Limits...
Compressing Spin-Polarized 3 He With a Modified Diaphragm Pump

View Article: PubMed Central - PubMed

ABSTRACT

Nuclear spin-polarized 3He 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 3He gas can rapidly produce highly polarized gas, but the best results are obtained at much lower pressure (~0.1 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 % 3He polarization using pure 3He gas and 35 % to 50 % 3He polarization using 3He-4He mixtures. By maintaining the storage cell at liquid nitrogen temperature during compression, the density has been increased by a factor of four.

No MeSH data available.


Related in: MedlinePlus

Free induction decay NMR signal from the cell Neptune, using a fully destructive π/2 rad tip angle. The cell was filled to a pressure of 0.13 kPa of pure 3He and polarized to 45 % by direct optical pumping. The first few data points are low because of the 3 ms lock-in time constant. The solid line shows a fit to an exponential decay for times greater than 0.016 s.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4862822&req=5

f7-j64gen: Free induction decay NMR signal from the cell Neptune, using a fully destructive π/2 rad tip angle. The cell was filled to a pressure of 0.13 kPa of pure 3He and polarized to 45 % by direct optical pumping. The first few data points are low because of the 3 ms lock-in time constant. The solid line shows a fit to an exponential decay for times greater than 0.016 s.

Mentions: Free induction decay (FID) [68,69] is used to measure the polarization of the compressed gas in the storage cell. In this method, the magnetization is tipped away from the quantization axis by applying a short pulse of radiation at the Larmor frequency (85 kHz for our typical operating field of 2.6 mT). After this pulse, the transverse component of magnetization freely precesses around the quantization axis at the Larmor frequency while decaying in magnitude. The time scale of the decay, T2, is related to dephasing of the individual nuclear spins due to inhomogeneity in the static magnetic field. The initial size of the signal is proportional to the magnetization. The free precession is detected with a resonant pickup coil, connected to a dual-phase lock-in amplifier. By calibrating the response of the pickup coil using optical polarimetry at a known low pressure, and measuring the pressure in the storage cell, the storage cell polarization is determined. Figure 7 shows an FID signal from 45 % polarized gas produced by direct optical pumping of pure 3He at a pressure of 0.13 kPa, which we will refer to as the calibration FID signal. Figure 8 shows an FID signal from a compressed gas mixture (33 % 3He, 67 % 4He) at a pressure of 100 kPa. Whereas a fully destructive π/2 rad tip angle was used for the calibration FID signal, a tip angle of 0.070 rad was used for the compressed gas. Accounting for the different pressures and tip angles, and the mixture ratio for the compressed gas, leads to Pstc = 0.51 for the data in Fig. 8. Although performing the optical calibration of the NMR system with the same mixture ratio as the compressed gas cancels out inaccuracy in the knowledge of the mixture ratio, we used pure 3He for the calibration FID signal because it yields both a larger FID signal and a larger optical signal.


Compressing Spin-Polarized 3 He With a Modified Diaphragm Pump
Free induction decay NMR signal from the cell Neptune, using a fully destructive π/2 rad tip angle. The cell was filled to a pressure of 0.13 kPa of pure 3He and polarized to 45 % by direct optical pumping. The first few data points are low because of the 3 ms lock-in time constant. The solid line shows a fit to an exponential decay for times greater than 0.016 s.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4862822&req=5

f7-j64gen: Free induction decay NMR signal from the cell Neptune, using a fully destructive π/2 rad tip angle. The cell was filled to a pressure of 0.13 kPa of pure 3He and polarized to 45 % by direct optical pumping. The first few data points are low because of the 3 ms lock-in time constant. The solid line shows a fit to an exponential decay for times greater than 0.016 s.
Mentions: Free induction decay (FID) [68,69] is used to measure the polarization of the compressed gas in the storage cell. In this method, the magnetization is tipped away from the quantization axis by applying a short pulse of radiation at the Larmor frequency (85 kHz for our typical operating field of 2.6 mT). After this pulse, the transverse component of magnetization freely precesses around the quantization axis at the Larmor frequency while decaying in magnitude. The time scale of the decay, T2, is related to dephasing of the individual nuclear spins due to inhomogeneity in the static magnetic field. The initial size of the signal is proportional to the magnetization. The free precession is detected with a resonant pickup coil, connected to a dual-phase lock-in amplifier. By calibrating the response of the pickup coil using optical polarimetry at a known low pressure, and measuring the pressure in the storage cell, the storage cell polarization is determined. Figure 7 shows an FID signal from 45 % polarized gas produced by direct optical pumping of pure 3He at a pressure of 0.13 kPa, which we will refer to as the calibration FID signal. Figure 8 shows an FID signal from a compressed gas mixture (33 % 3He, 67 % 4He) at a pressure of 100 kPa. Whereas a fully destructive π/2 rad tip angle was used for the calibration FID signal, a tip angle of 0.070 rad was used for the compressed gas. Accounting for the different pressures and tip angles, and the mixture ratio for the compressed gas, leads to Pstc = 0.51 for the data in Fig. 8. Although performing the optical calibration of the NMR system with the same mixture ratio as the compressed gas cancels out inaccuracy in the knowledge of the mixture ratio, we used pure 3He for the calibration FID signal because it yields both a larger FID signal and a larger optical signal.

View Article: PubMed Central - PubMed

ABSTRACT

Nuclear spin-polarized 3He 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 3He gas can rapidly produce highly polarized gas, but the best results are obtained at much lower pressure (~0.1 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 % 3He polarization using pure 3He gas and 35 % to 50 % 3He polarization using 3He-4He mixtures. By maintaining the storage cell at liquid nitrogen temperature during compression, the density has been increased by a factor of four.

No MeSH data available.


Related in: MedlinePlus