1. Performance of scintillation pixel detectors with MPPC read-out and digital signal processing Mihael Makek with D. Bosnar, V. Gačić, L. Pavelić, P. Šenjug and P. Žugec Department of Physics Faculty of Science, University of Zagreb
2nd Jagiellonian Symposium on Fundamental and Applied Subatomic Pysics, Krakow, 2017

2. Motivation and outline
Construct and test segmented detector arrays using state-of- the art SiPMs, scintillation materials and digital signal processing electronics; applications: PET, PALS, fundamental measurements
Outline
Simulation of detector efficiencies
Experimental setup
Performance results of LFS scintillator pixels and MPPC array
Krakow, 2017
Mihael Makek

3. Simulation of ideal detection efficiency takes into account: density, Zeff, resolution
Geant4, geometry: 8x8 pixels, g impinging on either of 4 central pixels
Showing fraction of detected/incoming 511 keV gammas
Detector LFS 20mm
Edep.>50 keV
Edep.~511keV
Single
0.59
0.46
Coincidence
0.35
0.21
Detector LYSO 20mm
Edep.>50 keV
Edep.~511keV
Single
0.57
0.43
Coincidence
0.32
0.18
Krakow, 2017
Mihael Makek

4. LFS scintillator Lutetium Fine Silicate (Zecotec patent)
Crystal/Property
LFS
LYSO
Density [gcm-3]
7.35
7.1
Zeff 64 66
Atten. Length [cm]
1.15
1.12
Decay constant [ns]
<33 41
Max. emission [nm]
425
420
Light yield [% NaI]
80-85
70-80
Refractive index
1.81
Hygroscopic No
Active
Yes
4x4 array
3.14 x 3.14 x 20 mm3
1 layer~0.06 mm
Krakow, 2017
Mihael Makek

5. MPPC arrays 4x4 MPPC array (S13361‐3050AE Hamamatsu) Main features:
Number of micro-cells 3584/pixel
Micro-cell pitch = 50 mm
Fill factor 74%
Epoxy window, n=1.55
Vbr ~ 53 V
PDE (typical) ~ 40%
Spectral range nm (max at 450 nm)
1 p.e. 2 p.e. 3 p.e.
Krakow, 2017
Mihael Makek

6. Amplifiers 16-channel Passive base:
selectable cable length
Matching base depending on the SiPM model and manufacturer
Sum output with selectable gain and offset
Output signals:
50 Ohm
maximum -2 V
Rise-time ~ ns (depending on cable length)
Krakow, 2017
Mihael Makek

7. Digitizers CAEN model V1743
16 channel, switched capacitor (based on SAMLONG chip)
1024 samples/channel
7 events/channel buffer
Up to 3.2 GHz sampling rate
Selectable trigger logic (after bugfixes on our request)
Multi-board synchronization still in experimental phase
Individual readout for all crystal channles
Krakow, 2017
Mihael Makek

8. Setup and trigger 16 ch. Amp. 16 ch. Amp. 22Na 16 ch. digitizerOR OR
TRG IN
AND
TRG IN
Krakow, 2017
Mihael Makek

9. Digitized signals Recorded at 1.6 GHz 625 ps samples Vop = Vbr+1.5 V
Rise-time ~ 15 ns
Energy reconstruction:
Amplitude
Integral
Krakow, 2017
Mihael Makek

10. Reconstructed energy of 511 keV gammas
DE/E=15%
DE/E=13%
Integral linear with amplitude
Integral provides superior energy resolution
Krakow, 2017
Mihael Makek

11. Non-linearity correction
Limited number of micro-cells causes saturation of large signals
The relation between incident photons (Nph)and fired cells (Nfired):
Apply correction:
where Nfired is obtained empirically:
M is the total number of micro-cells, PDE is photon detection efficiency calculated as product of QE(l), Pav(V,T) and fill factor
Original
Corrected
201 keV from 176Lu
306 keV from 176Lu
511 keV from 22Na
Krakow, 2017
Mihael Makek

12. Light sharing Calibration: sum of all fired channels = 511 keV
Light sharing significant bewteen adjacent pixels.
 0,06 mm reflector too thin!
Energy deposition:
Leading/total ~ 80%
S1st neighbors/total ~ 20%
2nd neighors/total ~ negligible
Single pixel energy resolution keV: 11%  13%
Krakow, 2017
Mihael Makek

13. One photo-electon amplitude
Measured on oscilloscope
Map temperature and voltage dependence for both detectors
Vbr(1) = 52.1 V
Vbr(2) = 52.2 V
Krakow, 2017
Mihael Makek

14. Number of fired micro-cells @511 keV
Mean number of fired cells at 511 keV vs:
Voltage
Temperature
Reflects how PDE changes with temperature and voltage
 Impact on energy resolution
Krakow, 2017
Mihael Makek

15. 511 keV photo-peak amplitude
Mean amplitude of the 511 keV photo-peak vs V and t
(Range limited by amplifier gain)
Change of amplitude with V is ~equally governed by number of fired cells and 1p.e. amplitude
Change of amplitude with temp. is dominantly governed by 1p.e. amplitude
Krakow, 2017
Mihael Makek

16. Energy resolution @ 511 keV
Energy resolution improves with U in the measured range
Negligible dependence on temperature in the measured range
Krakow, 2017
Mihael Makek

17. Time resolution Pixel-to-pixel timing
Determined by fitting a straight line to the signal rising – edge
Dt ~1.6 ns (FWHM)
Limited by amplifier rise-time of ~15 ns
Krakow, 2017
Mihael Makek

18. Summary
Simulation shows LFS has potential to improve sensitivity of PET compared to LYSO
LFS performace tests:
energy resolution satisfactory, but can improve by reducing the light sharing between the pixels and running at higher overvoltage
Time resolution must be checked with fast preamplifier
MPPC arrays show stable performance
Saturation correction done on event-by-event basis
Signal amplitudes scale linearly with voltage and temperature
Temperature variations under control without direct compensation
Krakow, 2017
Mihael Makek

19. BACKUP SLIDES
Krakow, 2017
Mihael Makek

20. Single pixel full spectrum
Krakow, 2017
Mihael Makek

21. Energy calibration procedure
Correct each channel for non-linearity (event-by-event)
Equilibrate all channels by scaling each channel’s photo- peak to the same value
Calibrate the sum of all fired channels to 511 keV
Repeat procedure for each run (approx 1h of data taking)
The self-calibration on coincidence data is stable wrt to temperature and voltage change  no need to pre-calibrate the setup
Krakow, 2017
Mihael Makek

22. Uniformity of the response with distance from the MPPC
A modified detector setup to test the uniformity:
Pb collimator
22Na
MPPC d5 d4 d3 d2 d1
20 mm
Krakow, 2017
Mihael Makek

23. Uniformity of the response with distance from the MPPC
Select photo-peak in the leading pixel
Leading pixel amplitude vs. distance
Leading/(sum of 1st neighbors) vs. distance
Homogeneous response!
Krakow, 2017
Mihael Makek

24. CeBr3 scintillator Crystal/Property CeBr3 LYSO Density [gcm-3] 5.10
7.1
Zeff
45.9 66
Decay constant [ns] 20 41
Max. emission [nm]
380
420
Light yield [% NaI]
160
70-80
Refractive index
2.09
1.81
Hygroscopic
Yes No
Active
Low
Surface
Fine ground
Polished
Krakow, 2017
Mihael Makek

25. Simulation of ideal detection efficiency takes into account: density, Zeff, resolution
Geant4, geometry: 8x8 pixels, g impinging on either of 4 central pixels
Showing fraction of detected/incoming 511 keV gammas
Detector CeBr3 20mm
Edep.>50 keV
Edep.~511keV
Single
0.45
0.21
Coincidence
0.20
0.04
Detector LYSO 20mm
Edep.>50 keV
Edep.~511keV
Single
0.57
0.43
Coincidence
0.32
0.18
Krakow, 2017
Mihael Makek