Wide Range Of Count Rates Research Articles

Testing device for neutron flux monitoring equipment

This report describes a testing device capable of emulating neutron fission chamber signals over their whole operating range. The device assists in the design and testing of neutron flux monitoring equipment including reactimeters, spectrometers, period meters and protection channels. Because of its hardware architecture, testing of neutron flux monitoring equipment can be done both in a laboratory environment and in-situ. Main characteristics of the device allow one to test the equipment over a wide range of count rates. A flexible mathematical algorithm, that is used to calculate the signal model of fission chambers, gives the opportunity for user to set the pulse shape and the charge spectrum such as those in a real fission chamber. It is also possible to set dynamic signals, for example, such as an exponentially varying count rate or a neutron point reactor kinetics signal with a configurable number of delayed neutron groups and constants. The device was verified by testing of the International Thermonuclear Experimental Reactor (ITER) neutron flux divertor monitor data acquisition system.

Non-linearity assessment of neutron detection systems using zero-power reactor transients

A new method for neutron detection system non-linearity assessment and correction is proposed and was tested at the VR-1 reactor. It is based on the known behaviour of a zero-power nuclear reactor during the asymptotic positive period, which is used as a source of a true (non-linearity unaffected) signal for detection system non-linearity correction. The proposed method can determine the non-linearity correction continuously for a wide range of count rates and does not require additional equipment but the detection line for which the non-linearity effect is to be evaluated.

Poisson errors and adaptive rebinning in X-ray Powder Diffraction Data.

This work provides a short summary of techniques for formally-correct handling of statistical uncertainties in Poisson-statistics dominated data, with emphasis on X-ray powder diffraction patterns. Correct assignment of uncertainties for low counts is documented. Further, we describe a technique for adaptively rebinning such data sets to provide more uniform statistics across a pattern with a wide range of count rates, from a few (or no) counts in a background bin to on-peak regions with many counts. This permits better plotting of data and analysis of a smaller number of points in a fitting package, without significant degradation of the information content of the data set. Examples of the effect of this on a diffraction data set are given.

Performance of Higher Order Campbell methods, Part I: review and numerical convergence study

This paper investigates, through numerical simulations, the performance of a signal analysis method by which a high temperature fission chamber can be used over a wide range of count rates. Results reported in a previous paper (Elter et al., 2015 [1]) indicated that the traditional Campbell method and the pulse mode cannot provide a sufficient overlap at medium count rates. Hence the use of the so-called Higher Order Campbell (HOC) methods is proposed and their performance is investigated.It is shown that the HOC methods can guarantee the linearity (i.e. correctness) of the neutron flux estimation over a wide count rate, even during transient conditions. The capabilities of these methods for suppressing parasitic noise (originating from various sources) are verified.

Optimization of statistical methods for HpGe gamma-ray spectrometer used in wide count rate ranges

The need to perform γ-ray measurements with HpGe detectors is a common technique in many fields such as nuclear physics, radiochemistry, nuclear medicine and neutron activation analysis. The use of HpGe detectors is chosen in situations where isotope identification is needed because of their excellent resolution. Our challenge is to obtain the “best” spectroscopy data possible in every measurement situation. “Best” is a combination of statistical (number of counts) and spectral quality (peak, width and position) over a wide range of counting rates. In this framework, we applied Bayesian methods and the Ellipsoidal Nested Sampling (a multidimensional integration technique) to study the most likely distribution for the shape of HpGe spectra. In treating these experiments, the prior information suggests to model the likelihood function with a product of Poisson distributions. We present the efforts that have been done in order to optimize the statistical methods to HpGe detector outputs with the aim to evaluate to a better order of precision the detector efficiency, the absolute measured activity and the spectra background.Reaching a more precise knowledge of statistical and systematic uncertainties for the measured physical observables is the final goal of this research project.

Performance evaluation of the Ingenuity TF PET/CT scanner with a focus on high count-rate conditions

This study evaluated the positron emission tomography (PET) imaging performance of the Ingenuity TF 128 PET/computed tomography (CT) scanner which has a PET component that was designed to support a wider radioactivity range than is possible with those of Gemini TF PET/CT and Ingenuity TF PET/MR. Spatial resolution, sensitivity, count rate characteristics and image quality were evaluated according to the NEMA NU 2–2007 standard and ACR phantom accreditation procedures; these were supplemented by additional measurements intended to characterize the system under conditions that would be encountered during quantitative cardiac imaging with 82Rb. Image quality was evaluated using a hot spheres phantom, and various contrast recovery and noise measurements were made from replicated images. Timing and energy resolution, dead time, and the linearity of the image activity concentration, were all measured over a wide range of count rates. Spatial resolution (4.8–5.1 mm FWHM), sensitivity (7.3 cps kBq–1), peak noise-equivalent count rate (124 kcps), and peak trues rate (365 kcps) were similar to those of the Gemini TF PET/CT. Contrast recovery was higher with a 2 mm, body-detail reconstruction than with a 4 mm, body reconstruction, although the precision was reduced. The noise equivalent count rate peak was broad (within 10% of peak from 241–609 MBq). The activity measured in phantom images was within 10% of the true activity for count rates up to those observed in 82Rb cardiac PET studies.

A cascaded model of spectral distortions due to spectral response effects and pulse pileup effects in a photon‐counting x‐ray detector for CT

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.

Optimization of pulse processing parameters for HPGe gamma-ray spectroscopy systems used in extreme count rate conditions and wide count rate ranges

The need to perform gamma-ray spectroscopy measurements at high count rates with HPGe detectors is more common than many believe. Examples exist in safeguards, radiochemistry, nuclear medicine, and neutron activation analysis. In other applications wide dynamic ranges in count rate may be encountered, for example samples taken after a nuclear accident are counted on a system normally used for environmental monitoring. In a real situation, it may not be possible to reduce count-rates by increasing the distance or using collimators. The challenge is to obtain the “best” data possible in the given measurement situation. “Best” is a combination of statistical (number of counts) and spectral quality (peak width and position) considerations over a wide range of count rates. The development of multichannel analyzers (MCA) using digital signal processing (DSP) has made possible a much wider range of values for shaping times as well as the processing of the detector signal in various ways to improve performance with pulse-by-pulse adjustments. The pulse processing time is directly related to the shaping time. The throughput is related to the pulse processing time and the duration of the detector signal. Longer shaping times generally produce better peak resolution. However, the longer shaping times mean larger dead times and lower throughput. The ability to select the best compromise between throughput and resolution is possible with DSP MCAs. In addition, the dead-time-per-pulse can be reduced by changing the digital filter without significant impact on the full-width at half-maximum. To evaluate the improvements and to suggest an approach to optimization of system performance, a small and a large GEM (p-type) coaxial HPGe detector were selected for measurements to determine the performance at various input count rates and wide range of rise times and flattops in the DSPEC 50 MCA. Results will be presented for the throughput measured at dead times from 30 to 99.9 % with and without the use of the ORTEC Enhanced Throughput Mode.

Long-term performance evaluation of positron emission tomography: analysis and proposal of a maintenance protocol for long-term utilization

Positron emission tomography (PET) scanners require periodic monitoring in order to maintain scanner performance. The aim of the present study was to examine the deterioration of PET scanner performance caused by aging. We retrospectively examined PET scanner performance alterations in terms of sensitivity, spatial resolution, false coincidences due to scatter and random coincidences based on 13 years of follow-up data, including data when the PET scanner underwent an overhaul at the 10th year after installation. Sensitivity and scatter fraction were calculated by using cross calibration factor (CCF) measurement data, which are collected routinely. Efficacy of the examining the sensitivity and scatter was confirmed by NEMA measurements. Trans-axial resolution was measured as full width at half-maximum (FWHM) and full width at tenth-maximum (FWTM) at 0-20 cm offset from the field of view (FOV) center at the time of installation, 8 years after installation, and immediately after the overhaul. Random coincidence rate fraction was measured in a wide range of count rates before and after the overhaul. The results indicated that the total reduction of sensitivity during the first 10 years was 41% of the initial value in terms of NEMA measurement, and that the annual reduction of sensitivity progressed at a rate of 4.7% per year in terms of CCF measurement data. The changes in sensitivity can be calculated using CCF measurement data. Regarding the spatial resolution, mean FWHM and FWTM values were increased by 1.7 and 3.6%, respectively, in 8 years after installation. The relative scatter fraction was significantly increased compared with that before the overhaul. The random fraction decreased by 10-15% after the overhaul within a certain range of random count rates (1-120 kcps). In the case of our scanner, the parameter that displayed the largest change was the sensitivity, and this change was thought to be caused by the reduction of photomultiplier tube (PMT) gain, although the changes in PMT gain can cause various types of performance deterioration, as investigated in this study. We observed that the sensitivity of our PET scanner generally deteriorated due to aging. Sensitivity monitoring using CCF measurements can be an easy and useful method for monitoring and maintaining the performance of PET scanners against aging. Since the data were obtained from a single scanner, the authors would encourage the initiation of a follow-up study involving various scanners.

Data acquisition system development for the detection of X-ray photons in multi-wire gas proportional counters

A new data acquisition system coupled to a backgammon-type gas proportional counter capable of single-photon counting over a wide range of count rates has been developed and replaces a CAMAC-based system. The new apparatus possesses improved architecture, interface technology, speed and diagnostic capability. System efficiency and throughput is significantly improved, especially in addressing earlier problems of hardware buffer downloads containing zero or repeat data and inefficient gating control. The new system is a PXI-based data acquisition apparatus including additional electronics, controlled by a graphical programming environment. It allows development of superior diagnostic tools for system optimisation and more stable performance. System efficiency is improved by 10% over a wide range of count rates (0.5 Hz–50 kHz). For the Backgammon Detector type, this represents a significant improvement in performance and applicability over previous systems. Characteristic and few-electron spectra collected on the new acquisition system are illustrated.

Development of backgammon-type multiwire proportional counters for the detection of x rays

A multiwire proportional counter (MWPC) based on a backgammon-type design has been developed for the detection of x-ray photons in two dimensions over the energy range of 4–6keV. The MWPC was designed for operation in an experimental spectroscopy application requiring single photon counting over a wide range of count rates, high photon detection efficiency and resolution, and a highly linear response function across the detector face. A model based on attenuation of the incident x-ray beam by the various components of the detector is presented and highlights key areas for efficiency optimization. The design, construction, testing procedure, and performance of the backgammon MWPC are presented and all objectives are met including a particularly impressive differential nonlinearity.

Noise analysis of an Si-drift detector system with time-variant shaping

We report on the temperature and count-rate dependent noise behavior of an Si-drift detector system using an on-sensor integrated JFET in a source-follower configuration. The readout chain of a 0.8-/spl mu/m BiCMOS chip consists of a fully differential low-noise postamplifier, current-mode shaper with gated integrator, and analog storage cell. Six channels are processed in parallel using a 6 : 1 multiplexer buffered by a 100-/spl Omega/ line driver. The readout chain's power dissipation is /spl sim/15 mW/channel. The indexes for parallel, serial, and 1/f noise of the time-variant signal processor are calculated using weighting functions. For a detector capacitance of /spl sim/140 fF and low count rates, the chip's and total electronics' input-referred equivalent noise charge is about 20 and 23 rms electrons, respectively. Due to an almost quadratical increase of the noise indexes with increasing count rate, the signal current deteriorates so that the spectral resolution of a Cu-K/sub /spl alpha//-emission line at 20/spl deg/C decreases from /spl sim/300 eV (full-width at half-maximum) at low count rates to /spl sim/850 eV at 600 kilocounts per second. The investigation of the temperature-dependent leakage current for different detectors leads to current densities between 1.5 pA/mm/sup 2/ and 3 pA/mm/sup 2/ at 20/spl deg/C. The simulated and experimental data verify the theoretical results for a wide range of count rates and sensor temperatures.

Developments in component-based normalization for 3D PET

Normalization in positron emission tomography (PET) is the process of ensuring that all lines of response joining detectors in coincidence have the same effective sensitivity. In three-dimensional (3D) PET, normalization is complicated by the presence of a large proportion of scattered coincidences, and by the fact that cameras operating in 3D mode encounter a very wide range of count-rates. In this work a component-based normalization model is presented which separates the normalization of true and scattered coincidences and accounts for variations in normalization effects with count-rate. The effects of the individual components in the model on reconstructed images are investigated, and it is shown that only a subset of these components has a significant effect on reconstructed image quality.

Formulation of data synthesis technique for Feynman-α method

We intend to develop a subcritically monitor based on the Feynman-α method, which detects a change in the prompt neutron decay constant caused by a change in the reactivity of a system containing nuclear fuel materials. For the practical use of this method, quick data processing is necessary to facilitate a real time measurement and it needs to be applicable to a wide range of count rates in the neutron detector. The Feynman-α method, based on pulse counting, has a serious problem relating to the counting statistics, namely the count loss effect due to the dead time of counter itself and the following counting circuit. To eliminate the count loss effect, especially in a high count rate region, a quick correction technique should be established to ensure accurate measurement. As a candidate for this, we have developed the data-synthesis technique which processes synthesized data generated by adding multiple time-series data acquired by independent neutron detectors. The present paper provides the derivation of a formula for this data-synthesis technique and the result of a numerical examination on its applicability.

THE ULTRA-LOW-ENERGY ISOTOPE SPECTROMETER (ULEIS) FOR THE ACE SPACECRAFT

The Ultra Low Energy Isotope Spectrometer (ULEIS) on the ACE spacecraft is an ultra high resolution mass spectrometer designed to measure particle composition and energy spectra of elements He-Ni with energies from ∼45 keV nucl-1 to a few MeV nucl-1. ULEIS will investigate particles accelerated in solar energetic particle events, interplanetary shocks, and at the solar wind termination shock. By determining energy spectra, mass composition, and their temporal variations in conjunction with other ACE instruments, ULEIS will greatly improve our knowledge of solar abundances, as well as other reservoirs such as the local interstellar medium. ULEIS is designed to combine the high sensitivity required to measure low particle fluxes, along with the capability to operate in the largest solar particle or interplanetary shock events. In addition to detailed information for individual ions, ULEIS features a wide range of count rates for different ions and energies that will allow accurate determination of particle fluxes and anisotropies over short (∼few minutes) time scales.

Signal‐to‐noise measurement in X‐ray photoelectron spectroscopy

AbstractA study of signal‐to‐noise (S/N) was made in a Kratos ES300 XPS system on a variety of materials using both monochromatic and conventional X‐ray sources covering a wide range of count rates. Equations relating S/N to total counts (S + B), background (B), and the signal‐to‐background (S/B) ratio were derived and tested for data obtained in a system limited by noise from counting statistics.

Application of Geiger-Müller counters over a wide range of counting rates

The useful dynamic range of Geiger-Müller counters is limited by counting losses at the upper end and by poor statistics at the lower end of the range. Methods for extending the dynamic range which have been studied at the Chalk River Nuclear Laboratories are described and also the limitations of these methods.

Details of dead time losses in scaling and multiscaling

An electrometer is employed to investigate dead time effects in scaling and multiscaling. The electrometer method as presented gives accurate values for the dead time over a wide range of counting rates. An expression is also developed for the ``between channel'' dead time of a multiscaler, and a simple method of determining this time is also given. The technique as outlined allows the exact determination of the nonlinearity caused by dead time losses.

A measurement of pulse separations for photoelectrons in the case of a Lorentz spectral line

Measurements of the width of a Lorentz spectral line using a digital photoelectron correlation device are compared with measurements made on the same gaussian light source by a pulse separation detector. The agreement of the two measurements over a wide range of count rates provides a test of recent theories of pulse separation measurements.

High resolution spectrometry with nuclear radiation detectors

A high resolution spectrometer system which contains optoelectronic charge restoration at the detector, direct coupled signal amplification, active signal filters and a versatile analogue-to-digital converter is briefly described. The performance of this system with semiconductor gamma ray and X-ray detectors, scintillation counters and proportional counters over a very wide range of counting rates is described. Owing to the unique characteristics of this spectrometer the performance with every detector tested is believed to be substantially better than that hitherto reported for any other spectrometer. It is shown that the overall performance is limited solely by noise at the input and by detector characteristics even with the best available detector.