Design of the Mainz Active Polarized Proton Target

At the MAMI electron accelerator in Mainz, Germany, the A2 Collaboration investigates the spin-polarizabilities of the proton by scattering experiments with spin-polarized energy-tagged photons. An active polarized proton target was developed to identify the reactions by detecting recoil protons inside the Mainz-Dubna dilution cryostat. Polarizable plastic scintillator disks are stacked in a target head made of wavelength-shifting material or of PMMA. Material and geometry of the head determine its scintillation light output. The light is then guided through a tube of borosilicate glass to silicon photomultipliers. All parts of the active polarized proton target are operated at cryogenic temperatures.