Count Rate Conditions Research Articles

Quantitative analysis of silicon photomultipliers for thermoluminescence measurement

The employment of silicon photomultipliers (SiPMs) in photon detection systems facilitates the reduction of detection module size, enhances portability and ruggedness, and lowers product costs. This study investigated the potential of SiPM in thermoluminescence (TL) measurements, focusing on its detection limits, temperature dependency, and dose responses. A 3 × 3 mm2 multi-pixel photon counter (MPPC, Hamamatsu MPPC) composed of 50 μm single-photon avalanche diodes (SPADs) was utilized in single-photon counting mode as the detector. This experimental setup was compared with a conventional photomultiplier tube (PMT) in TL readout case. To ensure comparable light exposure between the two detectors, a 3-mm diameter pinhole was positioned in front of the PMT. By using a thermoelectric (TE) cooling module to maintain the sensor temperature at −20 °C, signal reproducibility was achieved with a fluctuation of less than 1.2%, and a detection limit ranging from 30 to 300 μGy was assessed for four dosimeters: LiF:Mg,Ti, LiF:Mg,Cu,P, LiF:Mg,Cu,Si, and Al2O3:C. The dose response of the system was limited by significant signal saturation due to pile-up effects. We introduced a correction model with a 50 ns paralyzable deadtime, which enhanced the signal response by more than 20-fold. Furthermore, this model also correlated well with the observed count rates under high dark count rate (DCR) conditions up to 1.67 MHz. The findings from this study further contributes to the discussion regarding the perspective of SiPM as TL detectors.

Pushing the high count rate limits of scintillation detectors for challenging neutron-capture experiments

One of the critical aspects for the accurate determination of neutron capture cross sections when combining time-of-flight and total energy detector techniques is the characterization and control of systematic uncertainties associated to the measuring devices. In this work we explore the most conspicuous effects associated to harsh count rate conditions: dead-time and pile-up effects. Both effects, when not properly treated, can lead to large systematic uncertainties and bias in the determination of neutron cross sections. In the majority of neutron capture measurements carried out at the CERN n_TOF facility, the detectors of choice are the C6D6 liquid-based either in form of large-volume cells or recently commissioned sTED detector array, consisting of much smaller-volume modules. To account for the aforementioned effects, we introduce a Monte Carlo model for these detectors mimicking harsh count rate conditions similar to those happening at the CERN n_TOF 20 m flight path vertical measuring station. The model parameters are extracted by comparison with the experimental data taken at the same facility during 2022 experimental campaign. We propose a novel methodology to consider both, dead-time and pile-up effects simultaneously for these fast detectors and check the applicability to experimental data from 197Au(n, γ), including the saturated 4.9 eV resonance which is an important component of normalization for neutron cross section measurements.

Algorithms of pulse shape analysis for Gammasphere under high count rate conditions

The implementation of digital electronics for the multi-detector Gammasphere array has provided an opportunity to perform experiments which exceed the technical capabilities of its analog counterpart. The pulse shape analysis of HPGe detectors is presented for the purpose of determining the γ-ray energy under high-counting rates and short integration times with the aim of improving the data throughput. A revised trapezoidal algorithm is delineated which is able to determine the γ-ray energies during the offline analysis without being constrained by predetermined parameters. The performance of this algorithm is discussed, and compared with that of the analog system. The measured energy resolution meets expectations for operations with high-counting rates and short integration times.

Neutron/Gamma discrimination code based on trapezoidal filter

Neutron/gamma discrimination techniques are widely applied in scintillator-based neutron diagnostics for present nuclear fusion tokamak experiments (e.g. JET – Neutron Camera and compact neutron spectrometer) and will also be necessary for neutron diagnostics of up-coming machines (e.g. ITER Radial Neutron Camera). Neutron/gamma discrimination in scintillators relies on the fact that the detectors output pulses have different shapes depending on the impinging particle; several discrimination techniques are described in literature such as the charge-integration, curve-fitting and pattern recognition [1].This paper aims at describing a new technique for neutron/gamma discrimination in scintillators based on trapezoidal filtering, which targets Field Programmable Gate Array (FPGA) implementation due to its recursive nature. Furthermore its capability to restore the baseline of each detected pulse and the fact that the output signals are shorter than the correspondent incoming pulses, points this technique as a promising solution for applications in high count rate conditions. First results coming from the application of a real-time FPGA implementation of the trapezoidal filter to simulated neutron/gamma data including pile-up events and to real scintillator data will be presented.

Artificial neural network algorithms for pulse shape discrimination and recovery of piled-up pulses in organic scintillators

We developed two neural-network (NN)-based algorithms (fully-connected neural network (Fc-NN) and recurrent neural network (RNN)) to perform pulse shape discrimination (PSD) and identification of piled-up pulses produced by organic scintillators, upon interaction with neutrons and gamma rays. We tested the algorithms on measured and verification sets of data and compared their classification performances to standard approaches. At a high acquisition count rate (100,000 counts per second, cps), in the presence of a gamma-to-neutron ratio of approximately 400–1, the proposed NN-based algorithm achieves a fraction of misclassified neutron, gamma, and piled-up pulses of approximately 1%, 1.8%, and 0.6%, respectively. Compared to the traditional approach, it exhibits 3×, 14×, and 11× improved (lower) miscalculation rates for neutron, gamma, and piled-up pulses, respectively. We also demonstrate the capability of NN-based algorithms of successfully recovering and identifying neutron and gamma ray compositions from piled-up pulses in challenging, high pulse count rate conditions.

An alternative pulse height correction method for pole zero cancellation circuitry

CdTe diode radiation detectors have a problem called the “polarization effect”, which causes CdTe performance to degrade with time. The pulsed bias voltage shutdown technique, in which a bias voltage is turned off and on quickly to recombine the accumulated charges, is used to suppress the effect. When this technique is used with a charge sensitive amplifier equipped with a pole zero cancellation (PZC) circuit, an extra dead time is observed. After a 40ms shutdown, we have to wait for 130ms (net 170ms dead time) due to the saturation of the amplifier when equipped with a PZC stage while only 10ms (net 50ms dead time) is necessary without a PZC. However, PZC is essential to reduce the degradation of the energy resolution in high count rate conditions. Therefore, an alternative technique to avoid degrading the energy resolution in high count rate conditions without a PZC circuit was investigated. A present pulse height was corrected by using both the pulse heights and the elapsed times from the previous events. As a result, in a readout circuit having no PZC circuitry, an energy resolution of 4.7% for a 122keV photo peak at a count rate of 20kcps was obtained when using the correction, and a 9.1% one was obtained without the correction. In addition, an energy resolution of 2.7% for a 662keV photo peak at a count rate of 21kcps was obtained when using the correction, and a 5.3% one was obtained without the correction. It is shown that this new correction method has almost the same effect as PZC circuitry.

Application of a digital pileup resolving method to high count rate neutron measurements

The possibility of working in high count rate conditions is an important issue for neutron measurements applied to fusion experiments in order to extend the measurement dynamic range and time resolution. One of the main issues of high count rate operation is the overlapping of closely spaced events (pileup) that produces a distortion in their amplitude and therefore in the pulse height spectra. The usual way to deal with such pileup events using standard analog electronics is to reject them in the energy analysis process. Thanks to digital acquisition techniques, the waveforms of pileup events can be recorded and elaborated in order to reconstruct the contributing original single events. This work presents the results of the application of a pileup resolving method for the reconstruction and analysis of overlapping events to pulses acquired digitally from a NE213 liquid scintillator detector. The measurements have been carried out at the PTB accelerator facility with 14 MeV neutrons at total count rates over 400 kHz.

OS-EM and FBP reconstructions at low count rates: effect on 3D PET studies of [ 11C] WAY-100635

11C-labeled neuroreceptor ligands frequently require long scan durations to quantify ligand–receptor binding. In this paper, we compare the accuracy of two three-dimensional (3D) positron emission tomography (PET) reconstructions: ordered-subset expectation-maximization (OS-EM) versus filtered backprojection (FBP) under low count rate conditions exhibited by 11C neuroreceptor studies. Data were obtained from a dynamic 11C phantom acquisition as well as six dynamic human [ 11C] WAY-100635 studies, all acquired in 3D mode using the EXACT HR+ PET scanner. Model-based scatter correction of the phantom datum was found to overcorrect in low count rate situations producing a negative bias in FBP reconstruction and a positive bias in OS-EM reconstruction, the OS-EM bias attributed to the non-negativity constraint of sinogram values. In the phantom OS-EM and FBP, reconstruction bias occurred at activities less than 25 Bq/cm 3. In the human cerebellum, OS-EM deviated from FBP at activities less than 50 Bq/cm 3. The total volume of distribution ( V T), as determined from the metabolite corrected arterial input function and a two-tissue compartment kinetic model, was more sensitive to the positive bias of OS-EM than the negative bias of FBP at low count rates. To avoid reconstruction bias with 3D PET studies using the HR+, the scan duration should be limited so as to yield a final non-decay-corrected activity concentration of no less than 50 Bq/cm 3. In neuroreceptor studies, if such a low count rate cannot be avoided, FBP reconstruction is preferable to OS-EM to estimate V T.

Optimal dose of injection in activation study with O-15 water and PET.

In activation studies with the bolus method for O-15 water and PET, the radiotracer concentration may reach the limits of the system in terms of dead time correction and accidental coincidence. To obtain the optimal injection dose of O-15 water, we performed a normal volunteer study to evaluate the relationship between the injected dose and the radioactivity concentration in the brain and a phantom study to evaluate the performance of the PET scanner (PCT3600W) under high count rate conditions and the effect of averaging on the signal to noise ratio for the PET images. A linear relationship was noted between the injected dose (normalized for each body weight: x) and the mean radiotracer concentration in the brain measured by PET (y) (y = 2.52 + 30.1 x, n = 64, r = 0.87, p < 0.001). The percent error in the measurement of radioactivity with PET was within +/- 5% in the 100 to 2000 nCi/ml (3.7-74 KBq/ml) range. Below 100 nCi/ml (3.7 KBq/ml), the percent error increased due to the rapid increase in noise in the reconstructed images. Over 1000 nCi/ml (37 KBq/ml), on the other hand, the noise was almost unchanged. With our PET scanner, the optimal range of the radiotracer concentration in the brain is below 1000 nCi/ml (37 KBq/ml), corresponding to an injection dose of 33 mCi (1.22 GBq)/60 kg body weight. With the same total dose, the increment of number of repeated measurements for averaging provided the better signal to noise ratio. In designing a paradigm for an activation PET study, the injection dose and the number of repeated measurements for averaging should be considered.

Noise equivalent count measurements in a neuro-PET scanner with retractable septa

The noise-equivalent count-rate (NEC) performance of a neuro-positron emission tomography (PET) scanner has been determined with and without interplane septa on uniform cylindrical phantoms of differing radii and in human studies to assess the optimum count rate conditions that realize the maximum gain. In the brain, the effective gain in NEC performance for three-dimensions (3-D) ranges from >5 at low count rates to approximately 3.3 at 200 kcps (equivalent to 37 kcps in 2-D). The gains of the 3-D method assessed by this analysis are significant, and are shown to be highly dependent on count rate and object dimensions.

A high speed driftchamber readout system: “Tirus”

TIRUS (Time Interval Readout Using Scalers) is a fast and accurate, digitalwire chamber readout system which is capable of operating under high background and high real count rate conditions. The system can be configured for a variety of experiments in medium energy physics, including multiple arm spectrometer setups. Drift time is measured directly by a 500 MHz counter for each wire. Timing resolution is 1 ns, readout speed is 30 to 60 ns per event and relative channel efficiency variation is less than 3%. No delay adjustments are necessary. Current results from a 28 wire prototype are presented.

Evaluating the Performance of Multiplane Positron Tomographs Designed for Brain Imaging

The evaluation of a multiplane tomograph designed for brain imaging requires particular attention to certain design parameters that tend to be comprised in such designs. These parameters include resolution uniformity, axial slice thickness uniformity with emphasis on the interplane slices, scatter fraction, accidentals fraction, and system dead time. In addition, the final performance specifications should be relevant to the manner in which the system will actually be employed in the clinic. These specifications should be measured under realistic scatter and count rate conditions using the same reconstruction techniques that are employed in patient imaging. These parameters were evaluated on the NeuroECAT positron tomograph (1,2) and the effect of choice of manner of presentation and type of measurement condition on the apparent performance of the system is shown. The results indicate that clearly defined conditions are a necessity in evaluating the true performance of a positron tomograph.

Filtering and detection for doubly stochastic Poisson processes

Equations are derived that describe the time evolution of the posterior statistics of a general Markov process that modulates the intensity function of an observed inhomogeneous Poisson counting process. The basic equation is a stochastic differential equation for the conditional characteristic function of the Markov process. A separation theorem is established for the detection of a Poisson process having a stochastic intensity function. Specifically, it is shown that the causal minimum-mean-square-error estimate of the stochastic intensity is incorporated in the optimum Reiffen-Sherman detector in the same way as if it were known. Specialized results are obtained when a set of random variables modulate the intensity. These include equations for maximum a posteriori probability estimates of the variables and some accuracy equations based on the Cramér-Rao inequality. Procedures for approximating exact estimates of the Markov process are given. A comparison by simulation of exact and approximate estimates indicates that the approximations suggested can work well even under low count rate conditions.

An Investigation into the Harms Dead Time Correction Procedure for Pulse Height Analyzers Using Monte Carlo Modeling Techniques

An automatic dead time correction technique for multichannel pulse height analyzers that does not make use of the usual live time correction has recently been proposed by J. Harms. In order to study the validity of this new correction technique, a Monte Carlo code was written to simulate the response of multichannel pulse height analyzers, incorporating both the Harms and standard live time correction techniques, to a variety of pulse height spectra. Simulated pulse height spectra of the type one might encounter from nuclear radiation detectors under a variety of count rate conditions were generated by the code. It was found that the Harms correction technique gave valid results in all cases tested, whereas the standard live time correction technique did not work properly when the spectral shape changed drastically during the course of the measurement.