Octavian Sima

Accurate computation of coincidence summing corrections in low level gamma-ray spectrometry

The GESPECOR (Germanium Spectrometry Correction factors) software, previously developed for computing the self-attenuation and coincidence summing corrections, was applied to the computation of the coincidence summing correction factors for a well-type and two coaxial HPGe detectors. Cylindrical samples as well as Marinelli beaker samples were considered. The computed values are in good agreement with carefully measured values. A detailed study of the uncertainties assigned to the results was carried out. The analysis shows that the procedures used in GESPECOR are reliable and provide results with a well defined accuracy.

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.

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.

Monte Carlo simulation versus semiemperical calculation of autoabsorption factors for semiconductor detector calibration in complex geometries

Abstract Monte Carlo calculations of detection efficiency for gamma ray spectrometry using Marinelli geometry are reported. An algorithm based on the moments of the photon path length through the sample is implemented and autoabsorption factors in function of linear attenuation coefficient are computed in this way. The energy dependence of these factors is investigated. Their variation with detector parameters is presented. The results are compared with semiemperical estimations and an analytical formula for the equivalent thickness of the sample is reported.

Uncertainties in gamma-ray spectrometry

High resolution gamma-ray spectrometry is a well-established metrological technique that can be applied to a large number of photon-emitting radionuclides, activity levels and sample shapes and compositions. Three kinds of quantitative information can be derived using this technique: detection efficiency calibration, radionuclide activity and photon emission intensities. In contrast to other radionuclide measurement techniques gamma-ray spectrometry provides unambiguous identification of gamma-ray emitting radionuclides in addition to activity values. This extra information comes at a cost of increased complexity and inherently higher uncertainties when compared with other secondary techniques. The relative combined standard uncertainty associated with any result obtained by gamma-ray spectrometry depends not only on the uncertainties of the main input parameters but also on different correction factors. To reduce the uncertainties, the experimental conditions must be optimized in terms of the signal processing electronics and the physical parameters of the measured sample should be accurately characterized. Measurement results and detailed examination of the associated uncertainties are presented with a specific focus on the efficiency calibration, peak area determination and correction factors. It must be noted that some of the input values used in quantitative analysis calculation can be correlated, which should be taken into account in fitting procedures or calculation of the uncertainties associated with quantitative results. It is shown that relative combined standard uncertainties are rarely lower than 1% in gamma-ray spectrometry measurements.

Deep underground gamma spectrometric measurement of 26Al in meteorite samples.

26Al is routinely measured in meteorite samples by accelerator mass spectrometry (AMS). PTB participates in an international intercomparison between laboratories using AMS and gamma-ray spectrometry to improve the quality of such measurements. We performed gamma spectrometric measurements at the underground laboratory UDO using the GESPECOR software to calculate corrections for efficiency transfer, self-attenuation and coincidence summing. Four meteorite samples were measured for more than 30 days each but nevertheless, the uncertainty budgets are dominated by the uncertainties of the count rates.

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.

Equivalence of computer codes for calculation of coincidence summing correction factors – Part II

The aim of this study was to check for equivalence of computer codes that are capable of performing calculations of true coincidence summing (TCS) correction factors. All calculations were performed for a set of well-defined detector parameters, sample parameters and decay scheme data. The studied geometry was a point source of (133)Ba positioned directly on the detector window of a low-energy (n-type) detector. Good agreement was established between the TCS correction factors computed by the different codes.

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 IAEAs 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.

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.