Review Review on on Scintillators Scintillators William W. Moses Lawrence Berkeley National Laboratory December 7, 2003 Outline: – Current Trends – Benefits for SPECT – Benefits for PET 3+ 3+ 3+ Many New Ce Scintillators. Why Ce Many New Ce Scintillators. Why Ce3+?? Activator Requirements: • One optically-active electron when in preferred valence state (no electron-electron interactions). • Transition is spin-parity allowed (decay lifetime is short, quenching reduced). • Atomic diameter similar to heavy metal ions (“fits” into lattices of dense host compounds). • Not radioactive (no background signal). Lack Lack of of Cooperation Cooperation from from Chemists! Chemists! 3+ ⇒ Only Ce Meets These These Requirements Requirements ⇒ Only Ce3+ Meets • A. J. Wojtowicz, E. Berman, and A. Lempicki, IEEE Trans. Nucl. Sci. NS-39, pp. 1542–1548 (1989). In In the the Beginning Beginning (1989)… (1989)… CeF3: • Halide • Cerium is a main constituent (not a dopant) • Scintillation Properties: 30 ns primary decay lifetime 4,000 photons / MeV Cerium Cerium “Noticed” “Noticed” in in Scintillation Scintillation • D. F. Anderson, IEEE Trans. Nucl. Sci. NS-36, pp. 137–140 (1989). • W. W. Moses and S. E. Derenzo, IEEE Trans. Nucl. Sci. NS-36, pp. 173–176 (1989). In In the the 1990’s… 1990’s… Lu2SiO5:Ce • Oxide • Cerium is a dopant • Scintillation Properties: 40 ns primary decay lifetime 25,000 photons / MeV • Dramatic (6x) increase in luminosity Oxides Oxides Dominate Dominate • C. L. Melcher and J. S. Schweitzer, IEEE Trans. Nucl. Sci. NS-39, pp. 502– 504 (1992). Many Many Oxide Oxide Hosts Hosts Doped Doped with with Cerium Cerium There’s There’s Something Something About About Lutetium… Lutetium… • C. W. E. van Eijk, Proceedings of SCINT 97, the International Conference on Inorganic Scintillators and Their Applications, pp. 3–12 (1997). In In the the Early Early 2000’s… 2000’s… RbGd2Br7:Ce, LaCl3:Ce, LaBr3:Ce, CeBr3 • Halides • Cerium is a dopant or a constituent component • Scintillation Properties: 20 – 30 ns primary decay lifetime 50,000 – 70,000 photons / MeV • Further (2x – 3x) increase in luminosity There’s There’s Something Something About About Halides… Halides… • Summarized in W. W. Moses, Nucl. Instr. Meth. A-537, pp. 317–320 (2005). Today… Today… LuI3:Ce • Lutetium and Halide • Cerium is a dopant • Scintillation Properties: 25 ns primary decay lifetime 100,000 photons / MeV • Another (1.5x – 2x) increase in luminosity (now ~at fundamental limit) There’s There’s Something Something About About Lutetium Lutetium Halides? Halides? • K. S. Shah, J. Glodo, M. Klugerman, W. Higgins, et al., IEEE Trans. Nucl. Sci. NS-51, pp. 2302–2305 (2004). Energy Resolution (fwhm) Tomorrow??? Tomorrow??? 100% LaCl3 LaBr3 NaI:Tl Statistical 10% 1% 10 100 1000 Energy (keV) Improved Improved Energy Energy Resolution? Resolution? SPECT SPECT Scintillator Scintillator Requirements Requirements Planar Photon Detectors Collimators 140 keV Photons • High Light Output • • • • • (>35,000 photons / MeV) High Photofraction (>80% at 140 keV) High Density (>3.5 g/cc) Low Cost (<$15/cc) Wavelength Match to PMT (300–500 nm) Short Decay Time (<1 µs) NaI:Tl NaI:Tl Predominately Predominately Used Used Opportunities Opportunities for for SPECT SPECT Scintillators Scintillators • Better Energy Resolution – – – – Scatter Presently 9% fwhm for 140 keV Other Over 35% of SPECT events are scatter Scatter fraction linearly proportional to resolution Other effects dominate if resolution <4% fwhm • Higher Luminous Efficiency – Fewer PMTs for same intrinsic resolution NaI:Tl NaI:Tl Used Used for for >40 >40 Years... Years... True Promising Promising SPECT SPECT Scintillators Scintillators NaI RbGd2Br7 LaCl3 Ce/LaBr3 Natural Radioactivity? No Luminosity (ph/MeV) 38,000 Energy Resol. (@ 140 keV) 8% Density (g/cc) 3.7 Atten. Length (mm, 140 keV) 4.9 Photofraction (@ 140 keV) 84% Wavelength (nm) 415 Decay Time (ns) 230 Hygroscopic? Yes Yes 56,000 10% 4.7 3.5 82% 430 45 Yes No 50,000 10% 3.9 4.5 80% 350 20 Yes No 60,000 6% 5.0 5.3 3.8 79% 370 25 Yes ••CeBr CeBr33 & & LaBr LaBr33 have have Better Better Lums Lums & & Energy Energy Resol. Resol. ••No No Other Other Performance Performance Drawbacks! Drawbacks! PET PET Scintillator Scintillator Requirements Requirements Ring of Photon Detectors • Short Attenuation Length 511 keV Photons • • • • • (<1.2 cm at 511 keV) High Photofraction (>30% at 511 keV) Short Decay Time (<300 ns) Low Cost (<$30/cc) High Light Output (>8,000 photons / MeV) Wavelength Match to PMT (300–500 nm) BGO BGO & & LSO LSO Predominately Predominately Used Used Opportunities Opportunities for for PET PET Scintillators Scintillators • Better Energy Resolution 3-D PET – Scattered events often outnumber true events • Higher Luminous Efficiency True Random – Fewer PMTs for same spatial resolution Scatter • Better Timing Resolution – Reduce random events (up to 50% of total events) – Time-of-flight PET to reduce noise variance (by ~5x) ••There There is is Significant Significant Room Room for for Improvement Improvement (Even (Even Over Over LSO) LSO) Promising Promising PET PET Scintillators Scintillators BGO Luminosity (ph/MeV) Energy Resol. (@ 511 keV) Decay Time (ns) Density (g/cc) Atten. Length (mm, 511 keV) Photofraction (@ 511 keV) Wavelength (nm) Natural Radioactivity? Hygroscopic? 8,200 12% 300 7.1 11 43% 480 No No LSO Ce/LaBr3 LuI3 25,000 10% 40 7.4 12 34% 420 Yes No 60,000 100,000 3% 4% 25 5.0 30 5.3 5.6 24 18 14% 29% 370 470 No Yes Yes Yes CeBr CeBr33,, LaBr LaBr33 & & Lul Lul33 have have Better Better Energy Energy Resolution, Resolution, but but Worse Worse Attenuation Attenuation Length Length & & Photoelectric Photoelectric Fraction Fraction Low Low Density Density ⇒ ⇒ Radial Radial Elongation Elongation Penetration Blurs Image 20 Resolution (mm fwhm) 3 Attenuation Lengths Resolution vs. Position LaCl 3 NaI 15 LaBr 3 BaF RGB 2 LuI3 10 LuYAP LSO LuAP GSO BGO 5 0 0 5 10 15 Radial distance (cm) 20 25 Some Some Degradation Degradation with with LuI LuI33,, More More with with Ce/LaBr Ce/LaBr33 Low Low Photoelectric Photoelectric Fraction Fraction ⇒ ⇒ Low Low Coincidence Coincidence Efficiency Efficiency Both Photons Deposit >350 keV Compton 3 Atten. Lengths Scintillator Photoelectric LaCl3 NaI RGB LaBr3 BaF2 LuI3 GSO LuYAP LSO LuAP BGO 0 0.2 0.4 0.6 0.8 Relative Efficiency Some Some Degradation Degradation with with LuI LuI33,, More More with with Ce/LaBr Ce/LaBr33 1 Statistical Statistical Noise Noise in in PET PET If there are 106 counts in the image, SNR = 106 106 = 103 Signals Signals from from Different Different Voxels Voxels are are Coupled Coupled ⇒ ⇒ Statistical Statistical Noise Noise Does Does Not Not Obey Obey Counting Counting Statistics Statistics Very Very Visible Visible Reduction Reduction in in Noise Noise Non-TOF Non-TOF TOF TOF 35 cm dia. w/ 1 cm dia. 6:1 hot spot 300k events, T:S:R = 1:1:1, 500 ps fwhm Improvement Improvement Largest Largest for for Large Large Patients! Patients! Coincidence Coincidence Timing Timing Resolution Resolution BaF2 210 ps Scintillator RGB 330 ps LaCl3 265 ps LaBr3 260 ps LuI3 200 ps LSO 300 ps LuAP 360 ps 3000 ps BGO 0 100 200 300 400 500 Coincidence Timing Resolution (ps) •• New New Scintillators Scintillators Capable Capable of of Time-of-Flight Time-of-Flight •• 500 500 ps ps Resolution Resolution ⇒ ⇒ 5x 5x Reduction Reduction in in Noise Noise Variance Variance For SPECT: Conclusions Conclusions • CeBr3 and LaBr3 are compelling – Better light output & energy resolution than NaI:Tl – Shorter attenuation length than NaI:Tl – No other performance drawbacks! For PET: • LuI3 is very interesting, but has some tradeoffs – Energy resolution, light output, & timing excellent – Worse attenuation length & photoelectric fraction • LaBr3 and CeBr3 have more severe tradeoffs – Atten. length & photoelectric fraction much worse Economic Economic Growth Growth is is Absolutely Absolutely Necessary Necessary Thanks Thanks To: To: Dominic Rothan Saint Gobain Jinyi Qi Lawrence Berkeley National Laboratory Kanai Shah & Jarek Glodo Radiation Monitoring Devices, Inc.
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