1. Glasgow University Seminar October 2006 A Novel Photon Polarimeter Joseph Santoro CUA Glasgow University Seminar October, 2006

2. Glasgow University Seminar October 2006 WARNING IN THIS TALK, POLARIZATION WILL BE SPELLED WITH A “Z”!!

3. Glasgow University Seminar October 2006 Overview 1.Intro: Polarized photons and pair production 2.Polarimetry: Polarization estimation vs. polarization measurements 3.The Hall B polarimeter 4.Results from g8b polarized photon run

4. Glasgow University Seminar October 2006 Polarized Photons Circular PolarizationLinear Polarization X y E E y X P P tt Peaks and troughs are 90 ◦ out of phase Produced as long as the incident electron beam is polarized Produced via coherent brem. (goniometer) or via Compton backscattering Peaks and troughs are 0 ◦ out of phase

5. Glasgow University Seminar October 2006 Pair production e + e - production plane is correlated with the polarization of the incoming photon Yang, Berlin, and Madansky 1950 Olsen, Maximon 1961

6. Glasgow University Seminar October 2006 Why do we care about Polarized Photons? A polarized photon beam is important for Baryon spectroscopy experiments (such as those @ CLAS)  Theoretical calculations can include photon polarization   Asymmetry  (polarization-dependent) can be more sensitive than    measurement is experimentally easier than  (No acceptance calculation !!)  One part of obtaining double polarization observables (Beam-Target and Beam-Recoil or Beam-Meson)  Extraction of 6 more SDME’s than unpolarized data  Much higher sensitivity to N* interference effects

7. Glasgow University Seminar October 2006 Drift Chambers TOF Cherenkov Electromagnetic Calorimeter A touch of CLAS

8. Glasgow University Seminar October 2006 Polarized Photon Beamline Tagger Photon Polarimeter Goniometer CLAS Pair Spectrometer Electron Beam e-e- e+e+ Actually bends in z

9. Glasgow University Seminar October 2006 Polarized Photons at CLAS The polarized photons are produced in one of two linearly polarized orientations, which are parallel or perpendicular to the horizontal plane in the lab. The fine adjustment required to produce the coherent photon spectrum (on the order of μradians) is achievable with the Goniometer. Electron beam Coherent bremsstrahlung γ-rays produced by an electron beam on 50μm diamond crystal mounted on “Goniometer”. The Goniometer is aligned to the incident electron beam through a series of photon intensity scans. θ INCOH θ COH

10. Glasgow University Seminar October 2006 Polarimetry

11. Glasgow University Seminar October 2006 Estimating Photon Polarization The polarization can be estimated by a fit to the coherent energy spectrum Enhancement Data for PERP Calculation Polarization Photon Energy (MeV) Polarization from calculation Polarization from calculation Coherent Spectrum Photon Energy (MeV) Primary coherent edge K. Livingston

12. Glasgow University Seminar October 2006 Measuring Photon Polarization Need a minimum of 3 MSDs to resolve ambiguity in position of e + e - pair e-e- e+e+  Measure  polarization P  from pair production Pair polarimeters based on magnetic separation (circa 1960) of the e + e - pair sacrifice analying power Microstrip detectors get rid of the magnetic field!!!! or more specifically….. B. Wojtsekhowski, et. al. “A pair polarimeter For linearly polarized photons” NIM 515 (2003) 605-613

13. Glasgow University Seminar October 2006 Hall B Photon Polarimeter Top View Schematic The photon with polarization e, parallel to the X-axis, is incident on the thin converter along the z-axis and produces an e+ e- pair. The crossing points of the e+e-(P,N) and the angle between the polarization plane and the e+e- production plane ±±

14. Glasgow University Seminar October 2006 Active Target and Veto Counter Beam Active Target Veto counter Entrance window VC 1x1cm, 100  m-thick scintillator AT 1x1cm, 50  m-thick scintillator Photo-coupling to the PMTs through a reflective light box

15. Glasgow University Seminar October 2006 Silicon Microstrip Detectors 2x256 and 2x384 strips 50  m pitch On-board preamplifier chips

16. Glasgow University Seminar October 2006 MSD Housing

17. Glasgow University Seminar October 2006 All put together  The idea is take measurements with the detector at 0 ◦ and rotated by 90 ◦ at each E   +- ≈ m e c 2 /E  E  = 2.5 GeV r e+e- ~ 800  m

18. Glasgow University Seminar October 2006 Electronics and Readout 1. 2(3) PCB-mounted preamplifier chips 2. Custom-made signal control box (Ribbon to LEMO) 3. 2 CAEN V550 flash ADC cards and V551 B sequencer Each strip has one FADC channel and is read out sequentially

19. Glasgow University Seminar October 2006 Triggering on e + e - pairs e+ e- delayed Active Target Pair Spectrometer MSD  ~200 ns ~1ms readout / deadtime

20. Glasgow University Seminar October 2006 Trigger Flow Chart

21. Glasgow University Seminar October 2006 Position Distributions DEAD Noisy strips Optimal One-track events used for independent detector alignment Need at least 3 working detectors to simultaneously resolve the position of e + e - pair

22. Glasgow University Seminar October 2006 Distribution of pairs Circular distribution reflects the photon beam profile

23. Glasgow University Seminar October 2006 From Data to Asymmetry We need to know the polarization a priori at least once before we use the polarimeter!!! Polarimeter was “calibrated” with 100% polarized Compton back scattered photons at Spring-8 facility to obtain an “Analysing Power” A=0.11 Analysing power is a combo of MSD and converter thickness (multiple scattering) We actually measure product of analysing power and asymmetry   K. De Jager, et. al. “A pair polarimeter for linearly polarized, high-energy photons” Eur Phys J A (2004) 19, 275-278

24. Glasgow University Seminar October 2006  -Asymmetry   eff Proof of principle experiment Independent measurement of PARA and PERP polarization was desirable but unattainable A = 0.119 (from SPRING-8 test) CHECK OUT CLAS NOTE 2006-01!!

25. Glasgow University Seminar October 2006 Polarimeter++ Polarimetry is highly desirable for the upcoming FROST experiment but first some things need to be fixed: AT/VC need to be redesigned to provide better light collection Motor control assembly and detector housing needs to be redesigned, with two different motors and optical encoder position readback Replace 4 panel single-sided MSD design with 2 panel design incorportating a double-side and single sided panel

26. Glasgow University Seminar October 2006 Summary and Conclusions A scheme for photon polarimetry, based on field-free detection of an e+e- pair was presented Tested for the first time during the g8b experiment and due to “technical difficulties”, the test turned into a proof-of-principle experiment Comparisons were made between a calculation/fit to the coherent spectra and an effective asymmetry A future polarimeter design (FROST) will incorporate a new housing design, motor rotation mechanism and 2 MSD panels instead of 4.