Radon and Thoron Exhalation Rates From Surface Soil of Bangka - Belitung Islands, Indonesia

DOI:10.17014/ijog.2.1.35-42Radon and thoron exhalation rate from soil is one of the most important factors that can influence the radioactivity level in the environment. Radon and thoron gases are produced by the decay of the radioactive elements those are radium and thorium in the soil, where its concentration depends on the soil conditions and the local geological background. In this paper, the results of radon and thoron exhalation rate measurements from surface soil of Bangka Belitung Islands at thirty six measurement sites are presented. Exhalation rates of radon and thoron were measured by using an accumulation chamber equipped with a solid-state alpha particle detector. Furthermore, the correlations between radon and thoron exhalation rates with their parent nuclide (226Ra and 232Th) concentrations in collected soil samples from the same locations were also evaluated. The result of the measurement shows that mostly the distribution of radon and thoron is similar to 226Ra and 232Th, eventhough it was not a good correlation between radon and thoron exhalation rate with their parent activity concentrations (226Ra and 232Th) due to the environmental factors that can influence the radon and thoron mobilities in the soil. In comparison to a world average, Bangka Belitung Islands have the 222Rn and 220Rn exhalation rates higher than the world average value for the regions with normal background radiation.


Introduction
Radon ( 222 Rn) and thoron ( 220 Rn) are radioactive gases produced by the decay of 226 Ra and 224 Ra, which are themselves the decay products of 238 U and 232 Th series in the ground, respectively. 222 Rn and 220 Rn decay with the emission of alpha particles and produce daughter nuclei -polonium ( 218 Po, 216 Po, 214 Po, 212 Po), lead ( 214 Pb, 212 Pb, 210 Pb), and bismuth ( 214 Bi, 212 Bi, 210 Bi). These daughter nuclei emit alpha or beta particles. 222 Rn has a half-life of 3.825 days and is an alpha emitter; 220 Rn has a half-life of 55.6 s and is also an alpha emitter ( Figure 1) (Porstendorfer, 1994;Ramach-andan and Sathish, 2011). 222 Rn having an atomic number of 86, is the heaviest member of the rare gas group (∼ 100 times heavier than hydrogen and ∼ 7.5 times heavier than air). 220 Rn is an isotope of radon, that has an atomic number of 86, and mass number of 220. The main characteristic of 222 Rn and 220 Rn among the other natural radioactive elements is the fact that their behaviour is chemically inert (noble gases), not affected by chemical processes. The 222 Rn and 220 Rn are free to move through soil pores and rock fractures; then to escape into the atmosphere. 222 Rn and/or 220 Rn exhaled from the earth surface into the free atmosphere is rapidly dispersed and diluted by I J O G natural convection and turbulence (Mudd, 2008;Hassan et al., 2011). 222 Rn and 220 Rn in the ambient air depend on the soil conditions and the local geological background. 222 Rn and 220 Rn emanate mainly from the earth surface through the gap in soil to the atmosphere. 222 Rn and 220 Rn gases enter the house from various gaps in walls and open windows or doors. They decay producing isotopes of polonium ( 218 Po, 216 Po, 214 Po, 212 Po), lead ( 214 Pb, 212 Pb, 210 Pb), and bismuth ( 214 Bi, 212 Bi, 210 Bi) which are heavy metals chemically very active, that may exist briefly as ions and/or free atoms before forming molecules in a condensed phase or attached to airborne dust particles, forming radioactive aerosols. This fraction may be inhaled and deposited in the respiratory tract, in which they release all their α-emissions. It is, therefore, important to know the 222 Rn and 220 Rn exhalation, and these information are useful for presumption of a high 222 Rn-220 Rn concentration area.
Bangka and Belitung Islands have the geological potential of mineral resources, especially tin, with accessory minerals consisting of monazite, zircon, xenotim, ilmenite, magnetite, and pyrite spreading in almost all regions. Bangka Belitung Islands are known as tin producer places which form a part of Southeast Asia Tin Belt, the richest tin belt in the world which stretches along South China -Thailand -Myanmar -Malaysia to Indonesia (Schwartz et al., 1995). Meanwhile, there are some other mineral resources like: quartz sand, building construction sand, kaolin, granite, clay, and mountain stone. The geology of Bangka Belitung is structured by granites (hard stone). Itis generally covered by klabat granite which are devided into three catagories, i.e. biotite granite, granodiorite, and gneissic granite. The soil derived from granite will have a higher radioactivity than the soil from the other rock types (Saleh andRamli, 2013: Rani andSingh, 2005).
Generally, when the 226 Ra and 232 Th are high, the exhalation rates of 222 Rn and 220 Rn at that site are a relatively high too. Therefore, this study was conducted with the 222 Rn and 220 Rn exhalation rates from the ground surface in Bangka-Belitung. The objective of this work is to determine 222 Rn and 220 Rn exhalation rates from the soil surfaces of Bangka -Belitung Islands and to evaluate the correlation with their parent radionuclides ( 226 Ra and 232 Th). The study will

I J O
G help in understanding the status of indoor and outdoor 222 Rn, 220 Rn and the status of the exhalation of these gases from soil in Bangka-Belitung. This type of study has never been done before in Bangka Belitung Islands. Bangka-Belitung Islands are located at 104° 50' -109° 30' E and 0° 50' -4° 10' S which lie in east of Sumatra, northeast of South Sumatra Province (Figure 2).

Material
The field study was performed on thirty six sampling sites where twenty eight sites were at Bangka Island and ten sites were at Belitung Island. Geographical coordinates of the sampling points were determined using GPS Map 60CHx manufactured by Garmin. After finding the measuring site, any grass, gravel, and roots were removed to perform the measurements of 222 Rn and 220 Rn exhalation rates. Then, at least 2 -3 kg of soil was collected at each point using shovel and scoop. At a collection point the soil sample was wrapped in black plastic bag and then taken to the laboratory.

Measurement of 222 Rn and 220 Rn Exhalation Rates from Surface Soil
In this study, the method for 222 Rn and 220 Rn exhalation rate measurement is based on small accumulation chambers connected to a continuous Radon Gas Monitor, model RAD7 ( Figure  3), produced by Durridge Company Inc. (2010), equipped with a solid state alpha detector (RAD7, Durridge Co. Inc., Bedford, MA, USA). The accumulation chamber made of stainless steel as 222 Rn and 220 Rn accumulation chambers is connected to the RAD7 detector by vinyl tubing with a gas-drying unit filled with a desiccant (CaSO 4 with 3% CaCl 2 ) to maintain the relative humidity at <10% within the measurement system. The system is a closed loop in which the gas circulates continuously.
The accumulation chamber was placed on the ground surface from its open side, and its surroundings were covered to prevent any air exchange with environment. Tamping down the soil around the chamber is to prevent the leakage of fresh air into the sample acquisition path or down the outside of the chamber to sampling point. Following the localization of the measured points using GPS devices and clearing the rel-  It is noteworthy that the environmental parameters such as temperature, pressure, and relative humidity were recorded by the device during the period of measuring each site.
The changes in the 222 Rn and 220 Rn concentration in the chamber were used as a function of time to estimate the exhalation rate from the ground surface. The concentration of 222 Rn and 220 Rn exhaled from the sample increases exponentially until radioactive secular equilibrium is reached. The exhalation rate (E 0 ) from the sample can be calculated with equation (Tuccimei et al., 2006;Hassan et al., 2011): where: C is the net concentration (exhaled radon/ thoron less the background concentration) at accumulation time t (Bqm -3 ), λ is the decay constant (s -1 ), V is the effective air volume (m 3 ), and S is the sample surface area (m 2 ).

Measurement of 226 Ra and 232 Th in Surface Soil
The measurement of 226 Ra and 232 Th in the soil samples collected from the same site with 222 Rn and 220 Rn exhalation rates were carried out at a laboratory by using ORTEC P-type coaxial high purity Germanium (HPGe) detector with a relative efficiency of 60% and a resolution of 1.95  keV (full width at half maximum) for the peak of 1,33 keV. In the laboratory, the soil samples were dried in an oven at a temperature of 105 0 C to a constant weight to remove any available moisture. After being dried, the samples were crushed and sieved with a mesh having holes each of diameter of 2 mm in order to remove organic materials, stones, and lumps. Afterwards, the homogenized samples were packed to fill 1 liter marinelli beakers. The marinelli beakers were carefully sealed in order to prevent trapped radon gas from escape and allowed to stand for at least four weeks for secular equilibrium to be established between the long-lived parent nuclides of 226 Ra and 232 Th, and their short-lived daughters before measurement. The gamma energy peaks 352 keV of 214 Pb and 609.31 keV of 214 Bi were used to determine 226 Ra. The gamma energy peaks of 238.6 keV from 212 Pb, 911.2 and 969 keV gamma energy peak from 228 Ac and 583 keV gamma energy peak from 208 Tl were used to determine the 232 Th. The activity concentrations (A) of 226 Ra and 232 Th in Bq kg -1 for the samples were determined using the following expression (Knoll, 2000;Syarbaini et al., 2014): where: N e = net counts of a peak at energy E, ε f = the counting efficiency of the detector system at energy E, P γ = the gamma ray emission probability (gamma yield) at energy E, t c = sample counting time, M = mass of sample (kg).

Result and Discussion
The measurement result of 222 Rn and thoron exhalation rates is shown in Table 1. It can be seen in Table 1 that   exhalation rates by all thirty six data were obtained to be 48.11 mBq.m -2 . s -1 , 2008 mBq.m -2 . s -1 , respectively. The distribution of 222 Rn and 220 Rn exhalation rates was indicated in Figure 4. This figure shows that 222 Rn and 220 Rn exhalation rates vary widely from site to site.  Figure 4. The distribution graph of radon and thoron exhalation rate levels. Seventy five percent of 222 Rn exhalation rate measurement results are more than 10 mBq.m -2 . s -1 and 8.3 % of its are more than 100 mBq.m -2 . s -1 , however 7.5 % of its are lower than 10 mBq.m -2 . s -1 . Most of 220 Rn exhalation rates are higher than 222 Rn exhalation rate, where 47.2 % of its are more than 100 mBq.m -2 .s -1 and 52.8 % of results had a exhalation rate of more than 1000 mBq.m -2 . s -1 . It was admitted that the 220 Rn exhalation rate was about forty two times of the 222 Rn exhalation rate. In order to evaluate how 222 Rn and 220 Rn exhalation rates are influenced by the activity concentration of their precursors ( 226 Ra and 232 Th, respectively), the variation of 222 Rn and 220 Rn exhalation rates were observed to variations of radium and thorium concentrations in soil collected from the same site. The concentration of 226 Ra and 232 Th in soil samples collected from the same measurement sites were shown in Table 1, columns 6 and 7. Generally, the distribution of 226 Ra and 232 Th in soil showed the same tendency as 222 Rn and 220 Rn distribution. By using its concentration levels, the correlation between the 222 Rn exhalation rate and the 226 Ra concentration and between the 220 Rn exhalation rate and the 232 Th concentrations were presented as in Figures 5 and 6.
As can be seen in Figures 5 and 6, it was found a weak correlation between the 222 Rn  exhalation rate and the radium concentration (R = 0.67), and between the 220 Rn exhalation rate and the thorium concentration (R = 0.51).
The weak correlation may be influenced by environmental factors, such as weather, water content of soil, and geology. According to Sun et al. (2004), the main factors influencing 222 Rn diffusion in soil are the soil characteristics such as soil porosity and moisture. Advection takes place when there is pressure difference between the airs of pore space and ground surface. The most important factor affecting advection is the soil permeability. Other meteorological parameters like temperature difference between soil and surface air, wind velocity, and rainfall also affect the advection process. Hosoda et al. (2007) reported that the exhalation rate showed a decreasing tendency for the increase in the moisture content over 8 %.
At the measurement day, the soil condition was different from each other such as dry, semimoist, and wet. The weather conditions on the day of measurement were partly cloudy, partly sunny, and cloudy. Based on the conducted studies, it was observed that with reduced soil moisture and weather conditions changing into sunny, 222 Rn and 220 Rn exhalation rates increase.
In comparison to a world average, the results indicate that the 222 Rn and 220 Rn exhalation rates for the studied area are much higher than the worldwide average. UNSCEAR (2000) report shows that the world averages of the 222 Rn and 220 Rn exhalation rates are 26.2 mBq.m -2 . s -1 and 1000 mBq.m -2 .s -1 , respectively. Thus, the obtained value in this study were twice of values shown in UNSCEAR (2000) report. This fact suggests that the 222 Rn and 220 Rn exhalation rates in Bangka-Belitung Islands (Figure 7) must be considered to assess the radiological hazard of living in these areas. 222 Rn and/or 220 Rn exhaled from the surface soil may migrate into the structure of dwelling and accumulate indoors in sufficient quantities to pose a health hazard. WHO (2009) has classified them as carcinogenic to humans and the second most important cause of lung cancer after cigarette smoking. WHO (2009) recommended the levels of radon in the residental buildings as 100 Bqm -3 .

Conclussion
The 222 Rn and 220 Rn exhalation rates from surface soil of Bangka and Belitung Islands have been determined in situ measurement by using an accumulation chamber equipped with a solid-state alpha particle detector of RAD7. Then, the activity concentrations of parent radionuclides ( 226 Ra and 232 Th) in soil samples collected from the same site have been determined in the laboratory by using gamma-ray spectroscopy. The result of measurement showed that 222 Rn and 220 Rn exhalation rates as well as activity concentrations of 226 Ra and 232 Th varied widely from site to site. Mostly, the distribution of 226 Ra and 232 Th showed the same tendency as 222 Rn and 220 Rn distribution, but it was not any strong correlation due to the influence of environmental factors, such as weather, water content of soil, pressure, temperature, and humidity conditions. All the measurement result showed that the 220 Rn exhalation rate was higher than the 222 Rn exhalation rate. From this study, it was also found that Bangka Belitung Islands have the 222 Rn and 220 Rn exhalation rate higher than the world average value reported by UNSCEAR. The geology of Bangka Belitung Islands is covered by granite which has a higher radioactivity concentration level than the soil from the common areas world average. Due to high level of 222 Rn and 220 Rn exhalation rates in some areas of the BangkaBelitung Islands, necessary provisions should be considered for construction in these areas to avoid 222 Rn and 220 Rn entrance into residential buildings.