Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2002-03-27
2003-12-23
Doerrler, William C. (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S003100, C600S420000, C604S020000, C604S181000
Reexamination Certificate
active
06666047
ABSTRACT:
The invention concerns an apparatus with devices comprising glass for the polarization of noble gases in the sample cell. The invention further concerns a method of operating the apparatus.
Recent developments in magnetic resonance tomography (MRT) and in magnetic resonance spectroscopy (NMR) with polarized noble gases lead to the expectation of uses in medicine, physics and material sciences. High levels of polarization of nuclear spins of noble gases can be achieved by optical pumping by means of alkali metal atoms, as can be seen from the publication Happer et al, Phys. Ref. A, 29, 3092 (1984). Typically at the present time the alkali metal atom rubidium is used in the presence of a noble gas and nitrogen. It is possible in that way to achieve nuclear spin polarization of the noble gas xenon (
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Xe) of about 20 percent. Such a level of nuclear spin polarization is about 100,000 times greater than the equilibrium polarization in clinical magnetic resonance tomographs. The drastic increase in the signal-noise ratio, which this involves, explains why in future new possible uses are expected in medicine, science and technology.
The term polarization is used to denote the degree of orientation (order) of the spins of atomic nuclei or electrons. 100 percent polarization means for example that all nuclei or electrons are oriented in the same fashion. The polarization of nuclei or electrons involves a magnetic moment.
Polarized xenon is for example inhaled by or injected into a human being. Between 10 and 15 seconds later the polarized xenon collects in the brain. The distribution of the noble gas in the brain is detected by means of magnetic resonance tomography. The result is used for further analysis procedures.
The choice of the noble gas depends on the situation of use.
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Xenon involves a large chemical shift. If xenon is adsorbed for example on a surface its resonance frequency significantly changes. In addition xenon is dissolved in fat-loving (that is to say: lipophilic) liquids. If properties of that kind are wanted, xenon is used.
The noble gas helium scarcely dissolves in liquids. The isotope
3
helium is therefore used regularly when cavities are involved. The lung of a human being represents an example of such a cavity.
Some noble gases have valuable properties other than those referred to above. Thus for example the isotopes
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krypton,
21
neon and
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xenon have a quadrupole moment which is of interest for example in experiments in basic research or surface physics. Those noble gases however are very expensive so that they are not suitable for uses in which relatively large amounts are used.
It is known from the publication “B. Driehuys et al., Appl. Phys. Lett. 69, 1668 (1996)” for noble gases to be polarized in the following manner in a polarizer.
Circularly polarized light is provided by means of a laser, that is to say light in which the rotational impulse or spin of the photons are all in the same direction. The rotational pulse of the photons is transmitted to free electrons of alkali metal atoms. The spins of the electrons of the alkali metal atoms therefore have a great deviation from thermal equilibrium. The alkali metal atoms are consequently polarized. By a collision of an alkali metal atom with an atom of a noble gas, the polarization of the electron spin is transferred from the alkali metal atom to the atom of the noble gas. That results in polarized noble gas.
Alkali metal atoms are used as they have a high optical dipole moment which interacts with the light. In addition alkali metal atoms each have a free electron so that no detrimental interactions can occur between two and more electrons per atom.
Cesium would be a particularly well-suited alkali metal atom which is superior in relation to rubidium to achieve the above-indicated effects. At the present time however there are no lasers available with a sufficiently high level of power, as would be required for the polarization of xenon by means of cesium. There is however the expectation that in future lasers with power levels of around 100 watts on the cesium wavelength will be developed. In that case cesium will probably be preferably used for the polarization of xenon gas.
In accordance with the state of the art, a gas mixture is passed slowly under a pressure of typically between 7 and 10 bars through a cylindrical glass cell. The gas mixture comprises for example 98 percent of
4
helium, one percent of nitrogen and one percent of xenon. The typical speeds of the gas mixture are some ccm per second.
The gas mixture firstly flows through a vessel (hereinafter referred to as the “supply vessel”) in which there is about one gram of rubidium. The supply vessel with the rubidium therein is heated together with the adjoining glass cell to between about 100 and 150 degrees Celsius. The rubidium is evaporated by the provision of those temperatures. The concentration of the evaporated rubidium atoms in the gas phase is determined by the temperature in the supply vessel. The gas flow transports the evaporated rubidium atoms from the supply vessel into the cylindrical sample cell. A powerful, circularly polarized laser (100 watt output in continuous operation) irradiates the sample cell which is generally a cylindrical glass cell, axially, and optically pumps the rubidium atoms into a highly polarized condition. In this case the wavelength of the laser must be matched to the optical absorption line of the rubidium atoms (D 1—line). In other words: in order to optimally transfer the polarization of light to an alkali metal atom, the frequency of the light must coincide with the resonance frequency of the optical transition. The sample cell is in a static magnetic field B
0
of a few 10 Gauss, which is produced by coils—in particular a so-called Helmholtz coil pair. The direction of the magnetic field extends parallel to the axis of the cylindrical configuration of the sample cell or parallel to the laser beam direction. The magnetic field serves to guide the polarized atoms.
The rubidium atoms which are optically highly polarized by the light of the laser collide in the glass cell inter alia with the xenon atoms and deliver their high level of polarization to the xenon atoms. At the outlet from the sample cell the rubidium is deposited at the wall by virtue of the high melting point in comparison with the melting points of the other gases. The polarized xenon or the residual gas mixture is conducted from the sample cell into a freezing-out unit. This comprises a glass flask whose end is immersed in liquid nitrogen. The glass flask is further disposed in a magnetic field of a strength of between 1000 and 2000 Gauss. The highly polarized xenon gas is deposited in the form of ice at the inside glass wall of the freezing unit. At the outlet from the freezing unit the remaining gas (helium and nitrogen) is passed by way of a needle valve and finally discharged.
The flow speed in the entire arrangement can be controlled by way of the needle valve and measured with a measuring device. If the flow speed rises excessively greatly, there is no time remaining for transfer of the polarization effect from the rubidium atoms to the xenon atoms. No polarization is therefore achieved. If the flow speed is too low, too much time elapses before the desired amount of highly polarized xenon has frozen. More specifically due to relaxation the polarization of the xenon atoms decreases again. Relaxation of the xenon atoms is greatly slowed down by the freezing effect and due to the strong magnetic field to which the freezing unit is exposed. It is therefore necessary for the noble gas xenon to be frozen after the polarization operation as quickly as possible and without loss. Admittedly, it is not possible to avoid relaxation, by virtue of the freezing procedure. However, between 1 and 2 hours still remain before xenon polarization has fallen so greatly that further use of the initially highly polarized gas is no longer possible.
A polarizer of the above-indicated kind always has connecting locations. Connecting locations
Appelt Stephan
Halling Horst
Shah Nadim Joni
Unlu Timur
Zilles Karl
Doerrler William C.
Forschungszentrum Julich GmbH
Merchant & Gould P.C.
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