DEVELOPMENT OF MULTIGAP RESISTIVE PLATE CHAMBERMRPC FOR MEDICAL

DEVELOPMENT OF MULTI-GAP RESISTIVE PLATE CHAMBER(MRPC) FOR MEDICAL IMAGING Arnab Banerjee 1, Arindam Roy 1, Saikat Biswas 1, 2*, Subhasis Chattopadhyay 1, Sanjay Pal 1 , Ganesh Das 1 1 Variable Energy Cyclotron Centre, 1/AF Bidhan Nagar, Kolkata-700 064, INDIA 2 GSI Helmholtzzentrum für Schwerionenforschung Gmb. H, Darmstadt, Germany-64291 *S. Biswas@gsi. de FRONTIER DETECTORS FOR FRONTIER PHYSICS, 20 -26 May 2012, La Biodola, Isola d'Elba, Italy WHY MULTI GAP? INTRODUCTION The Resistive Plate Chambers (RPCs), made up of high resistive plates as electrodes, which help to contain the discharge created by the passage of a charged particle or an ionizing radiation in a gas volume, and pick-up strips (used to collect the resulting induced signals) has been developed for various high energy physics and nuclear physics experiments as a low cost tracking detector for its good position and timing resolutions [1]. Apart from its enormous use in high energy and nuclear physics experiments the new category of such detectors, called Multi-gap Resistive plate chamber (MRPC) , opens up a new possibility to use them in the field of nuclear medicine, e. g, in TOF-PET systems. Conventional PET scanners use expensive scintillators for detecting photon pairs with very high efficiency and excellent energy resolution. However, the scanners suffer from the limitations of a short Field of View (FOV) and relatively poorer position resolution. Extremely good time (~100 s ps) and position (~ mm) resolution make the relatively less expensive MRPC-based TOF-PETs to be an alternative candidate. ü Both side of the intermediate plate and inner surfaces of the other Bakelite sheets are coated with viscous silicone (coefficient of viscosity = 5. 5 pa. s at 230 C) to make the surface smooth [3]. Length : 230 mm Breath : 230 mm Width : 10 mm The operation of a single gap RPC is described in Ref. [1]. The typical efficiency and the time resolution obtained from the single gap RPC is 95% and 2 ns respectively. Grooving for Perspex gas flow (4 base mm X 2 mm) 10 mm Steps : ü The edges of the Bakelite sheets are sealed by applying a layer of the epoxy adhesive to prevent permeation of moisture. One problem with the conventional Single gap RPC is that extra large avalanche charge on a local spot on the surface results in local inefficiency. For a multi-gap RPC, if the internal plates are thin enough, a deposition of charge on a surface of one of the plates will cause a local decrease in electric field in one sub-gap, but the neighbouring sub-gap will have an increase in field. This compensation helps to produce a longer efficiency plateau for a MRPC [2]. 230 mm ü After proper cleaning, a graphite coating is made on the top surface of topmost Bakelite and lower surface of the lowermost sheet to distribute the applied voltage uniformly over the entire RPC. ü To collect the induced charges, pick up strips, made of copper (20 micrometer thick), are placed above the graphite coated surfaces. 2. Six gap Glass –based MRPC: ü The signal from different strips is sent through a ribbon cable, followed by RG-174/U coaxial cables using proper impedance matching. ü The silicone treated surfaces are kept under heat lamp for two hours to allow the viscous fluid to fill up all the micro-holes on the surface. Dimension : 200 mm X 80 mm Pick-up strip used in the experiment Glass: procured from GSI Glass Thickness: 600 μm Gas gap: 200 μm üThe MRPC module is leak- checked using Argon and Helium sniffer probes. Fabrication procedure is same as stated earlier except graphite paint is used as conductive material Test Results: Schematic representation of cosmic ray setup I-V Characteristics Four gap Bakelite MRPC 1. Four Gap Bakelite-based MRPC: Efficiency vs. High voltage for 4 -Gap Bakelite MRPC Time delay & FWHM for 4 -Gap Bakelite MRPC Six gap Glass MRPC M The curve shows two distinct slopes -- at lower voltage the inverse of the slope represents the resistivity of the polycarbonate spacer where as at higher voltage it represents the resistivity of the Bakelite sheets. Time spectrum for Six gap glass MRPC Time resolution : START with Master trigger STOP with MRPC [4] The efficiency increases with the increase of the applied voltage and reaches a plateau from 12. 5 k. V. The figure suggests that both FWHM and time difference decrease with voltage. Above 13. 5 k. V the time resolution appears to be ~ 2 ns. Summary and Outlook : Charge spectrum for Six gap glass MRPC Glass MRPC based PET imaging system Ø MRPCs are suitable candidates for PET imaging requiring good time and position resolution. Ø One prototype bakelite MRPC was build with 0. 6 mm × 4 gas gap and one prototype glass MRPC was build with 0. 2 mm × 6 gas gap. Ø Streamer mode operation of the bakelite MRPC gives time resolution (σ) ~ 900 ps Ø In the avalanche mode operation of the glass MRPC a time resolution (σ)~ 440 ps has been obtained. Ø Simulation to optimise the number of gaps and width of individual gap is in progress. References: [1] R. Santonico, R. Cardarelli, Nucl. Instr. and Meth. 187 (1981) 377. The time resolution (σMRPC) comes out to be ~ 440 ps The mean of the charge spectrum is found to be at 6 p. C The system is ready to test [2] E. Cerron Zeballos, et al. , Nucl. Instr. and Meth. A 374 (1996) 132. [3] S. Biswas, et al. , Nucl. Instr. and Meth. A 602 (2009) 749. [4] S. Biswas, et al. , Nucl. Instr. and Meth. A 617 (2010) 138.