Pile-up Rejection Research Articles

Smart pile-up consideration for evaluation of high count rate EDS spectra

This work describes a new pile-up consideration for the very high count rate spectra which are possible to acquire with silicon drift detector (SDD) technology. Pile-up effects are the major and still remaining challenge with the use of SDD for EDS in scanning electron microscopes (SEM) with ultra thin windows for soft X-ray detection. The ability to increase the count rates up to a factor of 100 compared with conventional Si(Li) detectors, comes with the problem that the pile-up recognition (pile-up rejection) in pulse processors is not able to improve by the same order of magnitude, just only with a factor of about 3. Therefore, it is common that spectra will show significant pile-up effects if count rates of more than 10000 counts per second (10 kcps) are used. These false counts affect both automatic qualitative analysis and quantitative evaluation of the spectra. The new idea is to use additional inputs for pile-up calculation to shift the applicability towards very high count rates of up to 200 kcps and more, which can be easily acquired with the SDD. The additional input is the 'known' (estimated) background distribution, calculated iteratively during all automated qualitative or quantitative evaluations. This additional knowledge gives the opportunity for self adjustment of the pile-up calculation parameters and avoids over-corrections which challenge the evaluation as well as the pile-up artefacts themselves. With the proposed method the pile-up correction is no longer a 'correction' but an integral part of all spectra evaluation steps. Examples for the application are given with evaluation of very high count rate spectra.

On gamma-ray spectrometry pulses real time digital shaping and processing

In gamma-ray medical imaging and nuclear spectroscopy instruments, deployment of high speed reconfigurable field programmable gate arrays circuits make it possible to implement signal processing algorithms for pulse shaping, pile up detection and correction, and determination of pulse amplitude and its time of detection in real time. Based on this idea, we have developed a digital pulse processing board for use in a gamma ray spectroscopy and later in positron emission tomography. The proposed digital pulse-processing core relies on digital trapezoidal or triangular pulse shaping, an automatic baseline correction and pileup rejection methods, all of which are implemented in FPGA to permit on-line event processing. Experimental results show that because of its pulse shaping method, system throughput and energy resolution significantly outperforms analog pulse processing systems.

HDL Based FPGA Interface Library for Data Acquisition and Multipurpose Real Time Algorithms

The inherent parallelism of the logic resources, the flexibility in its configuration and the performance at high processing frequencies makes the field programmable gate array (FPGA) the most suitable device to be used both for real time algorithm processing and data transfer in instrumentation modules. Moreover, the reconfigurability of these FPGA based modules enables exploiting different applications on the same module. When using a reconfigurable module for various applications, the availability of a common interface library for easier implementation of the algorithms on the FPGA leads to more efficient development. The FPGA configuration is usually specified in a hardware description language (HDL) or other higher level descriptive language. The critical paths, such as the management of internal hardware clocks that require deep knowledge of the module behavior shall be implemented in HDL to optimize the timing constraints. The common interface library should include these critical paths, freeing the application designer from hardware complexity and able to choose any of the available high-level abstraction languages for the algorithm implementation. With this purpose a modular Verilog code was developed for the Virtex 4 FPGA of the in-house Transient Recorder and Processor (TRP) hardware module, based on the Advanced Telecommunications Computing Architecture (ATCA), with eight channels sampling at up to 400 MSamples/s (MSPS). The TRP was designed to perform real time Pulse Height Analysis (PHA), Pulse Shape Discrimination (PSD) and Pile-Up Rejection (PUR) algorithms at a high count rate (few Mevent/s). A brief description of this modular code is presented and examples of its use as an interface with end user algorithms, including a PHA with PUR, are described.

Pulse Analysis for Gamma-Ray Diagnostics ATCA Sub-Systems of JET Tokamak

A new Data Acquisition (DAQ) sub-system for gamma-ray diagnostics was developed for Joint European Torus (JET). The system is based on the ATCA platform and is able to sample up to 400 MSPS with 14-bit resolution. This DAQ is used for gamma-ray diagnostics dedicated to the study of fast ions in fusion tokamak plasma experiments. The present work describes the development of pulse processing algorithms used to extract the pulse parameters from the DAQ free running ADC data streams. These algorithms are divided in three main functional blocks, namely, advanced triggering and segmentation, segmentation validation and finally, peak height analysis (PHA) and pile-up rejection (PUR). The developed algorithms perform the pulse segmentation, shaping and validation according to the noise level and characteristics of the digitized signal. Shaping is performed through a reconfigurable trapezoidal filter in order to optimize the spectral resolution with the required throughputs and a comprehensive study of this parameterization is presented. Calibration procedures are mandatory for an efficient real time application and its implementation in the FPGA is discussed and explained. The presented and discussed results refer to the analysis of the JET pulse files obtained during the recent campaigns where a spectral resolution of 4.5% for the 137Cs energy peak at 662 keV was achieved through FPGA real time processing. Results of high gamma-ray reaction rates experiments performed outside JET are also presented.

Performance enhancements of compound semiconductor radiation detectors using digital pulse processing techniques

Abstract The potential benefits of using compound semiconductors for X-ray and gamma ray spectroscopy are already well known. Radiation detectors based on high atomic number and wide band gap compound semiconductors show high detection efficiency and good spectroscopic performance even at room temperature. Despite these appealing properties, incomplete charge collection is a critical issue. Generally, incomplete charge collection, mainly due to the poor transport properties of the holes, produces energy resolution worsening and the well known hole tailing in the measured spectra. In this work, we present a digital pulse processing (DPP) system for high resolution spectroscopy with compound semiconductor radiation detectors. The DPP method, implemented on a PC platform, performs a height and shape analysis of the detector pulses (preamplifier output pulses), digitized by a 14-bit, 100 MHz ADC. Fast and slow shaping, automatic pole-zero adjustment, baseline restoration and pile-up rejection allow precise pulse height measurements both at low and high counting rate environments. Pulse shape analysis techniques (pulse shape discrimination, linear and nonlinear pulse shape corrections) to compensate for incomplete charge collection were also implemented. The results of spectroscopic measurements on a planar CdTe detector show the high potentialities of the system, obtaining low tailing in the measured spectra and energy resolution quite close to the theoretical limit. High-rate measurements (up to 820 kcps) exhibit the excellent performance of the pulse height analysis and the benefits of pulse shape techniques for peak pile-up reduction in the measured spectra. This work was carried out in the framework of the development of portable X-ray spectrometers for both laboratory research and medical applications.

Suppression of gamma-ray sensitivity of liquid scintillators for neutron detection

Methods to reduce gamma-ray sensitivity of a liquid scintillator EJ309 have been studied. Zero-crossing pulse shape discrimination method was used to separate events generated by neutron and gamma radiation between 60− keVee and 4 MeVee. The measurements were carried out under irradiation from an intense 137Cs source, yielding dose rate of 10 mR/h at the detector. A Pu–Be source was used to establish neutron integration window. Pile-up rejection (PUR) circuit was used to reduce gamma-ray induced events under irradiation from an intense gamma-ray source. Further, application of lead, tin and copper shields was done in order to decrease intrinsic gamma-neutron detection efficiency.

ASIC for SDD-Based X-Ray Spectrometers

We present an application-specific integrated circuit (ASIC) for high-resolution x-ray spectrometers (XRS). The ASIC reads out signals from pixelated silicon drift detectors (SDDs). The pixel does not have an integrated field effect transistor (FET); rather, readout is accomplished by wire-bonding the anodes to the inputs of the ASIC. The ASIC dissipates 32 mW, and offers 16 channels of low-noise charge amplification, high-order shaping with baseline stabilization, discrimination, a novel pile-up rejector, and peak detection with an analog memory. The readout is sparse and based on custom low-power tristatable low-voltage differential signaling (LPT-LVDS). A unit of 64 SDD pixels, read out by four ASICs, covers an area of 12.8 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and dissipates with the sensor biased about 15 mW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . As a tile-based system, the 64-pixel units cover a large detection area. Our preliminary measurements at -44°C show a FWHM of 145 eV at the 5.9 keV peak of a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">55</sup> Fe source, and less than 80 eV on a test-pulse line at 200 eV.

Parallel processing method for high-speed real time digital pulse processing for gamma-ray spectroscopy

A new data acquisition (DAQ) system was developed to fulfil the requirements of the gamma-ray spectrometer (GRS) JET–EP2 (joint European Torus enhancement project 2), providing high-resolution spectroscopy at very high-count rate (up to few MHz). The system is based on the Advanced Telecommunications Computing Architecture™ (ATCA™) and includes a transient record (TR) module with 8 channels of 14 bits resolution at 400 MSamples/s (MSPS) sampling rate, 4 GB of local memory, and 2 field programmable gate array (FPGA) able to perform real time algorithms for data reduction and digital pulse processing. Although at 400 MSPS only fast programmable devices such as FPGAs can be used either for data processing and data transfer, FPGA resources also present speed limitation at some specific tasks, leading to an unavoidable data lost when demanding algorithms are applied. To overcome this problem and foreseeing an increase of the algorithm complexity, a new digital parallel filter was developed, aiming to perform real time pulse processing in the FPGAs of the TR module at the presented sampling rate. The filter is based on the conventional digital time-invariant trapezoidal shaper operating with parallelized data while performing pulse height analysis (PHA) and pile up rejection (PUR). The incoming sampled data is successively parallelized and fed into the processing algorithm block at one fourth of the sampling rate. The following data processing and data transfer is also performed at one fourth of the sampling rate. The algorithm based on data parallelization technique was implemented and tested at JET facilities, where a spectrum was obtained. Attending to the observed results, the PHA algorithm will be improved by implementing the pulse pile up discrimination.

Gauss-Seidel Iterative Method as a Real-Time Pile-Up Solver of Scintillation Pulses

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The pile-up rejection in nuclear spectroscopy has been confronted recently by several pile-up correction schemes that compensate for distortions of the signal and subsequent energy spectra artifacts as the counting rate increases. We study here a real-time capability of the event-by-event correction method, which at the core translates to solving many sets of linear equations. Tight time limits and constrained front-end electronics resources make well-known direct solvers inappropriate. We propose a novel approach based on the Gauss-Seidel iterative method, which turns out to be a stable and cost-efficient solution to improve spectroscopic resolution in the front-end electronics. We show the method convergence properties for a class of matrices that emerge in calorimetric processing of scintillation detector signals and demonstrate the ability of the method to support the relevant resolutions. The sole iteration-based error component can be brought below the sliding window induced errors in a reasonable number of iteration steps, thus allowing real-time operation. An area-efficient hardware implementation is proposed that fully utilizes the method's inherent parallelism. </para>

Measuring total reaction cross-sections at energies near the coulomb barrier by the active target method

An experimental technique is described that is able to measure reaction cross-sections at energies around the Coulomb barrier by using low intensity beams and a Si detector as an active target. Set-up optimization was carefully investigated in terms of collimation, detector efficiency and pile-up rejection. The method has been tested by measuring the total reaction cross-section σ R( E) for the 7Li+ 28Si system in the energy range of E lab=12–16 MeV. The deduced excitation function σ R( E) agrees with the data obtained in a previous experiment. The presented technique can also be applied in order to determine total reaction cross-sections for low intensity radioactive beams at energies around the Coulomb barrier.

An artificial neural network based neutron–gamma discrimination and pile-up rejection framework for the BC-501 liquid scintillation detector

BC-501 is a liquid scintillation detector sensitive to both neutrons and gamma rays. As these produce slightly different signals in the detector, they can be discriminated based on their pulse shape (Pulse Shape Discrimination, PSD). This paper reports on results obtained with several PSD techniques and compares them with a method based on artificial neural networks (NN) developed for this application. Results indicated a large performance advantage of NN especially in the region of small deposited energy which typically contains the majority of the events. NN were also applied for discrimination of pile-up events with good results. This framework can be implemented on some of the most recent programmable data acquisition cards and it is suitable for real-time application.

Pile-up correction by Genetic Algorithm and Artificial Neural Network

Pile-up distortion is a common problem for high counting rates radiation spectroscopy in many fields such as industrial, nuclear and medical applications. It is possible to reduce pulse pile-up using hardware-based pile-up rejections. However, this phenomenon may not be eliminated completely by this approach and the spectrum distortion caused by pile-up rejection can be increased as well. In addition, inaccurate correction or rejection of pile-up artifacts in applications such as energy dispersive X-ray (EDX) spectrometers can lead to losses of counts, will give poor quantitative results and even false element identification. Therefore, it is highly desirable to use software-based models to predict and correct any recognized pile-up signals in data acquisition systems. The present paper describes two new intelligent approaches for pile-up correction; the Genetic Algorithm (GA) and Artificial Neural Networks (ANNs). The validation and testing results of these new methods have been compared, which shows excellent agreement with the measured data with 60Co source and NaI detector. The Monte Carlo simulation of these new intelligent algorithms also shows their advantages over hardware-based pulse pile-up rejection methods.

PeneloPET, a Monte Carlo PET simulation tool based on PENELOPE: features and validation

Monte Carlo simulations play an important role in positron emission tomography (PET) imaging, as an essential tool for the research and development of new scanners and for advanced image reconstruction. PeneloPET, a PET-dedicated Monte Carlo tool, is presented and validated in this work. PeneloPET is based on PENELOPE, a Monte Carlo code for the simulation of the transport in matter of electrons, positrons and photons, with energies from a few hundred eV to 1 GeV. PENELOPE is robust, fast and very accurate, but it may be unfriendly to people not acquainted with the FORTRAN programming language. PeneloPET is an easy-to-use application which allows comprehensive simulations of PET systems within PENELOPE. Complex and realistic simulations can be set by modifying a few simple input text files. Different levels of output data are available for analysis, from sinogram and lines-of-response (LORs) histogramming to fully detailed list mode. These data can be further exploited with the preferred programming language, including ROOT. PeneloPET simulates PET systems based on crystal array blocks coupled to photodetectors and allows the user to define radioactive sources, detectors, shielding and other parts of the scanner. The acquisition chain is simulated in high level detail; for instance, the electronic processing can include pile-up rejection mechanisms and time stamping of events, if desired. This paper describes PeneloPET and shows the results of extensive validations and comparisons of simulations against real measurements from commercial acquisition systems. PeneloPET is being extensively employed to improve the image quality of commercial PET systems and for the development of new ones.

Analysis of time resolution in a dual head [formula omitted] PET system using low pass filter interpolation and digital constant fraction discriminator techniques

PET systems need good time resolution to improve the true event rate, random event rejection, and pile-up rejection. In this study we propose a digital procedure for this task using a low pass filter interpolation plus a Digital Constant Fraction Discriminator (DCFD). We analyzed the best way to implement this algorithm on our dual head PET system and how varying the quality of the acquired signal and electronic noise analytically affects timing resolution. Our detector uses two continuous LSO crystals with a position sensitive PMT. Six signals per detector are acquired using an analog electronics front-end and these signals are processed using an in-house digital acquisition board. The test bench developed simulates the electronics and digital algorithms using Matlab ® . Results show that electronic noise and other undesired effects have a significant effect on the timing resolution of the system. Interpolated DCFD gives better results than non-interpolated DCFD. In high noise environments, differences are reduced. An optimum delay selection, based on the environment noise, improves time resolution.

A Mixed-Signal Spectroscopic-Grade and High-Functionality CMOS Readout Cell for Semiconductor X-$\gamma$ Ray Pixel Detectors

We present a mixed-signal CMOS front-end cell designed for semiconductor pixel detectors for X-gamma ray imaging and spectroscopy. The readout pixel cell (RPC) comprises an analog and a digital section to accomplish all the functionality of a spectroscopic-grade signal readout. The analog section includes a low noise charge preamplifier, a two stage shaper amplifier with digitally selectable shaping times from 1 mus to 10 mus, a baseline restorer, a high precision peak stretcher and an output amplifier. The digital section includes an amplitude discriminator with coarse and fine digital control of the threshold level, a peak discriminator, a current-mode trigger generator, a logic circuit to accomplish the pulse pile-up rejection and the RPC reset after the A/D conversion, the disable functions of the preamplifier and of the discriminators. The RPC has been designed and manufactured in 0.35 mum CMOS technology with a size of 300 mumtimes300 mum. The full functionality of the RPC has been successfully tested. At room temperature, the intrinsic equivalent noise charge is 15.7 electrons r.m.s. at 3.3 mus and the minimum noise slope is 22 electrons/pF at 8 mus, the linearity error is between -1.4% and +1.2% within 1.9 fC input signal dynamic. The RPC has separate voltage supplies (+3.3 V) and grounds for the analog and digital sections and the total power consumption is 495 muW.

Measurement of the thick target 7Li(p,n) neutron source spectrum using a 3He ionization chamber

Neutron spectra from the thick target 7Li(p,n)7Be reaction were measured using a 3He ionization chamber in the proton energy region of 1.95–2.30MeV. A digital pulse processing system was employed for pulse height analysis to achieve an enhanced pulse pile-up rejection. For analyses of measured spectra, an unfolding code was written according to the van Cittert iterative algorithm with Janssen constraint. When testing the unfolding with an ideal fluence spectrum, cross-correlation filtering was found to be useful for stabilizing the result with noisy data. Only the high-energy part of the measured spectra (above 794keV) was used in unfolding due to the interference from the thermal peak tail. Theoretical neutron spectra were also calculated by combining the analytical spectra from the 7Li(p,n) cross-section data with Monte Carlo simulation. A significant spectral modification was introduced by the lithium target holder assembly and the cooling water. The theoretical spectra were compared with the unfolded spectra and their patterns were consistent.

A Multiplexer Design for Position-Sensitive Avalanche Photodiode Detectors in a PET Scanner

A small-animal positron emission tomography (PET) scanner using PS-APD (position-sensitive avalanche photodiode) detectors has been developed for simultaneous PET/MRI imaging. In this scanner, up to 16 detector modules (one PS-APD per module) are used, and each detector module produces 4 signals to be digitized with their peak values. This leads to as many as 64 analog outputs to the data acquisition (DAQ) system, requiring 64 DAQ channels. In the future, the system will be extended to 32 modules, resulting in 128 channels. It is possible to sample all channels simultaneously, but most of them do not contain useful data, since only one coincidence event (producing data on 8 channels) is identified each time. The purpose of this work was to develop a general-purpose method for reducing the number of analog inputs to the data acquisition for the sparse fast analog signals produced in a PET scanner. To achieve this, a multiplexer board was designed to sample coincidence events from the PET scanner. The effect of the multiplexer on signal quality was evaluated and the average peak-to-valley ratios in detector flood histograms with and without multiplexer were 2.97 and 3.02 respectively. On-board coincidence, pile-up rejection and multiple coincidence rejection functions were implemented and worked as expected. The dead time performance was also characterized.

The control and data acquisition software for the gamma-ray spectroscopy ATCA sub-systems of the JET-EP2 enhancements

A local control and data acquisition (CDAQ) sub-system for gamma-ray spectroscopy diagnostics is being developed for the JET-EP2 enhancements. The Hardware implementation is based on the Advanced Telecommunications Computing Architecture™ (ATCA™) and will permit acquisition at a very high count rate (up to few MHz) with digital pulse processing (DPP) on a field programmable gate array (FPGA) performing pulse height analysis (PHA), pulse shape discrimination (PSD) and pile-up rejection (PUR) algorithms. This paper presents the CDAQ software implementation, which is based on the FireSignal platform developed by the Euratom/IST Association. The FireSignal is based on a client/server modular approach, where both the server and the hardware clients are defined by a generic XML description. Its event-driven operation is dynamic thereby permitting continuous control and data acquisition and ‘plug-and-play’ of clients/hardware. The FireSignal is being extended to interface the diagnostic sub-systems to the JET CODAS HTTP data/event transport layer and to integrate the diagnostics specific requirements including the hardware device drivers, real-time spectroscopy processing and local control operation.

Improved EDS Pileup Rejection at Low Energies and High Count Rates

Extract Extended abstract of a paper presented at Microscopy and Microanalysis 2007 in Ft. Lauderdale, Florida, USA, August 5 – August 9, 2007

A digital acquisition and elaboration system for nuclear fast pulse detection

A new digital acquisition and elaboration system has been developed and assembled in ENEA-Frascati for the direct sampling of fast pulses from nuclear detectors such as scintillators and diamond detectors. The system is capable of performing the digital sampling of the pulses (200 MSamples/s, 14-bit) and the simultaneous (compressed) data transfer for further storage and software elaboration. The design (FPGA-based) is oriented to real-time applications and has been developed in order to allow acquisition with no loss of pulses and data storage for long-time intervals (tens of s at MHz pulse count rates) without the need of large on-board memory. A dedicated pulse analysis software, written in LabVIEW TM , performs the treatment of the acquired pulses, including pulse recognition, pile-up rejection, baseline removal, pulse shape particle separation and pulse height spectra analysis. The acquisition and pre-elaboration programs have been fully integrated with the analysis software.