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

The NA49 Experimental Set-up

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NA49 is a large acceptance magnetic and tracking spectrometer, detecting about 70% of all created charged particles simultaneously in each event. Up to about 1200 hadrons are recorded and identified in Pb+Pb collisions. This is a formidable challenge to experimental technique. It is met with large volume gas-filled high-resolution detectors (Time Projection Chambers). The first two (Vertex TPC's) are placed inside huge dipole magnets with superconducting coils. From the bending of the trajectories in the magnetic field one derives the particle momentum. Two larger Main TPC's are placed behind the magnets to measure very accurately the ionization energy loss of the particles in the chamber gas and therefrom deduce the particle velocity. The time-of-flight measurement between the production target and two walls of scintillation counter tiles (TOF) provides another independent determination of the velocity for an important fraction of the particles. The combined knowledge of the momentum and the velocity allows us to identify the mass of the particles.
	Tracking detectors The tracking of charged particles is performed by time projection chambers (TPC). These consist of a large gas volume in which the particles leave a trail of ionisation electrons. A homogeneous vertical electric field is established by a surrounding field cage made of aluminised mylar strips that are kept at the appropriate electric potential. The electrons drift with constant velocity under the influence of the field towards the top plate where their position, arrival time, and total number are measured with proportional wire chambers. To achieve high spatial resolution the chamber surfaces must be granulated into pixels of one square centimeter area, a total of 180.000 for all TPC's. From the recorded arrival times of the track signals and the known pixel positions one gets a string of 3-dimensional measured points along the particle trajectories.
Main Time Projection Chamber Each Main TPC has a readout surface at the top of 3.9x3.9 m**2 and a depth of the field cage of about 1.1 m. It is filled with a gas mixture of Argon/Carbon Dioxide/Methane in the proportion 91/4.5/4.5. The track signals are read out by 25 proportional chambers providing up to 90 measured points and ionization samples on each particle trajectory. The accuracy of the measurement of the average ionization energy loss for a particle is about 4%.
	Vertex TPC Each Vertex TPC has a gasbox with 2.0x2.5 m**2 top surface area and 0.67 m depth. The inserted field-cage structures exclude the region of 0.12 m on either side of the beam line in which the particle density is so high that trajectories cannot be resolved. A gas mixture of Neon/Carbon Dioxide in the proportion 91/9 is employed. It minimizes the blurring of particle trajectories due to diffusion effects and keeps the distortion from the charge deposited in the gas negligibly small. The readout is performed by 6 proprtional chambers on the top which provide up to 72 measurements and ionisation samples on the particle trajectories. The pixels of the proportional chambers are connected to the readout electronics attached on their backside as can be seen from the picture of one of the assembled Vertex TPC's.
Superconducting Magnet The Vertex TPC's are mounted inside the two 450 ton superconducting magnets, which produce a vertical magnetic field of up to 1.5 Tesla strength. The magnetic field bends the trajectories of the charged particles. The particle momentum is deduced from this deflection.
	Front end electronics The TPC's are read out by highly integrated electronics. The front-end cards are plugged into the backside of the proportional readout chambers. They each perform time and charge measurement as well as digitization for 32 pixels. The data from up to 18 front-end cards are multiplexed on the control boards and transmitted via a single light fibre (orange cable) to the data recording building outside the experimental zone. There the up to 40 Megabytes of raw data per event are compressed to 8 Megabytes in the readout boards for recording on magnetic tape.
Time-of-flight measurement For part of the produced particles we measure the flight time between the production target and two 2.2 m^2 scintillation counter walls (TOF). Each consists of 891 closely stacked scintillators connected to their own photon multiplier tubes. The achieved time reslotion is 60 picoseconds.
	Calorimeters Downstream of the TPC's NA49 has a Ring Calorimeter and a Veto Calorimeter (not shown on the layout picture) The Ring Calorimeter is of the sampling type composed of sandwiched layers of lead or iron absorber and scintillator plates. The summed light signal from the scintillator plates is proportional to the energy of the particles absorbed in the calorimeter. The device has a cylindrical structure, coaxial with the incident beam, and its sensitive area is circular with an outer diameter of 3.0 m and a 56 cm central hole. It is segmented into 240 cells, 24 sectors in azimuth and 10 rings radially. The Ring Calorimeter was used to measure the energy flow due to the particles produced in Pb+Pb collisions. The Veto Calorimeter is of similar type. It is placed behind an iron collimator with a hole around the beam line designed to let pass only those fragments of the beam nuclei which have not interacted in the target foil. The recorded energy tells us which part of the beam nucleus interactead and thus how close the centers of the colliding nuclei approached during the collision.