1. Free carrier absorption as a tool for study of carrier dynamics in scintillators
G. Tamulaitis,
S. Nargelas,
A. Vaitkevicius,
Vilnius University,
Lithuania
M. Korjik, A. Borisevich, A. Fedorov, V. Mechinsky, Research Institute for Nuclear Problems, Minsk, Belarus
E. Auffray,
M.T. Lucchini
CERN

2. Outline Aim Experimental Carrier dynamics in GAGG:Ce
Fugure of merit for fast LSO:Ce

3. Stimulus
10 ps

4. Pump and probe technique in femtosecond domain
is indispensable tool to reveal differences in carrier trapping
and to select crystals with sufficiently fast scintillation build up

5. Characterization of carrier dynamics
GAGG:Ce,Mg

6. GAGG:Ce Kinetics of differential absorption in GAGG:Ce
under predominant excitation of Gd sublattice
Free carrier absorption is dominated by
absorption of free holes
The rise of FCA in GAGG:Ce, takes a few picoseconds
That is caused by delocalization of holes from Gd3+ sublattice to the valence band

7. Free carrier absorption in YAGG:Ce
Initial part of the kinetics Kinetics of optical density induced by short pulse excitation of YAGG:Ce crystal at 254 nm, probed at 1030 nm, for different excitation pulse energy densities (indicated)
No gadolinium, no slow rise time

8. GAGG:Ce Peak differential absorption of GAGG:Ce
as a function of probe photon energy
Ground state of lattice-building gadolinium ions in GAGG crystal is in the valence band
by ~1 eV from its top
In consistence with the conclusion reported in P. Dorenbos, ECS J. Solid State Sci. Technol. 2, R3001 (2013)] that the ground state of the 8S6D7/2,9/2 intracenter transition of Gd3+ ions is located within the valence band.

9. GAGG:Ce Peak differential absorption of GAGG:Ce
as a function of probe photon energy
Ground state of lattice-building gadolinium ions in GAGG crystal is in the valence band
by ~1 eV from its top
Holes relax to the bottom of the valence band with characteristic time of ps

10. GAGG:Ce Energy-level diagram for
GAGG crystal doped with Ce and codoped with Mg

11. in GAGG:Ce and GAGG:Ce,Mg
Kinetics of photoluminescence intensity (red and green curves) and free carrier absorption (blue and magenta)
in GAGG:Ce and GAGG:Ce,Mg
Mg cdoping:
Mg based defects acceleate recombination of nonequlibrium carriers, so the decrease of their density is enhanced
G. Tamulaitis, A. Vaitkevičius, S. Nargelas, R. Augulis, V. Gulbinas, P. Bohacek, M. Nikl, A. Borisevich, A. Fedorov, M. Korjik, E. Auffray, Subpicosecond luminescence rise time in magnesium codoped GAGG:Ce scintillator, NIMA, 870, 25–29 (2017)

12. Figure of merit for fast response
LSO:Ce

13. Examples of large-scale production for HEP experiments
L3 (CERN) CMS (CERN) PANDA at GSI (Darmstadt)
Material
BGO
PWO
PWO-II
Parameters
certified
dimensions,
optical transmission,
light yield
light yield,
scintillation kinetics,
light yield nonuniformity,
slope of cutoff,
radiation-induced absorption (estimated by sampling)
gated light yield in temperature range,
radiation-induced absorption (measured for all crystals)
Targets:
Good energy resolution of calorimeter and tolerance to radiation

14. Targeted features for scintillation elements
to be installed in BTL
high light yield
small distribution of decay time constants
low level of phosphorescence
tolerance to irradiation
minimization of the influence of trapping centers on the transfer processes affecting the scintillation response
high CTR
high LY homogenity
stability of time resolution
stable and homogeneous
time resolution
How to characterize this feature in mass production
to ensure homogeneous time response in BTL ?

15. Transfer of electronic excitation affecting the rise of scintillation; Ce3+ doped material
Free carrier generation
Free carrier capture by shallow traps and Ce3+
Selective photoexcitation by femtosecond pulses is a unique tool to study the carrier trapping
Trapping and detrapping of free carriers
influences the scintillation rise time

16. Differential absorption of three LSO ingots
The change in absorption due to material excitation by pump pulse
Differential absorption of three LSO ingots

17. Differential absorption of three LSO ingots
The change in absorption due to material excitation by pump pulse
Differential absorption of three LSO ingots
Are we able to identify a clear and reliably measurable
figure of merit ?

18. Differential absorption of three LSO ingots

19. Figure of merit Exponential decay time @1.4eV in different LSO samples
of three ingots

20. Figure of merit VALIDATION NEEDED Exponential decay time
@1.4eV in different LSO samples
of three ingots

21. Thank you for your attention
Collaboration has been facilitated by

22. Number of ingots to be studied
Number of elements in BTL
Element dimensions , mm
Element volume, material losses included, cm3
Volume of material needed, cm3
Number of ingots, cm3 each
256000
11x11x3
0.5
128000
180

23. Time consumption Total time per sample: 5 hours
Measurement time,
hours per sample
Time for data processing up to applicability conclusion,
Total average,
0.5
1.5 2
Setup arrangement, calibration, and rearrangement,
hours per batch
For 5 samples is a batch, time for rearrangements,
hours per sample 15
15/5 = 3
Total time per sample:
5 hours
Total time for 180 samples:
900 hours

24. Time consumption 900 h x 20 EUR/h = 18 kEUR
Measurement time,
hours per sample
Time for data processing up to applicability conclusion,
Total average,
0.5
1.5 2
900 h x 20 EUR/h = 18 kEUR
Setup arrangement, calibration, and rearrangement,
hours per sample
For 5 samples is a batch batches, time for rearrangements, 15
15/5 = 3
Total time per sample:
5 hours
Total time for 180 samples:
900 hours
Shipping and handling is not included

25. Time consumption 900 h x 20 EUR/h = 18 kEUR
Measurement time,
hours per sample
Time for data processing up to applicability conclusion,
Total average,
0.5
1.5 2
900 h x 20 EUR/h = 18 kEUR
To built the setup already available at Vilnius University: ~ kEUR
However, the setup is available without further investment
Setup arrangement, calibration, and rearrangement,
hours per sample
For 5 samples is a batch batches, time for rearrangements, 15
15/5 = 3
Total time per sample:
5 hours
Total time for 180 samples:
900 hours
Shipping and handling is not included