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Patricia Aguar Bartolomé Institut für Kernphysik, Universität Mainz PSTP 2013 Workshop, Charlottesville 11th September 2013.

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Presentation on theme: "Patricia Aguar Bartolomé Institut für Kernphysik, Universität Mainz PSTP 2013 Workshop, Charlottesville 11th September 2013."— Presentation transcript:

1 Patricia Aguar Bartolomé Institut für Kernphysik, Universität Mainz PSTP 2013 Workshop, Charlottesville 11th September 2013

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

3 Hydro-Moller PV Detector

4 Compton Scattering: Accurate enough at energies > 4GeV, but accuracy around 1% at low energies Not enough for PV-experiments Møller Scattering with ferromagnetic target Advantages: Beam energy independent High analyzing power (~ 80%) 2 particles with high energies in the final state detectable in coincidence eliminates background Disadvantages:  Low electron polarization ~ 8 %  Target heating Beam current limited to 2-3  A  Levchuk effect ~ 1%  Low P t dead time  Systematic errors on target polarization ~ 2% Polarimetry Methods

5 Møller Scattering with polarized atomic hydrogen gas, stored in a ultra-cold magnetic trap E.Chudakov and V.Luppov IEEE Trans. on Nucl. Sc., 51, 1533 (2004) Advantages: 100% electron polarization Very small error on polarization No dead time No Levchuk effect High beam currents allowed Continuous measurement Expected  P B /P B ≤ 0.5% Suitable for PV experiments Disadvantages  Technical complexity of the target R&D needed  Beam Impact depolarization effects

6 Magnetic field B splits H 1 ground state At B = 8T 0.3% Mixing angle tan 2  ≈ 0.05/B(T)

7 In a field gradient a force  Pulls into the strong field  Repels out of the strong field recombination (releasing ~ 4.5 eV) higher at low T cell walls coated with ~50 nm superfluid 4 He Gas density: cm % polarization of the electrons Storage Cell H+H H 2.

8 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 ~10 17 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 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 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 shipped from UVA to Mainz T = 300 mK of the atomic trap can be reached using a Dilution Refrigerator

11 New Dilution Refrigerator needs to be designed and produced!! Test superconducting solenoid

12 Superconducting Solenoid Test  Pre-cooling with Nitrogen  Cooling down with Helium?? T(K) t(min.)

13 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 3 He and 4 He gas needed - Volumes of all parts inside the DR (separator, evaporator, still) and also pumps and lines Produce new mixing chamber

14 Preliminary design of the new Dilution Refrigerator Heat Exchangers (HE)  Design of the HE is of major importance. The important parameters are: 1.Small volume to reach the equilibrium temperature very fast 2.Small thermal resistance between the streams to get good temperature equilibrium between them  Imperfections and impurities can influence the transport of heat  Thermal boundary resistance between helium and the HE material at T<1K Kapitza resistance ~ T 3

15 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

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

19 Liquid Helium Pre-cooling System Cooling Power falls exponentially with decreasing temperature Pumping on 4 He: ~ 1K Pumping on 3 He: ~ 0.3K

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

21 1. 4 He inserted into the separator Helium is separated in gas and liquid phases 2. Cooling down separator to T ~ 4 K by pumping 3. This outgoing gas pre-cools the incoming 3 He gas 4. Liquid helium from separator moves to evaporator incoming 3 He is liquified 5. Cooling down evaporator to T = 1.5 K by pumping helium

22 3 He- 4 He mixture cooled down by thermal contact with the still (T ~ 0.7K) Heat exchangers reduce the temperature of the liquid 3 He Gas enters mixing chamber where the diluted-concentrated phase separation is produced coldest point (T ~ 300 mK) Outgoing cold liquid from mixing chamber is employed to pre-cool the incomig 3 He

23 Below 0.3K the dilution refrigerator has much higher cooling power Cooling power:


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