JRA13-P3E: Polarized Electrons, Positrons and Polarimetry

Experimental discoveries in nuclear, hadron and particle physics are enabled by developments in accelerator and detector technologies. Facilities such as MAMI and MESA in Mainz, CEBAF 12 GeV and JLEIC in Newport News herald a new age for precision hadron physics. In order to optimally harness the discovery potential of these new facilities, the P3E Joint Research Program aims at pushing further the intensity frontier of polarized electron/positron sources and the precision frontier of electron polarimetry on the basis of novel methods and innovative technologies developed by the P3E partners.

The interest in both polarized and unpolarized intense positron beams for the experimental investigation of the physical world ranges from the macroscopic molecular scale accessible at eV energies down to the most elementary scale of fundamental symmetries probed with hundreds of GeV lepton beams. The production of high-quality polarized positron beams relevant to these many applications is one concern of the P3E project which will benefit facilities including but not limited to ALTO, MAMI, MESA, CEBAF, and JLEIC. It relies on the demonstrated PEPPo method developed by IPNO and TJNAF that is the efficient polarization transfer from a polarized electron beam to the positrons generated by the Bremsstrahlung radiation of the beam in a high Z target. The performance of this technique is limited only by the initial beam polarization and intensity. In addition, the modest initial energy of the required electron beam (> 5 MeV) makes PEPPo applicable at essentially any polarized electron beam facility.

The capabilities of polarized electron source are the initial driver of the PEPPo scheme for polarized positron beams. The availability of intense polarized electron source will correspondingly boost the intensity of the positron beam as well as the demands with respect to the production target. The objective of this task that will develop at TJNAF is to explore promising technologies for the extension of the charge lifetime of today’s polarized GaAs photoelectron guns from tens to thousands of Coulombs. This includes: the minimization of the ion damage effects via improved photogun vacuum quality and/or larger laser spot size; the minimization of the ion production rate by operating with higher photo-gun voltage and/or increased quantum efficiency of the photocathode. Achievements in that matter will benefit any facilities operating polarized electron beams.

Application of the PEPPo technique for a high intensity polarized positron beam at ALTO, MAMI, MESA, CEBAF and JLEIC involve similar electron beams in the 10-150 MeV energy range. Therefore, the production, collection and shaping schemes of the positron beam are common to all these facilities while the final stage of the source depends on the designated application. Our objective is to define and optimize the production of slow (for solid state and atomic physics) and fast positrons (for hadronic physics) on the basis of simulations and crucial experiments. The effort of the IPNO partner will focus on the simulation of the electron conversion target configuration, the collection of positrons, and their moderation or shaping for acceleration.

The high electron beam intensity put severe constraints on the target, particularly with respect to the received high thermic load. Furthermore, such a material stress happens in a radiative environment where the fatigue limits of the target materials do not apply any more. Therefore, material tests in such an environment are required and foreseen at MAMI. The goal is to understand and control the target material behaviour under exposure to a high peak energy deposition density (PEDD) comparable to the expected load at future high-intensity facilities. At MAMI, the UUH and JGU partners together with IPNO and TJNAF will investigate the long-term sustainability of heavily stressed target materials. The successful conduct of this R&D will benefit implementation of positron beams at any electron facility, especially CEBAF and JLEIC where the availability of polarized positron beams would significantly enhance the scientific reach. Precision experiments with polarized electrons and positrons require the best possible measurement of the polarization.

The current state of the art is an uncertainty of 0.6% for 1.165 GeV electrons achieved using the combination of a conventional Møller polarimeter and a Compton polarimeter for the Q-weak parity violation experiment at TJNAF. Future parity violation experiments such as P2 at the MESA accelerator in Mainz aim for a knowledge of the polarization with an uncertainty on the 0.1% level at much lower beam energies. This requires a new approach in the form of a Hydro- Møller Polarimeter. Here the cross-section asymmetry in electron-electron (Møller) scattering is used to access the beam polarization. Atomic hydrogen at cryogenic temperatures is magnetically trapped in a very strong (7T) field, leading to almost complete polarization of the bound electrons, thus eliminating systematic effects due to target polarization. To prevent recombination to molecular hydrogen, the cell walls need to be covered in suprafluid helium, necessitating a 3He/4He dilution cryostat. Detecting the outgoing electron pair requires a tracker tailored to the very special operating conditions close to a high field, high power beam and cryogenic environment. Background from electrons scattering off residual gas in the beam pipe can be suppressed by reconstructing the electron interaction vertex inside the trap volume, which in turn defines strict requirements on the tracker in terms of pointing resolution. At the relevant energies, this resolution is dominated by multiple Coulomb scattering in the detector and passive material, necessitating an extremely low mass detector. The further concern of the P3E project is the design of this detector based on the High Voltage Monolithic Active Pixel Sensors technology developed by the JGU partner.