Simple modification of Compton polarimeter to redirect synchrotron radiation

Abstract

Synchrotron radiation produced as an electron beam passes through a bending magnet is a significant source of background in many experiments. Using modeling, we show that simple modifications of the magnet geometry can reduce this background by orders of magnitude in some circumstances. Specifically, we examine possible modifications of the four dipole magnets used in Jefferson Lab’s Hall A Compton polarimeter chicane. This Compton polarimeter has been a crucial part of experiments with polarized beams and the next generation of experiments will utilize increased beam energies, up to 11 GeV, requiring a corresponding increase in Compton dipole field to 1.5 T. In consequence, the synchrotron radiation (SR) from the dipole chicane will be greatly increased. Three possible modifications of the chicane dipoles are studied; each design moves about 2% of the integrated bending field to provide a gentle bend in critical regions along the beam trajectory which, in turn, greatly reduces the synchrotron radiation within the acceptance of the Compton polarimeter photon detector. Each of the modifications studied also softens the SR energy spectrum at the detector sufficiently to allow shielding with 5 mm of lead. Simulations show that these designs are each capable of reducing the background signal due to SR by three orders of magnitude. The three designs considered vary in their need for vacuum vessel changes and in their effectiveness.