The Coherent-Bremsstrahlung Facility in Hall B at Jefferson

Lab

F.J. Klein a,1, B.L. Berman b, K. Livingston c, P.L. Cole d,2, M. Anghinolfi e,

S. Boyarinov f , W.J. Briscoe b, H. Crannell a, C. Cuevas f , K.S. Dhuga b,

J.P. Didelez g, L. Fichen g, A. Freyberger f , J. Kellie c, L.Y. Murphy b, P. Musico e,

E. Pasyuk h, J. Proffitt f , A. Puga 2, J. Santoro a, D. Sober a, D.J. Tedeschi j,

B. Vlahovic k,f , M.H. Wood j, B. Wojtsekhowski f

aThe Catholic University of America, Washington, DC 20064, USAbThe George Washington University, Washington, DC 20052, USA

cThe University of Glasgow, Glasgow G12 8QQ, UKdIdaho State University, Pocatello, ID 83209, USA

eINFN, Sezione di Genova, 16146 Genova, ItalyfThomas Jefferson National Accelerator Facility, Newport News, VA 23606, USA

gIN2P3, Institut de Physique Nucleaire, 91406 Orsay, FrancehArizona State University, Tempe, AR 85287, USA

iTexas Christian University, Fort Worth, TX 76129, USAjUniversity of South Carolina, Columbia, SC 29208, USA

kNorth Carolina Central University, Durham, NC 27707, USA

Abstract

The tagged-photon beam in Hall B at Jefferson Lab has been enhanced with the addition of a coherent-bremsstrahlung facility to produce a beam of linearly polarized tagged photons. The polarized photon beamis produced via coherent bremsstrahlung on a diamond radiator positioned by a goniometer and is stronglycollimated to enhance the linear polarization. This facility is used to perform experiments with linearlypolarized photons using the CEBAF Large Acceptance Spectrometer (CLAS).

Key words: coherent bremsstrahlung, goniometer, diamond radiator, collimator, pair polarimeter, JLabPACS: 41.50.+h, 78.70.Ck, 23.20.Ra, 07.60.F

1 Corresponding author.Address: Physics Department, The Catholic Univer-sity of America, 207 Hannan Hall, Washington,DC20064, U.S.A.; Tel : +1-202-319-6190, Fax : +1-202-319-4448; E-mail : kleinf@cua.edu (F.J. Klein).2 formerly : The University of Texas at El Paso, ElPaso, TX 79968, USA

1 Motivation

The study of photo-induced exclusive reac-tions is greatly improved when measuring notonly cross sections but also spin observableswhich give access to interference terms, i.e.,bilinear products of different amplitudes. The

Preprint submitted to Elsevier Preprint 23 May 2008

two basic modes of photon polarization, circularand linear, give access to different interferenceterms. However, spin observables involving lin-early polarized photons incident on a nucleontarget not only elucidate more terms, but alsoare more sensitive in general.

The two common methods for producing lin-early polarized photons are Compton backscat-tering and coherent bremsstrahlung. In Comp-ton backscattering [1], which is preferably em-ployed at facilities with storage rings, the elec-tron beam collides with a soft x-ray [2] or laserbeam of short wavelength [3], resulting in high-energy polarized photons. Technically, the un-dulator (and reflection mirror) or the laser sys-tem significantly limit the intensity and maxi-mum fractional photon energy x=Eγ/E0, whereE0 denotes the electron beam energy and Eγ thephoton energy.

Coherent bremsstrahlung has been success-fully employed at various facilities [4,5] to pro-duce high-energy polarized photons. In thisprocess [6], linearly polarized photons are pro-duced on a crystal at discrete fractional energiesx corresponding to specific momentum transfers~q of the electrons to the crystal nuclei accordingto the Laue condition, ~q = ~g, where ~g is thereciprocal-lattice vector of the crystal. By care-fully choosing the orientation of the crystal withrespect to the direction of the electron beam,one can select isolated reciprocal-lattice vectors,e.g., |~g| = 2π

a

√22 + 22 for Miller index [022]. The

constant a denotes the interplanar distance; inthe case of diamond, |~g| = 9.85 keV/c for [022].The energy of coherent bremsstrahlung dependson the longitudinal momentum transfer ql:

qminl ' m2

ec3

2E0

x

1 − x≤ ql ≤

m2ec

3

2E0(1 − x. (1)

We discard the transverse component of the mo-mentum transfer qt, which is effectively limitedto 0

<

− qt<∼ mec due to the rapid decrease of the

cross section with qt. Equation (1) together withthe Laue condition show that qmin

l increases withx until it exceeds the longitudinal component of

~g, leading to a discontinuity of the cross sectionat the corresponding fractional energy xd givenby:

xd =2E0ql

m2ec

3 + 2E0ql

. (2)

Photon Energy (MeV)1000 2000 3000 4000 5000

effi

cien

cy (

%)

0

20

40

60

80

100

Photon Energy (MeV)1000 2000 3000 4000 5000 5800

Po

lari

zati

on

(%

)

0

20

40

60

80

100

Photon Polarization

Photon Energy (MeV)1000 2000 3000 4000 5000 5800

Inte

nsi

ty (

arb

. un

its)

0

1

2

3

4

5

6

7

8

9

10

Intensity

5.8-GeV e-beam on 20 m diamond (mosaic spread 10 rad) µ µ

collimated to 45 radµuncollimated

- - - - - -______

Fig. 1. Calculated photon spectrum for a diamondcrystal of thickness 20 µm for an incident electronbeam of 5.8 GeV. The crystal is oriented such thatthe coherent peak (for reciprocal-lattice vector [022])is set to 2.0 GeV. The dashed lines show the effectof (tight) collimation to 1

2θchar.

Whereas the angular distribution of incoher-ent bremsstrahlung is nearly independent of thephoton energy, the emission angle of coherentbremsstrahlung is directly correlated to x anddecreases with increasing photon energy (x ≤xd):

θ2 =1 − x

x

xd

1 − xd

− 1 , (3)

where θ is the reduced angle in units of the char-acteristic angle θchar = mec

2/E0. By tightly col-limating the photon beam, the relative contribu-tion of coherent bremsstrahlung (in comparisonto the incoherent part) can be enhanced. In this

2

case, the maximum emission angle is limited bythe opening angle of the collimator θcoll, whichprovides a lower limit xmin for the coherent-bremsstrahlung energy passing the collimator:xmin ≤ x ≤ xd with

xmin =xd

1 + (1 − xd)θ2coll

/θ2char

. (4)

An example for the calculated intensity andpolarization of coherent bremsstrahlung froman incident 5.8-GeV electron beam is shownin Fig. 1. For this calculation, we included theeffects of thickness and mosaic spread (impu-rity)for an existing diamond radiator. The ef-fect of tight collimation to 1

2θchar ' 45 µrad is

depicted by the dashed lines.

2 Beamline for Producing Linearly Po-

larized Photons

The Hall-B beamline for experiments withcoherent bremsstrahlung is depicted in Fig. 2.Experiments with linearly polarized photons us-

π−

Pairpolarimeter

Pairspectrometer

CLAS 4 spectrometerπGoniometer(diamond radiator)

Photon tagger

CollimatorActive

e−

e+

22.9m

Fig. 2. The Hall-B beamline for the coher-ent-bremsstrahlung facility at Jefferson Lab (not toscale).

ing the CEBAF Large Acceptance Spectrometer(CLAS) [7] typically are run at photon energiesabove 1 GeV, produced from electron beams of4–6 GeV. In order to produce such energeticpolarized photons in a diamond radiator, theangle of incidence between the reciprocal-latticevector of the radiator and the electron beammust be aligned to approximately 0.1 mrad. Agoniometer is used to align diamond radiators

of thickness 20–50 µm. An instrumented col-limator, positioned 22.9 m downstream of thediamond radiator, is used to enhance the degreeof linear polarization by ensuring that the cen-tral portion of the photon beam passes throughthe collimator. The spectrum of the collimatedphoton beam incident on the target centered inCLAS is monitored by a pair spectrometer. Thedegree of polarization is calculated by comparingthe collimated photon spectrum with simulateddistributions, based on the anb code by F.A.Natter [8]. A direct measurement of the photonpolarization is obtained using a pair polarimeterbased on a four-layer silicon microstrip detectorpositioned in front of the pair spectrometer.

3 Goniometer and Diamond Radiators

The energy of coherent bremsstrahlung is di-rectly correlated to the crystal orientation, andthus the energy of the coherent peak can gradu-ally be changed by rotating the crystal slightly.In order to perform a proper alignment, the di-amond crystal is mounted on a goniometer thatallows horizontal and vertical motion of the crys-tal as well as rotation with high accuracy aboutall three independent axes. For convenience, athird translational axis has been installed tomove the radiator-target ladder centered in thecradle without re-adjusting the position of thegoniometer. The goniometer [9] in Hall B, asshown in Fig. 3, is a comparatively large device(44cm × 20cm × 76cm) that provides accuraciesof±25 µm (horizontal),±180 µm (vertical trans-lation), and ± 0.7 to 1.3 µrad for the rotationalaxes yaw (θv), pitch (θh), and roll (φ). The levelof reproducibility was found to be fully consis-tent with the accuracy of the end-stop switches.The device does not have independent mecha-nisms to verify the proper motion of the steppermotors, such as optical encoders. Therefore, thetagged-photon spectrum and rate asymmetriesof the instrumented collimator are continuouslymonitored during its in-beam operation.

3

φ

θ

θ

v

h

Fig. 3. The Hall-B goniometer. Diamond radiatorsare mounted on the target ladder centered in thecradle.

Natural and synthetic diamond radiators forthis facility are selected after passing quality-control tests in the form of rocking curves, i.e.,the intensity spectrum resulting from x-raydiffraction when the crystal is rotated throughthe Bragg angle. The width of the rocking curveis a direct measure of the mosaic spread (impu-rity) in the irradiated area of the crystal. Werequire that the mosaic spread of the selectedcrystals be small (≈ 10-20 µrad) over a largefraction of the crystal surface. The rocking curvein Fig. 4 shows that the mosaic spread for thissynthetic crystal is about 7 µrad, which is lessthan twice the natural width (labeled “FWHMtheory” in Fig. 4). The rocking curve was takenbefore thining the 4×4 mm2 sized crystal to

20 µm. The inset of Fig. 4 shows a polarizedphotograph of this crystal.

Fig. 4. Rocking curve and polarized photograph (in-set) of a diamond crystal. The mosaic spread of thissynthetic crystal is less than twice the natural width.

The alignment of the diamond radiator athigher energies is complicated by the fact thatthe crystal angles scale with 1/E0, thus requiringvery accurate settings. Our technique for align-ing the crystal by means of a series of scans is anextension of that of Lohmann [5], and is detailedin Ref. [10]. The alignment procedure entails ex-ecuting small angular movements of the crystaland recording the corresponding tagged-photonspectrum for each shift in θh (pitch) and θv

(yaw). A scan of a 20 µm-thick diamond crystalis shown in Figs. 5 and 6. The radial distancecorresponds to the photon energy. The dark ra-dial ridges on the plots trace the energy of thecoherent peak as the angle between the beamand the face of the crystal varies. Figure 5 showsthe initial scan of that crystal. The lines andtext denote the result of fitting the spectrum toa template composed of eight lines, spaced 45◦

apart. These lines correspond to the orientationof the crystal axes [022], [022] (bold lines), and[044], [044] (light lines). The second, final scan– shown in Fig. 6 – is close to a perfect four-foldsymmetry, showing that the crystal is very wellaligned with the beam.

4

hθ

vθ

= 0.75 deg0φ)=(4.92, 9.78) mrad,h, SBvBeam (SB)=(SBBeam-to-Crystal vector BC = -(S+SB) = (-5.81, 24.73) mrad

S

B

Scan radius = 20 mrad) =h, SvScan origin (S

(0.89, -34.51) mrad

Fig. 5. Scan of a 20 µm-thick diamond crystal, il-lustrating the alignment procedure by fitting a tem-plate. Here the face of the crystal, i.e., the [100]plane, is offset in θh and θv with respect to the di-rection of the incident electron beam.

4 Photon Tagger

The photon spectrum, as measured by theHall-B photon tagger [11], is the key diagnostictool for aligning the reciprocal-lattice vectorsof the diamond radiator with respect to the in-cident electron beam. To this end, the energycounters of the tagger hodoscope were improvedconsiderably; all 384 energy counters receivedimproved shielding, new bases, and individualHV controls; 90Sr sources were attached to arail located along the hodoscope to calibrate thecounters individually; the output signals are fedinto custom-built pre-amplifiers and VME-scalerarrays as well as into commercial FASTBUSmulti-hit TDCs. The scalers can be used both infree running and in gated mode; triggering witha device downstream of the collimator allows thedetermination of the photon spectrum incidenton the CLAS target. These improvements en-able accurate monitoring of the tagged-photon

hθ

vθ

= 45.00 deg0φ)=(0.00, 0.00) mrad,h, SBvBeam (SB)=(SBBeam-to-Crystal vector BC = -(S+SB) = (-5.81, 24.73) mrad

S B

Scan radius = 20 mrad) =h, SvScan origin (S

(5.81, -24.73) mrad

Fig. 6. Final scan of the same crystal, showing thatthe crystal is now well aligned.

spectrum as well as calculation of the degree oflinear polarization, as shown in Fig. 7.

1 50 100 150 200 250 300 350 3840

50

100

150

200

250

300

350

400

450

500

Tagger Energy Counter

Rel

ativ

e In

ten

sity

(%

)

Coherent edge

[022]

[044]

[066,022,004]- -

Fig. 7. Normalized photon energy spectrum. Highercounter numbers correspond to lower photon ener-gies. The spectrum produced in a diamond radia-tor has been normalized to a spectrum produced inamorphous carbon radiator.

5

Fig. 8. Normalized photon spectrum before and aftercollimation. Note that both spectra have the samevertical scale. This highlights the effect of tight col-limation. The fits to the photon spectra are basedon the code of Ref. [8].

5 Instrumented Collimator

An instrumented collimator, having an aper-ture of 2.0 mm in diameter, is installed in theHall-B beamline downstream of the tagger mag-net and located 22.9 m away from the diamondradiator. The collimator [12] serves to enhancethe degree of linear polarization, Pγ , within thecoherent peak. As shown in Fig. 8, the coherentdistribution, peaked at 1.90 GeV, is consider-ably enhanced by tightly collimating the photonbeam to one half of a characteristic angle. Thespectra were taken in July 2001 for an electronbeam energy of 5.7 GeV. Since the merit func-

tion scales as 1/Pγ2, the collimation as shown in

Fig. 8 enhances the quality of the polarizationdata by 30%.

The collimator is formed of a stacking arrange-ment of thirteen nickel disks, each of thickness15 mm with an outer diameter of 50 mm. Thedisks are stacked within a cylindrical sheath ofstainless steel (Fig. 9). Four 4-mm cubic scintil-lators are sandwiched between the first two disksto monitor the rate of e+e−-pairs produced byphotons outside the 2-mm core incident on thefirst nickel disk. The scintillators are read outby photomultipliers (Hamamatsu R1635), whichwere gain matched by lining up the first and sec-ond minimum ionizing peaks. The entire deviceis operated under vacuum.

Fig. 9. Side view of the instrumented collimator,showing a sheath containing the nickel disks and thefour photomultipliers connected to thin scintillatorspositioned between the first two disks.

Asymmetries between the count rates of thescintillators are directly related to shifts in thebeam position. Figure 10 compares the asym-metry of the top and bottom scintillators to thebeam position as determined by beam-positionmonitors (σ=10 µm) located just upstream ofthe goniometer. Such comparisons show thatthe asymmetries obtained from the instrumen-ted collimator track beam shifts to a sensitivity

6

better than 25 µm.

Vertical Beam Position

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

Co

llim

ato

r A

sym

met

ry

-0.55

-0.5

-0.45

-0.4

-0.35

-0.3

-0.25

0 1 2 3 4 5 6 7 8 9 10 x106

Event Number in Run 29216 (July 17, 2001)

Bea

m P

os.

Mo

nit

or

(mm

)

Fig. 10. Variation of the vertical beam position dur-ing a specific run of 90 minutes length. The upperplot shows the variation of the beam position mea-sured by the asymmetry of top and bottom scintil-lators in the instrumented collimator, the lower plotthe readout of a beam position monitor in front ofthe goniometer.

6 Photon Polarimeter

The photon polarization can be determinedby fitting the relative intensity spectrum for thediamond radiator with respect to the energyspectrum obtained from an amorphous radiator,(Icoh + Iincoh)/Iincoh, to a model that takes intoaccount all known contributions of coherent andincoherent bremsstrahlung, multiple scattering,mosaic spread, and beam divergence, as done forthe distributions shown in Fig. 8. This methodproduces information about the coherent peakposition and the photon polarization within ashort time. However, the accuracy is limited bythe accuracy of scaling the bremsstrahlung spec-trum obtained with the amorphous radiator.

A direct measurement of the photon polariza-tion employs the polarization dependence of the

differential cross section expressed as a functionof the azimuthal angle Φ between the analyzerand the photon polarization plane:

dσ

dΦ=

σ0

2π(1 − PγA cos 2Φ) , (5)

where σ0 is the total unpolarized cross section,Pγ is the degree of linear polarization of the pho-ton beam, and A is the analyzing power.

A pair polarimeter employs the polarizationdependence of the angle of the e+e− plane (cf.Eq.(5)) to measure the direction and the degreeof polarization directly. Such a device promisesa large analyzing power (up to ≈ 27%) for a verythin converter and optimal energy sharing be-tween the pair components. However, the smallopening angle of the e+e− pair causes a majorproblem in determining the angle of the e+e−

plane. In many existing pair polarimeters thisproblem has been overcome by using magneticseparation of the e+e− pair at the expense oflowering the analyzing power [13]. In our ap-proach [14,15], the area between the thin activetarget, a 50 µm thin scintillator, and the detec-tors is kept field-free. Solid-state microstrip de-tectors with 100 µm pitch are used to resolve theangular correlations. The detector device con-sists of four layers of silicon microstrip detectors,positioned at 0, ±60◦, and 90◦ with respect toeach other, which allows for resolving all possibleambiguities in the hit patterns. The pair spec-trometer downstream of the microstrip detectorsis used to identify symmetric e+e− pairs. Sincethe microstrip detectors have to be positioned onthe beam axis, special run configurations are re-quired to perform the polarimetry measurement.To this end, both target and microstrip detec-tors are mounted on stages, which are operatedunder high vacuum. Polarimetry measurementsduring the summer 2005 data taking [16] con-firmed the feasibility of this approach.

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7 Summary

The experimental program for photo-inducedreactions in Hall B at Jefferson Lab has been im-proved greatly by the coherent-bremsstrahlungfacility. Photon beams of up to ≈ 90% linear po-larization are being produced in the energy rangeof 0.8 ≤ E~γ ≤ 2.2 GeV. In conjunction withCLAS and liquid hydrogen and deuterium tar-gets as well as a frozen-spin target, the coherent-bremsstrahlung facility allows for (almost) com-plete measurements of spin observables for pion,eta, and kaon photoproduction.

Acknowledgements

We thank the CLAS collaborators who con-tributed in establishing this facility, as well as thestaffs of Hall B and of the accelerator group atJefferson Lab, for their continued support duringconstruction, installation, and commissioning ofthis facility. The project is supported by U.S.Department of Energy under grant DE-FG02-95ER40901, the National Science Foundation,and the Engineering and Physical Sciences Re-search Council (UK). The Southeastern Univer-sities Research Association (SURA) operates theThomas Jefferson National Accelerator Facilityfor the U.S. Department of Energy under con-tract DE-AC05-84ER40150.

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