Refrigeration – Cryogenic treatment of gas or gas mixture – Solidification
Reexamination Certificate
2002-08-19
2004-05-18
Doerrler, William C. (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Solidification
C062S003100, C062S600000, C062S637000
Reexamination Certificate
active
06735977
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of hyperpolarized inert gases. More particularly, the present invention relates to a method for minimizing the loss of
129
Xe polarization during the cryogenic accumulation process.
BACKGROUND OF THE INVENTION
As is known in the art, polarized
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Xe must be separated from the 4He carrier gas. Due to the high freezing/melting point of Xe, about 160K, separation from the carrier gas is typically accomplished by freezing the Xe out of the mixture in liquid nitrogen.
Gatzke et al., in Phys Rev Lett 70, 690, 1993, have established that fields of greater than 500 Gauss are required to achieve the longest possible polarization relaxation time of solid
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Xe of about 2-3 hours at 77K. Cates et al., in Phys Rev Lett 65, 2591, 1990, determined that, when frozen in liquid N
2
at 77K in a sealed Pyrex cell,
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Xe relaxation rates are essentially independent of magnetic field strength above 500G. In view of these findings, polarizers today position permanent magnets providing a field strength of about 500G around solid
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Xe.
However, further investigation supporting the present invention has revealed that this field strength, while sufficient for minimizing relaxation at 77K, is insufficient for minimizing relaxation at temperatures closer to the melting point. The relaxation rate of the solid
129
Xe can be very fast near its freezing point, limiting the amount of
129
Xe polarization which is retained throughout the accumulation process. Presently, a factor of 2 or more in polarization is lost in the freezing and thawing process.
There is therefore a need for a system and method of reducing nuclear-spin relaxation rates when melting frozen
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Xe.
SUMMARY OF THE INVENTION
In view of the needs of the art, the present invention provides a yoke supporting a magnet which provides a magnetic field of at least 2 kiloGauss about frozen
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Xe which is to be melted and collected.
The yoke of the present invention supports a magnet about the entire region of a collector where frozen
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Xe accumulates. The yoke thereby provides a holding magnetic field which extends through the volume of
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Xe accumulation. The collector is desirably constructed in the form of a ‘cold finger’ as is known in the art. The holding magnetic field may be induced by permanent magnets, electromagnets, superconducting magnets, or standard magnets. The holding field may further be provided by a yoke body which couples the magnetic field of the magnets about the cold finger.
The present invention further provides a method for reducing the relaxation rate of accumulated polarized
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Xe in a chamber. The method includes the step of positioning the chamber in a magnetic holding field having a magnetic field strength of greater than 2 kiloGauss. The method further maintains the accumulated
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Xe in the holding field as the accumulated
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Xe is thawed and subsequently recrystallized.
REFERENCES:
patent: 5809801 (1998-09-01), Cates et al.
patent: 5809901 (1998-09-01), Gutzmer
patent: 5860295 (1999-01-01), Cates et al.
patent: 6079213 (2000-06-01), Driehuys et al.
patent: 6199385 (2001-03-01), Driehuys et al.
patent: 6269648 (2001-08-01), Hasson et al.
patent: 6293111 (2001-09-01), Nacher et al.
patent: WO 00/23797 (2000-04-01), None
Gatzke, et al. Phys Rev Lett 70, 690, 1993.
Cates, et.al. Phys Rev Lett 65, 2591, 1990.
Kuzma Nicholas N.
Nelson Ian
Chisholm Robert F.
Doerrler William C.
Medi--Physics, Inc.
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