Observation of Polarized Positrons from an Undulator-Based Source

An experiment (E166) at the Stanford Linear Accelerator Center has demonstrated a scheme in which a multi-GeV electron beam passed through a helical undulator to generate multi-MeV, circularly polarized photons which were then converted in a thin target to produce positrons (and electrons) with longitudinal polarization above 80% at 6 MeV. The results are in agreement with Geant4 simulations that include the dominant polarization-dependent interactions of electrons, positrons, and photons in matter.

Figure 1

Conceptual layout (not to scale) of the E166 experiment. A 46.6-GeV electron beam entered from the left and was deflected by magnet $\mathsf{D}\mathsf{1}$ after traversing the undulator. Part of the beam of $\approx 8\mathrm{\text{−}}\mathrm{MeV}$ circularly polarized photons created in the undulator was converted to positrons in a target 35 m downstream of the undulator, and the rate and polarization of the positrons and unconverted photons were subsequently diagnosed in the spectrometer $\mathsf{D}\mathsf{2}$. $\mathsf{B}\mathsf{P}\mathsf{M}=\mathrm{\text{beam-position monitor}}$, $\mathsf{B}\mathsf{T}=\mathrm{\text{beam toroid}}$, $\mathsf{C}=\mathrm{\text{collimator}}$, $\mathsf{H}\mathsf{C}\mathsf{O}\mathsf{R}=\mathrm{\text{horizontal-correction magnet}}$, $\mathsf{H}\mathsf{S}\mathsf{B}=\mathrm{\text{hard-soft-bend magnet}}$, $\mathsf{O}\mathsf{T}\mathsf{R}=\mathrm{\text{optical-transition-radiation monitor}}$, $\mathsf{P}\mathsf{R}=\mathrm{\text{beam-profile monitor}}$, $\mathsf{T}\mathsf{1}=\mathrm{\text{target}}$, $\mathsf{W}\mathsf{S}=\mathrm{\text{wire scanner}}$.

Figure 2

Schematic of the photon and positron diagnostics. $\mathsf{A}\mathsf{1}$, $\mathsf{A}\mathsf{2}=\mathrm{\text{aerogel Čerenkov detectors}}$, $\mathsf{C}\mathsf{2}-\mathsf{C}\mathsf{4}=\mathrm{\text{collimators}}$, $\mathsf{D}\mathsf{2}=\mathrm{\text{dipole spectrometer magnet}}$, $\mathsf{C}\mathsf{s}\mathsf{I}=3\times 3$ array of CsI crystals, $\mathsf{G}\mathsf{C}\mathsf{A}\mathsf{L}=\mathrm{Si}\mathrm{\text{−}}\mathrm{W}$ calorimeter, $\mathsf{J}=\mathrm{\text{movable}}$ $\mathrm{W}$ jaws, $\mathsf{P}\mathsf{1}$, $\mathsf{S}\mathsf{1}$, $\mathsf{S}\mathsf{2}=\mathrm{Si}\mathrm{\text{-diode detectors}}$, $\mathsf{S}\mathsf{L}=\mathrm{\text{solenoid lens}}$, $\mathsf{T}\mathsf{1}=\mathrm{\text{positron production target}}$, $\mathsf{T}\mathsf{2}=\mathrm{\text{reconversion target}}$, $\mathsf{T}\mathsf{P}\mathsf{1}=\mathrm{\text{positron transmission polarimeter solenoid}}$, $\mathsf{T}\mathsf{P}\mathsf{2}=\mathrm{\text{photon transmission polarimeter solenoid}}$. The detectors were encased in lead and tungsten shielding (not shown).

Figure 3

Solid line: calculated photon number spectrum per beam electron of undulator radiation integrated over angle, plotted as a function of photon energy $E_{\gamma }$ for electron beam energy 46.6 GeV, undulator period 2.54 mm and undulator strength parameter $K=0.17$. The peak energy of the first-harmonic (dipole) radiation was 7.9 MeV. Dashed line: longitudinal polarization $P_{\gamma }$ of the undulator radiation as a function of energy.

Figure 4

Distributions of energy $E_{\mathrm{CsI}}$ observed in the central CsI crystal of the positron polarimeter from individual electron beam pulses with undulator-on (right peak) and undulator-off (left peak). The central positron energy was 6.7 MeV.

Figure 5

Positron-induced asymmetries ${\delta }_{e^{+}}$ in the central CsI crystal of the positron polarimeter for 220 pairs of 1500 undulator-on beam pulses with opposite magnetization in the polarimeter. The central positron energy was 6.1 MeV. The average asymmetry was 0.0108, as indicated by the horizontal line.

Figure 6

Longitudinal polarization $P_{e^{\pm }}$ as a function of energy $E_{e^{\pm }}$ of positrons and electrons as determined from the asymmetries ${\delta }_{e^{\pm }}$ observed in the central CsI crystal. The smaller error bars show the statistical uncertainty, and the larger bars indicate the statistical and systematic uncertainties combined in quadrature. Also shown are predictions by a Geant4 simulation of the experiment.