Large Acceptance Hadron Detector for an Investigation of Pb- and p-induced Reactions at the CERN~SPS

%NA49 %title\\ \\Experiment NA49 measures charged particle and neutral strange particle production over a large part of phase space in Pb and p beam reactions. The main aim is the search for evidence transition predicted by QCD for matter of sufficient energy density. The transient existence of a deconfined phase in the early stage of the collision is expected to modify the particle spectra and composition, the correlations and the space-time evolution of the final state as compared to a scenario of confined hadronic matter. In addition to high precision inclusive measurements of these quantities, the large particle multiplicity in Pb+Pb collisions and the wide acceptance of NA49 allow for the first time to measure the event by event fluctuations of observables like mean transverse momentum or temperature, the K/$\pi$ ratio, and the multiplicity. In order to study the effects of normal nuclear matter p+p and p+nucleus collisions are measured for comparison. The latter data will provide information on these reactions of yet unavailable precision and detail.\\ \\The detector of NA49 is a large acceptance magnetic spectrometer with dE/dx and time of flight measurement. The beam particle trajectory is accurately determined by small proportional chambers (BPD's). Two time projection chambers (VTPC1,2 of 3m$^{3}$ each) in large aperture (1m gap) analysis magnets of 4.5Tm bending power each and two TPC's downstream of the magnets (MTPCL,R of 20m$^{3}$ each) determine the trajectories and the specific energy loss dE/dx (4-5\% accuracy) of charged particles. The TPC's use a slow drift-velocity low-diffusion gas mixture of Ne/CO$_{2}$ 90:10 (VTPC's) and Ar/CO$_{2}$/CH$_{4 }$ 90:5:5 (MTPC's) in order to optimise tracking accuracy and 2-track resolution. Time of flight is measured by two scintillator tile walls (TOF-L1, TOF-R1) of 891 elements each and 60-70ps time resolution and by two scintillator grids (TOF-L2, TOF-R2) of 90ps time resolution. Particle identification over a wide momentum range is accomplished by combining the information from dE/dx and TOF. A cell structured ring calorimeter (RCAL) measures the transverse energy flow and event anisotropy. A forward calorimeter (VCAL) shielded by a collimator (COLL) detects the energy of the spectator nucleons in Pb beam collisions and allows to trigger on the impact parameter of the reaction.\\ \\Pestov counter time of flight arrays were constructed and placed at the sides of VTPC1 to extend the acceptance for KK interferometry and $\phi$ meson production studies. A special multiplicity detector around the target has been built and is used to select the impact parameter in p-nucleus collisions at the trigger level.\\ \\