Click on the pictures to see them on full scale.

General remarks:

In both experiments the main tracking device was a set of silicon detectors (silicon telescope). In WA97 (1995 setup) it consisted of 7 planes of Omega2 pixel detectors and of 10 planes of silicon microstrips with a pitch. In NA57 the silicon telescope consisted only of Omega2 and Omega3 silicon pixel planes. This tracking device was placed in order to accept particles at central rapidity () and medium transverse momentum (e.g. for ). The apparatus was placed in a magnetic field (1.8 Tesla provided by the Omega magnet for WA97 and 1.4 Tesla provided by the Goliath magnet for NA57). In WA97 three MWPC's with a pad cathode readout were used, as lever arm detectors, to improve the momentum resolution of fast tracks, while NA57 used a set of double-sided microstrip planes.

In both experiments the Pb+Pb events were triggered by a set of scintillators ("petals") which provided a fast estimation of the charged particle multiplicity. The microstrip multiplicity detectors (MSD) provided a more precise estimate of the event multiplicity to be used in the offline analysis of the centrality.

WA97 layouts:

NA57 layout:

Pixel detectors:

In WA97 and NA57 the track reconstruction in a high multiplicity environment relies mostly on the new technology of the silicon pixels developed by the CERN Microelectronics group (RD19 collaboration). This technique was applied in the construction of two generations of chips: the Omega2 (with cell size ) [1] and the Omega3 (with cell size ) [2]. The basic detector element is the ladder, i.e. a matrix of pixel cells. Several ladders were then mounted on a thin ceramic bus to form a half-plane. A plane of pixels is formed by two half-planes staggered in order to cover the dead areas. The plane active area is .

A total of 0.5×10⁶ channels were read-out in the WA97 experiment; In NA57 more than 1.1×10⁶ pixels were used.

In order to demonstrate the capability of the pixels to handle high multiplicity events, the figure below shows a no-field Pb+Pb event with 153 reconstructed tracks.

Multiplicity detectors (MSD):

The event multiplicity was measured by two planes of silicon microstrips which covered a pseudorapidity region and respectively.

Some photos of the NA57 experiment:

References:

1. M. Campbell et al., Nucl. Instr. And Meth. A342 (1994) 52.

2. E.H.M. Heijne et al., Nucl. Instr. And Meth. A383 (1996) 55.

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