Measurement Of Electron Beam Polarization Research Articles

A novel comparison of Møller and Compton electron-beam polarimeters

We have performed a novel comparison between electron-beam polarimeters based on M{\o}ller and Compton scattering. A sequence of electron-beam polarization measurements were performed at low beam currents ($<$ 5 $\mu$A) during the $Q_{\rm weak}$ experiment in Hall C at Jefferson Lab. These low current measurements were bracketed by the regular high current (180 $\mu$A) operation of the Compton polarimeter. All measurements were found to be consistent within experimental uncertainties of 1% or less, demonstrating that electron polarization does not depend significantly on the beam current. This result lends confidence to the common practice of applying M{\o}ller measurements made at low beam currents to physics experiments performed at higher beam currents. The agreement between two polarimetry techniques based on independent physical processes sets an important benchmark for future precision asymmetry measurements that require sub-1% precision in polarimetry.

Measurement of electron beam polarization from unstrained GaAs via two-photon photoemission

Two-photon absorption of 1560nm light was used to generate polarized electron beams from unstrained GaAs photocathodes of varying thickness: 625μm, 0.32μm, and 0.18μm. For each photocathode, the degree of spin polarization of the photoemitted beam was less than 50%, contradicting earlier predictions based on simple quantum mechanical selection rules for spherically-symmetric systems but consistent with the more sophisticated model of Bhat et al. (Phys. Rev. B 71 (2005) 035209). Polarization via two-photon absorption was the highest from the thinnest photocathode sample and comparable to that obtained via one-photon absorption (using 778nm light), with values 40.3±1.0% and 42.6±1.0%, respectively.

Upgraded photon calorimeter with integrating readout for the Hall A Compton polarimeter at Jefferson Lab

The photon arm of the Compton polarimeter in Hall A of Jefferson Lab has been upgraded to allow for electron beam polarization measurements with better than 1% accuracy. The data acquisition (DAQ) system now includes an integrating mode, which eliminates several systematic uncertainties inherent in the original counting-DAQ setup. The photon calorimeter has been replaced with a Ce-doped Gd2SiO5 crystal, which has a bright output and fast response, and works well for measurements using the new integrating method at electron beam energies from 1 to 6GeV.

Measurement of Electron Beam Polarization from Unstrained Bulk GaAs via Two Photon Photoemission

This paper describes measurements of the beam polarization and quantum efficiency for photoemission using two-photon excitation from unstrained bulk GaAs illuminated with pulsed, high intensity 1560nm laser light. Quantum efficiency is linearly proportional to 1560nm peak laser intensity, which was varied in three independent ways, indicating that the emitted electrons are promoted from the valence to the conduction band via two-photon absorption. Beam polarization was measured using a microMott polarimeter, with a value of 16.8(4)% polarization at 1560nm, which is roughly half the measured value of 33.4(8)% using 778 nm light.

Accurate measurement of the electron beam polarization in JLab Hall A using Compton polarimetry

A major advance in accurate electron beam polarization measurement has been achieved at Jlab Hall A with a Compton polarimeter based on a Fabry–Perot cavity photon beam amplifier. At an electron energy of 4.6 GeV and a beam current of 40 μ A, a total relative uncertainty of 1.5% is typically achieved within 40 min of data taking. Under the same conditions monitoring of the polarization is accurate at a level of 1%. These unprecedented results make Compton polarimetry an essential tool for modern parity-violation experiments, which require very accurate electron beam polarization measurements.

First electron beam polarization measurements with a Compton polarimeter at Jefferson Laboratory

A Compton polarimeter has been installed in Hall A at Jefferson Laboratory. This Letter reports on the first electron beam polarization measurements performed during the HAPPEX experiment at an electron energy of 3.3 GeV and an average current of 40 μA. The heart of this device is a Fabry–Perot cavity which increased the luminosity for Compton scattering in the interaction region so much that a 1.4% statistical accuracy could be obtained within one hour, with a 3.3% total error.

Measurement of electron beam polarization from the energy asymmetry of Compton scattered photons

We describe a new method for precision measurement (/spl sim/1%) of the longitudinal polarization of high-energy electron beams. The method relies on measuring the energy asymmetry of Compton scattered photons which result from collisions between the electron beam and a polarized laser beam. The measurement takes place near the electron-positron interaction point at the SLAC Linear Collider (SLC). The detector described here (PGC) is a threshold gas Cherenkov device which follows a lead radiator. The Cherenkov threshold (14 MeV) is essential for eliminating enormous synchrotron radiation-induced backgrounds at the position of interest in the SLC. We also describe the calibration of the PGC energy response in a test beam and its performance as a working polarimeter for the SLC/SLD physics program. Preliminary results from the 1996 run indicate that PGC measurement with total error at or below the 1% level is indeed achievable, and hence serves as a valuable crosscheck of the polarization scale.

A target for precise Møller polarimetry

In order to achieve better accuracy on the measurement of electron beam polarization employing ē - ē scattering, we have developed an improved polarized electron target. Using a pure iron foil saturated out-of-plane in a 4 T magnetic field one can reduce the systematical errors to the promille level. Measurement of the relative target polarization using polarized light and the Kerr effect allows to observe on-line the change in target polarization due to heating by the electron beam.

The intra-atomic motion of bound electrons as a possible source of the systematic error in electron beam polarization measurements by means of a Möller polarimeter

The role of the motion of bound electrons in target atoms in electron beam polarization measurements by means of Möller scattering is discussed. It is shown that this effect can result in an essential overestimate of the asymmetry. This overestimate considerably exceeds the uncertainly of the target polarization, and, therefore, the effect should be taken into account in experiments with polarized electrons and in the development of Möller polarimeters. Some recommendations are given to eliminate this systematic error.

Measurement of electron beam polarization at the SLC

The polarimeters needed to monitor and measure electron beam polarization at the Stanford Linear Collider are discussed. Two types of polarimeters, are to be used. The first is based on the spin dependent elastic scattering of photons from high energy electrons. The second utilizes the spin dependence of elastic electron-electron scattering. The plans of the SLC polarization group to measure and monitor electron beam polarization are discussed. A brief discussion of the physics and the demands it imposes on beam polarization measurements is presented. The Compton polarimeter and the essential characteristics of two Moeller polarimeters are presented. (LEW)