Hyperfine Interact (2011) 200:41–44DOI 10.1007/s10751-011-0277-9

A4 Compton polarimetryProgress since PAVI06

Jürgen Diefenbach

Published online: 27 April 2011© Springer Science+Business Media B.V. 2011

Abstract For the A4 parity violation experiment at MAMI we have installed aCompton laser polarimeter in our beam line. We have now operated this polarimeterat different beam energies of 315, 855, and 1508 MeV. A short overview of ourdetectors for scattered photons and electrons is given, together with a discussion ofthe extraction of Compton asymmetries using tagged photon spectra.

Keywords Polarized beams · Beam analyzers · Compton polarimetry · MAMI

1 Introduction

For the A4 parity violation experiment at the MAMI accelerator facility in Mainz,four different electron beam polarimeters are currently used to determine thelongitudinal spin polarization of the electron beam. One of them is the A4 laserCompton backscattering polarimeter.

The advantage of Compton backscattering is that it provides a non-destructivetool, capable of absolute beam polarization measurements while the parity experi-ment is running.

2 Experimental set-up

The A4 Compton polarimeter features the intra-cavity design which provides lumi-nosities comparable to those of Fabry-Perot cavity designs, e.g. [1]. For an overviewof the experimental set-up see [2].

J. Diefenbach (B)Institut für Kernphysik, Universität Mainz, Mainz, Germanye-mail: defi@kph.uni-mainz.de

42 J. Diefenbach

A magnetic chicane separates electron beam and scattered photons. Two detec-tors are used, a compact calorimeter to measure the energy of backscattered photons,and a bundle of scintillating fibres to detect the scattered electrons. Fibre detectorand magnetic chicane are used as a magnetic spectrometer for the scattered electrons.For the momentum calibration a TRANSPORT calculation is used [3].

3 Detectors and data acquisition

For detection of the backscattered photons we use a calorimeter consisting ofLYSO crystals of (20 × 20 × 200) mm3 arranged in a 3 × 3 matrix. The compact(X0 = 12 mm) scintillator material LYSO makes it possible to cover the energyrange from 1 to about 100 MeV. Therefore, one can use the same detector for alldifferent beam energies in the A4 experiment (315, 855, 1508 MeV, correspondingto maximum energies of backscattered photons of 3.6, 26.2, and 78.2 MeV). With adecay time constant of the scintillation light of 41 ns LYSO is a rather fast calorimetermaterial, so that pile-up can be kept low even under difficult beam conditions withhigh background. The crystals are read out by nine photomultipliers equipped within-house made active voltage dividers that ensure good linearity even at high rates.

The electron detector consists of 48 scintillating fibres, arranged in four stackedlayers [3]. The detector is divided into 24 channels, each of which corresponds to acertain scattered electron momentum range due to the dispersion of the magneticchicane.

Read-out of detectors is triggered by hits in the photon calorimeter. Events arehistogrammed online and, for each electron beam helicity, spectra of all photons(“unbiased”), of photons in coincidence with hits on the fibres (“coincidence”),and tagged photon spectra (spectra of photons in coincidence with each fibreindividually) are produced. The mean trigger efficiency of the fibre detector hasbeen determined to be about 95%, which is in good agreement with expectationsfrom simulations.

To avoid systematic uncertainties arising from modeling the detector responsewith GEANT4 simulations, we decided to analyze the tagged photon spectra, whenextracting the Compton asymmetry [2] from “unbiased” or “coincidence” spectra.For any photon energy the Compton asymmetry can be calculated from the Comptoncross section. A tagged photon spectrum contains only photons from a narrow energyrange (order of 2 MeV). If the trigger efficiency as a function of scattered electronenergy is known for every fibre, the analyzing power can be calculated from thecorresponding photon energy distribution. This avoids the need for a lower cut onthe energy spectrum of backscattered photons and makes the result of the analysisalmost independent of the shape of the detector response to monoenergetic photonsin the GEANT simulation.

The trigger efficiencies of the scintillating fibres have been simulated usingGEANT4 and parametrized (Fig. 1). From a parameter set (fibre detector position,effective dispersion of magnetic chicane, etc.) the tagged photon spectra can begenerated from simulated Compton events using the trigger efficiencies as weightingfunctions. Figure 2 shows one of the spectra of tagged photons at 855 MeV, together

A4 Compton polarimetry 43

Fig. 1 Left: Comparison between measured energy deposition spectrum of a fibre (histogram) forscattered electrons with the result of the GEANT4 simulation (dots with error bars), showing verygood agreement. Below channel 550 the spectrum is dominated by electronics noise and dark current,not included in the simulation. Right: Simulation of trigger efficiency of a fibre as a function ofscattered electron energy and its parametrization (red curve)

Fig. 2 “Unbiased” spectrum of backscattered photons (left) and spectrum of tagged photons (right)for one of the 24 fibres at 855 MeV of beam energy. The spectra for both electron beam helicitiesare shown in red and blue. The green curve in the right spectrum is the result of a simultaneous fitof the detector simulation to the measured spectra of tagged photons using the parametrized triggerefficiencies of the fibre detector as weighting functions

with the spectra reproduced from the detector simulation using a simultaneous fit tothe measured spectra.

In routine operation during our parity running we reached a statistical precision ofabout 1% in 24 h at 855 MeV beam energy and better than 1% in 12 h at 1508 MeV.From the results of the on-going analysis it is estimated that deadtime corrections willbe below 1% and pile-up corrections between 1 and 2% (both in terms of relativechanges of Compton asymmetries). Thus, the A4 laser Compton polarimeter willhave an important impact on the A4 parity violation results, lowering the uncertaintyof the polarization measurement by at least a factor of three.

44 J. Diefenbach

Acknowledgements This work is supported by the Deutsche Forschungsgemeinschaft (DFG) inthe framework of the SFB 443 and by the U.S. Department of Energy (DOE). This article comprisespart of the author’s PhD thesis.

References

1. Falletto, N., et al.: Nucl. Instrum. Methods A 459, 412 (2001)2. Diefenbach, J.: Compton polarimetry at JLab and MAMI. Eur. Phys. J. A 32, 255 (2007)3. Lee, J.H.: Concept and realization of the A4 Compton backscattering polarimeter at MAMI.

Ph.D. thesis, Universität Mainz (2008)