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Continuous Scintillator Slab withMicrochannel Plate PMT for PET

Heejong Kim1, Chien-Min Kao1, Chin-Tu Chen1,Jean-Francois Genat2, Fukun Tang2, Henry Frisch2,

Woong-Seng Choong3, William Moses3

1. Department of Radiology, University of Chicago, IL2. Enrico Fermi Institute, University of Chicago, IL3. Lawrence Berkeley National Laboratory, Berkeley, CA

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Outline

1. Introduction2. Material and Methods3. Experimental Tests4. Results5. Summary

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1. Introduction

The advantages of using Microchannel Plate(MCP) PMT. Position sensitiveness. Fast time response. Compact size than conventional PMT.

Monolithic scintillator slab. Higher packing fraction. Convenience in Crystal machining.

Transmission Line readout scheme. Readout both ends of the strip.

A PET detector design, using continuous slab LSO with MCP PMT, has been investigated. The preliminary results of Geant4 simulation study are presented here. The real tests to validate the simulation has been conducted with Photonis Planacon MCP(XP85022) and the results are also shown.

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MCP & Transmission Line board

Fig.1 - Photonis Planacon MCP(XP85022) with 1024(32x32) anodes(left) and Transmission line(TL) baord with 32 microstip(right). One microstrip is connected to one raw of MCP anode(32) and signals are readout at both ends of a TL.

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2.Material and Methods

One detector module consists of two layers of LSO slab and MCP assembly.

LSO slab dimension : 102x102x5mm3. MCP assembly dimension : 102x102x9.15mm3 It

includes photocathode and TL structure. (MCP with 8’’x8’’ area is under development.)

MCP is coupled to LSO slab at back side. An alternative configuration was also simulated.

Single layer of 10mm thickness slab. Two MCPs are coupled the LSO slab at both sides.

Distance between two detector modules : 5cm

A. Detector configuration

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Double layers vs Single layer

Fig. 2 Simulation set-up with tow detector modules. Two layers of scintillator slab(5mm thick) and MCP assembly( left). Bule colored part is the scintillator slab and MCP is shown in grey. For comparison, 10mm thick single scintillator slab are coupled with two MCPs at both sides.(right)

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B.Simulation Setup

Photon generation and transport was simulated by Geant4.

Two 511keV photons are generated back to back at the middle of two detector modules and sent to the detector centers.

The side edges of the slab was treated black to avoid the light reflection at the edge.

The surface between LSO slab and MCP glass was optically coupled with the optical grease.

LSO characteristics( simulation input parameters) Light yield : 30,000/MeV Decay time : 40ns Resolution : 10.4%( FWHM)

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Q.E & LSO emission spectra

Fig. 3 Quantum efficiency of XP85022 as a function of the wavelenth(left). Emissin spectra of LSO before(after) XP85022 Q.E applied : Geant4 generated(right).

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Signal Readout Scheme

Electrical signal was formed based on the measured XP85022 characteristics combining with the Geant4 simulation outputs: optical photon’s position and arrival time at photocathode.

For each individual photo electron, the measured single photo electron response assigned. Convolute pulses due to all the photo-electron within the area of TL strip.

TL signal then propagates to both ends of TL. In the first layer MCP, 24 TL strips run vertically. By

applying Anger logic to measured TL signals, X coordinate can be obtained.

TLs runs horizontally in the second layer to get Y coordinate.

In addition, the position can be measured form the measured time difference at both ends of TL.

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3. Experimental Tests

XP85022Chevron type, 10um pore

Textronix DPO7354 Oscilloscope recorded the waveform of TL at 10GS/s.

The charge of pulse are obtained by integrating TLwaveform.

The test set-up was built using a XP85022 MCP and TL board to measure the characteristics of the MCP. The measured single photo-electron response(SER) was fed to the simulation for electrical signal

Fig. 4 MCP/TL assembled for the real test. 4 TL channels were connected through SMA to the DPO7354 Oscilloscope. A LSO crystal with 1”x1” area is on top of MCP.

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A. Single Electron Response(SER)

SER was measured using the pulsed LED as a light source. The rise time of SER was measured ~560ps. The SER signal was spread in ~5 TL. ( may due to the gap between MCP out and anode)

Fig. 5 Integrated charge of SER waveforms(left). Averaged waveform of SER; the maximum TL signal only(right).

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The MCP gain vs HV

Absolute gain of the XP85022 was calculated from the integrated charge of SER.

The gain at HV = -2300V : 1.5 x 106

Fig. 6 XP85022 MCP gain as a function of HV.

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B. Responses to 511keV photon

MCP/TL coupled to 25x25x8.5mm3 LSO crystal. Hamamatsu R9800PMT with 6.2x6.2x25mm3 LSO for coincidence Use Na22 for positron source. Waveform recorded by Tektronix DPO7354 scope

Fig. 7 Test set-up for 511keV gamma coincidence(left) Energy distribution of R9800PMT(right).

E resolution =13.8% FWHM

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Energy( real test)

Energy sum of 3 TLs : only left side of TL. The peak is not at 511keV.

Light is widely spread and readout channel was only 3.Peak at ~75keV of 3 TL corresponds 511keV.

Energy sum of 3TL is well matched.

Fig. 8 Energy sum of 3 TL signal by 511keV photon.

Real Test

Simulation of Test Setup

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Coincidence Timing ( real test)

Event selection requirement for the coincidence timing. R9800PMT : 400 < E < 600 keVMCP 3TL Sum : 60 < E < 90keV

Coincidence timing resolution = ~590ps( FWHM)contribution from R9800PMT side = ~200ps (FWHM)

Real Test

Real TestSimulation of the test setup

Fig. 9 coincidence time distribution.

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4. Results - Energy

Sum of 5 TL signals Energy resolution : ~12% Use the measured XP85022 SER for the TL signal.

Fig 10. Energy distribution.

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Result - Coincidence Timing

The event time was extracted by Leading Edge(LE) to the maximum TL signal. ( Threshold : 3mV)

Energy window [400, 600] keV required for coincidence event. The detection efficiency : ~14%( ~37% for one module). Coincidence timing resolution : ~360 ps. Alternative configuration in Fig.2 : ~600ps

Fig. 11 Event time difference of coincident event.

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Results - Position

The centroid of the most energetic 5 TL signal. RMS of X reconstructed coordinate = ~2.3mm Degraded near edge due to light absorbing at the surface. Y coordinate using the time difference : ~3.2mm (RMS)

Fig. 12 Reconstructed X coordinate(left). The 511keV gamma injected 20mm off the center. The deviation of reconstructed position from the injection point: (X_recon - X_gamma)

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5.Summary

A PET detector design using continuous scintillator slab and MCP PMT with Transmission Line readout was studied.

Geant4 was used for optical photon simulation. Real test setup using XP85022 MCP and TL board

was built to measure SER of MCP. The measurement from the test set-up was used to

simulate TL signal with Geant4 output. The preliminary results from the study show

promising results. Energy resolution~12% at 511keV was obtained. The coincidence time resolution ~360ps with

~13% detection efficiency were estimated.

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References

1. J. L. Wiza, NIMA 162 (1979) 587-6012. F. Bauer et al, IEEE NSS/MIC CR(2006) 2503-25053. J. Anderson et al, IEEE NSS/MIC CR(2008) 2478- 24814. S. Agostinelli et al, NIMA 506(2003) 250-3035. http://www.photonis.com 6. J.S. Huber et al, IEEE TNS 48(2001) 684-6887. J.-F. Genat et al, NIMA 607(2009) 387-393