Considerations for the Optimal Polarization of 3He Targets

Brielin C. BrownUniversity of Virginia

October 10, 2008SPIN 2008

Background• The ideal target for probing the fundamental quark-

gluon structure of the nucleon is a free neutron target, but free neutrons are unstable and decay with a lifetime of under 15 minutes (885.7 ± 0.8s)

• The ground state wave function of a polarized 3He nucleus is predominantly an S state in which the spins of the protons are paired anti-parallel to each other

• In this state, the spins of the protons cancel out so that the nucleus is left mainly with the spin of the unpaired neutron

Background - cellPolarized 3He can be used as an effective neutron target

Background – Why higher polarization?

• Higher polarization allows more statistics to be collected in a shorter period of time.

Background – Why faster spin-up times?

• Adiabatic Fast Passage (AFP) spin flip losses – lose .3-.5% polarization per spin flip

• Need short enough spin-up times to recoup losses between spin flips

Relevance

• A 3He target will be used in 6 experiments run over the course of 2008-2009

• Higher statistics combined with record setting luminosity reduce error in experiments

• For Example: Transversity

Motivation

• Hybrid K-Rb cells provide faster spin-up times and polarization than Rb cells

• Faster spin-up times and higher polarizations are still desirable

• Governing equations show a strong temperature dependence in spin-up time and polarization percentage

• Higher temperatures increase frequency of atom-atom interactions

Motivation

• It has been shown that the gas-induced relaxation related to the interactions between the gasses ~ T4.25

• Implies significantly lower levels of sustainable polarization at higher temperatures.

Experiment Overview

• Use Spin-Exchange Optical Pumping to polarize a hybrid K-Rb cell at various temperatures

• At the highest temperature, use SEOP to polarize the cell at various laser powers

• Monitor the effects of temperature and laser power on spin-up time and maximum polarization using a combination of NMR and EPR measurements

Spin-Exchange Optical Pumping

• Circularly polarized light is used to polarize Rb in the pumping chamber of the cell

• The net result is that all electrons accumulate in the F = 3, mf = 3 sublevel; there is hyper-polarized Rb gas in the chamber.

Hybrid Spin-Exchange

• Spin exchange interaction between the Rb and the K cause the polarization of K. Similar interactions between K and 3He result in polarized 3He.

Experimental Setup-Pumping chamber held in an oven designed to hold temperatures of up to 300o C-Circularly polarized light from an optical system with 5 30 watt lasers optically pumps the cell

Optical Setup-Details of the orientation ofplates, plates, and mirrors used to direct the 795 nm light €

λ2

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λ4

Experiment Execution

• The cell “Rockport” was polarized at temperatures ranging from 190oC to 240oC in 10oC increments

• At 240oC the cell “Boris” was polarized at the laser powers: 90, 100, 125, and 150 watts

• During the spin-up procedure, a LabView program takes NMR measurements automatically every 3 hours for a total of 7 measurements at each temperature and laser power

• The program sweeps the holding field every 3 hours between 25 G and 32 G and records the signal induced in a pair of pickup coils

Experiment – cont’d

• Afterwards another program fits the peak signal heights to a logarithmic curve

• Having this fit then gives the expected maximum NMR and spin-up time of the cell

• An example fit:

NMR Normalization

• NMR measurements are used to obtain the maximum polarization and spin-up times during spin-up

• EPR is used for the normalization of NMR data• FOR each temperature and laser power configuration a

series of measurements to normalize NMR are taken:– NMR, then EPR, then NMR– quick succession (< 3 minutes apart) in order to minimize

depolarization• This data is used to normalize the NMR, and

extrapolated to the maximum NMR signal in the spin-up test via

EPR

Results

Conclusions• The increases in polarization and decreases in spin-up time

provided by operating at higher temperatures and laser powers can be extremely beneficial for the use of polarized 3He targets.

• Increasing the temperature or laser power too much has an adverse effect on polarization yet continues to lower spin-up time.

• Shorter spin-up time is advantageous because it allows for polarization to be quickly restored after AFP losses

• This increases the effective polarization, and allowing more frequent spin-flips

• At the highest temperature and laster power (240 ◦ C and 150 watts) polarization decreased substantially such that the decrease in spin-up time would not be advantageous

• The increased polarization itself allows for higher statistics during a shorter run-time in the experiment

Conclusions• A major drawback of operating at the higher temperature and laser power

is the decrease in cell lifetime• At higher temperatures, cell’s are known to lose polarization faster, and

the longevity of the cell is compromised• This would lead to more cell changes, which could offset the time gained

by faster spin-up times, and increase the costs of the experiment by requiring more 3He cells.

• The temperature data turned out somewhat different than anticipated.• A gradual climb in polarization, peaking at a certain high temperature and

power and then dropping off at about the same rate with spin-up times decreasing throughout the test band was expected

• This did occur in the laser power results, but not in the temperature test results

• While the results presented here are significant, the study should be conducted with more cells to eliminate possible instrumentation errors