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Physical Review A

Production of a nuclear-spin-polarized Ne21 sample

B. C. Grover

Published Oct 1, 1983• 6 citations
Publisher

Abstract

Nuclear-spin-polarized samples of up to 200 Torr $^{21}\mathrm{Ne}$ have been obtained through spin-exchange collisions with optically pumped Rb vapor. The exponential time constant associated with buildup and decay of this polarization along the quantization axis is in excess of 15 h in 3-cm-diam Pyrex cells. From the data and estimates given, samples of $^{21}\mathrm{Ne}$ with densities greater than 1 STP and polarization in excess of 1% appear feasible.

DOI
10.1103/physreva.28.2521

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Quantum-state preparation has a wide range of applications ranging from quantum optics to quantum metrology to fundamental physics precision measurements. In the context of nuclear $\ensuremath{\beta}$-decay correlation experiments such as the ones using argon, one needs to prepare a source of highly polarized atoms. Creating such a state-prepared sample amounts to generating a so-called stretched state, i.e., a state with the maximal projection of the total angular momentum along the quantization axis. This is typically achieved through optical pumping. Since this technique inherently depends on cycles of absorption followed by spontaneous emission, it may lead to undesirable heating and population loss. It is therefore crucial to devise polarization methods with both optimized efficiency and minimized drawbacks. We propose and compare various schemes, which we have been investigating numerically in the case of argon-35 atoms. We show that polarization degrees as high as $99.99%$ can be obtained within less than 140 $\ensuremath{\mu}\mathrm{s}$.

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Here we present a $\mathrm{Rb}\text{\ensuremath{-}}^{129}\mathrm{Xe}$ spin-exchange optical pumping polarizer capable of rapid generation of large volumes of highly polarized $^{129}\mathrm{Xe}$ gas. Through modeling and measurements we maximize the $^{129}\mathrm{Xe}$ nuclear spin polarization output to enable the generation of polarized $^{129}\mathrm{Xe}$ gas imaging volumes ($300\text{ }\text{ }{\mathrm{cm}}^{3}$) every 5 min within a clinical setting. Our model is verified by experiment to correctly predict the optimum Rb vapor density for maximum $^{129}\mathrm{Xe}$ nuclear polarization for a flux $3.4\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$ of circularly polarized Rb ${D}_{1}$ photons incident on an 80 cm long cylindrical optical cell. We measure a $^{129}\mathrm{Xe}$ magnetization production efficiency of ${\ensuremath{\eta}}_{\mathrm{pr}}=1.8%$, which approaches the photon efficiency limit ${\ensuremath{\eta}}_{\ensuremath{\gamma}}=3.3%$ of this system and enables the polarization of $2.72\ifmmode\times\else\texttimes\fi{}{10}^{22}$ $^{129}\mathrm{Xe}$ spins per hour, corresponding to $1013\text{ }\text{ }{\mathrm{cm}}^{3}$ of 100% polarized $^{129}\mathrm{Xe}$ at STP. This magnetization production rate is threefold higher than the highest previously published $^{129}\mathrm{Xe}$ magnetization production rate and has enabled routine clinical lung magnetic resonance imaging (MRI) with hyperpolarized $^{129}\mathrm{Xe}$ doses available on demand at run time, as well as high-SNR $^{129}\mathrm{Xe}$ MRI of the human brain and kidneys.

Physical Review Letters•Oct 8, 2018•95 citations
Nuclear-spin-polarized samples of up to 200 Torr 21Ne^{21}\mathrm{Ne}21Ne have been obtained through spin-exchange collisions with optically pumped Rb vapor. The exponential time constant associated with buildup and decay of this polarization along the quantization axis is in excess of 15 h in 3-cm-diam Pyrex cells. From the data and estimates given, samples of 21Ne^{21}\mathrm{Ne}21Ne with densities greater than 1 STP and polarization in excess of 1% appear feasible.
We report investigations of spin exchange between 21Ne^{21}\mathrm{Ne}21Ne nuclei and optically pumped Rb vapor. Polarization of 30% (〈Kz〉=0.46〈{K}_{z}〉=0.46〈Kz​〉=0.46) of 2\ifmmode\times\else\texttimes\fi{}1019{10}^{19}1019 atoms-${\mathrm{cm}}^{\ensuremath{-}3}$ has been achieved with a vapor pressure of 3\ifmmode\times\else\texttimes\fi{}1014{10}^{14}1014 atoms-${\mathrm{cm}}^{\ensuremath{-}3}$ of Rb. This high density of alkali metal can be efficiently optically pumped only when light trapping is suppressed by inclusion of at least 100 Torr of N2{\mathrm{N}}_{2}N2​. In addition, we have made the first accurate measurement of the binary-collision spin-exchange rate: $〈{\ensuremath{\sigma}}_{\mathrm{ex}}v〉=(4.66\ifmmode\pm\else\textpm\fi{}0.28)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$ cm3{\mathrm{cm}}^{3}cm3-${\mathrm{s}}^{\ensuremath{-}1}$. Extension of these techniques to 3He^{3}\mathrm{He}3He has yielded 5% polarization of 2\ifmmode\times\else\texttimes\fi{}1019{10}^{19}1019 atoms-${\mathrm{cm}}^{\ensuremath{-}3}$ in a preliminary experiment.
The spin-exchange cross section for Rb85{\mathrm{Rb}}^{85}Rb85-Rb87{\mathrm{Rb}}^{87}Rb87 collisions has been measured to be (1.70\ifmmode\pm\else\textpm\fi{}0.21)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}14}$ cm2{\mathrm{cm}}^{2}cm2 using the techniques of optical pumping and an interferometric method for measuring the Rb vapor density. The Rb87{\mathrm{Rb}}^{87}Rb87 constituent in a vapor of natural Rb was polarized through absorption of circularly polarized resonance radiation, and the Rb87{\mathrm{Rb}}^{87}Rb87 polarization was partially transferred to the Rb85{\mathrm{Rb}}^{85}Rb85 constituent through spin-exchange collisions. A phenomenological theory of the optical pumping plus the exchange polarization process which neglects nuclear spin was used to relate the cross section to the ratio of the signal strengths for the Rb87{\mathrm{Rb}}^{87}Rb87 and Rb85{\mathrm{Rb}}^{85}Rb85 Zeeman resonances, the relaxation time, and the Rb87{\mathrm{Rb}}^{87}Rb87 vapor density. The density was measured by comparing spectral profiles of the resonance radiation transmitted through the optical pumping cell at room temperature with the resonance radiation transmitted at the temperature at which the signal strengths were measured. These spectral profiles were obtained through use of a scanning Fabry-Perot interferometer.
Estimates of the Rb-131Xe^{131}\mathrm{Xe}131Xe spin-exchange cross section have been obtained by observing the nuclear spin relaxation of 131Xe^{131}\mathrm{Xe}131Xe in the presence of a Rb vapor. Nuclear spin relaxation of 131Xe^{131}\mathrm{Xe}131Xe due to the presence of a foreign buffer gas and the relaxation rate on the glass walls of the cell have also been observed.
Quantum-state preparation has a wide range of applications ranging from quantum optics to quantum metrology to fundamental physics precision measurements. In the context of nuclear $\ensuremath{\beta}$-decay correlation experiments such as the ones using argon, one needs to prepare a source of highly polarized atoms. Creating such a state-prepared sample amounts to generating a so-called stretched state, i.e., a state with the maximal projection of the total angular momentum along the quantization axis. This is typically achieved through optical pumping. Since this technique inherently depends on cycles of absorption followed by spontaneous emission, it may lead to undesirable heating and population loss. It is therefore crucial to devise polarization methods with both optimized efficiency and minimized drawbacks. We propose and compare various schemes, which we have been investigating numerically in the case of argon-35 atoms. We show that polarization degrees as high as 99.9999.99%99.99 can be obtained within less than 140 $\ensuremath{\mu}\mathrm{s}$.
In a hyperfine-optical-pumping experiment we employed the Franzen transient method to determine the total spin-exchange cross sections for Rb87{\mathrm{Rb}}^{87}Rb87-Rb87{\mathrm{Rb}}^{87}Rb87 and Rb87{\mathrm{Rb}}^{87}Rb87-Cs133{\mathrm{Cs}}^{133}Cs133 collisions at 78\ifmmode^\circ\else\textdegree\fi{}C. Values determined in this way were: $\ensuremath{\sigma}({\mathrm{Rb}}^{87}\ensuremath{-}{\mathrm{Rb}}^{87})=(1.9\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ cm2{\mathrm{cm}}^{2}cm2 and $\ensuremath{\sigma}({\mathrm{Rb}}^{87}\ensuremath{-}{\mathrm{Cs}}^{133})=(2.3\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ cm2{\mathrm{cm}}^{2}cm2. A large difference between the ground-state hyperfine populations was established by pumping with light absorbable by atoms in only one hyperfine level. In the presence of a second unpumped species, the resulting polarization relaxed with a characteristic rate of $\frac{1}{\ensuremath{\tau}}=\frac{1}{T}+\frac{1}{{T}_{S1}}+\frac{1}{{T}_{E1}}$, where TTT is the non-spin-exchange relaxation time and TS1{T}_{S1}TS1​ and TE1{T}_{E1}TE1​ are the self- and cross-exchange times. Then $\ensuremath{\tau}$ was obtained by fitting the signal to a single exponential and applying a small correction ($\ensuremath{\approx}10%$) to account for the fact that the signal is only approximately proportional to the polarization (and hence to a single exponential). The relaxation measurement utilized rapid data accumulation with a Kerr-cell shutter and pulse-height analyzer. The cross section can be easily deduced from the relaxation times if the corresponding densities are known. Each density was measured by determining the integral over all frequencies of the absorption coefficient by means of a scanning Fabry-Perot interferometer. We describe how to remove the effect of the Fabry-Perot on the true emission and absorption profiles.
Here we present a $\mathrm{Rb}\text{\ensuremath{-}}^{129}\mathrm{Xe}$ spin-exchange optical pumping polarizer capable of rapid generation of large volumes of highly polarized 129Xe^{129}\mathrm{Xe}129Xe gas. Through modeling and measurements we maximize the 129Xe^{129}\mathrm{Xe}129Xe nuclear spin polarization output to enable the generation of polarized 129Xe^{129}\mathrm{Xe}129Xe gas imaging volumes (300  cm3300\text{ }\text{ }{\mathrm{cm}}^{3}300  cm3) every 5 min within a clinical setting. Our model is verified by experiment to correctly predict the optimum Rb vapor density for maximum 129Xe^{129}\mathrm{Xe}129Xe nuclear polarization for a flux 3.4  W/cm23.4\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}3.4  W/cm2 of circularly polarized Rb D1{D}_{1}D1​ photons incident on an 80 cm long cylindrical optical cell. We measure a 129Xe^{129}\mathrm{Xe}129Xe magnetization production efficiency of ${\ensuremath{\eta}}_{\mathrm{pr}}=1.8%$, which approaches the photon efficiency limit ${\ensuremath{\eta}}_{\ensuremath{\gamma}}=3.3%$ of this system and enables the polarization of $2.72\ifmmode\times\else\texttimes\fi{}{10}^{22}$ 129Xe^{129}\mathrm{Xe}129Xe spins per hour, corresponding to 1013  cm31013\text{ }\text{ }{\mathrm{cm}}^{3}1013  cm3 of 100% polarized 129Xe^{129}\mathrm{Xe}129Xe at STP. This magnetization production rate is threefold higher than the highest previously published 129Xe^{129}\mathrm{Xe}129Xe magnetization production rate and has enabled routine clinical lung magnetic resonance imaging (MRI) with hyperpolarized 129Xe^{129}\mathrm{Xe}129Xe doses available on demand at run time, as well as high-SNR 129Xe^{129}\mathrm{Xe}129Xe MRI of the human brain and kidneys.