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

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.

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) …

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

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

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.

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.

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.

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

Intrinsic timing capability

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.

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

Temperature dependence Breakdown -30C +30C

Temperature dependence Dark count Breakdown -30C +30C

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

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

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%

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

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

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

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!

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 (www.hybrid-pet-mr.eu) DaSiPM2 - INFN (http://www.df.unipi.it/~fiig/) Both examples address the important issue: covering a large area with SiPMs.

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

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

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

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

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

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

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

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

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

DaSiPM2 functional tests Δ 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

DaSiPM2 functional tests Δ 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

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 PDE @ short wavelengths decrease dark count: difficult task new simplified interconnection with electronics

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