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

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

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

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

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 320-900 nm (max at 450 nm) 1 p.e. 2 p.e. 3 p.e. Krakow, 2017 Mihael Makek

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 ~ 10-15 ns (depending on cable length) Krakow, 2017 Mihael Makek

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

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

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

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

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

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 degradation @511 keV: 11%  13% Krakow, 2017 Mihael Makek

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

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

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

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

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

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

BACKUP SLIDES Krakow, 2017 Mihael Makek

Single pixel full spectrum Krakow, 2017 Mihael Makek

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

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

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

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

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