High-purity Germanium Detector Research Articles

Optimisation of gamma spectrometry measurements in atmosphere during nuclear emergencies.

During emergencies following a major nuclear accident a prompt and accurate characterisation of the radioactive cloud is one of the main task of any radioactivity monitoring system. This task is usually performed by means of γ-spectrometry High Purity Germanium (HPGe) measurements on atmospheric particulate samples gathered by means of high-volume pumps. The key parameters describing the performances of a monitoring system are the Minimum Detectable Activities (MDAs) of the most relevant radionuclides. These parameters depend on a number of factors related to the efficiency of the available germanium detector, the volume of air filtered by the sampling devices and the γ-decay scheme of each radionuclides as well. Besides the MDAs, another very important characteristic of a monitoring system, especially during an evolving emergency, is its capability of giving reliable results at a given and constant pace. It is therefore important to define the time resolution of the monitoring system, i.e. the minimum time needed to produce the data, namely the activity concentrations of the radionuclides in the atmosphere. The optimization of the measurements procedures are discussed in this work: in particular it is demonstrated that, being t the time resolution of the monitoring system, the lowest MDAs can be achieved with a sampling time given by (2/3)·t and a counting time of (1/3)·t. Finally, the MDAs achievable for a standard monitoring system based on a 30% HPGe detector are calculated for all the most important fission products.

Determination of internal gamma dose assessment due to natural radioactivity for river sand using a gamma-ray spectrometer.

River sand is an environmental component that is always used as a building material in India and hence the present study, activity concentrations of 226Ra, 232Th and 40K have been measured in sand samples collected from Ponnai river, Tamil Nadu using a high-resolution gamma-ray spectrometer consisting of high-purity germanium detector. Calculated mean specific activity is 31, 84 and 416Bqkg-1 for 226Ra, 232Th and 40K, respectively. These results reveal that 226Ra was found to be less than the world average value of 33Bqkg-1 whereas 232Th and 40K were higher than the global mean value of 30 and 400Bqkg-1, respectively. In order to assess the internal dose to population a standard index radium equivalent activity (Raeq) is calculated for these samples. Based on the obtained results, it is seen that these sand samples do not pose significant health hazards to the inhabitants of the houses constructed.

Characterization of a 4παβ(LS)-γ(HPGe) prototype system for low-background measurements

A ground-level prototype system for low-background measurements was developed and tested. The system consists of a high-purity germanium (HPGe) detector used for detecting γ rays and coupled to a liquid scintillator (LS) used for detecting α and β particles. Both detectors are surrounded by shielding materials and anti-cosmic detectors (“veto”) used to suppress background events. The energy and timestamp of detected α, β and γ emissions are recorded event-by-event and analyzed offline. By requiring timing coincidence between the HPGe and LS detectors, background events originating from outside the volume of the measured sample can be effectively rejected. The system performance was evaluated using liquid samples containing known activities of an α emitter (241Am) or a β emitter (60Co) whose decays are accompanied by γ rays. The LS detector was found to provide a solid angle of almost 4π for α and β particles. Compared to the traditional γ-singles mode, operating the system in coincidence mode (i.e., α-γ or β-γ) reduced the background counts by a factor of ∼100. Consequently, the minimal detectable activity for 241Am and 60Co was improved by a factor of 9, being 4 mBq and 1 mBq for an 11-d measurement, respectively. Furthermore, by applying a spectrometric cut in the LS spectrum that corresponds to α emission from 241Am, a background reduction factor of ∼2400 (compared to γ-singles mode) was achieved. Beyond low-background measurements, this prototype exhibits additional compelling features, such as the ability to focus on certain decay channels and study their properties. This concept for a measurement system may be of interest to laboratories that monitor environmental radioactivity, studies involving environmental measurements and/or trace-level radioactivity.

Updated and novel limits on double beta decay and dark matter-induced processes in platinum

A 510 day long-term measurement of a 45.3 g platinum foil acting as the sample and high voltage contact in an ultra-low-background high purity germanium detector was performed at Laboratori Nazionali del Gran Sasso (Italy). The data was used for a detailed study of double beta decay modes in natural platinum isotopes. Limits are set in the range {mathcal {O}}(10^{14}{-} 10^{19}) years (90% C.L.) for several double beta decay transitions to excited states confirming, and partially extending existing limits. The highest sensitivity of the measurement, greater than 10^{19} years, was achieved for the two neutrino and neutrinoless double beta decay modes of the isotope ^{198}Pt. Additionally, novel limits for inelastic dark matter scattering on ^{195}Pt are placed up to mass splittings of approximately 500 keV. We analyze several techniques to extend the sensitivity and propose a few approaches for future medium-scale experiments with platinum-group elements.

Photofission delayed gamma-ray measurements in a large cemented radioactive waste drum during LINAC irradiation

Nuclear materials characterization in 870 L cemented waste drums is challenging due to significant neutron and gamma ray attenuation. Among the existing non-destructive approaches, photon active interrogation is of particular interest. A LINAC accelerates electrons to energies greater than 10 MeV, leading to highly penetrating interrogating bremsstrahlung photons that can reach nuclear materials in the depth of the 870 L waste drum and allow the detection of delayed gamma rays emitted by photofission products. Because the interrogating photon and neutron fluxes are extremely high during irradiation, measurements with high purity germanium detectors are usually performed after a long irradiation time, for example by moving the detector or the waste drum. Therefore, such measurements are only sensitive to delayed gamma rays associated to photofission products having sufficient long half-life of at least tens of seconds, which cuts a valuable signal coming from fission products with a shorter half-life. In this paper, we report gamma rays measurements of very short lived photofission products using a 3 in × 3 in cylindrical LaBr3(Ce) fast scintillation detector. A depleted uranium sample was irradiated by a pulsed bremsstrahlung source with maximum energy 17.5 MeV in a macro pulsing mode (1 s irradiation, 2 s cooling). This allowed to acquire a strong delayed gamma signal within a time window of 2 s.A clear signature of nuclear materials was observed even in the depth of a concrete mock-up, with an excellent signal-to-noise ratio beyond 3.5 MeV where the background of nonnuclear activated materials is negligible. These results are compared to a semi analytical model that qualitatively agrees but underestimates the measurements by a constant factor 1.7, probably caused by a wrong beam intensity normalization. Indeed, in this work the LINAC pulse time structure (macro pulsing) was different from its nominal working point used to characterize the beam (usual pulsing mode). Since it allows direct calculations of the detector response to delayed gamma-ray photofission measurements, this model is a fast calculation alternative to time-consuming Monte Carlo simulations, in view of further studies that will follow this feasibility demonstration.

Characterization of the radiation field surrounding the Leksell Gamma Knife® and shielding applications

The aim of this study is to improve the characterization and modeling of the radiation field surrounding the Leksell Gamma Knife®-PerfexionTM. The improved characterization of the radiation field enables more accurate shielding calculations to be performed for the areas adjacent to the treatment room. With the aid of a high-purity germanium detector and a satellite dose rate meter, γ-ray spectra and ambient dose equivalent H*(10) data were acquired at various locations in the field of a Leksell Gamma Knife unit in a treatment room at Karolinska University Hospital, Sweden. These measurements were used to validate the results of the PEGASOS Monte Carlo simulation system with a PENELOPE kernel. The levels of the radiation that passes through the shielding of the machine (leakage radiation) are shown to be much lower than what is suggested by various bodies, e.g. the National Council on Radiation Protection and Measurements, to be used when calculating radiation shielding barriers. The results clearly indicate that Monte Carlo simulations may be used in structural shielding design calculations for γ rays from the Leksell Gamma Knife.

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A new study of double beta decay processes in natural Zr isotopes using Zr metal samples and an ultra-low-background high-purity germanium detectors was performed at Laboratori Nazionali del Gran Sasso of INFN (Italy). Two data sets were collected with a 736.35 g and a 129.94 g Zr metal over 59.7 days and 37.3 days, respectively, and were used for a detailed analysis. A new limit on the double beta decay of 94\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{94}$$\\end{document}Zr to the 1st excited level of 94\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{94}$$\\end{document}Mo was set as T1/2ββ>2.1×1020year\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$T_{1/2}^{\\beta \\beta } > 2.1\ imes 10^{20}~\ ext {year}$$\\end{document}, which is a factor of three better than the previous result. It is shown that the current radiopurity of Zr metal (tens of Bq/kg), produced via two methods, do not comply with the radiopurity requirements for low-background experiments. Hence, we propose to use a purified ZrCl4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{4}$$\\end{document} powder in an optimized geometry as a new approach for future experiments with extended sensitivity focused on studies of rare nuclear processes in Zr isotopes.

Baseline radioactivity in the five candidate sites for the second nuclear power plant in Bangladesh and concomitant hazards assessment

ABSTRACT To meet the increasing energy demands and foresee the shortage of fossil fuels as a source of energy, the Government of the People’s Republic of Bangladesh has decided to install a second Nuclear Power Plant in the southern region of Bangladesh. This study was conducted to assess the baseline radioactivity using the High Purity Germanium (HPGe) detector of the five candidate sites to construct a nuclear power plant in the southern regions of Bangladesh and to evaluate the associated radiological hazard parameters. This pioneering study demonstrates that the mean activity concentrations (range) of 226Ra, 232Th, and 40K in forty-five soil samples collected from the five candidate sites were 28.95 ± 2.72 (2.65–48.75), 30.10 ± 2.95 (10.35–46.65), and 278.82 ± 25.78 (126.05–418.61) Bqkg−1. The evaluated area poses insignificant radiological concerns regarding mean absorbed dose rate, radium equivalent activity, annual effective dose, external and internal hazard indices, and excess lifetime cancer risk values. This study revealed a thorough picture of radioactive dispersion in soil samples from Bangladesh’s southern areas, allowing policymakers to take appropriate measures in identifying viable candidate sites for constructing a Nuclear Power Plant. Moreover, the data of this study can be used as a reliable source of baseline radioactivity in the particular area.

Neutron Flux Determination for the NEAR Station at the CERN n_TOF Facility Using the SAND II Unfolding Code

The NEAR station, a new experimental area of the n_TOF facility, was established after the second long shut down of CERN in 2020. This new area was created in order to exploit in situ the high instantaneous neutron flux (originating from proton spallation into a lead target in bunches with momenta reaching up to 20 GeV/c). The reason for the creation of the NEAR station is to utilize the neutrons for experiments related to radiation damage on materials and nuclear astrophysics and the corresponding neutron flux was characterized via extensive neutron multiple foil activation measurements. The irradiated foils were subsequently measured using high purity germanium detectors (HPGe) to determine their induced activities. Finally, the widely used SAND II unfolding code was implemented for the characterization of the neutron flux, using the evaluated cross sections from the IRDFF library, along with the experimentally derived activities. The preliminary results concerning the neutron flux determined in the present work are compared with the corresponding FLUKA simulated ones.

Radioactivity measurements in eastern Lesvos, Greece

The radioactive background of the eastern side of Lesvos Island in Greece, an island rich in natural radioactivity, was studied by means of gamma ray spectroscopy. Dose rates and concentrations of natural radionuclides (232Th series, 226Ra and 40K) and 137Cs were measured in-situ and in the laboratory, respectively. A total of twenty soil samples and ten beach sand samples was collected and processed according to the IAEA protocol. For the in-situ measurement and the dose rate determination a portable NaI scintillation detector was utilized. The activity concentration calculations were held in the laboratory, using a high-purity germanium detector. These activity concentrations were also used to estimate dose rates, so as to compare the obtained results with the in-situ measurements. The maximum detected values of activity concentrations of 232Th series, 226Ra, 40K and 137Cs were found to be 190, 90, 960 and 70 Bq kg-1, respectively. As for the dose rates, both measured and estimated, the maximum values were 230 and 190 nGy h-1, respectively. A significant difference regarding activity concentrations and dose rates was observed between the two matrices (soil, beach sand), with the values of soil samples quite surpassing those of beach sand samples. That can be attributed to the granulometry of each matrix. Furthermore, the study attempts to compare the radioactive background between the eastern and the western side of Lesvos Island, with the larger values being found in the eastern part.

NORM measurements at Kalloni and Gera Gulfs, Lesvos Island, Greece

Natural radioactivity measurements were held in the beach sands of the two main gulfs (Gera, Kalloni) of Lesvos Island, Greece. These gulfs host thermal springs and are preferred tourist destinations throughout the year. Dose rates and concentrations of natural radioactivity (232Th series, 226Ra and 40K) and 137Cs were measured in-situ and in the laboratory by means of gamma ray spectroscopy. Ten beach sand samples were collected from each gulf. The in-situ measurement and the dose rate determination was achieved via a portable NaI scintillation detector (SpriID). The activity concentration calculations were realized in the laboratory, with the use of a high purity germanium detector, and then they were also used to estimate dose rates. The in-situ measured and estimated dose rates were compared to verify the different approaches. The highest values (activity concentrations and dose rates) were found at the beaches of Kalloni Gulf compared to those of Gera Gulf, which may be attributed to the granulometry.

Bayesian inference of high-purity germanium detector impurities based on capacitance measurements and machine-learning accelerated capacitance calculations

The impurity density in high-purity germanium detectors is crucial to understand and simulate such detectors. However, the information about the impurities provided by the manufacturer, based on Hall effect measurements, is typically limited to a few locations and comes with a large uncertainty. As the voltage dependence of the capacitance matrix of a detector strongly depends on the impurity density distribution, capacitance measurements can provide a path to improve the knowledge on the impurities. The novel method presented here uses a machine-learned surrogate model, trained on precise GPU-accelerated capacitance calculations, to perform full Bayesian inference of impurity distribution parameters from capacitance measurements. All steps use open-source Julia software packages. Capacitances are calculated with SolidStateDetectors.jl, machine learning is done with Flux.jl and Bayesian inference performed using BAT.jl. The capacitance matrix of a detector and its dependence on the impurity density is explained and a capacitance bias-voltage scan of an n-type true-coaxial test detector is presented. The study indicates that the impurity density of the test detector also has a radial dependence.

Activation cross sections of proton-induced reactions on manganese up to 30 MeV

The excitation functions of the 55Mn(p,x)51Cr, 54Mn reactions were measured up to 30-MeV proton energy. The experiment was performed at the RIKEN AVF cyclotron using the stacked-foil activation technique. High-purity germanium (HPGe) detectors were used for the gamma-ray spectrometry. Obtained results were compared with previous experimental data and the prediction of the TALYS nuclear reaction model code available in the TENDL-2021 library. Physical thick target yields of 51Cr and 54Mn were derived from the measured cross sections.

Numerical simulation of the nitrogen boil-off recondensation in hybrid cooling devices for HPGe detectors

This paper describes the results of a complex simulation of nitrogen boil-off recondensation in a hybrid cooling device for High Purity Germanium (HPGe) detectors. The OpenFOAM platform-based software was used. The proposed finite-volume axisymmetric 2D model combines cooling processes occurring in the considered three-phase medium, including a liquefier condenser (solid), nitrogen boil-off (gas) and the resulting condensate (liquid). At the beginning of the simulation condenser and gas temperatures are set above the condensation point while the end time is set such that the system reaches a quasi-steady state. For conical and hemispherical condensers, the energy consumption for the following processes occurring in the operating mode such as: condenser cooling; cooling of adjacent boil-off nitrogen vapors; their condensation; the gravity-driven flowing; condensate film subcooling and subsequent partial re-evaporation from its surface are compared. The visualizations of the temperature field of the finite-volume model and the velocity vectors of nitrogen vapors adjacent to the condenser are presented. The calculated integral mass recondensation rates were confirmed experimentally on a commercial hybrid cooling device, taking into account the actual equivalent characteristics of the liquefier on the Stirling cryocooler used.

A feasibility study of multi-electrode high-purity germanium detector for 76Ge neutrinoless double beta decay searching

Experiments to search for neutrinoless double-beta (0νββ) decay of 76Ge using a high-purity germanium (HPGe) detector rely heavily on background suppression technologies to enhance their sensitivities. In this work, we proposed a pulse-shape analysis method based on a neural network (NN) and a light gradient boosting machine (lightGBM; LGB) to discriminate single-electron (background) and double-electrons (0νββ signal) events in a multi-electrode HPGe detector. In this paper, we describe a multi-electrode HPGe detector system, a data-processing system, and pulse-shape simulation procedures. We built a fully connected (FC) neural network and an LGB model to classify the single- and double-electron events. The FC network is trained with simulated single- and double-electron-induced pulses and tested in an independent dataset generated by the pulse-shape simulation. The discrimination efficiency of the FC neural network in the test set for the 0νββ double-electron events signal was 77.4%, the precision was 57.7%, and the training time was 430 min. The discrimination efficiency of LGB model was 73.1%, the precision was 64.0%, and the training time was 1.5 min. This study demonstrated that it is feasible to realize single- and double-electron discrimination on multi-electrode HPGe detectors using an FC neural network and LGB model. These results can be used as a reference for future 76Ge 0νββ experiments.

Design and characterization of AmLi neutron sources for the LZ experiment

In this paper we describe the development, testing, and characterization of three low-emission rate AmLi neutron sources. The sources are used to calibrate the nuclear recoil response of the LUX-ZEPLIN (LZ) dark matter experiment. The sources' neutron emission rate was measured using 3He proportional tubes. The sources' gamma emissions were characterized using a high-purity germanium (HPGe) detector. Source-validated Geant4 Monte Carlo simulations allowed to calibrate the Ge and neutron detectors' responses.

Natural radioactivity Measurements of Dur-Kurigalzu's Ziggurat, Baghdad Governorate-Iraq using HPGe detector

Measurement of 238U, 233Th and 40K concentrations in soil samples collected from Ziggurat of Dur-Kurigalzu west region of Baghdad, have been evaluated, using high purity germanium detector. The activity concentrations of 238U, 233Th and 40K varied from 16.040 to 26.620 , 14.510 to 31.480 , and 153.820 to 266.320 with average values of 20.604±2.9 , 24.534±3.3 , 212.22±25.1 , respectively. The importance of these measurements lies in the estimation of radiation risk, radium equivalent, absorbed dose, annual effective dose, risk indices, gamma index, and cancer risk. The average value of the absorbed dose ranged from 33.187 outdoors to 63.111 indoors. The mean annual effective dose value was estimated to be 0.043 outdoors and 0.310 indoors from the study area. The total calculated lifetime risk of cancer ranged from 0.991×10-3 to 1.485×10-3, while the indoor lifetime cancer risk exceeded the recommended limit in samples 6, 8, 9, and 10. The areas examined in this study showed radioactive contamination less than the internationally permissible limits for the sites inhabited by the population.

True coincidence summing corrections in HPGe detectors

The true coincidence effect is studied in two High Purity Germanium (HPGe) detectors for a variety of isotopes, source geometries and source to detector configurations, via computational tools based on Monte–Carlo simulations. Τhe upgraded patch of MCNP code MCNP–CP and the 2018 version of PENELOPE, which take into account the decay scheme of each cascade emitter, are used to calculate the Full Energy Peak Efficiency (FEPE) for the corresponding gamma-ray energies. The true coincidence correction (TCC) factor is calculated as the ratio of the FEPE derived for each nuclide taking into consideration the true coincidence effect, to the FEPE estimated without considering the phenomenon. In all cases, a satisfactory agreement is observed between the TCC factors calculated using MCNP–CP and PENELOPE 2018. Moreover, the results of the calculations are compared against experimentally derived efficiency values. The correction factors obtained using the TrueCoinc software are applied on experimentally determined FEPE curves, based on measurements performed using reference sources, and consequently the corrected data are compared against the simulations for the "non-coincidence" case. The results of this work contribute to the validation of the computational tools and codes used to study the true coincidence effect and determine the corresponding correction factors, providing useful data for gamma–spectrometry studies of cascade emitters.

Natural radioactivity levels in some vegetable crops under greenhouse conditions

Estimating any radioactivity released into the environment is critical for public health protection, particularly if the radioactivity can enter the food chain. In the present work, the activity concentration of natural radionuclides in the soil, water, plants, and fruits of four vegetable crops, namely cucumber, sweet pepper, hot pepper, and tomato, growing under greenhouse conditions has been measured using a High Purity Germanium (HPGe) Detector. The measured activity concentrations for 226Ra, 232Th, and 40K in the studied soil samples ranged from 4.7 to 6.8, 3.4 to 6.1, and 63.9 to 112.4 Bq kg−1, respectively, while in plants, they ranged from Not Detected (ND) to 15.2, ND to 3.4, and 495.1 to 1467.4 Bq kg−1, respectively. The measured activity concentrations for 40K in the studied fruit samples ranged from 967.1 to 1459.1 Bq kg−1, while 226Ra and 232Th were not detected. The Transfer Factor (TF) of 226Ra, 232Th, and 40K from soil to plants and fruits has been evaluated, and the results from soil to plants varied from ND to 2.5, ND to 0.8, and 6.0 to 19.2, while the results for 40K in fruits varied from 8.7 to 18.4, while 226Ra and 232Th were not detected.

Charge trapping correction and energy performance of the Majorana Demonstrator

P-type point contact (PPC) high-purity germanium detectors are an important technology in astroparticle and nuclear physics due to their superb energy resolution, low noise, and pulse shape discrimination capabilities. Analysis of data from the MAJORANA DEMONSTRATOR, a neutrinoless double-beta decay experiment deploying PPC detectors enriched in $^{76}$Ge, has led to several novel improvements in the analysis of PPC signals. In this work we discuss charge trapping in PPC detectors and its effect on energy resolution. Small dislocations or impurities in the crystal lattice result in trapping of charge carriers from an ionization event of interest, attenuating the signal and degrading the measured energy. We present a modified digital pole-zero correction to the signal energy estimation that counters the effects of charge trapping and improves the energy resolution of the MAJORANA DEMONSTRATOR by approximately 30% to around 2.4 keV FWHM at 2039 keV, the $^{76}$Ge $Q$-value. An alternative approach achieving similar resolution enhancement is also presented.