Intercomparison of efficiency transfer software for gamma-ray spectrometry

High resolution gamma-ray spectrometry for routine measurements of environmental samples

A practical and comprehensive experimental approach for radioactivity measurements of voluminous environmental samples using high-resolution gamma-ray spectrometry with High Purity Germanium (HPGe) detector is presented. The radioanalytical procedure includes sample preparation, detector calibration, implemented corrections, quality control and assurance; and it is demonstrated with the determination of natural and anthropogenic radionuclides in water and soil standard samples. The methodology is verified, and the results are validated by intercomparison with the refence values provided as part of a worldwide proficiency test.

Atmospheric depositional fluxes of 210Pb in bulk precipitation at the Adriatic coast, Croatia

The atmospheric bulk depositional fluxes of 210Pb were measured at a station on the Adriatic coast, Croatia over 4 years period from March 2017 to December 2020. The monthly depositional fluxes followed oceanic deposition patterns with a lower flux between 0.0735 and 16.9 Bq m-2 month-1. The volume-weighted activities were 0.000514 and 1.35 Bq L-1 and decreased with increasing precipitation. A clear seasonal trend was observed with higher depositional flux in autumn and minimum value in the winter season. The average annual bulk depositional flux and volume-weighted activities of 210Pb were 73.8 Bq m-2 y-1 and 0.119 Bq L-1 respectively. The precipitation normalized enrichment factor (α) indicates higher depositional fluxes of 210Pb during summer and spring than desired value according to the amount of precipitation. We found that the 210Pb depositional fluxes in the coastal stations are lower due to 210Pb-depleted oceanic air masses and increase with the amount of precipitation.

Efficiency calibrations of radiation detectors for source localization and characterization at long source-detector distances for gamma and neutron sources

Deployment of radiation detectors under field conditions for the purposes of security, safety or response has increased in recent years. Effective use of such instruments in the field necessitates careful consideration of the efficiency of the detector - both peak and total - at distances which may extend beyond 100 m. Difficulties in addressing the determination of both peak and total efficiencies across the energy range of interest and at long distances reduces the utility of such systems in effectively characterising radiation sources in the field. Empirical approaches to such calibrations are difficult. Approaches such as Monte Carlo simulations can become challenging with respect to time and computational requirements as source-detector distances become greater and in consideration of total efficiency. This paper presents a computationally efficient method of calculating peak efficiency at distances more than 300 m using efficiency transfer from a parallel beam geometry to point sources at extended distances. The relationship between total and peak efficiency at extended distances is explored and means of estimating the total efficiency from the peak efficiency are discussed. The ratio of the total efficiency to the peak efficiency increases as a function of the source-detector distance. The relationship is linear at distances longer than 50 m and is independent of photon energy. Usefulness of the efficiency calibration as a function of the source-detector distance was demonstrated in a field experiment. Total efficiency calibration measurements were performed for a neutron counter. An AmBe source was then successfully localized and characterised using four measurements at arbitrary locations far away from the unknown source. This kind of capability is useful for the authorities responding to nuclear accidents or security events. It has important operational implications, including the safety of the personnel involved.

Radionuclide metrology: confidence in radioactivity measurements

Radionuclides, whether naturally occurring or artificially produced, are readily detected through their particle and photon emissions following nuclear decay. Radioanalytical techniques use the radiation as a looking glass into the composition of materials, thus providing valuable information to various scientific disciplines. Absolute quantification of the measurand often relies on accurate knowledge of nuclear decay data and detector calibrations traceable to the SI units. Behind the scenes of the radioanalytical world, there is a small community of radionuclide metrologists who provide the vital tools to convert detection rates into activity values. They perform highly accurate primary standardisations of activity to establish the SI-derived unit becquerel for the most relevant radionuclides, and demonstrate international equivalence of their standards through key comparisons. The trustworthiness of their metrological work crucially depends on painstaking scrutiny of their methods and the elaboration of comprehensive uncertainty budgets. Through meticulous methodology, rigorous data analysis, performance of reference measurements, technological innovation, education and training, and organisation of proficiency tests, they help the user community to achieve confidence in measurements for policy support, science, and trade. The author dedicates the George Hevesy Medal Award 2020 to the current and previous generations of radionuclide metrologists who have devoted their professional lives to this noble endeavour.

Study of coincidence summing effect using Monte Carlo simulation to improve large samples measurement for environmental applications

A new measurement model composed of High Purity Germanium detector HPGe including Lead Shield with a mixed source of Am-241, Cd-109, …in a soil matrix packed in a cylindrical container was developed by Monte Carlo N-Particles code MCNP5. The Monte Carlo models being largely used for efficiencies calculations in routine environmental radioactivity measurements of high volume soil samples. In order to validate the simulation, a multi-radionuclides soil matrix source in a cylindrical container is prepared. Comparing the simulated results to the experimental ones, allows us to correct coincidence summing in the soil standard and study its effect in activity measurements where a good agreement is found between corrected activities and results found by another geometry and show how uncorrected efficiency curves lead to erroneous activity calculation. Finally, the developed model is applied to study the distribution of natural and artificial radioactivity in a forest site in Bainem, northern of Algeria, these results are invested for radiological impact and soil erosion studies in the study Area.

Validation of EFFTRAN for efficiency transfer to distant geometries and volume sources

The efficiency transfer procedure from a geometry where a volume source was placed directly on the endcap of a germanium detector to three different distant geometries was carried out using the EFFTRAN code. One of these distant geometries included absorbers consisting of poly(methyl methacrylate). The efficiency transfer to this geometry therefore had to be realized as a two-stage transfer, since a direct efficiency transfer is not possible using EFFTRAN in such a case. Efficiency transfer to all three distant geometries yielded results which can be considered as fit-for-purpose in e.g. most of the applications of gamma ray spectrometry.

Verification of physical parameters of insity p-type HPGe detector by scan method using collimated low energy photon beam and MNCP simulation

In this work, the physical dimensions and the actual position of germanium crystal within a detector housing, the homogeneity of the crystal surface and outer dead layer thickness for a p-type HPGe detector were confirmed by the scan method using the collimated low energy photon beams combined with Monte Carlo simulation. The length and the diameter of the crystal were found to match with the values supplied by the manufacturer in discrepancy of about 3%. Only one mounting strap (Typical) for holding the crystal inside the mounting cup instead of two which is indicated in the detector drawing supplied by manufacturer was revealed by scanning along the lateral face of detector. Scanning on the front surface and around the lateral face of detector by the collimated 59.5 keV photon beam verified the outer dead layer thicknesses at the front surface and lateral face of the crystal averagely increases about 6.5% and 12% respectively. Adjusting the detector parameters for MCNP simulation by these verified values, the simulated peak efficiencies for different photon energies become being in accordance with the experimental peak efficiencies.

Efficiency calibration of HPGe detector in a PGNAA system for the measurement of aqueous samples

In this work, a method was proposed for efficiency calibration of HPGe detector employed in a prompt gamma-ray neutron activation analysis (PGNAA) system for large aqueous sample analysis. Experimental measurements of a reference surface gamma-ray source were combined with efficiency transfer method to obtain efficiency of a volumetric source. Then efficiency curve over the energy range 0.5-8.5 MeV were determined with prompt gamma-rays of chlorine. To evaluate the performance of the calibration curve, an internal standard method was used to determine the aqueous sample containing manganese based on the calibration curve. The 7.058 MeV and 7.243 MeV manganese gamma-ray peaks were used to calculate the mass of manganese. The relative deviations of calculated values and actual value were 5.0% and 6.5%, respectively.

Measurement of absolute γ-ray emission probabilities in the decay of 235U

Accurate measurements were performed of the photon emission probabilities following the α decay of ²³⁵U to ²³¹Th. Sources of highly enriched ²³⁵U were characterised in terms of isotopic composition by mass spectrometry and their activities were standardised by means of alpha-particle counting at a low defined solid angle. The standardised sources were subsequently measured by high-resolution γ-ray spectrometry with calibrated high-purity germanium detectors to determine the photon emission probabilities. Four laboratories participated in this work and reported emission probabilities for 33 γ-ray lines. Most of them agree with previously published evaluated data. In addition, new values are proposed for γ-lines which have been measured only once in the past.

A gamma-ray spectrometry analysis software environment

At the JRC-Geel's RadioNuclide Metrology sector, a Monte Carlo code based on EGSnrc, and a general purpose calculation sheet implemented in Microsoft Excel^®, have been developed to make the quantitative gamma-ray spectrometry analysis of samples simpler and more robust. The further aim is that the software can be used by non-experts in gamma-ray spectrometry e.g. external researchers using JRC-Geel's facilities through the EUFRAT transnational access scheme. This paper presents the developed Monte Carlo software and the functionality included in the calculation sheet.

A practical evaluation of measurement uncertainty in gamma-ray efficiency curve determination through an analytical approach

An example of uncertainty evaluation of an efficiency curve is reported. In the applied method, the efficiency curve is determined through the weighted least square method based on measured efficiencies employing a mixed-radionuclide standard source. The variance-covariance matrix of the efficiencies is determined by evaluating the uncertainties of the counting, the calibrated radioactivities, the correction factors for true coincidence summing, and the inadequacy of the fitting function. The proposed method can be a basis to develop a more general procedure.

Experimental HPGe coaxial detector response and efficiency compared to Monte Carlo simulations

The peak efficiency for photons hitting the frontal surface of a medium volume n-type HPGe coaxial detector is mapped using acutely collimated beams of energies between 31 and 383 keV from a (133)Ba radioactive source. Simulated values obtained with the Monte Carlo radiation transport code penelope, using a model that respected actual detector dimensions and physical constants while varying dead-layer thicknesses, allowed us to fit the experimental results in the detector bulk but not near its rim. The spectra of a (137)Cs source were measured using the detector shielded from the natural background radiation, with and without a broad angle collimator. The corresponding simulated spectra, using the fitted dead-layer thicknesses, underestimate the continuum component of the spectra and overestimate the peak efficiency, by less than ten percent in the broad angle collimator arrangement. The simulated results are sensitive to the photon attenuation coefficients.

Characterisation of the IAEA-375 Soil Reference Material for radioactivity

The Joint Research Centre Institute for Reference Materials and Measurements (JRC-IRMM) participated in a research project initiated by the International Atomic Energy Agency (IAEA) to upgrade some of its existing reference materials (RMs). The aim of the work described in this article was to determine the activity concentration of a series of radionuclides in the IAEA-375 soil RM with values traceable to the SI units. The radionuclides (40)K, (134)Cs, (137)Cs, (212)Pb, (212)Bi, (214)Pb and (214)Bi were measured by γ-ray spectrometry after drying the sample and placing it in a suitable container. The (90)Sr was assessed by liquid scintillation counting after dissolution of the soil by wet digestion and chemical separation of Sr by extraction chromatography. This soil RM was used later as basis for the 2010 EC Interlaboratory Comparison on Radionuclides in Soil.

Efficiency transfer using the GEANT4 code of CERN for HPGe gamma spectrometry

In this work we apply the GEANT4 code of CERN to calculate the peak efficiency in High Pure Germanium (HPGe) gamma spectrometry using three different procedures. The first is a direct calculation. The second corresponds to the usual case of efficiency transfer between two different configurations at constant emission energy assuming a reference point detection configuration and the third, a new procedure, consists on the transfer of the peak efficiency between two detection configurations emitting the gamma ray in different energies assuming a "virtual" reference point detection configuration. No pre-optimization of the detector geometrical characteristics was performed before the transfer to test the ability of the efficiency transfer to reduce the effect of the ignorance on their real magnitude on the quality of the transferred efficiency. The obtained and measured efficiencies were found in good agreement for the two investigated methods of efficiency transfer. The obtained agreement proves that Monte Carlo method and especially the GEANT4 code constitute an efficient tool to obtain accurate detection efficiency values. The second investigated efficiency transfer procedure is useful to calibrate the HPGe gamma detector for any emission energy value for a voluminous source using one point source detection efficiency emitting in a different energy as a reference efficiency. The calculations preformed in this work were applied to the measurement exercise of the EUROMET428 project. A measurement exercise where an evaluation of the full energy peak efficiencies in the energy range 60-2000 keV for a typical coaxial p-type HpGe detector and several types of source configuration: point sources located at various distances from the detector and a cylindrical box containing three matrices was performed.

Experimentally determined vs. Monte Carlo simulated peak-to-valley ratios for a well-characterised n-type HPGe detector

Measurements and simulations to investigate the contributing factors to the peak-to-valley (PTV) ratio have been both experimentally determined as well as Monte Carlo simulated for a well-characterised HPGe n-type detector together with a Cs-137 gamma source encapsulated in thin polystyrene. Measurements were carried out in a low-background gamma counting facility at Lund University. The results of the PTV ratio have been compared to distinguish what components or variables in the setup that significantly influence the ratio. In addition to manufacture specifications, the detector components have been examined using planar X-ray, source scanning and computer tomography in order to determine and verify component dimensions when necessary. In spite of these efforts a discrepancy of approximately 25% for thin absorbers in the PTV ratio between measurements and calculations is observed. However, this discrepancy becomes less significant for larger absorbing layers of copper (>1mm). This indicates that it would be difficult to achieve a field calibration for in-situ gamma spectrometry using the PTV ratio that could position a Cs-137 source in soil depth shallower than corresponding 1mm layer of copper. The results also showed that when building a detector in simulations part by part, the inner dead layer, and the contact pin are of great importance for the accuracy of the PTV ratio simulations.

Application of GEANT4 simulation on calibration of HPGe detectors for cylindrical environmental samples

The determination of radionuclide activity concentration requires a prior knowledge of the full-energy peak (FEP) efficiency at all photon energies for a given measuring geometry. This problem has been partially solved by using procedures based on Monte Carlo simulations, developed in order to complement the experimental calibration procedures used in gamma-ray measurements of environmental samples. The aim of this article is to apply GEANT4 simulation for calibration of two HPGe detectors, for measurement of liquid and soil-like samples in cylindrical geometry. The efficiencies obtained using a simulation were compared with experimental results, and applied to a realistic measurement. Measurement uncertainties for both simulation and experimental values were estimated in order to see whether the results of the realistic measurement fall within acceptable limits. The trueness of the result was checked using the known activity of the measured samples provided by IAEA.

Analytical approach for radioactivity correlation of disc sources with HPGe detector efficiency

The HPGe detector efficiency is measured as a function of source to detector separation using disc sources of (131)I with diameter ranging from 10 to 400mm. Detector efficiencies are characterized using single photon point-like standard sources at different distances; the calculated efficiencies for disc sources were analyzed by utilizing the double point detector model (DPDM) and the efficiency transfer method. The developed approach provided satisfactory results. The axial variation and radial dependence for disc sources efficiency determination in gamma-ray spectrometry were described with both gamma ray standard sources and measured samples as their extended sources.

Validation of efficiency transfer for Marinelli geometries

In the framework of environmental measurements by gamma-ray spectrometry, some laboratories need to characterize samples in geometries for which a calibration is not directly available. A possibility is to use an efficiency transfer code, e.g., ETNA. However, validation for large volume sources, such as Marinelli geometries, is needed. With this aim in mind, ETNA is compared, initially to a Monte Carlo simulation (PENELOPE) and subsequently to experimental data obtained with a high-purity germanium detector (HPGe).

The determination of the efficiency of a Compton suppressed HPGe detector using Monte Carlo simulations

A Compton suppressed high-purity germanium (HPGe) detector is well suited to the analysis of low levels of radioactivity in environmental samples. The difference in geometry, density and composition of environmental calibration standards (e.g. soil) can contribute to excessive experimental uncertainty to the measured efficiency curve. Furthermore multiple detectors, like those used in a Compton suppressed system, can add complexities to the calibration process. Monte Carlo simulations can be a powerful complement in calibrating these types of detector systems, provided enough physical information on the system is known. A full detector model using the Geant4 simulation toolkit is presented and the system is modelled in both the suppressed and unsuppressed mode of operation. The full energy peak efficiencies of radionuclides from a standard source sample is calculated and compared to experimental measurements. The experimental results agree relatively well with the simulated values (within ∼5 - 20%). The simulations show that coincidence losses in the Compton suppression system can cause radionuclide specific effects on the detector efficiency, especially in the Compton suppressed mode of the detector. Additionally since low energy photons are more sensitive to small inaccuracies in the computational detector model than high energy photons, large discrepancies may occur at energies lower than ∼100 keV.

Extended relative method of activity determination

An extension of the classical relative method of activity determination in gamma-ray spectrometry to standards that differ from the measured sample in size, density, composition and radionuclide contents is described. The approach is based on Monte Carlo simulations and was successfully tested against experimental data.

Improving the trade-off between simulation time and accuracy in efficiency calibrations with the code DETEFF

Quick and relatively simple procedures were incorporated into the Monte Carlo code DETEFF in order to consider the escape of Bremsstrahlung radiation and secondary electrons. The relative bias in efficiency values was thus reduced for photon energies between 1500 and 2000 keV, without any noticeable increment of the simulation time. A relatively simple method was also included to consider the rounding of detector edges. The validation studies showed relative deviations of about 1% in the energy range 10-2000 keV.

Application of the Monte Carlo method for the calibration of an in situ gamma spectrometer

A MCNP model was developed for the efficiency calibration of an in situ gamma ray spectrometry system based on a high purity germanium (HPGe) detector. The detector active crystal volume was adjusted semi-empirically against experimental measurements. Calculated full energy peak efficiency curves, over the photon energy range between 50 keV and 5 MeV, are presented for surface and slab source configurations. The effect of different collimator apertures and the contribution of different HPGe crystal regions in the detector response are also shown.

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.

Characterisation of the IAEA-152 milk powder reference material for radioactivity with assigned values traceable to the SI units

The Institute for Reference Materials and Measurements (IRMM) participated in a research project initiated by the International Atomic Energy Agency (IAEA) to upgrade some of its existing reference materials (RMs). The aim of the project is to improve the RM metrological status by establishing traceability of their assigned values to SI units. The purpose of the work described in this article was to establish traceability to the International System of Units (SI) of the activity concentrations of the radionuclides (134)Cs, (137)Cs, (40)K, and (90)Sr in the IAEA-152 milk powder RM. The choice of the particular RM was based on the concern about radioactivity levels in foodstuff. The sample preparation and the assaying of the activity concentrations in the milk powder, the methods used to achieve instrument calibrations and measurements traceable to the SI units, the data reduction and analysis, and finally, the results obtained are presented.

Analytical efficiency curve for coaxial germanium detectors

A new semi-empirical analytical expression of gamma-ray spectrometer efficiency curves has been proposed and a method for their parameters determination has been developed. The complete curve consists of two parts, first being the basic detector characterization curve of low number of parameters and second one being a corrective curve, which adjusts the final results to the required and achievable accuracy.