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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