Determining the probability of locating peaks using computerized peak-location methods in gamma-ray spectra as a function of the relative peak-area uncertainty.

The probabilities of locating peaks with a high relative peak-area uncertainty were determined empirically with nine types of peak-location software used in laboratories engaged in gamma-ray spectrometry measurements. It was found that it is not possible to locate peaks with a probability of 0.95, when they have a relative peak-area uncertainty in excess of 50%. Locating peaks at these relatively high peak-area uncertainties with a probability greater than 0.95 is only possible in the library-driven mode, where the peak positions are supposed a-priori. The deficiencies of the library-driven mode and the possibilities to improve the probabilities of locating peaks are briefly discussed.

Unperturbed, high spatial resolution measurement of Radon-222 in soil-gas depth profile.

This work presents a method for measuring the depth distribution of 222Rn activity in soil gas. The method is based on the capacity of polycarbonates to absorb 222Rn and on the possibility of performing sensitive measurements of 222Rn absorbed by the polycarbonates via liquid scintillation counting (LSC). The method is the following: cylindrical holes are drilled along a metal rod and Makrofol® N polycarbonate foils enclosed in polyethylene envelopes are placed in each hole. The rod is driven into the soil and kept for a certain time. As long as the rod is in the soil, the polycarbonate foils are exposed to the 222Rn concentration at their depth. At the end of the exposure the rod is pulled out and the foils are transferred to liquid scintillation (LS) vials filled with liquid scintillator. The 222Rn absorbed in the foils is then measured with a LS analyzer. The rod with the polycarbonate foils acts as a passive probe which senses the 222Rn concentration at different depths beneath the ground surface. The achievable minimum detectable 222Rn activity concentration with the equipment and conditions used in this study is around 12.5 kBq/m3. It can easily be lowered below 1 kBq/m3 if larger foils and low-background LS analyzers are used. Since the method does not require air sampling the depth distribution of 222Rn in the soil is unperturbed by the sampling. The spatial distribution and the maximum measurement depth are set by the distance between the holes and the depth to which the rod can be fixed into the ground. Results from in situ applications of the method in terrains with high 222Rn in soil-gas are reported, which demonstrate the feasibility and the usefulness of the proposed approach.

Decision threshold and relative uncertainty of the measured net signal in radiation measurement.

This work proposes a simple way to assess whether a physical effect due to the sample is present, via the relative uncertainty of the measured net signal. A possibility for evaluation of the respective relative limit for any particular (already measured) positive net signal is shown. This relative limit ('relative decision threshold') is strictly related to the basic definition of the decision threshold. The full agreement between the conclusions via the decision threshold and via the relative decision threshold, is experimentally demonstrated.

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.

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.

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.

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.

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.

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 %.

Effective thickness of bulk samples in "close" measuring gamma-ray spectrometry.

Analyzing the experimentally evaluated self-attenuation correction coefficients needed in activity determination by means of gamma-ray spectrometry, the effective thicknesses of bulk samples of three shapes were evaluated. The effective thicknesses deduced significantly differ from the geometrical sample thicknesses in "close" measuring geometry.

Corrections for self-attenuation in gamma-ray spectrometry of bulk samples.

On the basis of the measurement of over 70 radioactive standard bulk sources with different matrix density and different shapes, the gamma-ray self-attenuation corrections needed in activity determination by means of gamma-ray spectrometry are evaluated. The full-energy peak efficiency dependence on the density, and the self-attenuation correction dependence on the photon energy are described.