Murray Davies

Space-qualified silicon avalanche-photodiode single-photon-counting modules

Abstract A space-qualified silicon avalanche-photodiode (APD) based single-photon-counting-module (SPCM) was developed for the Geoscience Laser Altimeter System (GLAS) on board NASAs Ice, Cloud, and Land Elevation Satellite (ICESat). Numerous improvements were made over the commercially available SPCMs in both performance and reliability. The measured optoelectronic parameters include, 65% photon detection efficiency at the 532nm wavelength, 15–17 mega-counts per second (Mcps) maximum count rate and less than 200s−1 dark counts before exposure to space radiation.

LabPET II, an APD-based Detector Module with PET and Counting CT Imaging Capabilities

Computed tomography (CT) is currently the standard modality to provide anatomical reference for positron emission tomography (PET) in molecular imaging applications. Since both PET and CT rely on detecting radiation to generate images, using the same detection system for data acquisition is a compelling idea even though merging PET and CT hardware imposes stringent requirements on detectors. These requirements include large signal dynamic range with high signal-to-noise ratio for good energy resolution in PET and energy-resolved photon-counting CT, high pixelization for suitable spatial resolution in CT, and high count rate capability for reasonable CT acquisition time. To meet these criteria, the avalanche photodiode (APD)-based LabPET II module is proposed as the building block for a truly combined PET/CT scanner. The module is made of two monolithic 4×8 APD pixel arrays mounted side-by-side on a custom ceramic holder. Individual APD pixels have an active area of 1.1×1.1 mm2 at a 1.2 mm pitch. The APD arrays are coupled to a 12-mm high, 8 ×8 LYSO scintillator array made of 1.12 ×1.12 mm2 pixels also at a pitch of 1.2 mm to ensure direct one-to-one coupling to individual APD pixels. The scintillator array was designed with unbound specular reflective material between pixels to maximize the difference between refractive indices and enhance total internal reflection at the crystal side surfaces for better light collection, and the APD quantum efficiency was improved to ~ 60% at 420 nm to optimize intrinsic detector performance. Mean energy resolution was 20 ±1% at 511 keV and 41±4% at 60 keV. The measured intrinsic spatial and time resolution for PET were respectively 0.81 ±0.04 mm FWHM/1.57 ±0.04 mm FWTM and 3.6±0.3 ns FWHM with an energy threshold of 400 keV. Initial phantom images obtained using a CT test bench demonstrated excellent contrast linearity as a function of material density. With a magnification factor of 2, a CT spatial resolution of 0.66 mm FWHM/1.2 mm FWTM, corresponding to 1.18 lp/mm at MTF10%/0.67 lp/mm at MTF50%, was measured, allowing 0.75 mm air holes in an Ultra-Micro Hot Spot resolution phantom to be clearly distinguished.

LabPET II, an APD-based PET detector module with counting CT imaging capability

CT imaging is currently the standard modality to provide anatomical reference in PET molecular imaging. Since both PET and CT rely on detecting radiation to generate images, it would make sense to use the same detection system for data acquisition. Merging PET and CT hardware imposes stringent requirements on detectors, including wide dynamic range with high signal-to-noise ratio for good energy resolution in both modalities, high pixellisation for high spatial resolution, and very high count rate capabilities. The APD-based LabPET II module is proposed as the building block for a truly combined PET/CT scanner. The module is made of two 4 × 8 APD pixel monolithic arrays mounted side by side unto a custom ceramic holder, with each element having an active area of 1.1 × 1.1 mm2 at a 1.2 mm pitch, coupled to a 12-mm high LYSO scintillator block array. While a previous version of the module was made of pyramidal shaped crystals (1.35 × 1.35 / 1.2 × 1.2 mm2, top/bottom), a recent version was designed with a simpler rectangular geometry (1.2 × 1.2 mm2), better reflective material optimizing the shift of refractive index at crystal interface, and enhanced APD quantum efficiency to improve intrinsic detector performance. Mean energy resolution was improved to 20 ± 1% (formerly 24 ± 1%) at 511 keV and to 41 ± 4% (formerly 48 ± 3%) at 60 keV. These intrinsic detector performance characteristics make the LabPET II module suitable for counting CT imaging with efficient energy discrimination. Initial phantom images obtained from a CT test bench demonstrated excellent contrast linearity as a function of material density and spatial resolution of 0.61 mm FWHM / 1.1 mm FWTM, corresponding to 1.3 lp/mm at MTF10% / 0.73 lp/mm at MTF50%, which allowed 0.75 mm air holes in an Ultra Micro resolution phantom to be clearly distinguished.

Performance evaluation of a dual-crystal APD-based detector modules for positron emission tomography

Positron Emission Tomography (PET) scanners dedicated to small animal studies have seen a swift development in recent years. Higher spatial resolution, greater sensitivity and faster scanning procedures are the leading factors driving further improvements. The new LabPETTM system is a second-generation APD-based animal PET scanner that combines avalanche photodiode (APD) technology with a highly integrated, fully digital, parallel electronic architecture. This work reports on the performance characteristics of the LabPET quad detector module, which consists of LYSO/LGSO phoswich assemblies individually coupled to reach-through APDs. Individual crystals 2×2×~10 mm3 in size are optically coupled in pair along one long side to form the phoswich detectors. Although the LYSO and LGSO photopeaks partially overlap, the good energy resolution and decay time difference allow for efficient crystal identification by pulse-shape discrimination. Conventional analog discrimination techniques result in significant misidentification, but advanced digital signal processing methods make it possible to circumvent this limitation, achieving virtually error-free decoding. Timing resolution results of 3.4 ns and 4.5 ns FWHM have been obtained for LYSO and LGSO, respectively, using analog CFD techniques. However, test bench measurements with digital techniques have shown that resolutions in the range of 2 to 4 ns FWHM can be achieved.

LabPET II, a novel 64-channel APD-based PET detector module with individual pixel readout achieving submillimetric spatial resolution

A new avalanche photodiode (APD) detector module, the LabPET II, was developed to achieve submillimetre spatial resolution for small animal molecular imaging. The module consists of two monolithic APD arrays of 4 × 8 pixels, each with an active area of 1.1 × 1.1 mm2 at a 1.2 mm pitch. The two APD arrays are mounted in a custom ceramic holder and coupled to an 8 × 8 LYSO scintillator array designed to accommodate one-to-one coupling between individual APDs and crystal pixels. An analog test board adapted from the LabPET™ processing electronics and consisting of four 16-channel preamplifier ASICs, was designed for testing the LabPET II detector module. The devices have a broad operating range (over 200 V) with breakdown voltage of ∼350 V, at which a typical gain well above 200 is reached. Individual APD pixels have a capacitance of 3.7 ± 0.4 pF (including stray capacitance), a typical dark current of 30 ± 11 nA, a dark noise of 0.13 ± 0.03 pA/Hz½ and an equivalent noise charge (ENC) of 12 e− rms at a gain of 100. At a standard APD operating bias, a mean energy resolution of 27.5 ± 2.1% was typically obtained with a relative standard deviation of 13.8% in signal amplitude for the 64 individual pixels when irradiated with 511 keV photons. A global timing resolution of 5.0 ± 0.2 ns FWHM was measured with two modules in coincidence. Finally, an intrinsic spatial resolution of 0.82 ± 0.02 mm FWHM (1.54 ± 0.05 mm FWTM) was obtained by sweeping a 22Na point source between two rows of the detector array. By deconvolving the source size and non-collinearity, an expected 0.73 mm intrinsic geometric crystal resolution is obtained. The LabPET II detector module is demonstrating promising characteristics for dedicated small animal PET imaging at submillimetre resolution and, with some further optimization, would be suitable as the building block for a dual-modality combined PET/CT system.

Timing resolution and time walk in super low <italic<K</italic< factor single-photon avalanche diode—measurement and optimization

Abstract. Timing resolution (or timing jitter) and time walk are separate parameters associated with a detector’s response time. Studies have been done mostly on the time resolution of various single-photon detectors. As the designer and manufacturer of the ultra-low noise (ƙ-factor) silicon avalanche photodiode the super low K factor (SLiK) single-photon avalanche diode (SPAD), which is used in many single-photon counting applications, we often get inquiries from customers to better understand how this detector behaves under different operating conditions. Hence, here, we will be focusing on the study of these time-related parameters specifically for the SLiK SPAD, as a way to provide the most direct information for users of this detector to help with its use more efficiently and effectively. We will be providing the study data on how these parameters can be affected by temperature (both intrinsic to the detector chip and environmental input based on operating conditions), operating voltage, photon wavelength, as well as light spot size. How these parameters can be optimized and the trade-offs from optimization from the desired performance will be presented?

Timing resolution and time walk in SLiK APD: measurement and optimization

Timing resolution (or timing jitter) and time walk are separate parameters associated with a detector’s response time. Studies have been done mostly on the time resolution of various single photon detectors [1]. As the designer and manufacturer of the ultra-low noise (ƙ-factor) silicon avalanche photodiode the SLiK SPAD, which is used in many single photon counting applications, we often get inquiries from customers to better understand how this detector behaves under different operating conditions. Hence, here we will be focusing on the study of these time related parameters specifically for the SLiK SPAD, as a way to provide the most direct information for users of this detector to help with its use more efficiently and effectively. We will be providing the study data on how these parameters can be affected by temperature (both intrinsic to the detector chip and environmental input based on operating conditions), operating voltage, photon wavelength, as well as light spot size. How these parameters can be optimized and the trade-offs from optimization from the desired performance will be presented.

The APA pixel readout ASIC for an APD based fast 2D X-ray hybrid pixel detector

The XNAP project develops a two-dimensional counting pixel X-ray detector based on silicon avalanche photodiodes (APDs) with time resolution capabilities in the nanosecond range. Today, such a time scale is only accessible with single elements or linear arrays of fast devices. In order to read out APD arrays in the 1 k pixel range, a mixed-signal ASIC named APA was developed, which is bump-bonded via an interposer to the sensor to form a hybrid detector. The ASIC prototypes and different types of APD arrays with divided anode and divided cathode design were evaluated on 4 × 4 pixels and tested using 5.9 keY, 8 keY and 14.4 keY X-rays. We report on the system design and present first encouraging results on energy selectivity, position resolution, timing and maximum count rate.

Improved LabPET detectors using Lu 1.8 Gd 0.2 SiO 5 :Ce (LGSO) scintillator blocks

The scintillator is one of the key building blocks that critically determine the physical performance of PET detectors. The quest for scintillation crystals with improved characteristics has been crucial in designing scanners with superior imaging performance. Recently, it was shown that the decay time constant of high lutetium content Lu1.8Gd0.2SiO5:Ce (LGSO) scintillators can be adjusted between 30 ns and 48 ns by varying the cerium concentration from 0.025 mol% to 0.75 mol%, thus providing interesting characteristics for phoswich detectors. The large light output (90–120% NaI), the better spectral match and the high initial photoelectron rate (∼200 phe−/ns) of these scintillators with avalanche photodiode (APD) readout promise to provide superior energy and timing resolution. Moreover, their improved mechanical properties as compared to conventional LGSO (Lu0.2Gd1.8SiO5:Ce) make block array manufacturing readily feasible. To verify these assumptions, new phoswich block arrays made of LGSO-90%Lu with low and high mol% Ce concentrations were fabricated and assembled into LabPET modules. Typical crystal decay time constants were 32 ns and 48 ns, respectively. We therefore report on the initial evaluation of this modified version of the LabPET detector module. Phoswich crystal identification performed using a non-optimized digital pulse shape discrimination algorithm yielded an average 10% error. At 511 keV, energy resolution of 20 ± 2% and 15 ± 1% were obtained, while coincidence timing resolution between 4.9 ± 0.3 ns and 4.1 ± 0.1 ns were achieved. The improved characteristics of this new LGSO-based phoswich detector module are expected to enhance the LabPET scanner performance, first by improving sensitivity due to the overall higher stopping power of the detector module, and second by narrowing the coincidence time window, thus minimizing the random event rate. Altogether these two improvements will significantly enhance the noise equivalent count rate performance of an all LGSO-based LabPET scanner.