Nuclear Instruments and Methods in Physics Research A 367 (1995) 58-61 W&Z&* &MWNoDs IN PNVSCS REsEAncH ELSEVIER SectKXl A Liquid xenon multiwire chamber for positron tomography Wu. Chepel*, M.I. Lopes, H.M. Aratijo, M.A. Alves, R. Ferreira Marques, A.J.P.L. Policarpo zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM LIP-Coimbra and Departamento de Fisica da Universidade de Coimbra. 3oaO Coimbra, Portugal zyxwvutsrqponmlkjihgfed Abstract The liquid xenon multiwire chamber has been proposed as a detector of 5 11 keV gamma-photons for positron emission tomography (PET). Both the scintillation and the ionization signal, read from the anode wires, are used. A test chamber of about 300 cm3 and containing six multiwire ionization cells has been built. The cell is formed by two plane cathodes, 10 mm apart, and 20 anode wires spaced by 2.5 mm tied to pairs resulting in 10 channels, read out independently. The first experiments were performed for a single cell. The number of the anode wire on which the signal is induced gives the depth of interaction in the’ detector with a precision of 5 mm. This is important information allowing one to reduce the parallax error and, therefore, to decrease the tomograph size and improve the solid angle. The position sensitivity across the cell (or along the tomograph ring) is achieved by measurement of the drift time of the electrons from the point of primary ionization to the anode, triggered by the scintillation detected with a photomultiplier tube. The scintillation also supplies a fast trigger for the ‘coincidence analysis. 1. rntroduction From the point of view of instrumentation, the problem of positron emission tomography (PET) is to detect two back-toYback annihilation photons with high efficiency (-80% for a single photon), position resolution in the range of a few millimeters, time resolution as good as possible (it contributes to the decrease of random coincidences and, moreover, a very good time-of-flight resolution may be used for the image reconstruction), ability for the discrimi- nation of photons scattered in the object (by measuring their energy) and a count rate capability of the order of lo’-lo6 SC’ cm-’ of the detecting surface [l]. Liquid xenon fulfills these PET requirements very well. Indeed, it is known to be an excellent scintillator, having a light yield similar to NaI(T1) with decay times of about 3 and 25 ns. Furthermore, the free electrons from the particle track can be easily extracted and collected, allowing one to use various methods of position determination. The idea of using both the scintillation and the ionization properties of liquid xenon to build detectors for different applications was expressed by various authors (see e.g., Refs. [2-61). A few approaches were already undertaken in the 70s and early 80s [2,7,8] for the use of liquid xenon in medical imaging, showing good prospects as well as certain technological difficulties. During the last decade, important achievements in xenon purification and low-noise elec- * Corresponding author. E-mail vitaly@filip3.fis.uc.pt tronics have been made. Recently, a new readout technique with microstrip plates immersed into liquid xenon, allow- ing one to amplify the ionization signal in the detector, was developed [9]. Such progresses have stimulated new activity in this direction and a proposal of a liquid xenon detector for PET was published [5,6]. The use of ionization readout in addition to scintillation allows one to combine the fast timing in the ns range with high position precision. It also allows one to solve the parallax error problem and to use the whole field of view of the tomograph effectively. In the present paper we describe the experimental chamber built to investigate the stated principles, and which might be considered as an element of a prototype of a real detector for PET. First results are presented. 2. Multiwire ionization cell In the present design, the multiwire chamber working in the ionization mode has been chosen as the basic element of the large position sensitive detector. The multiwire chamber technology provides, in a very simple way, the fine granularity of the detecting space and allows one to build a detector of practically unlimited size. The basic element is schematically presented in Fig. 1. It consists of two parallel cathode planes, 1 cm apart, and a row of anode wires in the middle plane. The multiwire cell is perpendicular to the entrance window of the detector so that the y-photons enter the cell parallel to the wire plane. Each photon undergoes one or more interactions with the 01689002/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved S6DI 0168-9002(95)00528-5