Pile-up Correction Research Articles

Development of a correction method for the time-of-flight prompt gamma-ray analysis

A new analytical technique, time-of-flight prompt gamma-ray analysis, has been developed at the Japan Proton Accelerator Research Complex. In order to apply it to accurate elemental analysis, a set of Fe and Au reference samples were measured to examine the several factors which affect the number of detected events. It was found that major contributing factors included attenuations of neutrons and gamma rays in the sample, live-time fraction and signal pile-up correction. A simulation model was built for the estimation of neutron and gamma-ray attenuations. A simple empirical formula was proposed to calculate the signal pile-up correction factor. The whole correction method has proven to be accurate and reliable.

Mechanistic Study of the Wear of Ceramic Heads by Metallic Stems in Modular Implants

Titanium-based and cobalt–chrome alloys as well as some ceramics have been widely used in orthopaedic applications as these materials can significantly enhance the quality of human life as implant materials. However, the in vivo performance of some large-diameter metal-on-metal joints is unsatisfactory, and concerns have been expressed over the wear behaviour of the materials and released metal ions affecting local tissue and more distant organs. The longevity of these materials is highly influenced by their mechanical properties and this has driven the development of alternative ceramic components with greatly improved tribological performance. Even these novel materials are not immune to damage, for instance in some devices alumina-based ceramic components articulate with titanium alloy counterfaces (e.g. in the taper connections of titanium alloy stems and zirconia-toughened alumina femoral heads in modern modular designs) and damage has been reported of the harder ceramic surface by the softer titanium alloy component. In such contacts, the chemically inert ceramic component is not expected to corrode, so the electrochemical damage mechanisms often suggested for metal–metal contacts are not appropriate. This study attempts to understand why this wear might occur by investigating bulk and surface mechanical properties (such as hardness and Young’s modulus) of a number of hip implants and test samples using a Hysitron Triboindenter. AFM images were also obtained to determine the contact area and hence, pile-up correction factors for the metallic material. It was found that the alumina ceramic heads were generally subject to chemomechanical softening after exposure to water for an extended period whilst titanium alloy oxidised preferentially generating a hard oxide surface which was not softened by water. Furthermore, the oxidised titanium showed significantly higher hardness values therefore damaging the chemomechanically softened alumina material.

Baseline restoration and pile-up correction based on bipolar cusp-like shaping for high-resolution radiation spectroscopy

The baseline may shift in many radiation measurement systems. The shift is time variant and depends on the events. Also, with high count rates, pulses may overlap in random time intervals. These phenomena can affect the peak values of the pulses. Piled-up events are traditionally rejected, but this reduces the detection efficiency considerably. In other approaches, the corrupted events are reconstructed, and information about the single pulses is extracted. The peaks carry much of the basic information, so many shaping methods have been proposed so far. For pile-up mitigation, a narrow unipolar shaping is enough, but a baseline shift is eliminated by using bipolar shaping. However, the latter decreases the signal-to-noise ratio (SNR), which is critical for high-resolution spectroscopy. In this paper, we propose bipolar cusp-like shaping as a tradeoff between mitigating the baseline shift and pulse pile-up. A novel recursive algorithm, implementable on digital pulse processors (DPPs), is introduced and is then evaluated. Finally, the superior noise-reduction capability is studied by using Monte Carlo simulations, a real piled-up pulse stream shaped by using the algorithm, and the results show its advantages.

Neural-based pile-up correction and ballistic deficit correction of X-ray semiconductor detectors using the Monte Carlo simulation and the Ramo theorem

ABSTRACTPile-up distortion is a common problem in many nuclear radiation detection systems, especially in high count rates. It can be solved by hardware-based pile-up rejections, but there is no complete pile-up elimination in this way. Additionally, the methods can lead to poor quantitative results. Generally, time characteristics of semiconductor detector pulses are different from Scintillator detector pulses due to ballistic deficit. Hence, pulse processing-based pile-up correction in the detectors should consider this specification. In this paper, the artificial neural network pile-up correction method is applied for silicon detector piled-up pulses. For this purpose, the interaction of photons with a silicon detector is simulated by the MCNP4c code and the pulse current is calculated by Ramo's theorem. In this approach, we use a sub-Nyquist frequency sampling. The results show that the proposed method is reliable for pile-up correction and ballistic deficit in semiconductor detectors. The technique is remarkable for commercial considerations and high-speed, real-time calculations.

Run 2 upgrades to the CMS Level-1 calorimeter trigger

The CMS Level-1 calorimeter trigger is being upgraded in two stages to maintain performance as the LHC increases pile-up and instantaneous luminosity in its second run. In the first stage, improved algorithms including event-by-event pile-up corrections are used. New algorithms for heavy ion running have also been developed. In the second stage, higher granularity inputs and a time-multiplexed approach allow for improved position and energy resolution. Data processing in both stages of the upgrade is performed with new, Xilinx Virtex-7 based AMC cards.

Pileup Correction Algorithm using an Iterated Sparse Reconstruction Method

Every radioactive source can be characterized by an histogram obtained after collecting the energies of photons emitted from the source, also called energy spectrum. However, when the activity of this source is high, a physical phenomenon known as the pile-up effect distorts direct measurements, resulting in a significant distortion of the energy spectrum. We suggest in this letter an iterative algorithm to attenuate the pile-up effect and enhance the resulting energy spectra. It is based on iterations of a post-processed, non-negative, version of the Least Absolute Shrinkage and Selection Operator (LASSO). Results on simulations and real data illustrate the improvement obtained by the proposed method.

Pile-Up Correction of Mechanical Characteristics of Individual Phases in Various Steel by Depth Sensing Indentation

The Oliver–Pharr method has extensively been adopted for measuring hardness and elastic modulus by indentation techniques. However, the method assumes that the contact periphery sinks in, which limits the applicability to the materials pile-up [1]. This study proposes an improved methodology to calculate the real mechanical characteristics of individual phases in various steels with significant pile-up. Pile-up correction of mechanical characteristics is based on ratio of pile-up height and contact depth. Pile-up height was measured by atomic force microscopy (AFM). The effect of grain boundaries on the shape and size of the pile-up lobes was also discussed.

Sub-MeV band observation of a hard burst from AXP 1E 1547.0−5408 with the Suzaku Wide-band All-sky Monitor

Abstract The 2.1-s anomalous X-ray pulsar 1E 1547.0−5408 exhibited an X-ray outburst on 2009 January 22, emitting a large number of short bursts. The wide-band all-sky monitor (WAM) on-board Suzaku detected at least 254 bursts in the 160 keV–6.2 MeV band over the period of January 22 00:57–17:02 UT from the direction of 1E 1547.0−5408. One of these bursts, which occurred at 06:45:13, produced the brightest fluence in the 0.5–6.2 MeV range, with an averaged 0.16–6.2 MeV flux and extrapolated 25 keV–2 MeV fluence of about 1 × 10−5 erg cm−2 s−1 and about 3 × 10−4 erg cm−2, respectively. After pile-up corrections, the time-resolved WAM spectra of this burst were well-fitted in the 0.16–6.2 MeV range by two-component models; specifically, a blackbody plus an optically thin thermal bremsstrahlung or a combination of a blackbody and a power-law component with an exponential cut-off. These results are compared with previous works reporting the persistent emission and weaker short bursts followed by the same outburst.

Performance and upgrade of the CMS electron and photon trigger for Run 2

The CMS experiment implements a sophisticated two-level online trigger selection system that achieves a rejection factor of nearly 105. The first level (L1) trigger is based on coarse information coming from the calorimeters and the muon detectors while the high-level trigger combines fine-grain information from all sub-detectors. In the near future the LHC will increase its centre of mass energy to 13 TeV and progressively reach an instantaneous luminosity of 2 × 1034 cm−2s−1. In order to guarantee a successful and ambitious physics program under this challenging environment, the CMS trigger and data acquisition system must be consolidated. In particular the calorimeter L1 trigger hardware and architecture will be changed. The aim is to maintain the current thresholds and improve performance. This programme will be achieved using the μTCA architecture with fast optical links and latest generation FPGAs. Sophisticated object reconstruction algorithms, as well as online pile-up corrections, are being developed that will make use of these new hardware capabilities. For electron and photon reconstruction and identification, the first version of the new algorithms has been tested against the current algorithms. It shows a reduction of the trigger rate by a factor of two for isolated objects, an improved energy resolution of about 30%, and a position resolution reduced by more than four.

SoftKiller, a particle-level pileup removal method.

Existing widely used pileup removal approaches correct the momenta of individual jets. In this article we introduce an event-level, particle-based pileup correction procedure, SoftKiller. It removes the softest particles in an event, up to a transverse momentum threshold that is determined dynamically on an event-by-event basis. In simulations, this simple procedure appears to be reasonably robust and brings superior jet resolution performance compared to existing jet-based approaches. It is also nearly two orders of magnitude faster than methods based on jet areas.

Correction of dead-time and pile-up in a detector array for constant and rapidly varying counting rates

The effect of dead-time and pile-up in counting experiments may become a significant source of uncertainty if not properly taken into account. Although analytical solutions to this problem have been proposed for simple set-ups with one or two detectors, these are limited when it comes to arrays where time correlation between the detector modules is used, and also in situations of variable counting rates. In this paper we describe the dead-time and pile-up corrections applied to the n_TOF Total Absorption Calorimeter (TAC), a 4π γ-ray detector made of 40 BaF2 modules operating at the CERN n_TOF facility. Our method is based on the simulation of the complete signal detection and event reconstruction processes and can be applied as well in the case of rapidly varying counting rates. The method is discussed in detail and then we present its successful application to the particular case of the measurement of 238U(n, γ) reactions with the TAC detector.

Pulse pile-up and dead time corrections for digitized signals from a BaF2 calorimeter

The pulse pile-up and associated dead time effects in an 243Am(n,γ) cross section measurement with a BaF2 Total Absorption Calorimeter at the CERN n_TOF facility have been characterized. In this case, reliable corrections of these effects are crucial because of the complex detector configuration and of the high count rate induced by the sample activity of 50 MBq. The techniques, which have been developed for offline processing of the present data, may be of general interest for the analysis of other experiments using digital acquisition systems.

Pulse pileup correction in the presence of a large low-energy background

Pulse pileup correction is required for high count rate experiments. Spectral distortion due to pileup is strongly affected by the distribution of pulse heights. Methods used to limit the digitization rate – including energy thresholds and coincidence requirements – adversely affect attempts to correct the pileup-induced distortion. A simplified method for correcting the spectral distortion using a general pileup function can be used when the spectrum is dominated by a low-energy background. This method has particular relevance for nuclear resonance fluorescence experiments.

Multi-shape pulse pile-up correction: The MCPPU code

In spectroscopic measurements with high counting rate, pulse pile-up (PPU) is a common distortion of the spectrum. It is fully ascribable to the pulse handling circuitry of the detector and it is not comprised in the detector response function which is well explained by a purely physical model. Since PPU occurs after the transport inside the detector, this is the first correction to perform in case of spectrum unfolding. Many producers include electronic rejection circuits to limit the appearance of PPU, but it is never suppressed completely. Therefore, it is always necessary to correct PPU distortions after the measurement. In the present work, it is described the post-processing tool MCPPU (Monte Carlo Pulse Pile-Up), based on the MC algorithm developed by Guo et al. (2004, 2005). MCPPU automatically determines the dead time of the counting system and corrects for PPU effects even in the presence of electronic suppression. The capability of allowing a user defined pulse shape makes the code suitable to be used with any kind of detector. The features of MCPPU are illustrated with some examples.

Specimen homogeneity requirements for EDS in high count rate mode

This work describes limits with pile-up correction considering EDS spectra evaluation in the case of processing high count rate spectra from inhomogeneous specimen areas. The pulse processor pile-up rejection has some limitations with detection of X-ray event coincidences. Therefore, remaining pile-up artefacts are always present in measured spectra and it is necessary to consider these artefacts to avoid mistakes in qualitative and quantitative results. Any pile-up calculation is based on probabilities for coincidences of X-rays, some of which cannot be resolved by pile-up rejection due to the constantly limited time resolution. A fundamental assumption with all calculation approaches is the time-related random distribution of emitted X-rays. This rule is violated with spectra from inhomogeneous specimen areas. Examples are shown and opportunities to avoid the fundamental shortcoming are discussed.

A Building Block for Nuclear Medicine Imaging Systems Data Acquisition

Adapting acquisition electronics to new detector designs has often led to complications and compromises. As we developed depth-of-interaction detector designs based on both discrete crystal arrays (dMiCE) and monolithic crystals (cMiCE) concepts, we found that our previous electronics design was inadequate to the task and launched a design effort we have termed our Phase II electronics. The system is based on a basic card design (the Phase II board) that has a large field programmable gate array (FPGA) with sufficient static RAM to support a variety of pulse processing algorithms our group has developed-including timing estimation, pulse integration with pileup correction, and statistical estimation of the event location in the detector. Here we report on the initial development and testing of the Phase II digital board as a basic building block for data acquisition systems.

Mechanical property measurements of heterogeneous materials by selective nanoindentation: Application to LiMn2O4 cathode

Mechanical properties of composite materials for application as electrodes in batteries have been measured by means of selective statistical nanoindentation. The sample is strongly heterogeneous, as it consists of LiMn2O4 particles, carbon black and PVDF embedded in a soft and compliant epoxy matrix. For comparison, a similar composite sample of SiO2 particles in epoxy was prepared. The difference in terms of elastic modulus between the matrix and the particles is of one order of magnitude. Structural compliance and edge effects induce inconsistent tests which in return cause spurious measurements. A 2-step filtering method has been designed to overcome this problem. This automated method consists in identifying spurious tests and withdraw them from the final statistical analysis. First, nonquadratic load versus displacement curves are filtered out. Then, the Joslin–Oliver analysis is used to filter out tests with an apparent structural compliance. The method greatly improves the noise to signal ratio. After deconvolution, the E-modulus of the silica particles was measured as 69.8GPa (±1.2). It shows the reliability of the method. 105GPa (±7.5) was found for the LiMn2O4 particle E-modulus. After pile-up correction, the real E-modulus of LiMn2O4 particles is estimated to be 13% smaller. The developed method has been demonstrated to be an effective tool to investigate mechanical properties of composites.

Why does titanium alloy wear cobalt chrome alloy despite lower bulk hardness: A nanoindentation study?

Titanium-based and cobalt-chrome alloys have been widely used in orthopaedic applications as these materials can significantly enhance the quality of human life as implant materials. The longevity of these materials is highly influenced by their mechanical properties. In some devices cobalt chrome components articulate with titanium alloy counter faces (e.g. in the taper connections of stems and femoral heads in modern modular designs) and damage has been reported of the harder cobalt chrome by the softer titanium alloy component. This study attempts to understand why this might occur by investigating bulk and surface mechanical properties (such as hardness and Young's modulus) of a number of hip implants and test samples using a Hysitron Triboindenter. AFM images were also obtained to determine the contact area and hence, pile-up correction factors.The results were compared for samples before being used in the body, to account for surface mechanical response due to implant manufacture, and after, to account for the materials response to long-term cyclic loads. To assess the effects of oxidation, the alloys were treated electrochemically with Sodium-Chloride (NaCl) solution at body temperature. It was found that titanium oxidised preferentially compared with cobalt-chrome alloys. Furthermore, the oxidised titanium showed significantly higher hardness values therefore damaging the un-oxidised cobalt-chrome material. The implications for device design and manufacture are discussed.

Improved dead-time correction for PET scanners: application to small-animal PET

Pile-up and dead-time are two main causes of nonlinearity in the response of a PET scanner as a function of activity in the field of view (FOV). For a given scanner and acquisition system, pile-up effects depend on the material and size of the object being imaged and on the distribution of activity inside and outside the FOV, because these factors change the singles-to-coincidences ratio (SCR). Thus, it is difficult to devise an accurate correction that would be valid for any acquisition. In this work, we demonstrate a linear relationship between SCR and effective dead-time, which measures the effects of both dead-time (losses) and pile-up (gains and losses). This relationship allows us to propose a simple method to accurately estimate dead-time and pile-up corrections using only two calibration acquisitions with, respectively, a high and low SCR. The method has been tested with simulations and experimental data for two different scanner geometries: a scanner with large area detectors and no pile-up rejection, and a scanner composed of two full rings of smaller detectors. Our results show that the SCR correction method is accurate within 7%, even for high activities in the FOV, and avoids the bias of the standard single-parameter method.

Accurate Rutherford backscattering spectrocsopy measurement of aluminium composition in AlxGa1-xN crystal film

The AlxGa1-xN epitaxial film is grown on (0001)-oriented sapphire with a 20 nm thick aluminium nitride buffer layer by metal organic chemical vapor deposition. The thickness of AlxGa1-xN layer with high crystal quality is about 2 μrm determined by ultraviolet visible light transmittance spectrum analysis. The homogeneity of aluminium in AlxGa1-xN epitaxial film is tested through the energy Eg at the peak intensity in the deep ultraviolet photoluminescence spectrum. The epitaxial wafer with good homogeneity is used to determine aluminium composition by Rutherford backscattering spectroscopy (RBS). Six samples are measured by tow ion beam analysis laboratories, and the experimental data of RBS random spectrum are simulated by the software. The source of measurement uncertainty is analyzed including the sample homogeneity, pileup correction and counting statistics and so on. The research results show that when the alpha particle is used as incident ion, with 2000 keV energy and 165° scattering angle, the measurement uncertainty of RBS for the determination of aluminium composition (x=0.8) is 2.0% and the coverage factor k=2.