Calibrating GESPECOR model of computing the full-energy peak efficiency of coaxial high-purity germanium detectors by Monte Carlo simulation.

In this work, a classical approach was used for calibrating the GESPECOR detector model for computing the full-energy peak efficiency of p-type coaxial HPGe detectors that is based on the use of linear least squares optimization. The key element of the work is the multiplicative model developed for approximating the values of the full-energy peak efficiency provided by GESPECOR code. It was linearized using the logarithmic transformation to allow an easy use of the linear least squares optimization. A procedure was also developed to estimate the optimal values of the parameters, describing the p-type coaxial HPGe detectors. Its application to a Canberra detector GC3018 showed that it is possible to determine accurate values of the full-energy peak efficiency computed by GESPECOR code using the optimized parameter values.

A novel virtual point detector model for high purity germanium detectors.

A novel virtual point detector model was developed for cylindrical high purity germanium detectors that are widely used in gamma-ray spectrometry. According to this model, the full energy peak efficiency response of these detectors can be computed by replacing the detector with a virtual point detector located at the center of the detector crystal. In fact, the novel model makes the transfer of the FEP efficiency response from a reference point source to a given point source, both sources being located on a line that passes through the virtual point detector. The model was tested and provided accurate results.

Novel integral expressions for computing the full energy peak efficiency of gamma spectrometry systems.

A new theoretical approach was developed for computing the FEP efficiency of HPGe and scintillation detectors using the concept of probability function. Thus integral expressions for the FEP efficiency were developed using three probability functions. Starting from the integral expression of the FEP efficiency and using integral means and Chebyshev functional, new expressions of the FEP efficiency, detector response, and self-attenuation factors were obtained. Also, the new approach provides new insights useful for gamma spectrometry measurements. Two practical applications of this approach are described.

Efficiency transfer method applied to surface beta contamination measurements.

In this paper, the application of the efficiency transfer method to the evaluation of the surface beta contamination is described. Using efficiency transfer factors, the reference calibration factor of contamination monitors is corrected, to obtain the calibration factor for an actual contamination source. The experimental part of the paper illustrates the applicability of the method to the direct measurement of the surface beta contamination.

Determination of the neutron activation profile of core drill samples by gamma-ray spectrometry.

This paper provides guidance for determining the neutron activation profile of core drill samples taken from the biological shield of nuclear reactors using gamma spectrometry measurements. Thus, it provides guidance for selecting a model of the right form to fit data and using least squares methods for model fitting. The activity profiles of two core samples taken from the biological shield of a nuclear reactor were determined. The effective activation depth and the total activity of core samples along with their uncertainties were computed by Monte Carlo simulation.

A new approach in evaluating the surface beta contamination using the direct method of measurement.

This paper describes a new approach in evaluating the surface beta contamination using the direct method of measurement. It makes use of previous results obtained in numerical modeling of electron transport in planar geometry and is mainly based on the estimation of the efficiency of contamination sources for beta radiation and its standard uncertainty using the available information concerning the component materials of the sources and their main parameters. Experimental results illustrate the appropriateness of the new approach for surface beta contamination measurements.

Improved method of measurement for tritiated water standardization by internal gas proportional counting.

Tritiated water has been standardized by internal gas counting following its chemical reduction to tritiated hydrogen by using a new tritium generator and an improved method of measurement. This method is based on the equality between the molar activities of the tritiated water and the tritiated hydrogen and is valid only when the reduction yield is unitary. It is simpler and more accurate than the conventional method. Two samples of tritiated water from a sealed ampoule were measured by internal gas proportional counting using both the conventional method and the improved method of measurement. The obtained results demonstrated the validity of the improved method.

Standardization of tritiated water by two improved methods.

Tritiated water has been standardized in the framework of a French-Romanian cooperation by two improved methods: liquid scintillation counting based on the triple to double coincidence ratio method and the internal gas proportional counting used in conjunction with a tritium generator for chemical reduction of water. The uncertainties of measurement for both methods were smaller than 0.6% and the two results were consistent within these uncertainties, indicating that either method is equally suited for standardizing tritiated water.

Uncertainty assessment in the free release measurement by gamma spectrometry of rotating waste drums.

The assessment of uncertainty in free release measurements by integral gamma scanning method is described and applied to the measurement of homogeneous and heterogeneous waste drums. It is based on the propagation of distributions using the Monte Carlo method. In addition, two techniques for the uncertainty reduction are also described. The first technique makes use of containers constructed from two concentric cylinders and the second technique is based on the measurement of a group of waste drums. It is proved that the uncertainty of clearance measurements can be reduced using both techniques.

A novel method for the activity measurement of large-area beta reference sources.

A novel method has been developed for the activity measurement of large-area beta reference sources. It makes use of two emission rate measurements and is based on the weak dependence between the source activity and the activity distribution for a given value of transmission coefficient. The method was checked experimentally by measuring the activity of two ((60)Co and (137)Cs) large-area reference sources constructed from anodized aluminum foils. Measurement results were compared with the activity values measured by gamma spectrometry. For each source, they agree within one standard uncertainty and also agree within the same limits with the certified values of the source activity.

Modeling the transmission of beta rays through thin foils in planar geometry.

This paper is concerned with the modeling of the transmission of beta rays through thin foils in planar geometry based on the plane source concept, using Monte Carlo simulation of electron transport and least squares fitting. Applications of modeling results for calculating the efficiency of large-area beta sources, transmission coefficient of beta rays through thin foils and the beta detection efficiency of large-area detectors used in surface contamination measurements are also presented.

Interfacial conditions during evaporation or condensation of water.

Steady-state evaporation and condensation experiments have been conducted with water under conditions where buoyancy-driven convection is not present. The temperature profile in each phase has been measured. At the interface, independently of the direction of the phase change, a temperature discontinuity has been found to exist in which the interfacial vapor temperature is greater than that in the liquid. In a thin layer immediately below the interface the temperature is uniform in a layer ( approximately 0.5 mm) and below that the temperature profile is linear, indicating thermal conduction. The uniform temperature layer indicates a mixing process occurs near the interface that could result from surface-tension driven (Marangoni-Bénard) convection and/or from "energy partitioning" that is necessary to account for the measured temperature discontinuity near the interface. When the measured interfacial properties are used with the expression for the phase change rate that is obtained from statistical rate theory, it is found that the predictions are in close agreement with the measurements.

Importance of covariances for uncertainty estimates in measurement of radionuclide mixtures by multichannel counting.

Multichannel counting is often used when samples containing a mixture of known radionuclides are measured. Solutions to this method have been presented but to our knowledge, none of them took into account the covariances for evaluating the uncertainty of measurement. This paper presents new solutions for estimating uncertainties in measurement of radionuclide mixtures by multichannel counting and shows that covariances may contribute significantly to uncertainty of measurement so that they must be evaluated.

Analytical calculations of counting losses in internal gas proportional counting.

In internal gas proportional counting, the evaluation of counting losses is important in order to obtain high accuracy measurement results. In this paper, counting losses due to the wall effect and not counted beta particles with very low energy are evaluated by analytical calculations. The calculated and experimental results are compared and a very good agreement is found.

A simple method for determining the activity of large-area beta sources constructed from anodized aluminum foils.

A simple method has been developed for determining the activity of large-area beta reference sources in anodized aluminum foils. It is based on the modeling of the transmission of beta rays through thin foils in planar geometry using Monte Carlo simulation. The method was checked experimentally and measurement results show that the activity of large-area beta reference sources in anodized aluminum foils can be measured with standard uncertainties smaller than the limit of 10% required by ISO 8769.

Numerical modeling of large-area beta sources constructed from anodized-aluminum foils.

The numerical modeling of large-area beta sources constructed from anodized-aluminum foils is described in this paper. Based on a realistic model for the activity depth distribution, theoretical lower and upper bounds for the efficiency and the transmission coefficient were calculated and used to analyze the comparison method recommended by ISO 8769 for measuring the surface emission rate. The analysis shows that this method can provide measurement results with relative standard uncertainties smaller than 3% for high energy beta emitters such as (90)Sr-(90)Y, (36)Cl and (204)Tl.

A new approach in gamma-ray scanning of rotating drums containing radioactive waste.

The paper presents a new approach in the integral gamma scanning of rotating waste drums based on solving a first-kind Fredholm integral equation using a small number of measurements. Both numerical experiments and experimental results show that the nuclide inventory of waste drums can be determined accurately using the new approach especially when the waste drums contain multi-gamma emitters.

A new method for determining the efficiency of large-area beta sources constructed from anodized aluminum foils.

A new method has been developed for determining the efficiency of large-area beta sources in anodized aluminum foils using transmission measurements. The method was applied to the efficiency measurement of a (90)Sr-(90)Y large-area reference source. Measurement results show that the method can provide efficiency values for (90)Sr-(90)Y reference sources with standard uncertainties smaller than 2.9%, which are far below the limit of 10% required by ISO 8769.

A new model calculation of the peak efficiency for HPGe detectors used in assays of radioactive waste drums.

In this paper we present a new semi-empirical model calculation of the peak efficiency for unshielded HPGe detectors based on the virtual point detector and the attenuation factor concepts. The validity of the model calculation was checked by comparison with Monte Carlo efficiency values and experimental efficiencies determined for a HPGe detector type GEM 25P4 using a calibration drum. The discrepancy between experimental and calculated efficiencies is smaller than 10% in the energy range 122-1408 keV.

The 222Rn standard system established at IFIN-HH, Romania.

The paper presents the (222)Rn Standard System realized at the Radionuclide Metrology Laboratory from IFIN-HH, Romania. It contains a Pylon solid (226)Ra source, and a glass circuit for circulation and recovery of (222)Rn in glass ampoules, at the 77K temperature. The radon can be recovered both in ampoules with liquid scintillator (LS), for absolute standardization of (222)Rn by liquid scintillation counting (LSC) (Cassette et al., 2006), and in evacuated ampoules as gas, simultaneously or by the transfer of radon from gas into LS ampoules, in order to establish the traceability chain. The absolute standardization consists in the registration of the double coincidence counting rate in LS, due to the whole sequence of short life components of the (222)Rn chain, in equilibrium conditions. The main correction applied to the measurement results is due to the decay of (214)Po during the extendable dead time of the system. The following procedure was applied to take it into account. The value of the base duration of the dead time was precisely measured and used for the data corrections. The measurements of each source were repeated during a period of several days, and the decay curve parameters were calculated. If the dead time value and the correction formulae, presented throughout the paper, were adequate, then the re-determination of the (222)Rn half life, T(1/2)=(3.8232+/-0.0008) d, using the decay curve of the corrected counting rates should be correct. On the other hand, the effective dead time values obtained by parallel calculations were compared. The paper presents the results obtained in the standardization of several sources, by LSC, and the evaluation of the uncertainties associated to the method.