Chemistry: analytical and immunological testing – Metal or metal containing
Reexamination Certificate
2000-06-08
2003-07-08
Soderquist, Arlen (Department: 1743)
Chemistry: analytical and immunological testing
Metal or metal containing
C422S083000, C436S173000, C436S183000, C436S079000
Reexamination Certificate
active
06589793
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to devices and methods for measuring the absolute polarization of alkali metal atoms wherein the operation of a detection laser is improved by controlling the laser operating temperature and/or current. The polarized alkali metal atoms are contained within a sample cell disposed within magnetic fields, and the intensity of the detection laser light after passage through the sample cell is measured to determine the polarization.
BACKGROUND TO THE INVENTION
Recent developments in magnetic resonance tomography (MRT) and in magnetic resonance spectroscopy (NMR) with polarised inert gases can be expected to yield applications in medicine, in physics and in materials sciences. High nuclear spin polarisation levels in inert gases can be achieved by optical pumping using alkali metal atoms, as can be seen from the paper by Happer et al., Phys. Rev. A, 29, 3092 (1984). Typically at present, the alkali metal atom rubidium is used in the presence of an inert gas and nitrogen. In this way, it is possible to achieve a nuclear spin polarisation of ca. 20 percent in the inert gas xenon (
129
Xe). Such a nuclear spin polarisation is ca. 100,000 times greater than the equilibrium polarisation in clinical magnetic resonance tomographs. The consequent drastic increase in the signal-to-noise ratio explains why in the future new possible applications are expected in medicine, science and technology.
Polarisation is understood to mean the degree of alignment (ordering) of the spin of atomic nuclei or electrons. For example, 100 percent polarisation means that all nuclei or electrons are oriented in the same way. A magnetic moment is associated with the polarisation of nuclei or electrons.
Polarised xenon is for example inhaled by a person or injected into him. 10 to 15 seconds later, the polarised xenon collects in the brain. Using magnetic resonance tomography, the distribution of the inert gas in the brain is established. The result is used for further analyses.
The choice of the inert gas depends on the particular application.
129
Xenon displays a large chemical shift if xenon is for example adsorbed on a surface, then its resonance frequency changes significantly. Furthermore, xenon dissolves in fat-loving (i.e. lipophilic) liquids. When such properties are desired, xenon is used.
The inert gas helium is almost insoluble in liquids. The isotope
3
He is therefore regularly used when cavities are concerned. The lungs of a person are an example of such a cavity.
Some inert gases have valuable properties other than the aforesaid. Thus for example the isotopes
83
Krypton,
21
Neon and
131
Xenon have a quadrupole moment, which is for example of interest for experiments in fundamental research, namely in surface physics. However, these inert gases are very expensive, so that these are unsuitable for applications in which larger amounts are used.
From the paper “B. Driehuys et al., Appl. Phys. Lett., 69, 1668 (1996), the polarisation of inert gases in the following way is known.
Using a laser and, a &lgr;/4 plate positioned in the light beam from the laser, circularly polarised light is produced, that is to say light in which the angular momentum i.e. spin of the photons all point in the same direction. The angular momentum of the photons is transferred to the electrons of alkali metal atoms. Hence the spins of the electrons of the alkali metal atoms display a large deviation from the thermal equilibrium. Consequently, the alkali metal atoms are polarised. As a result of a collision of an alkali metal atom with an atom of an inert gas, the polarisation of the electron spin of the alkali metal atom is transferred to the nuclear spin of the inert gas. Polarised inert gas is thus produced.
Alkali metal atoms are used as these have a large optical dipole moment, which interacts with the light. Further, alkali metal atoms each have one free electron, so that no disadvantageous interactions between two and more electrons per atom or molecule can arise.
Caesium would be a particularly suitable alkali metal atom, which Is superior to rubidium for the production of polarised xenon. However, at present there are no lasers available with sufficiently high power, such as would be needed for the polarization of xenon using caesium. It is however to be expected that in the future lasers with power levels of about 100 watts at the caesium wavelength will be developed. Probably caesium will then be preferentially used for the polarisation of inert gases.
The state of the art is that a gas mixture at a pressure typically of 7 to 10 bars is slowly passed through a cylindrical glass cell. The gas mixture consists 98 percent of
4
Helium, one percent nitrogen and one percent of xenon. The typical flow rates for the gas mixture are a few cc per second.
The gas mixture first flows through a vessel (hereinafter termed “feed vessel”) which contains ca. one gram of rubidium. The feed vessel with the rubidium present in it, together with the glass cell connected to it, is heated to ca. 100 to 150 degrees centigrade. By the provision of these temperatures, the rubidium is vaporised. The concentration of the vaporised rubidium atoms in the gas phase is determined by the temperature in the feed vessel. The gas flow transports the vaporised rubidium atoms from the feed vessel into the cylindrical sample cell. A powerful, circularly polarised laser (100 watts power in continuous operation) irradiates the sample cell, which is generally a glass cell, axially and optically pumps the rubidium atoms into a highly polarised state.
Here, the wavelength of the laser must be matched to the optical absorption line of the rubidium atoms (Dl line) In other words: in order optimally to transfer the polarisation 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 located in a static magnetic field B
0
of a few tens of Gauss, which is created by coils, in particular a so-called Helmholtz coil pair. The direction of the magnetic field runs parallel to the cylinder axis of the sample cell, i.e. parallel to the direction of the laser beam. The magnetic field serves to control the polarised atoms.
The rubidium atoms optically highly polarised by the light of the laser collide in the glass cell inter alia with the xenon atoms and give up their high polarisation to the xenon atoms. At the exit of the sample cell, the rubidium is deposited on the wall, owing to its high melting point compared to the melting points of the other gases. The polarised xenon or the gas mixture is passed on from the sample cell into a freezing trap. This consists of a glass flask, the end of which is immersed in liquid nitrogen. The glass flask is moreover located in a magnetic field with a strength of 1000 to 2000 Gauss. The highly polarised xenon gas is deposited as ice on the inner glass wall of the freezing trap. At the outlet of the freezing trap, the remaining gas (helium and nitrogen) is passed through a needle valve and finally released.
The flow rate in the whole apparatus can be controlled with the needle valve, and measured with a gauge. If the flow rate increases too much, no time remains for the transfer of the polarisation from the rubidium atoms to the xenon atoms. Hence no polarisation is achieved. If the flow rate is too low, then too much time elapses before the desired amount of highly polarised xenon has been frozen. Thus the polarisation of the xenon atoms again declines through relaxation. The relaxation of the xenon atoms is greatly retarded by the freezing and by the strong magnetic field, to which the freezing trap is exposed. Hence it is necessary to freeze the inert gas as quickly and with as little loss as possible after the polarisation. The relaxation admittedly cannot be avoided by the freezing. However, there still remain 1 to 2 hours before the polarisation has declined so much that a subsequent use of the initially highly polarised gas is no longer possible.
A polariser of the aforesaid type alwa
Appelt Stephan
Shah Nadim Jone
Unlu Timur
Forschungszentrum Julich GmbH
Pearne & Gordon LLP
Soderquist Arlen
LandOfFree
Measurement device for the measurement of the absolute... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Measurement device for the measurement of the absolute..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Measurement device for the measurement of the absolute... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3005322