Polarized WACS Experiment (E12-14-006) Using Compact Photon Source (CPS) Donal Day, Dustin Keller, Jixie Zhang University of Virginia Feb. 18, 2016 1 1 1 1

Outline Experiment setup for polarized approved WACS (E12-14-006) Compact Photon Source design Summary Jixie Zhang Polarized WACS 2 2 2 2 2

E12-14-006 Setup: HMS + NPS 80% polarized beam at 4.4 GeV HMS: High Momentum Spectrometer NPS: Neutral Particle Spectrometer 80% polarized beam at 4.4 GeV Kinematic Range: E = 4.0 GeV, s= 8 GeV2 θCM = 60o and 136o 6% copper radiator mixed e−γ beam polarized proton target Target field sweeps away the electrons Jixie Zhang Polarized WACS 3 3 3 3 3

Challenge of WACS from PT Small cross sections Low photon flux Limited luminosity of DNP targets Mixed photon-electron beam degrades polarization Radiation damage Overhead Anneals TEs Swapping material Optics demands knowledge of the photon coordinates at the target (+/- 2 mm) Necessary collimation of photon beam reduces flux (depending on distance from radiator) Jixie Zhang Polarized WACS 4 4 4 4 4

The Fix - A Pure Photon Source Pure photon source opens new opportunities For polarized WACS a pure photon beam has the following advantages: Polarized target FOM is higher higher average polarization Reduced target maintenance/overhead time Possible to significantly increase incoming beam current from nominal 100 na to as high as 1 microA Minimal background from electron scattering, signal is cleaner Opens possibility of new WACS proposal with higher incoming beam energy, larger kinematic coverage Opens door for other high intensity photon beam experiments from from polarized targets (proton/deuteron) for Hall C and A Jixie Zhang Polarized WACS 5 5 5 5 5

Compact Photon Source Structure From Bogdan's Talk Jixie Zhang Polarized WACS 6 6 6 6 6

Compact Photon Source From Bogdan's Talk According to previous page, the total z length of magnet+shielding is 2.6m. That says, there will be 80cm thick shielding between magnet and target chamber. Therefore the distance from the radiator to the target should not less than 2.32 m. (Target chamber entrance window is 0.52m to target). Jixie Zhang Polarized WACS 7 7 7 7 7

Brem. Photon Angular Distribution Geant 4 simulation result: 6.6 Gev electron, 10% radiator (Cu) If the radiator is 2.32m from target, requires +/-2mm beam size at target will have ~18% flux lost Jixie Zhang Polarized WACS 8 8 8 8 8

Heat Load at Target: Electron Beam Geant 4 simulation result: 100 nA 6.6 Gev electron beam Jixie Zhang Polarized WACS 9 9 9 9 9

Heat Load at Target: Brem. Photon Geant 4 simulation result: 6.6 Gev electron, 10% radiator (Cu) Jixie Zhang Polarized WACS 10 10 10 10 10

Photon beam fixed in space, rotating, moving target No need to raster electron beam Full and uniform irradiation of target Photon spot fixed in space, target cell is moving up and down with rotation UVa group already rotating target for other studies Jixie Zhang Polarized WACS 11 11 11 11 11

F.O.M Change Using CPS Eliminate electron backgrounds: e-p elastic and ep events. Target averaged polarization increases from 70% to ~90%, FOM increases by ~1.7 Collimator reduces photon flux down to 82% . Heat load from one photon is a factor of 30 smaller than that from one electron. Beam current can be increased from 100 nA to 1000 nA (compromise between the target operation ). Overhead time will be greatly reduced (by 1/3): fewer anneals, target changes and TE measurements (associated with target changes). FOM could be improved by a factor of 17. Jixie Zhang Polarized WACS 12 12 12 12 12

Summary E12-16-004 was approved by PAC42 for 15 days of beam time With 4.4 GeV beam. Compact photon source will improve the FOM by a factor of 17, assuming use 1 uA beam current and radiator is 2.32m away from the target. It will allow us to propose polarized WACS at 6.6 GeV. Jixie Zhang, UVA Polarized WACS 13 13 13 13

Back up slides Jixie Zhang Jixie Zhang, UVA Polarized WACS 14 14 14 14 14 14

Photon Flux from Bremsstralung From review of particle data booklet 2012, p251 http://pdg.lbl.gov/2013/download/rpp-2012-booklet.pdf , where k is the energy of radiated photon The cross section: Number of photon between Kmin and Kmax emitted from an electron: Where d is the thickness of the radiator (d<<X0) For 10% radiator, Ymax=1.0 and Ymin = 0.5, N = 6.3% For 10% radiator, Ymax=1.0 and Ymin = 0.1, N = 29.0% Jixie Zhang, UVA Jixie Zhang Polarized WACS Polarized WACS 15 15 15 15

Electron|Positron Production at Target Incoming photon E=4.0 GeV. Jixie Zhang, UVA Polarized WACS 16 16 16 16 16