Full Energy Peak Efficiency Research Articles

The effect of front edge on efficiency for point and volume source geometries in p-type HPGe detectors

Monte Carlo (MC) simulations are increasingly being used as an alternative or supplement to the gamma spectrometric method in determining the full energy peak efficiency (FEPE) necessary for radionuclide identification and quantification. The MC method is more advantageous than the experimental method in terms of both cost and time. Experimental calibration with standard sources is difficult, especially for specimens with unusually shaped geometries. However, with MC, efficiency values can be obtained by modeling the geometry as desired without using any calibration source. Modeling the detector with the correct parameters is critical in the MC method. These parameters given to the user by the manufacturer are especially the dimensions of the crystal and its front edge, the thickness of the dead layer, dimensions, and materials of the detector components. This study aimed to investigate the effect of the front edge geometry of the detector crystal on efficiency, so the effect of rounded and sharp modeled front edges on the FEPE was investigated for <300 keV with three different HPGe detectors in point and volume source geometries using PHITS MC code. All results showed that the crystal should be modeled as a rounded edge, especially for gamma-ray energies below 100 keV.

Investigation of the gamma-ray efficiency for various scintillation detector systems

The calibrations of the full-energy peak efficiency, energy, and energy resolution are the main factors for successful quantitative gamma-ray analysis. The first purpose of this work is to present the calibration factors for 2"x2" ScintiPack Model 296 NaI(Tl). The detector was irradiated with radioactive sources ( 137 Cs, 60 Co, 152 Eu point sources, and 241 Am, 22 Na cylinder-shaped sources) that emit photons at different energies from 59 keV to 1408 keV. The detector efficiency was also evaluated using Geant4 based GATE simulation. The analytical equations for efficiency calibration were found out with their parameters. Since the results obtained from the experimental and simulation were compatible with each other and with literature results, then the simulation model was modified into the novel scintillation detector systems such as; LaBr3, GAGG(Ce), and SrI2 .

Hybrid analytical method for calibrating a standard NaI(Tl) gamma-ray scintillation detector using a lateral hexagonal radioactive source

In various fields of radiation and nuclear physics, research and its applications help in the management and control of human activities, especially in the analysis of nuclear waste and measurement of low activity environmental samples. The current study uses a hybrid analytical technique and a 152Eu lateral hexagonal radioactive source to calibrate a standard cylindrical NaI(Tl) γ-ray scintillation detector. The technique is a combination of experimental measurements and theoretical calculations because for each environmental and/or radioactive waste sample, a separate reference calibration source equivalent to samples of different compositions is required, which needs additional funding. This method is suitable for any source–detector geometry. It can be used to calculate the geometric, total, and full-energy peak efficiency and the P/T ratio using the model probability of detecting γ-radiation photons, in particular from a lateral hexagonal radioactive source. The detection probability can be mainly influenced by the direction of the γ-quantum motion. This can help build more realistic and complex models than the typical point source case, where simple integration is used to numerically calculate the efficiency of the detection system. Using this method, the self-attenuation and coincidence-summing effect can be calculated for any gamma source matrices. The results obtained can show how useful this technique can be for calibrating complex gamma-ray detectors with any type of gamma-ray source.

Calculating off-axis efficiency of coaxial HPGe detectors by Monte Carlo simulation

In beam geometries where a directed γ-ray beam hits the surface of a coaxial high purity germanium detector (HPGe), the detector efficiency is sensitive to the position where γ-rays initially hit the detector surface because the structure of the detector is nonuniform. This may cause inaccuracy of the detector efficiency when measured using standard sources that are point-like sources emitting γ-rays isotropically. Obtaining a precise estimation of the full energy peak efficiency of the coaxial HPGe detector in the beam geometry for on-axis and off-axis measurements requires a Monte Carlo simulation. We performed Monte Carlo simulations that calculate the detector efficiency in the beam geometry. The effects of the off-axis distance and γ-ray beam size on the efficiency are quantitatively analyzed. We found that the intrinsic efficiency in the beam geometry is maximized when the beam hits the detector at specific off-axis distances. Our Monte Carlo calculations have been supported by nuclear resonance fluorescence experiments using laser Compton scattering γ-ray beams.

Defining an equivalent geometry for Monte Carlo calculations of a high-purity Ge detector with high relative efficiency using a genetic algorithm

Detailed geometric information of a high-purity Ge (HPGe) detector is a very important issue for Monte Carlo simulation of the detector. Commonly, users have no geometric information about the detector and information given by the manufacturer is not completely valid for simulation. An equivalent geometry of detector, the parameters of which can be used for Monte Carlo simulation, is optimised using a genetic algorithm for a large-volume HPGe detector in this study. A mixed-point gamma calibration standard, emitting 12 useful gamma-radiation energies within 59.5–1836.1 keV, is placed at 74 different locations around the detector for this purpose. A high-quality solution is generated starting from an initial population of randomly-generated detector geometries using a genetic algorithm. Fitness of each geometry is obtained by comparing full energy peak efficiencies computed by Monte Carlo simulation with experimental values for each energy and position. Efficiencies with relative errors less than 5% for high energies and less than 7% for lower energies, except 59.5 keV, are obtained using optimised equivalent geometry parameters for the Monte Carlo simulation. Also, the necessity of using crystal dimensions smaller than real dimensions for Monte Carlo simulations of high-volume HPGe detectors is discussed. In addition, for Monte Carlo simulation of high-volume HPGe detectors, it is demonstrated that the use of smaller crystal dimensions than the real dimensions is necessary to obtain experimentally measured efficiencies of the detector.

A Practical Procedure to Determine Natural Radionuclides in Solid Materials from Mining

There are many regulations related to the radiological control of NORMs (Naturally Occurring Radioactive Materials) in activities such as mining, industry, etc. Consequently, it is necessary to apply fast and accurate methods to measure the activity concentrations of long-lived natural radionuclides (e.g., 238U, 234,232,230,228Th, 228,226Ra, 210Pb, 210Po, and 40K) in samples characterized by a wide variety of compositions and densities, such as mining samples (wastes, minerals, and scales). Thus, it is relevant to calculate the radioactive index (RI), which summarizes for all radionuclides the ratio between the activity concentration and its respective threshold activity concentration as established by regulations, in order to classify a material as a NORM. To proceed with the determinations of these radionuclides, two spectrometric techniques based on both alpha-particle and gamma-ray detections should be employed. In the case of gamma-ray spectrometry, it is necessary to correct the full-energy peak efficiency (FEPE) obtained for the calibration sample, εc, due to self-attenuation and true coincidence summing (TCS) effects. The correction is especially significant at low gamma emission energies, that is, Eγ &lt; 150 keV, such as 46 keV (210Pb) and 63 keV (234Th). On the other hand, in samples which contain radionuclides that are in secular disequilibrium with others belonging to the same series (238U or 232Th series), like wastes or intermediate products, it is necessary to measure some pure-alpha emitters (232Th, 230Th, 210Po) by employing alpha-particle spectrometry. A practical and general validated procedure based on both alpha and gamma spectrometric techniques and using semiconductor detectors is presented in this study.

A practical and general methodology for efficiency calibration of coaxial Ge detectors

The correct determination of artificial and natural radionuclides like 152Eu, 137, 134Cs, 60, 57Co, etc., and 234, 228Th, 228, 226Ra, 210Pb and 40K, respectively, is essential for many environmental science fields. For this, a general function was obtained for the full-energy peak efficiency (FEPE) by gamma-ray spectrometry using coaxial Ge detectors. Then, the experimental FEPE values, obtained fixing the energy, Eγ, were fitted varying the thickness, h, of cylindrical standards. The parameters resulted from these fittings were fitted, in turn, versus Eγ, obtaining a general efficiency function, εc (Eγ, h). εc (Eγ, h) was validated, obtaining very good zscore, except for Eγ affected by TCS effects. Consequently, a practical and general method was developed, recalibrating the detector by varying the sample-detector distance, d. εc (Eγ, h, d) was obtained, achieving very good zscore. Furthermore, this practical method was also employed to correct high self-absorptions and high dead times.

Performance of various mathematical functions for the in-situ relative detector efficiency towards its applicability for k0 IM-NAA

The in-situ relative detection efficiency strongly influences the characteristics of the k0-based internal monostandard neutron activation analysis (IM-NAA). In the present work, various mathematical functions were explored for the establishment of in-situ relative detector efficiency calibration and compared their performance based on the reduced chi-square (χ2) values. Among the various mathematical functions, the polynomial logarithm with 6th order was found to be associated with the minimum mean standard deviation for the experimental data and the lowest value of reduced χ2 after carrying out multiple iterations using Nelder–Mead algorithm. Quality assurance of the function was tested by carrying out elemental quantification of the NIST SRM 1633b coal fly ash. Gamma energies of the activation products, 152mEu, 59Fe, 140La, 24Na and 46Sc of the irradiated NIST standard were used for the in-situ relative full energy peak efficiency calibration of 30% HPGe detector. The sample was counted for different time intervals for the complete profiling of the elements present in the NIST SRM. The deviations for most of the elements were found to be within ±5% with respect to the certified values and ξ-score values were within ±2, demonstrating its better accuracy. This method was also applied satisfactorily to profile the elemental concentrations of alloy materials used in a thermal sensor guide tube of the steam generator in a test reactor.

Effect of front, lateral and back dead layer thicknesses of a HPGe detector on full energy peak efficiency

As well known, High Purity Germanium (HPGe) detectors have a wide range of applications in every area where radiation detection and measurements are involved. One of the most important parameters in this respect is the determination of the full energy peak (FEP) efficiency. There are certain parameters affecting the so-called FEP efficiency. Perhaps the most important one is the thickness of dead layers of the crystal itself. In the current study, we investigated the effects of the thickness of front, lateral and back dead layer thicknesses of the detector crystal on the FEP efficiencies for the energy range 30 keV–5 MeV using EGS4 Monte Carlo simulation package. It was shown that the change in the thickness of the front dead layer has significant effect on the efficiency values for the energy interval 30–400 keV. The change in the thickness of the lateral and back dead layer was shown to have significant effects on the efficiencies for the energy intervals 2–3 MeV and 3–5 MeV, respectively. The results were checked using GEANT4 and a good agreement was observed.

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.

Determination of the gamma emission intensities of 125Xe based on 125I radioactivity

The radionuclide 125Xe plays an important role in diagnosing deuterium-tritium (DT) fuel areal density in inertial confinement fusion (ICF). Accurate data for the gamma emission intensities of 125Xe are highly desired. In this paper, after the 125Xe and 125I samples were prepared, the activity of 125I was determined by the sum-peak method using a high resolution well-type HPGe (High Purity Germanium) detector, and the activity of 125Xe was obtained by the decay relationship between 125Xe and 125I. The full-energy peak efficiencies of gamma-rays of 125Xe were accurately calibrated using standard radioactive sources. As a result, the absolute emission intensities of 125Xe for 12 gamma-rays were determined experimentally for the first time.

Determination of L-X ray absolute emission intensities of 238Pu, 244Cm, 237Np and 233Pa radionuclides using a metallic magnetic calorimeter

Energy dispersive photon spectrometry is a common analysis technique to quantify radionuclides present in a sample, the analysis is based on the knowledge of the photon emission intensities specific to each radionuclide. Among them, actinides have in general intense emission of L X-rays during the decay. However, their emission intensities are not well known and not detailed in the nuclear and atomic data tables due to the complexity of their spectra that cannot be resolved by conventional semiconductor spectrometers. Using a dedicated high energy resolution cryogenic detector, based on metallic magnetic calorimeter sensor technology, the L X-ray spectra of the decays of 238Pu(α) → 234U, 244Cm(α) → 240Pu, 237Np(α) → 233Pa and 233Pa(β−) → 233U were measured with an energy resolution between 23 eV and 43 eV (full width at half maximum) given access to an unprecedented level of detail.Moreover, the detector was conceived to provide a quasi-constant efficiency in the energy range of the L X-rays, minimizing the uncertainty for relative L X-ray emission intensities. Prior to the measurements, the full energy peak efficiency was carefully characterized, which enabled the determination of absolute emission intensities, with an uncertainty of the order of 1%. Some corrections had to be introduced using Monte Carlo simulations, in particular to take into account the surface activity inhomogeneities of the sources. Total and group L X-ray emission intensities are compared with the available data and with the recommended values. In addition, nearly 30 individual L X-ray intensities are presented.

Development of a new hand-held type thyroid monitor using multiple GAGG detectors for young children following a nuclear accident

One of the most important lessons learned from the Fukushima Daiichi Nuclear Power Plant accident is that direct (in vivo) measurements of thyroid exposure to radioiodine (mainly, 131I) in the affected populations should be initiated in a timely manner. Furthermore, the existing commercial detectors are not necessarily suitable for the measurement of young children, who are especially vulnerable to radiation exposure and therefore important to screen. This paper presents the development of a new thyroid monitor for such measurements and the results from phantom-based experiments. The monitor has two unique probes having multiple high-energy-resolution type Gd3(Al,Ga)5O12(Ce) (GAGG) detectors that can be placed directly on the young subject's anterior neck. The crystal size of the GAGG detector is 1 cm3, and a probe consisting of a 4 × 1 or 4 × 2 detector array can be selected depending on the subject's body size. The thickness of the 4 × 1 array probe is 24 mm, which is less than that for a conventional NaI(Tl) survey meter having a crystal 1 inch in diameter and 1 inch long (38 mm). Experimental and computational calibrations of the new monitor using existing and virtual phantoms allowed us to determine the full energy peak efficiency for the 131I thyroid contents of different age groups from 3-mo-old to 10-yr-old and minimum detectable activity (MDA) values under various conditions. As a result, the attainable MDA for subjects age ≤5 years under a normal background level (∼0.05 μSv h−1) was found to be ∼30 Bq, which was low enough to identify children with thyroid-equivalent doses over 10 mSv up to about 25 days after the 131I intake. Our new monitor would be useful in direct thyroid measurements for vulnerable young children following a large nuclear accident.

Investigation of the effect of copper contact pin on efficiency in HPGe detectors using Monte Carlo method

The full energy peak efficiency (FEPE) determined by experimental or Monte Carlo (MC) simulation methods is a very important parameter in HPGe detectors. Since FEPE depends on the detector's geometric parameters, the parameters provided by the manufacturer are of great importance in modeling the detector with the MC method. The most important reason for the discrepancy between MC and experimental calculations is the lack of accurate information about the detector's geometric properties. The thickness of the copper contact pin in the middle of the detector hole is not given by the manufacturer. In this study, the effect of copper contact pin thickness on detector efficiency was investigated by using the PHITS 3.24 MC simulation program both at different copper contact pin radii and at different detector-source distances. The efficiency values were calculated for photons in the energy range of 59.5 keV-1408 keV, at 4 different distances, namely 5 cm, 13.25 cm, 15 cm, and 20 cm and for the radii of copper contact pins increased from 1 mm to 3.5 mm at 0.5 mm intervals. According to the results, it has been determined that the presence of copper contact pins causes a change in detector efficiency up to 1.9%, especially in the high energy region, and has no effect on the detector efficiency in the low energy region. In addition, it has been observed that the effect of copper contact pin thickness on detector efficiency is almost independent of the source-detector distance.

Dependence of photon registration efficiency on LaBr3(Ce) detector orientation for in situ radionuclide monitoring

Knowledge of a radiation detector's numerical characteristics allows its energy efficiency to be calibrated theoretically for any measurement geometry. Here, energy efficiency calibration is discussed for oriented LaBr3(Ce) detectors used for in situ radiation monitoring. Vertical and horizontal detector orientations relative to the ground surface are compared using efficiency calibrations based on the detector's numerical characteristics and are equally effective. Quantitative assessment of the different measurement geometries was performed using a new analytical approach: integrated absolute full energy peak efficiency.

An upgraded lab-based method to determine natural γ-ray emitters in NORM samples by using Ge detectors

The regulation existing on NORM activities requires the accurate and quick measurement of natural radionuclides in a wide range of matrices by gamma spectrometry. The aim of this work has been to develop a comprehensive and accurate methodology to determine natural long-lived radionuclides (210Pb, 228,226Ra, 234,228Th and 40K) by gamma spectrometry with Ge detectors in NORM samples. An exhaustive calibration method to obtain the full-energy peak efficiency (FEPE), for each gamma emission energy, as a function of the thickness, apparent density and matrix composition of the cylindrical sample, has been developed. The selected certified calibration standards contain only natural radionuclides belonging to the 238U- and 232Th-series as well as 40K (codes RGU-1, RGTh-1 and RGK-1 from IAEA). Then, the obtained FEPE curves were validated using Genie 2000 simulations and Certified Reference Materials, for which the self-attenuation corrections were considered. Finally, a study on 222Rn losses was done for several samples.

Investigation of shape effects and dead layer thicknesses of a coaxial HPGe crystal on detector efficiency by using PHITS Monte Carlo simulation

In gamma-ray spectrometry, the full energy peak efficiency can be determined by experimental, or by Monte Carlo (MC) simulation methods. In experimental methods, reference standard materials including a large number of gamma emitters or mixed gamma emitters in different geometries are requested to determine the efficiency curves. Besides, simulation by MC methods has advantages of modelling any geometries and energy range of interest. However, by simulation it is not easy to determine the efficiency curves due to the lack of information supplied from manufacturers. In this work, it is focused on the effect of dead layer thickness and other geometric parameters of detector crystal on the efficiency of the detector. The general-purpose MC code PHITS and specific-purpose MC code GESPECOR are used to simulate the detector. The experiments are performed by single gamma point sources at a 12.5 cm distance from the end cap of the detector. The results showed that the physical parameters of the detector are different from those provided by the manufacturer. After 15 years of operation of the detector, it is observed that the dead layer thickness increased from 0.7 mm to 1.29 mm. It causes a decrease in the efficiency by a reduction in the active volume of the detector. In addition, it is showed that the round shape of the front edge and bullet shape of the inner hole of the detector crystal have an undeniable effect on efficiency of detector especially for low energy photons. The study shows that the PHITS MC simulation program can be successfully used in obtaining efficiency calibration of HPGe detectors and determining the dead layer thickness.

Optimization of the n-type HPGe detector parameters using the “design of experiments” technique

This paper presents a new procedure to optimize the geometric parameters of a n-type coaxial HPGe detector. It is based on a statistical technique called “Design of Experiments” (DoE). This technique aims to identify the most influential parameters and to determine the optimal configuration. In this work, The effects of each parameter on the detector responses have been investigated by a fractional factorial design. Only the most influential factors contributing to the detector response have been selected. Precise modeling of these factors was then performed using a full factorial design. Based on the results obtained from this design, the full energy peak efficiencies according to the geometric parameters were modeled by a multiple-linear regression. These models have been statistically validated by analysis of variance (ANOVA). The optimal combination of the geometric parameters has been identified using the desirability function approach, which is a useful tool to optimize multi-response problems. A verification test was performed to validate the results obtained. It was observed that the relative deviation found between experimental and simulated values was less than 5%.

Characterisation of a well-type NaI(Tl) detector by means of a Monte Carlo simulation for radionuclide metrology application

A well-type NaI(Tl) detector was modelled and characterised by means of a Monte Carlo simulation, as part of a project to develop a 4πβ (Plastic Scintillator)-4πγ instrument to be used for the primary standardisation of radionuclides at ANSTO. The simulation based on GEANT4 was used to characterise the 4πγ detector in terms of potential dead layer/inactive materials, full energy peak efficiency, coincidence-summing correction, and energy resolution. An excellent agreement was obtained between the simulation results and the experimental measurements.

Measurements correlated to efficiency studies of NaI(Tl) scintillator with vertical hexagonal radioactive source

Nowadays, many scientists work on the geometry change between source-to-detector arrangement to calibrate the 3”× 3” γ-ray NaI(Tl) detector based on the available sample shape in the laboratory. The calculated full-energy peak efficiency is more sensitive in radiation activity determination and can be estimated build on complex analytical and numerical techniques. In the present study, the full-energy peak and total efficiencies beside the peak-to-total ratio (P/T) for γ-ray NaI(Tl) detector concerning vertical hexagonal ^152Eu radioactive source is calculated for all γ-rays involved in the cascade and estimate the suitable correction for coincidence summing effects (COI) by mean of the numerical simulation method (NSM). The extra factors which cause the γ-ray attenuation taken into consideration as well, such as, source composition materials, “self-attenuation within the source itself”, the detector end cap with the other supporting materials around the detector crystal, the source container material, and the holder used during the measurement process. The theory of this methodology depends on the efficiency transfer technique; calculate the effective solid angle, that be positioned in between the source-to-detector system, besides estimating the path lengths of the γ-rays inside the hexagonal ^152Eu radioactive sources and the active medium inside the detector itself as well. The results gained in the present work gave an agreement between simulation and measured data. The results show that to improve the accuracy of the γ-rays detector efficiency, many technical and methodological aspects of the present method are fitting and possible for applications inside the radiation laboratory, industrial, and medical sectors.