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 $\mu$A. 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

Aniol

B Frois

Barber

C Cavata

C.W de Jager

D Lhuillier

D Neyret

E Burtin

F Marie

Falletto

Gustavson

J Martino

J Mitchell

Jorda

M Baylac

Neyret

P.Y Bertin

Passchier

S Escoffier

T Pussieux

Woods

Physics Letters B

English

arXiv

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 muA. 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

Baylac, Maud

Burtin, E.

Cavata, C.

Escoffier, S.

Frois, B.

Lhuillier, D.

Marie, F.

Martino, J.

Neyret, D.

Pussieux, T.

Bertin, P.Y.

Jager, Kees de

Mitchell, J.

UNT (University of North Texas) Digital Library

arXiv:hep-ex/0203012 v1   8 Mar 2002First electron beam polarization measurements with a Compton polarimeter atJefferson LaboratoryM. Baylac, E. Burtin, C. Cavata, S. Escoffier, B. Frois, D. Lhuillier, F. Marie, J. Martino, D. Neyret, T. Pussieux∗CEA Saclay, DSM/DAPNIA/SPhN, F-91191 Gif-sur-Yvette Cedex, France, http://www.jlab.org/comptonP.Y. BertinUniversit Blaise Pascal et CNRS/IN2P3 LPC, 63 177 Aubière Cedex, FranceC.W. de Jager, J. MitchellJefferson Lab, 12000 Jefferson Avenue, Newport News, VA 23606, USA(March 8, 2002)A Compton polarimeter has been installed in Hall A atJefferson Laboratory. This letter reports on the first elec-tron beam polarization measurements performed during theHAPPEX experiment at an electron energy of 3.3 GeV andan average current of 40 µA.The heart of this device is a Fabry-Perot cavity which in-creased the luminosity for Compton scattering in the interac-tion region so much that a 1.4% statistical accuracy could beobtained within one hour, with a 3.3% total error.I. INTRODUCTIONNuclear physics experiments using a polarized electronbeam require an accurate knowledge of the beam polar-ization. However, polarization measurements often ac-count for the main systematic uncertainty for such exper-iments. The Continuous Electron Beam Accelerator Fa-cility (CEBAF), located at Jefferson Laboratory (JLab),is equipped with three types of beam polarimeters. AMott polarimeter, limited to low energy (few MeV), isavailable at the injector, while Møller polarimetry is usedat low currents (few µA) in all three experimental halls.The main limitation of both techniques is their invasivecharacter due to the solid target (Au and Fe, respec-tively) used, preventing beam polarization measurementssimultaneous to running an experiment. Moreover, nei-ther of these instruments is capable of operating in theenergy and/or current regime delivered at JLab. In con-trast, Compton polarimetry, which uses elastic scatteringof electrons off photons, is a non-invasive technique. In-deed, the interaction of the electron beam with the ”pho-ton target” does not affect the properties of the beam sothat the beam polarization can be measured upstream ofthe experimental target simultaneously with running theexperiment. The applicability of Compton polarimetryhas been demonstrated at high-current (> 20 mA) stor-age rings (NIKHEF [1], HERA [2]) and at high-energy (>50 GeV) colliders (SLAC [3], LEP [4]). However, at theCEBAF beam conditions (energy of several GeV, currentof up to 100 µA) the Compton cross section asymmetryis reduced to a few percent, which makes Compton po-larimetry extremely challenging.A pioneering technique was selected to increase theCompton interaction rate such that a 1% statistical er-ror measurement could be achieved within an hour: anoptical Fabry-Perot type cavity fed by an infrared photonsource. The Compton luminosity has been enhanced fur-ther by minimizing the crossing angle between the elec-tron and the photon beams, which required setting thecavity mirrors only 5 mm away from the electron beam.This unique configuration allows measurements of theelectron beam polarization at moderate beam energies(> 1 GeV) and currents (> 10 µA). The Fabry-Perotcavity [5], comprising two identical high- reflectivity mir-rors, amplifies the photon density at the Compton In-teraction Point (CIP) with a nominal gain of 7000. Theinitial laser beam (NdYAG, 300 mW, 1064 nm) is lockedon the resonant frequency of the cavity through an elec-tronic feedback loop [6]. The optical cavity, enclosed inthe beam pipe, is located at the center of a magneticchicane (16 m long), consisting of 4 identical dipoles.The chicane separates the scattered electrons and pho-tons and redirects the primary electron beam unchangedin polarization and direction to the Hall A beam line.The backscattered photons are detected in a matrix of25 PbWO4 crystals [7], which are read out by PMT’s.The PMT output signal is amplified and integrated be-fore being sent to ADC’s. This article reports on the firstpolarization measurements performed during the secondpart of the data taking of HAPPEX experiment [8] (July1999) with the Hall A Compton polarimeter at an elec-tron energy of 3.3 GeV and an average current of 40 µA.II. TOWARDS A POLARIZATIONMEASUREMENTFor the HAPPEX experiment CEBAF produced alongitudinally polarized electron beam with its helicityflipped at 30 Hz. This reversal induces an asymmetry,Aexp =N+−N−N++N− , in the Compton scattering events N±detected at opposite helicity. In the following, the eventsare defined as count rates normalized to the electronbeam intensity within the polarization window. The elec-tron beam polarization is extracted from this asymmetry1via [9]Pe =AexpPγAth, (2.1)where Pγ denotes the polarization of the photon beamand Ath the analyzing power. The measured raw asym-metry Araw has to be corrected for dilution due tothe background-over-signal ratio BS , for the backgroundasymmetry AB and for any helicity-correlated luminos-ity asymmetries AF , so that Aexp can be written to firstorder asAexp =(1 +BS)Araw −BSAB + AF . (2.2)The polarization of the photon beam can be reversedwith a rotatable quarter-wave plate, allowing asymme-try measurements for both photon states, A(R,L)raw . Theaverage asymmetry is calculated asAexp =ωRARraw − ωLALrawωR + ωL, (2.3)where ωR,L denote the statistical weights of the rawasymmetry for each photon beam polarization. Assum-ing that the beam parameters remain constant over thepolarization reversal and that ωR ' ωL, false asymme-tries cancel out such thatAexp 'ARraw − ALraw2(1 +BS). (2.4)III. DATA TAKING AND EVENTPRESELECTIONBefore starting data taking, the electron beam orbitand its focus in the central part of the chicane have tobe optimized to reduce the background rate observed inthe photon detector which was generated by a halo ofthe electron beam scraping on the Fabry-Perot mirrorports [11]. The vertical position of the electron beamwas adjusted until the overlap between the electron andthe photon beam was maximized at the CIP. This wasachieved by measuring the count rate in the photon de-tector while adjusting the field of the chicane dipoles.Because of a slow drift of the electron beam, its positionhad to be readjusted every couple of hours to minimizesystematic effects (cf section IV). Polarimetry data wereaccumulated continuously while running the HAPPEXexperiment. Each one hour run consisted of five pairsof signal - with the cavity resonant (cavity ON) - andbackground - when the cavity was intentionally unlocked(cavity OFF), approximatively one third of the time -runs. Signal and background events were recorded for al-ternating values of the photon beam polarization insidethe cavity. With an average power of 1200 W inside thecavity, the count rate observed in the photon detectorwas ∼ 90 kHz and the background rate of the order of 16kHz.For the data presented here, only the central crystalwas used in the backscattered photon detection. Theelectronics was triggered when the amplitude of a sig-nal recorded in the PMT coupled to that central crys-tal exceeded a preset threshold value. The state (Cav-ity ON/Cavity OFF) of a recorded event is determinedby the photon beam diagnostics outside the cavity andby the cavity-locking feedback parameters [6]. The firstevent selection imposed a minimum current of 5 µA andvariations smaller than 3 µA within each run as well asminor cuts to account for malfunctions of the experimen-tal setup. The modulation of the electron beam positionand energy [8] imposed by the HAPPEX experiment in-duced a significant loss of events (∼ 36 %) in limitingsystematic effects (cf section IV).IV. EXPERIMENTAL ASYMMETRYThe raw asymmetry normalized to the electron beamintensity, Araw , is determined at 30 Hz for each pair ofopposite electron helicity states. Averaging all pairs forboth photon polarizations yields the mean raw asymme-try (eq. 2.3) The mean rates with cavity ON (r1) andcavity OFF (r0) are obtained by averaging over both elec-tron and both photon states within each run. The dilu-tion factor of the asymmetry is thus (1+BS ) =r1r1−r0 . Theaverage signal and background rates were respectivelyabout 1.9 and 0.4 kHz/µA. The experimental asymme-try of about 0.013 was determined with a relative sta-tistical accuracy ∼ 1.4% within an hour. Fluctuationsof the signal over background ratio BS and of the statis-tical precison of the background asymmetry estimationAB within a run account for a small contribution to theerror budget (cf table I).Any correlation between the Compton scattering lu-minosity and the electron helicity induces a false asym-metry which adds to the physical asymmetry (eq. 2.2).For each pair i of opposite electron helicity, a system-atic difference of the transverse electron beam positionor angle ∆pi =p+i −p−i2(pi = x, y, θx, θy) gives rise to anasymmetryAip =1ri(p = pi)∂ri∂p∣∣∣∣i∆pi, with ri =NitiIi. (4.1)The two beams, crossing in the horizontal plane, eachhave a transverse size of ∼ 100 µm. The Compton lumi-nosity is therefore highly sensitive to the vertical positionof the electron beam (fig. 1), of the order of 0.2%/µm.Thus, a systematic position difference of ∼ 100 nm wouldgenerate a false asymmetry of 0.02%, corresponding to∼ 1.5% of the experimental asymmetry. To minmize thesystematic error, the second event selection accepts only2data lying within 50 µm from the luminosity optimum.Again, averaging over the two helicity states of the pho-ton beam cancels this contribution if the beam propertiesremain stable over the duration of the run. However, aslow drift of the beam parameters induces a change inthe count rate sensitivity. The asymmetry induced bya difference ∆pi can therefore be different for the twophoton states, thus creating a residual false asymmetry.This residual contribution is estimated by calculating Aipfor each pair and summing over the four positions andangles. Averaging over all pairs is done using the sta-tistical weights of the experimental asymmetry. Helicity-correlated false asymmetries represent on average 1.2% ofthe experimental asymmetry and remain its main sourceof systematic error (cf table I).V. PHOTON BEAM POLARIZATIONThe photon beam polarization can not be measuredat the CIP during the experiment since any instrumentplaced in the photon beam interrupts the build-up pro-cess. Instead, the polarization at the exit of the cavitywas monitored on-line during data taking. Absolute po-larization measurements were performed at the CIP andat the exit of cavity during maintenance periods when thecavity is removed. These measurements allow to modelthe transfer function of the light from the CIP to theoutside detectors, which then result in an evaluation ofthe photon beam polarization during the experiment [10].The polarization is found to be PR,Lγ = ±99.3+0.7−1.1%, forboth right- and left-handed photons.VI. ANALYZING POWERThe Compton scattering cross section has to be con-voluted with the response function of the calorimeter:dσ±smeareddk′r=∫ ∞odσ±cdk′g(k′ − k′r) dk′where k′ is the backscattered photon energy, k′r is theenergy deposited in the calorimeter,dσ±cdk′ is the helicity-dependent Compton cross section and g(k′) is the re-sponse function of the calorimeter. The latter is assumedto be gaussian with a width σres(k′) = a⊕ b/√k′ ⊕ c/k′where (a, b, c) are fitted to the data. The observed en-ergy spectrum (fig. 2) has a finite width at the threshold.This can be due either to the fact that the threshold levelitself oscillates or to the fact that a given charge can cor-respond to different voltage amplitudes at the discrim-inator level. To take this into account the threshold ismodelled using an error function p(k′s, k′r) = erf(k′r−k′sσs)in which σs is fitted to the data. Hence, the observedcount rate can be expressed as :N±(k′s) = L×∫ ∞0p(k′s, k′r)dσ±smeareddk′rdk′rwhere L is the interaction luminosity. An example of ameasured energy spectrum and a fit using the proceduredescribed in this section is shown in fig. 2. Finally, theanalyzing power of the polarimeter can be calculated:Ath =N+(k′s)− N−(k′s)N+(k′s) + N−(k′s)and is of the order of 1.7 %. In calculating the en-ergy spectrum, the raw ADC spectrum had to be cor-rected for non-linearities in the electronics, resulting ina systematic error of 1 %. Another 1 % comes fromthe uncertainty in the calibration which is performed byfitting the Compton edge. Variations of the parametera, b, c, k′s, σs around the fitted values where used to esti-mate the systematic error associated with our imperfectmodelling of the calorimeter response. This contributes1.9% to the systematic error which is mainly due to thehigh sensitivity of the analyzing power to the value of thethreshold (∼ 1%/MeV around 30 MeV). This representsthe main source of systematics in our measurement (cftable I).VII. RESULTS AND CONCLUSIONSWe have measured for the first time the JLab electronbeam polarization at an electron energy of 3.3 GeV andan average current of 40 µA. The 40 measurements of theelectron beam polarization, which was of the order of 70%[11], are displayed in fig. 3. The average error budget forthese data is given in table I. These results are in goodagreement with the measurements performed with theHall A Møller polarimeter [12]. Moreover, the Comptonpolarimeter provides a relative monitoring of the polar-ization with an error of 2% due to the absence of corre-lations between consecutive runs in the systematic errorsassociated with the photon detector (Ath) and with thephoton polarization (Pγ). These results show that theinstrument presented here is indeed capable of produc-ing a fast and accurate polarization measurement. Thechallenging conditions of JLab (100 µA, few GeV) havebeen succesfully adressed with a low-power laser coupledto a Fabry-Perot cavity. Moreover, the level of system-atic uncertainties reached in these first measurements,is encouraging for forth-coming achievements. Severalhardware improvements have been added to the setupsince then: new front-end electronic cards, feed-back onthe electron beam position and a detector for the scat-tered electron, consisting of 4 planes of 48 micro-strips.Data taken during recent experiments [13] [14] are beinganalyzed and are expected to reach a 2% total error at4.5 GeV and 100 µA.3VIII. ACKNOWLEDGMENTSThe authors would like to acknowledge Ed Folts andJack Segal for their constant help and support, as well asthe Hall A staff, the HAPPEX and Hall A collaborationand the Accelerator Operations group. This work wassupported by the French Commissariat à l’Energie Atom-ique (CEA) and the Centre National de la Recherche Sci-entifique (CNRS/IN2P3). The Southern Universities Re-search Association (SURA) operates the Thomas Jeffer-son National Accelerator Facility for the US Departmentof Energy under Contract No. DE-AC05-84ER40150.∗ Corresponding author.[1] I. Passchier et al., Nucl. Instr. Meth. A 414, 446 (1998).[2] D.P. Barber et al., Nucl. Instr. Meth. A 329, 79 (1993).[3] M. Woods, Proc. of the Workshop on High Energy Po-larimeters, Amsterdam, 1996, eds. C.W. de Jager et al.,p. 843.[4] D. Gustavson et al., Nucl. Instr. Meth. A 165, 177(1979).[5] J.P. Jorda et al., Nucl. Instr. Meth. A 412, 1 (1998).[6] N. Falletto et al., Nucl. Instr. Meth. A 459, 412 (2001).[7] D. Neyret, T. Pussieux et al., Nucl. Instr. Meth. A 443,231 (2000).[8] K. A. Aniol et al. [HAPPEX Collaboration], Phys. Lett.B 509, 211 (2001).[9] C. Prescott, SLAC internal report, SLAC TN 73 1.[10] N. Falletto, Ph.D. thesis (in french), Université GrenobleI; Report CEA/DSM/DAPNIA/SPhN-99-03-T.[11] M. Baylac, Ph.D. thesis (in french), Université Lyon I; Report CEA/DSM/DAPNIA/SPhN-00-05-T.[12] http : //www.jlab.org/moller.[13] S. Frullani, J. Kelly and A. Sarty (spokespersons), JLabexperiment E91-011.[14] E. Brash, M. Jones, C. Perdrisat and V. Punjabi(spokespersons), JLab experiment E99-007.TABLE I. Average relative error budget.Source Systematic StatisticalPγ 1.1%Aexp Statistical 1.4%B/S 0.5%AB 0.5% 1.4%AF 1.2%Ath Non-linearities 1%Calibration 1% 2.4%Efficiency/Resolution 1.9%Total 3.0% 1.4%y [µm]Rate [kHz/µA]0.60.811.21.41.61.822.2400 500 600 700 800 900 1000FIG. 1. Count rate normalized to the beam current versusvertical position of the electron beam.0100200300400500Events [a.u.]50 100 150 200Energy [MeV]0FIG. 2. Energy spectrum of Compton photons, detected inthe central PbWO4 crystal. The solid curve is the fit, whichaccounts for the calorimeter resolution and efficiency.ComptonMøllerDaysPolarization (%)66687072747678808284102 104 106 112 114FIG. 3. Polarization measured by the Compton (full cir-cles) and the Møller (stars) polarimeters during the HAPPEXexperiment. For the Møller data the total error is shown, forthe Compton the statistical and total error is indicated sepa-rately.4

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

de Jager, C.W.

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

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

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