Polarized 3He spin filters for neutron science

Abstract

The large spin dependence of the absorption cross section for neutrons by 3 He gas provides a method to polarize neutron beams. For certain applications, such polarized 3 He-based neutron "spin filters" have advantages over conventional neutron optical polarizing methods. Spin filters operate at all neutron wavelengths, 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 3 He spin filters at various neutron facilities worldwide. We focus on the development and application of SEOP-based neutron spin filters at the National Institute of Standards and Technology (NIST), Center for Neutron Research (NCNR). The combination of long relaxation time spin filter cells, high power spectrally narrowed diode lasers, and the use of Rb/K mixtures have allowed us to reach 3 He polarizations up to 85 % in spin filter cells ≈1 liter in volume. Studies have revealed limits to the achievable polarization from temperature-dependent relaxation and unexplained magnetic field dependence for relaxation in SEOP cells. 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-3 He scattering length was performed with a small, polarized 3 He cell located inside a neutron interferometer. A precision measurement of the neutron polarization for this experiment was also performed with a 3 He spin filter. Use of spin filters in high flux neutron beams have revealed beam-induced alkali-metal relaxation and long term effects on SEOP spin filter cells.

Systematic T1 improvement for hyperpolarized 129xenon

“…3 He, 129 Xe) up to six orders of magnitude beyond the thermal polarization has opened up several applications, which could hardly be carried out otherwise. These applications stretch from uses as varied as spin-polarized target in accelerator experiments [1,2], neutron filters [3,4] and spinclocks [5] in fundamental physics to contrast agents in clinical MRI [6][7][8] or materials science [9,10]. This enormous increase of the nuclear spin polarization is usually termed ''hyperpolarization'' (HP) and achieved either via Spin Exchange Optical Pumping (SEOP) [11] or Metastability Exchange Optical Pumping (MEOP) [12].…”

Optically polarizedHe3

“…20 shows the variation of the neutron polarization and transmission with the pressure-length-wavelength product for 3 He polarizations of 0.5, 0.75 and 1. The ideal opacity factor depends on the type of experiment and its optimization has been addressed with different approaches (Gentile et al , 2005a; Goossens and Cussen, 2002; Tasset and Ressouche, 1995; Williams, 1999); a typical value is 3 bar-cm-nm.…”

“…Increasing the range of applications will require NSFs with large and/or complex cells and/or greater tolerance to stray magnetic fields. Finally, some NSFs may provide the greatest impetus for achieving the highest possible 3 He polarizations for two reasons: 1) some NSFs are relatively simple, single, cells at pressures near one bar, for which the limiting polarization in a practical application can be close to that possible under ideal conditions and 2) high neutron polarization and/or analyzing power are typically desirable for neutron scattering experiments in which a small component of magnetic scattering is separated from a much large component of nuclear scattering (Gentile et al , 2005a). In contrast with the typical figure of merit proportional to $P_{\mathrm{H}e}^2$ for the running time of an experiment, such situations yield a stronger dependence that is closer to $P_{\mathrm{H}e}^4$.…”