Compressing spin-polarized 3He with a modified diaphragm pump

Nuclear spin-polarized He-3 gas at pressures on the order of 100 kPa (1 bar) are required for several applications, such as neutron spin filters and magnetic resonance imaging. The metastability-exchange optical pumping (MEOP) method for polarizing He-3 gas can rapidly produce highly polarized gas, but the best results are obtained at much lower pressure (similar to0.1 kPa). We describe a compact compression apparatus for polarized gas that is based on a modified commercial diaphragm pump. The gas is polarized by MEOP at a typical pressure of 0.25 kPa (2.5 mbar), and compressed into a storage cell at a typical pressure of 100 kPa. In the storage cell, we have obtained 20% to 35% He-3 polarization using pure He-3 gas and 35% to 50% He-3 polarization Using He-3-He-4 mixtures. By maintaining the storage cell at liquid nitrogen temperature during compression, the density has been increased by a factor of four.