1. A compact pair polarimeter and spectrometer
Ken Livingston, University of Glasgow, Scotland 1
NPE Seminar October 2016
Motivation
Pair production and polarimetry
Compact polarimeter design 1

2. The tagged photon facilities
Motivation*
Baryons: N*
Energy (GeV)
Mesons: GlueX
1.5
1.5 3 11 12
The tagged photon facilities
Hardon physics set for major breakthroughs in both
Pol observables + Ampitude analysis – require high precision measurements
Beam and target polarisation need to be known very accuarately. 2

3. Polarization observables
Linear Polarisation 
Circular polarisation 
Nucleon recoil
polarimeter x
+N → Π
→ Y
Hyperons are “self analysing” 
Transverse pol targets p,n
Longitudinally pol targets p,n 3

4. Polarization observables – Measuring asymmetries: a simple example, 4

5. Polarization observables – Measuring asymmetries: a simple example, 
Systematics of detector acceptance cancel out.
“Only” need to know Plin, the degree of linear polarization. 5

6. Linearly polarised photons
Tagged bremsstrahlung photons up to 6GeV.
Timing resolution < 1 beam bucket.
Circularly polarized
up to ~80% now standard
Linearly polarized
coherent bremsstrahlung up to >90%. 6

7. Linearly polarised photons
Tagged bremsstrahlung photons up to 6GeV.
Timing resolution < 1 beam bucket.
Circularly polarized
up to ~80% now standard
Linearly polarized
coherent bremsstrahlung up to 90%. 7

8. Pair production Photon incident on a thin scattering foil.
Pair production – recoil is a Nucleus
< 20% analysing power, high cross section ]
Triplet production – recoil is an atomic electron.
> 20% analysing power in recoil electron angle, easy to detect, low cross section
The distribution of azimuthal angle of the pair gives the degree of linear pol.
Selecting symmetric pairs increases the analysing power
Looks easy ! 8

9. Pair production – why it's not easydX 9

10. Pair production – why it's not easyX dX 10

11. Pair production For symmetric events (x+ ~ x- = x)
x >> dX: Measure Eg. Use only dY for polarimetry
dx/x ~ dE/E: Eg from tagger. Use dX and dY for polarimetry
Dx/x ~ d(B.dl)/B.dl 11

12. Halbach dipole array Timepix3
Polarimeter design
Halbach dipole array Timepix3 12

13. Simulation 13

14. Simulation Worst case Best case
Based on Mainz PS rates with ~10nA beam
Best case (High Eg resolution from Tagger.)
<2% error per tagger bin in 20minutes.
Worst case (No Eg from tagger, dY only)
<2% error per tagger bin in 5Hrs 14

15. Simulation Worst case Best case
Based on Mainz PS rates with ~10nA beam
Best case (High Eg resolution from Tagger.)
<2% error per tagger bin in 20minutes.
Worst case (No Eg from tagger, dY only)
<2% error per tagger bin in 5Hrs 15

16. Conclusion
Looks polarised to me 16

17. STFC1
Motivation – spectroscopy. Baryon – CLAS, MAMI, Bonn, Meson, GlueX
Pol observables – Lin pol coherent brem - polarimetry
3) Current method – distribution of tagged electrons. Good stats. Limit is systematics (%5) Not detecting the photon. Infer pol from dist of degraded electrons. Only improve by measuring the pol of the photon.
4) PS process. Detect the photon.
5) In beam or Mag field.
6) Why is this not in use. Rate calc. Size.
7) Design. Result.
8) Cost. 17