Effects of dark counts on digital silicon photomultipliers performance

Digital Silicon Photomultipliers (dSiPM) are novel light detector that integrates single-photon avalanche photodiodes and CMOS logic into a single silicon chip and have been used for developing new, high performance detectors for Positron Emission Tomography (PET). As a solid-state devices they suffer from thermal excitation what leads to the appearance of noise events called dark counts. However, it is unclear what effect the dark counts have on the count rate performance of dSiPM. Therefore, it is necessary to investigate the event loss caused by these dark counts and to come up with optimal configuration of these devices. Here, the effects of dark counts on the performance of are evaluated. Due to the trigger architecture of dSiPM, dark counts cause start of acquisition sequence of the device. Processing of these dark counts leads to dead time of dSiPM what cause the loss of true gamma events. We studied how trigger level, validation level and validation length influence the loss of events due to dark counts. We found that validation time should be kept long (40 ns) to minimize the loss of events. Use of high trigger level and validation level also reduce the event loss caused by dark counts. However, with the high validation level, detection of events with low number of optical photons is reduced as it more difficult for these events to pass the validation threshold. The RTL refresh option was also tested to reduce the effect of dark counts. We found that this option resulted in the achieving maximum sensitivity, i.e. the highest fraction of correctly recorded true events, of dSiPM regardless of used validation and trigger levels. In cases when the scintillation light is spread over several dies, we found that the use of RTL refresh option combined with a low validation level in order to guarantee the individual validation of all required dies ensures higher sensitivity than the use of Neighbor Logic (NL). Finally we verified the dead time of dSiPM and found that is longer than specified and equal to 50 ns

Performance of digital silicon photomultipliers for time of flight PET scanners

The performance of Digital Silicon Photomultipliers (dSiPM) coupled to a LYSO array containing 15×15 pixels with a size of 2×2×22 mm3 is evaluated to determinate their potential for whole body Time of Flight (TOF) PET scanners. The detector pixels are smaller in size than the light sensors and therefore light spreading is required to determine the crystal where interaction occurred. A light guide of 1 mm was used to spread the light and neighbor logic (NL) configuration were employed to ensure correct crystals identification. We studied the energy resolution and coincidence resolving time (CRT) for different trigger levels. The measured average energy resolution across detector was 14.5 %. Prior to measurements of time resolution skew time calibration of dSiPM was performed. The average CRT achieved using trigger level 1 option was 376 ps FWHM. Finally, we studied the amount of events that are disregarded due to dark count effects for different trigger levels and temperatures. Our studies show that a trade-off must be made between the detector’s CRT and sensitivity due to its vulnerability to dark counts. To employ dSiPM in TOF PET systems without 1:1 coupling effective cooling is necessary to limit dark count influence

A critical review on test methods for evaluating the resistance of concrete against sulfate attack

Sulfate attack comprises a series of chemical reactions between sulfate ions and the components of hardened concrete. As these reactions may lead to cracking, spalling or strength loss of concrete structures, appropriate test methods are needed to determine the resistance of cncrete under sulfate exposure. Accelerated test methods are most suitable since sulfate attack is a long term process. Current ASTM C1012 test method accelerates the attack mechanism by using a solution with a high sulfate concentration in which the samples are immersed. The SVA procedure uses smaller specimens to obtain results earlier. In the Wittekindt method not only smaller specimens are used but also the W/C-ratio is increased. However these tests still last for several months. Test methods such as ASTM C452 and the Chatelier-Anstett test use a mixture of cement and gypsum, simulating internal sulfate attack. Results are already obtained after two weeks, but the attack mechanism is no longer representing field conditions in a realistic way. Another problem relates to the way to quantify the degree of degradation. SVA, Wittekindt, Duggan, ASTM C1012 and C452 use expansion measurements. Mehta and Gjorv proposed to use decrease in compressive strength and in the rapid electrochemical test current is measured to determine degradation. Depending on the selected degradation measure, different conclusions can be drawn regarding the performance of concrete under sulfate attack. In this paper an overview of the existing test method is given and a critical discussion is performed

Design and Characterisation of an MRI Compatible Human Brain PET Insert by Means of Simulation and Experimental Studies

Positron emission tomography (PET) is a widely used in-vivo imaging technique to visualise metabolism, allowing for a broad spectrum of applications in oncology, cardiology and neuroscience. At present, an MRI compatible human brain PET scanner for applications in neuroscience is being constructed in the scope of a Helmholtz Validation Fund project. In this thesis, a detector for this novel PET device was designed. The detector concept combined three scintillator layers with a lightguide and digital silicon photomultipliers (dSiPMs). Monte Carlo simulations were used to optimise the dimensions of the scintillator arrays, so that the new scanner design yielded the maximum possible sensitivity. The benefit from the additional depth information, which can be acquired with three scintillator layers, was evaluated and proven to be higher compared to a less expensive two layer geometry. Since a more homogeneous spatial resolution was achieved in the whole field of view, this finding had a high relevance for the envisaged neuroscientific applications. In order to accurately acquire the depth information, new strategies for decoding the flood map during the calibration of a detector module were developed. This required realistic simulation data with ground truth information, so that the simulation toolkit GATE was extended to model the electronic readout of the dSiPMs. To overcome extended simulation times and to provide simulations on a statistically sound basis, the GATE studies were executed on the supercomputer JURECA. The simulated data were matched to measured data from test detectors. This allowed the determination of an optimum thickness of a lightguide between the scintillators and the dSiPMs. Moreover, the number of correctly identified scintillation events was evaluated by means of different event positioning approaches and different clustering methods during the calibration step. The highest amount of correctly identified events in a single detector block was achieved with model-based clustering and Maximum Likelihood positioning (61.5 %). By simulating the whole propagation and detection of scintillation photons including ground truth information, this study provides the opportunity to improve the positioning approaches and to enhance this number in future. The gained insights were further applied to select a surface finish of the scintillators. Measurements with crystal samples of the final detector dimensions showed that rough lateral crystal surfaces yielded the best signal separation in the calibration flood map. The experimental and simulation studies presented in this thesis had a major influence on the final detector design of the novel brain PET. The detailed simulations including the propagation and detection of scintillation photons were in good agreement with measured data, and could be a promising approach for future detector design studies

Hardware and control co-design enabled by a state-space formulation of cascaded, interconnected PID controlled systems

In recent years, more and more attention has been paid to the simultaneous optimization of hardware and control to achieve an optimal design of complex and interacting systems. To efficiently carry out this co-design optimization, there is a need for flexible methods to apply the variable hardware and control co-design aspects while also allowing a faster system response calculation compared to traditional methods. These time savings in the response calculations are of significant importance when using iterative optimization algorithms that typically require a large number of simulations to arrive at a solution. That is why this paper proposes a general methodology to create a closed-loop state-space model consisting of an open-loop process with an observer and extensive control loop structures. These structures comprise a combination of cascaded decentralized and distributed controllers, synchronizing controllers, and feedforward controllers while taking into account reference trajectories and input disturbances. It is shown that with the proposed methodology, response calculations for a motion application are much faster compared to traditional graphical programming tools that enable to apply flexible control architecture features. This shows that the presented methodology permits the efficient co-design optimization of hardware en control aspects

Dynamic range extension of SiPM detectors with the time-gated operation

The silicon photomultiplier (SiPM) is a novel detector technology that has undergone a fast development in the last few years, owing to its single-photon resolution and ultra-fast response time. However, the typical high dark count rates of the sensor may prevent the detection of low intensity radiation fluxes. In this article, the time-gated operation with short active periods in the nanosecond range is proposed as a solution to reduce the number of cells fired due to noise and thus increase the dynamic range. The technique is aimed at application fields that function under a trigger command, such as gated fluorescence lifetime imaging microscopy

The Digital Silicon Photomultiplier

The Silicon Photomultipliers (SiPMs) are the new step in the development of the modern detection structures in the area of low photon flux detection with a unique capability of detection up to the single photons. The Silicon Photomultiplier intrinsically represents a digital signal source on the elementary cell level. The materials and technology of SiPMs are consistent with the modern electronics technology. We present the realization and implementation of a fully digital Silicon Photomultiplier Imager with an enclosed readout and processing on the basis of modern 3D technology

Analysis of Photon Detection Efficiency and Dynamic Range in SPAD based Visible Light Receivers

We investigate the photon detection efficiency (PDE) and the dynamic range for digital silicon photomultipliers (dSiPMs) over a selection of design parameters: dSiPM unit cell dead time, PDE, unit cell area and fill factor, number of cells, and total dSiPM active area. Two receiver scaling scenarios are con-sidered: varying the number of cells for 1) a fixed unit cell area or 2) a fixed total dSiPM area. Theoretical and simulated results are confirmed with experimental data from a selection of dSiPMs realised on a test chip in130-nm CMOS process