Nationwide indoor 222Rn and 220Rn map for India: a review

Considering the role of radon in epidemiology, an attempt was made to make a nation-wide map of indoor ²²²Rn and ²²⁰Rn for India. More than 5000 measurements have been carried out in 1500 dwellings across the country comprising urban and nonurban locations. The solid state nuclear track detectors based twin cup ²²²Rn/²²⁰Rn discrimination dosimeters were deployed for the measurement of indoor ²²²Rn, ²²⁰Rn and their progeny levels. The geometric means of estimated annual inhalation dose rate due to indoor ²²²Rn, ²²⁰Rn and their progeny in the dwellings was 0.94 mSvy⁻¹ (geometric standard deviation 2.5). It was observed that the major contribution to the indoor inhalation dose was due to indoor ²²²Rn and its progeny. However, the contribution due to indoor ²²⁰Rn and its progeny was not trivial as it was found to be about 20% of the total indoor inhalation dose rates. The indoor ²²²Rn levels in dwellings was significantly different depending on the nature of walls and floorings.     

A theoretical approach to indoor radon and thoron distribution

A model based on the Finite Element Method was developed to simulate indoor behavior of radon ((222)Rn), thoron ((220)Rn) and their progeny, as well as, to calculate their spatial distributions. Since complex physical processes govern the distribution several simplifications were made in the presented model. Different locations of possible radon/thoron sources, diffusion of these gases, their radioactive decay, etc were taken into account. Influences of different parameters on thoron/radon as well as indoor distribution of their progeny, such as the geometry and room dimension, the presence of aerosols and their size distribution expressed through the diffusion coefficient, different kinds of ventilation, etc, were investigated. It has been found that radon is distributed homogeneously, while the thoron concentration is rather inhomogeneous and decreases exponentially with the distance from the source. Regardless of the source distribution, the distribution of radon was homogeneous, except at places near an air inlet and outlet. However, the distribution of thoron depends on the source distribution. If thoron emanates from walls or the floor, its concentration decreases with the distance from the wall. Moreover, the concentration gradient is much larger near walls. This suggests that the actual selection of the site effect should be taken into account when obtaining a representative value of indoor (220)Rn and their progeny for dose assessment. The simulation results of activities and their distribution were in accordance with the results of other studies and experiments.     

Development of an energy discriminate CR-39(®) nuclear track etch dosimeter for Radon-220 gas measurements

An energy discriminate CR-39^® nuclear track etch dosimeter for use in a ²²⁰Rn and ²²²Rn gas monitor has been developed and experimentally assessed. It utilises a thin film of Mylar^® C to attenuate the alpha particle energies to allow only the damage tracks created by the 8.785 MeV alpha particles emitted from ²¹²Po of the ²³²Th decay chain to be registered in the CR-39^® plaque, allowing for the direct measurement of ²²⁰Rn gas concentrations. The dosimeter was developed through a combination of experimental investigations and theoretical simulations using the Monte Carlo ion transport modelling program Stopping and Range of Ions in Materials (SRIM 2008). A film thickness of 54 μm has been shown to attenuate all alpha energies less then 7.7 MeV.     

Observations and modelling of thoron and its progeny in the soil-atmosphere-plant system

Samples of pasture vegetation, mainly Trifolium pratensis, were collected at the Botanic Garden of the University of Bologna during the period 1998-2000 and measured by gamma-spectrometry for determining thoron progeny. Concentrations of ²¹²Pb were between 1.5 and 20 Bq m⁻², with individual peaks up to 70 Bq m⁻². Soil samples were collected at the same location and physically characterised. Their chemical composition (particularly Th and U) was determined by X-ray fluorescence spectroscopy. Lead-212 on plants mainly originates from dry and wet deposition of this isotope generated in the lower atmosphere by the decay of its short-lived precursor ²²⁰Rn, which is produced in the upper soil layers as a member of the natural thorium decay chain and exhales into the atmosphere. Concentrations of ²²⁰Rn in the atmosphere depend on (1) the amount of Th present in soil, (2) the radon fraction which escapes from the soil minerals into the soil pore space, (3) its transport into the atmosphere, and (4) its redistribution within the atmosphere. The mobility of radon in soil pore space can vary by orders of magnitude depending on the soil water content, thus being the main factor for varying concentrations of ²²⁰Rn and ²¹²Pb in the atmosphere. We present a simple model to predict concentrations of thoron in air and its progeny deposited from the atmosphere, which takes into account varying soil moisture contents calculated by the OPUS code. Results of this model show close agreement with our observations.     

Dynamics, deterministic nature and correlations of outdoor (222)Rn and (220)Rn progeny concentrations measured at Bacău, Romania

The long-term variation, nature and correlations of outdoor ²²²Rn and ²²⁰Rn progeny concentrations measured during the period 1994-2009 were investigated. The time series of data were obtained within the framework of the monitoring program performed by the Environmental Radioactivity Monitoring Station (ERMS) Bac?u, a component part of the National Environmental Radioactivity Survey Network (NERSN), coordinated by National Environmental Protection Agency (NEPA). The measuring method is based on the total beta measurements of atmospheric aerosol filters, using a low background total beta counter and (⁹⁰Sr/Y) reference standard. Analysis of the time series of progeny concentrations in the low atmosphere makes evident different patterns of variation of these concentrations: diurnal, seasonal and annual. A possible relationship of progeny concentration increase with global warming is emphasized. In order to find the dominant frequency of the physical processes determining progeny concentration variability the power spectrum has been used. The deterministic nature of the time series of concentrations has been studied making use of the autocorrelation function and stationarity of the original data and of their phase randomized time series. Also, the correlations with meteorological parameters have been investigated using Pearson’s correlation coefficient with corresponding level of significance.     

Variation of short-lived beta radionuclide (radon progeny) concentrations and the mixing processes in the atmospheric boundary layer

Radon is emitted to the atmosphere with quasi constant emission rates depending on the radium concentration in the earth's crust and soil physical properties. In this way, the ²²²Rn and ²²⁰Rn concentration in air reflects significantly the thickness of the atmospheric boundary layer (ABL). The aerosol-associated, beta-emitting progeny nuclides of ²²²Rn were measured daily in the framework of the atmospheric radioactivity monitoring program of NIMH at Sofia. The ²¹⁴Pb concentration was estimated from the measured short-lived beta activity of 24-h filter samples, changed daily at 6:00 GMT. The impact of some meteorological factors such as wind direction, wind velocity, humidity, and temperature on short-lived beta radionuclides is estimated, and the results show no simple statistical relationship. A seasonal pattern was observed with winter minima and late summer-early autumn maxima. High variability in daily morning concentrations and mean monthly values was observed. There were well pronounced differences between years. The height of the convective ABL was estimated from daily radio-soundings at 12:00 GMT for the period 2001-2006 and from seven soundings per day during the experimental campaign in Sofia in October 2003. In general, concentrations of short-lived ²²²Rn progeny nuclides decreased with increased convective ABL height.     

Development and calibration of a portable radon sampling system for groundwater Rn activity concentration measurements

The assembling of a system for field sampling and activity concentration measurement of radon dissolved in groundwater is described. Special attention is given in presenting the calibration procedure to obtain the radon activity concentration in groundwater from the raw counting rate registered in a portable scintillation detector and in establishing the precision of the activity concentration measurements. A field procedure was established and the system tested during one year of monthly observations of ²²²Rn activity concentration in groundwater drawn from two wells drilled on metamorphic rocks exposed at Eastern São Paulo State, Brazil. The observed mean ²²²Rn activity concentrations are 374 Bq/dm³ in one well and about 1275 Bq/dm³ in the other one. In both wells the ²²²Rn activity concentrations showed a seasonal variation similar to variations previously reported in the literature for the same region.     

Possible effect of solar tides on radon signals

Large temporal variations of radon (²²²Rn) are often encountered in air in the geologic environment, at time scales from diurnal to annual. Interpretations as to the nature of these variations, unique to ²²²Rn, often invoke either above surface atmospheric variations, or the influence of subtle active geodynamic processes. So far the eventual geophysical drivers of the variation of ²²²Rn as well as its specific qualities enabling this temporal variation are not known. New insight on the temporal variation of ²²²Rn is gained by experimental simulation in confined air. Two short laboratory experiments, and one external experiment lasting over 3 years, were performed inside closed canisters and using natural and commercial ²²²Rn sources. Internal and external gamma and alpha detectors recorded variations of the radiation, up to around 20% of the equilibrium level. Radon signals of different time scale occurred with: a) periodic annual and semi-annual signals; b) non-periodic multi-day signals; c) periodic daily signals. Similar, related, inversely-related and dissimilar temporal patterns were manifested in the measured time series of the different sensors. Diurnal periodicity was dominated by the solar tide components S1, S2 and S3, exhibiting unlike relative amplitudes and different phases at the different sensors. A compound association occurs among the amplitudes and phases of the diurnal and seasonal periodicities of the daily ²²²Rn signal, linking the periodic phenomena to the rotation of earth around its axis and around the sun. ²²²Rn variation patterns in the frequency-time domain cannot be driven by the corresponding atmospheric variation patterns. These results, obtained under static and isolated conditions, are in disagreement with the expected radioactive equilibrium and its spatially uniform expression within and around the experimental volume. The external influence which drives the daily signals evolving from ²²²Rn inside the canister is non-atmospheric and seemed to be from a remote source and traversed a 5-cm thick lead shield. The similarities with observations on ²²²Rn signals from upper crustal levels imply that such an external influence, possibly as a component of solar irradiance, drives the ²²²Rn signals to a depth of at least 100 m. New combined prospects for the research are indicated in terms of the radioactive behavior of ²²²Rn in air and in terms of an above surface geophysical driver for this behavior.     

Dose estimation and radon action level problems due to nanosize radon progeny aerosols in underground manganese ore mine

One of the essential parameters influencing of the dose conversion factor is the ratio of unattached short-lived radon progeny. This may differ from the value identified for indoor conditions when considering special workplaces such as mines. Inevitably, application of the dose conversion factors used in surface workplaces considerably reduces the reliability of dose estimation in the case of mines.This paper surveyed the concentration of radon and its short-lived radon progeny and identified the unattached fraction of short-lived radon progeny. As well equilibrium factor during the month of August was calculated simultaneously at two extraction faces in a manganese ore mine.During working hours the average radon concentrations were 220 Bq m⁻³ and 530 Bq m⁻³ at Faces 1 and 2; the average short-lived progeny concentration was 90 Bq m⁻³ and 190 Bq m⁻³, the average equilibrium factors were 0.46 and 0.36, and the average unattached fractions were 0.21 and 0.17, respectively. The calculated dose conversion factor was between 9 and 27 mSv WLM⁻¹, but higher values could also be possible.     

Inhalation dose assessment of indoor radon progeny using biokinetic and dosimetric modeling and its application to Jordanian population

High indoor radon concentrations in Jordan result in internal exposures of the residents due to the inhalation of radon and its short-lived progeny. It is therefore important to quantify the annual effective dose and further the radiation risk to the radon exposure. This study describes the methodology and the biokinetic and dosimetric models used for calculation of the inhalation doses exposed to radon progeny. The regional depositions of aerosol particles in the human respiratory tract were firstly calculated. For the attached progeny, the activity median aerodynamic diameters of 50 nm, 230 nm and 2500 nm were chosen to represent the nucleation, accumulation and coarse modes of the aerosol particles, respectively. For the unattached progeny, the activity median thermodynamic diameter of 1 nm was chosen to represent the free progeny nuclide in the room air. The biokinetic models developed by the International Commission on Radiological Protection (ICRP) were used to calculate the nuclear transformations of radon progeny in the human body, and then the dosimetric model was applied to estimate the organ equivalent doses and the effective doses with the specific effective energies derived from the mathematical anthropomorphic phantoms. The dose conversion coefficient estimated in this study was 15 mSv WLM⁻¹ which was in the range of the values of 6-20 mSv WLM⁻¹ reported by other investigators. Implementing the average indoor radon concentration in Jordan, the annual effective doses were calculated to be 4.1 mSv y⁻¹ and 0.08 mSv y⁻¹ due to the inhalation of radon progeny and radon gas, respectively. The total annual effective dose estimated for Jordanian population was 4.2 mSv y⁻¹. This high annual effective dose calculated by the dosimetric approach using ICRP biokinetic and dosimetric models resulted in an increase of a factor of two in comparison to the value by epidemiological study. This phenomenon was presented by the ICRP in its new published statement on radon.     

Exerpts from the history of alpha recoils

Any confined air volume holding radon (²²²Rn) gas bears a memory of past radon concentrations due to ²¹⁰Pb (T_1/2 = 22 y) and its progenies entrapped in all solid objects in the volume. The efforts of quantifying past radon exposures by means of the left-behind long-lived radon progenies started in 1987 with this author’s unsuccessful trials of removing ²¹⁴Po from radon exposed glass objects. In this contribution the history and different techniques of assessing radon exposure to man in retrospect will be overviewed. The main focus will be on the implantation of alpha recoils into glass surfaces, but also potential traps in radon dwellings will be discussed. It is concluded that for a successful retrospective application, three crucial imperatives must be met, i.e. firstly, the object must persistently store a certain fraction of the created ²¹⁰Pb atoms, secondly, be resistant over decades towards disturbances from the outside and thirdly, all ²¹⁰Pb atoms analysed must originate from airborne radon only.For large-scale radon epidemiological studies, non-destructive and inexpensive measurement techniques are essential. Large-scale studies cannot be based on objects rarely found in dwellings or not available for measurements     

Radon concentration in soil gas around local disjunctive tectonic zones in the Krakow area

The purpose of this study was to investigate radon in the vicinity of geologic fault zones within the Krakow region of Poland, and to determine the influence of such formations on enhanced radon concentrations in soil. Radon ((222)Rn and (220)Rn) concentration measurements in soil gas (using ionization chamber AlphaGUARD PQ2000 PRO and diffusion chambers with CR-39 detectors), as well as radioactive natural isotopes of radium, thorium and potassium in soil samples (using gamma ray spectrometry with NaI(Tl) and HPGe detectors), were performed. Site selection was based on a geological map of Krakow. Geophysical methods (ground penetrating radar and shallow acoustic seismic) were applied to recognize the geological structure of the area and to locate the predicted courses of faults. Elevated levels of radon and thoron in soil gas were found in the study area when compared with those observed in an earlier survey covering Krakow agglomeration. For (222)Rn, the arithmetic mean of registered concentration values was 39 kBq/m(3) (median: 35.5 kBq/m(3)). For (220)Rn, the arithmetic mean was 10.8 kBq/m(3) and median 11.8 kBq/m(3).     

Inverse modeling of Asian Rn flux using surface air Rn concentration

When used with an atmospheric transport model, the ²²²Rn flux distribution estimated in our previous study using soil transport theory caused underestimation of atmospheric ²²²Rn concentrations as compared with measurements in East Asia. In this study, we applied a Bayesian synthesis inverse method to produce revised estimates of the annual ²²²Rn flux density in Asia by using atmospheric ²²²Rn concentrations measured at seven sites in East Asia. The Bayesian synthesis inverse method requires a prior estimate of the flux distribution and its uncertainties. The atmospheric transport model MM5/HIRAT and our previous estimate of the ²²²Rn flux distribution as the prior value were used to generate new flux estimates for the eastern half of the Eurasian continent dividing into 10 regions.The ²²²Rn flux densities estimated using the Bayesian inversion technique were generally higher than the prior flux densities. The area-weighted average ²²²Rn flux density for Asia was estimated to be 33.0 mBq m⁻² s⁻¹, which is substantially higher than the prior value (16.7 mBq m⁻² s⁻¹). The estimated ²²²Rn flux densities decrease with increasing latitude as follows: Southeast Asia (36.7 mBq m⁻² s⁻¹); East Asia (28.6 mBq m⁻² s⁻¹) including China, Korean Peninsula and Japan; and Siberia (14.1 mBq m⁻² s⁻¹). Increase of the newly estimated fluxes in Southeast Asia, China, Japan, and the southern part of Eastern Siberia from the prior ones contributed most significantly to improved agreement of the model-calculated concentrations with the atmospheric measurements. The sensitivity analysis of prior flux errors and effects of locally exhaled ²²²Rn showed that the estimated fluxes in Northern and Central China, Korea, Japan, and the southern part of Eastern Siberia were robust, but that in Central Asia had a large uncertainty.     

Radon mapping by retrospective measurements - an approach based on CDs/DVDs

This article points out the ability to map retrospective ²²²Rn concentrations by home stored CDs/DVDs. The method employs the high radon absorption ability of the polycarbonate material of CDs and DVDs and their track-etch properties. The principle steps for the application of the method are addressed. The possibility for individual a posteriori calibration is studied, demonstrating that better than 10% accuracy in retrospective measurements is potentially achievable. Results from retrospective measurements in three different regions are shown, demonstrating the potential of the method for large-scale radon mapping. Comparison with independently evaluated retrospective ²²²Rn concentrations in places with known radon history was made and very good correspondence was observed. The experience indicates that the method can be used for large scale retrospective radon mapping and its applications can be expanded towards mapping of radon concentrations in water and soil gas.     

Effect of moisture content on emanation at different grain size fractions - A pilot study on granitic esker sand sample

It is known that in soils and sediments moisture adsorbed on particle surfaces and in the pore system significantly affects the behaviour of recoiling radon (²²²Rn) atoms after decay of parent ²²⁶Ra, leading to increased ²²²Rn emanation. As a first step in an effort to characterize the ²²²Rn source term in mineralised sediments in the present study, complementing previous studies in the area, granitic esker sand samples were collected in order to test how moisture content affects ²²²Rn emanation at different grain size fractions. Emanation fractions measured for natural samples were compared with theoretical calculations. Six different grain size fractions were studied at 0%, 5% and 10% moisture contents relative to the mass of solids. In a further study necessary complementary information on the chemical and structural distribution of ²²⁶Ra was gained by selective leaching experiments. The results showed that ²²⁶Ra concentration increases from 50 Bq/kg at grain size 1-2 mm to 200 Bq/kg at grain size <0.063 mm. Respectively, the emanation factor increased from 0.12 to 0.30 at 5% moisture content. Both emanation factor and radium concentration increased significantly when grain size was below 0.125-0.250 mm. Above this fraction, the emanation fraction was approximately constant, 0.13 at 5% moisture content. In most of the grain size fractions, emanation reaches its maximum at 5% moisture content, being twice as high as in a dry sample. For the small particles (<0.063 mm) the ²²⁶Ra distribution is rather complex and depends on the mineral composition compared to larger particles wherein emanation from the internal pore system and the adjacent matrix is dominating over the contribution from external surface.     

Absorbed dose in target cell nuclei and dose conversion coefficient of radon progeny in the human lung

To calculate the absorbed dose in the human lung due to inhaled radon progeny, ICRP focussed on the layers containing the target cells, i.e., the basal and secretory cells. Such an approach did not consider details of the sensitive cells in the layers. The present work uses the microdosimetric approach and determines the absorbed alpha-particle energy in non-spherical nuclei of target cells (basal and secretory cells). The absorbed energy for alpha particles emitted by radon progeny in the human respiratory tract was calculated in basal- and secretory-cell nuclei, assuming conical and ellipsoidal forms for these cells. Distributions of specific energy for different combinations of alpha-particle sources, energies and targets are calculated and shown. The dose conversion coefficient for radon progeny is reduced for about 2mSv/WLM when conical and ellipsoidal cell nuclei are considered instead of the layers. While changes in the geometry of secretory-cell nuclei do not have significant effects on their absorbed dose, changes from spherical to conical basal-cell nuclei have significantly reduced their absorbed dose from approximately 4 to approximately 3mGy/WLM. This is expected because basal cells are situated close to the end of the range of 6MeV alpha particles. This also underlines the significance of better and more precise information on targets in the T-B tree. A further change in the dose conversion coefficient can be achieved if a different weighting scheme is adopted for the doses for the cells. The results demonstrate the necessity for better information on the target cells for more accurate dosimetry for radon progeny.     

Modeling the Pb activity concentration in the lower atmosphere

A worldwide radionuclide network of 80 stations, part of the International Monitoring System, is being setup to monitor compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The radioactivity sampled at these stations is primarily ²²⁰Rn progenies affecting the detection capability. A model linking the ²²⁰Rn emanation with the sampled ²¹²Pb activity was developed and is presented here. The model and the performed measurements show that the variation of the sampled ²¹²Pb activity can be fully explained by the variation of the local ²²⁰Rn activity concentration.     

Mean annual (222)Rn concentration in homes located in different geological regions of Poland: first approach to whole country area

The paper presents the results of year-long measurements of radon (²²²Rn) concentration inside 129 buildings in Poland in relation to the geological conditions of their foundation. The authors took into account the division of the country into tectonic units, as well as the lithology of the rocks forming the bedrock of these buildings. As expected, the highest value of mean annual ²²²Rn concentration (845 Bq/m³) was recorded in a building situated in the area of the Sudetes, while the highest geometric mean (characteristic of the expected log-normal data distribution) was calculated based on measurements from buildings located within the East-European craton, in the area of Mazury-Podlasie monocline, where it reached 231 Bq/m³. Such results reflect geological conditions - the occurrence of crystalline rocks (especially U- and Ra-enriched granites and orthogneisses) on the surface in the Sudetes, and of young post-glacial sediments containing fragments of Scandinavian crystalline rocks, also enriched with U and Ra, in the area of Mazury-Podlasie monocline. However, the least expected result of the investigations was finding out that, contrary to the hitherto widespread belief, none of the major tectonic units of Poland can be excluded from the list of those containing buildings with mean annual ²²²Rn concentration exceeding 200 Bq/m³. The mean annual concentration of radon for all the buildings were much higher than the mean concentration value (49.1 Bq/m³) of indoor radon in Poland quoted so far. These results cast a completely new light on the necessity to perform measurements of radon concentration in residential buildings in Poland, no more with reference to small areas with outcrops of crystalline rocks (especially the Sudetes, being the Polish fragment of the European Variscan belt), but for all the major tectonic units within Poland.     

A nationwide survey of outdoor radon concentration in Japan

Nationwide outdoor radon (222Rn) concentrations in Japan were measured to survey the environmental outdoor 222Rn level and to estimate the effective dose to the general public from 222Rn and its progeny. The 222Rn concentration was measured with a passive-type radon monitor. The 222Rn monitors were installed at about 700 points throughout Japan from 1997 to 1999. The annual mean 222Rn concentration in Japan was estimated from four quarters measurements of 47 prefectures in Japan. Nationwide outdoor mean 222Rn concentration was 6.1 Bq m(-3). This was about 40% of the indoor 222Rn concentration in Japan. The 222Rn concentration in Japan ranged from 3.3 Bq m(-3) in the Okinawa region to 9.8 Bq m(-3) in the Chugoku region, reflecting geological characteristics. Seasonal variation of outdoor 222Rn concentration was also found to be lowest in July to September, and highest in October to December. From the results of this 222Rn survey and previous indoor 222Rn survey program, the effective dose to the general public from 222Rn and its progeny was estimated to be 0.45 mSv y(-1).     

Seasonal variations of 222Rn concentrations in the air of a tunnel located in Nagano city

The seasonal variation of 222Rn concentrations in the air of tunnels constructed during World War II at Nagano City has been investigated. The determination of 222Rn concentrations in tunnel air was performed using a solid-state nuclear track detector technique. The monthly radon concentrations changed smoothly, decreasing towards winter and increasing towards summer, and it was found that the concentrations strongly correlate with the temperature difference between the inside and the outside of the tunnel. In the innermost areas of the tunnel, the maximum concentration was observed in July, its value being about 6500 Bq m (-3). The concentrations of radon in the tunnel air decrease exponentially towards the openings of the tunnel, which indicates that the radon concentration in the tunnel is basically governed by diffusion and mixing of radon gas with air. These observations lead to the conclusion that the seasonal variation of the radon concentration in the tunnel air is mainly caused by a convection current due to a stack effect induced by the temperature difference between the tunnel air and the outside air.