Electronics development for fast-timing PET detectors:

The multi-threshold discriminator Time of Flight PET system Contents 1. Introduction 2. Experimental Setup 3. Results 4. Summary Heejong Kim 3 , Jialie Lin 1 , Octavia Biris 1 , Chin-Tu Chen 3 , Woon-Seng Choong 4 , Henry Frisch 1,2 , Chien-Min Kao 3 , William Moses 4 , Fukun Tang 2 , Qingguo Xie 3 , Lin Zhou 2 June/5/2008 1. Department of Physics, University of Chicago, IL 2. Enrico Fermi Institute, University of Chicago, IL 3. Department of Radiology, University of Chicago, IL 4. Lawrence Berkeley National Laboratory, Berkeley, CA 1

1. Introduction

• Digitize PET event waveform and apply digital-signal processing (DSP).

• Potential advantages: • allow sophisticated information processing • easy to upgrade • inexpensive, high-performance digital components are common • A promising multi-threshold method is investigated by using digitized event waveforms obtained at a 20GHz sampling rate Q. Xie, etal ,

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Potential advantages of digi tally sampling scintillation pulses in timing determination in PET,

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NSS/MIC 2007 , pp. 4271-4274, 2007.

Multi-threshold method cf. PMT signal polarity is reversed for display purpose.

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Multi-threshold discriminator board

Threshold Setting Differential outputs  8 channels/board (4 channels installed): • ADCMP582 SiGe voltage comparator, 37ps rise/fall time • programmable threshold levels for individual comparators, adjustable from 0~-700mV and controlled by UMDDA-08HC DAC • Each channel provides differential LVDS outputs (high=1.26V, low=0.94V)  2 boards are implemented Input 3

2. Experimental Setup

Light Shied Box Na 22 1 2 3 3 4 Tektronix TDS6154C oscilloscope LSO Block diagram for result B&C • Three discriminator thresholds: -100, -200, -300mV.

• Tektronix TDS6154C oscilloscope: 4 input channels 15 GHz bandwidth 20GS/s sampling.

• Trigger for the oscilloscope Lecroy 623B discriminator, Lecroy 365AL coincidence.

PMT Holder from LBL 4

3. Results

A. Time offset and resolution of discriminator boards B. Pulse shape reconstruction Energy resolution Scintillation decay constant C. Timing uncertainty of the multi-threshold method D. Coincidence timing resolution F. Time readout using TDC (HPTDC) 5

A. Time offset and resolution of the discriminator board

Comparator 1 Comparator 2 Block diagram for result A TDS6154C oscilloscope Top : comparator outputs Bottom: Zoom in of the leading edge 6

A. Time offset and resolution of the discriminator board (Cont

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d)

board#1 D t (ps) s ( D t) ch1-ch2 194.9 7.2

ch1-ch3 404.8 7.6

ch1-ch4 613.6 8.4

board#2 ch1-ch2 209.5 7.1

ch1-ch3 414.8 7.8

ch1-ch4 630.4 8.0

D T( ch1-ch2) Ch4 is the closest to the input • Measurements used to correct time offset between channels • Time resolution of single channel was estimated to be ~13ps (FWHM) 7

B. Pulse shape reconstruction

   use three thresholds: -100, -200 and -300mV time-offset correction applied reconstruct pulse shape using 6 time measurements • linear fit on the leading • exponential fit on the falling Blue: PMT signal Red: multi-threshold timing, time-offset corrected Bottom : Zoom in of the leading part 8

B. Energy resolution

 Pulse height:  20Gs/S waveform: sample sum  multi-threshold: area under reconstructed curve  Energy resolution: • multi-threshold: 18% • 20Gs/S waveform: 13% Integrated charge 9

B. Decay constant

Decay constant  exponential fit on the tail gives the decay time constant  Results: mean: 43ns vs. 47ns width (FWHM): 14ns vs. 4ns 10

C. Timing uncertainty of the multi threshold method

(1) (2) D t(1-2) 49ps FWHM 11

D. Coincidence timing resolution

332ps FWHM Differential time The differential time has a 332ps FWHM cf. Timing resolution of ~300ps using CFD discriminator (Q. Xie et al. NSS/MIC 2007, p4271) 12

D. Time readout using HPTDC

Sample output from HPTDC HPTDC board (LBL) multi-threshold board    4 thresholds: -100, -200, -300, -400mV time-offset applied consistent with above results 13

Summary

     Multi-threshold discriminator boards with adjustable thresholds developed and evaluated Time offset between comparator channels measured and corrected; timing resolution of a single comparator: ~13ps (FWHM) Tested with LSO+R9800PMT and Na-22 • reconstruct event pulse using time outputs of the discriminators • 18% energy resolution at 511keV obtained by using 3 thresholds (-100mV, -200mV and -300mV), compared to 13% by using the full PMT waveform • the use of the multi-threshold method with three thresholds estimated to contribute a timing uncertainty of ~49ps FWHM.

A 332ps FWHM coincidence timing resolution obtained by using 2 thresholds (-100mV and -300mV), compared to ~300ps when using a CFD Initial successful results with HPTDC obtained 14