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Tracking and time of flight

Three highly segmented scintillating hodoscopes (H2, H3 [34], H4) and three multiwire proportional chambers (MWPC) with cathode-segment read-out (a pad chamber, PC, and two strip chambers, SC1 and SC2, each having a plane of vertical and a plane of horizontal strips) form the tracking system (see Fig.  3.1). In NA44, magnets and tracking detectors are separated in space so that tracks of charged particles are straight. There are no tracking detectors in the target area before the magnets.

Time of flight measurements with accuracy not worse than 100 ps are performed via H2, H3 [34], with the Cherenkov beam counter (described above in section  3.4.1) being the source of the start time reference with 35 ps accuracy. The hodoscopes are segmented along the momentum-sensitive coordinate (i.e. horizontally) into vertical slats, sufficiently narrow so as to be able to detect particle pairs with momentum difference of $50MeV/c$ at $p=10GeV/c$ in different slats. The scintillation light is read out from top and bottom of each slat by two PMTs, so that sensitivity to the vertical coordinate is gained from the comparison (though rendered obsolete with the addition of the strip chambers for the $Pb$ running).

The NA44 MWPCs measure ionization energy losses of charged particles in the 50%/50% $Ar/C_2H_6$ gas mixture. Primary ionization is ``amplified'' in the process of avalanche ionization in the highly non-linear electrostatic field near the 20 $\mu m$ anode wire. A cloud of charge, drifting under the influence of field, induces electrical pulses on the electrodes. The cathode is split into rectangular pieces, read out by separate channels of AMPLEX [30] chips. Normally, the same avalanche results in measurable inductions on several (4-8) channels, so that one can talk, in a sense, of an ``image'' of the avalanche, reconstructable off-line with good resolution: the required RMS of 2-3 hundred $\mu m$ along the direction of the anode wire has been easily achieved. More details about NA44's MWPCs are found in [35].