longterm measurement of the equilibrium factor has been proposed by Nikezic et al. (2004a) and Yu et al. (2005) using a bare LR 115 detector through the so called “proxyequilibrium factor” method. On the other hand, routine measurements on the radon and thoron gas concentrations can be measured simultaneously by using the “twins diffusion chamber method”. The measurements on the radon and thoron gas concentrations through the “twins diffusion chamber method” as well as the equilibrium factor through the “proxyequilibrium factor” method can be affected by many factors, which can potentially affect the accuracy of these measurements. In order to perform reliable measurements, the potential factors that affect the accuracies will be studied in the present investigation. These include (1) the sensitivities for radon and thoron inside the diffusion chamber; (2) the characteristics of and precautions in using the membrane filters used in the diffusion chamber; (3) the partial sensitivity of the bare detector in determination of the proxyequilibrium factor, and (4) the effect of thoron on the bare detector. It is found that the sensitivity of the LR 115 detector inside a diffusion chamber to the radon and/or thoron gas concentration is dependent on the actual removed active layer thickness of the LR 115 detector. The sensitivities for 222 Rn and 220 Rn inside the diffusion chamber have been experimentally determined for different removed active layer thickness. Monitoring of such removed active layer thickness is necessary, otherwise large percentage errors will result. Polyethylene (PE) membrane is commonly used as an antithoron filter. However, the thickness of the PE membrane used and the associated precautions are rarely specified in the literature. It is found in the present work that the number of alpha particle tracks registered by the LR 115 detector in a diffusion chamber covered with one layer of PE membrane is actually enhanced for radon exposure only. A sufficient thickness of the PE membrane is actually required to prevent signal enhancement due to the radon progeny deposited on the outside surface of the membrane. The optimal thickness of PE membranes was found to be 40 mm. This allows the largest amount of radon gas to diffuse into the diffusion chamber and at the same time screening out the largest amount of thoron gas. The applicability of the twins diffusion chamber method has been validated through experiments. The bare LR 115 detector can be used to determine the proxyequilibrium factor FP, which is defined as FP = (ρ/ρitCo)1 where ρi is the partial sensitivity. The corresponding equilibrium factor F can be found from the theoretical relationship between FP and F. In the present work, the partial sensitivity of the bare LR 115 detector has been experimentally determined for different removed active layer thickness. In addition, the applicability of the proxyequilibrium factor method has been validated through experiments. The proxyequilibrium factor method will be affected by the presence of thoron and its progeny in the ambient environment, which will enhance the number of tracks on the bare LR 115 detector. The partial sensitivity to thoron of the bare LR 115 detector has been experimentally determined for different removed active layer thickness. It is found that the amount of tracks due to thoron will be significant if the thoron concentration is high. Therefore, correction must be performed to eliminate this overestimation. After studying the potential factors that affect the accuracies of radon and thoron measurements, longterm measurements of radon gas (RC), thoron gas (TC) concentration and the equilibrium factor (F) have been performed in residential buildings. Two rounds of measurements in chosen residential building were made. The average values of RC, TC and F in the two rounds of measurements were found to be 39.9 Bqm 3 , 3.37 Bqm 3 and 0.490.58, respectively. The obtained results are commensurate with the results obtained in previous studies using active methods. In conclusion, we have successfully established a method combining the twins diffusion chambers and the proxyequilibrium factor to make longterm measurements of radon and thoron gas concentrations and the equilibrium factor.

Date of Award	3 Oct 2006
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kwan Ngok Peter YU (Supervisor)