Fig. 1. Picture of the OPERA cosmic ray test facility.

Fig. 2. Each trigger wall is composed by 16 rows, whose layout is shown in this figure. Four glass RPCs are faced to 8 Flat Cable Strips (FCS). Both the ends of the FCSs are connected to the LNGS cards that provide the fast digital OR of the strips. The fast signals from the front and rear side of each wall are sent to the TDC to determine the x-coordinate with the time difference method. The z-coordinate is measured by means of a standard digital chain.

Fig. 3. Event display of a crossing cosmic ray muon (front view).

Fig. 4. Single counting rate as a function of the applied voltage for the 128 glass RPCs of the trigger walls.

Fig. 5. Single counting rate as a function of the time. The applied voltage is 7.6 kV, for all the 128 glass RPCs.

Fig. 6. Current drawn by the glass RPCs as a function of time. The RPCs are divided into 8 groups of 16 detectors. The supplied voltage is 7.6 kV, apart the period indicated in the figure.

Fig. 7. Experimental setup to evaluate the resolution of the x-coordinate measured with the time difference method. The crossing muons are selected with a scintillator finger 5 mm wide, orthogonal to the FCSs. The LNGS cards signals () are sent to a TDC.

Fig. 8. Distribution of the time difference between the signals from the rear and front LNGS cards.

Fig. 9. Inefficiency map of a RPC. Well visible are the pinholes (top edge) and the spacers.