Maciej Kapusta

Intrinsic energy resolution of NaI(Tl)

The light output for +10 mm � 10 mm and +75 mm � 75 mm NaI(Tl) crystals and energy resolution were measured for g-ray energies ranging from 16–1333 keV. These measurements enabled the observation of the light yield nonproportionality behavior and allowed the determination of the intrinsic resolution after correctingfor the measured resolution for photomultiplier tube (PMT) statistics. The intrinsic resolution was then compared with the nonproportionality component. The latter was calculated usingmeasured electron response, Monte Carlo N Particle code (MCNP4B), and the simplified cascade sequence for NaI(Tl). This comparison allowed the identification of the intrinsic resolution component associated with d-rays. Consequently, it was shown that the d-ray component is the most dominant component of the NaI(Tl) intrinsic resolution. r 2002 Elsevier Science B.V. All rights reserved.

Fast Photomultipliers for TOF PET

Presently, a major of detectors for PET systems are based on scintillator crystals read by photomultipliers. In our previous work, a very good time resolution recorded with a 10x10x5 mm3 LSO crystal coupled to a Photonis XP20D0 and Hamamatsu R5320 photomultipliers was shown. Results for both detectors were almost identical and close to 170 ps but their properties such us quantum efficiency of the photocathode and time jitter were significantly different. The XP20D0 possessed high QE and the measured photoelectron number was 40% larger than that of R5320. The R5320 had a time jitter of 140 plusmn 7 ps at FWHM, three times better than that of XP20D0. The fact, that despite of large differences in parameters of the used PMTs a comparable time resolution was achieved, triggered our further study of the most important properties of the photomultipliers and their influence on timing and energy resolution towards to optimize TOF PET systems. Thanks to a close cooperation with Photonis, during last few years we gathered a large amount of information and experimental data of various types of PMTs. The aim of this paper is to present general conclusions and dependencies that were derived from these multiple experiments. This should help to develop an ultimate, PMT based detector, for TOF PET systems.

Characterization of Scintillators by Modern Photomultipliers—A New Source of Errors

The observed discrepancy in the light output, measured for a number of LSO, LYSO and BGO scintillators by different photomultipliers (PMTs), triggered studies to understand the problem. For that purpose the photoelectron number was measured by two different methods: the direct one based on a comparison of the full energy peak to that of the single photoelectron and by a method based on the pulse height resolution of the peak due to the light pulser. In this study, a significant number of different PMTs from Photonis and Hamamatsu were used. We concluded that the number of photoelectrons measured by means of the direct method was higher than the number of photoelectrons calculated from the pulse height resolution of the light pulser peak for all of the PMTs but XP2020Q. It leads to a large dispersion in the estimated light output for a given scintillator. In detail, the light output of BGO and LSO determined with the R6231 and R2059 PMTs is comparable to those measured with XP2020Q PMT and the S3590-18 pin photodiode, when photoelectron number is calculated from the pulse height resolution. Further in-depth studies of the photoelectron number at different bias voltages suggested that the effect is related to the space charge created in the dynode structure of the PMTs. Operation of PMTs at lower bias/gain minimizes this effect; thus, low noise electronics are recommended to determine the single photoelectron peak under these conditions. Moreover, the absolute light output of scintillators is affected by differences in the quantum efficiency calibrations by Photonis and Hamamatsu.

/sup 6/LiI(Eu) in neutron and /spl gamma/-ray spectrometry-a highly sensitive thermal neutron detector

Europium doped 6LiI crystals (enriched to 96%6 Li) have been studied in neutron detection and gamma-ray spectrometry. Two crystals, 50 mmtimes5 mm and 30 mmtimes3 mm in size were coupled to a calibrated Photonis XP5200 photomultiplier, were tested. The response of 6LiI(Eu) to neutrons emitted from a paraffin moderated Pu-Be source has been investigated and the thermal neutron peak has been found at a Gamma Equivalent Energy (GEE) of about 3.5 MeV. The high sensitivity of the 6LiI(Eu) crystal is demonstrated by the observation of a neutron peak at a dose equivalent as low as 0.05 muSv/h. Apart from the neutron response, the light output, energy resolution and nonproportionality of the 6LiI(Eu) crystals versus gamma-ray energies have been measured. The light yield of 1.5times104 ph/MeV (about 40% of NaI(Tl)) was obtained and the energy resolution at 662 keV (137 Cs) of 7.5plusmn0.1% was found for the large crystal. The linearity of the light output with energy has been measured and is better compared to NaI(Tl). Due to the high sensitivity to thermal neutrons and good proportionality against gamma-ray energy, the 6 LiI(Eu) crystal was tested with a few samples of nuclear materials. Shielded samples can be recognized via detection of neutrons following spontaneous fission

Energy resolution of small scintillation detectors with SiPM light readout

The development of silicon photomultipliers (SiPMs) with a large number of APD cells and improved linearity of the pulse height response prompted interest in their application to gamma spectrometry with scintillators. Hamamatsu MPPC sensors equipped with 3600 and 14400 APD cells were chosen in our study because of their well pronounced single photoelectron spectra, which allowed us to precisely measure the photoelectron numbers (PHE) or fired APD cells and then to discuss, in a quantitative manner, the obtainable energy resolution. The studied detectors were first characterized in direct detection of laser light pulses and then in gamma spectroscopy with LFS and CsI:Tl crystals. In the study with the laser light pulses the linearity of the MPPC response versus a light pulse intensity monitored with PMT was measured. Two different methods were used for an evaluation of the MPPC response expressed in the number of photoelectrons (PHE) generated by light illumination. The direct method (PHEdir), based on the comparison of the light peak position to that of the single photoelectron peak, determined the upper limit of the PHE. The lower limit of the PHE was derived from an analysis of the measured pulse height resolution under the assumption of Poisson statistics and MPPC excess noise factor (ENF) of 1. Furthermore, the ENF of the MPPC is discussed with respect to the contributions of device dead time, optical cross-talk and after-pulses to the results obtained. In the scintillation tests, measurements of energy resolution and non-proportionality of the light yield were performed with LFS and CsI:Tl crystals, and both types of 3 × 3 mm MPPC detectors were used for light readout. The results are discussed in a quantitative manner based on the measured PHE.

Breakthrough in quantum efficiency of bi-alkali photocathodes PMTs

Bi-alkali type photocathodes are commonly used in classical photomultipliers. They provide low noise and high blue and near UV sensitivity well matched to scintillation light. The first breakthrough enhancement of standard bi-alkali photocathodes brought to the market PMTs characterized by quantum efficiencies of 30 % at 400 nm. Through a detailed understanding of the creation process of the photocathode and its optimization PHOTONIS has successfully obtained a production process achieving an average quantum efficiency of 38 % for a PMT bi-alkali photocathode. Further enhancement of the PMTs quantum efficiency was obtained by utilizing a special tulip shaped PMT glass envelope to reflect back to the photocathode the light which initially passed through without interaction. This work reports on the spectral measurements of the new CLARITY XP family of PMTs and their performance in spectrometry with different scintillators.

Development of MRI-Compatible Nuclear Medicine Imaging Detectors

The value of pre-clinical PET and MRI is established, and SPECT also is now routinely used in drug development, disease treatment, and molecular biology research. The aim of this research is to prepare and test imaging prototypes of both SPECT and PET with similar form-factors for use within the core of a pre-clinical MRI. To test the imaging properties of the SPECT detector in a magnetic field a prototype based on CZT semiconductor detectors has been fabricated. This detector has a field-of-view of 2.54 times 12.7 cm2 and is fitted with a parallel hole collimator for mouse imaging. The prototype is designed to obtain a tomographic dataset by manual positioning within a 90-degree projection range. Shielding, support, positioning, and cooling materials were selected to minimally influence the magnetic field homogeneity. Fabrication of a stationary, polygonal ring of CZT has commenced. An APD-based, light-sharing detector has been evaluated for use in PET. The detector configuration is a 2 times 2 array of 10 mm times 10 mm Hamamatsu S8664-1010 APDs used to read out multi-crystal LYSO blocks. The design of the detector and front-end electronics is optimized for spectroscopic and timing performance, minimization of power dissipation, and low electromagnetic interference. Current plans are for assembling five rings, each with 17 modules, to give a 13.0 cm axial FOV with a 14 cm inner diameter. The results on the tests and performance of both types of modules are reported.

A Comparative Study of Fast Photomultipliers for Timing Experiments and TOF PET

The new 1 inch and 1.5 inch in diameter photomultipliers for timing applications from Photonis and Hamamatsu have been tested. Time resolution of XP1020, XP3060, R9800 and R9420 was measured with a 10 times 10 times 5 mm3 LSO crystal in coincidence experiments with 511 keV annihilation quanta from 22Na gamma source. Results were discussed in terms of measured photoelectron number and time jitter. Single photoelectron spectra were recorded and excess noise factor for each tube was also calculated. The final comparison of the tested tubes and their timing properties were presented in relation to large amount of experimental data of various types of PMTs collected during last few years. Especially, observed linear dependency between the time resolution normalized to the number of photoelectrons and time jitter was pointed out. Inconsistency of the data collected with the Hamamatsu R9420 PMT resulting from the overestimated photoelectron number was reported and further studied. Additional experiments with LED light source and different experimental set-ups were discussed and comparison of the two methods of the photoelectron number measurements was performed.

Energy resolution and nonlinearity of NaI(Tl), CaF 2 (Eu), and plastic scintillators measured with the wide-angle Compton-coincidence technique

Compton cameras are of general interest in various fields of operation. Because of the ability to locate and identify remote sources, homeland security supports the development of such devices in a rugged and reliable form. The decisions upon appropriate materials for the scatter- and absorber plane depend on performance and economical trade-offs. In order to estimate the expected performance of the Compton camera, simulations are necessary. Certain experimentally determined parameters have to be fed into simulations, such as the energy resolution of the detector. Two materials with low effective atomic number (Zeff), CaF2 and plastic, promise to be good candidates for the scattering plane. Those scintillators are known for quite some time, but not very well characterized with respect of energy resolution and nonlinearity. A modified Compton coincidence technique using a high purity Germanium (HPGe) detector in coincidence with the investigated scintillator is discussed in this paper: The wide-angle Compton-coincidence (WACC) setup provides a fast and reliable means for characterization of low-Z scintillators. For quality control purposes, the actual scatter detector can be monitored in-house using the WACC technique. This work presents results of different scintillator materials and sizes for validation and exploration of this method.

Comparative study of PP0275C hybrid photodetector and XP2020Q photomultiplier in scintillation detection

The properties of a hybrid photodetector (HPD), type PP0275C, produced by Delft Electronic Products B.V., for scintillation detection and spectrometry were studied and compared to a standard XP2020Q photomultiplier. The study was performed for several scintillators, such as NaI(Tl), CsI(Tl) and LSO(Ce) of different dimensions. The excellent capability of the HPD to resolve single photoelectron events was fully confirmed. However, the study of the HPD with the scintillators showed a dramatically reduced number of photoelectrons and a further deterioration of energy resolution, depending on the size (diameter or length) of the crystals. For a 10 mm diameter NaI(Tl) a number of 5000±250 photoelectrons/MeV was measured, which corresponds to about 56% of that observed with the XP2020Q with comparable quantum efficiency. An energy resolution of 9.2% for 662 keV γ-rays from a 137Cs source as measured with the HPD light readout indicated on a serious degradation, larger than that arising from the statistics of photoelectrons. In conclusion, the study showed that this HPD is optimized for single photon detection but its application to scintillation detection is very limited.