Bulletin of the American Physical Society
APS April Meeting 2015
Volume 60, Number 4
Saturday–Tuesday, April 11–14, 2015; Baltimore, Maryland
Session X15: Mini-symposium on Optically Pumped Polarized Targets, Sources and Methods I |
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Sponsoring Units: DNP Chair: Gordon Cates, University of Virginia Room: Key 11 |
Tuesday, April 14, 2015 10:45AM - 11:21AM |
X15.00001: Spin-Exchange Optical Pumping of He-3 Invited Speaker: Thad Walker Spin-exchange optical pumping, in which angular momentum is transferred from light to alkali atoms to He-3, is a powerful method for producing large quantities of highly polarized nuclei for electron scattering experiments and neutron spin filters/analyzers. Technical innovations (exquisite aluminosilicate glass cells with $T_1>>100$ hours, $>100 $ Watt frequency-narrowed diode lasers) and improved understanding of the atomic physics processes involved (precision polarimetry, hybrid spin-exchange, circular dichroism), have resulted in orders of magnitude scale-up from the nascent experiments in the late 1980s. Improved understanding of effects such as the X-factor and particulate formation are needed to further advance the conversion of spin-polarized light into seminal physics results. [Preview Abstract] |
Tuesday, April 14, 2015 11:21AM - 11:33AM |
X15.00002: Recent advances in polarized 3He based neutron spin filter development Wangchun Chen, Thomas Gentile, Ross Erwin, Shannon Watson, Kathryn Krycka, Qiang Ye Polarized 3He neutron spin filters (NSFs) are based on the strong spin-dependence of the neutron absorption cross section by 3He. NSFs can polarize large area, widely divergent, and broadband neutron beams effectively and allow for combining a neutron polarizer and a spin flipper into a single polarizing device. The last capability utilizes 3He spin inversion based on the adiabatic fast passage (AFP) nuclear magnetic resonance technique. Polarized 3He NSFs are significantly expanding the polarized neutron measurement capabilities at the NIST Center for Neutron Research (NCNR). Here we present an overview of 3He NSF applications to small-angle neutron scattering, thermal triple axis spectrometry, and wide-angle polarization analysis. We discuss a recent upgrade of our spin-exchange optical pumping (SEOP) systems that utilize chirped volume holographic gratings for spectral narrowing. The new capability allows us to polarize rubidium/potassium hybrid SEOP cells over a liter in volume within a day, with 3He polarizations up to 88{\%}, Finally we discuss how we can achieve nearly lossless 3He polarization inversion with AFP. [Preview Abstract] |
Tuesday, April 14, 2015 11:33AM - 11:45AM |
X15.00003: On the limits of spin-exchange optical pumping of $^{3}$He T.R. Gentile, W.C. Chen, Q. Ye, T.G. Walker, E. Babcock We have obtained improvement in the $^{3}$He polarization achievable by spin-exchange optical pumping (SEOP). These results were primarily obtained in large neutron spin filter cells using diode bar lasers spectrally narrowed with chirped volume holographic gratings. As compared to our past results with lasers narrowed with diffraction gratings, we have observed between 5{\%} and 11{\%} fractional increase in the $^{3}$He polarization $P_{He}$ . We also report a comparable improvement in $P_{He}$ for two small cells, for which we would not have expected an increase from improved laser performance. In particular, prior extensive studies had indicated that the alkali-metal polarization was within 3{\%} of unity in one of these cells. These results have impact on understanding the maximum $P_{He}$ achievable by SEOP, whether the origin of the improvement is from increased alkali-metal polarization or decreased temperature-dependent relaxation. We have observed $P_{He}$ of between 0.80 and 0.85 in several large cells, which marks a new precedent for the polarization achievable by SEOP. Recently we have obtained $P_{He}$ as high as 0.88 with increased laser power. We will discuss these results and tests performed to understand their origin. [Preview Abstract] |
Tuesday, April 14, 2015 11:45AM - 11:57AM |
X15.00004: RF detection with and electron polarization in an optically pumped multi-pass magnetometer Karen Sauer, David Prescott, Nezih Dural, Michael Romalis A magnetometer is constructed using optically pumped $^{87}$Rb in a crossed pump-probe configuration. To increase the signal size while maintaining a small volumetric footprint the off-resonant probe beam is passed back and forth through the cell 50 times within an active volume $<0.3$ cm$^3$. A small magnetic field tunes the magnetometer to radio-frequency (RF) signals on the order of a MHz and a sensitivity of 2 fT/$\sqrt{\mathrm{Hz}}$ is achieved. A pulsed pump beam is used to recover from a saturating RF pulse as might be used in magnetic resonance experiments and results in high atomic polarization, $>90$\%. We measure this polarization through different means and compare their results:(i) The number density, spin-destruction rate, and light narrowing is measured by varying the delay between the pump light pulse and a weak RF pulse used to create free induction decay signals. With these constants polarization is determined. (ii) The response after a 90$^\circ$ pulse exhibits multiple rotations in the Faraday rotation. The number of zero crossings serves as a metric of polarization independent of signal size or linewidth.(iii) The Faraday rotation observed when applying a relatively small DC magnetic field along the probe direction serves as another metric of polarization. [Preview Abstract] |
Tuesday, April 14, 2015 11:57AM - 12:09PM |
X15.00005: Online SEOP polarization of Large Area Neutron Spin Filters Earl Babcock, Zahir Salhi, Alexander Ioffe The Juelich Center for neutron Science has a program to use SEOP polarized 3He Neutron Spin Filters (3He NSF) on many neutron scattering instruments. The main applications are for polarization analysis of the scattered beam. As such, the devices must operate in close proximity to the neutron sample and sample environment which can include complicated cryostats, humidity and pressure cells, and high field magnets. Thus we have developed novel magnetic cavities to house the 3He NSF cells which allow for simultaneous optical pumping on the instruments. Further we continually develop and redevelop the laser sources which must be relatively narrow band and have long term (i.e. months) stability and robust operation. The first fully operational polarizer has been used continuously for over 2, 60-day reactor cycles at the FRM2. This device uses a 12.5 cm I.D. 3He cell, and has a 3He storage lifetime, including the cells lifetime, in excess of 200 hours with the cell about 60 cm from the sample position inside a 1.2 T electromagnet, and has achieved over 75{\%} 3He polarisation when fully optimized. This talk will describe the magnetic cavities, and laser sources as well as provide a description of the completed 3He polarizer devices. [Preview Abstract] |
Tuesday, April 14, 2015 12:09PM - 12:21PM |
X15.00006: Continuously pumped $^{3}$He neutron spin filter using spin exchange optical pumping for polarized neutron scattering in material science Fankang Li, Ke Li, Steven Parnell, Tianhao Wang, H. Yan, Xin Tong, P. Chenjang, Adam Washington, W. Chen, Roger Pynn, Michael Snow Nuclear polarized $^{3}$He can be used to polarise and analyse a neutron beam through a spin selective absorption of one neutron spin state. One exciting use of polarized neutrons is a technique known as Larmor labelling wherein a series of magnetic fields is used to separate the trajectories of two entangled spin states of each neutron. These states may visit different spatial locations within a scattering sample at different times so that interference between them gives information about time-dependent density correlations within the sample. Length and time scales that are not easily accessible by other techniques can be measured. We report on our experience running a $^{3}$He spin filter for three years. The device uses spin-exchange optical pumping of Rb to achieve high (\textgreater 70{\%}) $^{3}$He nuclear polarization. Because of their spatial uniformity, 3He spin filters permit the use of neutron beams with large cross section for Larmor labeling and eliminate artifacts that can be produced by alternative polarization methods based on supermirrors. We present examples to demonstrate these features. [Preview Abstract] |
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