Nuclear Instruments and Methods in Physics Research A 451 (2000) 406}413 The grid-geometry time-of-#ight detector used in the NA49 experiment at the CERN-SPS G. Pa H lla*, P. Csato H , Z. Fodor, F. Sikle H r, I. Szentpe H tery, J. Sziklai, Gy. Vesztergombi , A.D. Panagiotou KFKI Research Institute for Particle and Nuclear Physics of the Hungarian Academy of Sciences, P.O. Box 49, H-1525 Budapest, Hungary Department of Physics, University of Athens, Gr-157 71 Panepistimiopolis, Ilissia, Athens, Greece Received 19 April 1999; received in revised form 16 February 2000; accepted 19 February 2000 Abstract The high time and position resolution grid geometry time-of-#ight spectrometer is described. It is used for particle identi"cation as an element of the NA49 hadron detector system at the CERN-SPS. Performance and operational experience in high particle density environment of Pb#Pb collision are discussed.  2000 Elsevier Science B.V. All rights reserved. PACS: 29.90.#r Keywords: Scintillation detectors; TOF spectrometry; Integral method of particle identi"cation 1. Introduction The NA49 experiment aims to study the ha- dronic interactions via ultrarelativistic energy reac- tions induced recently by the most energetic Pb beam from the CERN-SPS, where the transition to a new state of matter, the quark-gluon plasma might occur. This experiment requires the large acceptance hadron detector system [1] consisting of a magnetic spectrometer combined with large Time Projection Chambers (TPC) and calori- meters, completed with large-area time-of-light (TOF) spectrometers suited to identify particles * Corresponding author. Tel.: #36-1-3959220; fax: #36-1- 3959151. E-mail address: palla@rmki.kfki.hu (G. Pa H lla). (, K, p, d, 2 ) in the momentum range below 8 GeV/c (particles of minimum ionization) (Fig. 1). 2. Detector concept The particle identi"cation by the TOF method is based on the relation for the particle mass m"p  ct  s !1  . It requires the measurement of the time of #ight (t  ) of the particles over a #ight path (s) and momentum (p) of the particles. The latter can be obtained from magnetic spectrometer and tracking as well from the geometry of the "xed target and detector system used in the experiment. 0168-9002/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 3 1 8 - 1