N1D1  Scintillation Detectors for Imaging

Monday, Nov. 2  16:30-18:10  Golden Pacific Ballroom

Session Chair:  Etiennette Auffray, CERN, Switzerland; Alan Janos, DNDO, United States

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(16:30) N1D1-1, LFS, LaBr3(Ce) and NaI(Tl) Performance Comparison for Single Photon Emission Imaging

A. Fabbri1,2, M. N. Cinti3,2, M. Galasso4,2, C. Borrazzo3,2, C. Trigila1,2, P. Bennati5, R. Pellegrini3,2, R. Pani3,2, F. de Notaristefani1

1Dept. Maths and Physics, University of "Roma Tre", Roma, Italy
2INFN, Roma, Italy
3Dept. of Molecular Medicine, University of "La Sapienza", Roma, Italy
4Dept. of Science, University of "Roma Tre", Roma, Italy
5KTH, Stockholm, Sweden
In this work we characterize a new 5 cm x 5 cm x 2 mm LFS crystal coupled to an R6231 PMT and to a H8500 PSPMT to evaluate energy resolution and imaging performance respectively. The crystal surfaces was analysed with a profilometre in order to investigate its roughness so a better modelization could be deployed. The results will be compared with previous ones obtained using a 5 cm x 5 cm x 4 mm NaI(Tl) crystal and LaBr3(Ce) in the same experimental setup. Both NaI(Tl) and LaBr3(Ce) were encapsulated with a 3 mm glass window due to their high hygroscopicity. Surface treatment were optimized for gamma imaging with the front surface covered with a white reflector and black absorber placed on the edge. Luthetium Fine Silicate (LFS) is a recent crystal characterized by a high light yield (85% of NaI) and a fast decay time, of about 33ns. Its high density provides an high efficiency with few millimetre thickness, moreover it is non-hygroscopic so a direct coupling with the PMT is possible. The reduced light path of this solution gives, at 140 keV, an intrinsic spatial resolution of 0.66 mm +- 0.08 mm in the cFoV. This spatial resolution can be achieved by the use of a least square fitting algorithm. An energy resolution of 15% at 140 keV was obtained even if it is affected by the intrinsic radioactivity of the crystal due to the Luthetium beta decay. The comparison with NaI(Tl) and LaBr3(Ce) puts the LFS as a valid alternative for single photon emission detector especially in small animal imaging application where the more important parameter is the intrinsic spatial resolution.

(16:50) N1D1-2, Detection of Soft and Hard X-Rays with High Spatial and Energy Resolution with a Novel Detector Combining a pnCCD and a CsI(Tl) Scintillator

L. W. J. Strueder1, M. Huth1, R. Hartmann1, A. Abboud2, M. Shokr2, U. Pietsch2, D. Schlosser1

1Radiation Detector Development, PNSensor, Munich, Germany
2Physics Dept., University of Siegen, Siegen, Germany
By combining a low noise 450 µm thick fully depleted pnCCD with a CsI(Tl) scintillator (700 µm thick) a detector can be realized with high quantum efficiency, high energy and position resolution. The X-ray radiation in a range of 1 keV up to 150 keV enters the detector system through the pnCCD first. If converted in the Silicon detector – called direct detection - the measured energy resolution is performed with the precision of a high resolution semiconductor detector of the order of 1 % at 122 keV and 5 % at 1 keV. If the interaction takes place in the scintillator coupled directly to the pnCCD, the generated optical photons (? = 550 nm) are recorded with the pnCCD through the homogeneous radiation entrance window, equipped with an anti-reflecting coating. In this case the measured energy resolution is approximately 10 % to 18 % at 122 keV depending on the depth of interaction in the scintillator (indirect detection). The position precision in pnCCD is better than the size of one single pixel (here: 75 µm), the position precision of the point of interaction determined with the scintillator is 30 µm in x and y (detector plane) and 50 µm in the z direction (depth of the scintillator). The detector system was modelled with GEANT4 to properly understand all details of the measured properties and derive proposals for further improvements. The results of the measurements and simulations will be presented; proposals for future improvements will be emphasized.

(17:10) N1D1-3, Simulation of Signal Losses in Highly Pixelated Scintillator Arrays Read Out by Discrete Photodetectors

F. Loignon-Houle1, M. Bergeron1, C. M. Pepin1, S. Charlebois2, R. Lecomte1

1Sherbrooke Molecular Imaging Center of CRCHUS, Dept. of Nuclear Medicine and Radiobiology, Universite de Sherbrooke, Sherbrooke, QC, Canada
2Institut Interdisciplinaire d'Innovation Technologique, Dept. of Electrical and Computer Engineering, Universite de Sherbrooke, Sherbrooke, QC, Canada
The performance of scintillation detectors strongly depends on the scintillation light transport from the crystal to the photodetector. In highly pixelated arrays of scintillators with a high form factor, the loss of signal is compounded with crosstalk effects, squandering valuable signal to adjacent pixels. The purpose of this simulation study is to uncover processes responsible for light losses in scintillator arrays. The Monte Carlo software Geant4 was used to simulate 511-keV photon interactions occurring in parallelepipedic Lu1.9Y0.1SiO5 (LYSO) scintillators assembled in a tightly-packed array and to track the propagation of the generated scintillation photons. Four sources of losses though crosstalk between pixels were identified, namely escaping photoelectrons to other pixels after photoelectric interactions, X-ray fluorescence and Auger emission, reflector transparency to scintillation light, and light leakage to other crystals due to adhesive material between reflectors and scintillators in which optical photons can propagate to other crystals. An important source of signal loss and energy resolution degradation was found to be related to the transmittance of the adhesive material used to bond reflectors to scintillators. Also, the angular distribution of scintillation photons impinging of the detection face was assessed in order to weigh the proportion of trapped photons through total internal reflection due to the refractive index difference between scintillators and optical coupling medium.

(17:30) N1D1-4, Developing High Spatial Resolution Fibre-Optic Dosimeters

E. Li1, M. Carolan2, M. Petasecca1, M. Lerch1, A. Rozenfeld1

1School of Physics, University of Wollongong, Wollongong, NSW, Australia
2ICCC, Wollongong Hospital, Wollongong, NSW, Australia
A high resolution, water equivalent, optical and passive x-ray dosimeter has been constructed using plastic scintillators and optical fibres. This dosimeter has a spatial resolution of 100 µm with a 10 µm dosimeter under investigation. This dosimeter was used to measure the beam profile and a depth dose curve of an x-ray beam generated by a linear accelerator. This dosimeter design has applications in x-ray multibeam radiation therapy where a high resolution is vital for accurate dose measurements and quality assurance if it is to be used in radiotherapy.

(17:50) N1D1-5, Growth and Scintillation Properties of Ce Doped Gd2Si2O7/SiO2 Eutectic as Phase Separated Scintillators for High Resolution Imaging

A. Yoshikawa1,2,3, K. Kamada2,3, S. Kurosawa1,2, Y. Yokota2, Y. Ohashi1

1Institute for Material Reseach, Tohoku University, Sendai, Japan
2New Industry Creation Hatchery Center, Tohoku University, Sendai, Japan
3C&A Corporation, Sendai, Japan
Recently submicron-diameter phase separated scintillator fibers (PSSFs) were reported and they possessed both the properties of an optical fiber and a radiation-to-light conversion. The PSSFs were fabricated using a directionally solidified eutectic (DSE) system. The DSE systems have been discovered in various materials for many applications [1,2]. In PSSFs, the light emitted from the scintillator fibers is confined and transported along the fiber direction by a total reflection mode, so that high-resolution radiation imaging can be achieved. CsI/NaCl [3] and GAP/a-Al2O3[4] have been already reported as PSSFs. Ce:Gd2Si2O7 (Ce:GPS) had a good light output of 30,000 photons/MeV and FWHM energy resolution of 6.0% at 662 keV at room temperature. However, bulk single crystal of Ce:GPS is not available to get as a commercial product. The main reason is the fact that GPS is not congruently melt. When the chemicals with stoichiometric composition is melted, initial melt has apatite phase. According to the previous report [5-8], Ce:GPS crystals need to be grown from the melt with heavy Ce-doping (approximately, 10 mol%) in order to make the melt congruent. However, such high Ce-concentration would lead to lower light output because of self-absorption or concentration quenching. Other possibilities to get Ce:GPS bulk single crystal are i) partial substitution of La in Gd site [9,10] or ii) solution growth using suitable flux. In this study, Ce doped GPS/SiO2 eutectics were grown by the micro pulling down (?-PD) method and their DSE system has been investigated. Luminescence and scintillation performances were also evaluated. In this Ce:GPS/SiO2 eutectics, heavy Ce-doping or La substitution is not necessary. SiO2 works as a flux as well as the separator. Moreover, as GPS has higher refractive index than that of SiO2, Ce:GPS phase can work as light guiding as well. Consequently, high resolution imaging can be expected.