Development of a "222Rn incremented method" for the rapid determination of air exchange rate using soil gas.

The air exchange rate (AER) is a critical parameter that governs the levels of exposure to indoor pollutants impacting occupants' health. It has been recognized as a crucial metric in spreading COVID-19 disease through airborne routes in shared indoor spaces. Assessing the AER in various human habitations is essential to combat such detrimental exposures. In this context, the development of techniques for the rapid determination of the AER has assumed importance. AER is generally determined using CO2 concentration decay data or other trace gas injection methods. We have developed a new method, referred to as the "222Rn incremented method", in which 222Rn from naturally available soil gas was injected into the workplace for a short duration (∼30 min), homogenized and the profile of decrease of 222Rn concentration was monitored for about 2 h to evaluate AER. The method was validated against the established 222Rn time-series method. After ascertaining the suitability of the method, several experiments were performed to measure the AER under different indoor conditions. The AER values, thus determined, varied in a wide range of 0.36-4.8 h-1 depending upon the ventilation rate. The potential advantages of the technique developed in this study over conventional methods are discussed.

A comprehensive modelling approach to estimate the transmissibility of coronavirus and its variants from infected subjects in indoor environments.

A central issue in assessing the airborne risk of COVID-19 infections in indoor spaces pertains to linking the viral load in infected subjects to the lung deposition probability in exposed individuals through comprehensive aerosol dynamics modelling. In this paper, we achieve this by combining aerosol processes (evaporation, dispersion, settling, lung deposition) with a novel double Poisson model to estimate the probability that at least one carrier particle containing at least one virion will be deposited in the lungs and infect a susceptible individual. Multiple emission scenarios are considered. Unlike the hitherto used single Poisson models, the double Poisson model accounts for fluctuations in the number of carrier particles deposited in the lung in addition to the fluctuations in the virion number per carrier particle. The model demonstrates that the risk of infection for 10-min indoor exposure increases from 1 to 50% as the viral load in the droplets ejected from the infected subject increases from 2 × 108 to 2 × 1010 RNA copies/mL. Being based on well-established aerosol science and statistical principles, the present approach puts airborne risk assessment methodology on a sound formalistic footing, thereby reducing avoidable epistemic uncertainties in estimating relative transmissibilities of different coronavirus variants quantified by different viral loads.

Translation-deformation coupling effects on the Rayleigh instability of an electrodynamically levitated charged droplet.

The breakup pathway of the Rayleigh fission process observed in the past experiments carried out using high-speed imaging of a charged drop levitated in an AC quadrupole trap has shown to exhibit several cycles of shape and center-of-mass oscillations followed by asymmetric breakup by ejecting a jet in the upward direction (i.e., opposite to the direction of gravity). We recently attempted to explain this using boundary integral simulations in the Stokes flow limit, wherein the position of the droplet and the polarity of the end cap electrodes were assigned using physical arguments, and the center-of-mass motion was not estimated consistently invoking quasi-static conditions. In this work, we explain the experimental observation of upward breakup of charged droplets in a quadrupolar field, using numerical calculations based on the boundary element method considering inviscid droplets levitated electrodynamically using quadrupole electric fields. The center-of-mass motion and the end cap are consistently calculated in the numerical scheme. The simulations show that the gravity-induced downward shift in the equilibrium position of the drop in the trap causes significant, large-amplitude shape oscillations superimposed over the center-of-mass oscillations of the drop. An important observation here is that the shape oscillations due to the applied quadrupole fields result in sufficient deformations that act as triggers for the onset of the instability below the Rayleigh limit, thereby admitting a subcritical instability. The center-of-mass oscillations of the droplet within the trap, which follow the applied frequency, are out of phase with the applied AC signal. Thus the combined effect of shape deformations and dynamic position of the drop leads to an asymmetric breakup such that the Rayleigh fission occurs upward via the ejection of a jet at the north pole of the deformed drop.

Relationship Between the Mobility of Aggregates and Fluid Penetration Depth Across a Range of Fractal Dimensions Using Stokesian Dynamics.

The hydrodynamic behavior of fractal aggregates plays an important role in various applications in industry and the environment, and has been a topic of interest over the past several decades. Despite this, crucial aspects such as the relationship of the mobility radius, Rm, with respect to the fractal dimension, df, and the fluid penetration depth, δ, have largely remained unexplored. Herein, we examine these aspects across a wide range of df's through a Stokesian dynamics approach. It takes into account all orders of monomer-monomer interactions to construct the resistance matrix for the entire cluster, which is assumed to be rigid. Statistical fractals created using algorithms such as diffusion limited aggregation (DLA), cluster-cluster aggregation (CCA), tunable Monte Carlo algorithm, and a deterministic Vicsek fractal, with df varying from 1.76 to 3, and the number of monomers ranging from 20 to 10 240 are considered. While confirming the expected asymptotic cluster-size independence of the hydrodynamic ratio, ß = Rm/Rg (where Rg is the radius of gyration of the cluster), this study reveals a monotonically increasing trend for ß with increasing df. The decay of the fluid velocity within the aggregate is quantified via the concept of penetration depth (δ). Analysis shows that the dimensionless penetration depth (δ* = δ/Rg) approaches asymptotic constancy with respect to cluster size in contrast to a weak dependency of the form δ* ∼ (Rg/a)-(df - 1)/2, predicted by the mean-field theory (a being the monomer radius). Furthermore, the penetration depth is found to decrease rapidly, in an exponential manner, with increasing ß. This establishes a quantitative relationship between the resistance experienced by the cluster and the degree of penetration of fluid into it. The implications of these results are further discussed.

Dose distribution to a random walker moving in a two-dimensional surface around a radioactive source.

BACKGROUND: Modeling of dose distribution of randomly moving population around a radioactive source is a complex problem. OBJECTIVE: The objective is to develop a model and solution techniques to estimate radiation absorbed dose to the population randomly moving around a radioactive source. METHODS: The problem is formulated using a second-order partial differential equation; different moments of the dose distribution function are defined related to physically realizable quantities, and solutions are obtained using standard moments methods. Alternatively, numerical simulations are performed to estimate the radiation doses using Monte Carlo approach for individual positions and random motions of the people around the source. RESULTS: A good agreement is found between average doses obtained from moments method and numerical simulations. A typical application of this model to different exposure conditions shows that the average dose is highly dependent on the population density. The study results show that average dose decreases with increase in the population density and movement area of random walker. SIGNIFICANCE: This mathematical model can be used as a rapid assessment tool by the emergency planners in resource optimization by providing quick estimates of likely exposures for triage and emergency response.

Subcritical asymmetric Rayleigh breakup of a charged drop induced by finite amplitude perturbations in a quadrupole trap.

The breakup pathway of Rayleigh fission of a charged drop is unequivocally demonstrated by continuous, high-speed imaging of a drop levitated in an AC quadrupole trap. The experimental observations consistently exhibited asymmetric, subcritical Rayleigh breakup with an upward (i.e., opposite to the direction of gravity) ejection of a jet from the levitated drop. These experiments supported by numerical calculations show that the gravity induced downward shift of the equilibrium position of the drop in the trap causes significant, large amplitude shape oscillations superimposed over the center-of-mass oscillations. The shape oscillations result in sufficient deformations to act as triggers for the onset of instability below the Rayleigh limit (a subcritical instability). The concurrently occurring, center-of-mass oscillations, which are out of phase with the applied voltage, are shown to lead to an asymmetric breakup such that the Rayleigh fission occurs upwards via the ejection of a jet at the pole of the deformed drop. As an important application, it follows by inference that the nanodrop generation in electrospray devices will occur, more as a rule rather than as an exception, via asymmetric, subcritical Rayleigh fission events of microdrops due to inherent directionality provided by the external electric fields.

Size distribution of virus laden droplets from expiratory ejecta of infected subjects.

For rebooting economic activities in the ongoing COVID-19 pandemic scenario, it is important to pay detailed attention to infection transfer mechanisms during interaction of people in enclosed environments. Utmost concern is the possibility of aerosol mediated infection transfer, which is largely governed by the size distributions of virus laden droplets, termed as virusols in this work, ejected from humans. We expand on the well-known theory of Poisson fluctuations which acts as statistical barrier against formation of virusols. Analysis suggests that for viral loads < 2 × 105 RNA copies/mL, often corresponding to mild-to-moderate cases of COVID-19, droplets of diameter < 20 µm at the time of emission (equivalent to ~ 10 µm desiccated residue diameter) are unlikely to be of consequence in carrying infections. Cut-off diameters below which droplets will be practically free of contamination, are presented as a function of viral loading. The median diameters of virus laden polydisperse droplet distributions will be 1.5 to 20 times higher depending upon the geometric standard deviation. The studies have implications to risk assessment as well as residence time estimates of airborne infections in indoor environments. Additionally, it will be also helpful for performance evaluation of sanitization and control technologies to mitigate infection risks in workplaces.

An innovative technique of harvesting soil gas as a highly efficient source of 222Rn for calibration applications in a walk-in type chamber: part-1.

The paper describes a novel technique to harvest 222Rn laden air from soil gas of natural origin as a highly efficient source of 222Rn for calibration applications in a walk-in type 222Rn calibration chamber. The technique makes use of a soil probe of about 1 m to draw soil gas, through a dehumidifier and a delay volume, using an air pump to fill the calibration chamber. 222Rn concentration in the range of a few hundred Bq m-3 to a few tens of kBq m-3 was easily attained in the chamber of volume 22.7 m3 within a short pumping duration of 1 h. A new technique referred to as "semi-dynamic mode of operation" in which soil gas is injected into the calibration chamber at regular intervals to compensate for the loss of 222Rn due to decay and leak is discussed. Harvesting soil gas has many important advantages over the traditional methods of 222Rn generation for calibration experiments using finite sources such as solid flow-through, powdered emanation, and liquid sources. They are: (1) soil gas serves as an instantaneous natural source of 222Rn, very convenient to use unlike the high strength 226Ra sources used in the calibration laboratories, and has no radiation safety issues, (2) does not require licensing from the regulatory authority, and (3) it can be used continuously as a non-depleting reservoir of 222Rn, unlike other finite sources. The newly developed technique would eliminate the need for expensive radioactive sources and thereby offers immense application in a variety of day to day experiments-both in students and research laboratories.

A periodic pumping technique of soil gas for 222Rn stabilization in large calibration chambers: part 2-theoretical formulation and experimental validation.

In an adjoining publication, we demonstrated the novel technique to harvest soil gas of natural origin as a highly efficient source of 222Rn for calibration applications in a large volume 222Rn calibration chamber. Its advantages over the use of conventional high strength 226Ra sources, such as the capability to serve as a non-depleting reservoir of 222Rn and achieve the desired concentration inside the calibration chamber within a very short time, devoid of radiation safety issues in source handling and licensing requirements from the regulatory authority, were discussed in detail. It was also demonstrated that stability in the 222Rn concentration in large calibration chambers could be achieved within ± 20% deviation from the desired value through a semi-dynamic mode of injection in which 222Rn laden air was periodically pumped to compensate for its loss due to leak and decay. The necessity of developing a theory for determining the appropriate periodicity of pumping was realized to get good temporal stability with a universally acceptable deviation of ≤ ± 10% in the 222Rn concentration. In this paper, we present a mathematical formulation to determine the injection periods (injection pump ON and OFF durations) for the semi-dynamic operation to achieve long term temporal stability in the 222Rn concentration in the chamber. These computed pumping parameters were then used to efficiently direct the injection of soil gas into the chamber. We present the mathematical formulation, and its experimental validations in a large volume calibration chamber (22 m3). With this, the temporal stability of 222Rn concentration in the chamber was achieved with a deviation of ~ 3% from the desired value.

Effect of noise on the stability of electrodynamically levitated one or many charged droplets.

The theory of the effect of external fluctuations on the stability and spatial distribution of mutually interacting and slowly evaporating charged drops, levitated in an electrodynamic balance, is presented using a classical pseudo-potential approach. The theory is supplemented with numerical simulations where the non-homogeneous modified Mathieu equation is solved for single-droplet as well as many-droplet systems. In this essentially non-equilibrium system a pseudo-potential is identified, and a Boltzmann-like pseudo-equilibrium distribution is suggested that describes the variance of the deterministic configuration of particles levitated in a quadrupolar trap. This formalism seems to explain the numerical results in a fairly close and convincing manner. A transition from a well-ordered Coulombic crystal to a randomly distributed liquid-like structure is observed above a threshold value of noise. A surprising finding of the present work is the observation that the strength of the threshold noise for the transition of a 2-particle system into a noise-dominated regime is identical to the critical noise required for a solid melting transition in a 100-particle system. The simulations could prove useful in analysing an ordered assembly of levitated micro- and nano-particles from the air streams using a contactless membrane.

A WALK-IN TYPE CALIBRATION CHAMBER FACILITY FOR 222Rn MEASURING DEVICES AND INTER-COMPARISON EXERCISES.

A walk-in type 222Rn calibration chamber of volume 22.7 m3, which has traceability to international standards, is established at the Centre for Advanced Research in Environmental Radioactivity, Mangalore University, India. It has a human-machine interface communication system, a programmable logic controller and sensor feedback circuit for controlling and data acquisition of relative humidity (RH) and temperature (T). An innovative method for the generation of desired 222Rn concentration (a few hundred Bq m-3 up to about 36 kBq m-3) using soil gas as a source was adopted. Leak rates of 222Rn from the chamber for the mixing fan ON and OFF conditions were determined to be 0.0011 and 0.00018 h-1 respectively. With the exhaust system fully turned on, the maximum clearance rate of the chamber was 0.58 ± 0.07 h-1. Excellent spatial uniformity in 222Rn concentration in the chamber was confirmed (with a mean value of relative standard deviation < 12%) through measurements at 23 locations using CR-39 film-based passive devices. Demonstration of calibration applications was performed using charcoal canister and PicoRad vials as the 222Rn adsorption devices. The study shows that gamma spectrometry is a convenient alternative approach to liquid scintillation analysis of PicoRad vials for 222Rn measurement.

Effect of the Quadrupolar Trap Potential on the Rayleigh Instability and Breakup of a Levitated Charged Droplet.

The experimental demonstration of Rayleigh instability that results in the breakup of a charged droplet, levitated in a quadrupole trap, has been investigated in the literature, but only scarcely. We report here the asymmetric breakup of a charged drop, levitated in a loose trap, wherein the droplet is stabilized at an off-center location in the trap. This aspect of levitation leads to an asymmetric breakup of the charged drop, predominantly in a direction opposite to that of gravity. In the present work, we report the evidence of successive events of the deformation and breakup of a charged drop and its subsequent relaxation after jet ejection using high-speed imaging at a couple of hundred thousand frames per second. Several relevant aspects of this phenomenon such as the effect of the electrodynamic (ED) trap parameters in terms of the applied potential as well as physical parameters such as the size of the drop, gravity, and conductivity on the characteristics of droplet breakup are explored. A clear effect of the trap strength on the deformation (both symmetric and asymmetric) is observed. Moreover, the cone angle at the pole undergoing asymmetric breakup is almost independent of the applied field investigated in the experiments. All of the experimental observations are compared with numerical simulations carried out using the boundary element method (BEM) in the Stokes flow limit. The BEM simulations are also extended to other experimentally achievable parameters. It is observed that the breakup in our study is mostly field-influenced and not field-induced. A plausible theory for the observations is reported, and a sensitive role of the sign of the charge on the droplet and the sign of the end-cap potential, as well as the off-center location of the droplet in the trap, is elucidated.

A study of temporal variations of 7Be and 210Pb concentrations and their correlations with rainfall and other parameters in the South West Coast of India.

As a part of establishing a regional database on natural radioactivity, the atmospheric concentrations of 210Pb and 7Be were measured over a three and half year period (2014-2017) in Mangalore and Kaiga in the South West Coast of India. A total of 99 air samples, collected in the different months of the year, were analysed in this study. The mean activity concentrations of 7Be and 210Pb were found to be 5.5 ±â€¯3.1 mBq m-3 and 1.1 ±â€¯0.73 mBq m-3, respectively. Both the radionuclides exhibited strong seasonal variations, with maximum concentration of 7Be occurring in the summer and that of 210Pb in the winter season. The concentration of both the radionuclides was minimum in the rainy season. Higher 210Pb concentration during winter was attributed to the ingression of continental air masses due to the wind regime from the North East. The sunspot number index of the solar activity also plays an important role in the increase and decrease of 7Be concentration in the air. A clear trend of increased and lowered concentration of 7Be with lower and higher solar activity (low and high sunspot number), respectively, in accordance with the 11-year solar cycle, was observed in this study. The temporal variation of PM10 concentration was also studied and it showed maximum value in the winter and minimum in the rainy season with an average of 56.9 µg m-3. Statistically significant positive correlation was observed between the PM10 and 210Pb activity concentration, whereas a weak correlation was observed between PM10 and 7Be. This is due to the fact that 7Be is largely associated with sub-micrometer size particles, whereas PM10 is contributed by larger sizes. The dependence of the activity concentrations of 7Be and 210Pb with meteorological parameters such as rainfall, temperature, and humidity was studied through linear regression analysis. A significant correlation was observed between 7Be and 210Pb concentrations with rainfall intensity (with identical correlation coefficients), which suggested that the removal mechanisms of these two radionuclides were similar. 7Be showed a strong correlation with temperature, whereas 210Pb with humidity. A comparison of the data obtained in the present study for the South West Coast of India with the global literature values of 7Be and 210Pb in aerosols showed that the values did not reflect the well-known latitudinal dependence of the 7Be tropospheric fluxes. Overall, the study provides an improved understanding of the correlation and variability of 210Pb and 7Be concentrations in the atmosphere in the South West Coast of India.

Thoron Mitigation System based on charcoal bed for applications in thorium fuel cycle facilities (part 1): Development of theoretical models for design considerations.

Regulating the environmental discharge of 220Rn (historically known as thoron) and its decay products from thorium processing facilities is important for protection of environment and general public living in the vicinities. Activated charcoal provides an effective solution to this problem because of its high adsorption capacity to gaseous element like radon. In order to design and develop a charcoal based Thoron Mitigation System, a mathematical model has been developed in the present work for studying the 220Rn transport and adsorption in a flow through charcoal bed and estimating the 220Rn mitigation factor (MF) as a function of system and operating parameters. The model accounts for inter- and intra-grain diffusion, advection, radioactive decay and adsorption processes. Also, the effects of large void fluctuation and wall channeling on the mitigation factor have been included through a statistical model. Closed form solution has been provided for the MF in terms of adsorption coefficient, system dimensions, grain size, flow rate and void fluctuation exponent. It is shown that the delay effects due to intra grain diffusion plays a significant role thereby rendering external equilibrium assumptions unsuitable. Also, the application of the statistical model clearly demonstrates the transition from the exponential MF to a power-law form and shows how the occurrence of channels with low probability can lower mitigation factor by several orders of magnitude. As a part of aiding design, the model is further extended to optimise the bed dimensions in respect of pressure drop and MF. The application of the results for the design and development of a practically useful charcoal bed is discussed.

Thoron Mitigation System based on charcoal bed for applications in thorium fuel cycle facilities (part 2): Development, characterization, and performance evaluation.

Exposure due to thoron (220Rn) gas and its decay products in a thorium fuel cycle facility handling thorium or 232U/233U mixture compounds is an important issue of radiological concern requiring control and mitigation. Adsorption in a flow-through charcoal bed offers an excellent method of alleviating the release of 220Rn into occupational and public domain. In this paper, we present the design, development, and characterization of a Thoron Mitigation System (TMS) for industrial application. Systematic experiments were conducted in the TMS for examining the 220Rn mitigation characteristics with respect to a host of parameters such as flow rate, pressure drop, charcoal grain size, charcoal mass and bed depth, water content, and heat of the carrier gas. An analysis of the experimental data shows that 220Rn attenuation in a flow through charcoal bed is not exponential with respect to the residence time, L/Ua (L: bed depth; Ua: superficial velocity), but follows a power law behaviour, which can be attributed to the occurrence of large voids due to wall channeling in a flow through bed. The study demonstrates the regeneration of charcoal adsorption capacity degraded due to moisture adsorption, by hot air blowing technique. It is found that the mitigation factor (MF), which is the ratio of the inlet 220Rn concentration (Cin) to the outlet 220Rn concentration (Cout), of more than 104 for the TMS is easily achievable during continuous operation (>1000 h) at a flow rate of 40 L min-1 with negligible (<1 cm of water column) pressure drop. The Thoron Mitigation System based on adsorption on charcoal bed offers a compact and effective device to remove 220Rn from affluent air streams in a space constrained domain. The prototype system has been installed in a thorium fuel cycle facility where it is being evaluated for its long-term performance and overall effectiveness in mitigating 220Rn levels in the workplace.

Estimation of critical supersaturation solubility ratio for predicting diameters of dry particles prepared by air-jet atomization of solutions.

Air-jet atomization of solution into droplets followed by controlled drying is increasingly being used for producing nanoparticles for drug delivery applications. Nanoparticle size is an important parameter that influences the stability, bioavailability and efficacy of the drug. In air-jet atomization technique, dry particle diameters are generally predicted by using solute diffusion models involving the key concept of critical supersaturation solubility ratio (Sc) that dictates the point of crust formation within the droplet. As no reliable method exists to determine this quantity, the present study proposes an aerosol based method to determine Sc for a given solute-solvent system and process conditions. The feasibility has been demonstrated by conducting experiments for stearic acid in ethanol and chloroform as well as for anti-tubercular drug isoniazid in ethanol. Sc values were estimated by combining the experimentally observed particle and droplet diameters with simulations from a solute diffusion model. Important findings of the study were: (i) the measured droplet diameters systematically decreased with increasing precursor concentration (ii) estimated Sc values were 9.3±0.7, 13.3±2.4 and 18±0.8 for stearic acid in chloroform, stearic acid and isoniazid in ethanol respectively (iii) experimental results pointed at the correct interfacial tension pre-factor to be used in theoretical estimates of Sc and (iv) results showed a consistent evidence for the existence of induction time delay between the attainment of theoretical Sc and crust formation. The proposed approach has been validated by testing its predictive power for a challenge concentration against experimental data. The study not only advances spray-drying technique by establishing an aerosol based approach to determine Sc, but also throws considerable light on the interfacial processes responsible for solid-phase formation in a rapidly supersaturating system. Until satisfactory theoretical formulae for predicting CSS are developed, the present approach appears to offer the best option for engineering nanoparticle size through solute diffusion models.

Deposition and spatial variation of thoron decay products in a thoron experimental house using the Direct Thoron Progeny Sensors.

Experiments have been carried out using the deposition-based Direct Thoron Progeny Sensors (DTPS) in a thoron experimental house. The objective was to study the thoron decay product characteristics such as the deposition velocities, spatial variability and dependence on aerosol particle concentrations. Since the deposition velocity is an important characteristic in the calibration of the DTPS, it is very important to study its dependence on aerosol concentration in a controlled environment. At low aerosol concentration (1500 particles/cm3) the mean effective deposition velocity was measured to be 0.159 ± 0.045 m h-1; at high aerosol concentration (30 000 particles/cm3) it decreased to 0.079 ± 0.009 m h-1. The deposition velocity for the attached fraction of the thoron decay products did not change with increasing aerosol concentration, showing measurement results of 0.048 ± 0.005 m h-1 and 0.043 ± 0.014 m h-1, respectively. At low particle concentration, the effective deposition velocity showed large scattering within the room at different distances from center. The attached fraction deposition velocity remained uniform at different distances from the wall. The measurements in the thoron experimental house can be used as a sensitivity test of the DTPS in an indoor environment with changing aerosol concentration. The uniform spatial distribution of thoron decay products was confirmed within the experimental house. This indicates that direct measurement of thoron decay product concentration should be carried out instead of inferring it from thoron gas concentration, which is very inhomogeneous within the experimental house.

Engineering of layered, lipid-encapsulated drug nanoparticles through spray-drying.

Drug-containing nanoparticles have been synthesized through the spray-drying of submicron droplet aerosols by using matrix materials such as lipids and biopolymers. Understanding layer formation in composite nanoparticles is essential for the appropriate engineering of particle substructures. The present study developed a droplet-shrinkage model for predicting the solid-phase formation of two non-volatile solutes-stearic acid lipid and a set of drugs, by considering molecular volume and solubility. Nanoparticle formation was simulated to define the parameter space of material properties and process conditions for the formation of a layered structure with the preferential accumulation of the lipid in the outer layer. Moreover, lipid-drug demarcation diagrams representing a set of critical values of ratios of solute properties at which the two solutes precipitate simultaneously were developed. The model was validated through the preparation of stearic acid-isoniazid nanoparticles under controlled processing conditions. The developed model can guide the selection of solvents, lipids, and processing conditions such that drug loading and lipid encapsulation in composite nanoparticles are optimized.

Ionizer induced 220Rn decay product removal in confined environment: Continuous vs. instantaneous source.

This paper presents an experimental approach to evaluate the effectiveness of unipolar ionizers in indoor environment for the removal of thoron (220Rn) daughter products. Both continuous and instantaneous source conditions were simulated during these experiments. Activity and aerosol related parameters were measured for these experiments and results were interpreted. Activity concentration was found to be reduced by a factor 6.6 and 34 for continuous and instantaneous source conditions, respectively. The particle size dependency of mitigation of particles using ionizer is also discussed. The effect of ionizer on activity size distribution has been directly measured for the first time. The ionizer induced changes in particle size distributions were coupled to Dose Reduction Factor (DRF) model and significant DRF values were obtained for both source conditions. This study discusses open issues which are important for establishing ionizer induced radioactivity mitigation as a technology application.

MEASUREMENT OF RADON CONCENTRATION IN DWELLINGS IN THE REGION OF HIGHEST LUNG CANCER INCIDENCE IN INDIA.

Indoor radon/thoron concentration has been measured in Aizawl district, Mizoram, India, which has the highest lung cancer incidence rates among males and females in India. Simultaneously, radon flux emanated from the surrounding soil of the dwellings was observed in selected places. The annual average value of concentration of radon(thoron) of Aizawl district is 48.8(22.65) Bq m-3 with a geometric standard deviation of 1.25(1.58). Measured radon flux from the soil has an average value of 22.6 mBq m-2 s-1 These results were found to be much below the harmful effect or action level as indicated by the World Health Organisation. On the other hand, food habit and high-level consumption of tobacco and its products in the district have been found to increase the risk of lung cancer incidence in the district.