Pulse Pile-up Rejection in Energy Dispersive XRF Analysis

Distortions of nuclear energy spectra at high counting rates have been efficiently reduced by two different signal processing methods: pile-up suppression by pulse shortening and pile-up rejection by detection and rejection of the piled-up pulses. By combining these methods new circuits have been developed increasing the limit for the quality gamma ray spectrometry from available, up to now, 100000 s-1 to the range of up to 500000 s-1.

This paper introduces a new method for neutron-gamma pulse shape discrimination (PSD) and pile-up signal rejection (PUR) based on multi-resolution analysis (MRA). The method uses a fast wavelet transform to decompose the original signal, extracting approximate components through low-pass filtering and detail components through high-pass filtering. Following reconstruction, the approximate and detail components are analyzed to achieve efficient PSD and PUR, respectively. The experiments used an EJ276 plastic scintillator detector to collect neutron and gamma signals from an Am-Be source for PSD analysis and mixed signals with pile-up pulses (with an additional Na-22 source) for PUR analysis. The MRA method with different wavelet bases was compared to the charge comparison (CC) method in terms of PSD performance across time gate widths and energy ranges. The results show that the CC method works well only with wide time gates and high energy ranges. Except for the Haar wavelet, the MRA method's PSD performance is more stable across different time gates, performing better with smaller time gates. The db3 wavelet provides the best discrimination at small time gates. The PUR results of the MRA method with the db3 wavelet were compared at time gates of 50 ns, 100 ns, 150 ns, and 200 ns. The 200 ns gate offers the best pulse discrimination but loses the most piled-up and neutron signals, increasing the computational load. The 50 ns gate retains the most neutron signals with the lowest load but has the least PSD. Based on users' needs, the ideal time gate for high count rate neutron-gamma PUR and PSD measurements is between 50 ns and 100 ns.

Comparisons of energy- and wavelength-dispersive spectrometry inevitably focus on the count rate limitations of the ED electronics. In some cases the use of amplifiers without pulse pileup rejection, or worse, misadjusted pileup rejection circuits, has led to erroneous results and assumptions regarding maximum tolerable count rates which are far too low.Proper evaluation of pileup rejection must consider both the count rate and energy dependence of the circuits. For example, using a typical ED spectrometer (EDAX, 154 eV, 10 mm2, 6 μsec amplifier time constant) we can first measure the total amplifier throughput curve as shown in Figure 1. This curve is only slightly dependent on photon energy, at least over the range from about 1-10 keV which usually contains the major part of the X rays, and shows a maximum stored count rate of slightly over 3000 cps for an input count rate of slightly less than 10,000 cps. Changing the amplifier shaping time constant will change the scale of the curve, but not its general shape.

Summary form only. A technique for the post-acquisition correction of distortion in radiation spectra due to pulse pileup is considered. Data have been obtained showing the application of this method in removing pileup distortion from Ge spectra acquired at high count rates. The results obtained have been compared with spectra acquired using a hardware pulse pileup rejector under identical conditions. A method for determining the effective pulse rise-time of a given MCA system (as seen by the system ADC) has been considered and the relevant theory has been examined. It is based on the comparison of low and high count rate spectra acquired by the system at known input count rates. >

Liquid-scintillation spectroscopy of α-particle emitters and detection of spontaneous fission events for on-line studies of actinide and transactinide elements

Our recent measurements with pixelated LaBr/sub 3/ Anger-logic detectors for use in time-of-flight (TOF) PET have demonstrated excellent energy resolution (5.1% at 511 keV) and coincidence time resolution (313 ps FWHM) with small prototype configurations. A full size detector module suitable for a whole-body 3D PET scanner has been constructed based on the prototype designs and consists of 1620 4/spl times/4/spl times/30 mm/sup 3/ LaBr/sub 3/ crystals. We have utilized simulations to guide experimental measurements with the goal of optimizing energy and time resolution in evaluating triggering configurations and pulse shaping needed in a full system. Experimental measurements with the detector module indicate energy and time resolution consistent with our earlier prototypes when measured at low count rate. At very high count rate the energy, time and spatial resolution degrade due to pulse pileup. While it is possible to reduce pulse pileup by using smaller PMTs (i.e., 39 mm instead of 50 mm), we are trying to limit the total number of PMTs needed for a full-scale PET scanner with a large axial field-of-view. Therefore, we have implemented a pulse shaping circuit to improve the detector response and performance at high count rate. Simulations of a complete LaBr/sub 3/ scanner indicate significant improvements in NEC and spatial resolution can be achieved using pulse shaping.

Investigations for the use of the fast digitizers with [formula omitted] detectors for radiative capture measurements at GELINA

Photon counting x-ray detectors (PCDs) with energy discrimination capabilities provide spectral information, can eliminate electronic and Swank noise, and image multiple contrast agents simultaneously with high resolution. However, with high flux on the detectors, pulse pileup leads to count loss and spectral distortion. Accurate description of the output count rate and spectrum behavior of a PCD is crucial to the development of further applications, e.g., material decomposition. In this work, a fully analytical model of the pulse pileup spectrum and count statistics for a nonparalyzable detector with nonzero pulse length is proposed, which accounts for seminonparalyzable behavior, i.e., retriggering of counts by pulses incident during the dead time. We use a triangle to approximate a realistic pulse shape and recursively compute the output spectrum of different pulse pileup orders. A count-rate and spectrum dependent expression of count statistics is further derived based on renewal theory. Our analytical model can then be used to predict material decomposition noise using the Cramer–Rao lower bound (CRLB). We compared our model predictions with Monte Carlo simulations. The results show that our model can accurately predict the count loss and spectral distortion resulting from pulse pileup and can be used in predicting material decomposition noise over a large range of count rates.

Probing the diffusion of molecules has become a routine measurement across the life sciences, chemistry and physics. It provides valuable insights into reaction dynamics, oligomerisation, molecular (re-)organisation or cellular heterogeneities. Fluorescence correlation spectroscopy (FCS) is one of the widely applied techniques to determine diffusion dynamics in two and three dimensions. This technique relies on the temporal autocorrelation of intensity fluctuations but recording these fluctuations has thus far been limited by the detection electronics, which could not efficiently and accurately time-tag photons at high count rates. This has until now restricted the range of measurable dye concentrations, as well as the data quality of the FCS recordings, especially in combination with super-resolution stimulated emission depletion (STED) nanoscopy.Here, we investigate the applicability and reliability of (STED-)FCS at high photon count rates (average intensities of more than 1 MHz) using novel detection equipment, namely hybrid detectors and real-time gigahertz sampling of the photon streams implemented on a commercial microscope. By measuring the diffusion of fluorophores in solution and cytoplasm of live cells, as well as in model and cellular membranes, we show that accurate diffusion and concentration measurements are possible in these previously inaccessible high photon count regimes. Specifically, it offers much greater flexibility of experiments with biological samples with highly variable intensity, e.g. due to a wide range of expression levels of fluorescent proteins. In this context, we highlight the independence of diffusion properties of cytosolic GFP in a concentration range of approx. 0.01–1 µm. We further show that higher photon count rates also allow for much shorter acquisition times, and improved data quality. Finally, this approach also pronouncedly increases the robustness of challenging live cell STED-FCS measurements of nanoscale diffusion dynamics, which we testify by confirming a free diffusion pattern for a fluorescent lipid analogue on the apical membrane of adherent cells.

The prompt gamma neutron activation analysis (PGNAA) technology is an online nondestructive material analysis technology widely used in industries. In order to achieve real-time measurements, the gamma ray count rate should reach 400 kcps. Due to the limitation of the detector's decay time, the signal output from the detector is heavily piled up. Meanwhile, the industrial site has a temperature fluctuation of 0-50 °C, causing the peak drift in the energy spectrum. This paper presents a readout electronic system specified for PGNAA and proposes series of algorithms that are dedicated for the gamma-ray spectrometric system. The system can achieve a count rate of 400 kcps, and it overcomes poor resolution, spectral distortion, and peak position shift, which are caused by pulse pileups and temperature fluctuations.

Raw-data–based material decomposition in spectral CT using photon–counting energy–selective detectors relies on a precise forward model that predicts a count–rate given intersection lengths for each material. This re- quires extensive system–specific measurements or calibration techniques. Existing calibrations either estimate a detected spectrum and are able to account for spectrally distorted assumptions or correct the predicted count rate using a correction function and can accommodate for count rate–dependent effects such as pulse pileup. We propose a calibration method that uses transmission measurements to optimize a correction function that, unlike existing methods, depends both on the photon energy and the count rate. It is thus able to correct for both kinds of distortions. In a simulated material decomposition into water and iodine, the error was reduced by 96 % compared to the best performing reference method if only pulse pileup was present and reduced by 23 % if additionally spectral distortions were taken into account. In phantom measurements using a Dectris SANTIS prototype detector, the proposed method allowed to reduce the error by 29 % compared to the best performing reference method. Artifacts were below the noise level for the proposed method, while the reference methods either showed an offset in the water region or ring artifacts.

Energy discriminating, photon-counting detectors (PCDs) are an emerging technology for computed tomography (CT) with various potential benefits for clinical CT. The photon energies measured by PCDs can be distorted due to the interactions of a photon with the detector and the interaction of multiple coincident photons. These effects result in distorted recorded x-ray spectra which may lead to artifacts in reconstructed CT images and inaccuracies in tissue identification. Model-based compensation techniques have the potential to account for the distortion effects. This approach requires only a small number of parameters and is applicable to a wide range of spectra and count rates, but it needs an accurate model of the spectral distortions occurring in PCDs. The purpose of this study was to develop a model of those spectral distortions and to evaluate the model using a PCD (model DXMCT-1; DxRay, Inc., Northridge, CA) and various x-ray spectra in a wide range of count rates. The authors hypothesize that the complex phenomena of spectral distortions can be modeled by: (1) separating them into count-rate independent factors that we call the spectral response effects (SRE), and count-rate dependent factors that we call the pulse pileup effects (PPE), (2) developing separate models for SRE and PPE, and (3) cascading the SRE and PPE models into a combined SRE+PPE model that describes PCD distortions at both low and high count rates. The SRE model describes the probability distribution of the recorded spectrum, with a photo peak and a continuum tail, given the incident photon energy. Model parameters were obtained from calibration measurements with three radioisotopes and then interpolated linearly for other energies. The PPE model used was developed in the authors' previous work [K. Taguchi et al., "Modeling the performance of a photon counting x-ray detector for CT: Energy response and pulse pileup effects," Med. Phys. 38(2), 1089-1102 (2011)]. The agreement between the x-ray spectra calculated by the cascaded SRE+PPE model and the measured spectra was evaluated for various levels of deadtime loss ratios (DLR) and incident spectral shapes, realized using different attenuators, in terms of the weighted coefficient of variation (COVW), i.e., the root mean square difference weighted by the statistical errors of the data and divided by the mean. At low count rates, when DLR < 10%, the distorted spectra measured by the DXMCT-1 were in agreement with those calculated by SRE only, with COVW's less than 4%. At higher count rates, the measured spectra were also in agreement with the ones calculated by the cascaded SRE+PPE model; with PMMA as attenuator, COVW was 5.6% at a DLR of 22% and as small as 6.7% for a DLR as high as 55%. The x-ray spectra calculated by the proposed model agreed with the measured spectra over a wide range of count rates and spectral shapes. The SRE model predicted the distorted, recorded spectra with low count rates over various types and thicknesses of attenuators. The study also validated the hypothesis that the complex spectral distortions in a PCD can be adequately modeled by cascading the count-rate independent SRE and the count-rate dependent PPE.

For a given sample material and experimental arrangement, there is an optimum sample size and shape. Samples must be large enough to give a high capture rate for good counting statistics in a reasonable measurement time but small enough to avoid inaccuracies that creep in with high counting rates, due to broadening and distortion of peaks caused by pulse pileup (Lindstrom and Fleming 1995). In addition, the sample should be small enough that bias from neutron absorption or scattering will be acceptably small. These considerations usually lead preferentially to small samples and long irradiation times. The composition, the experimental conditions, and the required accuracy determine what “small” means numerically.

A fuel rod scanner with an automatic decay correction system was manufactured to measure the reactor power density distribution by the Y-activity from an irradiated fuel rod. The detection head is a scintillation counter surrounded with Pb shield. A fuel rod travels on this head by the feed mechanism operated by compressed air force. Main advantages are that this mechanism does not give any oscillation to the fuel rod in moving and settles the measuring point accurately within ±0.01 mm. It is hardly possible to calculate the correction for decay of the fission product because it includes many kinds of decays and it has decay constant varying with time. Therefore, an automatic correction for decay time is highly desirable. A system of automatic decay correction, combining an automatic fuel rod scanner and electronic circuits, is used which effects the remote and automatic counting with minimal personnel. The effect of background counts in measuring and monitor channels and the pulse pile-up at high counting rate in electronic circuits are discussed in detail. In case of predominant background counts, it is preferable to make a measurement so as to satisfy the following equation, BM/B=P/NPri i where BM, B are background counts in monitor and measuring channels respectively and P, NPri i a preset count in monitor channel and an observed count in measuring channel.

Pulse pile-up correction by auto-regression on linear operations (ARLO) method: A comparison with integration-based algorithms