1. 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

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

3. 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

4. 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

5. 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

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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. Summary
E 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

14. Back up slides Jixie Zhang Jixie Zhang, UVA Polarized WACS14 14 14 14 14 14

15. Photon Flux from Bremsstralung
From review of particle data booklet 2012, p251
, 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

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