EP Method to Measure the Absolute Beam Energy in Hall A at Jefferson Lab

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The EP method was devoloped by the Clermont Ferrand group. The method requires the beam to be directed to a stand alone device in the beam line 17m upstream of the target. The EP device utilizes a target consisting of a rotating 10-30um thick tape of CH2. Measuring on the scattering angle of the electron and the recoil angle of the proton in the p(e,e'p) elastic reaction the beam energy can be determined by the following kinematic relation:

         E=Mp----------------------- +O(m2e/k'2)

Terms in O(m2e/k'2) are neglected. The error due to this approximation is one part in 10-8. qp is always fixed at 60 °, while qe will be in the range of 9 ° and 41 ° depending on the the beam energy which can range from 0.5 to 6GeV respectively.

The respective particles are detected in the the reaction plane using silcon strip detectors. Seven electron detectors are place at each arm, with the dimensions 12.8 x 12.8 mm2. The detectors are situated to detect scattering electrons in the range of 9 ° to 41 °, each being equipped with an associated scintillator as well as with a Cerenkenov counter. In addition there is a proton detector with the dimensions of 51.2 x 25.6 mm2 placed at exacly 60 ° in the reaction plane of each arm. Each proton detector has two scintillators for triggering and flight measurement purposes. Furthermore, each Si detector is equipped with an additional detector oriented perpendiculer to it. This detector is used to make measurements in the transverse plane, which is needed for final accuaracy as well as to distinguish between background and elastic events.

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