Application of WLS strips for position determination in Strip PET tomograph based on plastic scintillators J. Smyrski 1 , P. Moskal 1 , T. Bednarski 1 , P. Białas 1 , E. Czerwiński 1 , Ł. Kapłon 1,2 , A. Kochanowski 2 , G. Korcyl 1 , J. Kowal 1 , P. Kowalski 3 , T. Kozik 1 , W. Krzemień 1 , M. Molenda 2 , Sz. Niedźwiecki 1 , M. Pałka 1 , M. Pawlik 1 , L. Raczyński 3 , Z. Rudy 1 , P. Salabura 1 , N.G. Sharma 1 , M. Silarski 1 , A. Słomski 1 , A. Strzelecki 1 , W.WiĞlicki 3 , M. Zieliński 1 , N. Zoń 1 1 Institute of Physics, Jagiellonian University, Cracow, Poland 2 Faculty of Chemistry, Jagiellonian University, Cracow, Poland 3 ĝwierk Computing Centre, National Centre for Nuclear Research, Otwock-ĝwierk, Poland Keywords: Time-of-Flight Positron Emission Tomography, Scintillator Detectors Abstract: A method of determination of a gamma quantum absorption point in a plastic scintillator block using a matrix of wavelength-shifting (WLS) strips is proposed. Application of this method for improvement of position resolution in newly proposed PET detectors based on plastic scintillators is presented. The method enables to reduce parallax errors in reconstruction of images which occurs in the presently used Positron Emission Tomography scanners. Section 1: Introduction Plastic scintillators are characterized by relatively short light pulses with decay time on the order of a nanosecond and, therefore, they are widely used in nuclear and particle physics experiments for fast timing measurements. Typically, they have a form of a strip with rectangular cross-section and are read out at both ends by photomultipliers (see e.g. Ref.[1]). Also other solutions are used such as for example scintillator plates read out by arrays of photomultipliers [2,3]. The high timing resolution offered by the plastic scintillators is exploited in a newly invented type of positron emission tomograph (PET) using such scintillators for detection of the 511 keV gamma quanta originating from positron annihilation. Two alternative solutions of the tomograph were proposed [4]. One solution, referred to as the strip PET [5], contains scintillator strips read out by pairs of photomultipliers and arranged around a cylindrical surface forming a tomograph tunnel. Position of the gamma quantum interaction point in the strip - further on we call it shortly "the interaction point" - is determined on the basis of a time difference in propagation of light pulses registered by the pair of photomultipliers. The second solution, referred to as the matrix PET [6], uses plastic scintillator plates read out by arrays of photomultipliers. Registered amplitude and time of propagation of light pulses allow for localization of the interaction point in the plate. A key feature of both solutions is a high precision in measurement of difference in a time-of-flight (TOF) of the annihilation quanta allowing for determination of position of the positron annihilation along a line of response (LOR). This feature allows for substantial suppression of background in the reconstructed PET images and is one of the main advantages of the plastic scintillators compared to essentially slower inorganic crystals which are used in the contemporary commercial PET scanners. A disadvantage of the plastic scintillators compared with the inorganic crystals is