T. Boshkova

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.

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.

Experimental assessment of the coincidence summing corrections in gamma-ray spectrometry of bulk samples.

This work presents an experimental approach for estimation of the true coincidence-summing (TCS) correction for volume sources on the basis of TCS dependence on the source-to-detector distance. Firstly, it is shown that the TCS dependence on the source-to-detector distance can be linearized for point source geometry. If this linear dependence is established then TCS correction for an arbitrary source-to-detector distance can be obtained. In the case of a volume source a representative parameter d(V) can be formulated as the distance at which the point-source summing effect is the same as the one for the volume-source. Then if the TCS dependence on the source-to-detector distance is established for the point-source case and the volume-source d(V)-value is known, the TCS correction corresponding to the volume-source measuring geometry can be estimated. Experimental method and results are presented in the work too.

Metrological tests of a 200 L calibration source for HPGE detector systems for assay of radioactive waste drums.

In this work we present test procedures, approval criteria and results from two metrological inspections of a certified large volume (152)Eu source (drum about 200L) intended for calibration of HPGe gamma assay systems used for activity measurement of radioactive waste drums. The aim of the inspections was to prove the stability of the calibration source during its working life. The large volume source was designed and produced in 2007. It consists of 448 identical sealed radioactive sources (modules) apportioned in 32 transparent plastic tubes which were placed in a wooden matrix which filled the drum. During the inspections the modules were subjected to tests for verification of their certified characteristics. The results show a perfect compliance with the NIST basic guidelines for the properties of a radioactive certified reference material (CRM) and demonstrate the stability of the large volume CRM-drum after 7 years of operation.

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.

Design, production, metrological tests and certification of a large-volume (200L) calibration source for gamma-spectrometry systems for assay of radioactive waste drums

This work describes a concept for and a realization of a large-volume (200 L) certified source (calibration drum) for calibration of gamma-spectrometry systems used for Assay of Radioactive Waste Drums. The design of the calibration drum is based on the idea of modeling a homogenous source by a large number of point sources. We deal with the problem of finding an optimal number of point sources and their appropriate arrangement to obtain close modeling of a homogenous source. An original model for the arrangement of the sources is derived and presented. We demonstrate quantitatively that with the proposed arrangement of 448 point sources a very close modeling of a homogenous drum is achieved. A wooden matrix was constructed from 14 discs stuck together, in which 32 axial holes were drilled, following the abovementioned design. 470 standard sources — Certified Reference Materials — Modules (CRM-Modules) were produced by transferring equal masses of standardized 152Eu solution in each of them. The activity of each CRM-Module is 68,6 ± 1,4 kBq. The CRM-Modules are made from polycarbonate with cylindrical shape (Φ 25 × 56 mm). All modules were subjected to a large number of tests for validation of their individual metrological characteristics. The results from these tests allowed all CRM-Modules to be certified by the Bulgarian National Metrological Institute. 32 tubes, each containing 14 Certified Reference Materials — Modules (CRM-Modules) are placed in the holes. The calibration drum contains 448 CRM-Modules with overall activity 30,72 ± 0,65 MBq. After the final assembly, the calibration drum was subjected to a number of tests, which proved its metrological characteristics. As a result, the Large-volume source was also certified by the Bulgarian National Metrological Institute and it is in use for calibration of a system for assay of drummed radioactive waste.

An approach to study the distribution of radon in natural materials containing radium

This work aims to test an approach to study the distribution of 222Rn in natural materials containing radium. This distribution could be of interest for studies of radon exhalation from building materials and for archaeological dating of ceramics. To model the distribution, the diffusion length of 222Rn in the material is needed. In the work a method to determine experimentally the diffusion length is presented and tested with samples of archaeological ceramics. The applicability of the approach is evaluated. Furthermore, it is shown that for different samples of archaeological ceramics from the same site different ratios of the activities of 222Rn and 226Ra are found. Therefore, equilibrium between 222Rn and 226Ra in ceramics could not be generally assumed. Such an assumption could potentially influence the dating of ceramics by luminescence methods.