A simple technique for measuring the activity size distribution of radon and thoron progeny aerosols

Highlights

• Significant information on the activity size distribution of radon and thoron progeny in the environment can be measured.

• The impactor can work anywhere without AC supply.

• Solid-state nuclear track detectors are used as the impaction plate though they face a backup filter at the last stage.

• The dose conversion factor for radon and thoron progeny can be specified based on the measured activity size distribution.

Abstract

In this study, a portable cascade impactor was developed to more efficiently determine the activity size distribution of attached radon and thoron progeny in a natural environment. The developed impactor consisted of four stages with a backup filter stage for collection of the aerosol samples. The aerosol cut points were set for 10, 2.5, 1, and 0.5 μm at a sampling rate of 4 L min⁻¹. Five CR-39 chips were used as alpha detectors for each stage. To separate the alpha particles emitted from radon and thoron progeny, the CR-39 detectors were covered with aluminium-vaporized Mylar films. The thickness of each film was adjusted to allow alpha particles emitted from radon and thoron progeny to reach the surface of the CR-39 detectors. The particle cut-off characteristics of each stage were determined by mono-dispersive aerosols with particle sizes ranging from 0.1 to 1.23 μm from the collection efficiency curve. The test results showed that the respective cut-off size of stages 3 and 4 were close to the designed cut-points. Validation of the technique by comparison with two commercial devices confirmed that the developed technique could provide the necessary information to estimate the activity size distribution of attached radon and thoron progeny for dose assessment, especially, in a field survey where direct electric power is not available.

Introduction

Radon is an inert, naturally occurring radioactive gas, that is derived from the breakdown of uranium and thorium in the earth's crust, which provides a continuous source of radon. The two most common and naturally abundant isotopes of radon are ²²²Rn and ²²⁰Rn, which originate from the decay series of ²³⁸U and ²³²Th, respectively. The specific isotope ²²²Rn is usually referred to as radon and has a half-life of 3.8235 d, while ²²⁰Rn is commonly called thoron and has a shorter half-life (55.6 s). Radon can outflow from rocks and soils that make up the Earth's crust either by molecular diffusion or by convection and, as a consequence (Gundersen et al., 1991), ²²²Rn is present in the air of both outdoor and indoor environments (ICRU, 2012). Globally ²²²Rn is the most critical natural source of ionizing radiation, accounting for approximately 40% of the annual effective dose from all sources of radiation (Kranrod et al., 2020). In addition, ²²²Rn has also become recognized as one of the most important indoor air pollutants. The most significant source of natural radiation when taking into consideration the total annual dose in inhaled air is not caused by ²²²Rn and ²²⁰Rn gas but rather by their short-lived decay products (Kávási et al., 2009, 2011; Dung et al., 2014; Pornnumpa et al., 2015).

In epidemiological studies, ²²²Rn and its progeny are constantly present in indoor environments and their concentrations vary widely. They constitute the second known leading cause of lung cancer in the general population after cigarette smoking (ICRP, 2014; 2010; Tokonami, 2010; WHO, 2009). Additionally, ²²⁰Rn is present everywhere together with ²²²Rn, and the quantity of ²²⁰Rn can sometimes be much higher than ²²²Rn in certain positions in dwellings (De Jong et al., 2006). In comparing ²²²Rn and ²²⁰Rn, per unit radioactivity, ²²⁰Rn progenies convey a 13.6-fold larger potential alpha energy concentration (PAEC) than ²²²Rn progenies (De Jong et al., 2006). The alpha radiation from polonium isotopes (radon progenies: ²¹⁸Po and²¹⁴Po, and thoron progenies: ²¹⁶Po and ²¹²Po), which are heavy metals, and the short-lived decay products of ²²²Rn and ²²⁰Rn provide a much higher contribution to the radiologically significant dose, primarily because alpha particles deposit their energy within a range of 47–71 μm in soft tissue (Hofmann et al., 2020). As a consequence, the alpha energy is deposited in the relatively sensitive lung lining and also has a dense deposition pattern, which has a much greater biological impact (Yu, 1993). The biological effects are attributed to the sharp Bragg peak and high linear energy transfer (LET) of alpha particles. Thus, the interaction of alpha particles with cells can lead to direct DNA damage via double strand breaks.

The activity size distribution of radon progeny has been determined by tagging the natural aerosol particles with radon or thoron progeny. The observed data from many literatures have revealed that the size distribution consisted of ultrafine clusters with median diameters below 4 nm (unattached activity) and the progenies were associated with ambient aerosol particles in sizes ranging between 100 and 400 nm (attached activity). In aerosol-rich air, most of the radon and thoron progeny radionuclides attach on the surface of aerosol particles and form a radioactive aerosol. Therefore, the behavior of the airborne radionuclides is determined by the behavior of the aerosol particles in the atmosphere. Moreover, these radioactive aerosols (attached and unattached progenies) are inhaled into the human body during breathing, and accumulate in the lower respiratory tract, which consists of the trachea, bronchial tree, and lungs. The particle size determines the respiratory organ where the radioactive aerosols are deposited (Brenner, 1994; ICRP, 1994). Therefore, the lung dose assessment caused by radon and thoron progeny strongly depends on the aerosol size distribution (the activity median aerodynamic diameter, AMAD, and the standard deviation, σ_g), manner of breathing, and breathing rate (Mohamed et al., 2014; Porstendörfer, 1997; Tokonami, 2000). The unattached and ultrafine particles with sizes below 100 nm and with a high diffusion coefficient are considered to yield about 50% of the total radiation dose (Paquet et al., 2017).

In order to accurately assess the dose due to radon and thoron, one of the important physical parameters is the AMAD derived from the activity-weighted size distribution (AWSD), as well as the radon and thoron concentrations. In general, atmospheric aerosol particles follow a trimodal distribution (NRC, 1979): (1) the nucleation mode (from 3 to 70 nm, average 15 nm), (2) the accumulation mode (from 70–2000 nm, average 300 nm), and (3) the coarse mode (from 2000–36,000 nm, average > 10,000 nm). ²¹⁰Pb are preferentially attached to accumulation-mode aerosols (Sykora and Froehlich, 2009). Many radionuclides differently attach to various sizes of aerosols. For short-lived radon and thoron progenies, the size distribution was obtained as 12–19% in the nucleation mode, 81–88% in the accumulation mode, and none in the coarse mode. The AMADs of the accumulation mode was mixed between 332 nm (²¹⁸Po) and 347 nm (²¹⁴Po) for the radon progeny and between 382 nm (²¹²Po) and 421 nm (²¹²Pb) for the thoron progeny (Gründel and Porstendörfer, 2004).

The methods for measuring this physical parameter are different for radon and thoron progeny. Moreover, there are two types of device that has been widely used to measure the AWSD. These are screen diffusion batteries (Cheng and Yeh, 1980; Hopke et al., 1992) and low-pressure cascade impactors (Sorimashi et al., 2008a; Yamasaki and Suzuki, 1992). Cascade impactors, with various numbers of stages (Wołoszczuk and Skubacz, 2018; Mohamed and El-Hussein, 2005) and online impactors (Gründel and Porstendörfer, 2004), have been used to measure the size distribution of aerosol-attached radon and thoron decay products.

In order to easily determine the activity size distribution (ASD) of attached activity, a simple technique was developed using a standard four-stage portable impactor sampler and applying allyl diglycol carbonate (commercially known as CR-39) as the alpha detection system for radon and thoron progeny. This paper describes the performance of the developed portable impactor and its verification for use for assessment of the dose by measuring the ASD of both radon and thoron progeny in the environment.