Carsten Degenhardt

The digital silicon photomultiplier — Principle of operation and intrinsic detector performance

We developed a fully digital implementation of the Silicon Photomultiplier. The sensor is based on a single photon avalanche photodiode (SPAD) integrated in a standard CMOS process. Photons are detected directly by sensing the voltage at the SPAD anode using a dedicated cell electronics block next to each diode. This block also contains active quenching and recharge circuits as well as a one bit memory for the selective inhibit of detector cells. A balanced trigger network is used to propagate the trigger signal from all cells to the integrated time-to-digital converter (TDC). Photons are detected and counted as digital signals, thus making the sensor less susceptible to temperature variations and electronic noise. The integration with CMOS logic has the added benefit of low power consumption and possible integration of data post-processing. In this paper, we discuss the sensor architecture and present first measurements of the technology demonstrator test chip.

The digital Silicon Photomultiplier — A novel sensor for the detection of scintillation light

We developed a fully digital Silicon Photomultiplier (dSiPM) of 3.8 mm × 3.3 mm in size containing 8188 individual Geiger-mode cells. Each detected photon is directly converted into a digital signal in each of the Geiger-mode cells of the sensor. In addition, the complete trigger logic and a time-to-digital converter are integrated into the sensor. To show the performance of the sensor, LYSO crystals of different sizes were coupled to the sensor. The coincidence timing resolution for 3 mm × 3 mm × 5 mm LYSO crystals using a 22Na source amounts to 153 ps FWHM. The energy resolution at 511 keV was determined to be 10.7 % for 4 mm × 4 mm × 22 mm crystals. It is shown that saturation correction can be done without prior need for sensor calibration. The temperature dependence of the photon detection efficiency was found to be -0.6 %/°C including the temperature variation of the light output of LYSO.

The digital silicon photomultiplier — System architecture and performance evaluation

In this paper we present the first fully digital implementation of the Silicon Photomultiplier. The chip design is based on the technology demonstrator chip presented in [3]. The new sensor represents a self-contained detector including a JTAG controller for configuration and test, single-ended and differential clock and test input signals, an integrated acquisition controller and two serial data outputs. The sensor is based on a single photon avalanche photodiode (SPAD) technology integrated in a standard CMOS process flow. Photons are detected directly by sensing the voltage at the SPAD terminal using a dedicated cell electronics block next to each diode. This block also contains active quenching and recharge circuits as well as a one bit memory for the selective activation of individual detector cells. A balanced trigger network is used to propagate the trigger signal from all cells to the two integrated time-to-digital converters. Photons are detected and counted as digital signals, thus making the sensor less susceptible to temperature variations and electronic noise. The resulting data packets are transferred to the readout system through a serial data interface. In this paper, we discuss the new sensor architecture and evaluate its performance.

Arrays of digital Silicon Photomultipliers — Intrinsic performance and application to scintillator readout

Arrays of digital Silicon Photomultipliers (dSiPMs) have been developed. The arrays consist of 4 × 4 chips, each containing 2 × 2 dSiPMs, resulting in 8 × 8 dSiPMs on one array. With a pitch of 4 mm × 4 mm of the dSiPMs, the outer dimension of the array is 32 mm × 32 mm. To show the performance of the detector array, we investigated its intrinsic performance with respect to timing resolution using an external electronic trigger and picosecond laser pulses acting as an optical trigger. In addition, we show results of coupling arrays of 8 × 8 LYSO crystals with 4 mm × 4 mm pitch and 22 mm length to the detector arrays leading to a 1:1 coupling between the individual scintillator crystals and the dSiPM pixels. Two arrays have been operated in coincidence using 22Na as a source for annihilation gamma radiation in order to record energy and coincidence timing resolution.

Performance evaluation of a prototype Positron Emission Tomography scanner using Digital Photon Counters (DPC)

We show performance results of a prototype Positron Emission Tomography scanner based on digital SiPMs, or Digital Photon Counters (DPC), developed by the Philips Digital Photon Counting unit. The scalability of the DPC technology is demonstrated by an excellent system coincidence timing resolution of 266 ps FWHM and an energy resolution of 10.7 % FWHM. Even while using 4 mm × 4 mm × 22 mm LYSO crystals, the spatial resolution is close to 2.4 mm. Although not optimized yet, the image homogeneity is 5.8 %. We show that the system performance is maintained even at highest count rates encountered in PET scans.

Influence of chirped excitation pulses in an optical clock with ultracold calcium atoms

We have investigated the influence of frequency chirps of the excitation pulses in a time-domain optical Ramsey-Borde/spl acute/ atom-interferometer used in an optical clock with ultracold calcium atoms. With optical interferometry, we identify a ringing in the excitation pulses to be responsible for the observed frequency shifts. Using these results, the uncertainty of the latest frequency measurement could be reduced to 1.2/spl middot/10/sup -14/.

PhenoPET: A dedicated PET scanner for plant research based on digital SiPMs (DPCs)

In the framework of the German Plant Phenotyping Network (DPPN) we developed a novel PET scanner for imaging plants and crops. The observation of the carbon transport within the plant becomes possible by using 11CO2 as PET tracer. The use of the rather short living isotope C-11 asks for a scanner with high dynamic range. That means fast timing and high data rates are important features which let us choose the Philips Digital Photon Counter (DPC) as photo detector. Due to the fast photo detectors and the special crystal matrix arrangement the system will allow measurements with rather high activities. We could measure a coincidence resolution time of ~ 250 ps FWHM between two detector elements. This opens the opportunity to employ time-of-flight information for the first time on a PET scanner of this size. This paper presents very first results from a prototype single-ring system with a FOV of 18 cm diameter and 6.5 cm axial height.

Read-out electronics for digital silicon photomultiplier modules

This work has its focus on the development of fast read-out electronics for digital silicon photomultipliers (dSiPM -called Digital Photon Counter (DPC) by Philips).

Impact of intercrystal crosstalk on depth-of-interaction information in PET detectors

We have investigated the impact of intercrystal crosstalk on the spatial resolution of whole body PET detectors with different depth-of-interaction (DOI) readout schemes by validated Monte Carlo simulations. Readout schemes providing both discrete and continuous DOI information were analyzed. Detectors using scintillator crystals of different sizes and different stopping power (LYSO, LaBr3) were modeled. The results show a similar improvement for the spatial resolution at full width at half maximum across the field of view for all investigated methods. Concerning the resolution at full width at tenth maximum, the discrete approach using two offset layers of crystals is most affected by intercrystal scatter. Continuous DOI readout is least impaired by intercrystal scatter and shows the best results for the full width at tenth maximum.

Rate-dependence of the key performance parameters in an Anger logic based PET detector

The image quality of a PET scanner depends strongly on the spatial, energy, and time resolution of the detector. These parameters are usually specified for the case of low count rates. At higher rates, signals from subsequent events can overlap, and this pile-up deteriorates the resolution. In an Anger based detector, the high encoding ratio of scintillator pixels to photo multiplier tubes (PMTs) facilitates pile-up. The probability of pile-up depends on the detector geometry, the scintillator material, and the trigger scheme, and it increases with the count rate. In this paper, the key performance parameters of a whole-body PET detector are investigated as a function of the rate. Experimentally, we use scintillator arrays which can be coupled to PMT arrays with PMTs of different size, but otherwise similar characteristics. First results are presented which show-that signal pile-up leads to only small performance deterioration in typical /sup 18/F oncology studies, but has to be considered when using short lived tracers, e.g. based on /sup 11/C or /sup 15/O.