Full Energy Peak Efficiency Research Articles

FATIMA — FAst TIMing Array for DESPEC at FAIR

The components, working principle and characteristics of FATIMA (FAst TIMing Array), a fast-timing detector system for DESPEC at FAIR, are described. The core system includes 36 LaBr3(Ce) scintillator detectors, a mounting frame for the DESPEC station and a VME-based fast-timing data acquisition system. The current electronic timing circuit is based on V812 constant fraction discriminators and V1290 time-to-digital converters. Gamma-ray energies are measured using V1751 digitisers. Characteristics of the core FATIMA system including efficiency, energy, and coincidence resolving time, as well as limitations, are discussed on the basis of test measurements performed in the S4 cave at GSI, Germany. The coincidence γ-γ time resolution for the prompt 60Co cascade is determined to be ∼320 ps full width at half maximum. The total full energy peak efficiency at 1 MeV for the 36 detector array in the DESPEC setup is 2.9%. The energy-dependent prompt response centroid curve with the current CFD/TDC combination is shown to be smooth; the centroid shift method can be applied for the measurement of half-lives below 200 ps. An overview of applications of the FATIMA detectors as an ancilliary system in combination with other detector arrays during recent years is given. Data on the operation of the detectors in the presence of magnetic fields are presented.

A revision of the virtual point detector model for calculating Nai(Tl) detector efficiency

In present work, the validity of the virtual point detector (VPD) model for the NaI(Tl) detectors is studied and confirmed in the photon energy range of 60–1408 keV. The full energy peak efficiency (FEPE) of two NaI(Tl) detectors, which have scintillation crystal dimensions of 5.08 × 5.08 cm and 7.62 × 7.62 cm respectively, is measured for “point-like” radioactive sources on the symmetry axis with source-to-detector distances in the range of 2–40 cm. It is found that the VPD model is valid to fit too well to the experimental FEPE for the two surveyed NaI(Tl) detectors. The dependence of the VPD position on the incident photon energy for the NaI(Tl) detectors with different scintillation crystal dimensions is shown based on experimental data. A semi-empirical equation involving incident photon energy and source-to-detector distance is proposed to calculate the FEPE for the NaI(Tl) detectors. The calculated results for the two surveyed NaI(Tl) detectors by this equation are in a good agreement with experimental results for photon energies in the range of 344–1408 keV. However, the difference between experimental and calculated results is quite significant for source-to-detector close geometries for photon energies lower than 344 keV.

A large buoy-based radioactivity monitoring system for gamma-ray emitters in surface seawater

A buoy (shallow water light type) -based in situ gamma-ray spectrometry system with a 7.6 cmØ × 7.6 cm NaI(Tl) detector for remote real-time monitoring of gamma-ray emitting radionuclides in surface seawater is presented. To convert measured count rates to radioactivity, the full energy peak efficiency of the detector for radionuclides in seawater was estimated using Monte Carlo simulation with the MCNP code. The efficiency calibration was validated by comparing the results with a sampling analysis of 40K in seawater at the sites where the monitoring systems were deployed. The minimum detectable activity of the system for 137Cs, 134Cs and 131I with gamma-ray measurement time is discussed.

Coincidence summing corrections factors calculated for volume 152Eu sources in γ-ray spectromtery using Monte Carlo techniques

True coincidence summing (TCS) correction factors were determined for 152Eu volume sources used for γ-ray efficiency calibration as the ratio of full energy peak efficiency (e) simulated by MCNP5 code to the apparent full energy peak efficiency (eapp) determined experimentally. The results were compared to those calculated by TrueCoinc software and then validated using certified environmental radioactive samples. The dependance of 152Eu and 133Ba TCS factors on volume sample and matrix was investigated. Finally, as our n-type HPGe detector is very sensitive to low energy photons, the contribution of the γ–X coincidence summing to TCS factors for 152Eu and 133Ba sources in volume soil samples was studied.

Mô phỏng hệ đầu dò gamma triệt Compton bằng phương pháp Monte Carlo

In this study, two simulation toolkits including Geant 4 and MCNP-CP are used to investigate the configuration of the Compton suppression spectrometer at Nuclear Research Institute. The Compton suppression system consisting of 12 bismuth germanate (BGO) scintillation crystals surrounding one HPGe detector is used to reduce background from Compton scattering. The simulation results illustrate the good suitability between response functions for point source with each radioisotope such as Na-22, Co-60 and Cs-137. Moreover, the difference between the full-energy peak efficiency of two simulation toolkits is less than 1%. This research gives the initial results to optimize the configuration of Compton suppression system and to compare with experimental data in the future.

Photopeak Efficiency and Coincidence Summing Factors for HPGe Detectors Using Bulk Sources.

In the field of gamma-ray spectroscopy with high-purity germanium detectors, used to measure the activity when the sample to be measured has low radioactivity, volumetric sources are widely used. To determine the sample activity, the full-energy peak efficiency is needed. Bulk sources are very common for low-level radioactivity measurements in activation analysis and environmental samples. The main problem with these measurements is that coincidence effects are large for close source-detector geometries. Ignoring these effects can lead to an error typically of a factor of 2 in the determination of Co and Y activity, which was used in this work in the calibration process. Extensive experimental readings have been carried out, and bulk gamma aqueous sources containing several radionuclides covering the energy range from 60 to 1,840 keV were used. By comparison, the experimental (present work) and theoretical (published) full-energy peak efficiency values are in good agreement; the overall percentage error is less than 6%.

New mathematical approach to calculate the geometrical efficiency using different radioactive sources with gamma-ray cylindrical shape detectors

The geometrical efficiency of a source-to-detector configuration is considered to be necessary in the calculation of the full energy peak efficiency, especially for NaI(Tl) and HPGe gamma-ray spectroscopy detectors. The geometrical efficiency depends on the solid angle subtended by the radioactive sources and the detector surfaces. The present work is basically concerned to establish a new mathematical approach for calculating the solid angle and geometrical efficiency, based on conversion of the geometrical solid angle of a non-axial radioactive point source with respect to a circular surface of the detector to a new equivalent geometry. The equivalent geometry consists of an axial radioactive point source with respect to an arbitrary elliptical surface that lies between the radioactive point source and the circular surface of the detector. This expression was extended to include coaxial radioactive circular disk source. The results were compared with a number of published data to explain how significant this work is in the efficiency calibration procedure for the γ-ray detection systems, especially in case of using isotropic radiating γ-ray sources in the form of point and disk shapes.

Experimental and Monte Carlo determination of HPGe detector efficiency

Monte Carlo model has been developed using the GEANT code for calculation of the full energy peak efficiency of a HPGe detector operating in the Slovak Institute of Metrology. The model has been used for calculation of the HPGe detector efficiency for gamma-spectrometry measurements associated with the development of a national radon standard. The detector model was validated by comparison of simulated efficiencies with measured experimental values for point sources and for 450 mL Marinelli beaker. A reasonable, up to 5% agreement between the simulated and experimental results was achieved in the energy range from 100 to 1800 keV.

A low background gamma-ray spectrometer with a large well HPGe detector

A low background gamma-ray spectrometer was established in a ground-based laboratory. It consists of a large well (diameter: 28 mm, depth: 40 mm) HPGe detector and an anti-cosmic shielding system. The photon background of the present system was measured with and without the anti-cosmic mode, and compared with each other, and the detection capability with the detection efficiency calibration was estimated. The background with passive and active shields for the energy region between 40 keV and 3000 keV was reduced by 78% compared to one with just passive shield. The full energy peak efficiency for cylindrical samples with three different heights and the minimum detectable activity of the present system were tabulated for main radionuclides to be measured in environmental monitoring.

Characterizing low-activity waste containers: A case study for Compton Suppression Systems under challenging signal-to-noise ratio

Abstract This work investigates a compact Compton Suppression System (CSS) based on a High-Purity Germanium (HPGe) diode as a primary spectrometer. From a wide range of potential materials for the guard detector, including plastic scintillator, Thallium doped Sodium Iodide (NaI(Tl)), and Lanthanum III bromide (LaBr3), a bismuth germanate crystal (BGO) is selected on account of a high photon mass-attenuation coefficient, thus allowing more compactness. The authors then describe the complete design study of an HPGe/BGO CSS based on the post-processing of Monte Carlo particle track history files (MCNP6.1/PTRAC). The calculation program, simulating the filtering of coincident events on both primary and secondary detectors, is validated through a series of experimental measurements using Am-241, Cs-137, and Co-60 sources in various configurations. On the basis of this empirically validated calculation, an optimization study of the BGO-active shield is performed over two factors of merit: Compton Suppression Factor (CSF) and Full-Energy Peak Efficiency. Optimization steps subsequently involve the scaling of BGO internal and external radii, the thickness of the entrance window, and the diameter of a collimation hole. Eventually, performance figures of the CSS are illustrated in a case study involving the characterization of large waste containers, carrying low levels of uranium activity to seek among natural radiological background. Calculation results show a lowering of minimal detectable activities (MDA) by a factor of 2.5 based on the signature of 1001-keV gamma rays from U-238.

A two-point in situ method for simultaneous analysis of radioactivity in seawater and sediment

A new in situ radioactivity analysis method was developed to determine the fractional contributions of gamma-ray emitters in seawater and sediment from total measured counts. A semi-empirical formula to determine the fractional count contributions was derived using the variation characteristics of the gamma-ray attenuation rate and geometrical efficiency with the measurement points. The proposed method was employed to make in situ gamma-ray measurements using a CeBr3 detector for radioactivity analysis of seawater and sediment at a coastal area with a relatively high tidal range. The full energy peak efficiency of the detector at measurement site was obtained using the GEANT4 simulation code. The radioactivity concentration of 40K in seawater and sediment was determined using the proposed method and laboratory analysis with sampling. The MDA of the in situ measurement for 137Cs and 131I was also estimated assuming they were deposited on the sediment surface-layer. The validation of this method was demonstrated by comparing with the sampling analysis results.

Full-energy peak efficiency and response function of 1 cm3 CdZnTe detectors

The energy dependence of the full-energy peak (FEP) efficiency and response function of 1 cm3 CdZnTe detectors that use gamma rays were investigated in the present study. Specifically, full-energy peaks from 60 keV to 1.3 MeV were evaluated. 214Am, 133Ba, 137Cs, 54Mn, and 60Co radionuclides with good source activities were employed. The results showed that the absolute peak efficiencies decreased at higher energies, and the calculated (EGS5 code) efficiencies were nearly 70% greater than the experimental values when above 100 keV photon. Below 100 keV, disagreement is within 35%. The measured and calculated peak efficiencies peak agreed within 5% in a wide energy range when the “effective” of the crystal area was down to 0.8 x 0.8 cm2. Agreement corresponding to response function curve at the region of interest (full-energy peak) and Compton plateau region for 137Cs achieved once considering the 0.8 x 0.8 cm2 effective area of the crystal and the source plastic casing. The results of the present study provided important insights on the calibration of the FEP efficiency as a function of the gamma ray energy. The detectors must be individually calibrated to obtain reliable results.

HPGe detector efficiencies for extended sources using the Monte Carlo method

ABSTRACTTo determine the activity of radioactive sources, accurate efficiencies are always required since the data provided by the manufacturer for detector model is insufficient to calculate the efficiencies value and obtained strong discrepancies between simulated and experimental results, especially for low photon energies. For this purpose, a detailed detector model was developed to simulate the total and full energy peak efficiencies of a p-type coaxial High Purity Germanium (HPGe) detector using GEANT4. In the simulation, gamma aqueous sources containing several radionuclides were used, covering the energy range from 59.50 to 1836.01 keV. In the case of extended sources, the coincidence summing correction factors () for two nuclides () were calculated from the peak () and total efficiencies () at 15 points throughout the source volume obtained with GEANT4. A modified detector model including the summing effect, the simulated full energy peak efficiencies agreed with the experimental values within 1%, except for the high energy range where they are less than 2% between 122 and 1836 keV for the small extended source volume.

Study on the performance of some non-destructive methods to estimate the uranium enrichment in nuclear materials

Abstract 235U enrichment determination is one of the most important characteristics for nuclear materials verification. This paper focuses on the investigation of applicability and performance evaluation of two non-destructive methods, viz: the multi-group γ-ray analysis method for uranium (MGAU) and another method based on Monte Carlo simulation code (MCNP). The MGAU is the most widely applied non-destructive γ-ray spectrometry method. Its applicability and performance have been investigated, evaluated and compared with the MCNP method. The investigated samples in our work are U3O8 and UO2. The emitted gamma rays from these samples were measured using a planar high-purity germanium detector (HPGe) of high-resolution. Based on the measured gamma spectra, the MGAU method was used to estimate 235U enrichment of the measured samples through analysis of the net area under the peak of energy 185.7 keV. At the same time, the MCNP method based on the calculated full energy peak efficiency (FEPE) has been suggested to carry out the 235U enrichment calculations. Comparing the obtained results of both MGAU and MCNP with the reference values, it was concluded that the MGAU shows acceptable performance for enrichment measurements, but it gives non-accurate and erroneous results. However, the measurements have been done for a long time, its results of a maximum difference up to 16%; especially for shielded and low 235U amount samples. In addition to that, this method has some limitations and weaknesses coming from its requirements for prior system calibration with standard references having the same geometry as the assayed items. Besides, the dependence of its analysis algorithms on the x-ray region of interfering peaks is dependent on the detector performance as a function of its resolution. On the other side, the performance of MCNP results reveals a good agreement with the enrichments reference values within maximum difference up to 5%.

Investigation hybrid MCNP/Angle model for calculating the absolute full-energy peak efficiency of HPGe detector

The absolute full energy peak efficiency values have been obtained by Monte Carlo MCNP5 code and semi-empirical ANGLE3 calibration software for a coaxial HPGe detector. The two calibration methods were combined to build a hybrid MCNP5/ANGLE3 model. The hybrid model can give efficiency calibrations values for various gamma-ray source shapes and geometries and then generate efficiency calibrations for new geometries instantly without needing new standards. The model has been checked experimentally and theoretically and then approved to cover the various gamma detection measurements.

Nigeria research reactor-1 : Vertical detector efficiency calibration using conventional and semi-empirical approach for large samples NAA implementation

Detector efficiency calibration is mandatory for accurate measurement of induced activity in irradiated samples and for safe operation of the reactor with minimal uncertainty. This paper reported the efficiency calibration of vertically dIpstick High Purity Germanium detector, installed at the Centre for Energy Research and Training, Ahmadu Bello University Zaria for the purpose of large sample Neutron Activation Analysis (NAA) using Nigeria research reactor-1 (NIRR-1). The performance of the detector was evaluated for the radioisotope activity measurements during the reactor operation for large samples neutron activation analysis. The detector performance in terms of radioisotopes detection ability was inspected using the standard conventional and semi-empirical approaches. The full energy peak efficiencies were determined at the corresponding energies for three different geometries (source to detector distance of 1, 5 and 10 cm). The semi-empirical approach produced better and precise results that logically rhymed with theory than the traditional approach. Besides that, a consistency in the nature of the graphs and values were evidenced. The determined efficiencies and their corresponding energies revealed encouraging outcome and ensured the successful NAA for large samples of different material compositions.

Improved calibration for non destructive determination of manganese in metallurgical tap smelting slag using a high energy medical accelerator

An improved calibration method for the non-destructive determination of the manganese content in large metallurgical tap smelting slag samples using photon activation analysis is presented. A metallurgical tap smelting slag of archaeological interest from the Psaphaki site, Laconia, was irradiated under the mixed photon and parasitic neutron field produced by an 18 MV medical accelerator. The sample was 236 g in mass. Manganese was determined by the photon and neutron induced reactions 55Mn(g, n)54Mn and 55Mn(n,g)56Mn, measuring the 834.8 keV 54Mn and 846.8 keV 56Mn photopeaks, respectively. Gamma spectra from the large sample and calibration foils were acquired using a HPGe detector of 85% relative efficiency. The Efficiency Transfer method was applied to determine the Full Energy Peak Efficiency (FEPE) of the large sample. The results of the analysis showed that the manganese content in the slag was (39.5±15.8) % w/w and (39.7±2.6) % w/w measuring the 56Mn photopeak and the 54Mn photopeak, respectively. Therefore, a good agreement between the photon and neutron activation analysis techniques was observed. Advantages of the technique include improved representativeness of results, elimination of reference samples, irradiation versatility and convenience, as well as reduced contamination probability since a minimal sample preparation was required for the analysis. Moreover, the introduction of the relative calibration technique removes the dependency on parameters that are usually evaluated with larger uncertainties, such as detector efficiency, increasing the quality and reliability of the procedure.

Efficiency calibration for HPGe detector by Monte Carlo efficiency transfer method

In this paper, the Monte Carlo efficiency transfer method was used to calibrate the full energy peak efficiency (FEPE) of a coaxial p-type HPGe detector. The gamma standard radioactive sources including 22Na, 54Mn, 57Co, 60Co, 65Zn, 109Cd,133Ba, 137Cs, 154Eu, 207Bi, 241Am were measured at different positions on-center of detector with the distance of 5, 10, 15, 20, 25 cm. Besides, a cylindrical sample containing standard mixed nuclides solution was also measured at surface of the detector. The experimental FEPE curves as function of gamma energy for these geometries were determined with the coincidence-summing corrections. A HPGe detector model based on the specifications of manufacturer was built to directly calculate the FEPE for the geometries by Monte Carlo simulations with MCNP6 code. However, these simulated FEPEs show a quite high discrepancy from experimental FEPEs. Then, the FEPEs were calculated by the efficiency transfer method with the efficiency curve for point source at distance of 25 cm as the reference data. A good agreement was obtained between the calculated results by the Monte Carlo efficiency transfer method and experimental results. The comparisons between experimental and calculated FEPE showed that the relative deviations were mostly within +/-4% in the energy range of 53-1770 keV.

Calculation of self-absorption and coincidence summing correction factors for the extended sources using GEANT4

A detailed study of the full energy peak efficiency of a high purity germanium (HPGe) detector including the effect of source self-absorption and coincidence summing was performed using Monte Carlo simulation, as it is difficult and time-consuming to measure the full energy peak efficiency experimentally. Cylindrical water composition source was simulated with different characteristics, covering the energy range from 60 to 1836 keV. Self-absorption correction factors (SAFcal) were calculated for two source volumes and obtained good agreement with the experimental results except for (60Co and 88Y) nuclides. The simulation was performed for various samples with different densities and observed their effects on the full energy peak efficiency value of the detector. In the case of extended volumetric source, the coincidence summing correction factors (CSFcal) for two nuclides (60Co and 88Y) were estimated with the GEANT4 simulation toolkit. The effect of correction factors on different cylindrical source volumes was also investigated. With the self-absorption and coincidence summing effect, the best agreement was achieved between simulated and experimental results with discrepancy less than 2% for all of the radionuclides included in two source volumes.

MCNP model of the EEAE ACCUSCAN Whole Body Counter

Monte Carlo simulations and verification measurements for the efficiency calibration of the ACCUSCAN shadow-shield type Whole Body Counter (WBC) of the Greek Atomic Energy Commission (EEAE) are presented. A model of the counter and RMC-II anthropomorphic phantom was developed using the MCNP code. Full energy peak efficiencies for different phantom positions were calculated for 60Co and 137Cs sources. The deviations between computational and experimental efficiencies were found to be less than 12 % for 60Co and 4 % for 137Cs for the Ge detector and less than 25 % for 60Co and 4 % for 137Cs for the NaI detector. This work contributes to the accurate quantification of internal contamination in individuals accidentally exposed in Greece by the Greek Atomic Energy Commission Laboratories and moreover demonstrates the effectiveness of using computational tools for understanding the calibration of radiation detection systems used for in vivo monitoring.