Claude Pépin

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.

The vibration-rotation spectrum of methinophosphide: The overtone bands 2ν1 and 2ν3, the summation bands ν1 + ν2 and ν2 + ν3, and the difference band ν1 - ν2

Abstract The vibration-rotation bands 2 ν 1 , 2 ν 3 , ν 1 + ν 2 , ν 2 + ν 3 , and ν 1 - ν 2 of H 12 CP were recorded and analyzed. These data were combined with previously reported results for this molecule to obtain an improved and extended set of vibrational and rotational constants. All x ij and γ ij were calculated except those that require data from a summation band involving ν 1 and ν 3 .

High-resolution infrared spectrum of the ν3 and ν2 + ν3 − ν2 bands of 14N16O2

Abstract The infrared absorption spectrum of the ν 3 band of 14 NO 2 has been recorded with a resolution and a frequency accuracy much improved over the previous investigations. The K - and N -line assignments have been greatly extended and a more accurate set of spectroscopic constants derived. Several lines in the subbands with K a ≥ 3 have been observed to be doubled by spin-rotation interaction and spin-rotation interaction constants have been obtained. Several weak series of lines in the spectrum ( K a = 0, 1, 2, and 3) have been unambiguously assigned to the “hot band” ν 2 + ν 3 − ν 2 . Lines of the K a = 3, 4, 5, and 6 sub-bands of ν 3 have been found to be perturbed by a Coriolis interaction with the K a = 4, 5, 6, and 7 levels of 2 ν 2 .

The architecture of LabPET/spl trade/, a small animal APD-based digital PET scanner

The highly multiplexed, integrated analog processing front-end of current PET scanners yields high accuracy for timing and crystal identification, but also adds significant dead time and offers little flexibility for improvement. A new fully digital APD-based scanner architecture is proposed wherein nuclear pulses are sampled directly at the output of the charge sensitive preamplifier with one free-running ADC per channel. This approach offers the opportunity to explore new digital signal processing algorithms borrowed to command and control theory, as well as advanced heuristics such as neural networks. The analog front-end consists of a dedicated 0.18-mum, 16-channel CMOS charge sensitive preamplifier. Digitization is performed with off-the-shelf dual 8-bit 45-MHz analog-to-digital converters. Digital processing is concentrated in a dual processor FPGA and a digital signal processor (DSP), which can process the data from up to 64 parallel channels with no dead time. The FPGA deals with the initial signal analysis for energy measurement and time stamping, while crystal identification is deferred to the DSP running computation-intensive auto-regressive algorithms. The entire system is controlled serially through a Firewire link by a graphic user interface. The initial LabPETtrade implementation of the system is a dedicated small animal scanner holding up to 3072 APD channels at an averaged count rate of up to 10 000 events/s each

Real time digital signal processing implementation for an APD-based PET scanner with phoswich detectors

Recent progress in advanced digital signal processing provides an opportunity to expand the computation power required for real time extraction of event characteristics in APD-based positron emission tomography (PET) scanners. These developments are made possible by a highly parallel data acquisition (DAQ) system based on an integrated analog front-end and a high-speed fully digital signal processing section that directly samples the output of each preamplifier with a free-running, off-the-shelf, 45-MHz MAX1193 analog-to-digital converter that feeds the sampled data into a field programmable gate array VirtexII PRO from Xilinx. This FPGA features ~31,000 logic cells and 2 PowerPC processors, which allows up to 64 channels to be processed simultaneously. Each channel has its own digital signal processing chain including a trigger, a baseline restorer and a timestamp algorithm. Various timestamp algorithms have been tested so far, achieving a coincidence timing resolution of 3.2 ns FWHM for APD-LSO and 11.4 ns FWHM for APD-BGO detectors, respectively. Channels are then multiplexed into a TMS320C6414 DSP processor from Texas Instruments for crystal identification by an ARMAX recursive algorithm borrowed from identification and vector quantization theory. The system can sustain an event rate of 10 000 events/s/channel without electronic dead time

Scintillation light emission studies of LSO scintillators

UV and /spl gamma/-ray excited luminescence and nuclear spectroscopy were used to study the relationship between the scintillation mechanisms of LSO and the spectroscopic characteristics obtained with PMT and APD readouts at room temperature. No correlation was found between scintillation decay time and light output. Like other investigators, we observed the existence of two distinct luminescence centers, Ce1 and Ce2, that mainly give rise to short (420 nm) and long (440 nm) emission wavelengths. Our measurements showed that different LSO crystals excited by /spl gamma/-rays have emission spectra with largely different shapes and maxima depending on the relative population and luminescence efficiency of these centers. It was also found that the poor energy resolution of LSO and YSO scintillators is well correlated with the coexistence of the two competing luminescence mechanisms. The prevalence of either Ce1 or Ce2 luminescence tends to reduce the variance of light emission and, thus, to improve energy resolution. Inversely, the coexistence of the two centers increases variance and degrades energy resolution.

Time discrimination techniques using artificial neural networks for positron emission tomography

Relevant information in positron emission tomography is currently being obtained mostly by analog signal-processing methods. New digital PET scanner architectures are now becoming available, which offer greater flexibility and easier reconfiguration capability as compared to previous PET designs. Moreover, new strategies can be devised to extract more information with better accuracy from the digitized detector signals. Trained artificial neural networks (ANN) have been investigated to improve coincidence timing resolution with different types of Avalanche PhotoDiode (APD)-based detectors. The signal at the output of a charge-sensitive preamplifier was digitized with an off-the-shelf, free-running 100-MHz, 8-bit analog-to-digital converter and time discrimination was performed with ANNs implemented in field-programmable gate array (FPGA). Results show that ANNs can be particularly efficient with slow and low light output scintillators, such as BGO, but less so with faster luminous crystals, such as LSO. In reference to a fast PMT-plastic detector, a time resolution of 6.5 ns was achieved with a BGO-APD detector. With LSO, the ANN was found to be competitive with other digital techniques developed in previous works. ANNs implemented in FPGAs provide a fast and flexible circuit that can be easily reconfigured to accommodate various detectors under different signal/noise conditions.

Comparison of LSO, LGSO and MLS scintillators

The luminescence and nuclear spectroscopic properties of recent LSO, LGSO and MLS scintillators were investigated and compared to those of older LSO and LGSO crystals. UV-excited luminescent spectra outline important similarities between these scintillators. The two distinct luminescence mechanisms previously identified in LSO appear to be present in all crystals. The relative light yield and energy resolution were assessed using an avalanche photodiode (APD) readout. The energy resolution of all scintillators is found to be affected by an important systematic contribution that seriously limits the crystal resolution, irrespective of the light yield.

Subpicosecond excitation of strongly coupled porphyrin–phthalocyanine mixed dimers

Electrostatic heterodimers are formed in the liquid phase by pairing a zinc porphyrin with either a copper or an aluminium phthalocyanine bearing oppositely charged substitutents. The ground-state absorption spectra of such heterodimers are drastically changed with respect to the corresponding monomers, indicating the existence of a strong interaction between the two chromophores. Irrespective of the nature of the metal ions and of the peripheral substituents, the heterodimers present very similar ground-state spectra.The photophysical properties of the two heterodimers are investigated with the use of femto- and nano-second absorption spectroscopies. Excitation of the zinc porphyrin–aluminium phthalocyanine heterodimer at 565 or 620 nm is followed by a very efficient electron transfer from the porphyrin to the phthalocyanine moiety while a very efficient intersystem conversion takes place in the excited zinc porphyrin–copper phthalocyanine, leading to the final ‘triplet’ excimer. The difference in behaviour is analysed and explained in terms of the thermodynamics of the transfer process and of the peculiar properties of the paramagnetic copper phthalocyanine.

High resolution infrared measurements on the ν3 vibration-rotation band of 28SiH4

Abstract The ν 3 vibration-rotation band of silane ( 28 SiH 4 ) has been measured at high resolution (0.030–0.045 cm −1 ). The tetrahedral splitting has been partially resolved. The analysis of the band has yielded an improved set of molecular constants.