| Go Back |
Production of hyperpolarized (HP) 129Xe gas
The intensity of the NMR signal depends on the occupation difference between the Zeeman energy levels, i.e. the nuclear spin polarization. In thermal equilibrium the occupation numbers obey the Boltzmann distribution; near the room temperature and magnetic field of the NMR spectrometer (usually B = 2-18 T) the nuclear spin polarization is on the order of 10 ppm (~10-5). Due to the very low spin polarization the detection limit of the 129Xe NMR signal is about ~1018 nuclei. This restricts the use of the 129Xe NMR spectroscopy in materials research in which the quantity of the 129Xe nuclei may be very small. The purpose of the project is to produce so-called hyperpolarized 129Xe gas, where the nuclear spin polarization increases dramatically from the polarization in thermal equilibrium. The attained 129Xe spin polarization through hyperpolarizing is 1-70%, which means that at its best the 129Xe signal enhancement is over 50000 comparing to signal in thermal equilibrium.
The hyperpolarization process of 129Xe is based on spin-exchange optical pumping method. The optical pumping (by circularly polarized infrared laser light) creates large electron spin polarization of rubidium and due to spin-exchange interaction between rubidium and xenon the 129Xe nuclear spin polarization increases.
Polarization cell is placed inside an aluminium oven which is heated to 120-150°C so that rubidium inside begin to vaporize. High pressure (9 atm) inside the cell is used, because pressure broadening of the rubidium absorption linewidth leads to more efficient optical pumping. Unfortunately, it is not possible to gain high polarization of xenon in a system where the xenon pressure is high, so helium is used as a buffer gas. Circularly polarized light is directed through the cell in order to polarize electrons of rubidium, which in turn polarize xenon nuclei. Small amount of nitrogen (1%) in the gas mixture is needed to suppress the reradiation of light by quenching excited rubidium atoms. Polarized xenon gas flows then into the sample tube through PTFE tubing.
New continuous flow xenon polarizer is a mobile unit, so it is possible to operate with it on all of our four spectrometers.