S. Georgiev

Automatic Counting of Electrochemically Etched Tracks in Compact Discs. Application to Retrospective Measurements of Rn-222

This work describes an algorithm for automatic counting of alpha tracks in scanned images of electrochemically etched CDs and DVDs. It is demonstrated that the algorithm copes with the specific problems encountered in retrospective 222Rn measurements with CDs and DVDs; it reliably separates overlapping tracks and ignores artifacts in the image. The algorithm is validated by comparison to manual counting and a very good correlation is found. The measurement range of 222Rn concentrations in dwellings and soil gas is estimated. It is shown that the wide span of track densities that can be covered by the algorithm combined with the specific advantages of the compact disc method can provide reliable measurements in the whole range of 222Rn concentrations of practical interest. For retrospective measurements of 222Rn in dwellings the estimated range is from 10 Bq/m3 to 100 kBq/m3 for five years exposure time. For 222Rn in soil gas the estimated range is from 1 kBq/m3 to 10 MBq/m3 for ten days exposure time.

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

This article points out the ability to map retrospective (222)Rn 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 (222)Rn 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.

Determination of the diffusion coefficient and solubility of radon in plastics

This paper describes a method for determination of the diffusion coefficient and the solubility of radon in plastics. The method is based on the absorption and desorption of radon in plastics. Firstly, plastic specimens are exposed for controlled time to referent (222)Rn concentrations. After exposure, the activity of the specimens is followed by HPGe gamma spectrometry. Using the mathematical algorithm described in this report and the decrease of activity as a function of time, the diffusion coefficient can be determined. In addition, if the referent (222)Rn concentration during the exposure is known, the solubility of radon can be determined. The algorithm has been experimentally applied for different plastics. The results show that this approach allows the specified quantities to be determined with a rather high accuracy-depending on the quality of the counting equipment, it can be better than 10 %.

Liquid scintillation counting of polycarbonates: A sensitive technique for measurement of activity concentration of some radioactive noble gases

This work explores the application of the liquid scintillation counting of polycarbonates for measurement of the activity concentration of radioactive noble gases. Results from experimental studies of the method are presented. Potential applications in the monitoring of radioactive noble gases are discussed.

Determination of (222)Rn absorption properties of polycarbonate foils by liquid scintillation counting. Application to (222)Rn measurements.

This work demonstrates that a Liquid Scintillation Counting (LSC) technique using a Triple to Double Coincidence Ratio counter with extending dead-time is very appropriate for the accurate measurement of (222)Rn activity absorbed in thin polycarbonate foils. It is demonstrated that using a toluene-based LS cocktail, which dissolves polycarbonates, the (222)Rn activity absorbed in thin Makrofol N foil can be determined with a relative standard uncertainty of about 0.7%. A LSC-based application of the methodology for determination of the diffusion length of (222)Rn in thin polycarbonate foils is proposed and the diffusion length of (222)Rn in Makrofol N (38.9±1.3µm) and the partition coefficient of (222)Rn in Makrofol N from air (112±12, at 20°C) and from water (272±17, at 21°C) are determined. Calibration of commercial LS spectrometers for (222)Rn measurements by LSC of thin polycarbonate foils is performed and the minimum detectable activities by this technique are estimated.

Logistic of surveys of retrospective radon concentrations by home-stored CDs/DVDs

Recently, a method for rather precise retrospective (222)Rn measurements, based on home-stored compact disks (CDs)/DVDs, has demonstrated a promising potential for wide application. In Bulgaria, pilot surveys have been initiated based on voluntarily provided CDs/DVDs. The results showed that large-scale surveys could be efficiently organised. However, several problems were identified and are discussed in the report. The first is the relatively small proportion of participants (30-50 %) that actually provided disks. Other addressed topics include the compatibility between results of different disks from one place, the accuracy of dating CDs/DVDs and the possibility for individual a posteriori calibration of each disk. The possibility to follow year-to-year variations by disks of different age is also discussed.

Pilot Study of the Application of Plastic Scintillation Microspheres to Rn-222 Detection and Measurement

This work demonstrates that when plastic scintillation microspheres (PSms) synthesized from polystyrene are exposed to 222Rn in air or water, they concentrate 222Rn in their volume. A theoretical model, based on the solution of the diffusion equation is developed, in order to describe the 222Rn sorption and desorption processes in PSms. A macroscopic quantity termed “sampling efficiency” is used to quantify the absorption properties of the PSms. The sampling efficiency is estimated by exposure of PSms to 222Rn in air and water. Estimates of the counting efficiency for measurement of the exposed PSms by commercial scintillation counters are presented. The results show that polystyrene PSms are simultaneously good samplers and detectors of 222Rn. These properties indicate that sensitive approaches for the measurement of 222Rn based on absorption and scintillation counting of PSms can be developed.

A high-sensitivity method for the measurement of 222Rn based on liquid scintillation counting of polycarbonate powder

This work describes a technique for the measurement of 222Rn by absorption in polycarbonate (PC) powder and liquid scintillation counting (LSC). The work is an improvement of the recently proposed method for 222Rn measurements by LSC of exposed PC grains. It is demonstrated that the use of PC powder as a 222Rn sampler improves 13.6 times the sampling efficiency and leads to 6.5 times smaller minimmal detectable activity concentrations (MDAC) compared with the PC grains used so far. For a 40-h exposure of 7.4-g PC powder to 222Rn in air, the MDAC with a RackBeta 1219 LS counter is 62 Bq m(-3) (assuming a 8-h sample counting time and 24-h background time). For the same conditions the estimated 222Rn MDAC with a Quantulus 1220 LS counter is 20 Bq m(-3). The proposed technique is suitable for radon in air and radon in soil-gas measurements.

Retrospective Rn-220 measurements by compact discs

Recently, a method for retrospective 220Rn measurements by etching alpha-tracks in compact discs (CDs) or digital video discs (DVDs) has been proposed. In that method the tracks at the front surface (with respect to the reading laser beam) of the discs are analyzed. These measurements are influenced by 222Rn absorbed in the disc material and the signal must be corrected for 222Rn presence. In this work we study whether the influence of 222Rn can be eliminated if the back (labeled) side of the CDs is analyzed. Because of the coating on the back side of the disc, 222Rn absorption is prevented and its influence on 220Rn signal is significantly reduced. Moreover, it has been found that in most discs the signal at the back side is an order of magnitude greater than that at the front side, which significantly improves the sensitivity of the method. Thus, this work proposes a new practical approach for retrospective 220Rn measurements of improved sensitivity and reduced 222Rn influence.

Measurements of 131 I, 134 Cs and 137 Cs in environmental samples in Bulgaria after the Fukushima accident

This work reports results from measurements of ¹³¹I, ¹³⁴Cs and ¹³⁷Cs in environmental samples in Bulgaria after the Fukushima accident. Measurements performed in the period from 25 March to 2 May by high volume air sampling on glass fiber aerosol filters showed ¹³¹I concentrations in the range 31–2240 µBq/m³. Caesium-134 and ¹³⁷Cs were also detected in the air samples with concentrations in the ranges 33–456 µBq/m³ and 38–637 µBq/m³, respectively. Measurements of rainwater showed concentrations of ¹³¹I in the range 0.09–1.15 Bq/l. ¹³¹I was also found in raw milk (0.14–0.38 Bq/l) and in grass (0.33 Bq/kg). The measured concentrations are consistent with those in other European countries and do not require special actions for public protection. The measured peak concentrations in the air in Bulgaria were higher than those reported for Northern Greece and were reached on the same day (4–5 April) [3]. Long-term (15 days) air sampling have shown detectable concentrations of airborne activity concentrations of ¹³⁴Cs and ¹³⁷Cs with stable ¹³⁷Cs to ¹³⁴Cs ratio in the range 1.1 – 1.3. The results of the rainwater measurements indicate that washout was the dominant atmosphere scavenging mechanism during the rainfalls. Under these conditions roof sampling of rainwater in the early stages of the rain was found to be very appropriate for detection of trace quantities of ¹³¹I in the atmosphere. Dedicated comparison between roof sampling through a drain pipe and direct sampling shows that the roof sampling is more efficient as it appears to be more sensitive to ¹³¹I in the air.