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[PAST EVENT] Physics Colloquium "AT 3PM"
October 25, 2013
3pm - 4pm
Abstract:
The large spin dependence of the absorption cross section for neutrons by 3He gas provides a method to polarize neutron beams. For certain applications, such polarized 3He-based neutron "spin lters" have advantages over conventional neutron optical polarizing methods. Spin lters operate at all neutron wave-lengths, can cover a large angular range and/or a large energy range, and decouple neutron polarization from energy selection. Both spin-exchange optical pumping (SEOP) and metastability-exchange optical pumping (MEOP) are currently being employed to polarize 3He spin lters at various neutron facilities worldwide. I will focus on the development and application of SEOP-based neutron spin lters at the National Institute of Standards and Technology, Center for Neutron Research (NCNR) [1]. The combination of long relaxation time spin lter cells, high power spectrally narrowed diode lasers, and the use of Rb/K mixtures have allowed us to reach 3He polarizations up to 85% in spin lter cells 1 liter in volume. Studies have revealed limits to the achievable polarization from temperature-dependent relaxation [2] and unexplained magnetic eld dependence for relaxation in SEOP cells [3]. Applications include neutron scattering methods such as triple-axis spectrometry and small angle neutron scattering, and fundamental neutron physics. In most neutron scattering applications, cells are transported to the beam line and stored in a magnetically shielded solenoid or box. A recent focus has been apparatus for wide-angle neutron polarization analysis. A measurement of the spin-dependence of the neutron-3He scattering length was performed with a small, polarized 3He cell in a neutron interferometer and a 3He spin lter for accurate neutron polarimetry [4]. Use of spin lters in high ux neutron beams have revealed beam-induced alkali-metal relaxation and long term effects on SEOP spin lter cells [5].
The large spin dependence of the absorption cross section for neutrons by 3He gas provides a method to polarize neutron beams. For certain applications, such polarized 3He-based neutron "spin lters" have advantages over conventional neutron optical polarizing methods. Spin lters operate at all neutron wave-lengths, can cover a large angular range and/or a large energy range, and decouple neutron polarization from energy selection. Both spin-exchange optical pumping (SEOP) and metastability-exchange optical pumping (MEOP) are currently being employed to polarize 3He spin lters at various neutron facilities worldwide. I will focus on the development and application of SEOP-based neutron spin lters at the National Institute of Standards and Technology, Center for Neutron Research (NCNR) [1]. The combination of long relaxation time spin lter cells, high power spectrally narrowed diode lasers, and the use of Rb/K mixtures have allowed us to reach 3He polarizations up to 85% in spin lter cells 1 liter in volume. Studies have revealed limits to the achievable polarization from temperature-dependent relaxation [2] and unexplained magnetic eld dependence for relaxation in SEOP cells [3]. Applications include neutron scattering methods such as triple-axis spectrometry and small angle neutron scattering, and fundamental neutron physics. In most neutron scattering applications, cells are transported to the beam line and stored in a magnetically shielded solenoid or box. A recent focus has been apparatus for wide-angle neutron polarization analysis. A measurement of the spin-dependence of the neutron-3He scattering length was performed with a small, polarized 3He cell in a neutron interferometer and a 3He spin lter for accurate neutron polarimetry [4]. Use of spin lters in high ux neutron beams have revealed beam-induced alkali-metal relaxation and long term effects on SEOP spin lter cells [5].