The design of the new tagger hodoscope is restricted by the available space between the tagging magnet and the beam dump. For low energetic electrons, the focal plane lies between the pole shoes of the magnet, while for high energetic electrons, the focal plane lies in the beam dump. Therefore the tagger hodoscope is split into a horizontal component which mainly lies in the focal plane and a vertical component.
The actual tagger design includes 120 scintillators. This number is restricted due to the number of available high voltage channels. 54 are placed in the focal plane, while the other 66 are placed, due to spacing problems, in a vertical construction. In the horizontal component of the tagger, only ET Enterprise 9100SB PMTs are used, while ET9100SB and Hamamatsu R7400U PMTs are used in the vertical tagger component. The R7400U, which have a smaller diameter, a lower transit time and are more capable for higher rates, are used because the tracks of the bent electrons are getting closer and in addition due to the bremsstrahlung cross section the electron rate seen by the scintillators increases with higher electron energies.
The scintillators are placed such, that a tagging range of about 10%E0 -90%E0 is covered. Simulations of the primary electron trajectory show that 2 cm between the bent primary electron beam and the middle of the closest scintillator are left. Due to the limited space of 2 cm, it is not possible to increase the tagging range for higher electron energies.
For the new tagger, a design requesting coincidences with two PMTs will be used. The overlap of the scintillators is 55%. The resolution of the tagger is than defined by the overlapping areas of the scintillators. In the horizontal component, the width of the scintillators is chosen such, that the energy width covered by two neighbouring scintillators is 10 MeV.
To assure coincidences with two neighbouring scintillators in the vertical tagger component, at least three scintillators have to be placed in one plane. For higher electron energies the electron tracks get closer. To achieve an energy width of 10 MeV as in the horizontal component, the width of the scintillators has to become smaller and therefore to assure an overlap of 55% the number of scintillators placed behind each other increases. Due to the limited space to the beam dump, the energy width is increased (from 10 MeV to 15 MeV, 20 MeV, 30 MeV and 40 MeV) at four different positions in the vertical tagger component respectively four different electron energies ( 770 MeV, 1180 MeV, 1600 MeV and 1940 MeV, E0 = 2400 MeV), depending on the energy of the primary electron beam. This means the width of the scintillators has to be increased.
A simulation for a primary electron beam energy of 2400 MeV was done to determine the energy resolution of the current tagger design.