1. New developments of Silicon Photomultipliers (for PET systems)
Claudio Piemonte
FBK – Fondazione Bruno Kessler, Trento, Italy

2. Outline SiPMs for PET systems Critical SiPM properties: signal shape
intrinsic timing
photo-detection efficiency
temperature dependence
Energy and timing resolution
2 examples of innovative systems using SiPMs
TOF-PET/MR
multilayer detector
The data shown in the talk always refer to FBK SiPMs.

3. not sensitive to mag. fields
The (analog) SiPM
tiny micro GM-APD connected in parallel.
each element gives the same signal when fired by a photon
proportional information
with extremely high gain
Very fast response
Solid-state device
compact (thin)
robust
not sensitive to mag. fields
INNOVATIVE SYSTEMS
Some of the main producers:
FBK
Hamamatsu, (MPPC)
MPI-Munich
RMD (SSPM)
SensL (SPM)
ST microelectronics - Catania
Zecotek (MAPD) …

4. What is available? SiPM size: from 1x1mm2 up to 4x4mm2 Cell size:
- from 25x25 to 100x100um2
SEM picture
Most common technology:
epi silicon
poly silicon resistor

5. Single cell signal shape
CDn
RSn
VBDn
RQn
DIODE
CQn CG
nth MICRO-CELL
RQ = quenching resistor
CQ = parasitic cap.
CG = metal parasitic cap.

6. Single cell signal shape
CDn
RSn
VBDn
RQn
DIODE
CQn CG
nth MICRO-CELL
Current signal read out on 50W resistor
followed by a voltage amplifier:
RQ = quenching resistor
CQ = parasitic cap.
CG = metal parasitic cap.

7. Single cell signal shape
Current signal read out on 50W resistor:
1x1mm2 SiPM
CDn
RSn
VBDn
RQn
DIODE
CQn CG
nth MICRO-CELL
fast component
due to CQ
layout dependent
slow component
due to microcell
recharge
Temp. dependent
because of poly res.
RQ = quenching resistor
CQ = parasitic cap.
CG = metal parasitic cap.

8. Single cell signal shape
Current signal read out on 50W resistor:
1x1mm2 SiPM
CDn
RSn
VBDn
RQn
DIODE
CQn CG
nth MICRO-CELL
fast component
due to CQ
layout dependent
slow component
due to microcell
recharge
Temp. dependent
because of poly res.
3x3mm2 SiPM
larger cap. in parallel
to 50W reshapes the
signal from the
micro-cell:
- no fast comp.
- slower signal
RQ = quenching resistor
CQ = parasitic cap.
CG = metal parasitic cap.

9. Intrinsic timing capability
laser pulses
Device illuminated with ultra-short laser pulses at fixed repetition rate.
The fluctuations of the difference in time between successive 1 p.e. pulses have been measured. Dt
1x1mm2 SiPM
40x40um2 cell size
G. Collazuol
NIMA 581 (2007) 461–464

10. Intrinsic timing capability

11. Photo-detection efficiency
PDE = QE x Pt x FF
Quantum efficiency:
dielectric stack:
choose appropriate
dielectrics thickness
and material
doping profiles:
shallow implants for
blue light
Avalanche probability:
electron/holes
electrons should trigger
the avalanche
over-voltage
as high as possible
Fill factor:
each microcell has a
dead border region.

12. Photo-detection efficiency
PDE = QE x Pt x FF
Quantum efficiency:
dielectric stack
doping profiles
Avalanche probability:
electron/holes
over-voltage
Fill factor:
each microcell has a
dead border region.
50x50mm2 micro-cell
n-on-p structure
QE optimized at 420nm
(>90%) in air for
perpendicular light
FF~50%
Data obtained counting
pulses from uniform
low-level illumination

13. Temperature dependence
Breakdown
-30C
+30C

14. Temperature dependence
Dark count
Breakdown
-30C
+30C

15. Temperature dependence
Dark count
Breakdown
-30C
+30C
Quenching resistor

16. Temperature dependence
Dark count
Breakdown
-30C
+30C
Quenching resistor
Temperature must be stable and possibly low!

17. SiPMs in PET – energy resolution
dE/E ~ 1/sqrt(N)
LYSO
4x4x20mm3
Critical SiPM parameters:
photo-detection efficiency
- optical window
- internal QE
- triggering probability
- fill factor
density of microcells
dead time
4x4mm2 SiPM 50x50mm2 cell
Example of energy spectrum
with FBK SiPMs measured
by Philips Research Aachen
(corrected from saturation)
dE/E~14%

18. SiPMs in PET – timing resolution
Critical SiPM parameters:
intrinsic timing
extremely good -> no significant impact when used with LSO
photo-detection efficiency
statistics of emitted light plays a very important ->
we must “see” as much light as possible ->
PDE as high as possible
dark noise
for large SiPMs can be quite high

19. SiPMs in PET – timing resolution (2)
signal shape
output signal is the convolution of SiPM response and light emission
response to LSO (40ns dec. time)
for exponential SiPM current signal
with different time constants

20. SiPMs in PET – timing resolution
Two 3x3mm2 SiPMs in coincidence
LYSO 3x3x15mm3
CRT<430ps FWHM
Measurement at room temperature.
Decreasing temperature better results.
measurement by Philips Research Aachen

21. Real PET system with SiPMs?
Results are very good but they are still a bit worse than recent PMTs.
Possibility to build large area systems?
Cost?
probably present SiPM technology will not replace
PMTs in present PET technology!

22. Real PET system with SiPMs?
Results are very good but they are still a bit worse than recent PMTs.
Possibility to build large area systems?
Cost?
probably present SiPM technology will not replace
PMTs in present PET technology!
On the other side, due to its solid-state nature, the SiPM becomes an
essential component in innovative systems.
2 examples will be given:
HYPERImage - EU/FP7 funded (
DaSiPM2 - INFN (
Both examples address the important issue:
covering a large area with SiPMs.

23. First clinical whole body PET/MR investigations of breast cancer
HYPERImage project
consortium
Development of hybrid TOF-PET/MR test system with improved effective sensitivity
First clinical whole body PET/MR investigations of breast cancer
final goals

24. Research on ToF-PET/MR
Ultra compact solid-state
PET detector based on SiPMs
Why SiPMs?
Type
PMT
APD
SiPM
MR compliant no
yes
ToF compliant

25. Building block of the PET system
The stack
The SiPM tile
The ASIC tile
Mounting and measurements
at Uni. Heidelberg and Philips

26. The SiPM tile 32.7mm 32.7mm Overall fill factor ~ 84%
Flat surface for crystal
mounting
2x2 array of ~4x4mm2 SiPMs
700 working arrays have been
delivered by FBK

27. The stack works More results at next NSS, Orlando (FL), October 2009
M. Ritzert et al., “Compact SiPM based Detector Module for Time-of-Flight PET/MR”, presented at the Real Time Conference, May 10-15, Beijing, 2009
More results at next NSS, Orlando (FL), October 2009 27

28. DaSiPM2 project INFN Pisa Bari Bologna Perugia Trento
PET tomograph for small animals proposed by Pisa Univ.
S. Moehrs et al.,
Phys. Med. Biol,
pp. 1113–1127 (2006)
4 rotating heads
3 stacked layers:
4x4cm2
~5mm-thick scintillator (monolithic slab)
SiPM read-out
Use of monolithic SiPM matrices will:
improve spatial resolution and sensitivity
simplify the assembly 28

29. The DASiPM2 SiPM 8x8 array 1.5mm element pitch read-out on one side
1.2cm
1.3cm
Our largest area
monolithic array!!

30. DaSiPM2 SiPM: breakdown
IV curves of the 64 elements of one array

31. DaSiPM2 SiPM: breakdown
Vbd distributions on different wafers
IV curves of the 64 elements of one array
σ ~ 0.15÷0.4V
Vbd-Vbd_mean distributions in a matrix grouped by wafer
σ ~ 0.12V

32. DaSiPM2 functional testsΔ
Measurements at INFN Pisa
signal from all channels
is summed;
no gain correction
crystal just standing
on the SiPM, bad
optical coupling
More functional results in a following talk by G. Bisogni
A. Del Guerra., “Advantages and Pitfalls of the Silicon Photomultiplier (SiPM) as Photodetector for the Next Generation of PET scanners””, presented at the 11th Pisa Meeting on advanced detectors, La Biodola – Isola d’Elba- Italy, May 24-30, 2009 32

33. DaSiPM2 functional testsΔ
Measurements at INFN Pisa
signal from all channels
is summed;
no gain correction
crystal just standing
on the SiPM, bad
optical coupling
More functional results in a following talk by G. Bisogni
A. Del Guerra., “Advantages and Pitfalls of the Silicon Photomultiplier (SiPM) as Photodetector for the Next Generation of PET scanners””, presented at the 11th Pisa Meeting on advanced detectors, La Biodola – Isola d’Elba- Italy, May 24-30, 2009 33

34. Conclusion
Status:
The SiPM is becoming a reliable and competitive object:
performance is getting closer to PMT
large area monolithic arrays have been produced with
satisfactory yield and first large area systems are
under construction.
Room from improvement in many aspects.
Ongoing R&D at FBK
increase short wavelengths
decrease dark count: difficult task
new simplified interconnection with electronics

35. Acknowledgments FBK Mirko Melchiorri Alessandro Piazza
Alessandro Tarolli
Nicola Zorzi
HyperImage project
Philips
Volkmar Schulz
Torsten Solf
DaSiPM2 project
Alberto Del Guerra
Giuseppina Bisogni
Gabriela Llosa
Sara Marcatili
Gian-Franco Dalla Betta
Uni heidelberg
Peter Fischer
Michael Ritzer