Uncertainty and covariance matrix of the coincidence-summing correction factors due to decay-data uncertainties.

The uncertainty component of high efficiency gamma-ray spectrometry measurements due to the uncertainty of the decay parameters is evaluated. It is based on the analysis of a large set of random decay schemes, constructed using the values and uncertainties of the decay parameters taken from data libraries. The distributions of the coincidence-summing correction factors FC, of the factors p⋅FC (p = photon emission intensity), and of the correlations between the distribution of pairs of these factors are constructed and analysed.

On homogeneity approximation in calibration of gamma-spectrometry assessment of bulk samples.

The effect of the intrinsic inhomogeneity of a sample on the efficiency for the measurement of the sample by gamma-ray spectrometry is studied. The difference between the efficiency for the inhomogeneous sample and the efficiency for a sample made from the equivalent homogeneous matrix is evaluated.

Monte Carlo simulation of air sampling methods for the measurement of radon decay products.

A stochastic model of the processes involved in the measurement of the activity of the 222Rn decay products was developed. The distributions of the relevant factors, including air sampling and radionuclide collection, are propagated using Monte Carlo simulation to the final distribution of the measurement results. The uncertainties of the 222Rn decay products concentrations in the air are realistically evaluated.

Efficiency computation for gamma-ray spectrometry assessment of samples with intrinsic inhomogeneity.

The full energy peak efficiency for inhomogeneous samples, comprising a number of components with different activity and matrix is evaluated by Monte Carlo simulation. The distribution of the values of the efficiency due to the effects of inhomogeneity is constructed. In the particular case when one of the components is a highly active and highly attenuating medium, then the efficiency and its uncertainty at low energies is sensitive to the properties of this component, including the dimensions of the grains.

Application of GUM Supplement 1 to uncertainty of Monte Carlo computed efficiency in gamma-ray spectrometry.

The uncertainty of quantities relevant in gamma-ray spectrometry (efficiency, transfer factor, self-attenuation FA and coincidence summing FC correction factors) is realistically evaluated by Monte Carlo propagation of the distributions characterizing the parameters on which these quantities depend. Probability density functions are constructed and summarized as recommended in the GUM Supplement 1 and compared with the values obtained using the traditional approach (GUM uncertainty framework). Special cases when this approach encounters difficulties (FC uncertainty due to the uncertainty of decay scheme parameters, effect of activity and matrix inhomogeneity on efficiency) are also discussed.

Distribution of the ²²²Rn decay products from a ²²6Ra solution in a PTB ampoule--implications for calibration.

The spatial distribution of the activity in a flame-sealed PTB ampoule containing (226)Ra solution, incompletely filling the ampoule, was studied by autoradiography and by taking well-collimated gamma-ray spectra in 23 sections of the ampoule. A complex activity profile was observed and used for the Monte Carlo simulation of the ampoule by the GESPECOR code. The computed values of the efficiency transfer factors are in agreement with the experimental data. As a byproduct, the radon partition ratio in the solution-air phases was obtained.

Improved method for the assessment of 6°Co and ¹³4Cs point sources in samples with non-homogeneous matrix.

The correlation method (Suvaila et al., 2013) is extended to the assessment of (60)Co and (134)Cs point sources embedded in non-homogeneous samples. Experimental data reveal a strong correlation between the efficiency ε(1173 keV) and the ratio of the count rate R in the sum peak of 2505 keV and in the peak of 1332 keV. The correlation pattern and the measured R(2505)/R(1332) give unbiased values for ε(1173 keV), independent of source position and sample matrix. Monte Carlo simulations for (60)Co and (134)Cs support this conclusion.

Improving the assessment of activity in samples with non-uniform distribution using the sum peak count rate.

In this work a method for the evaluation of the activity when a point source containing (60)Co is located in an unknown position within a sample is developed. The method can be applied if the count rate in the 2,505 keV sum peak has an acceptable uncertainty. It is based on the correlation between the apparent efficiency for the 1,173 keV peak and the ratio of the count rate in the sum peak of 2,505 keV and in the 1,332 keV peak. The correlation was observed in the measurements of a (60)Co point source located in various positions in a soil sample. The measurements were done with a 47% efficiency n-type HPGe detector. The correlation is also observed in the measurements and simulations done with a Compton-suppressed spectrometer having a 100% n-type HPGe detector. The results obtained with the proposed method are less affected by the uncertainty of the position of the point source than the results obtained using the standard methods of activity evaluation.

Monte Carlo simulation by GEANT 4 and GESPECOR of in situ gamma-ray spectrometry measurements.

The application of GEANT 4 and GESPECOR Monte Carlo simulation codes for efficiency calibration of in situ gamma-ray spectrometry was studied. The long computing time required by GEANT 4 prevents its use in routine simulations. Due to the application of variance reduction techniques, GESPECOR is much faster. In this code specific procedures for incorporating the depth profile of the activity were implemented. In addition procedures for evaluating the effect of non-homogeneity of the source were developed. The code was validated by comparison with test simulations carried out with GEANT 4 and by comparison with published results.

Calibration of a low-level anti-Compton underground gamma-spectrometer by experiment and Monte Carlo.

In this work we present the experimental and Monte Carlo calibration of the Compton-suppressed spectrometer of the IAEA's Environment Laboratories, Monaco. For this purpose the GESPECOR code was extended to include the specific geometry and to implement the veto logic, integrated with the coincidence summing module of the code. The simulation results are in good accordance with experimental calibrations. The code is fast and user-friendly, able to evaluate the efficiency and the correction factors for nuclides with arbitrary complex decay schemes.

The examination of source distribution in a large sample by Monte Carlo simulation.

In this work, realistic Monte Carlo simulations were carried out for several distributions of activity in a waste drum to observe the dependence of the efficiency on the source distribution and to test whether the efficiency can be correlated with the shape of the spectra.

Precise measurement and calculation of coincidence summing corrections for point and linear sources.

Point sources of (60)Co, (133)Ba, (134)Cs and (152)Eu, calibrated at Physikalisch-Technische Bundesanstalt were measured in 13 positions on the axis of a 50% relative efficiency p-type detector. The peak and total efficiencies were calibrated using single photon emitting nuclides. Precise experimental values of the coincidence summing corrections were evaluated in each geometry. Synthetic linear source data, as well as the corresponding peak and total efficiency curves, were prepared using the dependence of the count rates on the position of the emitting point. The coincidence summing corrections for the linear sources were computed, analyzed with respect to different approximations and compared with simulations carried out with GESPECOR.

On within sample homogeneity testing using gamma-ray spectrometry.

Several problems concerning (133)Ba activity distribution in a cylindrical source were addressed by measurements with a 47% n-type HPGe detector and by simulation. Within-sample homogeneity was tested using the count rates from normal and from pure sum peaks. The sensitivity of front versus back source measurements to deviations from uniform distribution was studied. In the case of distributions symmetric with respect to the median plane of the source quantities that can provide information on the distribution of the activity were proposed.

Photon emission probabilities of 176Lu.

An aqueous solution containing dissolved lutetium nitrate with a natural isotopic composition was measured by means of gamma-ray spectrometry to determine emission probabilities of photons which are emitted as a consequence of the decay of the primordial isotope (176)Lu. The geometry and matrix of the sample were taken into account in terms of efficiency transfer factors computed with the aid of the upgraded Monte Carlo simulation software GESPECOR. A combination of these results, with the activity determination via liquid scintillation counting using the CIEMAT/NIST efficiency tracing method, yields absolute values of P(X)(54.61 keV)=0.095(4), P(X)(55.79 keV)=0.168(5), P(γ)(88.36 keV)=0.149(5), P(γ)(201.83 keV)=0.777(9) and P(γ)(306.84 keV)=0.929(9). In addition, the upper limit of the probability for a potential electron-capture decay of (176)Lu was improved using additional high resolution gamma-ray spectra taken with a planar HPGe detector.

Simulation studies of the response function of a radioactive waste assay system.

A simulation program based on GEANT 3.21 toolkit was developed to simulate the response function of ISOCART (Ortec) gamma-ray spectrometry system applied to radioactive waste drum assessment. In view of studying the effects of possible non-homogeneous radioactivity distribution in the drum, the volume of the drum was fictitiously divided into several spatial domains. The simulation program was applied repeatedly considering each time the source distributed in another domain. In this way the expected spectra in the energy range from 50 to 2000keV as well as the full energy peak and the total efficiencies were obtained for the case when the source is distributed in each domain and also for the case when the source is uniformly distributed in the entire drum.

Application of response functions to make efficient Monte Carlo simulations of germanium detectors.

A new Monte Carlo procedure to compute the efficiency and the coincidence summing corrections for closed end HPGe detectors was developed. In this procedure detector specific response functions that give the probability of getting a signal in the peak or in the total spectrum for photons incident on the end cap of the detector are used. The procedure is time efficient in the case of large scale computations because part of the simulation is circumvented by the use of pre-computed response functions.

ETNA software used for efficiency transfer from a point source to other geometries.

The quality of the results of gamma spectrometry measurement depends directly on the accuracy of the detection efficiency in the specific measurement conditions. The purpose of this work is to examine the applicability of the efficiency transfer method using ETNA software for the computation of the efficiency in various measurement geometries on the basis of the measured efficiency for reference point source geometry located at 100 mm distance from the high purity germanium (HPGe) detector.

On the Monte Carlo simulation of HPGe gamma-spectrometry systems.

Typical applications of Monte Carlo simulations in low-level gamma-ray spectrometry are presented. The current state of coincidence summing computations is briefly reviewed. Several problems concerning direct computation of the efficiency by Monte Carlo simulations are discussed.

Comprehensive software for the assessment of 222Rn and 220Rn decay products based on air sampling measurements.

A computational tool dedicated to the measurement of (222)Rn and (220)Rn decay products by air sampling is presented. alpha- or gamma-spectrometry measurements, gross alpha or beta counting, as well as a combination of them are considered. Special attention is given to the evaluation of the uncertainty budget of the results. Besides typical applications in the analysis of experimental data, the software can be used for assessing the expected quality of a measurement protocol and for optimizing it, by generating and analyzing sets of realistic synthetic data.

A tool for processing decay scheme data that encompasses coincidence summing calculations.

A description is given of the tool implemented in GESPECOR to process coincidence summing corrections and derive decay scheme data. The method of analysis produces relevant decay scheme data and the joint emission probability for any group of photons (gamma emissions, X-rays, annihilation photons resulting from beta(+) decay) emitted in the decay of any nuclide with less than 100 levels.