Considerations for the Optimal Polarization of 3He Targets

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Transcript Considerations for the Optimal Polarization of 3He Targets

Considerations for the Optimal
Polarization of 3He Targets
Brielin C. Brown
University of Virginia
October 10, 2008
SPIN 2008
Background
• The ideal target for probing the fundamental quarkgluon 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 - cell
Polarized 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 hyperpolarized 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 of 2
plates,  plates, and mirrors
4
used to direct the 795 nm

light

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 spinup 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