Pussieux, T

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Electron beam polarization with theCompton Polarimeter at JLabT. Pussieuxfor the Compton Polarimeter collaborationCEA Saclay, DAPNIA/SPhN, 91191 Gif sur Yvette Cedex, France.http://www.jlab.org/comptonAbstract. We built and operated a new kind of Compton polarimeter to measurethe electron beam polarization of the Thomas Jeerson National Accelerator Facility(Virginia, USA) to 3% total error within an hour. The heart of this polarimeter is thecoupling of a High Finesse monolithic Fabry-Perot cavity to the particle accelerator.Its purpose is to amplify a primary 300 mW laser beam to increase the luminosity atthe Compton interaction point. The measured Finesse and amplication gain of thecavity are F = 26000 and G = 7300. We have used this facility during the HAPPEX(April-July 1999) experiment.INTRODUCTIONAmong all the possibilities to measure the polarization of an electron beam, (Mottand Moller polarimeters), Compton polarimetry presents the advantage of provid-ing a measurement while the main experiment is running. Our group has completedthe installation of the Jlab Hall A Compton polarimeter in feb 1999. It has given itsrst results for the HAPPEX experiment [1] with a 4 GeV electron beam of 50 A.Whereas Compton polarimeters operating at higher energies (SLAC, HERA) orhigher currents (NIKHEF) use a high power laser photon source, the challenge ofthe Compton polarimeter at Jlab was to operate a Fabry-Perot cavity to amplifythe photon beam in order to achieve a statistical precision of 1% within 1 hour.I THE COMPTON POLARIMETERThe hearth of this polarimeter is a high power (PL' 1:4kW ) Fabry-Perot cavityinjected by a 300 mW NdYAG infra-red laser [2]. The light inside the Cavity iscircuraly polarized, and its helicity can be reversed thanks to a rotatable =4 plate.The polarization for both helicity is PR;L= 99:3 1:1% [3].A magnetic chicane steers the electron beam to the center of the optical Cavity.Backscattered photon are dectected by a PbWO4calorimeter [4]. Counting ratesfor events with photon energy k0above ks0' 30MeV are recorded by a scaler. Fora small fraction of events (1 %), the energy deposited in the calorimeter k0ris alsomeasured thanks to a charge ADC.The beam polarization Peis extracted from the Compton experimental asymme-try, Aexp, via Pe=1P< Ac>Aexp; where < Ac> is the analyzing power (A.P.).In practise we measure a raw conting rate asymmetry Araw=r+ r r++r given byAexp=1 +BSAraw+BSAB+ApF: (1)In order to measure the background over signal ratioBS' 0:3 and the backgroundasymmetry AB, we take data alternatively with the cavity on (' 5 minutes) ando (' 2 minutes). To reduce systematic errors associated with the luminosityasymmetries ApF, the photon polarization is set alternativly right or left.In addition to the determination of the Light polarisation, the two steps to extractthe polarization are the determinaitions of the experimental asymmetry Aexpfromthe measured raw asymmetry Arawand of the analyzing power < Ac>.II MEASUREMENT OF THE EXPERIMENTALASYMMETRY AEXPFor each photon helicity state (R;L), a raw asymmetry AR;Lrawis determined us-ing the measured rates (normalized to the beam intensity) (rR;L1) at 30 Hz whenthe cavity is On. If all the parameters were stable between two consecutive pho-ton helicity, then averaging the two raw asymmetries AR;Lrawcancels out the falseasymmetries (background and luminosity)Aexp'1 +BS12hARraw ALrawi: (2)The background dilution was estimated by using the rate r0measured when thecavity is o 1 +BS=r1r1  r0. With this method, we have measured experimentalasymetries close to Aexp' 1:2 % with a relative statistical error of 1.4% fora typical one hour run. Variations with time of the rates and the backgroundasymmetry result in a contribution to the relative systematic error of 0.5 % for thedilution and 0.5 % for the background asymmetry.There is also a contribution to the systematic error from the non complete can-cellation of the luminosity asymmetry between the two photon helcity states. Falseluminosity asymmetry can be traced back to the fact that we are crossing twobeams with small transverse sizes (' 100m) in the vertical direction (y). We thusexpect our luminosity to be very sensitive to the vertical electron beam position.For a non optimal crossing (e.g. o-centering by 50 m), the luminosity exhibitsa sensitivy to the electron beam vertical position1r@r@Ythat can reach the 0:1%=level. This eect, combined with the fact that there might be systematic dierentbeam positions for the two electron helcity y = y+  y of the order of 100 nm,could results in a relative false asymmetry close to 1%. To keep this systematicsat a small level, roughly 3% of the data for which the beams were o-centered bymore than 50 were rejected. Again, in practise, a false asymmetry is computedfor each photon polarization stateAR;Lp=Xp=x;y;a;y 1r@r@p!R;LpR;L(3)and the residual systematic error to the experimental asymmetry using Eq. 2 isgiven by AFp' 1=2ARp ALp: This is the largest contribution (' 1:2%) to therelative systematics error on the experimental asymmetry.III DETERMINATION OF THE ANALYZING POWERIn terms of backscattered photon energy k0, the A.P. is dened by< AC>=Rk0max0(k0)d+cdk0 Rk0max0(k0)d cdk0Rk0max0(k0)d+cdk0+Rk0max0(k0)d cdk0(4)where k0max= 190MeV is the Compton edge, (k0) is the calorimeter eciencyanddcdk0the known Compton helcity dependent cross-sections [2]. In practise, weonly measure the energy deposited in the calorimeter k0r, which diers from thebackscattered photon energy k0for various reasons (statistical uctuations, non-linearity, ...). The A.P. has thus to be computed using this measured energy< AC>=R10(k0r)d+cdk0r R10(k0r)d cdk0rR10(k0r)d+cdk0r+R10(k0r)d cdk0r(5)In order to determine the dependance of the cross-sectiondcdk0rand the eciency with the reconstructed energy k0rwe use the background substracted ADCspectrum. This spectrum is rst corrected for the non-linearity of the elec-tronics, and then calibrated in energy using the compton edge to get the en-ergy reconstructed spectrum. We then t this energy reconstructed spectrum byp(k0rs;sr)(k0r)"dcdk0 g(a;b;c)(k0)#where the Compton cross-sectiondcdk0is convolutedby a gaussian resolution g(a;b;c)with  = a  b=pk0 c=k0and the calorimetereciency is parametrized by an erf function centered around the threshold k0rswith a slope proportionnal to sr. With this procedure we extract for each run theA.P., < AC>' 1:7%. To estimate the systematic error associated with this proce-dure, we allow the parameters of the t (e.g the three parameters for the resolutionComptonMollerJoursPolarisation (%)66687072747678808284102 104 106 112 114FIGURE 1. Compton and Mller measurement as a function of time.(a; b; c) and the two parameters of the response function k0rs; sr) to vary within alarge range and assign as a systematic error on the A.P. the maximum absolutedierence between the A.P. determined using the nominal parameters of the t andthe ones determined with each combinaition of the parameters in the above range.This contributes to a relative systematics errors of 1.9 %, and is largely dominatedby the sensitivity of the A.P. to the threshold value. Eventually, we have added a1% contribution to the relative systematic error associated with the non-linearityof the electronics, along with a 1% contribution to take into account the calibrationprocedure. This ends up with a total relative systematics errors of 2.4 % associatedto the A.P.IV RESULTSThe 40 polarization measurement are given on gure 1. There are in good agree-ment with the Mller measurements. The typical total error is around 3:3%. Inaddition to these absolute measurements, the Compton was able to monitor thepolarization at the 1.5 % and ruled out possible large variation of the polarizationbetween two Moller measurements.REFERENCES1. HAPPEX Collab (K.A. Aniol et al.) Phys.Rev.Lett. 82 :1096 -1100 (1999)2. J. P. Jorda et al., Nucl. Instrum. Meth. A412 (1998) 1.3. N. Falletto, PhD Thesis, Grenoble University, (1999), unpublished.4. D. Neyret et al., ton polarimeter," Nucl. Instrum. Meth. A443, 231 (2000)

Electron beam polarization with the Compton polarimeter at JLAB

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