Henri Dautet

Photon counting techniques with silicon avalanche photodiodes.

The properties of avalanche photodiodes and associated electronics required for photon counting in the Geiger and the sub-Geiger modes are reviewed. When the Geiger mode is used, there are significant improvements reported in overall photon detection efficiencies (approaching 70% at 633 nm), and a timing jitter (under 200 ps) is achieved with passive quenching at high overvoltages (20-30 V). The results obtained by using an active-mode fast quench circuit capable of switching overvoltages as high as 15 V (giving photon detection efficiencies in the 50% range) with a dead time of less than 50 ns are reported. Larger diodes (up to 1 mm in diameter) that are usable in the Geiger mode and that have quantum efficiencies over 80% in the 500-800-nm range are also reported.

Properties of LYSO and recent LSO scintillators for phoswich PET detectors

The luminescence and nuclear spectroscopic properties of the new cerium-doped rare-earth scintillator lutetium-yttrium oxyorthosilicate (Lu/sub 0.6/Y/sub 1.4/Si/sub 0.5/:Ce, LYSO) were investigated and compared to those of both recent and older LSO crystals. UV-excited luminescent spectra outline important similarities between LYSO and LSO scintillators. The two distinct Ce1 and Ce2 luminescence mechanisms previously identified in LSO are also present in LYSO scintillators. The energy and timing resolutions were measured using avalanche photodiode (APD) and photomultiplier tube (PMT) readouts. The dependence of energy resolution on gamma-ray energy was also assessed to unveil the crystal intrinsic resolution parameters. In spite of significant progress in light output and luminescence properties, the energy resolution of these scintillators appears to still suffer from an excess variance in the number of scintillation photons. Pulse-shape discrimination between LYSO and LSO scintillators has been successfully achieved in phoswich assemblies, confirming LYSO, with a sufficient amount of yttrium to modify the decay time, to be a potential candidate for depth-of-interaction determination in multicrystal PET detectors.

Investigation of depth-of-interaction by pulse shape discrimination in multicrystal detectors read out by avalanche photodiodes

The measurement of depth of interaction (DOI) within detectors is necessary to improve resolution uniformity across the FOV of small diameter PET scanners. DOI encoding by pulse shape discrimination (PSD) has definite advantages as it requires only one readout per pixel and it allows DOI measurement of photoelectric and Compton events. The PSD time characteristics of various scintillators were studied with avalanche photodiodes (APD) and the identification capability was tested in multi-crystal assemblies with up to four scintillators. In the PSD time spectrum of an APD-GSO/LSO/BGO/CsI(Tl) assembly, four distinct time peaks at 45, 26, 88 and 150 ns relative to a fast test pulse, having resolution of 10.6, 5.2, 20 and 27 ns, can be easily separated. Whereas the number and position of scintillators in the multi-crystal assemblies affect detector performance, the ability to identify crystals is not compromised. Compton events have a significant effect on PSD accuracy, suggesting that photopeak energy gating should be used for better crystal identification. However, more sophisticated PSD techniques using parametric time-energy histograms can also improve crystal identification in cases where PSD time or energy discrimination alone is inadequate. These results confirm the feasibility of PSD DOI encoding with APD-based detectors for PET.

A short-wavelength selective reach-through avalanche photodiode

A new reach-through avalanche photodiode, designed for use with sources of short-wavelength light such as scintillators, is described. The device has a double junction p/sup +/-p-n-p/sup -/-n/sup +/ structure in which the central three layers, which comprise about 99% of the device thickness, are fully depleted. The p/sup +/ light-entry surface extends across the whole device and can be placed in contact with a scintillator. The multiplying p-n junction is buried and is located about 4 /spl mu/m below the p/sup +/-layer so that only primary photo-electrons generated by short-wavelength (i.e., strongly absorbed) light are fully multiplied. The p/sup -/-n/sup +/ junction, or array of junctions, is located at the back of the wafer and is surrounded by a guard-ring. Typical characteristics for a device 120 /spl mu/m thick and having a 25 mm/sup 2/ sensitive area, are a quantum efficiency (Q.E.) of 80% at 480 nm, a capacitance of 30 pF, operating voltage of <500 V, a speed of response of /spl sim/3 ns, a noise current of less than 1 pA/Hz/sup 1/2/ at a gain of 100, and an effective k value of .030.

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.

Measurement of proton radiation damage to Si avalanche photodiodes

The effects of proton radiation damage on EG&G C30902S Si avalanche photodiodes (APDs) were measured. The APD bulk leakage current increased at 0.29 fA/rad, or about 1800 dark photoelectrons per rad(Si) at -10/spl deg/C under 16.2 MeV protons. There was little change in the breakdown voltage with the radiation doses up to 30 krad(Si). The increase in the total dark currents below the breakdown voltage was insignificant until 3 krad(Si).

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.

An LSO BLOCK detector for PET using an avalanche photodiode array

A Block detector, consisting of a four by four array of lutetium oxyorthosilicate (LSO) scintillator crystals coupled to a two by two avalanche photodiode (APD) array was built and tested. The detector block was 8.5 by 8.5 by 10 millimeters so that the crystals were on 2.13 millimeter centers. The APD array has an active area of approximately 9 by 9 millimeters divided into four diodes of 4.5 by 4.5 millimeters each. A standard reach-through process was used to fabricate the diodes. Each of the sixteen crystals in the block was easily identified. The average FWHM of the peaks in ratio space was 4% of full scale. The average energy resolution was 16.2% for 511 keV gamma rays. A single LSO crystal coupled to an APD and put in time coincidence with a plastic detector produced a time coincidence of 1.9 nanoseconds FWHM.

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.

Investigation of the properties of new scintillator LYSO and recent LSO scintillators for phoswich PET detectors

The luminescence and nuclear spectroscopic properties of the new cerium-doped rare-earth scintillator lutetium-yttrium oxyorthosilicate (Lu/sub 0.6/Y/sub 1.4/SiO/sub 5/:Ce, LYSO) were investigated and compared to those of both recent and older LSO crystals. UV-excited luminescent spectra outline important similarities between LYSO and LSO scintillators. The two distinct Ce1 and Ce2 luminescence mechanisms previously identified in LSO are also present in LYSO scintillators. The energy and timing resolutions were measured using avalanche photodiode (APD) and photomultiplier tube (PMT) readouts. The dependence of energy resolution on gamma-ray energy was also assessed to unveil the crystal intrinsic resolution parameters. In spite of significant progress in light output and luminescence properties, the energy resolution of these scintillators appears to still suffer from an excess variance in the number or scintillation photons. Pulse-shape discrimination between LYSO and LSO scintillators has been successfully achieved in phoswich assemblies, confirming LYSO to be a potential candidate for depth-of-interaction determination in multi-crystal PET detectors.