Coaxial High Purity Germanium Detector Research Articles

Modeling of N and P-Type of Coaxial Ge Detectors Using MCNPX and the Effect of Dead Layer Variation on Its Response Function

Abstract This study assessed the response function of a p-type and n-type coaxial high-purity germanium (HPGe) detector via Monte Carlo N-Particle eXtended (MCNPX). MCNPX was employed to model the Coaxial Ge detectors, and for a precise simulation, the dimensions of the dead layer of germanium crystals were added. The dead layer was separated into front and lateral surfaces, and the thickness of each dead layer was modeled. In this work, the simulated detectors have been performed at different energy lines using a radioactive source Eu-152 to study the response function of each with dead layer variations for the front dead layer and study the range of relative deviation of the Monte Carlo simulation outputs from the manufactured declared data. The results proved that the n-type coaxial HPGe detector is more sensitive to the dead layer change than the p-type with a thick change of 0.01 mm. This research has significant effects on the efficiencies of the radiation detection systems in the energy range ∼ (120–1410) keV.

High-purity germanium semiconductor modeling in the detector response function toolkit

We have extended the detector response function toolkit (DRiFT) to provide modeling capabilities of semiconductor sensors. DRiFT provides realistic nuclear instrumentation response by post-processing Monte-Carlo N-particle (MCNP®) radiation transport outputs. MCNP® is capable of modeling radiation transport in complex environments, but has limited detector physics and readout electronics modeling capabilities. Semiconductor detector response can be calculated with a high-fidelity for a flexible range of environments by utilizing MCNP® to simulate radiation interactions inside of detector volumes, and then using DRiFT to model charge transport and signal formation in the semiconductor, as well as the readout electronics. DRiFT models charge transport in the semiconductor, the preamplifier, shaping amplifier, pulse pile-up, and electronic noise to generate detector response. The semiconductor application in DRiFT can model a range of semiconductor materials, shapes, and sizes; and is demonstrated here for a large volume coaxial high-purity germanium (HPGe) detector. Here, we compare detector response functions of a coaxial HPGe detector with measurement of 60Co, 133Ba, and 137Cs at varying count rates, and we conduct a parameter study to demonstrate the effect of changing parameters in the DRiFT simulation. The HPGe detector response function shows excellent agreement with measurements of difference sources with varying dead times and count rates.

Spatial distribution of 137Cs; 40K, 226Ra, 232Th in soils of Monchegorsk city (Murmansk region)

The purpose of the study was to establish the patterns of spatial distribution of radioactivity of technogenic 137Cs and natural radionuclides 232Th, 226Ra, 40K in urban soils of Monchegorsk. Location and time of the study. Soil samples, were collected in summer 2018 in the Monchegorsk city, Murmansk region, Russia. To study the specific activity of radiocesium and natural radionuclides, soil samples were taken from topsoil (0-5 cm) at 82 sites, including 68 sites within the city territory (urban soils) and 14 sites beyond the city limits in the subtorch areas strongly affected by the Severonickel factory. Similar studies of urban soils in Monchegorsk had not been carried out previously. Methods. During expeditionary work and laboratory research, soil, radiological, and mathematical and statistical methods were used. When carrying out gamma spectrometric measurements, radionuclides 137Cs, 226Ra, 232Th and 40K were determined on a low-background semiconductor gamma spectrometer ORTEC (USA) based on a high-purity coaxial germanium detector GEM10P4–70 (HPGe) with a SBS-75 pulse signal processor and Gamma-software. The resolution of the gamma spectrometer along the 1.33 MeV line (60Co) was 1.75 keV, and the relative efficiency was 15%. Calibration and quality control of gamma spectrometric measurements was carried out using volumetric activity measurements with 1-L Marinelli vessels of different densities (RITVERZ, Russia-Germany). Main results. The values of specific activity of radionuclides in soil samples were as following: in the technogenic 137Cs up to 31.3 Bq/kg and in the subtorch soils up to 63.4 Bq/kg. Specific activity of natural radionuclides in urban soils reached to 14.2 Bq/kg for 226Ra, whereas in the subtorch soils it reached 21.6 Bq/kg. The specific activity of 232Th in urban soils reached 18.3 Bq/kg, with 17 Bq/kg registered for the subtorch soils. As for 40K, its specific activity reached 498 Bq/kg in the urban soils and 317 Bq/kg. Conclusion. The gamma survey of the Monchegorsk territory did not reveal any anomaly in radiation. The dose rate of gamma radiation in the city does not exceed 0.074 μSv/hour, which is less than the values elsewhere in Russia, ranging from 0.04 to 0.20 μSv/hour. The obtained data about specific activity of technogenic 137Cs and natural radionuclides 232Th, 226Ra, 40K in the Monchegorsk soils are typical for the Murmansk region.

Calibrating GESPECOR model of computing the full-energy peak efficiency of coaxial high-purity germanium detectors by Monte Carlo simulation

In this work, a classical approach was used for calibrating the GESPECOR detector model for computing the full-energy peak efficiency of p-type coaxial HPGe detectors that is based on the use of linear least squares optimization. The key element of the work is the multiplicative model developed for approximating the values of the full-energy peak efficiency provided by GESPECOR code. It was linearized using the logarithmic transformation to allow an easy use of the linear least squares optimization. A procedure was also developed to estimate the optimal values of the parameters, describing the p-type coaxial HPGe detectors. Its application to a Canberra detector GC3018 showed that it is possible to determine accurate values of the full-energy peak efficiency computed by GESPECOR code using the optimized parameter values.

Radiation doses due to smoking different types of tobacco leaves grown in Turkey.

The activity concentrations of 238U, 232Th, 40K and 137Cs radionuclides in 25 samples of tobacco leaves grown in Turkey have been measured using a high purity co-axial germanium detector. The measured activity concentrations for 238U ranged from 3.6 to 22.5Bqkg-1 with an average of 9.8Bqkg-1, for 232Th from 0.8 to 11.0Bqkg-1 with an average of 2.4Bqkg-1, and for 40K from 360.3 to 1,479.5Bqkg-1 with an average of 810.0Bqkg-1. The concentrations of 137Cs, the residue of fallout from the Chernobyl reactor accident, were found to range from 0.2 to 2.1Bqkg-1 with an average of 0.5Bqkg-1. The results indicate that the average activity concentrations of 238U and 232Th lie within the range of measured values worldwide, whereas for 40K the average was found to be higher. The average annual effective radiation doses from the inhalation of the corresponding tobacco smoke by an adult smoking 20 cigarettes per day was deduced to be 93.3 μSv y-1 for 238U, 193.2 μSv y-1 for 232Th, 5.6 μSv y-1 for 40K, and 292.1 μSv y-1 in total. The average annual effective dose from 137Cs was calculated to be 8.5 nSv y-1. The resulting average annual effective radiation doses of 0.3mSv due to the inhalation of natural radiation sources in tobacco were compared with studies worldwide and found much smaller than the worldwide average effective dose of 1.26mSv. It is concluded that smoking may contribute a non-negligible fraction to the annual effective dose depending on cigarette consumption and activity concentration in tobacco. This study is the first in which natural and anthropogenic radioactivity in tobacco in Turkey was measured. Therefore, it is considered as a baseline study. Further studies are needed to investigate the role of tobacco refinement in the observed reduction of 238U concentration of refined tobacco.

Implementation of a machine learning technique for estimating gamma direction using a coaxial High Purity Germanium detector

We demonstrate the ability to obtain the direction of the gamma rays using a standard coaxial high purity germanium (HPGe) detector using the direction-sensitive information embedded in the shape of the pre-amplified HPGe signals. We deduced the complex relationship between the shape of the signal and the direction from which the gamma-ray enters the detector active volume using a two-step machine learning technique. In the first step, we collected pulses from the HPGe detector due to a 133Ba radioactive source placed in four distinct positions around the detector while keeping the distance from the center of the detector crystal constant. A subset of the pulses collected with radioactive source kept at the four positions was used to train an artificial neural network (ANN) called a self-organizing map (SOM) to cluster the HPGe waveforms based on their shape. The trained SOM network was then utilized to produce direction-specific maps corresponding to pulses generated when the 133Ba source is at a specific location with respect to the detector. In the second step, we used the SOM-generated direction-specific maps to train another network composed of a single feedforward layer for predicting the direction of the gamma ray from the pulses produced by the HPGe detector due to the gamma energy deposition. Our results show that even without employing complex methodologies, a standard coaxial HPGe detector can estimate the direction of incoming gamma rays and thus, provide initial guidance on the gamma-emitting radioactive source direction with reference to the detector.

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.

Evaluation the nonlinear response function of a HPGe detector for 59 keV to 10.7 MeV gamma-rays using a Monte Carlo simulation and comparison with experimental data

Modeling of High Purity Germanium (HPGe) detector on a wide energy range is important in gamma-ray spectroscopy. The precisely modeled detector can be used for proton-induced prompt gamma-ray spectroscopy. In this work, we used both the gamma-rays of calibration sources and prompt gamma-rays emitted in proton capture reactions to model a coaxial p-type HPGe detector using Geant4 Monte Carlo simulation for the gamma-ray energy range of 59–10764 keV. The calibration sources were 137Cs, 241Am, 60Co, 152Eu, and 133Ba, while the prompt gamma-rays were due to the gamma-ray cascades following the 27Al(p,γ)28Si reaction capture at the resonant energies of 992, 1317 and 2483 keV, as well as the 23Na(p,γ)24Mg reaction capture at the resonant energies of 1417 and 1395 keV. According to the results obtained, the presented simulated and experimental spectra show a good agreement. In addition, the mean and maximum relative deviations between experimental and simulated efficiencies corresponding to 27 major peaks are smaller than 5% and 15%, respectively.

Cross section measurements for (n,2n), (n,α), and (n,p) reactions on rhenium isotopes around 14 MeV neutrons and their theoretical calculations of excitation functions* *Supported by National Natural Science Foundation of China (11605099, 11575090)

Cross-section data of the 185Re(n,2n)184mRe, 185Re(n,2n)184gRe, 185Re(n,α)182m1+m2+gTa, 187Re(n, 2n)186g,(m)Re, 187Re(n,α)184Ta, and 187Re(n,p)187W reactions were measured at four neutron energies, namely 13.5, 14.1, 14.4, and 14.8 MeV, by means of the activation technique, relative to the reference cross-section values of the 93Nb(n,2n)92mNb reaction. The neutrons were generated from the T(d,n)4He reaction at the K-400 Neutron Generator at China Academy of Engineering Physics. The induced γ activities were measured using a high-resolution γ-ray spectrometer equipped with a coaxial high-purity germanium detector. The excitation functions of the six above-mentioned nuclear reactions at neutron energies from the threshold to 20 MeV were calculated by adopting the nuclear theoretical model program system Talys-1.9 with the relevant parameters properly adjusted. The measured cross sections were analyzed and compared with previous experiments conducted by other researchers, and with the evaluated data of BROND-3.1, ENDF/B-VIII.0, JEFF-3.3, and the theoretical values based on Talys-1.9. The new measured results agree with those of previous experiments and the theoretical excitation curve at the corresponding energies. The theoretical excitation curves based on Talys-1.9 generally match most of experimental data well.

Automated method for offline correction of spectrometry data affected by time instability

A new automated method has been developed with the aim to correct measured spectra for sudden gain and offset drifts due to temporal instrumentation instabilities. It is based on discrete cross-correlation that alleviates the need for Gaussian-shaped peaks and is capable of correcting spectra that change their shape in time. Potential of this method has been tested on decay and in-beam γ-ray spectroscopy data measured with an array of three n-type coaxial High-Purity Germanium detectors albeit its use is not limited only for γ-ray spectroscopy.

Geometrical optimisation of a segmented HPGe detector for spectroscopic gamma emission tomography—A simulation study

Segmented coaxial HPGe (High Purity Germanium) detectors have recently been shown to be feasible for Gamma Emission Tomography (GET). This type of detector allows for a combination of high efficiency and high energy resolution in gamma spectrometry of irradiated nuclear fuel. The ultimate aim of developing segmented HPGe for GET measurements is to achieve a high spatial resolution to facilitate imaging of rod-internal features and interrogation of smaller irradiated fuel samples.In this work, we present the optimisation of a segmented HPGe detector through a simulation study using the Monte Carlo particle transport code MCNP. Constraints to each dimension of the detector were identified, from manufacturing limitations and requirements arising from the use of a finite-sized collimator slit. In particular, a relationship between the minimum inner radius of the coaxial detector and the segments azimuthal dimension was derived based on the identified constraints. Segment azimuthal and radial dimensions have been varied (based on the derived relationship between the azimuthal and radial dimension) and the full energy efficiency and misidentification rate were evaluated to obtain the optimal dimensions. The optimal ranges of the segment dimensions were determined.

Precision efficiency calibration of a high-purity co-axial germanium detector at low energies

Following work done in the energy region above 100 keV, the high-precision calibration of a co-axial high-purity germanium detector has been continued in the energy region below 100 keV. Some of the previous measurements and Monte-Carlo simulations have been repeated with higher statistics and a new source measurement with 169Yb has been added. For the energy range from 40 keV to 100 keV, an absolute precision for the detection efficiency of ±0.2% has been reached, as previously obtained for energies above 100 keV. The low-energy behaviour of the germanium detector was further scrutinised by studying the germanium X-ray escape probability for the detection of low-energy photons. In addition, one experimental point, a γ ray at 2168 keV from the decay of 38K, has been included for the total-to-peak ratios agreeing well with simulations. The same γ ray was also added for the single- and double-escape probabilities. Finally, the long term stability of the efficiency of the germanium detector was investigated by regularly measuring the full-energy peak efficiency with a precisely calibrated 60Co source and found to be perfectly stable over a period of 10 years.

Cross-sections for , and reactions on molybdenum isotopes in the neutron energy range of 13 to 15 MeV * * Supported by the National Natural Science Foundation of China (11875016, 11565012, 11165007)

Given the insufficient cross-sectional data regarding the 14-MeV-neutron experiment of molybdenum, the vital fusion reactor structural material, and the significant heterogeneities among the reported values, this study examined the (n,2n), (n,α), (n,p), (n,d), and (n,t) reaction cross sections in molybdenum isotopes based on the neutrons produced via a T(d,n)4He reaction carried out in the Pd-300 Neutron Generator at the China Academy of Engineering Physics (CAEP). A high-resolution gamma-ray spectrometer, which was equipped with a coaxial high-purity germanium detector, was used to measure the product nuclear gamma activities. In addition, 27Al(n,α)24Na and 93Nb(n,2n)92mNb reactions were utilized as the neutron fluence standards. The experimental 92Mo(n,2n)91Mo, 94Mo(n,2n)93mMo, 100Mo(n,2n)99Mo, 98Mo(n,α)95Zr, 100Mo(n,α)97Zr, 92Mo(n,p)92mNb, 96Mo(n,p)96Nb, 97Mo(n,p)97Nb, 98Mo(n,p)98mNb, 92Mo(n,d)91mNb, and 92Mo(n,t)90Nb reaction cross sections were acquired within the 13–15 MeV neutron energy range. Thereafter, we compared and analyzed these obtained cross sections based on the existing IAEA-EXFOR database-derived experimental data, together with evaluation results corresponding to ENDF/B-VIII.0, JEFF-3.3, BROND-3.1, and CENDL-3.1 and the theoretical outcomes acquired through TALYS-1.95 and EMPIRE-3.2.3 (nuclear-reaction modeling tools).

Efficient machine learning approach for optimizing the timing resolution of a high purity germanium detector

We describe here an efficient machine-learning based approach for the optimization of parameters used for extracting the arrival time of waveforms, in particular those generated by the detection of 511 keV annihilation γ-rays by a 60 cm3 coaxial high purity germanium detector (HPGe). The method utilizes a type of artificial neural network (ANN) called a self-organizing map (SOM) to cluster the HPGe waveforms based on the shape of their rising edges. The optimal timing parameters for HPGe waveforms belonging to a particular cluster are found by minimizing the time difference between the HPGe signal and a signal produced by a BaF2 scintillation detector. Upon applying these variable timing parameters to the HPGe signals, we achieved a γ–γ coincidence timing resolution of ∼4.3 ns at the 511 keV photopeak (defined as 511 ± 50 keV) and a timing resolution of ∼6.5 ns for the entire γ-ray spectrum—without rejecting any valid pulses. This timing resolution approaches the best obtained by analog nuclear electronics, without the corresponding complexities of analog optimization procedures. We further demonstrate the universality and efficacy of the machine learning approach by applying the method to the generation of secondary electron time-of-flight spectra following the implantation of energetic positrons on a sample.

Comparison of gamma-ray spectral analysis methods for thick-walled UF6 cylinders

Three data analysis methods were compared for gamma ray spectra collected by planar and coaxial high-purity germanium detectors (HPGe) on thick-walled uranium hexafluoride cylinders. The cylinder contents range from natural to low-enriched uranium (4.95% 235U). Each spectrum was analyzed with the traditional 185.7 keV enrichment meter method, a 185.7/1001 keV ratio method, and the Fixed Energy Response Function Analysis with Multiple Efficiency (FRAM) software that utilizes low- and high-energy gamma rays and X-rays. When results for all cylinders were compared, the enrichment meter method and the 185.7/1001 keV peak ratio method consistently provided the most accurate data for all detector types. The error on the enrichment calculations analyzed with FRAM did not meet the international target value whereas those calculated with the enrichment meter and ratio method did. Enrichments calculated by FRAM were found to skew to higher values on natural uranium and to lower values on low-enriched uranium. FRAM calculated the least-precise enrichments; replicate analyses of single cylinders resulted in 235U values that varied by as much as 19%. FRAM does have the significant advantages in that it does not require the use of calibration cylinders, infinite thickness criteria are not necessary, and it can be applied to any HPGe detector. If calibration cylinders are available, the enrichment meter and 185.7/1001 peak ratio are the recommended methods as they are the most accurate and precise.

Cross sections for (n, 2n), (n, p), (n, a) and (n, n′) reactions on hafnium isotopes around the neutron energy of 14 MeV

In view of the fact that the fast-neutron experimental cross-section data of hafnium which is an important structural material of fusion reactor are few and there are the great differences in their evaluation values, several activation cross sections of (n, 2n), (n, p), (n, a) and (n, n′) reactions on hafnium isotopes have been investigated by neutrons generated from the T(d, n)4He reaction at the K-400 Neutron Generator at China Academy of Engineering Physics (CAEP). The gamma activities of the product nuclei were measured by a high-resolution gamma-ray spectrometer with a coaxial high-purity germanium detector. The 93Nb(n, 2n)92mNb reaction was used as neutron fluence standard, these experimental cross sections of the 174Hf(n, 2n)173Hf, 176Hf(n, 2n)175Hf, 178Hf(n, α)175m+gYb, 178Hf(n, p)178gLu, 178Hf(n, p)178mLu, 180Hf(n, 2n)179m2Hf, 180Hf(n, α)177m+gYb and 180Hf(n, n′)180mHf reactions were obtained in the neutron energy range of 13.5–14.8 MeV. These measured cross sections were discussed and compared with some previous experimental results from the literature and with the evaluation data from ENDF/B-VIII.0, CENDL-3.1, JEFF-3.3 and the theoretical results by using the computer code system Talys-1.9.

Measurements of the Cross-sections of Produced Short-Lived Nuclei Induced by Neutrons around 14MeV on Isotopes of Tungsten**Supported by the National Natural Science Foundation of China under Grant Nos 11605099 and 11575090.

New experimental cross-section data for the 180W(n,2n)179mW, 186W(n,2n)185mW and 186W(n,p)186Ta reactions at the neutron energies of 13.5 and 14.4 MeV are obtained by the activation technique. The neutron beams are produced by means of the 3H(d,n)4He reaction. The gamma activities of the product nuclei are measured by a high-resolution gamma-ray spectrometer with a coaxial high-purity germanium detector. The neutron fluence is determined using the monitor reaction 93Nb(n,2n)92mNb. The results in the current work are discussed and compared with the measurement results found in the literature. It is shown that these higher accuracy experimental cross-section data around the neutron energy of 14 MeV agree with some previous experimental values from the literature within experimental uncertainties.

Identification of activation products and radiation dose estimates in Havar® foils for a GE PETtrace800 medical cyclotron via high resolution gamma ray spectroscopy and MCNPX simulation

During production of F-18 in a medical cyclotron, high energy protons and secondary neutrons hit the highly reactive components of the target assembly resulting to activation products. In determining the presence of activation products and quantifying their respective activities, high resolution gamma spectroscopy using high purity germanium (HPGe) was used. From the results of the spectroscopy analysis, the doses were estimated using two methods: calculation method and MCNPX simulations. For the gamma spectroscopy analysis, the sample was prepared using aqua regia and was sent to PNRI. The analysis was done using GEM Series HPGe Coaxial Detector System. The results of the gamma spectroscopy showed the presence of 56Co, 57Co, 58Co, and 54Mn after a cooling period of eighty (80) days. For the MCNP simulations, the Havar® window foil was simulated as an isotropic disc source and a soft - tissue mimicking box detector was used and positioned at 100 cm from the center of the simulated window foil. *F8 tally was used to estimate the effective dose arising from a single activation product homogeneously distributed in the window foil and was done under the assumption that no other radiation sources were present in the simulation. Using calculation method, the effective dose estimates of 56Co, 57Co, 58Co, and 54Mn were found to be of minimal values during replacement of 18O vial at one meter. Moreover, it was found out that the highest contributors of dose during replacement of an 18O vial and maintenance of cyclotron are 56Co and 58Co. After a period of one year, the guiding activation products become 57Co and 54Mn due to their long half - lives. Furthermore, the annual dose estimates of these activation products were found to not exceed the annual dose limits for the whole body and for the extremities. As for disposal and storage, a period of four years will allow the Havar® window foil to reach the exemption levels set by PNRI.

Investigation of the Natural Occurring Radioactive Material (NORM) in the cyprinus carpio fishes breeding in artificial lakes of Baghdad governorate

The steady consumption of fish led many researchers to study it preferences over other foods, especially for radioactivity content. The specific activity concentration (S.A) of natural occurring radioactive materials (NORM) have been measured for Cyprinus carpio fishes collected from several industrial fishes' lakes located in Baghdad governorate using gamma spectroscopy doped with high purity germanium coaxial detector (HPGe). Thirteen fishes' samples were collected from industrial lakes, three samples were collected from cages, and two samples were collected from Trigger River. The last two types of samples were collected in order to compare the results with it. The measured overall averages of S.A for Ra-226, Th-232, and K-40 were 58.9 Bq/kg, 14.1 Bq/kg, and 388.6 Bq/kg, respectively. Some radiation health hazard indices have been estimated, such as the annual effective ingestion dose for an adult member of the public due to the intake of radionuclide through ingestion of fish (H). The results of H showed that the cage fish is better than river fishes and lakes fishes. However, the results of S.A and H strongly suggested that the studied fish is safe to use for human consumption from radiological point of view.

Absolute efficiency calibration of a coaxial HPGe detector for quantitative PGAA and T-PGAA

Following international guidelines by the IEEE and a well-established calibration procedure developed at the Hungarian Institute of Isotopes in Budapest, the experimental absolute efficiency calibration of an Ortec n-type coaxial high-purity germanium (HPGe) detector (GMX Profile series) has been performed, in view of direct application to quantitative PGAA and T-PGAA investigations of Cultural Heritage multicomponent manufacts.