The CLAS12 program requires the ability to detect neutral and charged pions at high momenta. Simulations have shown that the existing
electromagnetic calorimeter (EC) of CLAS will not be able to absorb the full energy of the electromagnetic showers produced by electrons
and photons with momenta above 5 GeV. The leakage from the back of the calorimeter would result in a significant loss of energy resolution.
For the simple kinematics of π0 decay, above a momentum of 5.5 GeV, the opening angle of the decay photons becomes too small
to be resolved with the existing EC, which are at a distance of about 7 m from the target. The readout segmentation of the EC is only ~10 cm.
Simulations with the GEANT software for the CLAS12 geometry show that these pions would be seen as a single cluster (i.e., one photon) by the
event reconstruction software.
To reconstruct the energy of high-energy showering particles and to separate high energy π0's and photons, a pre-shower detector (PCAL),
with finer granularity, will be built and installed in front of the current EC. The PCAL will have a similar geometry as the current EC. It will be
a lead-scintillator sandwich with three stereo readout planes. Simulations have shown that 15 layers of 1-cm thick scintillators, segmented into
4.5-cm wide strips, sandwiched between lead sheets of 2.2-mm thickness corresponds to about 5.5 radiation lengths, and will be sufficient to address
issues arising at high energies. Simulations show that good π0 identification can be obtained with full coverage of the EC front
surface. In addition to the improved performance in the reconstruction of electromagnetic showers, the increase of the overall scintillator thickness
in the CLAS12 forward electromagnetic calorimeters will increase the detection efficiency for neutrons.
Last modified: May 31, 2013
Daniel S. Carman