The OPERA magnetic spectrometer

The OPERA neutrino oscillation experiment foresees the construction of two magnetized iron spectrometers located after the instrumented lead-nuclear emulsion targets. The spectrometer consists of a dipolar magnet without air gaps. The driving coils are located in the return yokes which are connected by two columns of iron slabs interleaved with Resistive Plate Chambers. The particle trajectories are measured by layers of vertical drift tube planes located before and after the magnet. In this paper we review the construction of the spectrometers. In particular, we describe the results obtained front the magnet and RPC prototypes and the installation of the final apparatus at Gran Sasso. We present the ballistic techniques employed to calibrate the field in the bulk of the magnet and the results in term of field uniformity and magnetic properties of the iron. Moreover, we demonstrate that a field calibration at the level of 3% can be reached and we discuss the limiting systematics. The mass production of RPC started in Jan 2003. Results of the tests and issues concerning the mass production are reported. Finally, the expected physics performance of the detector is described; estimates rely on numerical simulations and the outcome of the tests described above.