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This article reviews the physics and technology of producing large quantities of highly spin-polarized, or hyperpolarized, $^3$He nuclei using spin-exchange (SEOP) and metastability-exchange (MEOP) optical pumping, and surveys applications of polarized $^3$He. Several recent developments are emphasized for each method. For SEOP, the use of spectrally narrowed lasers and Rb/K mixtures has substantially increased the achievable polarization and polarizing rate. MEOP in high magnetic fields has likewise significantly increased the pressure at which this method can be performed, and has led to the observation of a light-induced relaxation mechanism. In both methods the increased capabilities have led to more extensive study and modeling of the basic underlying physics. New unexplained dependences of relaxation on temperature and magnetic field have been discovered in SEOP cells. Applications of both methods are also reviewed, including targets for charged particle and photon beams, neutron spin filters, magnetic resonance imaging, and precision measurements.
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The Zeeman splittings and EPR frequencies of alkali-metal atoms are shifted in the presence of a polarized noble gas. For a spherical geometry, the shift is enhanced over what is expected classically by a dimensionless atomic parameter $kappa_0$ that is unique to each alkali-metal atom - noble-gas pair. We present a precise measurement of $kappa_0$ for the $^{39}$K-$^3$He system with a relative accuracy of better than 1%. A critical component of achieving sub-percent accuracy involved characterizing the shape of our samples using both MRI and CT medical-imaging techniques. The parameter $kappa_0$ plays an important role in establishing the absolute polarization of $^3$He in a variety of contexts, including polarized targets for electron scattering experiments and MRI of the gas space of the lungs. Our measurement more than doubles the accuracy possible when using $kappa_0$ for polarimetry purposes. Just as important, the work presented here represents the first {it direct} measurement of $kappa_0$ for the $^{39}$K-$^3$He system; previous values for $kappa_0$ in the $^{39}$K-$^3$He system relied on a chain of measurements that were benchmarked by previous measurements of $kappa_0$ in the Rb-$^3$He system.
We propose a new method to detect short-range textit{P-} and textit{T-} violating interactions between nucleons, based on measuring the precession frequency shift of polarized $^3$He nuclei in the presence of an unpolarized mass. To maximize the sensitivity, a high-pressure $^3$He cell with thin glass windows (250 $rmmu m$) is used to minimize the distance between the mass and $^3$He. The magnetic field fluctuation is suppressed by using the $^3$He gas in a different region of the cell as a magnetometer. Systematic uncertainties from the magnetic properties of the mass are suppressed by flipping both the magnetic field and spin directions. Without any magnetic shielding, our result has already reached the sensitivity of the current best limit. With improvement in uniformity and stability of the field, we can further improve the sensitivity by two orders of magnitude over the force range from $10^{-4}-10^{-2}$ m.
A standard method to detect thermal neutrons is the nuclear interaction $^3$He(n,p)$^3$H. The spin-dependence of this interaction is also the basis of a neutron spin-polarization filter using nuclear polarized $^3$He. We consider the corresponding interaction for neutrons placed in an intrinsic orbital angular momentum (OAM) state. We derive the relative polarization-dependent absorption cross-sections for neutrons in an $L=1$ OAM state. The absorption of those neutrons results in compound states $J^pi=0^-$, $1^-$, and $2^-$. Varying the three available polarizations tests that an OAM neutron has been absorbed and probes which decay states are physically possible. We describe the energetically likely excited states of $^4$He after absorption, due to the fact that the compound state has odd parity. This provides a definitive method for detecting neutron OAM states and suggests that intrinsic OAM states offer the possibility to observe new physics, including anomalous cross-sections and new channels of radioactive decay.
We present the development of high-performance polarized $^3mathrm{He}$ targets for use in electron scattering experiments that utilize the technique of alkali-hybrid spin-exchange optical pumping. We include data obtained during the characterization of 24 separate target cells, each of which was constructed while preparing for one of four experiments at Jefferson Laboratory in Newport News, Virginia. The results presented here document dramatic improvement in the performance of polarized $^3mathrm{He}$ targets, as well as the target properties and operating parameters that made those improvements possible. Included in our measurements were determinations of the so-called $X$-factors that quantify a temperature-dependent and as-yet poorly understood spin-relaxation mechanism that limits the maximum achievable $^3mathrm{He}$ polarization to well under 100%. The presence of this spin-relaxation mechanism was clearly evident in our data. We also present results from a simulation of the alkali-hydrid spin-exchange optical pumping process that was developed to provide guidance in the design of these targets. Good agreement with actual performance was obtained by including details such as off-resonant optical pumping. Now benchmarked against experimental data, the simulation is useful for the design of future targets. Included in our results is a measurement of the $mathrm{K}$-$^3mathrm{He}$ spin-exchange rate coefficient $k^mathrm{K}_mathrm{se} = left ( 7.46 pm 0.62 right )!times!10^{-20} mathrm{cm^3/s}$ over the temperature range 503 K to 563 K.
We present the conceptual design of a polarized $^3$He target to be used for high luminosity scattering experiments within high magnetic field environments. This two-cell target will take advantage of advancements in optical pumping techniques at high magnetic field to create 60% longitudinally polarized $^3$He gas in a pumping cell within a uniform magnetic field above 1 T. By transferring the polarized gas to cryogenic target cell, the gas density is increased to create a target thickness suitable for high luminosity applications. We discuss the general design of this scheme, and plans for its application in Jefferson Labs CLAS12 detector.
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