Patricia Aguar BartoloméInstitut für Kernphysik, Universität Mainz

PSTP 2013 Workshop, Charlottesville11th September 2013

Physics Motivation

Polarized Atomic Hydrogen Targets

Status of the Mainz Hydro-Møller Target

Summary

Hydro-Moller

PV Detector

• 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 independentHigh 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 ALevchuk effect ~ 1%Low Pt dead timeSystematic errors on target polarization ~ 2%

Polarimetry Methods

• 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 polarizationVery small error on polarizationNo dead timeNo Levchuk effectHigh beam currents allowed Continuous measurement ExpectedPB/PB ≤ 0.5% Suitable for PV experiments

Disadvantages Technical complexity of the target R&D needed Beam Impact depolarization effects

Magnetic field B splits H1 ground state

At B = 8T 0.3%

Mixing angle tan 2≈ 0.05/B(T)

• 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 4He

• Gas density: 3 10-15 cm-3

• 100 % polarization of the electrons

Storage Cell

H+H H2

.

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)

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

< 10-16

Dilution refrigerator and magnet shipped from UVA to Mainz

T = 300 mK of the atomic trap can be reached using a Dilution Refrigerator

New Dilution Refrigerator needs to be designed and produced!!

Test superconducting solenoid

Superconducting Solenoid Test

Pre-cooling with Nitrogen

Cooling down with Helium??

T(K

)

t(min.)

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

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 fast2.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 ~ T3

• 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

BACKUP

Dynamic Equilibrium and Proton Polarization

As a result, the cell contains predominantlyIn a dynamic equilibrium, P ~ 80 % in about 10 min.

Liquid Helium Pre-cooling System

Cooling Power falls exponentially with decreasing temperature

Pumping on 4He: ~ 1KPumping on 3He: ~ 0.3K

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

1. 4He 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 3He gas

4. Liquid helium from separator moves to evaporator incoming 3He is liquified

5. Cooling down evaporator to T = 1.5 K by pumping helium

• 3He-4He mixture cooled down by thermal contact with the still (T ~ 0.7K)

• Heat exchangers reduce the temperature of the liquid 3He

• 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 3He

Below 0.3K the dilution refrigerator has much higher cooling power

Cooling power: