A benchmark for Monte Carlo simulations in gamma-ray spectrometry Part II: True coincidence summing correction factors.

The goal of this study is to provide a benchmark for the use of Monte Carlo simulation when applied to coincidence summing corrections. The examples are based on simple geometries: two types of germanium detectors and four kinds of sources, to mimic eight typical measurement conditions. The coincidence corrective factors are computed for four radionuclides. The exercise input files and calculation results with practical recommendations are made available for new users on a dedicated webpage.

Measurement of the absolute gamma-ray emission intensities from the decay of 103Pd.

Palladium-103 decays through electron capture to excited levels of 103Rh, and especially to the 39.748-keV metastable state. A high activity palladium chloride solution was standardized by liquid scintillation, using the Triple-to-Double Coincidence Ratio method. The absolute photon emission intensities were determined by gamma-ray spectrometry using point sources prepared with the standard solution. Different detectors and measuring conditions were used to cross-reference the results. The most intense photon emission intensities are derived with about 1% relative combined standard uncertainty.

Measurement of the absolute gamma-ray emission intensities from the decay of 147Nd.

The 2011 Decay Data Evaluation Project (DDEP) evaluation for 147Nd includes recommended absolute emission intensities for the two main gamma-rays at 91.105 (2) keV and 531.016 (22) keV of 0.284 (18) and 0.127 (9) respectively, i.e. with uncertainties of 6.3% and 7.1%. These large uncertainties stem from inconsistencies in the published data and are unfit for modern purposes, since the production of 147Nd is used as an important neutron flux dosimeter. The LNE-LNHB has undertaken new absolute gamma-ray emission intensity measurements. The results of these measurements will be presented, along with a full uncertainty budget, and their effect on the recommended data uncertainties will be discussed.

Consistency test of coincidence-summing calculation methods for extended sources.

An internal consistency test of the calculation of coincidence-summing correction factors FC for volume sources is presented. The test is based on exact equations relating the values of FC calculated for three ideal measurement configurations. The test is applied to a number of 33 sets of FC values sent by 21 teams. Most sets passed the test, but not the results obtained using the quasi-point source approximation; in the latter case the test qualitatively indicated the magnitude of the bias of FC.

Activity measurements and determination of nuclear decay data of 166Ho in the MRTDosimetry project.

Holmium-166 is a high-energy ß--emitter radionuclide (~ 1.8 MeV) with a short half-life (~26.8h) that offers great potential as an alternative to 90Y for the treatment of liver cancer based on radioembolization. The possibility of quantitative Single Photon Emission Computed Tomography (SPECT) imaging of the main γ-ray emission at 80.6 keV, in addition to strong paramagnetic properties suitable for Magnetic Resonance Imaging (MRI), complement this therapeutic potential. The present paper describes the measurements carried out in three European radionuclide metrology laboratories for primary standardization of 166Ho and new determinations of X- and γ-ray photon-emission intensities in the framework of the European EMPIR project MRTDosimetry. New half-life measurements were also performed.

A benchmark for Monte Carlo simulation in gamma-ray spectrometry.

Monte Carlo (MC) simulation is widely used in gamma-ray spectrometry, however, its implementation is not always easy and can provide erroneous results. The present action provides a benchmark for several MC software for selected cases. The examples are based on simple geometries, two types of germanium detectors and four kinds of sources, to mimic eight typical measurement conditions. The action outputs (input files and efficiency calculation results, including practical recommendations for new users) are made available on a dedicated webpage.

Study of scattering effects in low-energy gamma-ray spectrometry.

The photon-scattering effect was studied in the low-energy range 15-80keV with planar and coaxial germanium detectors. Different experimental tests have been conducted with a point source, progressively reducing the matter around the radioactive deposit, to investigate the origin and characteristics of the different spectrum components due to scattered photons (bumps). These tests were completed by Monte Carlo simulations. Finally, a peak-shape calibration was performed using several radionuclides ((109)Cd, (241)Am, (133)Ba, (137)Cs and (152)Eu). This allowed in identifying the main contributions to the bumps and improving the spectra processing.

Measurement of (241)Am L X-ray emission probabilities.

Measurements of (241)Am L X-ray emission probabilities were conducted using both HPGe and Si(Li) detectors. The efficiency calibrations of these detectors were performed by means of a tunable, monochromatic photon beam and the determination of the thickness of absorbing materials inside the detector. These efficiency calibrations were obtained without any reference to radionuclide decay data, and with 0.8% relative standard uncertainty. The complex L X-ray region was processed using Voigt functions and by taking account of the detector response function established with the monochromatic beam. Twenty-two components of the L X-ray group were identified and quantified. The present results are compared with previously published data.

Comparative study of two drying techniques used in radioactive source preparation: freeze-drying and evaporation using hot dry nitrogen jets.

Quantitative solid sources are used widely in the field of radionuclide metrology. With the aim to improve the detection efficiency for electrons and x-rays, a comparative study between two source drying techniques has been undertaken at LNE-Laboratoire National Henri Becquerel (LNE-LNHB, France). In this paper, freeze-drying using commercial equipment is compared with a system of drying using hot jets of nitrogen developed at Institute for Reference Materials and Measurements (IRMM, Belgium). In order to characterize the influence of self-absorption, the detection efficiencies for (51)Cr sources have been measured by coincidence counting and photon spectrometry.

An intercomparison of Monte Carlo codes used in gamma-ray spectrometry.

In an intercomparison exercise, the Monte Carlo codes most commonly used in gamma-ray spectrometry today were compared with each other in order to gauge the differences between them in terms of typical applications. No reference was made to experimental data; instead, the aim was to confront the codes with each other, as they were applied to the calculation of full-energy-peak and total efficiencies. Surprising differences between the results of different codes were revealed.

Consistency of photon emission intensities for efficiency calibration of gamma-ray spectrometers in the energy range from 20keV to 80keV.

The efficiency calibration for different high-purity germanium detectors in the low-energy range was established by the conventional method, using standard radioactive sources. The peak shapes were carefully analysed taking account of natural linewidth, full-energy width at half maximum and scattering. Complementary information was obtained by Monte Carlo simulation using the PENELOPE code, after optimization of the geometrical parameters. This was used to measure photon emission intensities of some low-energy emitting radionuclides, including 133Ba, and compared to the tabulated values.

Improvement of the activity measurement method for solid dosimeters emitting X-rays.

Today, there is growing interest for neutrons in the intermediate energy range between 100keV and 1MeV, which are responsible for damaging materials in reactor. To improve this deficiency, we use rhodium and niobium which, through the inelastic neutron scattering reaction, leads to the formation of 103mRh and 93mNb low-energy X-emitters. This paper describes the improvements and validation made on this type of complex measurement by X spectrometry: self-attenuation, fluorescence correction, and emission intensity were poorly known previously.

Measurement of absolute K X-ray emission intensities in the decay of 103mRh.

A new experiment was designed to measure the photon emission intensities in the decay of 103mRh. The rhodium samples were activated in the ISIS experimental nuclear reactor at CEA Saclay. The procedure includes an absolute activity measurement by liquid scintillation counting using the Triple-to-Double Coincidence Ratio method, followed by X-ray spectrometry using a high-purity germanium detector to determine the photon emission intensities. The new result (IX = 0.0825 (17)) is derived with a significant reduction of the uncertainty.

Standardization of 222Rn by LSC and comparison with alpha- and gamma-spectrometry.

Liquid scintillation counting (LSC) was used for the measurement of 222Rn in equilibrium with its daughters, with detection efficiency close to 5. The appropriate corrections were considered, including one related to the probability that the 165-micros half-life 214Po decays during the dead time of the counter initiated by the disintegration of his parent nuclide, 214Bi. The dead-time determination of a commercial LS counter is also presented using a 222Rn standard source. The LSC 222Rn sources were prepared by transfer of 222Rn produced by a solid 226Ra source into LSC cocktail frozen at 77K, flame-sealed afterwards. They were measured using the LNHB triple coincidence counter with adjustable extending-type dead-time unit, between 8 and 100 micros; two different procedures were used to calculate an effective dead time and then to deduce the counting rate extrapolated to zero dead-time value. The LSC results were compared with those obtained by cryogenic alpha-particle spectrometry (LNHB system) and by gamma-ray spectrometry for the same radon source in the LSC vial; the geometry transfer coefficient was calculated using the ETNA software. Measurement results and uncertainties are discussed.

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.

Measurement of a 103Pd solution using the TDCR method by LSC.

Palladium 103 is a radionuclide used in brachytherapy sources for the treatment of prostate cancers and also for other medical applications. It decays through electron capture to excited levels of 103Rh. This paper describes the calculation method used to compute the detection efficiency in the framework of the triple to double coincidence ratio model. The calculation of the energy transferred to the scintillator is made by considering the various atomic events following the electron capture and the electron conversion. The energy deposited in the scintillator after the absorption of X-rays is calculated using the PENELOPE stochastic calculation code. The main contributors to the final uncertainty and their covariance matrix are discussed. As the calculation method cannot be reduced to an explicit function, this paper describes the Monte Carlo method used for the evaluation of uncertainties.

Intercomparison of efficiency transfer software for gamma-ray spectrometry.

The EUROMET project 428 examines efficiency transfer results for Ge gamma-ray spectrometers when the efficiency is known for a reference point source geometry. For this, different methods are used, such as Monte Carlo simulation or semi-empirical computation. The exercise compares the application of these methods to the same selected experimental cases to determine the usage limitations versus the requested accuracy. For carefully examining these results and trying to derive information for improving the computation codes, this study was limited to a few simple cases. The first part concerns the simplest case of geometry transfer, i.e., using point sources for 3 source-to-detector distances: 2, 5 and 20 cm; the second part deals with transfer from point source geometry to cylindrical geometry with three different matrices. The general conclusion is that the deviations between the computed results and the measured efficiencies are mostly within 10%. The quality of the results is rather inhomogeneous and shows that these codes cannot be used directly for metrological purposes. However, most of them are operational for routine measurements when efficiency uncertainties of 5-10% can be sufficient.

Standardization of xenon-127 and measurement of photon emission intensities.

Xenon-127 was standardized by internal gas counting using three proportional counters in a differential arrangement to eliminate edge effects. The detection efficiency of the proportional counters was calculated by considering the cascade of events following the electron capture and associated gamma transitions. Activity per unit volume was measured with 0.7% relative standard uncertainty. Gamma-ray spectrometry was performed and absolute photon emission intensities were derived. This study shows that (127)Xe could be a surrogate for (133)Xe for the calibration of remote radio-xenon monitoring stations.

Photon emission intensities in the decay of 108mAg and 110mAg.

This study focuses on two radioisotopes of silver, (108m)Ag and (110m)Ag, characterized by a complex decay scheme. Each isotope has two disintegration modes, the isomeric transition leading to the daughter isotope ((108)Ag and (110)Ag, respectively) with a short half-life. The radioactive solution was obtained by neutron activation on silver powder enriched in (109)Ag. Gamma-spectrometry was carried out using a calibrated high purity germanium detector. The main relative photon emission intensities for both radionuclides were obtained and compared with previously published values.

Equivalence of computer codes for calculation of coincidence summing correction factors.

The aim of the study was to check for equivalence of computer codes that can perform calculations of true coincidence summing correction factors. All calculations were performed for a set of well-defined detector and sample parameters, without any reference to empirical data. For a p-type detector model the application of different codes resulted in satisfactory agreement in the calculated correction factors. For high-efficiency geometries in combination with an n-type detector and a radionuclide emitting abundant X-rays the results were scattered.