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

2. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 2 Presentation Overview Photon beam properties Competing factors and optimization Electron beam requirements Beam monitoring and instrumentation Diamond crystal requirements

3. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 3 I. Photon Beam Properties Direct connections with the physics goals of the GlueX experiment: Energy Polarization Intensity Resolution 9 GeV 40 % 10 7  /s 10 -3 EE E solenoidal spectrometer meson/baryon resonance separation 2.8GeV/c 2 forward m X production up to 2.8GeV/c 2 adequate for resolving resonances opposite dominant exchanges with opposite dominant exchanges PWA provides sufficent statistics for PWA on major channels with ~1yr running all-charged modes matches resolution of the GlueX spectrometer in all-charged modes

4. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 4 No other solution was found that could meet all of these requirements at an existing or planned nuclear physics facility. Coherent Bremsstrahlung with Collimation 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. Unique:

5. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 5 Kinematics of Coherent Bremsstrahlung effects of collimation at 80 m distance from radiator incoherent (black) and coherent (red) kinematics effects of collimation: to enhance high-energy flux and increase polarization

6. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 6 circular polarization  transfer from electron beam  reaches 100% at end-point linear polarization  determined by crystal orientation  vanishes at end-point Polarization from Coherent Bremsstrahlung Linear polarization arises from the two-body nature of the CB kinematics Linear polarization has unique advantages for GlueX physics: a requirement not a requirement Circular polarization is potentially useful as well, but not a requirement to achieve the physics goals of GlueX.

7. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 7 Photon Beam Intensity Spectrum 4 nominal tagging interval Rates based on: 12 GeV endpoint 20  m diamond crystal 100nA electron beam Leads to 10 7  /s on target (after the collimator) Design goal is to build an experiment with ultimate rate capability as high as 10 8  /s on target.

8. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 8 II. Optimization photon energy vs. polarization crystal radiation damage vs. multiple scattering collimation enhancement vs. tagging efficiency Understanding competing factors is necessary to optimize the design

9. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 9 Optimization: chosing a photon energy 8 GeV10 GeV A minimum useful energy for GlueX is 8 GeV, 10 GeV would be better for several reasons, steep function of peak energy 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. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 10 Optimization: choice of diamond thickness 20  m 10 -4 rad.len Design calls for a diamond thickness of 20  m which is approximately 10 -4 rad.len. thinning Requires thinning: special fabrication steps and $$. Impact from multiple- scattering is significant. up to a point… Loss of rate is recovered by increasing beam current, up to a point… The choice of 20  m is a trade-off between MS and radiation damage.

11. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 11 Optimization: scheme for collimation The argument for why a new experimental hall is required for GlueX the short answer: because of beam emittance virtual electron spot 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. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 12 Optimization: radiator – collimator distance a < 20 mr s 0 < 1/3 c d > 70 m With increased collimator distance:  polarization grows  low-energy backgrounds shrink  tagging efficiency  tagging efficiency drops off

13. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 13 Optimization: varying the collimator diameter linear polarization effects of collimation on polarization spectrum collimator distance = 80 m 5 figure of merit: effects of collimation on figure of merit: rate (8-9 GeV) * p 2 @ fixed hadronic rate

14. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 14 peak energy 8 GeV 9 GeV 10 GeV 11 GeV N  in peak 185 M/s 100 M/s 45 M/s 15 M/s peak polarization 0.54 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 collimator 5.3 W 4.7 W 4.2 W 3.8 W power on H 2 target 810 mW 690 mW 600 mW 540 mW total hadronic rate 385 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) Results: summary of photon beam properties the maximum current a factor of 10 lower 1.Rates reflect a beam current of 3  A which corresponds to 10 8  /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. 2.Total hadronic rate is dominated by the nucleon resonance region. 3.For a given electron beam and collimator, background is almost independent of coherent peak energy, comes mostly from incoherent part. 2,3 1 1 1

15. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 15 III. Electron Beam Requirements beam energy and energy spread range of deliverable beam currents beam emittance beam position controls upper limits on beam halo Specification of what electron beam properties are consistent with this design

16. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 16 Electron Beam Energy effects of endpoint energy on figure of merit: rate (8-9 GeV) * p 2 @ fixed hadronic rate  The polarization figure of merit for GlueX is very sensitive to the electron beam energy. Requirement: >12 GeV  Decreasing the upgrade energy by only 500 MeV would have a substantial impact on GlueX.

17. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 17 Electron Beam Energy Resolution 0.01 % r.m.s. beam energy spread  E/E requirement: < 0.01 % r.m.s. 0.06 % compares favorably with best estimate: 0.06 % absolute energy scale determination to 0.1% via known meson masses in the GlueX detector: eg.  (1020)  p  +  +  - n 1.tied to the energy resolution requirement for the tagger 2.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. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 18 3  A upper bound of 3  A projected for GlueX at high intensity corresponding to 10 8  /s on the GlueX target. 5  A with safety factor, translates to 5  A for the maximum current to be delivered to the Hall D electron beam dump I =300 nA during running at a nominal rate of 10 7  /s : I = 300 nA 100 pA total absorption counter 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 Range of Required Beam Currents

19. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 19 Electron Beam Emittance <10 -8 m r requirement : < 10 -8 m r all expressions are r.m.s. values derivation :  virtual spot size: 500  m  radiator-collimator: 76 m  crystal dimensions: 5 mm In reality, one dimension (y) is much better than the other ( x 2.5) This is a key issue for achieving the requirements for the GlueX Photon Beam Optics study: goal is achievable, but close to the limits according to 12 GeV machine models

20. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 20 Hall D Optics Conceptual Design Study energy12 GeV r.m.s. energy spread7 MeV transverse x emittance10 mm µr transverse y emittance2.5 mm µr minimum current100 pA maximum current5 µA x spot size at radiator1.6 mm r.m.s. y spot size at radiator0.6 mm r.m.s. x spot size at collimator0.5 mm r.m.s. y spot size at collimator0.5 mm r.m.s. position stability±200 µm Summary of key results:

21. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 21 Must satisfy two criteria: 1.The virtual electron spot must be centered on the collimator. 2.A significant fraction of the real electron beam must pass through the diamond crystal.  x <  m criteria for “centering”:  x <  m ~100m upstream controlled by steering magnets ~100m upstream Electron Beam Position Controls 1.Using upstream BPM’s and a known tune, operators can “find the collimator”. 2.Once it is approximately centered (  5 mm ) an active collimator must provide feedback.

22. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 22 Electron Beam Halo two important consequences of beam halo: 1.distortion of the active collimator response matrix 2.backgrounds in the tagging counters Beam halo model:  central Gaussian  power-law tails Requirement: r /  central Gaussian power-law tail central + tail 1 234 5 Integrated tail current is less than of the total beam current. 10 -5 ~  -4

23. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 23 Photon Beam Position Controls conventional BPM’s provide coarse centering 100  m r.m.s.  position resolution 100  m r.m.s. 4mm r.m.s. at the collimator  a pair separated by 2m : 4mm r.m.s. at the collimator can find the collimator  matches the collimator aperture: can find the collimator primary beam collimator is instrumented  provides “active collimation” 30mm  position sensitivity out to 30mm from beam axis 200  m r.m.s.  maximum sensitivity of 200  m r.m.s. within 2mm

24. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 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. 1.1 mm 3.5 mm 1  contour of electron beam at radiator

25. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 25 Active Collimator Design 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 primary collimator (tungsten) incident photon beam

26. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 26 Active Collimator Simulation 12 cm5 cm

27. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 27 12 cm x (mm) y (mm) current asymmetry vs. beam offset 20% 40% 60% Active Collimator Simulation

28. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 28 Detector response from simulation inner ring of pin-cushion plates outer ring of pin-cushion plates beam centered at 0,0 10 -4 radiator I e = 1  A

29. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 29 Active Collimator Position Sensitivity using inner ring only for fine-centering ±200  m of motion of beam centroil on photon detector corresponds to ±5% change in the left/right current balance in the inner ring

30. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 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. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 31 Other Photon Beam Instrumentation visual photon beam monitors total absorption counter safety systems

32. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 32 V. Diamond crystal requirements how much to say here? should probably be brief

33. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 33 Diamond Crystal Properties limits on thickness from multiple-scatteringrocking curve from X-ray scattering natural fwhm

34. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 34 Diamond crystal: goniometer mount temperature profile of crystal at full operating intensity oCoC

35. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 35 Summary review highlights

36. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 36 Tagging spectrometer focal plane Requirements 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. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 37 Tagging spectrometer: two dipole design radiator quadrupole dipole 1 dipole 2 photon beam full-energy electrons focal plane  final bend angle:13.4 o  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. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 38 Tagger focal plane: fixed array  141 counters, spaced every 60 MeV (0.5% E 0 )  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. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 39 Tagger focal plane: microscope focal plane electron trajectory SiPM sensors scintillating fibers clear light fibers Design parameters 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 SiPM devices  readout with silicon photomultipliers (SiPM devices)  dispersion is 1.4 mm / 0.1% at 9 GeV  2D readout to improve tagging efficiency

40. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 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. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 41 Beam line simulation Hall D collimator cave tagger building vacuum pipe Fcal Cerenkov spectrometer cut view of simulation geometry through horizontal plane at beam height

42. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 42 Beam line simulation overhead view of collimator cave cut through horizontal plane at beam height collimators sweep magnets iron blocks concrete lead 12 m air vac

43. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 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. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 44 Beam position sensitivity Sensitivity is greatest near the center. Outside the central 1 cm 2 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  m 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. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 45 Beam line: shielding overhead view of collimator cave cut through horizontal plane at beam height collimators sweep magnets iron blocks concrete lead 12 m air vac

46. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 46 Beam line: physics simulation GEANT-based Monte Carlo 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. 1.g,N scattering 2.g,A single nucleon knockout 3.g,p pion photoproduction

47. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 47 Photon beam polarimetry Several methods have been considered by GlueX  hadronic reactions coherent  0 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.A546:376-384,2005F. AdamianH. HakobianZh. ManukianA. SirunianH. VartapetianYerevan Phys. Inst.R. JonesConnecticut U.  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. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 48 Polarimetry: method 0 measure azimuthal distribution of  0 photoproduction from a spin-0 target  sometimes called “coherent  0 photoproduction”  elegant – has 100% analyzing power  analysis is trivial – just N(  ) ~ 1 + P cos(2  )  coherent scattering is not essential  only restriction: target must recoil in a spin-0 state practical example: 4 He scattering  must detect the recoil alpha in the ground state  requires gas target, high-resolution spectrometer for E  ~ GeV  cross section suppressed at high energy not competitive at GlueX energies

49. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 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. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 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

51. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 51 Polarimetry: method 3 angular distribution from pair production on atomic electrons  sometimes called “triplet production”  polarization reflected in a number of observables  azimuthal distribution of large-angle recoil electrons is a preferred observable  analyzing power is roughly constant with photon energy  no need for precision tracking of forward pair  pair spectrometer still needed to select symmetric pairs systematic errors  reduced dependence on atomic physics – “exact” calculations  analyzing power sensitive to kinematic cuts  relies on simulation to know acceptance

52. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 52 Polarimetry: method 4 analysis of the shape of the CB spectrum  not really polarimetry – this you do anyway  probably the only way to have a continuous monitor of the beam polarization during a run  polarimetry goal would be to refine and calibrate this method for ultimate precision  takes advantage of tagging spectrometer  requires periodic measurements of tagging efficiency using a total absorption counter and reduced beam current systematic errors  atomic form factor used to calculate CB process  model of electron beam and crystal properties  photon beam alignment on the collimator

53. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 53 Measurements at YerPhI in 2005 beam time obtained for CB measurements  electron beam energy 4.5 GeV  4-week run during Fall 2005  goniometer, crystal, pair spectrometer commissioned installed to test IPP method  results anticipated soon YerPhI measurements supported by CRDF grant

54. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 54 Photon Beam: coherent bremsstrahlung Criteria Criteria  high energy 8-9 GeV  linear polarization 0.5  high flux 10 8 s -1  energy resolution 10 MeV r.m.s.  low background Design options Design options A.laser backscatter B.synchrotron backscatter C.bremsstrahlung D.coherent bremsstrahlung energy     polarization     flux     resolution (  ) (  ) (  ) (  ) background    ABC (  ) with tagging D “figure of merit” = (rate in 8-9 GeV window) * (polarization) 2 @ constant b.g.

55. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 55 Active collimator overview active stabilization required for collimation  distance from radiator to collimator 75 m  radius of collimator aperture 1.7 mm  size of real image on collimator face 4 mm r.m.s.  size of virtual image on collimator face 0.5 mm r.m.s.  optimum alignment of beam center on collimator aperture ±0.2 mm in x and y steering the electron beam  BPMs on electron beam measure x and y to ±0.1 mm  BPM pairs 2 m apart gives ±4 mm at collimator  BPM technology might be pushed to reach alignment goal, under the assumption that the collimator is stationary in this ref. sys.

56. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 56 Design criteria for photon monitor radiation hard (up to 5 W of gamma flux) require infrequent access (several months) dynamic range factor 1000 good linearity over full dynamic range gains and offsets stable for run period of days sampling frequency at least 60 Hz at operating beam current, 600 Hz desireable fast analog readout for use in feedback loop

57. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 57 Beam line simulation 3d view of primary collimator with segmented photon rate monitor in front

58. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 58 Detector response the photons are incident on the back side of the pin- cushions (opposite the pins) showers start in the base plates (~2 radiation lengths) showers develop along the pins, leaking charges into the gaps charge flow is asymmetric (more e - than e + ) due to high- energy delta rays called “knock-ons” asymmetry leads to net current flow on the plates proportional to the photon flux that hits it SLAC experience shows that roughly 1-2 knock-ons are produced per incident electron 1  A * 10 -4 rad.len. * ln(E 0 /E 1 )  1 – 2 nA detector current