Bulletin of the American Physical Society
40th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 54, Number 7
Tuesday–Saturday, May 19–23, 2009; Charlottesville, Virginia
Session B4: Polarized Atoms |
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Chair: Tom Gentile, National Institute of Standards and Technology Room: Clark Hall 108 |
Wednesday, May 20, 2009 10:30AM - 10:42AM |
B4.00001: Preparation of Laser-Polarized Xenon at High Xe Densities and High Resonant Laser Powers Provided by Volume Holographic Grating-Narrowed LDAs Boyd Goodson, Nicholas Whiting, Panayiotis Nikolaou, Neil Eschmann, Michael Barlow The xenon nuclear spin polarization ($P_{Xe})$ achieved via alkali metal spin-exchange optical pumping (SE OP) is normally expected to be limited at high xenon cell densities because of decreased rubidium electron spin polarization ($P_{Rb})$ resulting from increased Rb/Xe collisions. Surprisingly high $P_{Xe}$ values (e.g., $\sim $55{\%}, $\sim $32{\%}, $\sim $23{\%}, and $\sim $11{\%} at 50, 300, 500, and 2000 torr Xe) were obtained with batch-mode OP and a $\sim $29 W VHG-narrowed laser by exploiting a sensitive and unexpected interdependence between the optimal cell temperature and the Xe partial pressure. The OP dynamics can be further investigated as a function of wavelength offset and optical power using frequency-narrowed lasers (with on-chip Bragg gratings or TEC-controlled VHGs) able to tune over the entire range of the Rb D$_{1}$ absorption profile independently of laser flux. These effects are studied using \textit{in situ} time-dependent nuclear polarimetry and optical $P_{Rb}$ measurements based on magnetic field-dependent transmission of the polarizing laser beam. [Preview Abstract] |
Wednesday, May 20, 2009 10:42AM - 10:54AM |
B4.00002: Characterization of a Low Pressure, High Capacity $^{129}$Xe Flow-Through Polarizer Brian Saam, Geoffry Schrank, Zayd Ma, Allison Schoeck Hyperpolarized $^{129}$Xe produced {\it via} spin-exchange optical pumping continues to be an interesting physical system to study and is useful in many NMR and MRI applications. The generation of large quantities of highly polarized $^{129}$Xe is complicated by xenon's large cross section for spin destruction of the alkali-metal electron. This problem has been addressed in recent years by the development of flow-through xenon polarizers, which operate with a gas mixture that is lean in xenon flowing continuously through the optical pumping cell. We describe here our own flow-through xenon polarizer that is based on the University of New Hampshire design: it operates at low pressure, employs counterpropagating laser beam and gas flow, and has a long narrow optical pumping region. In our version, the systems for heating and Rb vapor generation have been simplified. We examine both the output $^{129}$Xe polarization by NMR and the {\it in situ} $^{85}$Rb polarization by optically detected EPR as a function of position in the meter-long cell. Under near-optimal conditions with 28 W of frequency-narrowed laser light, we achieve $^{129}$Xe polarizations $>30$\% with a flow of 5 bar$\cdot$cm$^3$/min of natural xenon. We compare our results with a numerical model. [Preview Abstract] |
Wednesday, May 20, 2009 10:54AM - 11:06AM |
B4.00003: Polarized $^{3}$He Gas for Neutron Scattering Wangchun Chen, Ross Erwin, Shannon Watson, Thomas Gentile, Changbo Fu Nuclear spin polarized $^{3}$He gas, produced by either metastability-exchange (MEOP) or spin-exchange optical pumping (SEOP), can be used to polarize or analyze neutron beams because of the strong spin dependence of the absorption cross section for neutrons by $^{3}$He. Such neutron spin filters (NSFs) have applications in both neutron scattering and fundamental neutron physics. The advent of practical NSFs with $^{3}$He polarization values exceeding 70{\%} has led to substantial increase in these devices in the neutron scattering community worldwide. $^{3}$He NSFs are advantageous over conventionally used neutron polarizers in that they can polarize a broad wavelength band of neutrons and polarize large area and widely divergent neutron beams. $^{3}$He cells are often polarized off-line and transported to neutron instruments, hence long lifetime cells are critical. In addition NSFs are required to be compact and relatively insensitive to $^{3}$He relaxation induced by external magnetic fields. In the talk I will present the current status of the polarized $^{3}$He user program for neutron instruments at the NIST Center for Neutron Research. Topics include the production rate of highly polarized $^{3}$He by SEOP, cell development, magnetostatic cavity development, and interfaces between $^{3}$He spin filter devices and a variety of instruments. [Preview Abstract] |
Wednesday, May 20, 2009 11:06AM - 11:18AM |
B4.00004: Application of Polarized 3He Cells in Neutron Interferometry Michael Huber, Fred Wietfeldt, Muhammad Arif, Thomas Gentile, WangChun Chen, Dmitry Pushin, Liang Yang, Daniel Hussey, Timothy Black We report a novel use of polarized $^3$He for studies of nucleon-nucleon (NN) interactions via neutron interferometry. Phenomenological NN models fail to accurately predict properties such as binding energies and scattering lengths of nuclear systems larger than two nucleons. Many different three nucleon interactions (3NI) exist that can be included with NN models to correct for some (but not all) of discrepancies between theory and experiment. We will present the results of a recent precision measurement done at the National Institute of Standards and Technology of the n-$^3$He incoherent scattering length, which plays an important role in the testing of 3NI models. In this experiment a small polarized $^3$He cell, polarized off-line by spin exchange optical pumping, was located within a neutron interferometer. The phase difference for the neutron polarization parallel and anti-parallel to the $^3$He polarization was measured. In an auxiliary measurement, polarized $^3$He cells were also employed to measure the neutron polarization and spin flipper efficiency to $<0.08$\,\% relative uncertainty. The incoherent scattering length was determined with a relative uncertainty of $<1$\,\%. [Preview Abstract] |
Wednesday, May 20, 2009 11:18AM - 11:30AM |
B4.00005: Using MR imaging methods for measurement of 3-He cells during pumping Steven Parnell, Martin Deppe, Stephen Boag, Mark Boyce, Selma Ajraoui, Juan Parra-Robles, Jim Wild We present details of a system for 1-D MR imaging and diffusion measurements of 3He cells during optical pumping. We have implemented at 10G a 1-D gradient system and we use a gradient echo pulse for imaging and bi-polar gradients to allow measurements of the diffusion constant. We perform 1-D imaging for monitoring the spatial distribution of the gas polarisation in the cell (important in 129Xe). For diffusion we measure a number of cells. In free diffusion, as is the case in large cells the diffusion constant can be used as a measure of cell pressure and temperature. The system is used to measure the pressure in sealed cells and compared with neutron transmission measurements. Finally we investigate the effects of heating from the laser in the cells. [Preview Abstract] |
Wednesday, May 20, 2009 11:30AM - 11:42AM |
B4.00006: Online SEOP Polarized 3He for Neutron Applications Earl Babcock, S. Boag, T.E. Chupp, T.R. Gentile, A. Petoukhov, T.G. Walker $^{3}$He spin filters provide unique capabilities in various areas of neutron science. Since polarized $^{3}$He experiences nuclear relaxation there is motivation to polarize the 3He in place with SEOP to obtain constant polarization levels and increased time average performance. Aspects of the SEOP process under the most extreme conditions in neutron science, with respect to incident neutron flux, present new challenges and processes to explore. Recently work to explore the effects on the SEOP process of the highest intensity neutron beams available have been done. In that work alkali-metal spin-relaxation processes over an order of magnitude faster than typical alkali-metal spin-relaxation rates were discovered. These rates were higher than one would have expected from prior work with charged particle beams. These high rates, along with a time constant of several minutes for a substantial portion of the alkali-metal relaxation, has lead us to consider mechanisms such as molecule and cluster formation in the cells. An overview and status of that work will be presented. [Preview Abstract] |
Wednesday, May 20, 2009 11:42AM - 11:54AM |
B4.00007: $^3$He Relaxation Time Measurements at $\sim$400mK for the neutron electric dipole moment (nEDM) experiment Qiang Ye In the new neutron electric dipole moment (nEDM) experiment planned to be carried out at the SNS, the neutron storage cell will be made of dTPB-dPS (wavelength shifting material) coated acrylic and filled with superfluid $^4$He at $\sim$400mK. The experiment will use the nuclear magnetic resonance technique to measure the neutron precession frequency by comparing with that of the polarized $^3$He using the spin-dependent nuclear process: $\vec{n}+\vec{^{3}He}\rightarrow p+t+764$ keV. The polarized $^3$He will also be used as a co-magnetometer to monitor \emph{in situ} the magnetic field during the experiment. Understanding the relaxation mechanism of polarized $^3$He in the storage cell under the experimental conditions and maintaining the $^3$He polarization is crucial. Following our earlier study[1] of the $^3$He relaxation time in a dTPB-dPS coated cylindrical acrylic cell at a temperature of 1.9K in the presence of superfluid $^4$He with a magnetic holding field of 21 G, more measurements at $\sim$400mK have been carried out in cylindrical and rectangular acrylic cells using a dilution refrigerator at TUNL with the magnetic holding field of $\sim$7 G. Preliminary results will be presented. [1] Q. Ye \textit{et al.} Physical Review A, 77:053408, 2008 [Preview Abstract] |
Wednesday, May 20, 2009 11:54AM - 12:06PM |
B4.00008: Spatial Polarization Profile in an Optically Pumped Alkali Vapor Ben Olsen, Brian Patton, Yuan-Yu Jau, Will Happer Spin-Exchange Optical Pumping (SEOP) is a technique used to polarize nuclei in gases, and more recently in solids, in excess of their equilibrium limit. SEOP is achieved by optically pumping an alkali vapor which subsequently transfers angular momentum to the nuclei of interest. The efficiency of SEOP is governed by optical pumping and relaxation of the alkali atoms, relaxation of the target nuclei, and interactions between the alkali and target substance. In this work we investigate the relationship between optical pumping and relaxation in cesium vapor with absorption spectroscopy at high magnetic field (2.7~T). Cesium vapor within a cylindrical glass vapor cell is optically pumped with a strong laser resonant with a $D2$ transition. The ground-state population of the vapor is measured at various positions along a diameter of the cell with a small, weak $D1$ laser beam which translates mechanically. The resulting polarization profile elucidates the interplay between optical pumping, diffusion in the buffer gas, and relaxation at the walls of the vapor cell. We report measurements of the spatial polarization profile in vapor cells with bare Pyrex walls and cells coated with paraffin (an anti-relaxation coating) or CsH salt (a target substance for SEOP), and compare them to numerical simulations. Further investigation might yield a new method for characterizing surface relaxation in vapor cells. [Preview Abstract] |
Wednesday, May 20, 2009 12:06PM - 12:18PM |
B4.00009: Photon Budget in Spin Exchange Optical Pumping Brian Lancor, Robert Wyllie, Thad Walker Standard models of spin-exchange optical pumping that include all known collisional spin relaxation and assume excited-state nuclear spin conservation consistently under-represent the amount of optical pumping light required to produce large quantities of polarized gas. The extremely large optical depths ($\sim$100) used in these experiments require high transparency for fully polarized alkali atoms. Should polarized atoms continue to absorb light, even at a small rate, the effects on photon usage are dramatic. We are currently investigating the frequency dependence of the circular dichroism of the Rb D1 resonance. This effect may be of particular importance for broad linewidth laser sources. In addition, we have quantitatively modeled the evolution of the Rb nuclear spin during the optical pumping process, and find that the standard assumption of nuclear spin conservation in the excited state is violated and has a significant effect on photon usage. [Preview Abstract] |
Wednesday, May 20, 2009 12:18PM - 12:30PM |
B4.00010: Direct measurement of the dwell time of spin polarized Rb atoms on coated Pyrex glass surfaces using light shift K.F. Zhao, M. Schaden, Z. Wu We present a simple method for directly measuring the average dwell time of spin polarized Rb atoms on coated Pyrex glass surfaces. The method relies on the light shift of the Zeeman resonances of spin polarized Rb atoms pumped and probed by evanescent waves, and does not depend on the microscopic details of surface interactions. We use a cell whose length is adjustable between 70 and $500\,\mu m$. The inverse of the difference in peak frequencies for $\sigma^+$ and $\sigma^-$ pump beams depends linearly on the cell length, from which we obtain the average dwell time $\tau_s$. For a Pyrex glass cell coated with octadecyltrichlorosilane (OTS) we find that $\tau_s=0.53\pm 0.03\,\mu s$ at a cell wall temperature of 103 $^\circ$C. The temperature dependence of $\tau_s$ is fitted to $\tau_s=\bar \tau_0 \exp(E_a/kT)$, yielding a pre-exponential factor $\bar\tau_0=(3.1\pm 0.1)\times 10^{-9}\,\rm s$ and an activation energy $E_a=0.18\pm 0.3\,\rm eV$. [Preview Abstract] |
Wednesday, May 20, 2009 12:30PM - 12:42PM |
B4.00011: Universal Long-Time Behavior of Nuclear Spin Decays in Solid Hyperpolarized Xenon Brian Saam, Steven W. Morgan, Boris V. Fine We have observed a universal long-time behavior of $^{129}$Xe FIDs and solid echoes in polycrystalline hyperpolarized xenon at 77~K. In all cases, a decay of the form $F(t) = Ae^{-\gamma t}\cos(\omega t + \phi)$ sets in after just a few times $T_2$; the behavior is universal in the sense that the decay constant $\gamma$ and the beat frequency $\omega$, which together characterize the long-time decay are the same for the FID and for solid echoes having different interpulse delay times $\tau$. These findings reveal a fundamental property of nuclear spin dynamics and are thus relevant to theoretical efforts that have been ongoing for decades to understand NMR lineshapes in solids. Moreover, the functional form and universality of this behavior were previously predicted on the basis of analogy with resonances in classical chaotic systems [2]. While we expect this behavior to be characteristic of nuclear-spin solids in general, $^{129}$Xe is an ideal system to examine it with high precision because of the relatively long $T_2\approx 1$~ms and because spin-exchange optical pumping can be used to achieve greatly enhanced magnetization, allowing precise examination of the decay over 3-4 orders of magnitude. [1] S.W. Morgan, {\it et al.}, PRL {\bf 101}, 067601 (2008). [2] B.V. Fine, PRL {\bf 94}, 247601 (2005). [Preview Abstract] |
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