Low-energy Tailing Research Articles

An adaptive digital filter for quasi-Gaussian pulse shaping

The radiation detector output is often shaped into Gaussian or quasi-Gaussian shape to improve the measurement performance. An adaptive digital Step-Gaussian filter is presented for quasi-Gaussian pulse shaping in this paper. It can be applied for both exponential decay input and step input. The transfer function of the filter is established. The amplitude-frequency features of the new filter are compared with the PZC-(RC) n filter and the CR-(RC) n filter. The results indicate that the new filter has the same amplitude-frequency features as the PZC-(RC) n filter and it possesses more effective filter performance than the CR-(RC) n filter. An X-ray fluorescence measurement system based on a FAST-SDD detector is set up for experiments. The proposed filter is verified by the measured pulses that are generated from a manganese sample. The shape of the filter output shows that the Step-Gaussian filter can eliminate the undershoot that exists in the CR-(RC) n filter output. The amplitude spectrums with different shaping times are created to study the shape of the characteristic peak. The results show that the Step-Gaussian filter can cancel the low-energy peak tailing of the 5.89 keV peak. The peak shape parameter, FWTM/FWHM ratio, is also introduced to evaluate the performance of peak shape improvement. The 5.89 keV peaks of the spectrums that are generated by using Step-Gaussian filter are closer to Gaussian distribution than that of the CR-(RC) n filter.

Formula omitted]-ray energy spectrum response tailing in CdZnTe detector

Affected by the characteristics of carrier transport, the γ-ray energy spectrum of the cadmium zinc telluride (CZT) detector usually has a low-energy tail phenomenon The traditional γ -ray energy spectrum response simulation model is mainly aimed at detectors with a uniform internal electric field, not suitable for non-uniform internal electric fields, and cannot accurately characterize the low-energy tailing phenomenon of the γ -ray energy spectrum of CZT detectors. In this study, for the quasi-hemispherical and coplanar-grid CZT detectors, the Geant4 program was used to establish the energy deposition model, and the γ-ray energy deposition and the carrier distribution in the two detectors were obtained. On this basis, the finite element analysis method was used to construct the internal electric field of the two detectors, and the charge collection efficiency model is established according to the transport law of carriers in the detectors. Finally, the above model was used to simulate the γ energy spectrum response of the four nuclides 241Am, 133Ba, 137Cs, and 60Co, and the simulation results were verified through experiments. The results show that the method in this paper can accurately describe the low-energy tail of the γ-ray energy spectrum of the two CZT detectors, and the obtained simulated γ-ray energy spectrum is basically consistent with the full-spectrum characteristics of the measured energy spectrum.

Monte Carlo modelling of a compact CZT-based gamma camera with application to 177Lu imaging

BackgroundSemiconductor gamma-camera systems based on cadmium zinc telluride (CZT) detectors present new challenges due to an energy-response that includes effects of low-energy tailing. In particular, such energy tails produce effects that need to be considered when imaging radionuclides with multiple emissions such as ^{177}{mathrm {Lu}}. Monte Carlo simulation can be used to investigate the behaviour of such systems and optimise their use, provided that the detector model closely reflects the real physical detector. The aim of this work is to develop a CZT model applicable for simulation of CZT-based gamma cameras.MethodsThe equations describing the charge transport and signal induction are considered in three dimensions and are solved numerically, and the CZT model is then realised by coupling the detector-response to the photon-transport handled by the SIMIND Monte Carlo program. The CZT model is tuned to reproduce experimentally measured energy spectra of a hand-held gamma camera system for multiple radionuclides (^{99mathrm {m}}{mathrm {Tc}}, ^{123}{mathrm {I}} and ^{177}{mathrm {Lu}}) and parallel-hole collimators (MEGP, LEHR) as well as an uncollimated system.ResultsOverall, the model results agree well with measurements across the range of experimental conditions. The applicability of the model is demonstrated by separating energy spectra into components to investigate the interference of high-energy photons on lower energy-windows, where pronounced effects of low-energy tailing for ^{177}{mathrm {Lu}} are observed.ConclusionsThe developed model provides understanding of the specifics of the camera response and is expected to be helpful for future optimisation of gamma camera applications.

Peak tailing cancellation techniques for digital CR-(RC)n filter

The CR-(RC)n filter is used for Semi-Gaussian pulse shaping in analog spectroscopy. A new digital CR-(RC)n filter is presented based on the recursive models of the CR filter and RC filter. The spectroscopic signal reconstruction technique is proposed. This technique is applied to digital exponential decay signals obtained by the digitization of analog signals that represent the combined response of a FAST-SDD detector and the associated front-end circuit. Compared with the direct application of the digital CR-(RC)n filter, the technique eliminates the undershoot of the Semi-Gaussian pulse which is the primary cause of the low-energy peak tailing. And, the digital pole-zero cancellation filter follows digital CR-(RC)n filter to reduce the long tail of the shaped Semi-Gaussian pulse which leads to the high-energy peak tailing. Experimental results based on XRF signals of manganese sample show that the proposed techniques can cancel peak tailing due to the shaped Semi-Gaussian pulse improving FWTM/FWHM ratio and throughput capability.

P2. Impact of the modelling of charge collection on the simulation of SPECT recordings from CZT semiconductors camera

Introduction CZT cameras, such as the DSPECT one (Spectrum Dynamics®), involve a principle of direct detection to convert γ photon into an electric signal. SPECT-recordings from the DSPECT camera were already simulated with the GATE platform [1] , but without considering certain physical properties of semiconductors, in particular the partial collection of charges between anode and cathode leading to a low energy tailing effect [2] . This study aimed to model this property by using the Hecht equation [3] putting in relation the amount of collected charge with the depth of interaction of the γ photon within the CZT crystal. Materials and methods The Hecht equation was used to model the response of the 9216 pixels of the DSPECT camera. A comparison was planned between the energy spectra provided by simulated and actually recorded data from a source of 99 mTc setting in the air or in a diffusing environment (water cylinder of a 8.5 cm diameter). Results A good agreement was documented between the energy spectra from simulated and actually recorded data in the air as well as in the diffusing environment. An under-estimation was however documented for the low-energy photons obtained with the simulation, particularly in diffusing environment and in the lower part of the recording window of the 99 mTc photopeak (relative decreases of −7.5% in the air and of −3.6% in the diffusing environment for the interactions simulated at 126 keV and compared to actual data). Conclusion The modelling of charge collection allows improving the Monte-Carlo simulation of SPECT-recording with energy spectra being close to those actually observed with CZT cameras.

Design of a new comprehensive continuous monitoring system for environmental radioactive aerosol

In order to comprehensive monitoring the radioactive isotopes from nuclear facilities, we developed a dual channel spectral monitoring instrument, and realized synchronous measurement for alpha, beta and gamma radionuclides. This article focuses on how to ensure its accuracy, stability and efficiency. First is the accuracy. In order to lower the interference of environmental and detector performance variation, the zero phase shift filter was designed to ensure the accuracy of characteristic peak position. Lorenz fitting algorithm was designed to reduce the effect of spectral low-energy tailing. Multi thread processing was introduced to ensure that there was sufficient time to complete our complex algorithms. Second is the stability. The complicated measuring process was decomposed into several sub-states. A state monitoring method was set up to timely dispose the abnormal operation. Third is the efficiency. Sampling process and measurement process were designed in synchronous to save monitoring time, which is especially useful for environmental low level radioactive monitoring. Continuous test for seven days shows that the detection limit is less than 0.0003Bq/m3 for U, 239Pu, and less than 0.048Bq/m3 for beta and 137Cs.

Typical uncertainties in alpha-particle spectrometry

Alpha-particle spectrometry is routinely performed with the aim of measuring absolute activities, activity ratios between different alpha-emitting nuclides or decay data such as branching factors, alpha emission probabilities and relative half-lives. It is most commonly performed with ion-implanted silicon detectors. Strong features of the technique are the low background levels that can be achieved due to low sensitivity to other types of radiation, the intrinsic efficiency close to 1 which reduces the efficiency calculations to a geometrical problem and the uniqueness of the energy spectra for each α-decaying nuclide. The main challenge is the limitation to the attainable energy resolution, even with thin and homogenous sources, which causes alpha energy peaks to be partially unresolved due to their width and low-energy tailing. The spectral deconvolution often requires fitting of analytical functions to each peak in the alpha spectrum. True coincidence effects between alpha particles and subsequently emitted conversion electrons cause distortions of the alpha spectra which lead to significant changes in the apparent peak area ratios. Optimum energy resolution can only be achieved on very thin sources, which puts constraints on the source preparation techniques. Radiochemical separations may be needed to extract the alpha emitters from voluminous matrices and efficiency tracing is performed by adding in another isotope by known amounts. Typical uncertainty components are discussed by means of some hypothetical examples.

Core-level spectra from graphene

We calculate core-level spectra for pristine and doped free-standing graphene sheets. Instructions for how to perform the calculations are given in detail. Although pristine graphene is not metallic the core-level spectrum presents low-energy tailing which is characteristic of metallic systems. The peak shapes vary with doping level in a characteristic way. The spectra are compared to experiments and show good agreement. We compare to two different pristine samples and to one doped sample. The pristine samples are one with quasi-free-standing epitaxial graphene on SiC obtained by hydrogen intercalation and one with a suspended graphene sheet. The doped sample is a gold supported graphene sheet. The gold substrate acts as an acceptor so the graphene sheet gets p-doped.

Anomalous excitation-power-dependent photoluminescence of InGaAsN/GaAs T-shaped quantum wire

The selected InGaAsN/GaAs T-shaped quantum wire (T-QWR) fabricated by metal organic vapor phase epitaxy has been investigated by microphotoluminescence (µ-PL) and excitation-power-dependent µ-PL. The optical characteristics of one-dimensional structure were taken at low-temperature (4 K) and room temperature (RT) to clarify the intersection of two familiar quantum wells (QWs) in the [001] and [110] directions, named QW1 and QW2, respectively. For the excitation-power-dependent measurement, the intensity of the excitation source was used in the range of 0.001I0 to I0. The result shows that all of emissions related to QW1 and QWR peaks have a nonsymmetric line shape as evidenced by the tailing on the lower-energy side. All peaks shift to higher-energy side (blueshift) with the increase of the excitation power intensity. The blueshift and the low-energy tailing of PL peaks are attributed to the alloying effect. However, the emission peak related to QWR region shows a larger blueshift rate than that of QW1 on increasing of the excitation power intensity. This is an anomalous characteristic for the low-dimensional structure, affected by the large fluctuation state in the QWR region. This fluctuation state is combined of both edges of QWs (QW1 and QW2).

Development of Decay Energy Spectroscopy for Radionuclide Analysis Using Cryogenic 4π Measurements

We report the recent progress in the development of decay energy spectroscopy for radionuclide analysis using a metallic magnetic calorimeter. In the present analysis, sample radionuclides were completely enclosed by a 4π steradian absorber. The use of a 4π absorber composed of gold foil guarantees that the total energy associated with radioactive decay is converted into thermal energy in the absorber. A paramagnetic temperature sensor was attached to the absorber to accurately measure the temperature change due to radioactive decay. The plutonium isotopes 238Pu, 239Pu, and 240Pu were readily identified in the decay energy spectrum because each isotope creates a single peak at its characteristic Q value. Two clear peaks were observed for 239Pu and 240Pu, and a 6.3 keV FWHM was obtained. The energy resolution of the method was affected by the low-energy tail of the spectrum at the left-hand side of the peaks. A 4.1 keV FWHM of a Gaussian fit was obtained for the right-hand side of the peak. Slow heat release to the absorber due to heat flow mechanisms is discussed to explain the low-energy tailing effect.

Improved x-ray spectroscopy with room temperature CZT detectors

Compact, room temperature x-ray spectroscopy detectors are of interest in many areas including diagnostic x-ray imaging, radiation protection and dosimetry. Room temperature cadmium zinc telluride (CZT) semiconductor detectors are promising candidates for these applications. One of the major problems for CZT detectors is low-energy tailing of the energy spectrum due to hole trapping. Spectral post-correction methods to correct the tailing effect do not work well for a number of reasons; thus it is advisable to eliminate the hole trapping effect in CZT using physical methods rather than correcting an already deteriorated energy spectrum. One method is using a CZT detector with an electrode configuration which modifies the electric field in the CZT volume to decrease low-energy tailing. Another method is to irradiate the CZT surface at a tilted angle, which modifies depth of interaction to decrease low-energy tailing. Neither method alone, however, eliminates the tailing effect. In this work, we have investigated the combination of modified electric field and tilted angle irradiation in a single detector to further decrease spectral tailing. A planar CZT detector with 10 × 10 × 3 mm3 size and CZT detector with 5 × 5 × 5 mm3 size and cap-shaped electrode were used in this study. The cap-shaped electrode (referred to as CAPture technology) modifies the electric field distribution in the CZT volume and decreases the spectral tailing effect. The detectors were investigated at 90° (normal) and 30° (tilted angle) irradiation modes. Two isotope sources with 59.6 and 122 keV photon energies were used for gamma-ray spectroscopy experiments. X-ray spectroscopy was performed using collimated beams at 60, 80 and 120 kVp tube voltages, in both normal and tilted angle irradiation. Measured x-ray spectra were corrected for K x-ray escape fractions that were calculated using Monte Carlo methods. The x-ray spectra measured with tilted angle CAPture detector at 60, 80 and 120 kVp tube voltages were compared to corresponding theoretical spectra. The low-energy tailing was nearly completely eliminated from 59.6 and 122 keV isotope spectra, and 60, 80 and 120 kVp x-ray spectra, when CAPture detector was used with 30° tilted angle irradiation. It is concluded that using a CZT detector with modified electric field in tilted angle configuration resolves problem of the tailing effect in CZT detectors, opening promising possibilities in gamma-ray and x-ray spectroscopy applications.

Quality Assurance Challenges in X-ray Emission Based Analyses, the Advantage of Digital Signal Processing

There is a large scatter in the results of X-ray analysis with solid-state detectors suggesting methodological origin. In order to improve the methodology, detector response functions have been investigated by many researchers and analysts. This was necessary as the departure of the response function of some detector-signal processing electronics from the normally assumed Gaussian line shape can exceed 100% in area. Several detector models have been proposed to improve understanding and establish a firm basis for quantitative work. After reviewing some contradictory results, we describe a signal processor that offers quality assurance, by producing two spectra for each measurement. One is the normal spectrum of accepted events, while the second spectrum contains all of the rejected events. For each measurement, therefore, all X-ray events are recorded, enabling quality control. In addition to this improvement, the digital signal processor of Cambridge Scientific, Canada, delivers a high throughput rate, excellent resolution, decreased low energy tailing and a line shape justified by the physics of the detector. Comparative measurements are presented to demonstrate the improved rejection of background from gamma rays as well as a significant improvement in pile-up recognition. The rejected events spectrum gives insight into the origin of the response function, which suggests that the flat plateau of the frequently used Hypermet function, normally attributed to detector dead layers, originates from pile-up with the low energy noise events. A detailed analysis demonstrates how the relative intensities of the X-ray lines can change in a varying noise environment, thus potentially explaining the unacceptable large scatter in the experimental data currently found in the literature. The comparison of the accepted and rejected events adds the possibility of monitoring the electronic efficiency of signal recognition that has generally been ignored in this field.

Soft X-ray emission spectra of argon atoms doped in solid matrices

Soft X-ray emission spectra in the Ar L region of various solid substrates doped with Ar atoms were measured to investigate the chemical states of the Ar atoms in the solid matrices. Ar ions were implanted into the solid matrices of Si(1 1 1), SiO 2, highly oriented pyrolytic graphite (HOPG), Ti, Cr, Ni, and Cu with an acceleration voltage of 5 kV at room temperature. Soft X-ray emission spectra in the Ar L region were measured using synchrotron radiation. Low-energy tailing was observed at the L 3–M 1 and L 2–M 1 X-ray emission peaks in Ar-doped transition metals. The density of states (DOS) of the Ar 3s orbitals indicated that the low-energy tailed DOS can be formed by hybridization with 3d orbitals in the overpressurized Ar clusters.

Energy distribution of the ternary alpha's emitted in 235U(n th, f) and 252Cf(SF)

Charged particles emitted in ternary fission appear to have a Gaussian-shaped energy distribution, except for the α particles, for which mostly an important non-Gaussian low-energy tailing is reported. We realised good experimental conditions for studying the 235U(n th, f) ternary α energy distribution at the reactor of the ILL in Grenoble. Thanks to an intense and clean neutron beam, a small, thin sample of highly enriched U could be used, with an activity of only 1.6 Bq, so the measurements could be done without absorber in between the sample and the Δ E – E detector. With the resulting low detection limit of 6 MeV, a clearly asymmetric energy distribution was obtained, deviating by 6% from a Gaussian shape. This energy distribution has been deconvoluted into components corresponding to “true” ternary α's and α's originating from the decay of 5He particles. Ternary α emission yield data have been re-evaluated accordingly.

Evaluation of low-energy tailing parameters of a HPGe X-ray detector to be used in GUPIX software library for PIXE analysis

Accurate description of the response function of X-ray detectors is important for quantitative PIXE analysis. For analytical description of low-energy tailing in PIXE spectra, the GUPIX approach was used, i.e. a combination of Hypermate type functions. The low-energy tailing parameters of a low-energy high purity germanium X-ray detector in the region 1.3–9.2 keV was determined by systematic fitting of X-ray spectra from thin MicroMatter standards. Except for the main Gaussian peak and germanium escape peaks, one shelf, one truncated shelf, one exponential function and KMM radiative Auger feature was applied to fit the spectral data collected with this low-energy Ge X-ray detector. The overall trend of different tail parameters shows similar characteristic as those for Si(Li) X-ray detectors used in this energy range, i.e. very strong tailing at low X-ray energies and a rapid fall off with increasing X-ray energy. The fitted parameters, as a function of X-ray energy, were then incorporated into the GUPIX library for PIXE analysis.

On the response function of solid‐state detectors, based on energetic electron transport processes

AbstractIn studies of the response function of state‐of‐the‐art Si(Li) and HPGe detectors to low‐energy (2–20 keV) mono‐energetic x‐rays, various characteristic features have been observed. To trace their origin we have studied the effect of the signal processing electronics on the evolution of the electronic signal, and measured the spectral shape of energetic electrons originating from the surfaces of Si and Ge crystals and front contact materials, using a high‐energy range electrostatic electron spectrometer. From the measured electron spectra leaving the surfaces of the detector and front contact materials, and from the details of electron transport processes, the origin of the distinct features is identified and an approximate analytical form for the representation of the detector response function has been derived. It gives an excellent description of the response function and may replace the frequently applied Hypermet function. On the basis of this study, an explanation is given of the origin of the exponential tail and other various discrete structures on the low‐energy side of the primary peak. The difference in low‐energy side tailing between Si and HPGe detectors is explored. A detailed analysis of the so‐called dead layer concept is given. Characteristic structures, ‘kinks’, are predicted in the spectra measured by solid‐state detectors. Copyright © 2003 John Wiley & Sons, Ltd.

The X-ray response of InP: Part B, synchrotron radiation measurements

In this, the second part of a detailed study into the X-ray response of InP, we present results of a series of X-ray measurements on a 3.142 mm 2×180 μm thick semi-insulating InP detector at the BESSY II and HASYLAB synchrotron radiation research facilities. Photon metrology was carried out at energies ranging from 8 to 100 keV. Additional measurements were made using radioactive and fluorescent target sources. At −60°C, under full-area illumination, the FWHM energy resolution was 2.4 keV at 5.9 keV rising to 8.5 keV at 59.54 keV. Under pencil-beam illumination, the measured resolutions were generally less, being 2 keV FWHM at 8 keV rising approximately linearly to 5 keV at 100 keV. Analysis of the energy resolution function indicates that poor charge transport presently limit the performance of InP detectors and specifically hole trapping. This is borne out by the observed low-energy tailing of the pulse height spectra at intermediate and high energies. At very low count rates, it was found that the device was linear to within 1%. However, for count rates above ∼100 s −1 and energies above ∼50 keV, the detector began to display time variable gain shifts indicative of polarization effects. The spatial uniformity of the detector was investigated by raster scanning a 50×50 μm 2, 15 keV monoenergetic X-ray beam across the active area. Apart from a few localized areas, the detector response was found to be reasonably uniform at the 10% level, although inconsistent with that expected from counting statistics alone (2%).

Simulation of the collection properties of CdTe strip detectors

CdTe is an attractive material for X-ray and gamma-ray detectors, but the poor transport properties of holes affect the performance by introducing a low energy tailing in the observed spectra. A possible solution to this problem is to optimize the electrode geometry, for example, by reducing the dimension of the anode with respect to the cathode. In this way, the charge signal in the external circuit is mostly due to the electrons moving toward the anode that is where the weighing field becomes localized. The optimization of the electrode geometry can be approached by a numerical analysis as the problem of charge collection is strictly connected to trapping and detrapping processes, which are difficult to analytically treat in nonuniform electric fields. In this paper, we present a numerical simulator based on a finite difference numerical method (which follows the weighing field approach) and on a Monte-Carlo procedure, which is able to analyze, in two dimensions, the effect of different electrode configurations: single strip, multiple strip, a single strip with a lateral extended cathode, and, for comparison, the uniform geometry. The results are analyzed in terms of the maps of the local charge collection efficiency and their histograms, equivalent to spectra due to high-energy X-rays.

Long-term radiation damage to a spaceborne germanium spectrometer

Abstract The Transient Gamma-Ray Spectrometer aboard the Wind spacecraft in deep space has observed gamma-ray bursts and solar events for four years. The germanium detector in the instrument has gradually deteriorated from exposure to the ≈10 8 p / cm 2 / yr (>100 MeV ) cosmic-ray flux. Low-energy tailing and loss of efficiency, attributed to hole trapping and conversion of the germanium from n- to p-type as a result of crystal damage, were observed. Raising the detector bias voltage ameliorated both difficulties and restored the spectrometer to working operation. Together, these observations extend our understanding of the effects of radiation damage to include the previously unsuccessfully studied regime of long-term operation in space.

Radiation effects on the proportional counter X-ray detectors on board the NEAR spacecraft

The X-ray proportional counters on board the Near Earth Asteroid Rendezvous (NEAR) spacecraft have exhibited a resolution degradation and recovery phenomenon several times during the long cruise phase of the mission. The resolution is checked periodically by commanding an 55Fe source into the window area. The degradation is seen as a low energy tailing of the 5.9 keV photopeak. Two events have occurred which provided good spectral data for better understanding the degradation phenomenon. In November 1997 a large solar particle event occurred that degraded the resolution and excited copper in the collimator. Eventually the detectors returned to normal. In January 1998 the spacecraft performed an Earth swingby gravity assist maneuver. The near Earth environment excited the magnesium and aluminum in the filter elements. The copper line was also produced. The NEAR spacecraft was launched in February 1996 and will rendezvous and orbit the asteroid 433 Eros in early 1999.