Development of a Dedicated Precision Polarimeter for Charged Particle EDM searches at COSY

Abstract

The international JEDI (Julich Electric Dipole moment Investigation) collaboration is preparing a first-ever direct measurement of the deuteron Electric Dipole Moment (EDM), using the COSY storage ring at Forschungszentrum Julich (Germany). A new polarimeter is required to detect the very slow and minuscule polarization change with time: starting in 2016, we have designed, built and commissioned a new modular type storage ring polarimeter based on LYSO inorganic scintillator crystals. The polarimeter concept exploits LYSO modules (3$\times$3$\times$8 cm$^3$), individually coupled to modern large area SiPM arrays which are operating at low voltage compared to conventional vacuum photo tubes. The detector system and its vacuum system have radial symmetry and a thin exit window, making the polarimeter very efficient for online up-down and left-right asymmetry measurements. After several tests at the external COSY beam, we have recently installed the complete system in the COSY ring for use with internal beams. We have done two commissioning beam times to test the detector at various polarized beam conditions. After that, it will be employed as the general purpose polarimeter for JEDI and possibly other users at COSY.