Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Reexamination Certificate
1998-08-14
2001-09-04
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
C062S051200, C062S055500, C062S919000
Reexamination Certificate
active
06282920
ABSTRACT:
The present invention relates to an apparatus and method for producing polarized high-pressure vapor-phase helium-3, particularly for NMR imaging, as well as the application of said apparatus.
Polarized helium-3 is an isotope known for the properties which have been exploited particularly for the measurement of extremely small variations in the terrestrial magnetic field, particularly for the fabrication of magnetometers. The state-of-the-art methods for producing dense polarized helium-3 are relatively difficult to implement. A first method, referred to as the “brute-force” method, is comprised of placing a helium-3 sample in an intense magnetic field at low temperature. This method is only effective for solid helium-3 at a pressure greater than 36 atmospheres or for helium-3 that is highly diluted in liquid helium-4, at a temperature of several millikelvin in a magnetic field of several teslas. These conditions pose noteworthy technological problems and prevent production under conditions that are economically acceptable for applications requiring production within a reasonably short period of time of large volumes of polarized helium-3.
Also proposed in the prior art has been polarization by spin exchange with a gas of alkaline atoms oriented by optical pumping. This method is also very slow because the exchange collisions that assure the transfer of polarization from the alkaline to the helium are of low efficacy and constitute an insurmountable limiting factor. This method requires a duration of 10 hours for the polarization of 150 cubic centimeters of helium at a pressure of 10 bars and a polarization of 10%.
Another method of the prior art, which constitutes the state of the art that is closest to the invention, comprises compressing gaseous helium-3 polarized by optical pumping using a mobile-piston compressor. This type of device is very delicate to fabricate and operate. Extremely rigorous precautions need to be taken to avoid relaxation during the compression phase, upon passage into the pump. Specifically, it is necessary to avoid use of lubricants that could contaminate the polarized gas and to select materials for the pump walls such that they reduce the relaxation of the polarized isotopes. This method can be implemented for research applications in laboratories but it can not be used at present for producing under economically acceptable conditions noteworthy quantities of strongly polarized helium-3 within brief time periods.
The object of the present invention is to resolve the drawbacks of the methods and apparatus of the prior art by proposing a method enabling the rapid production of highly polarized helium-3 in the form of dense gas, at a pressure on the order of atmospheric pressure or higher than atmospheric pressure, using a low-cost apparatus with uncomplicated operations.
To this end, the invention relates first of all to an apparatus for producing polarized high-pressure vapor-phase helium-3, particularly for NMR imaging. The apparatus comprises a means for injecting helium into an optical pumping cell, a means for liquefying the polarized gas from the optical pumping cell and a tank for storing the polarized liquid-phase helium-3, characterized in that the storage tank is cooled to a temperature that can be set between a storage temperature Ta and an evaporation temperature Te, and communicates alternately with the optical pumping cell and with a high-pressure gas-phase helium discharge duct.
The apparatus according to the invention operates according to a purely cryogenic compression principle, and employs solely the thermal effects for displacing the atoms. It is thus possible to avoid in particular all of the problems stemming from the use of a pump for the compression of the polarized gas.
Preferably, the optical pumping cell communicates with the helium-3 source via an adjustable-flow valve. In comparison with the spin-exchange method with optically polarized alkaline atoms, the major advantage is the rapidity of the production of the orientation by means of direct optical pumping of the helium.
According to a preferred mode of implementation, the optical pumping cell has a means for determining the nuclear polarization of the gas.
This means, for example a device for measuring the polarization of the florescence radiation emitted by the gaseous mixture in the optical pumping cell, enables control of the polarization rate of the gaseous mixture in the pumping cell and the adjustment of certain settings of the apparatus so as to maximize this rate. To this end, the apparatus has means for setting the flow rate of the pumping cell's feed valve and/or the temperature Ta of the storage tank as a function of the nuclear polarization in the optical pumping cell (indicated by dotted lines in the figure.
According to a preferred mode of implementation, the optical pumping cell is excited by a laser emitting a beam with a wavelength of 1083 nanometers.
For example, a laser diode with a power of 50 milliwatts enables preparation of 100 cubic centimeters of a mixture containing 10% of helium-3 under a pressure of 1.2 bar in 15 minutes.
The invention also relates to a method for producing polarized vapor-phase helium-3 comprising the injection of helium-3 into an optical pumping cell and the storage of the polarized helium-3 in liquid phase in a storage tank, characterized in that the polarized gas from the optical pumping cell is collected in a storage chamber cooled to a temperature that can be set between a helium-3 liquefaction temperature Ta and a helium-3 evaporation temperature Te.
Advantageously, a mixture of helium-3 and helium-4 is injected into the optical pumping cell. It has been found that the use of a mixture rather than pure helium-3 results in the lengthening of the time period during which the polarization can be maintained. Said mixture preferably contains between 3 and 30% of helium-3.
According to an advantageous variant, part of the polarized helium contained in the storage tank is evaporated. This variant is manifested by an enrichment in polarized helium-3 in the polarized gaseous mixture produced by the apparatus. In addition, use can be made of enrichment techniques that employ the thermodynamic effects characteristic of superfluid helium.
The invention also relates to the application of an apparatus in accordance with the preceding to NMR imaging. The body to analyze is penetrated by a polarized gaseous mixture stemming from an apparatus according to the invention and NMR imaging of said body is performed.
REFERENCES:
patent: 2982106 (1961-05-01), Ambler
patent: 4977749 (1990-12-01), Sercer
patent: 5073896 (1991-12-01), Reid et al.
patent: 0 471 586 (1992-02-01), None
patent: 2 598 518 (1987-11-01), None
patent: WO 91 07668 (1991-05-01), None
patent: WO 95 27438 (1995-10-01), None
Nacher Jean-Pierre
Tastevin Geneviève
Capossela Ronald
Centre National de la Recherche Scientific-CNRS
Schnader Harrison Segal & Lewis LLP
LandOfFree
Apparatus and method for producing polarized vapor-phase... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Apparatus and method for producing polarized vapor-phase..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Apparatus and method for producing polarized vapor-phase... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2451289