Refrigeration – Low pressure cold trap process and apparatus
Reexamination Certificate
2000-02-15
2001-11-20
Bennett, Henry (Department: 3744)
Refrigeration
Low pressure cold trap process and apparatus
C062S637000
Reexamination Certificate
active
06318092
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention to methods of hyperpolarizing noble gases. More particularly, the invention relates to methods of high efficiency optical pumping methods for hyperpolarizing noble gases.
It is known that noble gases such as
3
He and
129
Xe can be “hyperpolarized” using laser techniques. Such polarization methods include spin-exchange optical pumping, by which an alkali metal vapor is optically polarized, followed by “exchange” of this polarization with the noble gas (Bouchiat et al. 1960; Bhaskar et al. 1982; Happer et al. 1984; Zeng et al. 1985; Cates et al. 1992). Other polarization methods employ metastability exchange, in which noble gas nuclei (typically helium-3 (
3
He)) are directly optically pumped without an alkali metal intermediary (Schearer 1969; Laloë et al. 1984). Systems for producing polarized noble gases are described in U.S. Pat. Nos. 5,642,625 and 5,617,860, the complete disclosures of which are incorporated herein by reference.
Hyperpolarized noble gases can be used for numerous purposes. Historically, polarized
129
Xe has been used for fundamental symmetry studies (Chupp et al. 1994), nuclear spin relaxation studies of solids (Gatzke et al. 1993), high resolution nuclear magnetic resonance spectroscopy (NMR) (Raftery et al. 1991), and cross-polarization to other nuclei (Gatzke et al. 1993; Long et al. 1993). Polarized
3
He is also an important nuclear target (Anthony et al. 1993; Middleton (1994)).
Most recently, the enhanced NMR signals of laser polarized
129
Xe, which are about five orders of magnitude larger than those from thermally polarized
129
Xe, have made possible the first high-speed biological magnetic resonance imaging (MRI) of a gas (Albert et al. 1994). Helium-3 has also proven to be an excellent nucleus for gas phase MRI (Middleton et al. 1995). U.S. Pat. No. 5,545,396 describes the use of
129
Xe,
3
He, and other noble gas nuclei for biological MRI. These striking advances are now opening many new avenues of research.
The principal limitation in these applications of polarized noble gases has been the availability of sufficient quantities of the gases to meet the demand. Accordingly, attention has been directed to improving the rates of polarized noble gas production. Apparatus has been devised by which larger quantities of polarized gas can be produced on a continuous or batch mode basis. See U.S. Pat. No. 5,642,645. Methods for limiting depolarization of noble gases by interactions with container surfaces have been addressed by providing polymers as coatings. U.S. Pat. No. 5,612,103. Apparatus has also been developed to permit storage of frozen polarized
129
Xe. See U.S. application Ser. No. 08/622,865, filed on Mar. 29, 1996, the complete disclosure of which is incorporated herein by reference.
Even with these advances, the processes by which noble gases can be polarized are capable of further improvement, as many parameters have not been optimized. For example, efficiency of polarization is limited by the physical properties of the materials used to construct the polarizing apparatus. Moreover, an incomplete understanding of theoretical considerations underlying the physics of spin exchange in various systems implies that opportunities exist to identify systems with greater efficiencies.
From a practical perspective, hyperpolarization efficiency is related to laser power, while the cost of laser installation and maintenance often increases directly with delivered power. Accordingly, polarization systems for producing higher amounts of polarized noble gases can require significantly more expensive lasers. Therefore, it would be desirable to enable the artisan to increase the polarization yield of a given laser, and thereby to mitigate expense in scaled-up systems.
Accordingly, it is one of the purposes of this invention to overcome the above limitations in the art of spin-exchange optical pumping methods, by providing methods by which polarization efficiently is significantly improved using currently available apparatus. It is another purpose of the invention to provide the artisan with materials and methods that enable a wider variety of apparatus useful for polarizing noble gases.
SUMMARY OF THE INVENTION
It has now been discovered that these and other objectives can be achieved by the present invention, which in one embodiment is a method of hyperpolarizing a noble gas by spin-exchange optical pumping, comprising:
providing a polarization cell containing a noble gas and an alkali metal hybrid, wherein the alkali metal hybrid comprises a primary alkali metal and an auxiliary alkali metal; and
illuminating the polarization cell with radiation having a wavelength appropriate to optically polarize the primary alkali metal; thereby enabling spin-exchange interaction among the primary alkali metal, the auxiliary alkali metal, and the noble gas; whereby spin transfer to the noble gas yields hyperpolarized noble gas.
In the invention, the ratio of the primary alkali metal to the auxiliary alkali metal in the condensed phase can be from about 1:100 to about 100:1, and is preferably from about 1:25 to about 25.1. The ratio of the primary to the auxiliary alkali metals in the vapor phase can be from about 1:100 to about 10:1, more preferably from about 1:30 to about 1:1. It is preferred that an auxiliary alkali metal has greater efficiency than the primary alkali metal in polarizing the noble gas.
In one preferred alkali metal hybrid, the primary alkali metal is rubidium and the auxiliary alkali metal is potassium. In this case, a preferred ratio of the rubidium to the potassium is about 5:95. In an alternative alkali metal hybrid, the primary alkali metal is potassium, the auxiliary alkali metal is sodium. In another alternative alkali metal hybrid, the primary alkali metal is sodium, and the auxiliary alkali metal is potassium. The use of potassium and sodium enhances removal of the alkali metal from the hyperpolarized gas, since sodium and potassium have lower vapor pressures.
A preferred noble gas useful according to the invention is
3
He. The polarization cell can further contain a buffer gas and/or a quenching gas.
In another embodiment, the invention is a method of hyperpolarizing a noble gas, comprising:
a) optically polarizing a primary alkali metal by illumination with radiation having a wavelength that is resonant with an electronic transition in the primary alkali metal;
b) transferring polarization of the primary alkali metal to an auxiliary alkali metal; and
c) delivering polarization of the auxiliary alkali metal to a noble gas, thereby providing a noble gas having increased polarization.
Preferably, the primary alkali metal is rubidium, and the auxiliary alkali metal is potassium. Alternatively, the primary alkali metal is potassium, and the auxiliary alkali metal is sodium.
In another embodiment, the invention is apparatus for hyperpolarizing a noble gas by spin-exchange optical pumping, comprising:
a polarization cell containing an alkali metal hybrid, wherein the alkali metal hybrid comprises a primary alkali metal and an auxiliary alkali metal, provided that the primary alkali metal is capable of substantial optical polarization at a polarizing wavelength of light while the auxiliary alkali metal is not capable of significant polarization at the polarizing wavelength.
In the apparatus, the alkali metal hybrid is preferably an alloy of rubidium and potassium, more preferably an alloy comprising about 5% rubidium and about 95% potassium. Alternatively, the alkali metal hybrid is an alloy of sodium and potassium.
In another embodiment, the invention is a method of hyperpolarizing a noble gas by a spin-exchange optical pumping, comprising:
illuminating a polarization cell containing an alkali metal and a noble gas with radiation of a wavelength resonant with an electronic transition of the alkali metal under conditions and for a time sufficient to optically polarize alkali metal atoms, whereupon spin exchange between optically polarized alkali metal atoms and noble gas atoms yields polarized no
Cates, Jr. Gordon D.
Erickson Christopher J.
Happer William
Romalis Mikhail V.
Bennett Henry
Jones Melvin
Myers Bigel Sibley & Sajovec P.A.
The Trustees of Princeton University
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