Status of the experiment

The detector stand supporting 240 CsI crystals was designed at PSI during 1993 (Figure 2). The stand will be enclosed in a cosmic muon veto house in the final setup. The first 25 CsI crystals have been delivered and tested. Several tests were performed in a four week beam period in the E1 area in the summer of 1993. The analysis of the data from this run gave following results:

1. A prototype of the active fiber target was tested successfully. A high sensitivity and low cross talk between individual fibers () were measured. Although only 35 of 140 fibers were equipped with photomultipliers, the whole pion stopping distribution was measured by moving the target perpendicular to the beam direction in separate measurements. Analysis showed that 90 % of the stopped pions are contained in a spot of 20 mm diameter. This led to a redesign of the target with a reduced number of fibers of 33 mm cross section each, covering an area of 27 mm diameter.

2. Uniformity of the light output from several crystals was tested by placing them in a 350 MeV beam with their main body axis perpendicular to the beam. By optimizing surface treatment and cover of the front face of the crystals, a uniformity of was achieved (Figure 3).

3. A 12-crystal section of the CsI calorimeter was assembled and placed in a 70 MeV e beam. The measured energy resolution of about 5 % FWHM agrees well with a Monte Carlo calculation (Figure 4).