Planar High-purity Germanium Detectors Research Articles

Estimation of 241Am and 239Pu activity embedded in the tissue using portable planar HPGe detector.

This paper describes a procedure for the estimation of 241Am and 239Pu activity present in the human tissue by measuring the depth of contaminant using a portable Planar High Purity Germanium detector (HPGe). The ratios of photopeak counts of X-rays or gammas obtained with the detector coupled to collimator are calculated for the estimation of depth of the contaminant and the optimum one is determined. Since Minimum Detectable Activities (MDA) for the detector coupled to a collimator are higher than that of bare detector, activity must be estimated using bare detector, after locating the contaminant. Two methods are described for the estimation of plutonium coexisting with 241Am: (i) Abundance and isotopic correction for 239Pu (ii) and 239Pu:241Am ratio. The procedure to estimate 239Pu when plutonium isotopes alone are present is also established. An optimum monitoring period to detect the minimum value of intake for both radionuclides corresponding to chelation therapy and excision is also derived.

Study of the effect of photon self-absorption on the detection efficiency of HPGe detector in measurements of an activated uranium sample

The photon self-absorption effect of uranium on the full-energy peak (FEP) efficiency calibration of high purity germanium (HPGe) detector was studied as a function of energy using an activated uranium sample has a thickness of 1 mm. The absolute efficiencies of the detector were obtained with the use of gamma standard sources and gamma rays of radionuclides created by irradiating the uranium sample with secondary neutrons. The measurements were carried out with the use of a Planar HPGe detector. MCNP simulation code was used to verify the experimental results. The comparison of the empirical results with the simulation results showed a good agreement. To interpret the efficiencies, the reaction rates were calculated using two methods: (1) - the use of the efficiency curve from the standard sources combined with the self-absorption correction of the uranium material and (2) - the use of the efficiency curve obtained from gamma rays in the sample.

Optimization approach of gamma spectrometer measurements for accurate radioactive materials characterization

The materials characterization based on gamma measurements is one of the main features for verifying the radioactive materials for safeguards purposes. This work focuses on the optimization approach in order to find solutions that include some influence parameters, which should be taken into consideration during measurements. The considered samples in our work are certified reference nuclear materials of chemical composition (U3O8) with different enrichment ratios ranging from 0.31 wt.% to 4.46 wt.%. The gamma spectrometer based on a planar high-purity germanium detector of high-resolution was used in the current study. The study of various setups was experimentally carried out for different cans at different positions for different energy lines. The verified parameters influencing the characterization of the measured samples have been estimated. This approach for the measurement setup of the measured gamma spectra has been successfully explored to be very affected by different parameters such as the source to detector position and the various enrichment ratios. Consequently, the verified and characterized samples could be estimated based on this optimization approach.

Simulation and validation of a planar HPGe detector signal database for use in pulse shape analysis

Due to their excellent spectroscopic capabilities and sensitivity to the position of γ-ray interactions, segmented High-Purity Germanium (HPGe) detectors are frequently used in γ-ray imaging applications. The quality of produced images is heavily dependent on the precision of the identification of interaction position within the detector that can be achieved through segmentation and the application of pulse shape analysis (PSA) techniques. In this work, a simulated database of signals was produced for a HPGe planar detector used in a multi-tiered Compton camera system. The impurity concentration and temperature-dependent mobility parameters of the simulation were optimised and validated by evaluating the pulse-shape response at select positions using experimentally-measured collimated γ-beam data. A grid-search algorithm was developed for analysis of single and double-site γ-ray interactions using signal comparison PSA and is shown to improve the position resolution within the detector.

Characterization of high-purity germanium detectors with amorphous germanium contacts in cryogenic liquids

For the first time, planar high-purity germanium detectors with thin amorphous germanium contacts were successfully operated directly in liquid nitrogen and liquid argon in a cryostat at the Max-Planck-Institut für Physics in Munich. The detectors were fabricated at the Lawrence Berkeley National Laboratory and the University of South Dakota, using crystals grown at the University of South Dakota. They survived long-distance transportation and multiple thermal cycles in both cryogenic liquids and showed reasonable leakage currents and spectroscopic performance. Also discussed are the pros and cons of using thin amorphous semiconductor materials as an alternative contact technology in large-scale germanium experiments searching for physics beyond the Standard Model.

Search for variations of [formula omitted] decay constant

Exploration of possible anomalies in the decay of different radionuclides is an active research area in the fields of astrophysics and nuclear physics. The most significant changes in decay rates can be expected for β−, β+ or electron capture decay nuclides, due to the proposed influence of the solar neutrino flux by some research groups. In this work, the nuclear decay rate of β+ decay of N22a was investigated during the time interval of ~33 days. For this purpose, a coincidence system based on a planar high purity germanium detector and a plastic scintillation detector was designed and used for the acquisition of experimental N22a coincidence spectrum. The corresponding experimental conditions: temperature, atmospheric pressure, and relative humidity, which may affect the measurement system, were monitored. In addition, Monte-Carlo simulations of this system were performed in order to obtain the N22a simulated coincidence spectrum. The time-dependent coincidence spectra in 0.5 h bins were collected by the multiparameter MPA-3 system. The fluctuations in the decay rate (i.e. possible variations of the decay constant) were analyzed by Fourier and Lomb-Scargle algorithms. No significant evidence for periodic behavior in decay rate or aperiodic anomalies was found during the acquisition period.

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%.

Lung tissue substitute: synthesis, characterization and attenuation studies for low energy photons

Towards development of lung tissue substitute used in the calibration of lung counters, seven sets of polyurethane (PU) foam were synthesized exhibiting a density range of 0.14–0.42 g/cm3 by varying the quantity of foaming agent (water) in the formulation. Foam sets were characterized by Fourier transform infrared spectroscopy, elemental analysis and thermo gravimetric analysis. Effect of density on the properties of PU foam is investigated. In addition, attenuation behavior of synthesized material for low energy (<100 keV) gamma rays was checked by using 241Am & 133Ba sources and planar high purity germanium detector. Moreover, the linear and mass attenuation coefficients of all sets were calculated and were found to be in agreement to the reported values.

Isotopic composition analysis and age dating of uranium samples by high resolution gamma ray spectrometry

Non-destructive methods were applied to determine the isotopic composition and the time elapsed since last chemical purification of nine uranium samples. The applied methods are based on measuring gamma and X radiations of uranium samples by high resolution low energy gamma spectrometric system with planar high purity germanium detector and low background gamma spectrometric system with coaxial high purity germanium detector. The “Multigroup γ-ray Analysis Method for Uranium” (MGAU) code was used for the precise determination of samples’ isotopic composition. The age of the samples was determined from the isotopic ratio 214Bi/234U. This ratio was calculated from the analyzed spectra of each uranium sample, using relative detection efficiency. Special attention is paid to the coincidence summing corrections that have to be taken into account when performing this type of analysis. In addition, an alternative approach for the age determination using full energy peak efficiencies obtained by Monte Carlo simulations with the GESPECOR code is described.

Uranium Enrichment Determination Using a New Analysis Code for the U XKα Region: HyperGam-U

Abstract HyperGam-U was recently developed to determine uranium enrichment based on γ- and X-ray spectroscopy analysis. The XK α region of the uranium spectrum contains 13 peaks for 235 U and 238 U and is used mainly for analysis. To describe the X-ray peaks, a Lorentzian broadened shape function was used, and methods were developed to reduce the number of fitting parameters for decomposing the strongly overlapping peaks using channel-energy, energy-width, and energy-efficiency calibration functions. For validation, eight certified reference material uranium samples covering uranium enrichments from 1% to 99% were measured using a high-resolution planar high-purity germanium detector and analyzed using the HyperGam-U code. When corrections for the attenuation and true coincidence summing were performed for the detection geometry in this experiment, the goodness of fit was improved by a few percent. The enrichment bias in this study did not exceed 2% compared with the certified values for all measured samples.

Compton imaging with AGATA and SmartPET for DESPEC

DESPEC (DEcay SPECtroscopy) is a spectrometer, currently under construction, which is to be used at the FAIR (Facility for Antiproton and Ion Research) facility at GSI Darmstadt, Germany, as part of the NuSTAR (Nuclear STructure, Astrophysics and Reactions) project. Its goal is to analyse the decay of exotic nuclei produced via the Super-FRS (SUPERconducting FRagment Separator). The optimal configuration of certain elements of the spectrometer, namely a HPGe (High Purity Germanium) tracking array, is still under consideration. Work currently being carried out at the University of Liverpool using a segmented, coaxial HPGe detector (AGATA B009) and a pixelated, planar HPGe detector (SmartPET 1) in a Compton camera configuration aims to inform this process by providing insight into the potential performance of such a configuration. This will determine its suitability for the intended purpose, i.e. Compton reconstruction of gamma rays emitted from exotic nuclei implanted into the DESPEC implantation detector, AIDA (Advanced Implantation Detector Array). Preliminary results from this investigation, along with plans for the future direction of this work, are presented.

Charge collection performance of a segmented planar high-purity germanium detector

High-precision scans of a segmented planar high-purity germanium (HPGe) detector have been performed with a range of finely collimated gamma ray beams allowing the response as a function of gamma ray interaction position to be quantified. This has allowed the development of parametric pulse shape analysis (PSA) techniques and algorithms for the correction of imperfections in performance. In this paper we report on the performance of this detector, designed for use in a positron emission tomography (PET) development system.

Position sensitivity of segmented planar HPGe detectors for the DESPEC project at FAIR

One of the proposed germanium detector systems for the upcoming DESPEC array at the FAIR facility consists of triple modules of electrically segmented planar high-purity germanium detectors. In this work, the position sensitivity obtained by means of pulse shape analysis (PSA) for simulated γ-ray interactions has been studied for the possible segmentation patterns for such detectors—the double-sided strip detector (DSSD) and the one-sided pixelated geometry. An over-all similar number of readout channels has been considered for each case, resulting in 8+8 strips and 16 pixels. It has been found that the higher physical granularity of the DSSD results in a significantly higher position resolution, as well as in a somewhat lower probability of merging multiple interaction points. A drawback of the DSSD geometry is the inability to assign the interactions to the correct voxels for certain complex multiple interaction patterns. This effect has been studied in detail. Finally, a comparison has been made with the existing 25-pixel planar detector at KTH [A. Khaplanov, et al., Nucl. Instr. and Meth. A 580 (2007) 1075; L. Milechina, B. Cederwall, Nucl. Instr. and Meth. A 550 (2005) 278)].

Evaluation of lead shielding for a gamma-spectroscopy system

Three types of lead shielding assemblies were investigated to establish which provides the lowest background as preliminary work for determining the efficiency of a High Purity Germanium (HPGe) detector. The background continuum and peaks were examined using gamma spectroscopy with a coaxial and a planar HPGe detector. The 210Pb concentration in each lead sample taken from the assemblies was quantitatively determined. A French lead sample was found to have the lowest background in this relative comparison with a 210Pb concentration of 29.1±1.3Bqkg−1.

Segmented high-purity germanium detectors

Segmented planar high-purity germanium detectors were developed. Segmentation of p-n junction was performed by implantation of boron (for the n-type Ge) and mechanical grooving of the Li diffusion layer (for the p-type Ge). The electric and spectrometric characteristics of each individual segment and interactions between segments were investigated. The reliability and stability of the detectors were improved by using special passivation of the crystal surfaces.

Determination of HPGe detector response using MCNP5 for 20–150 keV X-rays

The Monte Carlo method was used to calculate the efficiency, escape and Compton curves of a planar high-purity germanium detector (HPGe) in the 20–150 keV energy. These curves were used for the determination of photons spectra produced by an X-ray machine in order to allow a precise characterization of photon beams applied to medical diagnosis. The detector was modeled with the MCNP5 computer code and validated by comparison with experimental data. The air kerma calculated after the spectra stripping was compared with ionization chamber measurements.

A study of fast neutron interactions in high-purity germanium detectors

The interaction properties of fast neutrons in a closed-end coaxial and a planar high-purity germanium detector (HPGe) have been studied. Digitized waveforms of HPGe preamplifier signals were recorded for time-of-flight separated neutrons and γ -rays emitted by a 252Cf source. The experimental results were compared to simulations of pulse shapes and to GEANT4 simulations of the interactions of the neutrons and γ -rays in the detectors. In the analysis, special emphasis was given to elastically scattered neutrons, which for HPGe energy thresholds below about 50 keV give the largest contribution to the interaction probability, and which may be an important effect to take into account in future γ -ray spectrometers based on γ -ray tracking. No differences between the pulse shapes of detected neutrons and γ -rays were observed. The general agreement between the experimental and simulated results was found to be good.

Nondestructive Inspection of Thin, Low-Z Samples using Multiplexed Compton Scatter Tomography

ABSTRACTThe viability of a Compton scattering tomography system for nondestructively inspecting thin, low Z samples for corrosion is examined. This technique differs from conventional x-ray backscatter NDI because it does not rely on narrow collimation of source and detectors to examine small volumes in the sample. Instead, photons of a single energy are backscattered from the sample and their scattered energy spectra are measured at multiple detector locations, and these spectra are then used to reconstruct an image of the object. This multiplexed Compton scatter tomography technique interrogates multiple volume elements simultaneously. Thin samples less than 1 cm thick and made of low Z materials are best imaged with gamma rays at or below 100 keV energy. At this energy, Compton line broadening becomes an important resolution limitation. An analytical model has been developed to simulate the signals collected in a demonstration system consisting of an array of planar high-purity germanium detectors. A technique for deconvolving the effects of Compton broadening and detector energy resolution from signals with additive noise is also presented. A filtered backprojection image reconstruction algorithm with similarities to that used in conventional transmission computed tomography is developed. A simulation of a 360–degree inspection gives distortion-free results. In a simulation of a single-sided inspection, a 5 mm × 5 mm corrosion flaw with 50% density is readily identified in 1-cm thick aluminum phantom when the signal to noise ratio in the data exceeds 28.

Temperature sensitivity of surface channel effects on high-purity germanium detectors

The temperature sensitivity of surface channel effects on planar high-purity germanium detectors was measured using 60-keV gamma-ray scanning techniques, as part of a radiation damage study. When measured in this manner, the surface effects on most detectors showed extreme temperature sensitivity in the 72–95 K region. The effect of the surface channel increased with increasing temperature to such an extent that the efficiency, as measured by the count rate in the 1332-keV peak from a 60Co source, decreased by a factor of over two in some cases. Since the peak efficiency for the 1332-keV gamma ray decreased as the temperature increased throughout the operating range (72–120 K) the effect of the surface channel must continue to increase beyond the temperature (95 K) at which the 60-keV scan loses its sensitivity because of the strong attenuation of these much lower energy gamma rays. Radiation damage had no measurable effect on the surface characteristics. No correlation between the surface effects and the resolution changes of the 1332-keV peak was observed.

Temporal instabilities associated with a planar high purity germanium detector

Experiments have been performed which show that a grooved planar high purity germanium semiconductor detector may give rise to time variant counting rates in energy regions below prominent spectral peaks. It is postulated that this effect is due to prolonged or incomplete charge collection in the vicinity of the bottom of the groove, and is caused by distortions in the applied electric field in this region. Such effects may lead to changes in the minimum detectable count rate in X-ray fluorescence measurements; these can be reduced by collimating incident photons on the central region of the detector but the resulting reduction in efficiency lowers the overall sensitivity.