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A growing interest in the development of dual modality PET/CT scanners prompts the comparative study of numerous scintillators to select the best one, which could be used simultaneously in PET detectors working in the pulsing mode and in the CT detectors working in the current mode. In the comparative measurements, done in the same experimental conditions, various samples of BGO, GSO, GSO:Ce, Zr, LGSO, LSO, LYSO, MLS, La[C1.sub.3], La[Br.sub.3] and CWO scintillators were tested. The measurements covered a determination of the light output, energy resolution, non-proportionality of the light yield, decay times of the light pulses and for the selected crystals their time resolution for 511 keV annihilation quanta. Moreover, a comparative study of afterglow, induced by 60 keV [gamma]-rays from a strong [sup.241]Am source (13.9 GBq), was done in the second range of time. The LSO-like crystals are the best in the PET scanners application. However, they do not fit to the CT requirements, due to a high afterglow. The studies conclude that besides of the well known BGO, only GSO:Ce and most likely La[Br.sub.3] might be considered for the simultaneous PET/CT detector. Index Terms--Image analysis and processing, NMIS, PET instrumentation, reconstruction algorithms.

The growing interest in time-of-flight PET triggered the study of the time resolution obtainable with a 4 x 4 x 20 [mm.sup.3] LSO crystal coupled directly to the center of a 52 mm in diameter Photonis XP20D0 photomultiplier as well as the time resolution obtainable with the use of an 11 mm thick Lucite light diffuser that simulates the conditions in typical PET block detectors. The LSO crystal directly coupled to the PMT yielded a time resolution of 166 [+ or -] 5 ps, while in the case of light readout with the use of the light diffuser it degraded to 196 [+ or -] 5 ps and 277 [+ or -] 6 ps in the center and at the edge of the PMT, respectively. The light diffuser was coated on the sides with black tape to absorb light and to approximate in this way the realistic performance of a future block detector. Similar time resolution was obtained by coupling the LSO crystal either to the Photonis XP20D0 PMT or to a very fast 25 mm diameter Hamamatsu R5320 PMT. These results illustrate the advantages of the very low time jitter of the Hamamatsu PMT on one side, and high quantum efficiency and a screening grid at the anode of the Photonis PMT, on the other. This study strongly suggests that time-of-flight PET based on LSO crystals is a realistic proposition for the further development. Index Terms--Fast PMTs, fast timing, LSO scintillator, time-of-flight PET.

The properties of CdW[O.sub.4] (CWO) crystals in gamma spectrometry were studied. Several small samples of 10 x 10 x 3 mm size, typically used in CT X-ray detectors, were tested and then compared to the performance of a larger crystal of 20 mm in diameter and 20 mm in height. The light output, energy resolution, and nonproportionality of the CWO response versus gamma-ray energy, were measured and compared with those of a small BGO to discuss further the origin of the intrinsic resolution of pure undoped scintillating crystals. A high light output of 6500 [+ or -] 200 phe/MeV and a good energy resolution of 6.6 [+ or -] 0.2% for 662 keV gamma rays from a [sup.137]Cs source were measured for the small samples coupled to an XP3212 photomultiplier. Common nonproportionality curves and consequently common intrinsic resolutions of small CWO and BGO suggest that they represent fundamental characteristics of the heavy oxide scintillating material themselves. Index Terms--CdW[O.sub.4], energy resolution, non-proportionality, scintillators.

This work presents the results from experiments performed on [Lu.sub.2]Si[O.sub.5] :Ce (LSO:Ce). It looks for potential correlations of light yield nonproportionality and other scintillation properties such as intrinsic energy resolution with thermoluminescence properties. The analyzed samples were chosen from various crystal batches having significantly different properties. Single crystal LSO:Ce samples were coupled to an XP2020Q photomultiplier and to large area avalanche photodiodes and measured as a function of [gamma]-ray energy between 14.7 and 1770 keV at room temperature and also near liquid nitrogen temperature. To explain the experimental results obtained from spectrometric methods, the properties of the samples were further studied using thermoluminescence techniques within the temperature range from 250 K to 600 K. The relationships between data obtained from these two types of experiments are reported.

The performance of a new Photonis XP1472 ninechannel photomultiplier (PMT) was studied with the main emphasis to the photoelectron numbers and photoelectron collection efficiency, as measured with a 10 mm diameter NaI(TI) crystal coupled to the separate channels. Simultaneously, energy resolution and crosstalk were measured for all channels with a a37Cs source. Time jitter and time resolution for an LSO crystal coupled to the different channels were measured for 511 keV annihilation quanta and compared to those measured earlier with XP2020Q PMTs. The studies confirmed high photoelectron collection efficiency in the PMT due to a new design of the input focalization system. Index Terms--Fast timing, PET detectors, multi-channel photomultipliers, photomultipliers.

Performances of new fast XP20Y0/DA and XP20Y0Q/DA 8 stage photomultipliers (PMTs) equipped with a screening grid at the anode were studied. The time jitter of the new PMTs was measured using a fast light pulser based on an XP22 light emitting diode. The timing properties in coincidence experiments were measured with Ba[F.sub.2], LSO, and La[Cl.sub.3] : Ce crystals for [sup.60]Co and [sup.22]Na gamma rays and discussed in terms of the measured photoelectron numbers. Particularly, the normalized time resolutions to the number of photoelectrons measured with Ba[F.sub.2] crystals were comparable to that measured in the past with the XP2020Q in a superior dynode-timing mode. A significant improvement of timing up to a factor of 1.2 due to the grid at the anode was shown also in the measurements with LSO and La[Cl.sub.3]:Ce crystals. Index Terms--Fast timing with Ba[F.sub.2], La[Cl.sub.3] : Ce, and LSO scintillators, photomuitipliers, scintillation detectors.

We have studied the performance of finger-like LSO:Ce (LSO) crystals coupled one by one to pixels of avalanche photodiode detector (APD) arrays during their operation in coincidence at [sup.12]C ion beams of parameters being typical for tumor irradiations. In a first step of these experiments the parameters of the detectors and the signal processing setup have been characterized off-beam, i.e., by means of [[beta].sup.+] radioactive sources ([sup.22]Na, [sup.68]Ge). Afterwards, the apparatus was installed at the medical beam line of the heavy ion synchrotron (SIS) of the Gesellschaft fur Schwerionenforschung (GSI) at Darmstadt, Germany. Here the [[beta].sup.+] activity produced by nuclear fragmentation reactions of [sup.12]C 200.3 AMeV heavy ion beams with polymerized methyl methacrylate phantoms were measured. Furthermore, a [sup.68]Ge source was included into the in-beam experiment, in order to check the stability of the setup and to compare energy and time resolution before, during, and after the phantom irradiations. Additionally, it could be demonstrated by means of high resolution [gamma]-ray spectroscopy that LSO is not activated by the light projectile fragments escaping the patients downbeam during therapeutic irradiations The experimental results indicate that LSO scintillator is a suitable material for in-beam positron emission tomography (PET) and, furthermore, the LSO/APD array is a feasible detector concept for in-beam monitoring of the dose application by means of PET.

The intrinsic energy resolution and nonproportionality of the light yield as a function of gamma ray energies, in the energy range of 14 keV to 1.33 MeV, were studied for small BGO ([Bi.sub.4] [Ge.sub.3] [O.sub.12]) crystals at room and liquid nitrogen (L[N.sub.2]) temperatures. The study showed that the intrinsic resolution of BGO and the light yield nonproportionality as a function of energy do not depend on the crystal temperature. High light outputs of 14000 [+ or -] 300 electron-hole pairs and energy resolution of 6.5% [+ or -] 0.2% for 662 keV gamma rays were measured with the 9 mm diameter, 4 mm thick crystals, coupled to large area avalanche photodiodes and cooled down to L[N.sub.2] temperature. Special attention was paid to analyzing the energy resolution of the escape peaks, which were well separated from the full-energy peaks due to the good energy resolution of BGO at L[N.sub.2] temperatures and the energy of bismuth KX-rays. The intrinsic energy resolution of the BGO crystal for escape peaks does not show deviations from analysis of total energy absorption photopeaks. This is in spite of the fact that the contributions of X-rays and Auger electron cascade in creation of escape peaks are much smaller than in full-energy peaks. In the small volume crystal, mainly electrons produced in the photoelectric absorption create the escape peaks. Index Terms--Avalanche photodiodes, BGO, energy resolution, light yield nonproportionality, scintillators.

Three pure NaI crystals grown in different batches by Scionix (The Netherlands) were studied at room and liquid nitrogen temperatures. The room temperature studies performed with photomultiplier readout covered measurements of emission spectra, photoelectrons yield and energy resolution. Moreover, the scintillation light pulses were measured by single photon method. Two crystals showed the photoelectron yield of about 1000 phe/MeV measured at room temperature with an XP2020Q photomultiplier. The energy resolution of the 662 keV photopeak was measured to be about 16% and was limited mainly by a low number of photoelectrons and a large nonproportionality of the light yield. The fast light pulse had the main decay time components of 1 and 5 ns. The high initial photon intensity of the tested NaI crystals revealed a good timing resolution of about 140 ps obtained for [sup.60]Co [gamma]-rays. At liquid nitrogen temperatures the measurements of light yield and energy resolution were performed using an Advanced Photonix, Inc. avalanche photodiode readout. A high light output above 80 000 ph/MeV was measured for the best of the tested crystals. The energy resolutions of the 662 keV photopeak of the three crystals differ a lot, varying from an excellent value of 3.8% to 6.2%. This suggests that the measured energy resolutions depend on the purity of the scintillator samples reflected in the emission spectra. The studies that were performed demonstrate the attractive properties of pure NaI at both room and [LN.sub.2] temperature. Index Terms--Avalanche photodiodes (APDs), energy resolution, nonproportionality of light yield, pure NaI, scintillators.

In this paper, we evaluate the performance of recently released avalanche photodiode arrays from Advanced Photonix, Inc., for scintillator matrices readout. The large-area avalanche photodiode (LAAPD) quadrant device is a monolithic 2 x 2 pixel structure with an active area of 5.3 [mm.sup.2] per pixel. The device allows stable operation with high gains up to 200 and detection efficiency of 73~ [+ or -] 10% for 420-nm photons. It is furthermore characterized by a pixel linear fill factor of 90% and low noise equal to 16 electrons equivalent noise charge measured at room temperature. We have measured pixel-to-pixel gain nonuniformity smaller than 1.5% and interpixel crosstalk of 1.7% in pulse-mode operation with a device gain of 50. An energy resolution of 12.3 [+ or -] 0.5% was achieved for the 511-keV photopeak from a [sup.22]Na source placed on top of a 2 x 2 x 10 [mm.sup.3] LSO crystal coupled to one pixel of a quadrant LAAPD. An array of LSO crystals was later coupled to the device allowing individual crystal identification. A coincidence timing resolution of 1.9 [+ or -] 0.1 ns full-width at half-maximum has been obtained for quadrant pixel with LSO scintillator for the 511-keV peak from a [sup.22]Na source. Finally, we compared the characteristics and readout performance of the LAAPD array with published results of some commercially available avalanche photodiode arrays. Index Terms--Position sensitive avalanche photodiodes, positron emission tomography, scintillation matrices.

We report on the performance of beveled-edge large-area avalanche photodiodes (LAAPDs) operating at liquid nitrogen (L[N.sub.2]) temperatures. The twofold purpose was to evaluate performance characteristics of LAAPD devices and feasibility of their application for scintillation light read out at L[N.sub.2] temperatures. The measurements were carried out with unpackaged 5-and 16-mm-diameter bare LAAPD devices cooled in a modified cryostat used for Si(Li) X-ray detectors. Light pulses from a green light-emitting diode and X-rays from a [sup.55]Fe source were used to characterize the performance of LAAPDs. Dark current below 1 pA was measured for a 16-mm LAAPD at a gain of 5000. The ratios of the LAAPD gain for X-rays and light pulses, the excess noise factor, and the dark noise contribution of the LAAPD preamplifier system were measured in device gain range up to 4000. Pulse-height spectra of about three photons were recorded with a 5-mm LAAPD operating at 100-K temperature and gain of 4000. A pure NaI crystal coupled to a 16-mm LAAPD was investigated, showing energy resolution of 5.9 [+ or -] 0.2% for 662-keV [gamma]-rays from a [sup.137]Cs source. Index Terms--Avalanche photodiodes at liquid nitrogen temperatures, pure NaI, scintillation detectors.

Prompted by recent interest in positron emission tomography scanners equipped with LuAP:Ce (LuAl[O.sub.3]:Ce) scintillators read by avalanche photodiodes, we have measured timing properties of this scintillating material with large-area avalanche photodiodes (LAAPDs) from Advanced Photonix, Inc. (API). We have obtained a time resolution of 1.16 [+ or -] 0.06 ns for 511-keV peak from a [sup.22]Na source with energy threshold set at 400 keV and 680 [+ or -] 35 ps for [sup.60]Co source with energy threshold set at 1 MeV. The dependence of time resolution on LAAPD gain was measured showing improved timing at gains over 200, but at the cost of higher excess noise. Comparing the time resolution of LuAP:Ce and LSO:Ce measured with LAAPD and photomultipliers, we discuss the dependence of timing properties on the light output and decay time of these scintillators. Index Terms--Avalanche photodiodes (APDs), lutetiium compounds, scintillation detector, timing.

The energy resolution of small NaI(T1), CsI(T1), BGO, GSO, YAP and LSO crystals has been studied using 16 mm diameter large area avalanche photodiodes (LAAPD) and a 52 mm diameter photomultiplier. The best result of 4.8% for 662 keV [Gamma]-rays from a 137Cs source was obtained with a 9 mm in diameter by 9 mm high CsI(T1) scintillator coupled to an LAAPD. Measuring the number of primary electron-hole pairs produced in the LAAPD and photoelectrons in the photomultiplier, as well as the noise contribution of the LAAPD, allowed a quantitative discussion of the results. The energy resolutions measured with LAAPDs are comparable to, or significantly better (at certain emission wavelengths) than, those obtained with the photomultiplier. At energies above 100 keV the energy resolution measured with the majority of crystals and the LAAPD was weakly affected by the photodiode noise contribution. The advantages and limitations of LAAPDs in energy spectrometry with scintillation detectors are also discussed.

The absolute light outputs of BGO, CsI(T1) and some new Ce-doped crystals have been measured to an accuracy of about [+ or -]5% using calibrated XP2020Q photomultipliers and standard S3590-03 and S2740-03 photodiodes. The use of small crystals, 9 mm in diameter and 1 mm thick, reduces the corrections for imperfections in the light collection process and in the photoelectron collection by the photomultipliers. The measured light output of 8500 [+ or -]350 ph/MeV (photons/MeV) for the BGO crystals agrees well with the earlier measurement done by Holl et al. The carried out study highlighted the importance of the spread in the published emission spectra of the crystals which seems to limit measurement accuracy. Finally, a simple comparative method of measuring light output to an accuracy of [+ or -]10%, is proposed using an uncalibrated XP2020 photomultiplier and a 1 mm thick BGO crystal as a standard.

Scintillation detectors consisting of LSO, LuAP and YAP crystals fitted to bevelled-edge large area avalanche photodiodes of 10 mm in diameter characterised by a high quantum efficiency up to 68% at 350 nm wavelength were studied. Among the properties measured were the number of electron-hole pairs, energy resolution and noise contribution of the LAAPDs at shaping time constants down to 20 ns at different gains. High electron-hole pair numbers of 11000[+ or -]550 e-h/MeV and 19500[+ or -]980 e-h/MeV for the YAP and LSO crystals were measured, respectively. The energy resolution for 137Cs [Gamma]-rays of 5.9% in FWHM for the YAP crystal, much better than that of 9.9% for the LSO crystal were measured at 0.5 [[micro]seconds] shaping time constant and the APD gain of 40. The same study done with a fast shaping of 20 ns and the APD gain of 160 showed the energy resolution of 6.7%.

The detection properties of 10 mm and 16 mm diameter bevelled-edge avalanche photodiodes fitted with LSO crystal were measured using [Gamma]-sources. Among the properties measured were the electron-hole pair yield, time resolution and energy resolution. These detectors provide excellent time resolution of 570[+ or -]17 ps for 60Co [Gamma]-rays with the energy threshold set at 1 MeV. The energy resolution of the 662 keV [Gamma]-peak from a 137Cs source was measured to be 11% FWHM with a shaping time constant of 0.1 [[micro]seconds]. A comparative discussion of the observed time resolution of these large area APDs emphasizes that APD gain, its quantum efficiency, and capacitance are important in timing applications.