Contents lists available at ScienceDirect Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad Determination of the thoron emanation coefficient using a powder sandwich technique K. Danyłec * , J. Mazur, K. Kozak, D. Grządziel Institute of Nuclear Physics Polish Academy of Sciences, PL-31342, Kraków, Poland ABSTRACT Thoron ( 220 Rn) is a natural radioactive gas, tasteless, odourless, colourless, undetectable without proper equipment. This gas is carcinogenic, just like radon ( 222 Rn) but due to the short half-life (55.6s) and a small amount in the environment, its share in the absorbed radiation dose is often neglected. However, in areas rich in thorium ( 232 Th), the radiation dose from the thoron can be much larger and quite significant. The problem is to measure the concentration of the thoron due to its short decay time as well as the fact that it is alpha-emitting as radon. An even greater challenge is to determine the emanation coefficient for the thoron. The method used in this experiment was developed by S.D. Kanse based on the work of D.J Greeman and adapted to the equipment used in Laboratory of Radiometric Expertise IFJ PAN. In the technique used to determine the thoron emanation coefficient, a closed loop system is used in which thoron is pushed out by means of a flow system from the sample and measured by a AlphaGuad DF2000 detector that is adapted to determine concentration of this gas. A sample of the material is placed between 2 filters in the geometry of the sandwich. This arrangement ensures that the thickness of the powder sample is significantly less than the length of the thoron diffusion, thus avoiding significant loss of the thoron due to intergranular absorption and facilitates the complete removal of this gas escaping from the powder. Using this technique, it is important to determine the concentration of 226 Ra and the 232 Th, since for the AlphaGuard detector, the ratio between thoron and radon should not exceed 5:1 for proper determination of the thoron concentration. Measurements of 226 Ra and 232 Th activity were carried out using gamma spectroscopy (HPGe detector). It was examined how the type of filter and grain size of sample affects the obtained results. 1. Introduction Radon (Rn) belongs to the family of noble gasses in the periodic table. It is tasteless, odourless, colourless, undetectable without proper equipment. There are four radon isotopes naturally present in the en- vironment ( 222 Rn, 220 Rn, 219 Rn, 218 Rn). They are chemically the same but there are differences in radiological properties (Haynes, 2011). One of them is thoron ( 220 Rn), which comes from the thorium 232 Th day series. This gas is carcinogenic but its share in the absorbed radiation dose is often neglected due to a small amount in the environment and the short half life (UNSCEAR, 2000; World Health Organization (WHO, 2009).). However, in areas rich in thorium ( 232 Th), the radiation dose from the thoron can be much larger and quite significant. Thoron is released from building materials and the earth's crust. The process of gas release from grains into the atmosphere is divided into three stages: emanation, transport and exhalation. Exhalation depends in a large extent on the emanation, which is influenced by many factors such as grain size, porosity, radioactive isotope content, distribution of 232 Th within the grain or humidity. The emanation process largely depends on recoil of 220 Rn atom after decay of 224 Ra, by which the 220 Rn atoms get released into the pore space of the matrix (Fleischer, 1983). There is a high probability that the recoil energy of the 220 Rn atom is not enough to get out of the grain or the recoil energy of the 220 Rn atom is sufficient but it will be absorbed by the adjacent grain or by the water located between the grains (Sahoo, 2013; Greeman and Rose, 1996). To de- termine the emanation, a parameter called the emanation coefficient (f) is introduced. It is a ratio between 220 Rn atoms that reach out of grain into pore volume to that of the total 220 Rn atoms that are produced in the sample matrix (Greeman and Rose, 1996). Currently there are many articles about emanation, transport and exhalation of radon ( 222 Rn) and factors affecting it, but there is not so many about the same issue for thoron ( 220 Rn). Unfortunately, the study of emanation of 220 Rn is quite a difficult task due to the short half life, which, unlike 222 Rn, 3.82 days, is very short, 55.6 s. There are articles describing 220 Rn emanation but they are mainly based on techniques used for 222 Rn. Due to short half life of thoron these methods may not be always suitable. To specify the emanation coefficient, a commonly used technique is an accumulation chamber associated with alpha particle detector. The thoron concentration is determined by means of the 216 Po peak and the radon concentration by means of the 218 Po peak, which allows to obtain a rapid equilibrium between radon and polonium nuclei, then the emanation coefficient is calculated by determining the 224 Ra content by means of gamma spectroscopy (Tuccimei et al., 2006). Another technique is the use of https://doi.org/10.1016/j.jenvrad.2018.10.004 Received 27 August 2018; Received in revised form 10 October 2018; Accepted 10 October 2018 * Corresponding author. E-mail addresses: karolina.danylec@ifj.edu.pl (K. Danyłec), Jadwiga.Mazur@ifj.edu.pl (J. Mazur), Krzysztof.Kozak@ifj.edu.pl (K. Kozak), Dominik.Grzadziel@ifj.edu.pl (D. Grządziel). Journal of Environmental Radioactivity 195 (2018) 109–113 0265-931X/ © 2018 Elsevier Ltd. All rights reserved. T