Alberto Gola

Sub-100 ps coincidence time resolution for positron emission tomography with LSO:Ce codoped with Ca

The coincidence time resolution (CTR) becomes a key parameter of 511 keV gamma detection in time of flight positron emission tomography (TOF-PET). This is because additional information obtained through timing leads to a better noise suppression and therefore a better signal to noise ratio in the reconstructed image. In this paper we present the results of CTR measurements on two different SiPM technologies from FBK coupled to LSO:Ce codoped 0.4%Ca crystals. We compare the measurements performed at two separate test setups, i.e. at CERN and at FBK, showing that the obtained results agree within a few percent. We achieve a best CTR value of 85 ± 4 ps FWHM for 2 × 2 × 3 mm(3) LSO:Ce codoped 0.4%Ca crystals, thus breaking the 100 ps barrier with scintillators similar to LSO:Ce or LYSO:Ce. We also demonstrate that a CTR of 140 ± 5 ps can be achieved for longer 2 × 2 × 20 mm(3) crystals, which can readily be implemented in the current generation PET systems to achieve the desired increase in the signal to noise ratio.

Analog circuit for timing measurements with large area SiPMs coupled to LYSO crystals

The most common method for time pick-off from signals coming from SiPMs coupled to scintillator crystals in PET applications is the Leading Edge Triggering. In this work we propose a new filtering scheme to be applied before the discriminator. It implements a type of baseline compensation aimed at the reduction of the time jitter due to the dark counts of the detector and is based on a simple analog filter, which is well suited for an ASIC implementation. We describe a circuit, built with discrete components, that implements the filter. Finally we report on the Coincidence Resolving Time measurements performed coupling the circuit to the real detector, composed of a SiPM built at FBK and a LYSO crystal, for the detection of 511 keV photons. The results obtained are promising, showing that the method is quite effective in reducing the impact of the detector noise on the timing performance of the system.

Characterization of new FBK SiPM technology for visible light detection

This paper presents the characterization of the new n-on-p SiPM technology developed at Fondazione Bruno Kessler (FBK, Trento-Italy). Several device aspects such as dark count rate, photo detection efficiency, breakdown voltage uniformity, and temperature stability have been significantly improved with respect to the original FBK SiPM technology. The modifications introduced involve the internal device structure and are based on an electric-field engineering approach. We report on the dark characterization, the visible light detection efficiency and 511 keV gamma ray energy resolution, when reading out small LYSO or Ce:GAGG crystals, of the new devices. In parallel, a comparison to the original SiPMs is done in order to underline the main advancements that have been obtained. We refer this new technology to as RGB-SiPMs because of the high detection efficiency for the whole red, green, and blue part of the spectrum.

Characterization of the First FBK High-Density Cell Silicon Photomultiplier Technology

In this paper, we present the results of the characterization of the first high-density (HD) cell silicon photomultipliers produced at FBK. The most advanced prototype manufactured with this technology has a cell size of 15 × 15 μm2 featuring a nominal fill factor of 48%. To reach this high area coverage, we developed a new border structure to confine the high electric-field region of each single-photon avalanche diode. The measured detection efficiency approaches 30% in the green part of the light spectrum and it is above 20% from 400 to 650 nm. At these efficiency values, the correlated noise is very low, giving an excess charge factor below 1.1. We coupled a 2 × 2 × 10- mm3 LYSO scintillator crystal to a 2.2 × 2.2- mm2 silicon photomultiplier, obtaining very promising results for PET application: energy resolution of less than 11% full-width at half maximum (FWHM) with negligible loss of linearity and coincidence resolving time of 200-ps FWHM at 20°C.

The DLED Algorithm for Timing Measurements on Large Area SiPMs Coupled to Scintillators

One of the main factors limiting the precision of timing measurements with silicon photomultipliers coupled to scintillators is the dark noise, especially in the case of large devices. In order to cope with it, a suitable signal processing should be employed. The method we propose is called differential leading edge discriminator (DLED) and allows an effective compensation of the baseline fluctuations due to the dark counts of the detector. In this paper we show a comparison between the measurements obtained using the traditional Leading Edge Discriminator technique and the DLED. The improvement we observe is remarkable. Combining this baseline correction algorithm with low temperatures, we were able to reach a coincidence resolving time of 180 ps FWHM, using 4 mm × 4 mm SiPMs produced at FBK coupled to 3.8 mm × 3.8 mm × 5 mm Teflon-wrapped LYSO crystals.

State of the art timing in TOF-PET detectors with LuAG, GAGG and L(Y)SO scintillators of various sizes coupled to FBK-SiPMs

Time of flight (TOF) in positron emission tomography (PET) has experienced a revival of interest after its first introduction in the eighties. This is due to a significant progress in solid state photodetectors (SiPMs) and newly developed scintillators (LSO and its derivatives). Latest developments at Fondazione Bruno Kessler (FBK) lead to the NUV-HD SiPM with a very high photon detection efficiency of around 55%. Despite the large area of 4×4 mm2 it achieves a good single photon time resolution (SPTR) of 180±5ps FWHM. Coincidence time resolution (CTR) measurements using LSO:Ce codoped with Ca scintillators yield best values of 73±2ps FWHM for 2×2×3 mm3 and 117±3ps for 2×2×20 mm3 crystal sizes. Increasing the crystal cross-section from 2×2 mm2 to 3×3 mm2 a non negligible CTR deterioration of approximately 7ps FWHM is observed. Measurements with LSO:Ce codoped Ca and LYSO:Ce scintillators with various cross-sections (1×1 mm2 - 4×4 mm2) and lengths (3mm - 30mm) will be a basis for discussing on how the crystal geometry affects timing in TOF-PET. Special attention is given to SiPM parameters, e.g. SPTR and optical crosstalk, and their measured dependency on the crystal cross-section. Additionally, CTR measurements with LuAG:Ce, LuAG:Pr and GGAG:Ce samples are presented and the results are interpreted in terms of their scintillation properties, e.g. rise time, decay time, light yield and emission spectra.

Characterization of Single-Photon Time Resolution: From Single SPAD to Silicon Photomultiplier

In this paper, we report on the characterization of the single-photon time resolution (SPTR) of the RGB (Red-Green-Blue) type silicon photomultipliers (SiPM) produced at FBK. We measured and compared single-photon timing jitter of 1 ×1 mm2 and 3 ×3 mm2 SiPMs, and also of square SPADs with integrated passive quenching, identical to the cells composing the SiPMs. We reached a single-photon time resolution of about 180 ps full-width at half-maximum for 3 ×3 mm2 SiPM, 80 ps for 1 ×1 mm2 SiPM and less than 50 ps for single cells. From measurements with pinholes placed in front of 1 ×1 mm2 detector we see a very good cell-to-cell uniformity: it is not a limiting factor for time resolution. We also characterized the timing jitter of SiPMs as a function of the number of photons per laser pulse (N) finding that it does not decrease exactly with the square root of N because of the optical crosstalk between cells.

NUV Silicon Photomultipliers With High Detection Efficiency and Reduced Delayed Correlated-Noise

In this paper, we present the characteristics and performances of new silicon photomultipliers (SiPMs), produced at FBK, for the near-ultraviolet (NUV) light detection, with reduced afterpulsing and delayed optical crosstalk. To study these components of the correlated noise, we manufactured SiPMs on silicon wafers featuring different substrate minority-carrier lifetime. This parameter proved to be crucial in determining the amount of delayed optical crosstalk and afterpulsing caused by photo-generated carriers diffusing from the substrate to the cell active region. With a very low substrate lifetime, we were able to minimize this correlated noise component to few percent at room temperature. Besides reducing the excess noise factor, the lower delayed correlated noise allows biasing the SiPM at higher voltages, reaching higher values of photon detection efficiency.

Performance of FBK high-density SiPM technology coupled to Ce:LYSO and Ce:GAGG for TOF-PET

This paper presents the performance, in terms of energy and timing resolution, of high-density silicon photomultipliers (SiPMs) produced at Fondazione Bruno Kessler for time-of-flight positron emission tomography application. The new SiPM technology allows us to produce devices with a small cell size maintaining a high fill factor (FF). The sensors considered in this paper are composed by 30 × 30 μm(2) cells with a FF exceeding 70% to cover a total area of 4 × 4 mm(2). The SiPM performance was evaluated using two types of scintillators (Ce:LYSO and Ce:GaGG) both with a short height (5 mm) in order to minimize the time jitter caused by light propagation in the crystal. With Ce:LYSO, an energy resolution of 9.0% FWHM at 511 keV and a coincidence resolving time (CRT) of 125 ps FWHM were obtained at -20 °C. With Ce:GaGG, an energy resolution of 6.4% FWHM and a CRT of 260 ps FWHM were achieved at the same temperature. The novel SiPM technology, combining a high PDE with a low correlated noise (i.e., crosstalk and afterpulse), allows us to improve the state-of-the-art of energy and timing resolution with both the tested crystals.

Performance of NUV-HD Silicon Photomultiplier Technology

In this paper, we present the full characterization of a new high-density (HD) cell silicon photomultiplier (SiPM) technology for ultraviolet (UV) and blue light detection, named near UV HD SiPM. Thanks to an optimized border region around each cell, we were able to develop devices having a very high detection efficiency and, at the same time, a high dynamic range. We produced SiPMs with a square cell pitch of 15, 20, 25, and 30 μm featuring a peak efficiency in the violet region ranging from 40% to 55%, according to the cell size. We tested this technology for time-of-flight positron emission tomography. Using two 4 × 4 mm2 SiPMs with a 25 × 25 μm2 cell pitch coupled to 3 × 3 × 5 mm3 LYSO scintillators, we reached for the first time 100-ps full-width at half-maximum coincidence time resolution. This result was independent of the temperature in a range from 20 °C to -20 °C. At the same time, thanks to the high dynamic range and low correlated noise, we obtained an energy resolution lower than 9% for 511-keV γ-rays.