Production of Highly Polarized Positrons Using Polarized Electrons at MeV Energies

The Polarized Electrons for Polarized Positrons experiment at the injector of the Continuous Electron Beam Accelerator Facility has demonstrated for the first time the efficient transfer of polarization from electrons to positrons produced by the polarized bremsstrahlung radiation induced by a polarized electron beam in a high-$Z$ target. Positron polarization up to 82% have been measured for an initial electron beam momentum of $8.19\mathrm{MeV}/\mathrm{c}$, limited only by the electron beam polarization. This technique extends polarized positron capabilities from GeV to MeV electron beams, and opens access to polarized positron beam physics to a wide community.

Figure 1

Schematic of the PEPPo line and apparatus illustrating the principle of operation of the experiment based on the processes sequence $\overrightarrow{e^{-}}\stackrel{\mathrm{T1}}{\to }\stackrel{\circlearrowleft }{\gamma }\stackrel{\mathrm{T1}}{\to }\overrightarrow{e^{+}}\stackrel{\mathrm{T2}}{\to }\stackrel{\circlearrowleft }{\gamma }\stackrel{\mathrm{S3}}{\to }\gamma$ described in the text. The setup footprint is about $3\times 1.5{\mathrm{m}}^2$.

Figure 2

Measured energy spectra in one of the annihilation detectors (AD) once T1 is inserted in (solid line) or removed from (dashed line) the electron beam path; the top right corner shows the difference between these spectra.

Figure 3

Electron and positron analyzing powers of the central crystal of the polarimeter. Statistical uncertainties were combined quadratically with systematic uncertainties taken from $P_e$, $\overline{P_T}$, and $A_C^e$ to determine actual error bars.

Figure 4

PEPPo measurements of the positron polarization (top panel) and polarization transfer efficiency (bottom panel); statistics and systematics are reported for each point, and the shaded area indicates the electron beam polarization.