Patricia Aguar Bartolomé, Kurt Aulenbacher, Valery Tyukin Institut für Kernphysik, Universität Mainz

PEB Workshop, MIT

15th March 2013

Physics Motivation Polarized Atomic Hydrogen Targets Hydro-Møller Physics Principle and Components

Status of the Mainz Hydro-Møller Target

Summary

Polarimetry Methods

• 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 mA

Levchuk effect ~ 1%

Low Pt dead time

Systematic errors on target polarization ~ 2%

• 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

Sufficient rates ~ x 0.005 no dead time

Hydrogen gas target

No Levchuk effect

High beam currents allowed Continuous measurement

Expected DPB/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 2q ≈ 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

Dilution refrigerator and magnet

shipped from UVA to Mainz

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

Below 0.3K the dilution refrigerator has much higher cooling power

Cooling power: ~ T2

• Liquid 4He pre-cooling system

• Uses the enthalpy of a mixture of liquid 3He-4He to cool down

Cooling by Dilution

• Upper 3He diffuses into the 4He layer below Temperature fall

~ 100% 3He

~ 6.6% 3He,

mostly 4He

• Phase separation into 3He rich and 3He poor phase below T ~ 800 mK

• 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

New Dilution Refrigerator needs

to be designed and produced!!

Test superconducting solenoid

Test superconducting solenoid

Pre-cooling with Nitrogen

Cooling down with Helium??

T(K

)

t(sec.)

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

- Conduction and viscous heating in the low temperature parts

- 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 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 ~ 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

BACKUP

Dynamic Equilibrium and Proton Polarization

As a result, the cell contains predominantly

In a dynamic equilibrium, P ~ 80 % in about 10 min.

Liquid Helium Pre-cooling System

Cooling Power falls exponentially

with decreasing temperature

Pumping on 4He: ~ 1K

Pumping 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