The GlueX Photon Beam

1. GlueX Photon Beamline-Tagger Review Jan. 23-25, 2005, Newport News The GlueX Photon Beam presented by Richard Jones, University of Connecticut GlueX Tagged Beam Working Group University of Glasgow University of Connecticut Catholic University of America

2. Presentation Overview • Photon beam properties • Competing factors and optimization • Electron beam requirements • Beam monitoring and instrumentation • Diamond crystal requirements

3. 107g/s dE 10-3 E I. Photon Beam Properties Direct connections with the physics goals of the GlueX experiment: • Energy • Polarization • Intensity • Resolution solenoidal spectrometer meson/baryon resonance separation forward mX production up to 2.8GeV/c2 9 GeV 40 % adequate for resolving resonances with opposite dominant exchanges provides sufficent statistics for PWA on major channels with ~1yr running matches resolution of the GlueX spectrometer in all-charged modes

4. Coherent Bremsstrahlung with Collimation No other solution was found that could meet all of these requirements at an existing or planned nuclear physics facility. Unique: • A laser backscatter facility would need to wait for new construction of a new multi-G$ 20GeV+ storage ring (XFEL?). • With the closure of SLAC, Jefferson Lab is the unique place where high-energy polarized photons can be found. • The continuous beams from CEBAF are ideal for an experiment seeking to detect high-multiplicity final states. • By upgrading CEBAF to 12 GeV, a 9 GeV polarized photon beam can be produced with high polarization and intensity.

5. incoherent (black) and coherent (red) kinematics Kinematics of Coherent Bremsstrahlung effects of collimation: to enhance high-energy flux and increase polarization effects of collimation at 80 m distance from radiator

6. Polarization from Coherent Bremsstrahlung Linear polarization arises from the two-body nature of the CB kinematics • linear polarization • determined by crystal orientation • vanishes at end-point • circular polarization • transfer from electron beam • reaches 100% at end-point Linear polarization has unique advantages for GlueX physics: a requirement Circular polarization is potentially useful as well, but not a requirement to achieve the physics goals of GlueX.

7. 4 nominal tagging interval Photon Beam Intensity Spectrum • Rates based on: • 12 GeV endpoint • 20mm diamond crystal • 100nA electron beam • Leads to 107g/s on target • (after the collimator) Design goal is to build an experiment with ultimate rate capability as high as 108g/s on target.

8. II. Optimization Understanding competing factors is necessary to optimize the design • photon energy vs. polarization • crystal radiation damage vs. multiple scattering • collimation enhancement vs. tagging efficiency

9. Optimization: chosing a photon energy • A minimum useful energy for GlueX is 8 GeV, 10 GeV would be better for several reasons, for a fixed endpoint of 12 GeV, the coherent gain factor is a steep function of peak energy. • CB polarization is a key factor in the choice of a peak energy of 9 GeV for GlueX but

10. Optimization: choice of diamond thickness • Design calls for a diamond thickness of20mmwhich is approximately10-4 rad.len. • Requires thinning: special fabrication steps and $$. • Impact from multiple-scattering is significant. • Loss of rate is recovered by increasing beam current, up to a point… The choice of 20mm is a trade-off between MS and radiation damage.

11. Optimization: scheme for collimation The argument for why a new experimental hall is required for GlueX • the short answer: because of beam emittance • a key concept: the virtual electron spot on the collimator face. It must be much smaller than the real photon spot size for collimation to be effective but the convergence angle a must remain small to preserve a sharp coherent peak. Putting in the numbers…

12. Optimization: radiator – collimator distance • With increased collimator distance: • polarization grows • low-energy backgrounds shrink • tagging efficiencydrops off a < 20 mr s0 < 1/3 c d > 70 m

13. Optimization: varying the collimator diameter effects of collimation on polarization spectrum collimator distance = 80 m effects of collimation on figure of merit: rate (8-9 GeV) * p2 @ fixed hadronic rate linear polarization 5

14. Results: summary of photon beam properties peak energy8 GeV 9 GeV 10 GeV 11 GeV N in peak185 M/s 100 M/s 45 M/s 15 M/s peak polarization0.54 0.41 0.27 0.11 (f.w.h.m.)(1140 MeV) (900 MeV) (600 MeV) (240 MeV) peak tagging eff.0.55 0.50 0.45 0.29 (f.w.h.m.)(720 MeV) (600 MeV) (420 MeV) (300 MeV) power on collimator5.3 W 4.7 W 4.2 W 3.8 W power on H2 target 810 mW 690 mW 600 mW 540 mW total hadronic rate385 K/s 365 K/s 350 K/s 345 K/s (in tagged peak) (26 K/s) (14 K/s) (6.3 K/s) (2.1 K/s) 1 1 1 2,3 • Rates reflect a beam current of 3mA which corresponds to 108g/s in the coherent peak, which is the maximum current foreseen to be used in Hall D. Normal GlueX running is planned to be at a factor of 10 lower intensity, at least during the initial running period. • Total hadronic rate is dominated by the nucleon resonance region. • For a given electron beam and collimator, background is almost • independent of coherent peak energy, comes mostly from incoherent part.

15. III. Electron Beam Requirements Specification of what electron beam properties are consistent with this design • beam energy and energy spread • range of deliverable beam currents • beam emittance • beam position controls • upper limits on beam halo

16. Requirement: >12 GeV Electron Beam Energy effects of endpoint energy on figure of merit: rate (8-9 GeV) * p2 @ fixed hadronic rate • The polarization figure of merit for GlueX is very sensitive to the electron beam energy. • Decreasing the upgrade energy by only 500 MeV would have a substantial impact on GlueX.

17. gp p+p+p- n Electron Beam Energy Resolution • beam energy spread dE/E requirement: < 0.01 % r.m.s. • compares favorably with best estimate: 0.06 % • absolute energy scale determination to 0.1% via known meson masses in the GlueX detector: eg. f(1020) • tied to the energy resolution requirement for the tagger • derived from optimizing the missing-mass resolution in the channels with a missing final-state particle. Typical channel analyzed using the missing mass technique:

18. Range of Required Beam Currents • upper bound of 3 mA projected for GlueX at high intensity corresponding to 108g/s on the GlueX target. • with safety factor, translates to 5 mAfor the maximum current to be delivered to the Hall D electron beam dump • during running at a nominal rate of 107g/s : I =300 nA • lower bound of 100 pA is required to permit accurate measurement of the tagging efficiency using a in-beam total absorption counter during special low-current runs. • similar to low-current operation in Hall B

19. Electron Beam Emittance This is a key issue for achieving the requirements for the GlueX Photon Beam • requirement : <10-8 m•r • all expressions are r.m.s. values • derivation : • virtual spot size: 500 mm • radiator-collimator: 76 m • crystal dimensions: 5 mm • In reality, one dimension (y) is much better than the other (x 2.5) Optics study: goal is achievable, but close to the limits according to 12 GeV machine models

20. Hall D Optics Conceptual Design Study Summary of key results: energy 12 GeV r.m.s. energy spread 7 MeV transverse x emittance 10 mm µr transverse y emittance 2.5 mm µr minimum current 100 pA maximum current 5 µA x spot size at radiator1.6 mm r.m.s. y spot size at radiator 0.6 mm r.m.s. x spot size at collimator 0.5 mm r.m.s. y spot size at collimator 0.5 mm r.m.s. position stability ±200 µm

21. Electron Beam Position Controls • Must satisfy two criteria: • The virtual electron spot must be centered on the collimator. • A significant fraction of the real electron beam must pass through the diamond crystal. • criteria for “centering”: dx < s / 2  200 mm • controlled by steering magnets ~100m upstream Using upstream BPM’s and a known tune, operators can “find the collimator”. Once it is approximately centered ( 5 mm ) an active collimator must provide feedback.

22. Electron Beam Halo • two important consequences of beam halo: • distortion of the active collimator response matrix • backgrounds in the tagging counters • Beam halo model: • central Gaussian • power-law tails • Requirement: central Gaussian power-law tail central + tail ~q-4 Integrated tail current is less than of the total beam current. 10-5 r / s 5 2 3 4 1

23. Photon Beam Position Controls • conventional BPM’s provide coarse centering • position resolution 100mm r.m.s. • a pair separated by 2m : 4mm r.m.s. at the collimator • matches the collimator aperture: can find the collimator • primary beam collimator is instrumented • provides “active collimation” • position sensitivity out to 30mm from beam axis • maximum sensitivity of 200mm r.m.s. within 2mm

24. Overview of Photon Beam Stabilization • Monitor alignment of both beams • BPM’s monitor electron beam position to control the spot on the radiator and point at the collimator • BPM precision in x is affected by the large beam size along this axis at the radiator • independent monitor of photon spot on the face of the collimator guarantees good alignment • photon monitor also provides a check of the focal properties of the electron beam that are not measured with BPMs. 3.5 mm 1s contour of electron beam at radiator 1.1 mm

25. Active Collimator Design primary collimator (tungsten) • Tungsten pin-cushion detector • used on SLAC coherent bremsstrahlung beam line since 1970’s • SLAC team developed the technology through several iterations • reference: Miller and Walz, NIM 117 (1974) 33-37 • SLAC experiment E-160 (ca. 2002, Bosted et.al.) latest users, built new ones • performance is known active device incident photon beam

26. Active Collimator Simulation 12 cm 5 cm

27. Active Collimator Simulation current asymmetry vs. beam offset y (mm) 20% 40% 60% x (mm) 12 cm

28. Detector response from simulation beam centered at 0,0 10-4 radiator Ie = 1mA inner ring of pin-cushion plates outer ring of pin-cushion plates

29. Active Collimator Position Sensitivity using inner ring only for fine-centering ±200 mm of motion of beam centroil on photon detector corresponds to ±5% change in the left/right current balance in the inner ring

30. Photon Beam Quality Monitoring • tagger broad-band focal plane counter array • necessary for crystal alignment during setup • provides a continuous monitor of beam/crystal stability • electron pair spectrometer • located downstream of the collimation area • sees post-collimated photon beam directly after cleanup • 10-3 radiator located upstream of pair spectrometer • pairs swept from beamline by spectrometer field and detected in a coarse-grained hodoscope • energy resolution in PS not critical, only left+right timing • coincidences with the tagger provide a continuous monitor of the post-collimator photon beam spectrum.

31. Other Photon Beam Instrumentation • visual photon beam monitors • total absorption counter • safety systems

32. V. Diamond crystal requirements • how much to say here? • should probably be brief

33. Diamond Crystal Properties limits on thickness from multiple-scattering rocking curve from X-ray scattering natural fwhm

34. Diamond crystal: goniometer mount temperature profile of crystal at full operating intensity oC

35. Summary • review highlights

36. Tagging spectrometer focal plane • Requirements • spectrum coverage 25% -- 95% in 0.5% steps 70% -- 75% in 0.1% steps • energy resolution 10 MeV r.m.s. • rate capability up to 500 MHz per GeV

37. Tagging spectrometer: two dipole design radiator quadrupole dipole 1 dipole 2 full-energy electrons photon beam focal plane • final bend angle: 13.4o • dipole magnetic field: 1.5 T • focal plane length: 9.0 m • energy resolution: 5 MeV r.m.s. • gap width: 3 cm • pole length: 3.1 m • dipole weight: 38 tons • coil power: 30 kW

38. Tagger focal plane: fixed array • 141 counters, spaced every 60 MeV (0.5% E0) • plastic scintillator paddles oriented perpendicular to rays • not designed for complete recoil electron coverage • conventional phototube readout • essential for diamond crystal alignment • useful as a monitor of photon beam • can run in counting mode at full source intensity

39. Tagger focal plane: microscope • Design parameters • square scintillating fibers • size 2 mm x 2 mm x 20 mm • clear light guide readout • aligned along electron direction • exploits the maximum energy resolution of the spectrometer • readout with silicon photomultipliers (SiPM devices) • dispersion is 1.4 mm / 0.1% at 9 GeV • 2D readout to improve tagging efficiency SiPM sensors clear light fibers scintillating fibers focal plane electron trajectory

40. Beam line simulation • Detailed photon beam line description is present in HDGeant • beam photons tracked from exit of radiator • assumes beam line vacuum down to a few cm from entry to primary collimator, followed by air • beam enters vacuum again following secondary collimator and continues down to a few cm from the liquid hydrogen target • includes all shielding and sweep magnets in collimator cave • monitors background levels at several positions in cave and hall • simulation has built-in coherent bremsstrahlung generator to simulate beam line with a realistic intensity spectrum • The same simulation also includes the complete GlueX target and spectrometer, detector systems, dump etc.

41. Beam line simulation cut view of simulation geometry through horizontal plane at beam height Hall D collimator cave Fcal tagger building Cerenkov vacuum pipe spectrometer

42. Beam line simulation overhead view of collimator cave cut through horizontal plane at beam height 12 m collimators concrete air vac vac sweep magnets iron blocks lead

43. Electronics and readout • tungsten plate is cathode for current loop • anode is whatever stops the knock-ons • walls of collimator housing • primary collimator • for good response, these must be in contact • tungsten plate support must be very good insulator – boron nitride (SLAC design) • uses differential current preamplifier with pA sensitivity • experience at Jlab (A. Freyberger) suggests that noise levels as low as a few pA can be achieved in the halls • requires keeping the input capacitance low (preamp must be placed near the detector) • differential readout, no ground loops

44. Beam position sensitivity • Sensitivity is greatest near the center. • Outside the central 1 cm2 region the currents are non-monotonic functions of the coordinates. • A fitting procedure has been demonstrated that could invert the eight currents to find the beam center to an accuracy of ±350 mm anywhere within 3 cm of the collimator aperture. • Using BPMs and survey data, the electron beam must be able to hit a strike zone 6 cm in diameter from a distance of 75 m.

45. Beam line: shielding overhead view of collimator cave cut through horizontal plane at beam height 12 m collimators concrete air vac vac sweep magnets iron blocks lead

46. Beam line: physics simulation • GEANT-based Monte Carlo • based on a coherent bremsstrahlung generator • good description of electromagnetic processes • extended to include hadronic photoproduction processes • complete description of beam line including collimator and shielding • integrated with detector sim. • g,N scattering • g,A single nucleon knockout • g,p pion photoproduction

47. Photon beam polarimetry • Several methods have been considered by GlueX • hadronic reactions • coherent p0 photoproduction on spin-0 target • rho or omega photoproduction (helicity conservation) • pair production from a crystal • incoherent pair production • triplet production • coherent bremsstrahlung spectrum analysis • Detailed comparisons have been carried out • A PHOTON BEAM POLARIMETER BASED ON NUCLEAR E+ E- PAIR PRODUCTION IN AN AMORPHOUS TARGET, F. Adamian, H. Hakobian, Zh. Manukian, A. Sirunian, H. Vartapetian (Yerevan Phys. Inst.), R. Jones (Connecticut U.),. 2005. 9pp. Published in Nucl.Instrum.Meth.A546:376-384,2005 • Polarimetry of coherent bremsstrahlung by analysis of the photon energy spectrum,   S. Darbinyan, H. Hakobyan, R. Jones, A. Sirunyan and H. Vartapetian,Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, In Press, Corrected Proof, Available online 26 August 2005.

48. Polarimetry: method 0 • measure azimuthal distribution of p0 photoproduction from a spin-0 target • sometimes called “coherent p0 photoproduction” • elegant – has 100% analyzing power • analysis is trivial – just N(f) ~ 1 + P cos(2f) • coherent scattering is not essential • only restriction: target must recoil in a spin-0 state • practical example: 4He scattering • must detect the recoil alpha in the ground state • requires gas target, high-resolution spectrometer for Eg ~ GeV • cross section suppressed at high energy • not competitive at GlueX energies

49. Polarimetry: method 1 • pair production from a crystal • makes use of a similar coherent process in pair production as produced the photon in CB • requires counting of pairs, but not precision tracking • analyzing power increases with energy (!) • second crystal, goniometer needed • asymmetry is in rate difference between goniometer settings • can also be done in attenuation mode, using a thick crystal • sources of systematic error • sensitive to choice of atomic form factor for pair-target crystal • shares systematic errors with calculation of CB process • in addition to theoretical uncertainty, it involves a model of the beam and crystal properties

50. Polarimetry: method 2 • angular distribution from nuclear pair production • sometimes called “incoherent pair production” • polarization revealed in azimuthal distribution of plane of pair • requires precise tracking of low-angle pairs, which becomes increasingly demanding at high energy • analyzing power is roughly independent of photon energy • analyzing power depends on energy sharing within the pair • best analyzing power for symmetric pairs • requires a spectrometer for momentum analysis • systematic errors • atomic form factor • multiple scattering in target, tracking elements or slits • simulation of geometric acceptance of detector