1. Improving Scintillation Performance of Ce-doped Garnet Crystals by Defect Engineering
Denys Solodovnikov
Marc H. Weber, Kelvin G. Lynn
Drew Haven, David F. Bahr, M. Grant Norton, Jalal M. Nawash, Romit Dhar, Michelle Robinson

2. Scintillation detectors
Scintillation detectors operate through emission of light flashes that are detected by a photosensitive device usually a photomultiplier or a silicon photodiode.
PMT
scintillator
gamma-ray
Light photons

3. Scintillation mechanism
Incoming high energy photon is transformed into scintillation light through a number of electron–hole pairs created in the conduction and valence bands, followed by their radiative recombination at suitable luminescence centers.
* M. Nikl, V. V. Laguta, and A. Vedda, Physica Status Solidi (b) 245, (2008)

4. Ce:YAG scintillators
The Ce-doped Yttrium Aluminum Garnet - Y3Al5O12 (Ce:YAG) single crystals were reported as excellent candidates for fast scintillators as early as 1978*.
Luminescence in Ce:YAG is produced by 5d→4f transition of Ce3+ ion which is peaking at 550 nm in the YAG host.
In spite of all the advances these materials fall short of performing close to their theoretical limits. Defects competing with Ce3+ ions in the energy transfer process are responsible for decreased performance. However, their origins are only partly understood.
* Autrata, R., Schauer, P. & Kuapil, J. Journal of Physics E 11, 707–708 (1978).

5. Optical excitation
Measurements performed at Pacific Northwest National Laboratory (PNNL)

6. Direct optical excitation of Ce3+ ions

7. Cathodoluminescence of Ce:YAG
* Rotman, S.R., Tuller, H.L. & Warde, C. Journal of Applied Physics 71, (1992)

8. Defects in YAG
Among the point defects studied in YAG are: F and F+ centers (two and one electron in an oxygen vacancy), cation vacancies and YAl antisites. The latter linked to UV emission near 300 nm.
Study of the UV emission was made by Zorenko et al.* using time-resolved emission spectroscopy. Comparison of the bulk single crystals with single crystalline films grown by Liquid Phase Epitaxy showed that films grown at substantially lower temperature (~1000 °C) are free of UV emitting defects.
* Zorenko, Y. et al. Journal of Luminescence 114, (2005).

9. Thermoluminescence

10. Thermoluminescence from UV laser damage
Letters were created by 266 nm laser beam at room temperature.
Then the sample was heated to 100 ºC to produce TL.

11. Aluminum in-diffusion
Before Ar anneal
After Ar anneal
1 mm
1 mm

12. Positron annihilation spectroscopy
Longer positron diffusion length (L+) corresponds to lower defect concentration

13. Scintillation results
Ce:YAG on PMT
Ce:YAG on Si photodiode
The resolution significantly improved on a PMT after O2 anneal followed by Al in-diffusion but remained low on a Si photodiode.

14. From Ce:YAG to Ce:GGG a novel class of scintillators
Transition from YAG to GGG host provides a significant increase in atomic number and density and adds neutron detection capability.
YAG
GGG
Chemical formula
Y3Al5O12
Gd3Ga5O12
Density[g/cm3]
4.57
7.08
Zeff4 (x106)[g/cm3] 5 80
Hardness [Mho]
8.5
7.5
Crystal structure
Cubic
Melting point [°C]
1970
1730

15. Ce:GGG crystal growth

16. Ce:GGG scintillation results

17. Ce3+ absorption in different hosts
Partial overlap of GGG band edge with absorption levels of activator is believed to be one of the reasons for low scintillation performance
Other wider band gap materials were tested as activator host: YGG (Yttrium Gallium Garnet), YGGG (Yttrium Gadolinium Gallium Garnet), and YGGAG (Yttrium Gadolinium Gallium Aluminum Garnet)

18. Ce:Tb:YGGAG and commercial Ce:YAG

19. Supercooling
Due to supercooling that is also observed in YAG (Yttrium Aluminum Garnet) YGGAG prefers to follow metastable crystallization which leads to crystallization of opaque solid mixture of non garnet phases instead of a garnet structure.

20. Conclusions
The scintillation output of Ce:YAG crystals under gamma-ray or X-ray excitation can be significantly increased with a post growth treatment only by increasing their UV scintillation.
GGG crystals doped with Ce show high output from direct activator excitation but when exited by gamma- or X-rays they have very low scintillation output.
A new mixed garnet material – YGGAG is proposed to be a better host for doping with Ce and Tb ions to produce a yellow scintillator. These crystals already demonstrate a very promising performance.

21. Future work
Stoichiometric composition studies
Optimization of activator concentration
Reducing oxygen vacancy concentration at a growth stage
Improvement of crystals’ optical quality

22. Acknowledgements
Kelvin G. Lynn Marc H. Weber M. Grant Norton Gary S. Collins Matthew D. McCluskey David F. Bahr, Jalal M. Nawash, Romit Dhar, Michelle Robinson, Drew Haven All members of CMR II – VI Foundation National Science Foundation, Award No. ECCS Department of Homeland Security, Domestic Nuclear Detection Office, Award No DN-077-ARI029-02

23. Thank you!

24. Photosensitivity of a PMT and a Si PD
QE=30%
XP2020/Q
Most of the scintillation light of Al in-diffused samples is emitted in a wavelength region detected only by the PMT, i.e., in the UV region.

25. Al diffusion in GGG
Analysis by Microprobe
800 micron step size