High nuclear polarization of ³ He at low and high pressure by metastability exchange optical pumping at 1.5 tesla

Abstract

Metastability exchange optical pumping of helium-3 is performed in a strong magnetic field of 1.5 T. The achieved nuclear polarizations, between 80% at 1.33 mbar and 25% at 67 mbar, show a substantial improvement at high pressures with respect to standard low-field optical pumping. The specific mechanisms of metastability exchange optical pumping at high field are investigated, advantages and intrinsic limitations are discussed. From a practical point of view, these results open the way to alternative technological solutions for polarized helium-3 applications and in particular for magnetic resonance imaging of human lungs.

Fundamentals of metastability exchange optical pumping in helium

“…They are additionally sensitive to laser light detuning from the centre of the composite pump line. For instance, at M =0 and low pump intensity the PEs are computed, and have been checked [31,35], to be of order 1 for the f ± 2 lines and 0.5 for the f ± 4 lines. The PEs can be significantly increased by detuning the pump laser away from the less efficient component(s).…”

“…For B=0, the coefficients are L ± =±1/3. In high fields they weakly differ from 1, which reduces the flow rate of angular momentum: a large number of ME collisions is required to transfer a unit of angular momentum between ground state and 2 3 S state atoms [24,35].…”

“…imaging (MRI) system [13,14], and up to 4.7 T [15]. Operating at higher pressures would make subsequent compression significantly easier to perform.…”

“…This model has been also used to discuss OP experiments with a broadband laser [20,21]. Extensions to specific situations have been proposed, but not fully developed, for OP in 3 He- 4 He isotopic mixtures 1 [23] and for OP in high magnetic fields [12,13,24]. Recently, we have developed a more comprehensive model of MEOP.…”

Magnetic Resonance Imaging: From Spin Physics to Medical Diagnosis

“…This is the case for several applications including lung MRI, and it introduces demanding requirements on the gas handling and compressing devices. However, it was recently shown that the range of operating pressures can be extended to several tens of mbar by performing MEOP in a high magnetic field, such as the 1.5 T field of an MRI system [54,55], which could make subsequent compression less difficult to perform.…”