Applications of scintillators
Development and performance evaluation of high energy resolution Compton-PET hybrid detector based on Ce:Gd3Ga2.7Al2.3O12, Eu:SrI2 and CeBr3 scintillators (#4037)
M. Yoshino1, 2, K. Kamada3, 2, Y. Shoji2, 1, S. Kurosawa3, 7, A. Yoshikawa3, 1, K. Shimazoe5, A. Koyama5, M. Uenomachi5, H. Takahashi5, N. Nakada5, K. Fujiwara6, M. Takahashi6, T. Momose6
1 Tohoku University, IMR, Sendai, Japan
Among all molecular imaging modalities, positron emission tomography (PET) and single photon emission computed tomography (SPECT) have made a significant contribution for many years as to evaluate the physiological function and biochemical changes of molecular targets. However, it is difficult to obtain PET/SPECT images in simultaneous measurements because the SPECT detectors are unable to discriminate between photoabsorption of SPECT tracers and Compton scattering of PET tracers. In this study, the Compton-PET hybrid detectors which consist of the PET detector and the Compton camera were developed with no physical collimator. It is feasible to discriminate PET/SPECT tracers by measuring incident radiation energy with both scatter and absorber of Compton camera. In this study, we evaluated the scintillation properties itself of several scintillators (Eu:SrI2, Ce:GGAG and CeBr3) coupled with various photodetectors such as photomultiplier tube and MPPC. Next, we fabricate the three types of Compton-PET hybrid detectors which scatters are composed of different kind of scintillators such as Eu:SrI2, Ce:GGAG and CeBr3. Then, we report the principle proof of the Compton-PET hybrid camera detectors and the influence of the scintillation properties for the performance of Compton-camera.
Keywords: Scintillators, PET, SPECT, Compton-PET
Depth-of-Interaction Bias in the Achievable Ultra-Fast Timing of Detectors for TOF-PET (#3414)
M. Toussaint1, 2, F. Loignon-Houle2, J. - P. Dussault1, R. Lecomte2
1 Université de Sherbrooke, Department of Computer Science, Sherbrooke, Québec, Canada
In Positron Emission Tomography (PET), deep crystals (>20 mm) must be used to enhance detection efficiency and increase overall scanner sensitivity. However, for fast time-of-flight (TOF) scanners, this may come at a cost for the achievable coincidence time resolution (CTR), since the propagation time of annihilation photons differs depending on the depth-of-interaction (DOI) location. In the literature, DOI effect in CTR computation is modeled by incorporating the attenuation probability density function (PDF) with the PDFs for the scintillation pulse emission, propagation and detection. However, the resulting PDF would not describe accurately the variation, in timestamps distribution, due to annihilation photon DOI. In this study, we propose to investigate the DOI bias effect on CTR for a typical and a near-ideal scintillation detector. We also calculated the CTR of some estimators (k-th trigger, average of first-k triggers, and Gauss-Markov estimator) based on the ordered primary detected scintillation photons. The root-mean-square error (RMSE) of the coincidence detection process is a straightforward metric to compare the quality of different estimators. The RMSE of the different estimators was calculated using DOI probability in coincidence. A small difference in the calculated CTR was found for a typical LYSO/SiPM scintillation detector when assessing RMSE; this was expected since the DOI error influence remains negligible against other parameters. However, the difference becomes crucial for near-ideal scintillation detectors: the standard method that does not correctly account for the DOI bias predicts 10-20 ps CTR, but RMSE remains above ~55 ps CTR, which seems more likely since the difference in TOF introduced by the crystal length can almost be of the same magnitude. Thus, in the absence of DOI measurement, investigation of the achievable ultra-fast CTR in TOF-PET detectors should include the bias introduced by DOI.
Keywords: Coincidence timing resolution, Scintillation detectors, ToF-PET, Depth of Interaction
Influence of Compton Interactions on the Achievable Time Resolution in Scintillation Detectors (#3389)
F. Loignon-Houle1, M. Toussaint1, 2, R. Fontaine3, R. Lecomte1
1 Universite de Sherbrooke, Department of Nuclear Medicine and Radiobiology, Sherbrooke, Québec, Canada
The gain in image signal-to-noise ratio in Time-of-flight Positron Emission Tomography (TOF-PET) is critically dependent on the coincidence time resolution (CTR). The stochastic nature of the production, propagation, and detection of scintillation photons limits the achievable lower bound on CTR, as was pointed out in many recent investigations. A fixed number of detected scintillation photons based on complete 511 keV energy deposit in the crystal is generally considered in the reported calculations. However, most TOF-PET scanners use an energy window extending to lower energies than the photopeak in order to enhance sensitivity. Therefore, we calculated the Cramér-Rao lower bound (CRLB) on CTR of coincident scintillation detectors accepting lower energy Compton scattered interactions. Including these lower-light yield events leads to degraded achievable CTR by ~15-50 ps, depending on the detector response and efficiency. Such differences may be substantial in the quest for ultra-fast TOF-PET devices. Individual timestamps and Gauss-Markov estimators CTR was investigated as a function of the number of primary triggers. A strong asymmetry of the primary photon time distributions with increasing trigger index in a single detector appears when Compton interactions are included. In coincidence, the asymmetry induces non-gaussian profiles with growing FWTM/FWHM ratio as the trigger index increases. For all estimators and photon indexes, it was found that including Compton interactions increases the CTR and the Gauss-Markov estimator loses its rapid convergence capability to the CRLB. Indeed, the Gauss-Markov estimator relies on the assumption of gaussian detected photon time profiles, which is no longer the case with a wide energy window. These findings highlight the probable difficulty for achieving ultra-fast timing and high sensitivity in realistic scanner conditions.
Keywords: Scintillation detectors; TOF-PET; Coincidence time resolution; Compton interactions
Compact Solid State Neutron-Gamma Detectors for Backpack or Handheld Instruments (#1289)
L. Soundara-Pandian1, M. Spens1, P. O'Dougherty1, J. Tower1, A. Gueorguiev1, J. Glodo1, K. S. Shah1
1 Radiation Monitoring Devices, Inc., Radiation Detection and Imaging Group, Watertown, Massachusetts, United States of America
Cs2LiYCl6 (CLYC) and Cs2LiLa(Br,Cl)6 (CLLBC) crystals from the elpasolite family are dual-mode detectors that show excellent gamma-ray resolution and efficient neutron detection with pulse shape discrimination capability for gamma-neutron separation. Large volumes of these scintillators are commercially available from RMD and have become potential replacement detectors for a combination of gamma detectors and 3He tubes for neutron detection. These detectors, when made compact, can become an ideal choice for dual-mode detection in pager, handheld, and backpack instruments. The high gamma-ray equivalent energy (> 3 MeV) of the thermal neutron peak allows for pulse height discrimination with very good gamma rejection, which simplifies the electronics of the instrument. The excellent energy resolution (CLLBC 3% and CLYC 4% at 662 keV) makes them effective for radio isotope identification.
In this paper, we report on the characterization of detectors constructed at RMD using 1-inch, 1.5-inch, and 2-inch diameter right cylinders of CLYC and CLLBC crystals coupled to arrays of silicon photomultipliers (SiPM). Detectors constructed with small volume CLLBC crystals coupled to a 12 mm x 12mm total area SiPM array show an excellent energy resolution of ~3% at 662 keV and a large volume CLYC crystal when coupled to a 24 mm x 24 mm area array shows an energy resolution of ~6.6% at 662 keV. Both detectors show pulse shape discrimination capabilities.
Keywords: Scintillators, neutron detectors, silicon photomultipliers, gamma detectors, pulse shape discrimination
Multi-Signature Composite Detector System for Nuclear Non-proliferation (#1293)
U. Shirwadkar1, A. Gueorguiev1, E. van Loef1, G. Markosyan1, K. S. Shah1, J. Glodo1, J. Tower1, S. A. Pozzi2, M. Bourne2, S. Clarke2
1 Radiation Monitoring Devices, Inc, Watertown, Massachusetts, United States of America
Radiation Monitoring Devices (RMD) has developed a composite detector technology for multi-mode detection, based on incorporation of the inorganic scintillation material into a pulse shape discrimination (PSD) - capable plastic scintillator, mainly for handheld and backpack applications. The plastic scintillator serves as an optical light guide, and provides gamma and fast neutron detection along with PSD. The advantage of this technology is the lower cost compared to high quality large single crystals of inorganic scintillators for building highly efficient, large volume detectors. The technology is scalable since composite detector with multiple elements of inorganic scintillator behaves as a large volume single crystal, and the size is not limited by the crystal growth yield issues. We have constructed composite detectors (up to 2” x 2”) using PSD plastic scintillator as a host and CLYC as the inorganic component with up to 20% loading. A permanent detector probe was constructed using this sample. We achieved 5.7% energy resolution (ER) at 662 keV, and 4.8% ER of the thermal neutron peak at gamma equivalent energy of 3 MeVee. Due to the differences in the decay times of the gamma and neutron events detected in plastic and CLYC, we obtained excellent PSD between all signatures. The photopeak gamma efficiency was comparable to a 1” x 1” NaI:Tl scintillator. In order to support performance optimization of large composite systems in terms of scintillation light transport, refractive index matching, and loading factor, our collaborators at University of Michigan performed calculations using the MCNPX-PoliMi and Geant4 models. The results were able to reproduce experimental observations very accurately. For applications where superior energy resolution or higher efficiency is desired, other high performance scintillators such as CLLBC and TLYC have also been explored. Modeling and experimental data on these composite systems will be presented in this paper.
Keywords: Plastic scintillator, CLYC, Composite detector, inorganic-organic composite
Effects of Detector Cell Size on Dose Rate Measurements using Organic Scintillators (#3515)
C. A. Miller1, S. Clarke1, S. A. Pozzi1
1 University of Michigan, Ann Arbor, United States of America
Measurement of dose rates from both neutrons and photons inform radiation safety procedures in many areas of nuclear technology. The technique for using an organic scintillator to measure both neutron and photon dose rates in a mixed radiation field with a single detector cell has been previously demonstrated. This method can be applied for dose measurements in a wide variety of applications, although different size detectors may be required depending on expected flux. The dose rate measured should not depend on the size of the detector used, although many different factors can affect this dependence. This work examines the effect of detector size on dose rate, and provides insights into effects of the single scatter approximation on dose rate measurements. Measurements of laboratory neutron sources with different size organic scintillators are supported by Monte Carlo MCNPX-PoliMi simulations for additional detector response information. Preliminary results indicate that larger detector cells overestimate the measured neutron dose rate. This study of the detector size influence on dose rate measurements will inform future adjustments to current measurement methods. A dose rate measurement technique that can account for different size detectors will allow for use in many different areas of radiation safety.
Keywords: Dose, Organic Scintillator, Detector Response