rotated by ≈ 45 degrees Beam Diaphragm Many PMT’s Light condensor Mirror geometry:

Example 1: “Rugby ball” half axes: a = 50 mm, b=80 mm Detector at 30 cm distance from beam axis 100 mm radial 100 mm azimuthal Spot along beam (mm) Spot around beam (mm) Old simulation simplified Too big (?)

“Rugby ball” half axes: a = 100 mm, b=80 mm Detector at 20 cm distance from beam axis 40 mm radial 120 mm azimuthal Spot along beam (mm) Spot around beam (mm) Example 2: Border with neighbours Old simulation simplified

Full simulation with mirrors perpendicular to beam: 40 x 120 mm PM #1 X at PM [mm] Y at PM [mm] Ellipsoid:a=80 mm, b=100 mm

Radial offset [mm] Azimuthal offset [mm] All 8 PM’s overlaid (and rotated to same position): 120x40 mm

X at PM [mm] Y at PM [mm] All 8 positions overlaid in real space Overlap only because size too large if mirrors not tilted (radius only 100 mm)

Full simulation with mirrors tilted by 45 o : Ellipsoid:a=80 mm, b=100 mm X at PM [mm] Y at PM [mm] Very narrow (plane seen from the side) Now no overlaps!

Azimuthal offset [mm] Radial offset [mm] All 8 PM’s overlaid and rotated to overlap: 40x80 mm

Other example: a = 50 mm b = 80 mm r = 200 mm 80 x 100 mm Radial offset Azim. offset

Or with a spherical mirror (r=80 mm) ay 200 mm from axis: 80 x 70 mm Azim. offset Radial offset

Allows to study finer segmentation of counters over light spot Possibility to use individual small PMT’s rather than MAPMT’s No need to have mirrors of extreme quality