JEDI Polarimeter (JePo) installed in COSY: A Precision Instrument for EDM Searches
After a careful design, optimization and test phase at an external beamline of the Cooler Synchrotron COSY of Forschungszentrum Jülich (FZJ), the JEDI polarimeter (JePo) has recently been installed into the storage ring. It replaces the venerable EDDA detector, which was one of the earliest experimental installations in COSY, used for investigations of unpolarized, single and double polarized proton-proton elastic scattering experiments. For the past 10 years or so, it´s plastic scintillator system was used as the beam polarimeter for JEDI and other experiments. More recently, it was joined by the forward detector system of the WASA detector.
Both EDDA and WASA will not suffice the requirements for an EDM polarimeter, which is supposed to detect the miniscule change in vertical polarization of an originally horizontally polarized beam due to an electric dipole moment (EDM) of the particles under consideration (proton, deuteron). Thus the JEDI collaboration decided some time ago to design and build a dedicated modular new polarimeter for precision measurements, based on the inorganic scintillator material LYSO (Lutetium-yttrium oxyorthosilicate). This material is used, e.g., in modern PET scanners. The implementation was taken over by a JEDI subgroup, comprising Georgian scientists at IKP-2, led by Irakli Keshelashvili, and at the SMART|EDM_Lab at Tbilisi State University (TSU, Georgia), headed by David Mchedlishvili.
The JePo concept exploits LYSO modules (3x3x8 cm3), individually coupled to modern large area silicon photosensor (SiPM) arrays, which are operating at low voltage. The detector system has radial symmetry and an almost unhindered view onto the target inside the COSY ring, realized by a very thin vacuum window. This makes the polarimeter very efficient for up-down and left-right asymmetry measurements, which constitute the basis for the determination of the beam polarization. Commissioning of the JePo at various defined polarized beam conditions of COSY has demonstrated its functionality. In future it will be employed as the polarimeter for JEDI and possible other users for SPIN physics experiments.
Figure: Photograph of the JePo set-up in the COSY-ring. The beam enters from the left, hitting a target (e.g. carbon) inside the target chamber. The LYSO modules outside the COSY beam pipe view the target through a thin vacuum window. © Forschungszentrum Jülich
The photograph shows part of the JEDI collaboration involved in the project assembled around the JePo: (from left to right) M. Tabidze (HEPI TSU, Georgia), N. Lomidze (HEPI TSU, Georgia), H. Ströher (JARA-FAME, Jülich), O. Javakhishvili (SMART|Lab TSU, Georgia), J. Pretz (JARA-FAME, Jülich, RWTH Aachen), A. Kacharava (Jülich), N. Canale (Ferrara Uni), F. Müller (Jülich), I. Keshelashvili (Jülich), D. Shergelashvili (SMART|Lab TSU, Georgia) and D. Mchedlishvili (SMART|Lab TSU, Georgia). In this picture the COSY beam will come from the back. © Forschungszentrum Jülich
The motivation of the current R&D project is based upon the requirements of the JEDI international collaboration (web link) aiming to measure Electric Dipole Moments (EDMs) of charge particles in storage rings (srEDM). An ERC Advanced Grant (No. 694340, "Electric Dipole Moments using storage rings") for this project was granted to Prof. Dr. Hans Ströher in 2017. The essential contributions of the Georgian group to the JePo via the SMART|EDM_Lab of the Georgian German Science Bridge (GGSB) were made possible by the sizable financial support of the Georgian government (Grant from the Shota Rustaveli National Science Foundation of the Republic of Georgia (SRNSFG)).