CLAS-Note 96-011 The reaction p(e,e'p)π0 to calibrate the Forward and the Large Angle Electromagnetic Shower Calorimeters

The energy calibration of the two electromagnetic shower calorimeters (forward and large angle) of CLAS is proposed using the reaction p(e,e’p)π° like a uniform source of monochromatic pions. A preliminary study to understand the reaction kinematics, the energy range and the calibration feasibility as well as an estimate of the counting rate is presented. 1.1 F.E.S.C in CLAS The CLAS Forward angle Electromagnetic Shower Calorimeter [Br95] consist of six triangular submodules which are assembled together into a hexagonal endcap. The active area of this assembly covers scattering angles of 8 to 45 degrees. The detector is a sampling calorimeter comprised of 39 layers of scintillator strips alterned with lead, divided into two stages of longitudinal read-out. The strips vary in length from 20 to 470 cm. Each submodule is constructed in a pointing geometry, such that the scintillator volume of any three mutually overlapping strips forms a truncated triangular pyramid with its apex at the target location. The detector as constructed consists of 16.5 radiation length of material (95% in the lead) and 0.98 nuclear interaction lengths (48% in the scintillator) for perpendicular incidence. The purposes of the forward electromagnetic shower calorimeter include: to provide significant electron-pion discrimination; measurement of time, energy and direction of neutrons and photons; and the identification of electrons at the trigger level with good position uniformity. 1.2 L.A.E.S.C. in CLAS The Large Angle Electromagnetic Shower Calorimeter [Ta95] is the extension of the forward calorimeter of CLAS for angles larger than 45° in the laboratory system. It is a multilayer calorimeter (see fig. 1) made by lead sheets and plastic scintillator bars; in each layer of scintillators, the bars are oriented at 90° with respect to the neighbour one, giving a measurement of X and Y coordinates of the energy deposition. Also Z is partly accessible being LAESC splitted into two parts (Inner and Outer) independently read out. It adds capability to FESC in e/π separation, π°/η mass reconstruction and neutron detection.