Progress Report on the A4 Compton Backscattering Polarimeter

Transcription

1 A4 Progress Report on the A4 Compton Backscattering Polarimeter Yoshio Imai, Institut für Kernphysik, Universität Mainz International Workshop on Parity Violation and Hadronic Structure, LPSC Grenoble 1. Introduction 2. Compton Polarimetry 3. Layout of the Polarimeter 4. Status and Results 5. Outlook and Summary

2 The A4 experiment physics goal : method : 1. Introduction determine strange-quark contribution to the nucleon properties measure the parity violating crosssection-asymmetry in elastic eletronnucleon scattering with polarized beams Calorimeter: PbF 2 e Target: H 2 Transmission Polarimeter Dump E = 855 MeV, 57 MeV I = 2µA P = 8% (cf. talk by C. Weinrich) measured quantity: A exp = P A phys e exp. asymmetry physics asymmetry beam polarization Need to measure the absolute beam polarization Method: Compton backscattering polarimetry page 1

3 2. Compton Polarimetry Scattering of photons on leptons f g e g g Detector e g Compton cross-section: 1 Q VP e long long VP Lipps, Tolhoek Physica XX(1954) e trans cos trans Q,V : Stokes Parameters (initial photon polarization) Q : linear contribution V : circular contribution (+1 : right circular, -1 : left circular) With circular light: asymmetry between scattering of right- and left-handed photons average over f (longitudinal polarization) A d d d d long right right left left V P e long Asymmetry proportional to beam polarization page 2

4 Angular distribution of cross-section and asymmetry: ds s /dw W [Mb/sr] q g q lab [mrad] 4 asymmetry q lab [mrad] backscattered photons concentrated to small cone q g E = MeV g = k in = 2.41 ev q g k fmax =.6 mrad = 26.2 MeV most calorimetric detectors will average over f Measuring time counting rate asymmetry energy spectrum asymmetry t 1 1 t 2 L A L A 2 Luminosity requirements Chen, Bardin, Cavata et al. Conceptual Design Report... DP/P [%] t [min] L [khz/b]: 855 MeV 57 MeV green light (514.5nm), 8% electron polarization page 3

5 Luminosity for colliding beams Laser a Electron depends on: numerical results: L v rel 1 ( x) 2( x) d - beam focusing - crossing angle 3 x L [khz/barn] Laser focusing: z= R.5m z= 1.m R z= 1.5m R z= 2.m R z= 2.5m R z= 3.m R a [mrad] assumptions: laser light, nm laser power 1W gaussian beams E L hor E e vert E e = 26p p µm mrad =7.8p p µm mrad =1.p p µm mrad antiparallel geometry is desirable more laser power needed page 4

6 3. Layout of the A4 Polarimeter Methods of increasing the laser power 1. Fabry-Pérot external cavity (e.g. JLab Hall A) resonance buildup intensity gain up to 1 drawback: gain-bandwidth-product g const frequency stabilization necessary 2. Internal cavity (A4 Polarimeter) extend cavity make all mirrors high reflective no frequency stabilization necessary but: maximum intensity lower than with external cavity page 5

7 magnetic chicane (Poster by Jeong Han Lee) Schematic View of the Polarimeter fibre detector Stokes parameter measurement M3 wire scanners M45 lens waveplate (rotatable) 2.42 m 2.7 m waveplate M2 g detector plasma tube Ar-Ion laser l=514.5nm M1 quadrant diode quadrant diode for laser beam stabilization (Poster by J. Diefenbach) total cavity length interaction zone : 7.8 m : 2.7 m page 6

8 Optical System - design based on commercial laser crate - subject to boundary conditions: 1. beam profile in laser medium unchanged 2. optics accessible for maintenance 3. system to fit into chicane 4. profile matching for high luminosity despite vibrations in the optical system Problem: - Sensitivity of beam axis to optics vibration depends on optics spacing. - Sensitivity of luminosity to beam axis fluctuations depends on beam focusing perform MC-simulations to find compromise Mean luminosity as function of tilt noise amplitude: L [Hz/barn] max[ rad] with stabilization (preliminary) laser focusing: z R2 =.5m z R2 =1.m z R2 =1.5m z R2 =2.m z R2 =2.5m z =3.m R2 without stabilization (preliminary) compromise: z = 2.5m R2 L = 2.1 khz/barn per 1W max page 7

9 Polarization - Polarization of the laser light enters into asymmetry A V maximize V (=circular polarization) measure polarization state 1. Resonator analysis with Jones/Stokes-formalism - need two waveplates (because within resonator) - analyze resulting polarization when rotating one waveplate 1.5 round-trip attenuation Stokes parameter V -.5 V=+/-1 possible [ ] 2. Stokes parameter measurement - use vacuum window as beamsplitter - method: rotating waveplate and linear polarizer - result: intensity modulation, amplitudes proportional to Stokes parameters Glan- Laserprism waveplate photodiode I ( 1 4 Q ) (2I Q) 2V sin2 Usin4 cos4 linear circular linear linear page 8

10 Photon arm Detector NaI calorimeter, 3 crystals, 4PM each - length: 12 X - radius : 2.2r M Electron arm - involved electron loses energy - dispersion in dipole magnets leads to displacement behind chicane detect electron with SciFi-array and measure photons in coincidence with electrons background reduction page 9

11 4. Status and Results - successfully installed magnetic chicane in MAMI Hall 3 (Dec 22) - no degradation of beam quality on A4 target - successfully installed laser and optical system - at first, operated without waveplates (Dec 22) P = 7 W (max) - resolved problems with stress birefringence in the vacuum windows (Mar 24) - installed the waveplates (Mar 24) P = 9 W (max) intracavity power [W] : with waveplates : without waveplates tube current[a] page 1

12 - performed first successful overlap tests and measured backscattered photons with the NaI (Aug 23) pola pola Entries NaJ Spectrum with 2 mua, laser off NaJ Spectrum with 2 mua, laser on, 49W intracavity power No coincidence pola ADC channel Difference laser on - laser off No coincidence installed a SciFi array behind the chicane array is operational and has been used for background reduction in a test beamtime (May 24) Kanal ohne Laser µa, Laser off 2 µa, Laser on, 63W with coincidence Kanal Differenz difference with coincidence without fibre detector Compton rate: background rate: SNR: 2.6 khz 18.6 khz 1:7.11 with fibre detector Compton rate: background rate: SNR: despite non-optimal experimental conditions: - imperfect overlap - no laser stabilization 6.5 Hz 125 Hz 1:2.7 page 11

13 Scanner 21A: 2354_5 Up Scanner 21B: 2354_5 Up Scanner 21C: 2354_5 Up Medusa Door Medusa Door Medusa Door Down Down Down - installed a beam stabilization system (Nov 23)) system has been tested and being prepared for routine operation within the polarimeter system (cf. poster contribution by J. Diefenbach) page 12

14 - installed a Stokes parameter measurement system (Aug 23) laser light from 45 window stepping motor waveplate Glanlaserprism photodiode - system has been tested and is being prepared for routine operation within the polarimeter Stokes diode 1 (intensity normalized) page 13

15 5. Summary and Outlook - planned and successfully installed a magnetic chicane for a Compton backscattering polarimeter - planned and successfully installed an optical system, laser intensities up to nm - commissioned a detector and measured first backscattered photons - planned and successfully installed a stabilization system for the laser optics - commissioned an electron detector and improved SNR from 1:7 to 1:2 Next steps: - refine laser polarization measurement and measure the Compton asymmetry with circular light - upgrade the vacuum system ready for longitudinal asymmetry program - upgrade to transverse spin measurement: use position-sensitive detector to measure spatial Compton asymmetry ready for entire physics program page 14