Patricia Aguar Bartolomé Institut für Kernphysik, Universität Mainz

1. MøllerPolarimetry with Polarized Atomic Hydrogen at MESA Patricia Aguar Bartolomé Institut für Kernphysik, Universität Mainz PSTP 2013 Workshop, Charlottesville 11th September 2013

2. Outline • Physics Motivation • Polarized Atomic Hydrogen Targets • Status of the Mainz Hydro-Møller Target • Summary

3. Physics Motivation Goal: To perform low energy PV electron scattering experiments at MESA with a systematic accuracy of < 0.5 % for the beam polarization measurements Hydro-Moller PV Detector

4. Physics Motivation Polarimetry Methods • ComptonScattering:Accurateenough at energies > 4GeV, butaccuracy • around1% at lowenergiesNotenoughfor PV-experiments • MøllerScatteringwithferromagnetic target • Advantages: • Beamenergyindependent • High analyzingpower(~ 80%) • 2 particleswithhighenergies in the final state detectable in coincidenceeliminatesbackground • Disadvantages: • Lowelectronpolarization~ 8 % • Target heatingBeamcurrentlimitedto2-3 mA • Levchukeffect~ 1% • Low Pt dead time • Systematicerrorson target polarization~ 2%

5. Physics Motivation • MøllerScatteringwithpolarizedatomichydrogen gas, stored in a ultra-cold • magnetictrap • E.ChudakovandV.Luppov IEEE Trans. on Nucl. Sc., 51, 1533 (2004) • Advantages: • 100%electronpolarization • Verysmall error onpolarization • No dead time • No Levchukeffect • HighbeamcurrentsallowedContinuousmeasurement • Expected DPB/PB ≤ 0.5% Suitablefor PV experiments • Disadvantages • Technicalcomplexity of the target R&D needed • BeamImpactdepolarizationeffects

6. Polarized Atomic Hydrogen Target Magnetic field B splits H1 ground state Mixing angle tan 2q ≈ 0.05/B(T) At B = 8T 0.3%

7. Polarized Atomic Hydrogen Target Storage Cell • In a field gradient a force • Pulls into the strong field • Repels out of the strong field H+H H2 • recombination • (releasing ~ 4.5 eV) higher at low T • cell walls coated with ~50 nm superfluid 4He . • Gas density: 3 10-15 cm-3 • 100 % polarization of the electrons

8. Polarized Atomic Hydrogen Target Gas Lifetime in the Cell • Loss of hydrogen atoms from the cell due to: • Thermal escape through the magnetic field gradient dominates at T > 0.55 K • Recombination in the gas volume negligible up to densities of ~1017 cm-3 • Recombination in the cell surface constant feeding the cell with atomic hydrogen E.Chudakov and V.Luppov IEEE Trans. on Nucl. Sc., 51, 1533 (2004)

9. Polarized Atomic Hydrogen Target Contamination and Depolarization of the Target Gas No Beam • Hydrogen molecules • High energy atomic states and • Excited atomic states • Helium and residual gas empty target measurement • with the beam < 10-16 Beam Impact • Depolarization by beam generated RF field • Gas heating by beam ionization losses • Depolarized ions and electrons contamination • Contamination by excited atoms Expected depolarization

10. Dilution Refrigerator and Magnet T = 300 mK of the atomic trap can be reached using a Dilution Refrigerator Dilution refrigerator and magnet shipped from UVA to Mainz

11. Status of the Atomic Hydrogen Target New Dilution Refrigerator needs to be designed and produced!! Test superconducting solenoid

12. Status of the Atomic Hydrogen Target Superconducting Solenoid Test • Pre-cooling with Nitrogen • Cooling down with Helium?? T(K) t(min.)

13. Status of the Atomic Hydrogen Target Preliminary design of the new Dilution Refrigerator • General considerations • Obtaining low temperature (T=300mK) and high cooling power (Q= 15mW) • Optimization by a careful calculation: • - Heat exchangers • - Pressure drop in the pumping lines • - Condensation of the mixture • - Amount of 3He and 4He gas needed • - Volumes of all parts inside the DR (separator, evaporator, still) • and also pumps and lines • Produce new mixing chamber

14. Status of the Atomic Hydrogen Target Preliminary design of the new Dilution Refrigerator • HeatExchangers (HE) • Design of the HE is of majorimportance. Theimportantparameters are: • Small volumetoreachtheequilibriumtemperatureveryfast • Small thermalresistancebetweenthestreamstogetgoodtemperature • equilibriumbetweenthem • Imperfections and impurities can influencethetransport of heat • Thermalboundaryresistancebetweenhelium and the HE material at T<1K • Kapitzaresistance~ T3

15. Summary/Outlook • PV electron scattering experiments at MESA are planned systematic • accuracy of < 0.5% for the beam polaization measurements • Atomic Hydrogen gas, stored in a ultra-cold magnetic trap can provide this • accuracy • A solenoid and a dilution refrigerator were shipped from the University of Virginia • to Mainz • Cooling down of the solenoid will be performed in the next weeks • New dilution refrigerator design and production is needed • Production of a new mixing chamber and a atomic hydrogen dissociator is also • planned • Geant4 simulation of the detector system in progress

16. BACKUP

17. MESA - layout

18. Polarized Atomic Hydrogen Target Dynamic Equilibrium and Proton Polarization As a result, the cell contains predominantly In a dynamic equilibrium, P ~ 80 % in about 10 min.

19. Physics Principles of the Dilution Refrigerator Liquid Helium Pre-cooling System Cooling Power falls exponentially with decreasing temperature Pumping on 4He: ~ 1K Pumping on 3He: ~ 0.3K

20. Physics Principles of the Dilution Refrigerator Dilution Refrigerator Employs the enthalpy of a mixture of liquid 3He-4He to cool down Phase separation into 3He rich and 3He poor phase below T ~ 800 mK

21. Working Principle of the Dilution Refrigerator 1. 4He insertedintotheseparator Heliumisseparated in gas and liquid phases 2. Coolingdownseparatorto T ~ 4 K bypumping 3. Thisoutgoing gas pre-coolstheincoming 3He gas 4. Liquidheliumfromseparatormovesto evaporatorincoming3He isliquified 5. Coolingdownevaporatorto T = 1.5 K bypumpinghelium

22. Working Principle of the Dilution Refrigerator • 3He-4He mixturecooleddownbythermalcontactwiththestill(T ~ 0.7K) • Heatexchangers reduce thetemperature of theliquid3He • Gas entersmixingchamberwherethediluted-concentratedphaseseparation • isproducedcoldestpoint (T ~ 300 mK) • Outgoingcoldliquidfrommixingchamberisemployedto pre-cooltheincomig3He

23. Physics Principles of the Dilution Refrigerator Cooling power: Below 0.3K the dilution refrigerator has much higher cooling power