Polarized Photon Beam Instrumentation for GlueX

1. GlueX collaboration meeting Dec. 11-13, 2003, Newport News Polarized Photon Beam Instrumentation for GlueX Richard Jones, University of Connecticut Part 1: active collimation Part 2: polarimetry

2. GlueX activity at Connecticut: Part 1 • simulation and software • Geant simulation • experiment geometry description • software tools • crystal radiator • diamond quality control • crystal mount • photon tagger instrumentation • tagging microscope • photon beam instrumentation • beam line shielding • beam position control • photon beam polarimetry

3. Active collimator project 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.

4. Active collimator project overview • best solution: monitor alignment of both beams • monitor on electron beam position is needed anyway to control the spot on the radiator • 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. 1.9 mm 1s contour of electron beam at radiator 0.5 mm

5. This work is the senior project of Connecticut undergraduate Chris Gauthier

6. 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.

7. 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

8. 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

9. Beam line simulation 3d view of primary collimator with segmented photon rate monitor in front

10. 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

11. Design choice • Segmented scintillator • used for the Hall B collimator (lower currents) • not very rad-hard • Ion chamber • requires gas system and HV • good choice for covering large area • Tungsten pin-cushion detector • used on SLAC coherent bremsstrahlung beam line in 1970’s • SLAC team developed the technology through several iterations, refined construction method • reference Miller and Walz, NIM 117 (1974) 33-37 • SLAC experiment E-160 (ca. 2002, Bosted et.al.) still uses them, required building new ones • performance is known

12. Simulation geometry 12 cm 5 cm

13. 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 mA * 10-4rad.len.* ln(E0/E1)Þ 1 – 2 nA detector current

14. 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

15. Beam 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

16. Beam position sensitivity • Sensitivity is greatest near the center. • Outside the central 1 cm2 region the currents are non-monotonic functions of the coordinates. • CG demonstrated a fitting procedure 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 can be steered to hit a strike zone 6 cm in diameter from a distance of 75 m.

17. 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

18. Present status and future plans • A prototype detector is under construction. • aluminum housing already fabricated at Connecticut • mechanical drawings of tungsten pin-cushions exist • two options for fabrication • cut pins from tungsten wire and manually mount them into machined tungsten wedges (first method tried at SLAC) • start off with a thick tungsten wedge and remove the excess material using Electrostatic Discharge Machining leaving only base plate and pins (superior result, SLAC) • The goal is to build and instrument a prototype with two opposing pin-cushions and test it in the Hall B photon beam line. • Budget estimate for prototype is about 15K$, and about 20K$ additional to complete the full detector and electronics.

19. GlueX activity at Connecticut: Part 2 • simulation and software • Geant simulation • experiment geometry description • software tools • crystal radiator • diamond quality control • crystal mount • photon tagger instrumentation • tagging microscope • photon beam instrumentation • beam line shielding • beam position control • photon beam polarimetry

20. Photon beam polarimetry • Polarimetry is the least worked out aspect of the GlueX beam line Conceptual Design. • CDR section copied Hall B plan (1998) • no follow-up in terms of measurements • no criteria for precision • Argument for linear polarization for GlueX is qualitative • provides initial state with definite parity • separates natural/unnatural exchange production • Making a quantitative argument would involve trying to guess what the critical channels will be and what backgrounds will be important to eliminate. • Alternative to a physical argument for precise polarimetry would be common sense: build a state-of-the-art device and be prepared to push the precision if the analysis requires it.

21. CB polarimetry project at YerPhI • Facilites with experience in photon polarimetry • Saskatoon • Mainz • Hall B – organized Photon Polarimetry Workshop (1998) • Yerevan synchrotron (YerPhI) • Connecticut / YerPhI collaboration established 2002 • develop precise photon polarimetry for coherent bremsstrahlung beams • funded by CRDF grant AP2-2305-YE-02 (64K$ for 2 years) • Progress so far • review of 4 methods, two articles in draft form • beam time allocated at YerPhI to test 1 method in 2004

22. 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

23. 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

24. 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

25. 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

26. 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

27. Planned activities at YerPhI in 2004 • beam time approved for CB measurements • electron beam energy 4.5 GeV • 4-week run during summer 2004 • goniometer, crystal, pair spectrometer already installed to test IPP method • results anticipated for next fall • visits of YerPhI physicists to Jlab • travel support provided in CRDF grant • should be timed to coincide with GlueX meeting

28. Present status, future plans • visit to Yerevan 11/03 • R. Jones visited Yerevan for 8 days in November, 03 • accelerator is quiet (and dark) but group is still active • both theory and experimental expertise in CB • good response to seminar on GlueX • possible role for YerPhI in GlueX • Deputy Director A. Sirunian has expressed interest • group seeks invitation and open projects in GlueX • polarimetry would be an obvious place to start