Experimental studies of rubidium absolute polarization at high temperatures

Abstract

We report on measurements of the absolute rubidium ͑Rb͒ polarization, optically pumped by a high-power diode laser array, up to temperatures of 180°C. The penetration of the pump laser light into a high-pressure cell has been studied experimentally and theoretically. The experimental results are compared to a model, which describes the local Rb polarization and optical pumping rate in the cell. © 1999 American Institute of Physics. ͓S0003-6951͑99͒03229-5͔In nuclear magnetic resonance and in other fields of science such as human MRI, 1-4 material and surface physics, 5,6 the production of hyperpolarized noble gases through spin exchange with optically pumped alkali-metal atoms has gained importance. For some potential applications such as 129 Xe brain imaging, 4 very high nuclear polarizations are required to circumvent the limitations of the delivery method 7 and to avoid the anaesthetic effects of large quantities of Xe in the brain.We have built a Rb-Xe hyperpolarizing unit, similar to that described by Driehuys et al.,8 for the production of several grams of polarized Xe ice. In order to optimize the Rb ͑and thus Xe͒ polarization, a low-cost and compact polarization imaging device was developed which enables measurement of the distribution of the absolute Rb polarization in the pumping cell. The methodology and first experimental results of absolute Rb polarization imaging in high-pressure optical pumping cells at a temperature of Tϭ100°C were recently published by Baranga et al. 9 and Young et al. 10 Spin exchange optical pumping at temperatures higher than 100°C causes the spin exchange rate between Rb and Xe to be larger and therefore the time required to polarize Xe atoms is significantly shorter.Experimental results of the absolute 85 Rb polarization in high pressure optical pumping cells for temperatures up to 180°C are presented. The measured, volume-averaged absolute Rb polarization sets a limit on the maximum achievable Xe nuclear polarization at high temperatures. Furthermore, we studied the penetration of the pump laser light into the cell at a temperature range from 120-180°C. Figure 1 shows a schematic diagram of the compact, low-cost 85 Rb polarization imaging unit. For the sake of clarity, components of the Rb-Xe polarizer necessary for operating in a flowing mode and to freeze out the Xe gas are not shown. The optical pumping cell consisted of a glass cylinder with flat glass windows in order to avoid lens effects on the pump laser beam. The cell ͑inner diameter 24 mm, length 70 mm͒ was filled with approximately 1 g of Rb, 7 atm of 4 He, and 0.07 atm of N 2 and Xe gas. For optical pumping parallel to the magnetic field B 0 (z axis͒, we use a fiberoptically coupled diode laser array ͑Opto Power Corp., OPC 100͒ with a maximum output power of 100 W and a linewidth of 2 nm. The absolute 85 Rb polarization was measured by detecting, optically in the transverse mode, the Zeeman transitions of 85 Rb in a high magnetic field. The Zeeman spectrum was obtained by sweeping the magnetic field B 0 from 53....

Depolarization of nuclear spin polarized 129 Xe gas by dark rubidium during spin-exchange optical pumping

“…Then, we evaluate the presence and the impact of optically dark Rb vapor in the outlet of the optical cell on final 129 Xe polarization. While seemingly obvious when one compares the better performance of polarizer designs that incorporate cooling regions in the front of the optical cell to condense Rb vapor[15–17,21] with those that do not[18,19,22,24], the effects of dark Rb regions on xenon polarization have never been investigated. In order to perform these measurements, we designed and constructed a LabVIEW-based, ultra-low field NMR spectrometer as well as an optical spectrometer, which are also detailed in this manuscript.…”

Measuring the spin polarization of alkali-metal atoms using nuclear magnetic resonance frequency shifts of noble gases

“…The temperature sensitivity of an NMR gyroscope is theoretically estimated to be on the order of 1 deg/h/mK. 24,25 Therefore the temperature stability of…”

Predictive thermal model for indirect temperature measurement inside atomic cell of nuclear magnetic resonance gyroscope

“…The degree of polarization is highly dependent on the cell temperature, and temperature fluctuations in the cell can therefore create undesirable shifts in the signal. The temperature sensitivity of an NMR gyroscope is theoretically estimated to be on the order of 1 deg/h/mK [5][6]. Therefore the temperature stability of the cell is required to be better than 1 mK to achieve the targeted sub-degree per hour gyroscope performance.…”

“…87 Rb isotope is chosen for its high vapor pressure at relatively low temperatures. A cell temperature of 120ºC is chosen to achieve high polarization with uniform distribution in the cell [6]. In addition, the light source, a Vertical-Cavity Surface-Emitting Laser (VCSEL), must operate at temperatures below 80ºC to avoid degradation and reduced lifetime.…”