Experimental technique to measure thoron generation rate of building material samples using RAD7 detector

doi:10.1016/j.radmeas.2013.07.003

Radiation Measurements

Volume 59, December 2013, Pages 201-204

https://doi.org/10.1016/j.radmeas.2013.07.003 Get rights and content

Highlights

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RAD7 is used for the determination of thoron generation rate (emanation).
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The described model takes into account the thoron decay and attenuation.
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The model describes well the experimental results.
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A single point measurement method is offered at a determined sample thickness.

Abstract

Thoron (²²⁰Rn) is the second most abundant radon isotope in our living environment. In some dwellings it is present in significant amount which calls for its identification and remediation. Indoor thoron originates mainly from building materials. In this work we have developed and tested an experimental technique to measure thoron generation rate in building material samples using RAD7 radon-thoron detector. The mathematical model of the measurement technique provides the thoron concentration response of RAD7 as a function of the sample thickness. For experimental validation of the technique an adobe building material sample was selected for measuring the thoron concentration at nineteen different sample thicknesses. Fitting the parameters of the model to the measurement results, both the generation rate and the diffusion length of thoron was estimated. We have also determined the optimal sample thickness for estimating the thoron generation rate from a single measurement.

Introduction

The only gaseous isotope of the ²³²Th decay chain is ²²⁰Rn (thoron) which, similarly to ²²²Rn (radon) in the ²³⁸U decay chain, is omnipresent in our living environment. Its contribution to the average effective dose to people from natural background radiation is estimated to be about an order of magnitude smaller than that of radon (UNSCEAR, 2000) due to its short half-life (55.6 s). However, several studies (e.g. Sciocchetti et al., 1992, Németh et al., 2005, Shang et al., 2005, Yamada et al., 1999, Yonehara et al., 1999) show high thoron activity concentrations in some dwellings. In most of these cases thoron originates from building materials made by soil and mud, like in case of adobe dwellings in Hungary. Significant amount of thoron can enter buildings from this type of walls; hence the measurement of thoron generation rate in building material samples can be highly useful to characterize radiation safety of these materials (Tuccimei et al., 2006). In one of our ongoing study, in which tens of Hungarian adobe building material samples are tested for radiation safety purposes (e.g. Szabó et al., 2013), the development of an appropriate experimental method to measure thoron generation rate became necessary. Ujić et al. (2008) published a method using SSNTD detectors, in this paper we present an experimental technique developed for this purpose using RAD7 radon-thoron detector.

Section snippets

Experimental setup

The schematic view of the experimental setup is shown on Fig. 1. It consists of a cylindrical shape aluminum sample holder with cross-sectional area of A = 38.5 cm² and height of H = 9.5 cm; plastic tubing; a gas-drying unit filled with desiccant (CaSO₄ with 3% CoCl₂ as indicator); an aerosol filter and a RAD7 radon-thoron detector (Durridge Co., 2013). All connections are insulated by parafilm (product of the Pechiney Plastic Packaging Company). Unlike radon, thoron cannot leak in significant

Model

In this section we derive the formula to calculate the thoron concentration (C, (Bq m⁻³)) in the air-filled volume of the sample holder, which is measured and displayed by the RAD7 detector in our experimental setup. Because of the cylindrical symmetry of the sample holder, thoron concentration in the pore space of the sample, C(z), depends only on the elevation coordinate (z). Diffusion of thoron in the sample is described by the steady state transport equation: $\frac{d^{2} C (z)}{ⅆ z^{2}} = \frac{1}{D} (- G + λ β C (z)),$ where D

Measurements

For experimental validation of the method we have selected an adobe building material sample (originated from Gyomaendrőd, Békés County, SE-Hungary), and measured the dependence of thoron activity concentration as a function of the sample thickness. The sample has a comparably hard, stable structure which makes it possible to cut and rasp it to the appropriate sizes and carry out the measurements. The sample was shaped to the size of the best possible fit in the sample holder with the maximum

Results and discussion

Experimental results of thoron concentration measurement as a function of sample thickness is presented on Fig. 2 together with the curve of best fit of the model formula. Origin data analysis and graphing software was used to fit the model.

We found that the developed formula describes very well the experimental results. It is possible to determine the thoron generation rate, as well as the γ parameter for the selected adobe sample via a non-linear curve fitting. We have also tried to fit the

Conclusions

In this work we have developed and validated a new experimental technique to measure thoron generation rate in building material samples using RAD7 radon-thoron detector. The developed mathematical model of the measurement technique described well the thoron concentration response of RAD7 as a function of the sample thickness. Fitting the parameters of the model to the measurement results, both the generation rate and the diffusion length of thoron in the sample material was estimated. For the

Acknowledgment

We would like to thank very much the help of local people in the sample collection, Péter Völgyesi in the sample preparation, Dániel Breitner in the discussion of the results and Lithosphere Fluid Research Lab and Department of Atomic Physics, Eötvös University, Budapest, Hungary providing the environment of laboratory work and the RAD7 detector. We are very thankful for the constructive review of the manuscript. ZsSz is highly thankful for Szabina Török to provide an inspiring environment for

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Experimental technique to measure thoron generation rate of building material samples using RAD7 detector

Highlights

Abstract

Introduction

Section snippets

Experimental setup

Model

Measurements

Results and discussion

Conclusions

Acknowledgment

Japan. Int. Congr. Ser.

J. Environ. Radioact.

Appl. Radiat. Isot.

Radiat. Meas.

Entry of Soil Gas and Radon into Houses

RAD7 RADON DETECTOR User Manual (Revision 7.2.1.)