1. Scintillation Science and Nuclear Medical Imaging
William W. Moses Lawrence Berkeley National Laboratory
October 21, 2014
Outline:
Nuclear Medicine Needs & Opportunities
Scintillator Development History / Science
The NSS / MIC

2. Opportunities for SPECT Scintillators
Better Energy Resolution
Presently 9% fwhm for 140 keV
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
Advanced detector designs (small animal)
Collimator Limits Spatial Resolution & Efficiency
NaI:Tl Dominant for >40 Years...

3. Opportunities for PET Scintillators
3-D PET
Better Energy Resolution
Scattered events often outnumber true events
Higher Luminous Efficiency
Fewer PMTs for same spatial resolution
Advanced detector designs (small animal)
Better Timing Resolution
Reduce random events (up to 50% of total events)
Time-of-flight PET to reduce noise variance (by ~5x)
There is Significant Room for Improvement (Even Compared to LSO)

4. Cerium “Noticed” in Scintillation
In the Beginning (1989)…
CeF3:
Halide
Cerium is a main constituent (not a dopant)
Scintillation Properties:
30 ns primary decay lifetime
4,000 photons / MeV
Cerium “Noticed” in 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).

5. In the 1990’s… Oxides Dominate
Lu2SiO5:Ce
Oxide
Cerium is a dopant
Scintillation Properties:
40 ns primary decay lifetime
25,000 photons / MeV
Dramatic (6x) increase in luminosity
Oxides Dominate
C. L. Melcher and J. S. Schweitzer, IEEE Trans. Nucl. Sci. NS-39, pp. 502–504 (1992).

6. Many Oxide Hosts Doped with Cerium
Not a Lu Host
There’s Something About Lutetium…
C. W. E. van Eijk, Proceedings of SCINT 97, the International Conference on Inorganic Scintillators and Their Applications, pp. 3–12 (1997).

7. There’s Something About Halides…
In the Early 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 Something About Halides…
Summarized in W. W. Moses, Nucl. Instr. Meth. A-537, pp. 317–320 (2005).

8. There’s Something About Lutetium Halides?
Present Status…
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
There’s Something About Lutetium Halides?
K. S. Shah, J. Glodo, M. Klugerman, W. Higgins, et al., IEEE Trans. Nucl. Sci. NS-51, pp. 2302–2305 (2004).

9. The Ce3+ Revolution 1989: CeF3
30 ns 4,000 photons / MeV
1990’s: Lu2SiO5:Ce (& Many Other Oxides)
40 ns 25,000 photons / MeV
2000’s: RbGd2Br7:Ce, LaCl3:Ce, LaBr3:Ce, CeBr3
20 – 30 ns 50,000 – 70,000 photons / MeV
Now: LuI3:Ce
25 ns 100,000 photons / MeV
At Luminosity Limit for Cerium Scintillators
Future Progress Will Be In Density, Z, Cost, etc.

10. “Classical” Scintillation Mechanism
Ionic Bonding / Transitions Dominate
Transfer of Excitation from Host Ions to Activator

11. Why is Ce3+ the Favorite Dopant?
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).
Only Ce3+ Meets These Requirements
A. J. Wojtowicz, E. Berman, and A. Lempicki, IEEE Trans. Nucl. Sci. NS-39, pp. 1542–1548 (1989).

12. How Do We Make A Luminous Scintillator?
Lattice Conduction Band
Ce 5d
Energy
Band Gap
Ce 4f
Lattice Valence Band .
Small band gap
Ce 4f-5d levels in band gap, close to lattice energy
Good lattice transport & latticeCe transport
Empirical observations guide, many materials tried

13. “Semiconductor” Scintillator Mechanism
Data courtesy of S. Derenzo, LBNL
Covalent Bonding / Transitions Dominate
Many More Host & Dopants Available
Intrinsically Fast Mechanism

14. Scintillator Puzzle BGO LSO
662 keV
9.9% fwhm
662 keV
9.4% fwhm
LSO Has ~4x More Light Than BGO, But Same Resolution
Why Isn’t Resolution Dominated by Counting Statistics?

15. Light Yield Not Constant Depends on Particle Energy & Type
Non-Proportionality
Light Yield Not Constant
Depends on Particle Energy & Type

16. How Does Non-Proportionality Degrade Energy Resolution?
Scintillator Crystal
Auger Electrons, Fluorescent X-Rays
Compton Electron
Incident Gamma Ray
Compton Photon W
K L M Valence
Compton Photon
Compton Electron
Knock-On Electron
Multiple Energy Deposits Combined With Different Efficiency for Each Deposit

17. Cascade After Photoelectric Interaction
Photoelectron
Initial Gamma
Fluorescent X-Ray
Auger Electrons
Non-Proportionality + Multiple Energy Deposit
 Degraded Energy Resolution

18. What Creates the Shape of the Electron Response Curve?
Oxides
Alkali Halides
Decrease at High Ionization Density
(All Materials)
Decrease at Low Ionization Density
(Alkali Halides)
Key Empirical Observation:
Light Output Depends on Ionization Density

19. Competing Processes for e/h Recombination
One Voxel
Electron Ionization Track
Non-Radiative Trapping
Exciton Formation
Electron / Hole / Exciton Interactions
Luminescence h e
Many Ways for e/h Pairs in a Voxel to Recombine
Not All Recombinations Produce Light

20. Developments That Revolutionized PET
First Presented at the NSS/MIC
(Poster 4F9, 1991 NSS/MIC)
(Poster 2M10, 1991 NSS/MIC)
(Oral 11M01, 1993 NSS/MIC)
(Oral M501, 1998 NSS/MIC)
Discovery of LSO Scintillator
Septaless “3-D” Whole-Body PET Imaging
OSEM Reconstruction Algorithm
PET/CT Multimodal Imaging

21. Alberto Del Guerra and the NSS/MIC
79 NSS/MIC Conference Record Articles
43 TNS or TMI Articles
1996 Guest Editor
1999 NSS Chairman
2004 NSS/MIC General Chair (In Roma — second NSS/MIC in Europe)
2011 MIC Chairman
2011–2014 Elected AdCom Member (Representing MIC)