1. The MESA-Project and itS electron Polarimeters
Kurt Aulenbacher

2. Outline The MESA project Planned and possible experiments at MESA
Polarimetry at MESA
Some considerations concerning FOM in the 10MeV range
MESA = Mainz Energy recovering Superconducting Accelerator
…is a „follow up“ project to the existing MAMI facility …and has to deal with
the constraints of accelerator physics at universites…

3. Accelerators at universities: Why and how?
KPH-Mainz
Blue: MAMI
CERN-LHC
JGU-Mainz
1,3 km
1,3km
9km
0,2km
MAMI-accelerator
in underground
area at KPH in JGU
University facilities: limits of growth have been reached
Education and fore-front Research – how do they fit in?
Physikalisches Kolloquium des FB08

4. Expansion of MAMI???? 4 th stage of MAMI:
“HDSM” (Polytron of 2nd order)
RTM-1-3+HDSM=MAMI-C
Ideal accelerator for hadron
Physics at the GeV scale
(“perturbative QCD regime”)
R=130m
“MAMI-D” (Herminghaus 1991)
Polytron of 21nd order
E=15GeV, c.w.
to botanical
garden
To physics institute
Physikalisches Kolloquium des FB08

5. MESA: Real estate conditions
No new buildings necessary
MAMI continues separately for
hadron structure exp. ~1GeV scale
MESA takes over “low energy”
experiments ~100MeV scale
MESA-Hall-1b
MESA-Hall-1a
Shielding
MESA-Hall 2
(Experimental Hall)
High power beam dump
Shaft building
MESA advantages and innovations:
dedicated set-up‘s
New experimental technique based
on Energy Recovery Linac (ERL)-principle
Physikalisches Kolloquium des FB08

6. Initial Concept for MESA and experiments
Hall-A
Hall-B
Hall-Ex1
P2-exp
PIT-exp
Bunker-2
Bunker-1
To EB-
Beam
dump
n.c. injector „MAMBO“
SRF-Cryo-modules
EB mode: P2-experiment
155MeV, 150 muA external polarized beam
Goal: precision measurement of sin2qW
ERL mode : „PIT“-experiment
105 MeV >1mA beam with energy recovery
Goal: Search for dark photons
…obviously installation becomes quite crowded
D. Simon

7. Expansion Concept for MESA?
D. Simon
Possible Additional experiments:
EB mode: BdX in parasitic mode to P2
ERL mode: Extension of PIT to double spectrometer device „MAGIX“
Several others being discussed e.g. polarized positrons, upgrade MESA...eEDM?
More space is needed!
Successfull request for extension by research building („Forschungsbau“) June 2015,
Extension building + P2 and MAGIX experiment hardware are funded

8. A possible MESA layout with the extension building
Instrumentation
Bunker
Experiment
Extension Area
MESA-A
(Appendix)
BdX
MAGIX
Access
shaft P2
D. Simon
Old
New

9. Using the Appendix Hall for small storage ring?
14 MeV pol e-beam
from MAMI-A?
(limited peak current!)
10 m
Instrumentation
Bunker
BdX
MAGIX
Access
shaft P2
Old
New

10. Polarimeters at MESA 10 m 3 BdX P2 2 1
Instrumentation
Bunker 3
BdX
Access
shaft P2 2 1
1.) Double scattering Mott (0,1 MeV)
PKA2 test set-up for MESA)
2.) Single scattering Mott
(existing, MAMI)
3.) Hydro-Möller Polarimeter
(planned)

11. Polarimeters at MESA 1.) Double scattering Mott (0,1 MeV)
PKA2 test set-up for MESA
Instrumentation
Bunker 3
BdX
Access
shaft 2 1
A. Gellrich and J.Kessler
PRA (1991)
Double Mott Scattering polarimeter:
Determination of effective analyzing power
by direct (theory-free) Measurement
Goal DP/P = 0.3%
Test source set-up at PKA2
Apparatus in operation
Crucial: Elimination of false asymmetry
at 0.1% level
Cross checks for consistency ongoing!
Invasive technique, measurement time ~1min
limited dynamic range, Ip < ~1mA
M. Molitor

12. Polarimeters at MESA 3.) Hydro Möller (155MeV, online capability)
R&D project at MESA
Instrumentation
Bunker
E. Chudakov, V. Luppov,
EPJ A 24 (2005) 123–126 3
BdX
Access
shaft 2 1
Hydro Möller polarimeter:
Goal DP/P = 0.5%
Completely e-polarized Hydrogen gas,
Axial trapping by solenoid
Radial “trapping” by Suprafluid-Helium film
Measurement time ~2 hours (online)
Demanding technology, never used at
accelerator so far

13. Polarimeters at MESA 2.) Mott Polarimeter
Currently in operation at MAMI ILAC (3,5 MeV)
Instrumentation
Bunker 3
BdX
Access
shaft 2 1
Single scattering polarimeter @ 5MeV
Large dynamic range mA
Good reproducibility
V. Tioukine et al.
Rev. Sc. Instrum.
(2011)
V. Tioukine et al.
Rev. Sc. Instrum.
(2011)
R. Barday et al. 2011
J. Phys. Conf. Ser

14. Polarimeters at MESA 2.) Mott Polarimeter
Currently in operation at MAMI ILAC (3,5 MeV)
Instrumentation
Bunker 3
BdX
Access
shaft 2 1
Single scattering polarimeter @ 5MeV
Large dynamic range mA
Good reproducibility
V. Tioukine et al.
Rev. Sc. Instrum.
(2011)
V. Tioukine et al.
Rev. Sc. Instrum.
(2011)
R. Barday et al. 2011
J. Phys. Conf. Ser

15. Sensitivity issues for polarimeters in the ~10 MeV range
Concerning FOM
Situation in Mott Scattering is unfavorable for operation at multi MeV!
Note:
Cross section scales (very roughly)
~1/E2 * cot(q/2)
Motz, Olsen, Koch, Rev. Mod. Phys. 36, 881, (1964)

16. Sensitivity issues for polarimeters in the ~10 MeV range
Concerning FOM Situation in Mott Scattering
is unfavorable for operation at multi MeV !
Measurements at MAMI RTM-1, Etot=14.8 MeV, lead target
J.Sromicki et al. Phys. Rev. Lett. 82 p.52 (1999)
dsrel=2,8
dsrel=1
dsrel=0.056
Due to strong decrease of cross section
With scattering angle (~cot(q/2))
 Guess: Optimum ~145 deg. t=200mg/cm2
DW~100msr, ds/dW~0,3b/sr
Nd/Np ~ 2*10-5
FOM=4*10-8
Reduction of Analyzing Power with
Target thickness!

17. Sensitivity issues for polarimeters in the ~10 MeV range
Moller Scattering :
Spin parallel symmetric scattering in CM-System
Is FORBIDDEN bei Pauli Principle!
E/2
p/2 e E
E/2
CM-system
Lab– system
(fixed target)
B. Wagner et al.
Nucl. Instr. Meth. A 294 , 541 (1990)
-Analyzing power in ER-Limit
- Symmetric scattering (coincident detection for background suppression)
- optimum for long. polarization (7/9)
- But iron target yield only ~ 2/26*7/9~0,06  Seff similar to Mott scattering
- What about cross section ?

18. Sensitivity issues for polarimeters in the ~10 MeV range
Cross section for Moller Scattering ?
For symmetric scattering cross section is
~1/12 of Mott-scattering (ER-Limit)
Möller Scattering is much more efficient compared to Mott!
Target thickness effect requires attention, but probably not worse than Mott
Conclusion: FOM in Moller-Scattering
could be several orders of magnitude
larger, perhaps O(10-5)
B. Wagner et al.
Nucl. Instr. Meth. A 294 , 541 (1990)

19. Sensitivity issues for polarimeters in the ~10 MeV range
Cross section for Moller Scattering ?
For symmetric scattering cross section is
~1/12 of Mott-scattering (ER-Limit)
Due to lower lab angle, Möller Scattering is much more efficient compared to Mott!
Target thickness effect requires attention, but probably not worse than Mott
Conclusion: FOM in Moller-Scattering
could be several orders of magnitude
larger, perhaps O(10-5)
B. Wagner et al.
Nucl. Instr. Meth. A 294 , 541 (1990)

20. Conclusion MESA is a new polarized electron beam facility
High precision polarimeters are in development for experiments at MESA
Sensitivity/FOM of Mott polarimeter is limited in the 10MeV regime
Möller polarimeter could be better

21. Conclusion
Thank you…