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P olarized P ositrons at the J efferson Lab oratory

P olarized P ositrons at the J efferson Lab oratory. Jonathan Dumas (JLab/LPSC) , Joe Grames (JLab) , Arne Freyberger (JLab) , Serkan Golge (ODU) , Charles Hyde (LPC) , Eric Voutier (LPSC). Idaho State University, Idaho Accelerator Center, Jefferson Lab,

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P olarized P ositrons at the J efferson Lab oratory

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  1. Polarized Positrons at the Jefferson Laboratory Jonathan Dumas (JLab/LPSC), Joe Grames (JLab), Arne Freyberger (JLab), Serkan Golge (ODU), Charles Hyde (LPC), Eric Voutier (LPSC) Idaho State University, Idaho Accelerator Center, Jefferson Lab, LPC Clermont-Ferrand, LPSC Grenoble, Old Dominion University • A group of physicists is developping the possible concepts and technics for a high intensity positron source at JLab. • In this effort, the LPSC team is studying the production of polarized positrons from polarized electrons. • J. Dumas is a Grenoble PhD student co-funded by JLab & IN2P3.

  2. Physics Motivations Eric Voutier DVCS Beam Charge Dependent Cross Section DVCS Beam Spin Dependent Cross Section • The beam charge asymmetry (BCA) in deeply virtual Compton scattering (DVCS) isolates the real part of the DVCS amplitude. It is however very unlikely that e+ and e- will be delivered and measured simultaneously in the experimental halls. A polarized positron beam having the quality features of the CEBAF polarized electron beam will provide, via the beam spin asymmetry, a unique way to master systematics and insure a better determination of the Re,Im[DVCS*BH] terms from separate e+ and e- data.

  3. Principe of Operation Eric Voutier Unique Challenges for CW Positron Beams Proposed JLab Scheme • The e- source would consist of a 5-10 MeVhigh intensity longitudinally polarized e- beam. • The polarized e-’s create circularly polarized g’s by Bremsstrhalung radiation in a W target. • Within the same target, the polarized g’s create longitudinally polarized e+’s. Simulations for a 5 MeV longitudinally polarized (100 %) e- beam on 250 µm tungsten foil Simulations for a 5 MeV circularly polarized (100 %) g beam on 250 µm tungsten foil Positron longitudinal polarization Photon circular polarization

  4. GEANT4 Simulations Eric Voutier • At 5 MeV/c, 85% of polarization and a 250 µm tungsten foil, the integrated production rate in the forward direction is ~7 x 10-5 e+/e-, in absence of a magnetic collection system. Positron longitudinal polarization The longitudinal polarization of positrons can be as high as 70%, depending on the positron energy, and is ~ 40% on the average.

  5. 100 nm 14 pairs chekc Technical Challenges Eric Voutier The determinant elements for the achievement of a high intensity high polarization e- beam are: a performant electron gun, a very good photocathode, and a powerful laser. QE ~ 1% & P ~ 85% @ 780nm Strained Superlattice GaAs Load-lock gun Fiber-based laser system Improved electron gun with ultra-high vaccuum J. Grames et al., AIP CP 915 (2007) 1037 Pw ~ 1W @ 780 nm & dt ~ 50 ps @ 499 Hz A beam current of 1 mA was demonstrated for 10 h. Beam polarization not measured.

  6. Test Experiment Eric Voutier • A test experiment is being designed in order to validate this concept. • The angular, energy and polarization distributions of positrons will be measured. • A possible detection system would constitute of a selecting dipole magnet followed by a Compton transmission polarimeter. • 2009 : design completion and construction. • 2010 : data taking.

  7. JPos09 International Workshop on Positrons at Jefferson Lab Jefferson Lab, March 25-27, 2009 Contact: grames@jlab.org Organizing Committee: L. Ceraul (JLab) L. Elouadrhiri (JLab) T. Forest (Idaho State University) J. Grames (JLab) W. Melnitchouk (JLab) E. Voutier (LPSC Grenoble) Internationnal Advisory Committee: X. Artru (IPN Lyon) L. Cardman (JLab) P. Cole (Idaho State University) A. Freyberger (JLab) P. Guichon (CEA Saclay) R. Holt (Argonne National Laboratory) A. Hunt (Idaho Accelerator Center) C. Hyde (LPC Clermont Ferrand) M. Klein (University of Liverpool) K. Kumar (University of Massachusetts) M. Poelker (JLab) J. Sheppard (SLAC) A. Variola (LAL Orsay) Selected Topics: Technology of positron sources Polarized positrons Electron/photon drivers Positron & electron polarimetry Generalized parton distributions Electromagnetic form factors Positron annihilation spectroscopy

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