A high performance Josephson binary counter implemented in Nb and NbN technology

Josephson 1-kbit random access memories (RAM's) have been fabricated using Nb multilayer planarization technology with Nb/AlO/sub x//Nb junctions and Mo resistors. The RAM design has been reported previously. The RAM consists of a 32*32-bit nondestructive readout (NDRO) memory cell array and peripheral circuits. The NDRO memory cell consists of a loop storing three flux quanta and two 3-junction interferometer gates. The peripheral circuits consist of decoders with address inverters, drivers, a sense circuit and reset circuits, where resistor-coupled Josephson logic (RCJL) circuits are used as basic circuits. The RAM circuit size is 4.4*4.4 mm/sup 2/, and the memory cell size is 65*65 mu m/sup 2/. About 10000 Nb/AlO/sub x//Nb junctions with 1030-A/cm/sup 2/ critical current density were contained in the RAM. Minimum line and space widths were 3 and 2 mu m, respectively. The Mo resistors had 1.2 approximately 1.3 Omega sheet resistance. About 40 percent of the bits were successfully operated with a +or-18-percent bias margin. A minimum 570-ps access time with 13-mW power dissipation was obtained for the highest peripheral circuit bias conditions. >

Evaluation of count rate performance (CRP) is an integral component of gamma camera quality assurance and system deadtime (τ) may be utilized for image correction in quantitative studies. This work characterizes the CRP of three modern gamma cameras and estimates τ using two established methods (decay and dual source) under a variety of experimental conditions. For the decay method, uncollimated detectors were exposed to a Tc-99m source of relatively high activity and count rates were sampled regularly over 48 h. Input count rate at each time point was based on the lowest observed count rate data point. The input count rate was plotted against the observed count rate and fit via least-squares to the paralyzable detector model (PDM) to estimate τ (rates method). A novel expression for observed counts as a function of measurement time interval was derived, taking into account the PDM and the presence of background but making no assumption regarding input count rate. The observed counts were fit via least-squares to this novel expression to estimate τ (counts method). Correlation and Bland-Altman analyses were performed to assess agreement in estimates of τ between the rates and counts methods. The dependence of τ on energy window definition and incident energy spectrum were characterized. The dual source method was also used to estimate τ and its agreement with estimates from the decay method under identical conditions was also investigated. The dependences of τ on the total activity and the ratio of source activities were characterized. The observed CRP curves for each gamma camera agreed with the PDM at low count rates but deviated substantially from it at high count rates. The estimates of τ determined from the paralyzable portion of the CPR curve using the rates method and the counts method were found to be highly correlated (r = 0.999) but with a small (~6%) difference. No statistically significant difference was observed between the estimates of τ using the decay or dual source methods under identical experimental conditions (p = 0.13). Estimates of τ increased as a power-law function with decreasing ratio of counts in the photopeak to the total counts. Also, estimates of τ increased linearly as spectral effective energy decreased. No significant difference was observed between the dependences of τ on energy window definition or incident spectrum between the decay and dual source methods. Estimates of τ using the dual source method varied as a quadratic on the ratio of the single source to combined source activities and linearly with total activity. The CRP curves for three modern gamma camera models have been characterized, demonstrating unexpected behavior that necessitates the determination of both τ and maximum count rate to fully characterize the CRP curve. τ was estimated under a variety of experimental conditions, based on which guidelines for the performance of CRP testing in a clinical setting have been proposed.

This paper reports on the last year's two major activities of our nuclear instrumentation group it the field of high rate and high resolution gamma spectrometry which were mainly devoted to the needs of activation analysis of short-lived nuclides. The first of the projects was the completion of a state-of-the-art spectrometry system for very high counting rates which has been installed at the fast inrradiation and transport facility of the TRIGA reactor and now is the main instrument for the short-lived work of our radiochemistry group. Based on a laboratory-designed gated integrator pulse processing system and equipped with an Ortec Gamma-X detector of 20% relative efficiency with cooled FET and transistor reset preamplifier, it exhibits a basic resolution of 2.3 keV at 1332 keV which at a counting rate of 1.1 million cps of60Co degrades to 3.4 keV. An essential feature of the system is a novel “quantitative” pileup rejector of the pulse counting type which has been specially designed for high rejection efficiency and at the same time, for the reliable exemption of valid events, and thus is a necessary prerequisite for quantitative real-time correction of counting losses by means of the Virtual Pulse Generator method. The second project included the successful implementation of the novel Preloaded Filter Technique (applied for patent), a new method for high resolution and high throughput processing of nuclear detector signals which, in contrast to conventional techniques, does not rely on a fixed pulse processing time per event which up to now was the main reason for pulse pileup and limited throughput, but, at the latest, terminates the filtering process of an individual event at the instant of arrival of the next event which results in optimized throughput and, at the same time, in a self-adapting, counting rate dependent shaping time. To that aim, the delta-noise filter of the system must be preloaded with the best estimate of the final result of the filtering process which is simply the unfiltered signal amplitude, to make sure that at the instant of termination of the filtering process the output of the filter deviates from the final value not more than by the decaying noise amplitude. Complemented by counting rate dependent step-noise filtering, this technique made possible the creation of “a spectrometry system for all purposes” which at low to medium counting rates is comparable to the best of the semi-Gaussian amplifiers and at high counting rates to the gated integrator. An experimental implementation of the Preloaded Filter combined with an Ortec Gamma-X detector of 15% relative efficiency resulted in a basic resolution of 1.9 keV at 1332 keV at a counting rate of 5000 cps slowly degrading to 3.2 keV at a counting rate of 650 000 cps of60Co.

A new true Gaussian digital shaper of detector pulses is analised for applications with soft X-ray spectrometers at the highest count rates. The true Gaussian shaper allows for shaping detector pulses into a symmetrical form which width can be considerably less than the rise time of the input pulses. Therefore, the dead time of the true Gaussian shaper can be noticeably reduced in regards to that of trapezoidal shaper. Less dead time provides a higher count rate of the true Gaussian pulses. The true Gaussian shaper is compared to the standard trapezoidal shaper in terms of count rate, amplitude resolution and biasing the output shaped pulses in a wide range of the input count rates. The maximal output count rate of the true Gaussian shaper has been found to exceed the rate of the trapezoidal shapers in several times. The true Gaussian shaper provides biasing-free measurements of pulse amplitudes with better resolution than that provided by the trapezoidal shapers at high count rates. The tests have been performed by modeling the output signals of silicon drift detector (SDD) H7 VITUS equipped with AXAS-D pre-amplifier developed by KETEK GmbH for measurements of soft X-ray spectra at high count rates.

Thick reach-through silicon single-photon avalanche diodes (SPADs) are popular low-noise high efficiency single-photon detectors commonly used in actively quenched detector modules, and are useful at count rates < 10^7 /s. We demonstrate that these devices can achieve significantly higher count rates in a high-speed gating system. With gate durations of approximately 500 ps produced by a 1 GHz sinusoidal bias, we measure a reduction in the average charge per avalanche of more than 100x compared to an actively quenched device, enabling count rates above 10^8 /s without damage. We use a multi-harmonic interferometric readout to detect the minute avalanches at the SPAD anode. In our current setup we observe detection efficiencies above 0.55 (2) at 850 nm at count rates below 10^7 /s, and a count-rate dependent reduction in efficiency at higher rates; at 10^8 /s the efficiency is 0.35 (2). We discuss how to mitigate this effect to achieve higher efficiency at high rates.

The increasing luminosity of accelerators requires a higher counting rate capability of detectors. Previous studies have mostly simulated the detection efficiency of RPCs at a high counting rate but rarely involved the time resolution. In this paper, Geant4 software is used to simulate the interaction between particles and detectors, avalanche multiplication (considering the effect of space charge), and the generation, amplification, and shaping of the induction signal. A model is used to describe the establishment and recovery of the internal electric field in an MRPC at a high rate. We obtain the detection efficiency and time resolution of MRPCs with different structures at different counting rates. The simulation results are in good agreement with the experimental results, which verifies the feasibility of the model. At the same time, it has been confirmed that MRPCs built with the low resistivity glass developed by Tsinghua University can achieve a time resolution of 60–80 ps and a 90% efficiency at a flux up to 70 kHz/cm2 both in the experimental and simulation results.

We recently proposed a design of a modular cylindrical cardiac SPECT system. One special feature of this system is an integrated provision for transmission imaging. To meet the clinical demands of obtaining transmission images, this system must be able to achieve a very high count rate (CR). To explore methods for achieving a high CR capability on a modular cylindrical detector system, we have used our existing modular cylindrical brain SPECT system to examine the feasibility of two approaches. First, we use digital-signal-processing (DSP) boards, in parallel, to execute real time position calculations. Second, we use local encoding and triggering circuits to perform analog signal processing, including identifying the detector module and digitizing the pulse signals. The results of our preliminary investigations indicate that applying the multiple-DSP parallel position calculation and local triggering techniques in a modular SPECT system can improve the CR capability significantly. Applying local triggering increased the CR capability by 15% at a CR capability of 200 kcps. Because we have used slow-speed DSP boards during this proof-of-concept testing, we have not yet met the CR requirements for transmission imaging. However, these results indicate that by using state-of-the-art DSP boards the CR capability of this modular SPECT system can be increased to over 300 kcps.

Characterization of the count-rate performance of scintillation cameras should include not only the specification of count losses. At high count rates, there is also an image distortion due to the mispositioning of pile-up events. In this paper a simple and clinically relevant procedure to quantify this distortion is presented. The images of a square uniform technetium-99m phantom at high and low count rates are used. The fraction of the total counts being correctly positioned is determined as the peripheral count density divided by the total average count density. This ratio, corrected for the camera non-uniformity at low count rates, is called the 'positioning ability'. According to the National Electrical Manufacturers' Association (NEMA), the 'system count rate performance with scatter' should be reported as the measured count rate giving 20% count losses. In this paper it is suggested that this measure be complemented by a measure of the fraction correct positioned events at this count rate. This fraction, the 'high count rate positioning ability', can be easily and accurately measured using our method. The method has been tested on two different scintillation cameras. For one of them the high count rate positioning ability was determined as 91% at a measured count rate of 30,000 s-1 with 20% count losses. For the other camera, the corresponding figures were 88% at 59,000 s-1 and close to 100% at 38,000 s-1, before and after the installation of a new pile-up rejection circuit, respectively.

Photon counting x-ray detectors (PCXDs) offer several advantages compared to standard energy-integrating x-ray detectors, but also face significant challenges. One key challenge is the high count rates required in CT. At high count rates, PCXDs exhibit count rate loss and show reduced detective quantum efficiency in signal-rich (or high flux) measurements. In order to reduce count rate requirements, a dynamic beam-shaping filter can be used to redistribute flux incident on the patient. We study the piecewise-linear attenuator in conjunction with PCXDs without energy discrimination capabilities. We examined three detector models: the classic nonparalyzable and paralyzable detector models, and a ‘hybrid’ detector model which is a weighted average of the two which approximates an existing, real detector (Taguchi et al 2011 Med. Phys. 38 1089–102 ). We derive analytic expressions for the variance of the CT measurements for these detectors. These expressions are used with raw data estimated from DICOM image files of an abdomen and a thorax to estimate variance in reconstructed images for both the dynamic attenuator and a static beam-shaping (‘bowtie’) filter. By redistributing flux, the dynamic attenuator reduces dose by 40% without increasing peak variance for the ideal detector. For non-ideal PCXDs, the impact of count rate loss is also reduced. The nonparalyzable detector shows little impact from count rate loss, but with the paralyzable model, count rate loss leads to noise streaks that can be controlled with the dynamic attenuator. With the hybrid model, the characteristic count rates required before noise streaks dominate the reconstruction are reduced by a factor of 2 to 3. We conclude that the piecewise-linear attenuator can reduce the count rate requirements of the PCXD in addition to improving dose efficiency. The magnitude of this reduction depends on the detector, with paralyzable detectors showing much greater benefit than nonparalyzable detectors.

A novel photon counting method for nonparalyzable counting and its implementation in the new PILATUS3 ASIC are presented. Pulse pile-up significantly affects the observed count rate at high photon fluxes in single-photon counting x-ray detectors and can lead to complete paralyzation of the counting circuit. In PILATUS single-photon counting hybrid-pixel x-ray detectors, count rate correction is applied in order to compensate for the counting loss at high count rates. However, counter paralyzation limits the maximum usable count rate of the PILATUS2 ASIC to typically 2.106 photons per second and pixel. In order to overcome this limitation, instant retrigger technology is introduced as a new photon counting method that results in non-paralyzable counting and achieves improved high-rate counting performance. Instant retrigger technology re-evaluates the pulse signal after a predetermined dead time interval after each count and potentially retriggers the counting circuit in case of pulse pile-up. The respective dead time interval is adjustable and accounts for the width of a single photon pulse. As a result, the counting becomes non-paralyzable and enhanced count rate correction can be applied in order to achieve improved data quality at high count rates. The new PILATUS3 ASIC features instant retrigger technology with adjustable dead time. The implementation of this new approach and experimental results are presented. The new ASIC additionally features counter overflow handling, improved pixel uniformity, reduced crosstalk, reduced readout time, and compatibility with CdTe sensors. With the new design, higher count rates can be measured and better data quality is achieved up to rates of more than 107 photons per second and pixel.

The rate limitation of a photon counting germanium solid-state detector system is largely determined by the shaping time of the amplifier, which is in turn set by the required system resolution. This rate limitation arises because when an event passes into the shaping amplifier, it is paralyzed for a time referred to as the dead time, which is a function of the shaping time. If any further events pass into the shaping amplifier during this time, one or both events may be corrupted or lost. This phenomenon is known as pulse pileup. However, if one uses an incident count rate monitor (ICR) which gives an accurate indication of the incident photon rate but no energy information, one can correct for any pulses lost in the shaping amplifier due to pileup. This allows one to run the detector system at higher rates and still retain throughput linearity. This has been shown by the work of Zhang et al. The authors also showed that with respect to EXAFS, the loss of linearity due to pulse pileup at high rates has two main effects: (1) The EXAFS oscillations and edge step height are reduced and (2) noise and glitches present in I0 do not normalize out. This work has been extended to higher input count rates and in addition to using the paralyzable model to correct for the loss of throughput linearity as the input rate increases, was also used the ICR monitor to give an I0 value which when itself corrected can give improvements over an ion chamber I0 in normalizing noise and glitches out. Data has been taken with station 9.3 on the SRS at Daresbury Laboratory using 18 mM CuNO3 solution at ICR values of 20, 47, 84, 120, and 180 kHz. In addition, data has also been taken for 0.02 at. % As implanted to 1 μm depth in a-Si using reflexafs. This data was collected at 120 kHz as opposed to the usual 40 kHz and normalized using these techniques. It will be shown using this data that using pileup correction and an ICR I0, the linearity of the system can be retained at higher count rates and thus significant improvements in the signal-to-noise ratio can be gained.

Recently the practical X-ray measurement system is demanded energy distinction function with the high count rate and the high energy resolution used photon-counting X-ray sensor. Photon-counting CdTe semiconductor detectors have a high energy resolution in a low count rate at room temperature. However, the energy resolution is decreased by pile-up phenomenon in a high count rate. It is because that the photon counting sensor was used the signal-processing algorithm that the photon energy was estimated by integration of the output waveform from the CdTe detector. Moreover, the photon counting sensor is required to maintain the property of a high energy resolution even if the pile-up occurred. This paper purposes to maintain the high energy resolution by changing the signal-processing algorithm which derived the digital energy value from the pulse rise height of the output waveform generated the pile-up from the CdTe detector. As a result, the pulse rise time required to estimate the pulse rise size was shorter than 500ns. As the result of calculating energy spectrum by using this data, the FWHM of about 4keV (at 60keV) when the count rate of 3counts/60us in the pulse attenuation constant of 60μs was obtained (the pile-up is occurred in this condition). Furthermore, for confirmation of this signal processing in high count rate condition, we used white X-ray together with Am241 in order to measure energy spectrums in a high count rate. As a result of measurement energy spectrum in a high count rate, the FWHM was about 11keV (at 60keV) when the count rate of 500kcps.This result is indicated the possibility that the photon counting sensor has application for the high count rate imaging without decrease of the high energy resolution.

The benefit of time-of-flight (TOF) in PET reconstruction has been established for clinical imaging, and the signal-to-noise benefit with TOF has been demonstrated to increase with higher randoms fractions. TOF information mitigates and reduces image artifacts arising from errors in the corrections, but it may not account for all changes with count rate. Standard performance measures of image quality are carried out at low or clinical count rates for whole-body scanning, but the dependence of TOF quantitative performance on count rate has not been well-studied. All scanners with detector modules that employ photosensor-sharing experience pile-up of detected events with increasing count rate. We investigated the quantitative accuracy of TOF PET using cardiac and uniform phantoms imaged over a range of activities. At very high count rates the cardiac phantom showed significantly degraded image quality and quantification errors without TOF; TOF reconstruction was more stable to image artifacts and biases at high activities. For the uniform phantom, however, when the same number of prompt-delay events (trues+scatters) was reconstructed at different activities, a systematic negative bias in a large region was observed with increasing activity for TOF reconstructions. This bias was significantly larger than that for non-TOF reconstructions. This finding correlates with a cool center seen in the TOF images and suggests an overcorrection for scatter. At high count rates, object scatter modeled by the TOF-modified single scatter simulation (SSS) does not change, but detector pile-up leads to an increase in detected events with higher energies and long TOF tails than at low activities, neither of which is modeled by TOF-SSS. Deadtime correction factors implicitly include these sources of bias but depend on the algorithm (TOF vs. non-TOF) and likely the activity distribution as well through the inaccurate scatter estimate.

Liquid scintillation detectors are widely used in nuclear/high-energy physics and nuclear fusion for spectral measurements in mixed radiation fields due to their compactness, fast response and neutron/gamma discrimination capabilities. The use of response functions evaluated for the specific system and of appropriate methods of data analysis allows such systems to be used as broadband spectrometers for photons and neutrons. System stability and ability to reach high throughput count rates are key challenges for several applications (e.g., neutron spectrometry for nuclear fusion devices), but standard analog electronics limits the operation of liquid scintillation neutron spectrometers to low count rates ( ~ 3 ldr 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ) . The count rate capabilities of a liquid scintillation neutron spectrometer (NE213 detector) from the Physikalisch-Technische Bundesanstalt (PTB) has been extended up to ~ 4.2 ldr 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , by coupling it to a digital acquisition system developed at ENEA-Frascati. Measurements have been carried out at PTB using gamma sources and accelerator-produced 2.5 MeV and 14 MeV neutrons. For 14 MeV neutron measurements, digital pulse height spectra (PHS) obtained at high count rates have been compared to PHS recorded with standard analog electronics. The results show that stable PHS (within 1%) can be obtained at high count rate despite the high sensitivity of the gain of photomultiplier tubes to count rate variations.

Highly reliable 3He gas detectors are most often used in neutron scattering experiments but have disadvantages such as low count rates and low position resolutions. In this study, a 3He gas detector system called NEUNET-HCR was developed, whose count rate is approximately 10 times that of the current NEUNET system, which was developed for a high position resolution with a low count rate. NEUNET-HCR achieved a maximum count rate of 535 kcps per detector, with a loss of approximately 30%. As the NEUNET-HCR system was developed based on the NEUNET system, various improvements were required to achieve the high count rate. Further, a high count rate was achieved in the NEUNET-HCR system without significantly degrading the position resolution.