Nuclear Polarization inHe3Gas Induced by Optical Pumping and Dipolar Exchange

Abstract

OCTOBER 15, I960 R(H ||c) is small because electrons with k ± <fi rotate in a sense opposite to that for electrons with k jL >p and because co c tends to zero as k ± approaches j3.For CdS Thomas et al. 5 have estimated an upper bound of the order of a few hundredths of an ev for the displacement of any energy minima below E(0) in the conduction band. In the toroidal model this energy displacement equals A/3 2 . Setting A/3 2 = 0.04 ev, one obtains A« 6 at T = 77°K. Thus, one might hope to observe a large anisotropy in the Hall effect, if the model is correct.It should be emphasized that the model results from an approximation. Extension of the 5»p perturbation calculation to higher orders will lower the symmetry of the surfaces to one of sixfold rather than continuous rotational invariance. Whether or not the corrections are important will depend upon the size of /3 and the proximity of other bands. For A/3 2 = 0.04 ev and A = (2m)" 1 , 0 is typically of the order of one-tenth the shortest distance in k space between the point 5=0 and any of the reduced zone boundaries parallel to the k z axis.Added note. The toroidal model is a direct consequence of the spin-orbit interaction. The nature of £(S) in the limit of zero spin-orbit coupling is described in the earlier work of Rashba, 11 Although almost complete polarization of alkali metal nuclei can be produced by optical pumping utilizing a buffer gas and a filter to remove the D 2 resonance light, 1 * 2 the number of atoms polarized is necessarily very small and is limited by optical opacity of the sample vapor. The usual range of pressure is 10" 7 to 10~3 mm for successful detection and production of optical polarization. The Overhauser nuclear polarization effect 3 involving dipolar interactions 4 " 8 between such an optically polarized atom and the nucleus of a suitable buffer gas should provide a mechanism for the transfer of polarization to a gas at considerably higher pressure than that of the alkali vapor. In other words, the Overhauser effect should work not by the dipole-coupled re-who has recently given independent consideration to the effects of including the interaction. 12 1 R. laxation of a saturated paramagnetic impurity toward a polarized equilibrium, but by the relaxation of an optically polarized impurity toward a nearly depolarized equilibrium. We have observed this effect in He 3 gas used as the buffer gas for the optical pumping of natural rubidium vapor.The nuclear magnetic relaxation time in a liquid or a monatomic gas depends inversely on both the square of the magnetic moment of the nucleus and the square of the moment of other spins causing the relaxation. If He 3 is contaminated with "dissolved" Rb atoms having a moment 10 3 times as great as the He 3 , a concentration of more than 1 part in 10 6 would dominate the relaxation. The dipole interaction terms in-

Functional lung imaging using hyperpolarized gas MRI

“…The valence electron subsequently exchanges polarization with the gas nucleus. This process is known as spin-exchange optical pumping (32)(33)(34), and typically takes 14 -20 hours to provide ϳ1.3 liter of 40% polarized He-3 at atmospheric pressure. Another polarization process for He-3 nuclei is known as metastability exchange (35).…”

Large Production System for Hyperpolarized 129Xe for Human Lung Imaging Studies

“…Similarly, the low body tissue density and susceptibility mismatched interfaces inside the lungs present a challenge to proton MRI of the lungs. Hyperpolarized gases are polarized by alternative methods, such as spin-exchange optical pumping (SEOP) [1] or metastability exchange optical pumping (MEOP) [2], to extremely high values to compensate for the density difference, and they offer great potential as imaging agents. They can be introduced into the body non-invasively by breathing and imaged with no background.…”

“…In its simplified form the equation for Xe polarization during the SEOP process can be written as [21]: (1) where P A is the alkali vapor polarization, Γ is the noble gas nuclear spin-destruction rate, and γ SE is the spin-exchange rate between the alkali atoms and gas nuclei. Similar to the alkali, the noble gas nuclei can lose their polarization interacting with the container walls or with the other atoms in the composition.…”

Temperature‐Controlled Molecular Depolarization Gates in Nuclear Magnetic Resonance

“…Amplifying the available magnetization has been the focus of many studies, leading to various hyperpolarization (hp) techniques such as hp noble gases (1,2), para-hydrogen induced polarization transfer (3,4), or dynamic nuclear polarization (5). Exploiting chemical exchange of nuclei of a hyperpolarized reservoir in combination with a mechanism of gated transfer onto the molecule of interest would provide optimized, controlled 'utilization' of the hyperpolarization (Fig 1a), hence avoiding polarization losses during transfer reactions.…”