U.S. patent application number 13/188246 was filed with the patent office on 2012-06-28 for non-cryogenic storage cell for hyperpolarized 129xe.
This patent application is currently assigned to University of Utah. Invention is credited to Benjamin C. Anger, Brian T. Saam, Geoffrey Schrank.
Application Number | 20120160710 13/188246 |
Document ID | / |
Family ID | 41340898 |
Filed Date | 2012-06-28 |
United States Patent
Application |
20120160710 |
Kind Code |
A1 |
Saam; Brian T. ; et
al. |
June 28, 2012 |
NON-CRYOGENIC STORAGE CELL FOR HYPERPOLARIZED 129XE
Abstract
A system is disclosed for producing and storing gaseous spin
polarized .sup.129Xe including: a polarizer configured to produce
gaseous spin polarized .sup.129Xe, and a storage apparatus for
non-cryogenically storing gaseous spin polarized .sup.129Xe.
Inventors: |
Saam; Brian T.; (Salt Lake
City, UT) ; Schrank; Geoffrey; (Salt Lake City,
UT) ; Anger; Benjamin C.; (Houston, TX) |
Assignee: |
University of Utah
|
Family ID: |
41340898 |
Appl. No.: |
13/188246 |
Filed: |
July 21, 2011 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12994326 |
|
|
|
|
PCT/US2009/044884 |
May 21, 2009 |
|
|
|
13188246 |
|
|
|
|
61055819 |
May 23, 2008 |
|
|
|
Current U.S.
Class: |
206/.6 |
Current CPC
Class: |
G01R 33/282 20130101;
G01R 33/5601 20130101 |
Class at
Publication: |
206/6 |
International
Class: |
B65D 85/00 20060101
B65D085/00 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED R&D
[0002] This invention was made with government support under
PHY-0134980 awarded by National Science Foundation. The Government
has certain rights to this invention.
Claims
1. A storage apparatus for non-cryogenically storing gaseous spin
polarized by .sup.129Xe comprising: a storage vessel comprising an
interior surface substantially surround a storage volume; and a
magnet which produces a substantially uniform magnetic field within
the storage volume; wherein the longitudinal spin relaxation rate
of gaseous spin polarized .sup.129Xe contained in the storage
volume due to interactions with the interior surface is about equal
to or less than the longitudinal spin relaxation rate of the
gaseous spin polarized .sup.129Xe due to intrinsic mechanisms.
2-25. (canceled)
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The application claims benefit of U.S. Provisional Patent
Application Ser. No. 61/055819, filed May 23, 2008, the content of
which is incorporated herein by reference in its entirety.
BACKGROUND
[0003] This disclosure is related to the storage of spin polarized
(e.g. hyperpolarized) gasses (e.g. .sup.129Xe).
[0004] Noble-gas isotopes having non-zero nuclear spin may be
optically polarized to levels approaching unity via the techniques
such as of spin-exchange optical pumping (SEOP) [1, 2], whereby the
notoriously weak signal generated by nuclear moments is enhanced by
several orders of magnitude. Even after several decades of work by
many groups, hyperpolarized gasses continue to be studied and
applied in a wide variety of magnetic-resonance experiments; we
cite a few recent examples [3-5]. In a typical implementation,
circularly polarized laser light is incident on a glass cell
containing a macroscopic amount of an alkali-metal (usually Rb),
the noble gas, and a small quantity of nitrogen to promote
collisional de-excitation of the excited states generated by
resonant absorption of the laser light by the alkali-metal vapor at
the first principle (D.sub.1) electric-dipole transition [6]. (This
corresponds to a wavelength of 795 nm for Rb.) The alkali-metal
vapor density is controlled by adjusting the cell temperature from
room temperature up to .apprxeq.500 K in the presence of a
macroscopic amount of alkali metal in the closed cell. The
selection rule for absorption of circularly polarized light and
collisional mixing of the excited-state magnetic sublevels lead to
rapid and efficient spin polarization of the valence electron of
the alkali-metal vapor. Collisions with noble-gas atoms then lead
to an exchange of angular momentum between the alkali-metal
electron and the noble-gas nucleus. The time-dependent build-up of
nuclear polarization P.sub.N(t) in such a sample that occurs while
the laser is on is given by:
P N ( t ) - P A .gamma. se .gamma. se + .GAMMA. [ 1 - exp ( - t
.gamma. se + .GAMMA. ) ] , ##EQU00001##
where P.sub.A is the time- and volume-averaged alkali-metal
polarization, .gamma..sub.se is the spin-exchange rate, and .GAMMA.
is the longitudinal relaxation rate of the noble gas due to to all
other mechanisms. Note, in Eq. (1) we have ignored the anomalous
excess relaxation that scales with alkali-metal density recently
observed for SEOP of .sup.3 He [7].
[0005] It is clear from Eq. (1) that F limits the ultimate nuclear
polarization for a fixed value of .gamma..sub.se, the latter being
limited by available laser power and the photon efficiency (the
number of polarized nuclei produced per photon absorbed in the cell
volume) [8] for a given alkali-metal-noble-gas pair and laser/cell
geometry, whereby one generally maintains P.sub.A close to unity.
An understanding of the mechanisms responsible for the relaxation
rate r is thus essential to the efficient production of
hyperpolarized gasses. Note,
[0006] While SEOP is typically used to polarize either of the
stable spin-1/2 noble-gas isotopes, .sup.3He and .sup.129Xe, the
examples presented below will deal specifically with the relaxation
mechanisms that limit the polarization of .sup.129Xe. The
relaxation rate F may be written [9]:
.GAMMA.=.GAMMA..sub.t+.GAMMA..sub.p+.GAMMA..sub.g+.GAMMA..sub.w,
where .GAMMA..sub.i=.GAMMA..sub.t+.GAMMA..sub.p is the intrinsic
rate due to the sum of contributions from transient and persistent
Xe.sub.2 dimers; and .GAMMA..sub.e=.GAMMA..sub.g+.GAMMA..sub.w is
the extrinsic rate due to the sum of contributions from atomic
diffusion through gradients in the applied magnetic field [10, 11]
and interactions with the cell surface (wall relaxation). In most
cases involving SEOP of xenon, some combination of .GAMMA..sub.p
and .GAMMA..sub.w dominates the relaxation. For xenon densities as
low as 0.1 amagat, .GAMMA..sub.g is usually negligible [9],
although there is size limitation for hyperpolarized-xenon storage
cells in a given Helmholtz geometry due to this mechanism (see Sec.
4.5). For xenon densities .apprxeq.1 amagat and larger,
.GAMMA..sub.t sometimes makes a small but non-negligible
contribution to the total relaxation rate. Based on the present and
previous work [9, 12, 13], we have developed a semi-empirical
formula for the intrinsic relaxation rate l.sub.i of .sup.129Xe as
a function of xenon density [Xe] (amagats), temperature T (Kelvin),
applied magnetic field B.sub.0 (Tesla), and gas composition. This
formula applies for [Xe]>0.3 amagat at all reasonable values of
B.sub.0:
.GAMMA. i - 1 = [ Xe ] 56.1 h ( T 0 T ) 1 / 2 + 1 4.59 h ( T 0 T )
2 [ 1 + ( 3.65 .times. 10 - 3 ) B 0 2 ] ( 1 + r [ B ] [ Xe ] ) ,
##EQU00002##
where the first teen is clue to persistent dimers and the second is
due to transient dimers; T.sub.0=293 K, [B] is the density of a
second gas in the mixture, and r.ident.k.sub.B/k.sub.Xe is the
ratio of the persistent-dimer breakup coefficient for the second
gas to that for xenon. We have measured r=0.51 for nitrogen, which,
along with helium, is most often present with xenon in SEOP
situations. For helium, Chann, et al. have measured r=0.25. The
transient-dimer term in Eq. (3) is based on the results of
Moudrakovski, et al. [13]; we have estimated its temperature
dependence by considering that, in the weak-interaction limit, the
probability for a spin transition is approximately proportional to
the rate of binary collisions and to the square of the collision
duration. Hence, we should have .theta., .varies.1/.nu., where
.nu..varies.T.sup.1/2 is the mean thermal velocity of the xenon
atoms. The uncertainty in the relaxation time calculated from Eq.
(3) is about 10%.
[0007] Longitudinal relaxation also plays a key role in the
accumulation and storage of hyperpolarized gasses. Storage times of
several hours or more are directly relevant to applications such as
magnetic resonance imaging (MRI), where the gas must often be
transported to the MRI scanner with minimal polarization loss. In
the case of .sup.129Xe, a relatively long longitudinal relaxation
time T.sub.1.ident..GAMMA..sup.-1 is also important for the
accumulation stage in a flow-through xenon polarizer [14, 15], the
current state-of-the-art scheme for the versatile production of
liter-quantities of highly polarized .sup.129Xe for any
application. In these devices, a gas mixture lean in xenon is
passed continuously through a glass cell, in which it is polarized
by SEOP with a laser, and subsequently frozen as a polycrystalline
solid at 77 K in a liquid-nitrogen trap. This basic scheme has
proven effective in dealing with the inherently low (7%) Rb--Xe
spin-exchange efficiency. i.e., the rate at which angular momentum
is transferred to the noble-gas nucleus divided by the rate at
which it is lost by the alkali-metal atoms [16]. The source of this
low efficiency is the strong spin-rotation interaction of the
rubidium valence electron with the electron cloud of the xenon
atom, whereby P.sub.A begins to plummet for xenon densities
[Xe]>1 amagat. Hence, .sup.129Xe (unlike .sup.3He) is not
readily polarized in large batches at high density. Cryogenic
accumulation of xenon as it flows out of the polarizing cell serves
two purposes. First, it separates out the other gasses, typically
nitrogen and helium, making it possible to prepare pure xenon
samples. Second, since most or all of the polarization survives the
phase transition [15, 17], large quantities of hyperpolarized xenon
can be accumulated from the low-density flow and stored for times
on the order of T.sub.1.apprxeq.2.5 h at 77 K in an applied
magnetic field B.sub.0.gtoreq.0.1 T [18] before being
revolatilized. This method evolved, in part, because of the
reliable 2.5 h storage time, although it became clear in later work
the gas must be quickly and completely frozen to 77 K [15]; at
higher temperatures, particularly those approaching the xenon
melting point (161 K), relaxation rates increase dramatically due
to vacancy diffusion in the solid [19], resulting in polarization
losses in the freeze/thaw cycle.
[0008] Hyperpolarized .sup.129Xe is now used for research variety
of disciplines such as medical imaging, biological assays, and pore
characterization [1A, 2A, 3A]. Many of these experiments require
on-demand production of large (liter-per-hour and more) quantities
of hyperpolarized .sup.129Xe. The state-of-the-art method for such
production is the flow-through polarizer/accumulator [4A, 5A]. One
manufacturer claims the ability to produce 10 L/h of hyperpolarized
xenon [6A]. These techniques require diluting Xe to a small percent
of gas mixture, usually of order 1%.
[0009] Presently, the only method for separating the hyperpolarized
Xe from the other buffer gasses is using cryogenic freezing. This
method capitalizes on the high Xe melting temperature (161 K)
compared with other gasses in the mixture. Xenon freezes out of the
gas stream as a polycrystalline solid and deposits in some holding
cell; the hyperpolarization generally survives the phase
transition. The cryogenic cell is also used to store the Xe, as the
relaxation time of Xe at 77 K in an applied field of 2 kG is of
order 2 hours [7A]. The xenon can be accumulated and stored as a
solid for about this amount of time before it is revolatilized and
used as a gas in an experiment or application.
[0010] Cryogenic separation is disadvantageous because it is a
stepped method. One must accumulate Xe for some amount of time from
a flow through polarizer and then divert or stop the flow when
ready to volatilize the solid. It would advantageous for a number
of experiments to have the ability to separate the Xe continuously,
so that a steady stream of pure hyperpolarized Xe could be directed
to an experiment or application. Further disadvantantages of cryo
separation are discussed below.
SUMMARY OF THE INVENTION
[0011] Techniques are described for storing large quantities of
hyperpolarized (HP) .sup.129Xe gas. In various embodiments, an
apparatus may include a large (10 cm diam or larger) valved glass
container (cell), the interior of which is coated with a silicone
or paraffin-like compound to inhibit longitudinal relaxation of the
.sup.129Xe nuclei. The cell contains no alkali-metal. The cell sits
in a modest magnetic field (about 3 millitesla) generated by a
Helmholtz coil pair. The cell is designed to receive HP xenon gas
from a current state-of-the-art device, a .sup.129Xe flow-through
polarizer/accumulator based on the established method of spin
exchange optical pumping, whereby laser light and an alkali-metal
vapor are used to transfer spin angular momentum to .sup.129Xe in
the gas phase. The inventors have developed a thorough
understanding of gas-phase relaxation of .sup.129Xe nuclei in the
presence of other xenon atoms (intrinsic relaxation), as well as
due to collisions with the cell wall (extrinsic relaxation). Part
of this understanding is that the wall relaxation rate scales as
the surface-to-volume ratio of the cell: larger spherical cells
have slower relaxation rates. Cells may be produced in which the
storage lifetime of the HP xenon gas is 2-3 times longer than the
current state-of-the-art storage method and requires no cryogenic
freezing of the xenon or associated large magnetic fields.
Moreover, such cells may be used in conjunction with gas centrifuge
separators to provide pure hyperpolarized xenon without need for
cryogenic separation.
[0012] In one aspect, a storage apparatus is disclosed for
non-cryogenically storing gaseous spin polarized .sup.129Xe
including: a storage vessel including an interior surface
substantially surrounding a storage volume; and a magnet which
produces a substantially uniform magnetic field within the storage
volume; where the interior surface is characterized in that the
longitudinal spin relaxation rate of the gaseous spin polarized
.sup.129Xe due to interactions with the interior surface is about
equal to or less than the longitudinal spin relaxation rate of the
gaseous spin polarized .sup.129Xe due to intrinsic mechanisms.
[0013] Some embodiments include a heater for maintaining the
storage vessel at a temperature greater than room temperature. In
some embodiments, the heater is configure to maintain the storage
vessel at a temperature greater than about 100 degrees
centigrade.
[0014] In some embodiments, the magnet includes a pair of coils in
the Helmholtz configuration.
[0015] In some embodiments, the interior surface consists of a
layer of material substantially free of alkali-metal.
[0016] In some embodiments, the vessel includes glass, and the
interior layer is on the glass. In some embodiments, the interior
layer includes a silane- or siloxane-based coating.
[0017] In some embodiments, the vessel includes a plastic material
and the interior surface consists of the plastic material. In some
embodiments, the plastic material includes Teflon.
[0018] In some embodiments, the ratio of the area of the interior
surface to the storage volume is less than about 1 cm.sup.-1. In
some embodiments, ratio of the area of the interior surface to the
storage volume is less than about 0.5 cm.sup.-1.
[0019] In some embodiments, the substantially uniform magnetic
field within the storage volume has a magnitude of about 3
milliTesla or less.
[0020] In some embodiments, the storage vessel is characterized by
a relaxation time for the gaseous spin polarized .sup.129Xe of
greater than about five hours, the relaxation time corresponding to
a density of the spin polarized .sup.129Xe of greater than about
one amagat.
[0021] In some embodiments, the storage vessel is characterized by
a relaxation time for the gaseous spin polarized .sup.129Xe of
greater than about seven hours, the relaxation time corresponding
to a density of the spin polarized .sup.129Xe of greater than about
one amagat.
[0022] In another aspect, a system is disclosed for producing and
storing gaseous spin polarized .sup.129Xe including: a polarizer
configured to produce gaseous spin polarized .sup.129Xe; and a
storage apparatus for non-cryogenically storing gaseous spin
polarized .sup.129Xe as described above. The storage apparatus is
in communication with the polarizer to receive and store the spin
polarized .sup.129Xe.
[0023] In some embodiments, the polarizer is a spin exchange
optical pumping polarizer. In some embodiments, the polarizer
includes one or more volumes in which Xe is in the presence of
alkali-metal, and the storage apparatus stores the gaseous spin
polarized .sup.129Xe received from the polarizer in a substantially
alkali-metal free environment.
[0024] Some embodiments include: a gas centrifuge separator. The
separator is in communication with the polarizer to receive a
mixture of gaseous spin polarized .sup.129Xe and other gasses from
the polarizer. The separator is configured to separate
substantially pure gaseous spin polarized .sup.129Xe from the
mixture. The storage apparatus is in communication with the
separator to receive and store the substantially pure gaseous spin
polarized .sup.129Xe.
[0025] In some embodiments, the substantially pure gaseous spin
polarized .sup.129Xe is at least about 90% pure. In some
embodiments, the substantially pure gaseous spin polarized
.sup.129Xe is substantially free of alkali-metal.
[0026] In another aspect, a method of non-cryogenically storing
gaseous spin polarized .sup.129Xe is disclosed including: providing
storage vessel including an interior surface substantially
surrounding a storage volume; providing a substantially uniform
magnetic field within the storage volume; and introducing gaseous
spin polarized .sup.129Xe into the storage volume. The interior
surface is characterized in that the longitudinal spin relaxation
rate of the gaseous spin polarized .sup.129Xe due to interactions
with the interior surface is about equal to or less than the
longitudinal spin relaxation rate of the gaseous spin polarized
.sup.129Xe due to intrinsic mechanisms.
[0027] Some embodiments include maintaining the storage vessel at a
temperature greater than room temperature. In some embodiments, the
temperature greater than room temperature is greater than 100
degrees centigrade or greater than 200 degrees centigrade, or
more.
[0028] In some embodiments, introducing gaseous spin polarized
.sup.129Xe into the storage volume includes polarizing gaseous
.sup.129Xe in a polarizer to produce gaseous spin polarized
.sup.129Xe and transferring the gaseous spin polarized .sup.129Xe
to the storage vessel.
[0029] In some embodiments, transferring the gaseous spin polarized
.sup.129Xe to the storage vessel includes: passing a mixture of
gaseous spin polarized .sup.129Xe through one or more gas
centrifuge separators to produce substantially pure gaseous spin
polarized .sup.129Xe; and introducing the substantially pure
gaseous spin polarized .sup.129Xe into the storage volume. In some
embodiments, the centrifuge is configured separate polarized xenon
without substantially destroying the polarization.
[0030] Various embodiments may include any of the above described
features, either alone or in combination.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1A is a schematic of a cell for non-cryogenic storage
of gaseous spin polarized .sup.129Xe.
[0032] FIG. 1B a schematic of a system for producing and storing
gaseous spin polarized .sup.129Xe.
[0033] FIG. 1C is a flow diagram of a process for producing and
storing gaseous spin polarized .sup.129Xe.
[0034] FIG. 1 is a plot of persistent dimmer relaxation rate versus
total gas density.
[0035] FIG. 2 shows a plot of 2K(M.sup.sr+M.sup.csa) extracted from
the fits in FIG. 1 (see Table I) vs. the square of the applied
magnetic field B.sub.0.
[0036] FIG. 3 is a plot of the .sup.129Xe persistent-dimer
relaxation rate .GAMMA..sub.p at 8.0 T vs. 1/T.sup.2.
[0037] FIG. 4 is a plot of NMR signal intensity vs. time for cell
113B at room temperature in an applied field of 14.1 T.
[0038] FIG. 5 shows a plot of .GAMMA..sub.w vs. B.sub.0 at room
temperature.
[0039] FIG. 6 shows pressure profiles in stages of the
centrifugation process, for 1 stage (A), 3 stages (B), 5 stage(C),
and 8 stages (D) of centrifugation.
[0040] FIG. 7 shows the time evolution of the concentration of Xe
gas in a cylinder.
[0041] FIGS. 8a-8g are photographs showing exemplary storage
cells.
DETAILED DESCRIPTION
[0042] FIG. 1A shows a storage cell 100 or non-cryogenically
storing gaseous spin polarized.sup.129. Cell 100 includes a storage
vessel 102 including an interior surface 104 substantially
surrounding a storage volume 106. Storage volume 106 may be
accessed using valve 107.
[0043] A magnet 108 produces a substantially uniform magnetic field
within the storage volume 106. As shown, the magnet 108 is an
electromagnet which includes a pair of coils in the Helmholtz
configuration, driven by power source 110. In other embodiments,
and suitable type of magnet may be used including e.g. a permanent
magnet or an electromagnet in another configuration (e.g. a
solenoid surrounding all or a portion of vessel 102). In some
embodiments, the substantially uniform magnetic field within the
storage volume has a magnitude of about 3 milliTesla or less, or
about 1 milliTesla or less.
[0044] As described in detail below, interior surface 104 is made
of a material which inhibits longitudinal spin relaxation caused by
interactions (e.g. collisions) between the .sup.129Xe and the
surface. For example, in some embodiments, surface 104 is
characterized in that the longitudinal spin relaxation rate of the
gaseous spin polarized .sup.129Xe stored in volume 106 due to
interactions with the interior surface is about equal to or less
than the longitudinal spin relaxation rate of the gaseous spin
polarized .sup.129Xe due to intrinsic mechanisms.
[0045] In some such embodiments, the interior surface 104 is wholly
or partially made up of material which is substantially free of
alkali-metal. For example the vessel 102 may be made of glass, and
the interior surface 104 may be material layer on the glass. In
some embodiments, the material layer includes a silane- or
siloxane-based coating. Suitable coatings may be provided using any
techniques know in the art.
[0046] In some embodiments, the vessel 102 is made of an
alkali-metal free plastic material (e.g. Teflon) and the interior
surface 104 consists of this plastic material (e.g. no coating is
provided on the layer).
[0047] Some embodiments of cell 100 include a heater 112 for
maintaining the storage vessel at a temperature greater than room
temperature. The heater 112 may operate to heat vessel 102 using
any suitable method including contact heating, convection heating,
radiative heating, etc. Heater 112 may include a control system,
e.g. a thermostat for maintaining a set temperature. In some
embodiments, the heater 112 maintains the storage vessel at a
temperature, e.g., greater than room temperature, or greater than
about 100 degrees centigrade, 200 degrees centigrade, 300 degrees
centigrade, or more.
[0048] Although, as shown, the vessel 102 is a spherical bulb, in
various embodiments the vessel 102 may be formed as any suitable
shape. As described in detail below, it is advantageous to minimize
the ratio of the area of the interior surface 104 to the storage
volume 106. For example. In some embodiments, the ratio of the area
of the interior surface to the storage volume is less than about 1
cm.sup.-1, less than about 0.5 cm.sup.-1, or even less. FIGS. 8a
through 8g show exemplary vessels having various dimensions.
[0049] Cell 100 can be used for long term, non-cryogenic storage of
gaseous spin polarized .sup.129Xe. As describe in detail below, the
use of an alkali free interior surface and applied the magnetic
field reduces extrinsic relaxation, and allows for long storage
times. For example, in some embodiments, the storage vessel is
characterized by a relaxation time for the gaseous spin polarized
.sup.129Xe of greater than about five hours, greater than about
seven hours, or even more at a of greater than about one amagat or
more.
[0050] Referring to FIG. 1B, a system 200 may be used for producing
and storing gaseous spin polarized .sup.129Xe. System 200 includes
a polarizer 202 which operates on gas mix 203 to produce gaseous
spin polarized .sup.129Xe. In some embodiments, the polarizer 202
is a spin exchange optical pumping polarizer, e.g. of the type
described in detail below.
[0051] Spin polarized .sup.129Xe is transferred from the polarizer
to a storage cell 100 of the type described above for
non-cryogenically storage. Thus, the storage cell 100 is in
communication, directly or indirectly, with the polarizer 202 to
receive and store the spin polarized .sup.129Xe. The polarizer may
include one or more volumes in which Xe is in the presence of
alkali-metal, while storage cell 100 stores the gaseous spin
polarized .sup.129Xe received from the polarizer in a substantially
alkali-metal free environment. Any suitable system, e.g. system of
valves and transfer chambers, may be employed to transfer the spin
polarized .sup.129Xe from the polarizer 202 to the storage cell 100
while maintaining the alkali free environment of the cell 100.
[0052] Some embodiments optionally include separator 204, which may
be a gas centrifuge separator. The separator 204 is in
communication with the polarizer 202 to receive a mixture of
gaseous spin polarized .sup.129Xe and other gasses from the
polarizer 202. The separator 204 separates substantially pure
gaseous spin polarized .sup.129Xe from the mixture. As described in
detail below, the separator 204 may operate to effect the
separation without substantially reducing the spin polarization of
the polarized .sup.129Xe. For example, in some embodiments, the
separator 204 may be constructed with substantially alkali-metal
free inner surfaces to reduce extrinsic relaxation resulting from
collisions of the gas with these surfaces.
[0053] The storage cell 100 is in communication with the separator
202 to receive and store the substantially pure gaseous spin
polarized .sup.129Xe. For example, the substantially pure gaseous
spin polarized .sup.129Xe may be at least about 80%, about 90%
pure, about 95% pure, about 99% pure, or more. In some embodiments,
the substantially pure gaseous spin polarized .sup.129Xe
transferred to cell 100 is substantially free of alkali-metal.
[0054] FIG. 1 C shows a flow diagram illustrating steps for a
process 300 of polarizing and storing .sup.129Xe using the system
200. In step 301, polarizer 202 receives an un-polarized gas mix
203 containing Xe. In step 302, polarizer 202 polarizes at least
portion of the gas mix to produce gaseous spin polarized
.sup.129Xe. In optional step 303, separator 204 separates the
gaseous spin polarized .sup.129Xe from other gasses present in the
mix 203. In step 304, the gaseous spin polarized .sup.129Xe is
stored in storage cell 100. Step 304 may include the following
substeps. In substep 304a, the gaseous spin polarized .sup.129Xe is
contained in the alkali-metal free environment of vessel 102. In
substep 304b and 304c, a desired magnetic field and temperature is
maintained in the vessel 102.
[0055] While not intending to be bound by theory, the following
provides additional detail regarding devices and techniques for
non-cryogenic storage of gaseous spin polarized .sup.129Xe.
[0056] Accumulation and storage of hyperpolarized xenon near room
temperature in the gas phase is desirable in that it would
eliminate the need for large magnetic fields, the cryogenic
apparatus, and freeze/thaw cycles. The historical problem with this
approach has been that .sup.129Xe gas-phase relaxation is
relatively fast and notoriously irreproducible, whereby wall
relaxation plays a crucial role. Some progress was made in
understanding wall interactions, particularly in cells treated with
silane- or siloxane-based surface coatings in fields on the order
of 1 mT [20, 21], where T.sub.1=20-60 min was observed. Others
observed T.sub.1>3 h for some coated cells at 9.4 T, an
indication that wall relaxation is suppressed at high field [13,
22]. These studies all had in common relatively small cells (1-3 cm
dia.) that contained macroscopic amounts of rubidium along with the
coating, meaning that the gas was polarized by SEOP in the same
cell in which T.sub.1 was subsequently measured. While it is well
known that .sup.3He relaxation on uncoated glass is reliably
suppressed by the presence of alkali metal [23, 24, 25], this is
apparently not the case for .sup.129Xe, where in fact, the
interaction of the alkali metal with the surface coating,
particularly when heated to 100.degree. C. or more during SEOP can
lead to erratic and generally increasing relaxation rates [9]. Wall
relaxation in xenon cells is not relaxation-site limited at the
usual SEOP densities, i.e., xenon atoms are not inhibited from
interacting with wall sites due to their occupation by other xenon
atoms. Hence, in the regime for which the wall contribution to
T.sub.1 is long compared to the mean time for a xenon atom to
diffuse across the cell (easily realized in all of our experiments
and most others), the wall-relaxation rate is independent of [Xe]
and depends linearly on the surface-to-volume ratio S/V.
Accordingly larger-diameter coated cells containing no alkali metal
may be used as a way of reducing the .sup.129Xe gas-phase
wall-relaxation rate.
[0057] Gas-phase .sup.129Xe relaxation due to persistent Xe.sub.2
dimers has been shown[12]. These van der Waals molecules are formed
in three-body collisions and have a mean lifetime
.tau..sub.p.about.1 ns [12, 9] before being destroyed by another
collision. The maximum relaxation time for a pure xenon sample due
to this mechanism alone was shown to be .apprxeq.4 h and
independent of [Xe] for low applied magnetic field B.sub.0 (a few
millitcsla). The density independence arises both because the
fraction of xenon atoms bound in molecules and the molecular
formation/breakup rate .tau..sub.p.sup.-1 have the same linear
dependence on [Xe], and because the fast-fluctuation limit
.OMEGA..sup.2.tau..sub.p.sup.2<<1, where .OMEGA. is the
.sup.129Xe Larmor frequency, holds for all reasonable values of
[Xe] and B.sub.0<1 T; see Eq. (4) below. This density
independence effectively mimics wall relaxation, and it has
undoubtedly confounded some earlier work in measuring
.GAMMA..sub.w, particularly since the the minimum intrinsic rate
.GAMMA..sub.p is much larger than previously believed [13, 26]. We
have verified and extended this work at low [Xe] and B.sub.0=8.0 T,
which straddles the fast- and slow-fluctuation regimes. We showed
that persistent-dimer relaxation is strongly suppressed at this
field for sufficiently low xenon densities 0.1 amagat) and large
magnetic fields. Indeed, we observed extraordinarily small
gas-phase relaxation rates in our alkali-metal-free, coated cells,
with measured T.sub.1's exceeding 25 h in some cases.
[0058] Increased understanding of gas-phase relaxation of
.sup.129Xe allowns for significant improvements in cell performance
vis-a-vis hyperpolarized gas production, accumulation, storage, and
transport for the various applications. We have extended our study
of this relaxation to a wide range of applied magnetic fields and
temperatures, with an eye towards a large-diameter (.gtoreq.20 cm)
coated cell that could store several liters of hyperpolarized xenon
with a T.sub.1.gtoreq.7 h in an applied magnetic field of
.apprxeq.3 mT, tripling the storage time of solid xenon at 77 K and
eliminating the need for high-field cryogenic accumulation. The
work is divided into three main parts described herein:
[0059] (1) The study of the magnetic suppression of the
persistent-dimer mechanism in a range of magnetic fields from 1.5 T
to 14.1 T. This data allowed us to deduce the relative strength of
the spin-rotation (SR) and chemical-shift-anisotropy (CSA)
interactions via the B.sub.0.sup.2-dependence of the CSA
contribution. This, in turn, generates an independent estimate for
the maximum low-field pure-xenon relaxation time of T.sub.1=4.6
h.
[0060] (2) The study of wall relaxation over the same range of
B.sub.0 and further on down to 3 mT. This is made possible by a
thorough understanding of the persistent-dimer mechanism with which
wall relaxation often competes. Wall-relaxation times in our
alkali-metal-free coated cells varied from .apprxeq.10 h at 3 mT to
>100 h at 14.1 T, suggesting a high-field decoupling of a wall
mechanism that has to do with interactions of .sup.129Xe atoms with
unpaired electrons at the surface or inside of the coating
[20].
[0061] (3) The study of the temperature dependence of the
persistent-dimer rate .GAMMA..sub.p in the fast-fluctuation limit
in the range of 20-100.degree. C. The inverse-square dependence of
.GAMMA..sub.p on temperature T is consistent with our theoretical
model and results in an increase of .apprxeq.60% in the relaxation
time due to persistent dimers at 100.degree. C. compared to room
temperature.
[0062] Intrinsic longitudinal relaxation of .sup.129Xe gas in the
SEOP regime of pressure and temperature is dominated by the SR [27]
and CSA [28, 29] interactions modulated by the formation and
breakup of persistent Xe.sup.2 dimers in three-body collisions. The
theory is discussed in detail in Refs. [9, 12]. In brief, the
persistent-dimer relaxation rate is given by
.GAMMA. p = ( 2 K [ Xe ] ) ( M sr + M csa ) ( .tau. p 1 + .OMEGA. 2
.tau. p 2 ) , ##EQU00003##
where K.ident.[Xe.sub.2]/[Xe].sup.2 is the chemical equilibrium
coefficient, M.sup.sr and M.sup.csa are the interaction strengths
(second moments) of the SR and CSA interactions, respectively, and
.tau..sub.p.sup.-1 is the molecular formation rate (equal to the
breakup rate in chemical and thermal equilibrium). This equation
can be reparameterized and added to the wall relaxation rate
.GAMMA..sub.w to obtain for the total relaxation rate:
.GAMMA. ( [ G ] ) = .GAMMA. w + 2 K ( M sr + M csa ) ( .alpha. k
.alpha. [ G ] 2 k .alpha. 2 [ G ] 2 + .OMEGA. 2 ) ,
##EQU00004##
where [G] is the total gas density, .alpha..ident.=[Xe]/[G] is the
xenon concentration, and k.sub..alpha. is the molecular breakup
coefficient for the particular gas composition. In this work,
nitrogen is the only other gas in the mixture, and
1 .tau. p = k .alpha. [ G ] = k Xe [ Xe ] + k N [ N 2 ] ,
##EQU00005##
where k.sub.Xe and k.sub.N are the breakup coefficients for xenon
and nitrogen as third bodies, respectively.
[0063] At high gas densities, in the fast-fluctuation limit
.OMEGA..sup.2<<k.sub..alpha..sup.2.left brkt-top.G.right
brkt-bot..sup.2 of Eq. (5), the persistent-dimer relaxation rate is
independent of [G] for a given gas composition, as first observed
by Chann, et al. [12] for B.sub.0=2.0 mT and also by our group for
B.sub.0=8.0 T [9]. At lower densities the rate is suppressed due to
the increasing relevance of the .OMEGA..sup.2 term in Eq. (5).
Whereas M.sup.st is independent of the applied field B.sub.0 [27],
M.sup.csa is proportional to B.sub.0.sup.2. Hence, acquiring a set
of relaxation curves as a function of [G] that are fitted to Eq.
(5), where each curve is at one of several values of B.sub.0,
allows M.sup.sy to be separated from M.sup.csa.
[0064] The temperature dependence to .GAMMA..sub.p comes
predominantly through chemical equilibrium coefficient K and the
mean persistent-dimer lifetime .tau..sub.p in Eq. (4). The chemical
equilibrium coefficient is given by [30]
K = 1 2 ( h 2 2 .pi. .mu. kT ) 3 / 2 Z , ##EQU00006##
where h is the Planck constant, k is the Boltzmann constant,
Z=.SIGMA..sub.i(2N.sub.i+1)e.sup.-E.sup.i.sup./kT is the partition
function for the internal ro-vibrational states of the Xe.sub.2
dimer, and .mu. is its reduced mass. The portion of this expression
that multiplies Z is the ratio of translational partition function
for a single dimer to that for two free atoms in the classical
high-temperature and low-density limit. We neglect here the weak
temperature dependence of Z at room temperature and above, where
T>E.sub.i/k.apprxeq.280 K [31]. Classically, .tau..sub.p is
inversely proportional to the mean relative velocity of the gas
molecules, which is proportional to T.sup.1/2. We treat here only
the fast-fluctuation limit,
.OMEGA..sup.2.tau..sub.p.sup.2<<1, relevant to high-density
xenon storage cells in small magnetic fields. Since the product
K.tau..sub.p appears in this limit, we expect the relaxation rate
.GAMMA..sub.p to depend on T.sup.-2. We have ignored here any
temperature dependence of the collisional cross sections or of the
interaction strengths M.sup.sr and M.sup.csa.
[0065] As will be understood by those skilled in the art, some of
the experimental procedures described here are related to those
described in detail in our previous work [9]. Some of the
measurements of the longitudinal relaxation time T.sub.1 for
.sup.129Xe in xenon gas were done in a single borosilicate-glass
(Pyrex) "measurement" cell, designated 113B shown in FIGS. 8d-e. It
is a 6.7 cm diam sphere connected via a 10 cm length of capillary
(0.5 mm diam) to a glass valve and sidearm used for evacuation and
refilling. A 4 cm length of 6 mm glass tubing (the stem) extends
from the sphere opposite the capillary entrance. The cell contains
no alkali-metal, but the interior was coated with
dimethyldichlorosilane, which inhibits wall relaxation in a manner
similar to silicone coatings previously introduced [20, 21].
[0066] Hyperpolarized xenon was generated in one of several
"pumping" cells, which have a geometry similar to the measurement
cells and also contain Rb metal for SEOP. Our high-vacuum
gas-handling system [32] is used to measure cell volumes, evacuate
cells, and to refill pumping cells with a precise mixture of xenon
(isotopically enriched to 86%; Spectra Gasses, West Branchburg,
N.J.) and nitrogen. Unless otherwise noted, the xenon concentration
.alpha.=0.91.+-.0.02 throughout this work, where the error reflects
variation in multiple preparations of the mixture in the pumping
cells. The effects of varying a are consistent with the theory
presented above and were studied previously [9, 12].
[0067] Xenon gas, polarized by SEOP to 10% in a pumping cell was
then transferred (at the known value of .alpha.) to the measurement
cell using a glass transfer manifold and mechanical vacuum pump for
evacuating dead space. In the case of the 1.5 T and 8.0 T fields,
the cell was immediately inserted into a NMR probe and the probe
assembly was inserted into the magnet. In the case of the 4.7 T and
14.1 T fields, the polarized measurement cell was transported in a
portable 2 mT battery-powered solenoidal coil to an NMR facility.
(Less than 10% of the magnetization was lost during transport.) All
magnets (with the exception of the 1.5 T magnet) had a wide-bore
(89 mm diam) vertical configuration. The probes were capacitively
tuned saddle coils (one to two turns) placed along the stem of the
cell; the respective resonance frequencies corresponded to the
.sup.129Xe gyromagnetic ratio of 11.8 MHz/T. In the 1.5 T field
(provided by a 30 cm diam horizontal-bore imaging magnet), the cell
was situated horizontally at the magnet isocenter with a
surface-coil probe placed underneath it.
[0068] NMR measurements were conducted with an Aries (Tecmag)
spectrometer with a homebuilt rf section (1.5 T and 8.0 T),
Chemagnetics CMX200 (4.7 T), and Varian
[0069] Infinityplus 600 (14.1 T). For measurements above
room-temperature, the 8.0 T probe was insulated and heated with air
flowing across a filament heater located away from the magnet. In
addition, several low-field (B.sub.0.apprxeq.3 mT) measurements
were made using a homebuilt low-frequency spectrometer [33],
whereby the cell and NMR probe were placed in a oven (similarly
heated with flowing hot air) located at the isocenter of a
Helmholtz pair. In all cases the longitudinal relaxation rate
.GAMMA. was measured by periodic acquisition of a free-induction
decay (FID) induced by a single rf pulse. A negligible fraction of
the magnetization was destroyed by each pulse. Either the height or
the area under the the peak of each Fourier-transfoinied FID was
plotted as a function of time and a least-squares fit was used to
extract .GAMMA..
[0070] The relaxation rate .GAMMA. was measured as a function of
total gas density [G] for the four different magnetic fields. The
data were fit in each case to Eq. (5) using the appropriate value
of the Larmor frequency .OMEGA., with the wall-relaxation rate F
the interaction-strength term 2K(M.sup.sr+M.sup.csa) extracted as
free parameters; see Table 1. Since the xenon concentration .alpha.
and, hence, the breakup coefficient k.sub..alpha. are
field-independent, the value
k.sub..alpha.=(3.54.+-.0.28).times.10.sup.-10 cm.sup.3/s,
[0071] was determined from a global fit to the four data sets, and
this value was then used as a fixed parameter for each of the fits
to the individual data sets.
[0072] FIG. 1 shows a plot of the room-temperature .sup.129Xe
persistent-dimer relaxation rate vs. total gas density for a fixed
xenon concentration .alpha.=0.912 at four different applied
magnetic fields. The wall relaxation rate 64 .sub.w and the product
2K(M.sup.sr+M.sup.csa) are extracted from fits of the measured
relaxation rates .GAMMA.([G]) to Eq. (5) for each field (see Table
1). The corresponding value of .GAMMA..sub.w has been subtracted
from all the data sets in this plot to show clearly the behavior of
the persistent-dimer rate .GAMMA..sub.p. The high-density
fast-fluctuation limit results in a density-independent relaxation
rate (asymptote) that increases with field due to the increasing
strength of the CSA interaction; the magnetic suppression of the
persistent-dimer mechanism with decreasing density starts at higher
densities and happens more gradually for higher fields. The
field-independent molecular breakup coefficient
k.sub..alpha.=(3.54.+-.0.28).times.10.sup.-10 cm.sup.3/s was
extracted from a global fit to all four data sets.
[0073] The plot in FIG. 1 shows the persistent-dimer rate
.GAMMA..sub.p=.GAMMA.-.GAMMA..sub.w plotted vs. [G] for all four
fields along with the respective best fits. The errors in the free
parameters were, in general, underestimated by our non-linear
fitting routines and had to be handled with some care. They were
determined for a given field and temperature by allowing
k.sub..alpha. to vary over its error range and observing the effect
in the fit on 2K(M.sup.sr+M.sup.csa) and .GAMMA..sub.w.
TABLE-US-00001 TABLE I Free parameters extracted from the fits of
the data shown in FIG. 1 to Eq. (5). Errors are given in
parentheses for the least significant figure(s). B.sub.0
2K(M.sup.sr + M.sup.csa) .GAMMA..sub.w (T) (10.sup.-14
cm.sup.3/s.sup.2) (10.sup.-6 s.sup.-1) 1.5 2.02(17) 18.5(9) 4.7
2.53(14) 4.1(5) 8.0 2.87(15) 3.70(8) 14.1 3.86(9) 1.60(19)
[0074] The effect of the CSA interaction is shown in FIG. 1 by the
monotonic increase of the asymptotic high-density rate with
increasing magnetic field. To determine the relative contributions
of the SR and CSA interactions, the interaction-strength parameter
2K(M.sup.sr+M.sup.csa) is plotted vs. the square of the applied
field B.sub.0 in FIG. 2. FIG. 2 shows a plot of
2K(M.sup.sr+M.sup.csa) extracted from the fits in FIG. 1 (see Table
I) vs. the square of the applied magnetic field B.sub.0. A linear
fit to the data yields the relative contributions of the SR and CSA
interactions as a function of B.sub.0, as given in Eqs. (9) and
(10), where the intercept is proportional to the field-independent
spin-rotation interaction strength M.sup.sr, which can then be used
to deduce the limiting low-field pure-xenon relaxation rate due to
persistent dimers.
[0075] The data are consistent with a linear B.sub.0.sup.2
dependence. The slope and intercept from a linear least-squares fit
yield, respectively,
2KM.sup.csa[(8.26.+-.0.73).times.10.sup.-17
cm.sup.3/s.sup.2T.sup.2]B.sub.0.sup.2,
2KM.sup.sr=(2.24.+-.0.10).times.10.sup.-14 cm.sup.3/s.sup.2.
[0076] The inset graph to FIG. 2 shows the fraction of the the
total interaction strength that is due to the SR interaction as a
function of B.sub.0; the SR and CSA interactions contribute equally
for B.sub.0.apprxeq.16.5 T. A correction to the empirical formula
based on this result appears as a factor in the persistent-dimer
term of the empirical formula in Eq.(3). Moudrakovski, et al. [13]
made a similar measurement at very high xenon densities (>30
amagat), in the transient-dimer regime, and found that the SR and
CSA interactions contribute equally for B.sub.0=12 T. Although our
measurement was made at much lower density in the persistent-dimer
regime, there is no apparent reason that the relative strength of
the two interactions should be different in the two cases.
[0077] The result in Eq. (10) can be used to calculate the
density-independent persistent-dimer relaxation rate for pure xenon
gas in the high-density low-field limit, where only the SR
interaction contributes; this is almost always the relevant regime
for SEOP. Here we follow the notation originally introduced by
Chann, et al. [12] for this characteristic limiting rate:
.GAMMA. vdW Xe = 2 KM sr k Xe . ##EQU00007##
[0078] We use the value of k.sub..alpha. in Eq. (8) and the value
of the nitrogen breakup coefficient
k.sub.N=(1.9+0.2).times.10.sup.-10 cm.sup.3/s measured in our
previous work [9] in Eq. (6) to calculate
k.sub.Xe=(3.70+0.31).times.10.sup.-10 cm.sup.3/s. This represents
only a small correction to our value of k.sub..alpha., since our
samples are over 90% xenon. Finally, using Eqs. (10) and (11), we
obtain
.GAMMA..sub.vdW.sup.Xe=(6.05.+-.0.57).times.10.sup.-5 s.sup.-1,
corresponding to a relaxation time of 4.59.+-.0.43 h, which is the
value that appears in the persistent-diener term of Eq. (3). This
value is in good agreement with 4.1 h measured by Chann, et al.
[12]. It is smaller than 5.45 h deduced from measurements in our
previous work [9]; however, most of this discrepancy can be traced
to using different data to calculate the relative contributions of
the SR and CSA interactions to the total interaction strength. Some
of our previous work was done at B.sub.0=8.0 T, where we took the
measured rate and divided it by the fraction of the interaction
strength that is due to the SR interaction in order to obtain a
value appropriate in the low-field limit. This fraction was
determined from the measurements of Moudrakovski, et al. [13], to
be 71% at 8.0 T. A similar calculation based on the data presented
here [see Eqs. (9) and (10)] yields an 81% contribution for the SR
interaction at 8.0 T, which would lower the relaxation time in our
previous work [9] to 4.8 h, in much better agreement with the
present result.
[0079] We performed a series of individual relaxation measurements
in temperature range 20-100.degree. C. at 8.0 T for [G]=0.35
amagat, well into the density-independent fast-fluctuation limit.
Relaxation due to transient dimers is negligible for this low
density, but the measured rates at all temperatures have been
corrected by subtracting the room-temperature wall-relaxation rate
at 8.0 T (see Table I). The higher-temperature points are likely
over-corrected, since .GAMMA..sub.W should become smaller at higher
temperatures due to decreasing residence time on the coating,
assuming that this time is governed by an Arrhenius relationship
[20]. However, the correction is small in any case, corresponding
to a relaxation time of .apprxeq.75 h, so we use it as a best
approximation at all temperatures.
[0080] FIG. 3 is a plot of the .sup.129Xe persistent-dimer
relaxation rate .GAMMA..sub.p at 8.0 T vs. 1/T.sup.2, where the
absolute temperature T ranges between 293 K and 373 K. The measured
rates were corrected by subtracting the relatively small
room-temperature wall-relaxation rate .GAMMA..sub.w. The quality of
the one-parameter fit forced through the origin indicates that this
simple inverse-square model for the temperature dependence of
.GAMMA..sub.p based on the arguments presented herein is reasonably
valid at and above room temperature.
[0081] The corrected data are plotted in FIG. 3 vs. the
inverse-squared absolute temperature. The one-parameter
least-squares linear fit to this data (forced through the origin)
supports the simple theory of a linear dependence of the
persistent-dimer relaxation rate on 1/T.sup.2, which comes from the
temperature dependence of the product K.tau..sub.p in the
fast-fluctuation limit of Eq. (4). The slope of fitted line is
6.2.+-.0.2 s.sup.-1K.sup.2 . The slope can be corrected for the
low-field limit by multiplying by 81%, the fraction of the
interaction strength due to SR at 8.0 T (see end of previous
section and FIG. 1). Using the corrected slope to calculate
.GAMMA..sub.p at T=293 K yields 5.85.times.10.sup.-5 s.sup.-1, in
good agreement with the minimum relaxation rate for pure xenon
given in Eq. (12). We performed these measurements at 8.0 T to
clearly separate .GAMMA..sub.p from any temperature-dependent wall
relaxation, but the results should be equally valid in the
low-field limit and contribute to longer overall relaxation times
at higher temperatures. Based on these results, we include the
factor of (T.sub.0/T).sup.2 in the persistent-dimer term of Eq.
(3), which predicts an intrinsic maximum T.sub.1 for pure xenon of
7.45 h at T=100.degree. C.
[0082] The extracted wall-relaxation rates .GAMMA..sub.w in Table I
decrease dramatically with increasing applied field. At 14.1 T, in
the low-density regime where the persistent-dimer rate is highly
suppressed, we measured T.sub.1=99.4 h for [G]=0.012 amagat. The
wall-relaxation time extracted from the fit is an extraordinary 174
h.
[0083] FIG. 4 is a plot of NMR signal intensity vs. time for cell
113B at room temperature in an applied field of 14.1 T. The cell
contains xenon at 12.0 mbar and nitrogen at 1.09 mbar. To our
knowledge, this is by far the longest gas-phase relaxation time
ever recorded for .sup.129Xe and results from the simultaneous
suppression of the intrinsic persistent-dimer mechanism and the
wall-relaxation mechanism at 14.1 T.
[0084] The plot of recorded NMR signal intensity vs. time in FIG. 4
and shows that the slope actually trends slightly downward over the
course of this measurement, corresponding to T.sub.1=105 h for the
first 50 h and T.sub.1=82 h for the last 45 h. This may have to do
with a gradual increase in oxygen concentration (due to very slow
outgassing or leakage) into the cell over the course of the long
measurement. If this gradual increase in relaxation rate were due
solely to collisions with paramagnetic oxygen atoms, it would
correspond to an oxygen partial pressure of .apprxeq.10.sup.-3 mbar
[34] developing over the course of the measurement.
[0085] FIG. 4: Plot of NMR signal intensity vs. time for cell 113B
at room temperature in an applied field of 14.1 T. The cell
contains xenon at 12.0 mbar and nitrogen at 1.09 mbar. To our
knowledge, this is by far the longest gas-phase relaxation time
ever recorded for .sup.129Xe and results from the simultaneous
suppression of the intrinsic persistent-dimer mechanism and the
wall-relaxation mechanism at 14.1 T.
[0086] FIG. 5 shows a plot of .GAMMA..sub.w vs. B.sub.0 at room
temperature. In an attempt to obtain a more complete picture of the
field-dependence of .GAMMA..sub.w, data were acquired for three
additional values of the applied field B.sub.0 made in an
electromagnet (0.91 T, and 2.0 T) and a Helmholtz pair (2.8 mT).
For these three data points, .GAMMA..sub.w was not extracted from a
fit. Instead, cell 113B was filled with nearly pure xenon (a from a
flow-through xenon polarizer (built in our laboratory) to a density
.apprxeq.1 amagat. In this density and magnetic-field regime, the
persistent dimer rate .GAMMA..sub.p=.GAMMA..sub.vdW.sup.Xe.
According to Eq. (2), .GAMMA..sub.w was then determined by
subtracting our deduced value of .GAMMA..sub.vdW.sup.Xe in Eq. (12)
from the measured rate for each of the three additional values of
B.sub.0.
[0087] Note that in FIG. 5 the points with small error bars are
extracted from the density-dependence curves shown in FIG. 1; the
weighted fit to Eq. (13) is almost entirely determined by these
points. The other points result from single measurements on pure
xenon in the fast-fluctuation limit, where the persistent dimer
relaxation rate
.crclbar..sub.vdW.sup.Xe=(6.05.+-.0.57).times.10.sup.-5 s.sup.1 has
been subtracted from the measured relaxation rate. The error
propagation from this subtraction leads to much larger error bars.
The fit yields a correlation time for the wall interaction of
.apprxeq.4 ns, consistent with interaction of .sup.129Xe with
fluctuating paramagnetic sites on or in the wall coating.
[0088] We model the high-field wall relaxation as
.GAMMA. w = M w ( .tau. c 1 + .OMEGA. 2 .tau. c 2 ) ,
##EQU00008##
where M.sup.w is the strength of the wall interaction and
.tau..sub.c.sup.31 1 is its correlation time, presumed to be due to
fluctuating paramagnetic spins at the surface. This is a simplified
version of the model proposed by Driehuys, et al. [20] based on the
expected field dependence of the relaxation due to the coupling of
the .sup.129Xe spin I with a wall spin S [35], which contains
additional terms in the power spectrum of Eq. (13) involving the
Larmor frequency of the spin S in addition to the .sup.129Xe Larmor
frequency SZ. In the range of applied field B.sub.0<10 mT
studied in that work, Driehuys, et al. [20] were able to fit their
relaxation data to a sum of two terms involving protons and
paramagnetic sites, respectively, as the spin S . They determined
that .sup.129Xe relaxes due to coupling with the protons in the
surface coating with an associated correlation time
.tau..sub.c.apprxeq.8 .mu.s. The proton-induced relaxation, which
was directly verified with a double-resonance experiment, cannot be
explained by a simple adsorption model; rather, xenon atoms must be
trapped within the coating for times .gtoreq.8 .mu.s. The second
term yielded a much shorter correlation time .tau..sub.c.apprxeq.8
ns, which is a reasonable relaxation time for paramagnetic surface
spins at room temperature.
[0089] For the much larger applied fields in our work, the
relaxation due to protons is completely suppressed. For relaxation
due to paramagnetic sites, the terms in the power spectrum
involving the paramagnetic resonance frequency are negligible, due
to the .apprxeq.10.sup.3 larger gyromagnetic ratio for electrons
compared to .sup.129Xe, leading to the simple form of Eq. (13). A
least-squares fit of the data to this functional form is also shown
in FIG. 4, and yields a correlation time .tau..sub.c.apprxeq.4 ns
(corresponding to a characteristic decoupling field .apprxeq.3 T),
in reasonable agreement with the predicted correlation time for
interaction with paramagnetic spins at the surface or inside of the
coating.
[0090] To explore the implications of the above results for a
practical low-field hyperpolarized-xenon storage cell at ambient
pressures, additional experiments were done at B.sub.0.apprxeq.3 mT
at both room temperature and T=100.degree. C. Again, the
flow-through xenon polarizer provided nearly pure xenon
(.alpha.=1), and cells were filled to a density .apprxeq.1 amagat.
We also used three additional alkali-metal-free coated cells. Two
of these (designated 105B and 113A) were similar in size to cell
113B; the other was also similar except that its diameter (12.7 cm)
is double that of the other cells. The cells all showed increases
in the measured relaxation time T.sub.1 of 50-100% at the elevated
temperature. Our results are summarized in Table II, which displays
measured relaxation times T.sub.1 and the inferred wall-relaxation
times based on subtracting from the measured rate both
.GAMMA..sub.p and .GAMMA..sub.t (the latter is a 10% correction at
most), as calculated from Eq. (3). It is difficult to extract
precise information concerning wall-relaxation times, particularly
at the elevated temperature, since .GAMMA..sub.p and .GAMMA..sub.w
are comparable at these low fields (unlike at B.sub.0=8.0 T) and
both decrease with increasing temperature (see Sec. 4.3 above).
However, it is clear that a significant improvement was realized
for the cell with larger S/V; the measured T.sub.1 in this cell of
5.75 h at T=100.degree. C. approaches our predicted limit of 7.45 h
and is a factor of two or more longer than any previously recorded
.sup.129Xe relaxation time in the low magnetic fields typical of
SEOP.
[0091] Table II shows low-field relaxation times (in hours) of four
cells at both room temperature and 100.degree. C. The first three
have a diameter .apprxeq.6.7 cm and were measured at B.sub.0=2.8
mT; the last cell has a diameter .apprxeq.12.7 cm and was measured
at B.sub.0=3.1 mT. The cells all contained pure xenon at the
indicated density (in amagats). Uncertainties are given in
parentheses for the least significant figure(s). The last two
columns show the room-temperature wall-relaxation time derived from
subtracting the relevant persistent- and transient-dimer rates [Eq.
(3)] from the measured rate. The elevated temperature increases the
measured T.sub.1 by 50-100%.
TABLE-US-00002 TABLE II T.sub.1 T.sub.1 T.sub.1 (wall) T.sub.1
(wall) Cell [Xe] 293 K 373 K 293 K 373 K 105B 1.5(1) 2.40(5)
3.66(11) 5.8(8) 8.7(1.1) 113A .apprxeq.1.5 1.30(4) 2.45(5) 1.9(1)
4.0(2) 113B 1.1(1) 2.57(15) 4.53(13) 6.6(1.3) 14.5(3.0) 139 0.7(1)
3.40(22) 5.75(23) 16(7) 35(18)
[0092] Even larger cells with a correspondingly larger xenon
storage capacity are possible.
[0093] In some embodiments, the size will eventually be limited by
magnetic field gradients far away from the center of a pair of
Helmholtz coils, but this limit is not terribly stringent for
xenon. As a guideline, we assume Helmholtz pair of radius (and
separation) R and a cell having radius no larger than R/3 . We have
estimated the gradient-induced relaxation for such a cell to be
[36]
.GAMMA. g .apprxeq. 0.01 D R 2 . ##EQU00009##
Although the calculation is done for an ideal Helmholtz geometry
(actual gradients might be larger), the estimate in Eq. (14)
applies only to the outer edge of a cell whose radius is as large
as R/3, and so remains fairly conservative for the entire cell. For
[Xe]=0.1 amagat at (a conservative estimate of the density during
the filling process), D=8.2.times.10.sup.-5 m.sup.2 /s at
100.degree. C. [37]. If we take R=0.50 m, a 0.33 m diam spherical
cell containing pure-xenon should have
.GAMMA..sub.g.sup.-1.gtoreq.85 h from the gradient mechanism alone;
this time would increase by an order of magnitude as the cell is
filled to 1 amagat. Such a cell would have a 19 L storage
capacity.
[0094] For completeness, we note that dilution of xenon with a
second gas lowers the rate .GAMMA..sub.p significantly for those
gasses that can form and break up persistent Xe.sub.2 dimers with
an efficiency comparable to Xe itself. Referring to Eq. (4), the
second gas decreases the persistent-dimer lifetime .tau..sub.p
without changing the fraction of xenon atoms bound in molecules.
The effects of adding a second gas were studied thoroughly by
Chann, et al.
[0095] [12] and in our previous work [9]. Nitrogen has the largest
breakup coefficient measured (besides xenon); .GAMMA..sub.p is
reduced by about one-third for a 50-50 mixture. We have included
the effects of a second gas in our semi-empirical formula for the
total intrinsic relaxation rate in Eq. (3).
[0096] In summary, we have presented a systematic study of both
intrinsic persistent-dimer relaxation and wall relaxation of
.sup.129Xe, including temperature and magnetic-field dependence; we
conclude that it should be possible to develop a xenon storage cell
that has a measured T.sub.1.gtoreq.7 h at 3.0 mT and 100.degree. C.
for pure xenon at densities up to a few amagats. These cells are
silicone-coated but alkali-metal-free and show relatively long and
robust wall-relaxation times of up to tens of hours. They can be
utilized in state-of-the-art flow-through xenon polarizers, whereby
storage times for polarized xenon can be increased by a factor of
three or more compared with state-of-the-art cryogenic schemes, and
cryogenic storage and associated freeze/thaw cycles can be
eliminated. We note that if producing pure hyperpolarized xenon is
required for a given experiment, then separation of xenon from
other gasses in the mixture (which comes naturally with cryogenic
accumulation) might be a limitation of the room-temperature
accumulation scheme proposed here. One approach would be to use the
cryogen for gas separation only, followed by immediate
volatilization and transfer to a storage cell. However, other
cryogen-free separation schemes are possible, as described below.
The use of a small gas centrifuge (on the order of 0.1 m diam) has
already been demonstrated for the continuous separation of methane
from CO.sub.2 on the time scale of minutes [38, 39]; such a device
utilizing suitable materials and/or a surface coating that does not
depolarize .sup.129Xe could presumably accomplish continuous
separation of xenon from the other much lighter gasses typically
found in a flow-through polarizer.
[0097] As noted above, non-cryogenic storage does not allow for
easy separation of .sup.129Xe from other gasses through cryogenic
solidification. However, Gas-centrifuge separation may be used in
conjunction with a non-cryogenic storage cell to provide
purification. This is a process where separation is brought about
by rotating the gasses at high speed. Gasses with higher molecular
weights are pushed to the walls of the centrifuge, while lighter
gasses remain in the center. This is usually done in a
continuous-flow mode. One typically flows the gas mixture through
several centrifuge stages in order to achieve desired separation.
Gas centrifuges have been used to separate uranium isotopes for use
in nuclear fission [8A]. Such separations are time intensive
because of the small mass separation between the two isotopes of
uranium. Gasses with greater mass ratio separate more easily.
[0098] Centrifuge devices are most effective when using an axial
countercurrent flow, whereby one gains both enhanced separation and
shorter equilibrium times [9A]. We present a simple centrifuge
model with no countercurrent flow for use in separating
hyperpolarized Xe from buffer gasses. Systems using axial
countercurrent flow would perform better than what is presented
here. The radial partial pressures of gasses in a centrifuge are
given by [10A]
p.sub.i=p.sub.i0e.sup.A.sup.i.sup.r.sup.2,
where p.sub.i is the partial pressure of the i.sup.th gas in the
mixture, p.sub.i0 is its pressure in the center of the centrifuge,
r is the radial distance from the center, and
A i = M i .omega. 2 2 RT , ##EQU00010##
where M.sub.i the molar mass of the i.sup.th gas in the mixture,
and .omega. is the angular speed of the centrifuge. It can be shown
that the relationship between the center pressure and the partial
pressure of the gas when not rotating is [10A]
p 0 i = A i R 2 A i R 2 - 1 p fi , ##EQU00011##
where R is the radius the centrifuge chamber.
[0099] Using these equations, we can determine the final gas
fraction profiles for a given set of initial gas partial pressures.
We apply this to a typical mixture of Xe in a flow-through
polarizer. The simulated centrifuge was spinning at
5.times.10.sup.4 RPM and had a radius of 10 cm. We simulated
removing the gas between 9 cm and 10 cm radius and injecting the
mixture into another centrifuge stage with the same parameters.
After a single stage, the Xe concentration is increased from 1% to
about 3%. After three stages, the concentration increased to 27%,
and after eight stages, the concentration has increased to 98%.
FIG. 6 shows some of the resulting pressure profiles in stages of
the centrifugation process. In particular, FIG. 6 shows the
normalized gas pressure for 1 stage (A), 3 stages (B), 5 stage(C),
and 8 stages (D) of centrifugation. The intial gas mixture is
composed of 1% Xe, 10% N.sub.2, and 89% He. Xe is in grey, N .sub.2
is in black, and He is in lighter grey.
[0100] In some embodiments, it is important to understand how long
the gas mixture will spend in each stage of the centrifuge so that
one can plan the volume of the stages and estimate losses in Xe
polarization due to relaxation. The gas mixture will quickly gain
angular momentum and establish a pressure gradient. Diffusion will
then establish the equilibrium concentration profile.
[0101] The diffusion equation for the heavy gas in two-component
system in a cylindrical centrifuge is given by [11A]
.differential. .differential. t ( Px RT ) + 1 r .differential.
.differential. r { P RT D [ ( A heavy - A light ) r 2 x ( 1 - x ) -
r .differential. x .differential. r ] } = 0. ##EQU00012##
This is a nontrivial partial differential equation typically
requiring numerical methods to approximate the solution. A simpler
approach is to have the initial conditions be the known equilibrium
profile of the rotating system and then use the non-rotating
diffusion equation to determine the time it takes for the system to
relax. It is reasonable to assume that these two processes take
place on similar time scales.
[0102] The diffusion equation for a non-rotating system is
.differential. x .differential. t - D ( .differential. 2 x
.differential. r 2 + 1 r .differential. x .differential. r ) = 0.
##EQU00013##
Using Comsol FEMLAB 3.1 diffusion package, we started with the
pressure profile given in Eq. 1a and allowed the system to relax to
equilibrium. Xenon was taken to have a uniform concentration of 1%
at equilibrium. FIG. 7 shows the time evolution of the
concentration of Xe gas in a 10 cm radius cylinder at r=9.9 cm. The
concentration profile in initially that of Xe in a centrifuge
spinning at 50000 rpm. The system relaxes with a characteristic
time on the order of .apprxeq.10 s and should be completely relaxed
in .apprxeq.60 s, comparable with numerical simulations done on
other gasses in similar centrifuge systems.
[0103] Centrifuge gas separation of hyperpolarized .sup.129Xe from
flow-through systems is a feasible alternative to cryogenic
separation. The above analysis indicates that one could reasonably
enrich an initial 1% Xe mixture to >90% purity using 8
centrifuge stages in about 8 minutes. In order to realize a
separator, one needs to find a material that is sufficiently strong
to take the stress of high speed rotation and has long enough wall
relaxation rates such that the polarized .sup.129Xe does not
appreciably decay. Alternatively, a suitable high-strength material
could be coated with a silane- or siloxane-based coating, such as
those used with glass polarization cells [12A, 13A], or with some
other suitable non-relaxivc coating.
[0104] A number of embodiments of the invention have been
described. Nevertheless, it will be understood that various
modifications may be made without departing from the spirit and
scope of the invention. For example, although the storage cells
described above are made of glass with an interior coating free of
alkali-metals, any material may be used which is characterized in
that the longitudinal spin relaxation rate of the gaseous spin
polarized .sup.129Xe due to interactions with the interior surface
(the wall rate) is about equal to or less than the longitudinal
spin relaxation rate of the gaseous spin polarized .sup.129Xe due
to intrinsic mechanisms. For example, plastic materials (e.g.
fluoropolymer plastics) such as Teflon of UItem 1000 should have a
wall rate comparable to or less than that of the coated glass
surfaces used in the examples above.
[0105] Although devices described in the examples above operate at
a defined temperatures (e.g. room temperature or 100.degree. C.),
other temperatures or temperature ranges may be used. As
demonstrated in the examples above, feasible storage times increase
with increasing temperature. Thus, in some embodiments, storage
cells may be maintained at temperatures of, for example, a few
hundred degrees centigrade or more to provide improved performance.
In general, this operating temperature is limited only by the
material properties (e.g. melting point) of the storage cell.
[0106] Although the devices described above feature storage cells
with substantially spherical volumes, any other shape may be
used.
[0107] Although the devices described above employ Helmholtz coils
to provide a uniform magnetic field, it is to be understood that
any other suitable magnet may be used (e.g. a solenoid, a permanent
magnet, etc.). Although specific magnetic field strengths are
described in the examples above, a field may be provided with any
suitable strength, e.g. 3 mT or more, 100 mT or more, 1000 mT or
more, etc. In general, increased field strength will improve the
performance of the storage cell by decreasing the
hyper-polarization relaxation rate.
[0108] Any of the techniques described above may be used in
conjunction with known applications of hyperpolarized gasses,
including but not limited to medical imaging (e.g. medical
MRI).
[0109] In some embodiments storage cells of the type described
above may be used in conjunction with a cryogenic apparatus used
for separation of hyperpolarized xenon, but not for storage. For
example, one could receive a polarized gas mixture in small
batches, freeze it long enough to separate the xenon from the other
gasses in the mixture (e.g., a minute or two), and then immediately
volatilize it into a storage cell.
[0110] The devices and techniques described herein may be extended
to the non-cryogenic storage of hyperpolarized materials other than
1.sup.29Xe, e.g. any other material which experiences inhibited
wall relaxation in an alkali free environment.
[0111] Additional discussion related to the devices and technique
B.C. Anger, et al., Gas-phase spin relaxation of 129Xe, Phys. Rev.
A 78 043406 (2008), which is incorporated by reference herein it
its entirety.
[0112] Additional material is attached in an appendix and/or
incorporated by reference. It is to be understood that in the case
that any technical definitions presented in the main body of this
application conflict with those presented in the appendix or
incorporated references, the definition in the main body holds.
REFERENCES
[0113] [1] T. G. Walker and W. Happer, Rev. Mod. Phys. 69, 629
(1997).
[0114] [2] S. Appelt, A. Ben-Amar Baranga, C. J. Erickson, M. V.
Romalis, A. R. Young, and W. Happer, Phys. Rev. A 58, 1412
(1998).
[0115] [3] B. Driehuys, J. Walker, J. Pollaro, G. P. Cofer, N.
Mistry, D. Schwartz, and G. A. Johnson, Magn. Reson. Med. 58, 893
(2007).
[0116] [4] L. Schroder, T. J. Lowery, C. Hilty, D. E. Wemmer, and
A. Pines, Science 314, 446 (2006).
[0117] [5] S. Pawsey, I. Moudrakovski, J. Ripmeester, L.-Q. Wang,
G. J. Exarhos, J. L. C. Rowsell, O. M Yaqhi, J. Phys. Chem. C 111,
6060 (2007).
[0118] [6] W. Happer, Rev. Mod. Phys. 44, 169 (1972).
[0119] [7] E. Babcock, B. Chann, T. G. Walker, W. C. Chen, and T.
R. Gentile, Phys. Rev. Lett. 96, 083003 {2006).
[0120] [8] E. Babcock, I. Nelson, S. Kadlecek, B. Driehuys, L. W.
Anderson, F. W. Hersman, and T. G. Walker, Phys. Rev. Lett. 91,
123003 (2003).
[0121] [9] B. N. Berry-Pusey, B. C. Anger, G. Laicher, and B. Saam,
Phys. Rev. A 74, 063408 (2006).
[0122] [10] G. D. Cates, S. R. Schaefer, and W. Happer, Phys. Rev.
A 37, 2877 (1988).
[0123] [11] L. D. Shearer and G. K. Walters, Phys. Rev. 139, A1398
(1965).
[0124] [12] B. Charm, I. A. Nelson, L. W. Anderson, B. Driehuys,
T.G. Walker, Phys. Rev. Lett. 88, 113201 (2002).
[0125] [13] I. L. Moudrakovski, S. R. Breeze, B. Simard, C. I.
Ratcliffe, J. A. Ripmeester, T. Seideman, and J. S. Tse, J. Chem.
Phys. 114, 2173 (2001).
[0126] [14] B. Driehuys, G. D. Cates, E. Miron, K. Sauer, D. K.
Walter, and W. Happer, Appl. Phys. Lett. 69, 1668 (1996).
[0127] [15] I. C. Ruset, S. Ketel, F. W. Hersman, Phys. Rev. Lett.
96, 053002 (2006).
[0128] [16] X. Zeng, Z. Wu, T. Call, E. Miron, D. Schreiber, and W.
Happer, Phys. Rev. A 31, 260 (1985).
[0129] [17] G. D. Cates, D. R. Benton, M. Gatzke, W. Happer, K. C.
Hasson, and N. R. Newbury, Phys. Rev. Lett. 65, 2591 (1990).
[0130] [18] M. Gatzke, G. D. Cates, B. Driehuys, D. Fox, W. Happer,
and B. Saam, Phys. Rev. Lett. 70, 690 (1993).
[0131] [19] N. N. Kuzma, D. Babich, and W. Happer, Phys. Rev. B 65,
134301 (2002).
[0132] [20] B. Driehuys, G. D. Cates, and W. Happer, Phys. Rev.
Lett, 74, 4943 (1995).
[0133] [21] X. Zeng, E. Miron, W. A. van Wijngaarden, D. Schreiber,
and W. Happer, Phys. Lett. 96A, 191 (1983).
[0134] [22] S. R. Breeze, S. Lang, I. Moudrakovski, C. I.
Ratcliffe, J. A. Ripmeester, G. Santyr, B. Simard, and I. Zuger, J.
Appl. Phys. 87, 8013 (2000).
[0135] [23] W. A. Fitzsimmons, L. L. Tankersley, and G. K. Walters,
Phys. Rev. 179, 156 (1969).
[0136] [24] W. Heil, H. Humblot, E. Otten, M. Schafer, R. Surkau,
and M. Leduc, Phys. Lett. A 201, 337 (1995).
[0137] [25] R. E. Jacob, S. W. Morgan, B. Saam, and J. C. Leawoods,
Phys. Rev. Lett. 87, 143004 (2001).
[0138] [26] E. R. Hunt and H. Y. Can, Phys. Rev. 130, 2302
(1963).
[0139] [27] P. S. Hubbard, Phys. Rev. 131, 1155 (1963).
[0140] [28] H. M. McConnell and C.H. Holm, J. Chem. Phys. 25, 1289
(1956).
[0141] [29] H. W. Spiess, D. Schweizer, U. Haeberlen, and K. H.
Hausser, J. Magn. Reson. 5, 101 (1971).
[0142] [30] F. Reif, Fundamentals of Statistical and Thermal
Physics (McGraw-Hill, N.Y., 1965).
[0143] [31] M. Hanni, P. Lantto, N. Runeberg, J. Jokisaari, and J.
Vaara, J. Chem. Phys. 121, 5908 (2004).
[0144] [32] R. E. Jacob, S. W. Morgan, and B. Saam, J. Appl. Phys.
92, 1588 (2002).
[0145] [33] B. Saam and M. S. Conradi, J. Magn. Reson. 134, 67
(1998).
[0146] [34] C. J. Jameson, A. K. Jameson, and J. K. Hwang, J. Chem.
Phys 89, 4074 (1988).
[0147] [35] A. Abragam, Principles of Nuclear Magnetism (Oxford
Science Publications, New York, N.Y., 1961).
[0148] [36] See EPAPS Document No. TBD for a discussion of
gradient-induced relaxation in a Helmholtz-coil geometry. For more
information on EPAPS, see
http://www.aip.org/pubservs/epaps.html.
[0149] [37] J. Kestin, K. Knierim, E. A. Mason, B. Najafi, S. T.
Ro, and M. Waldman, J. Phys. Chem. Ref Data 13, 229 (1984).
[0150] [38] R. van Wissen, M. Golombok and J. J. H. Brouwers, Chem.
Eng. Sci. 60, 4397 (2005).
[0151] [39] M. Golombok and L. Chewter, Ind. Eng. Chem. Res. 43,
1743 (2004).
[0152] [1A] Li-Qoing Wang, Yongsoon Shin, W. D. Samuels, Gregory J.
Exarhos, I. L Moudrakovksi, V. V. Terskikh, and Ripmeester.
Magnetic resonance studies of hierarchically ordered replicas of
wood cellular structures prepared by surfactant-mediated
mineralization. J. Phys. Chem. B, 107(50):13793-13802, 2003.
[0153] [2A] Thomas J. Lowery, Seth M. Rubin, E. Janette Ruiz, Megan
M. Spence, Nicolas Winssinger, Peter G. Schultz, Alexander Pines,
and David E. Wemmer. Applications of laser-polarized .sup.129Xe to
biomolecular assays. Mag. Res. Imag., 21:1235-1239, 2003.
[0154] [3A] R. W Mair, M. I. Hrovat, S. Patz, M. S. Rosen, I. C.
Ruset, G. P. Tupulos, L. L. Tsai, J. P. Butler, F. W. Hersman, and
R. L. Walsworth. .sub.3 He lung imaging in an open access,
very-low-field human magnetic resonance imaging system. Mag. Res.
Med., 53:745-749, 2005.
[0155] [4A] B. Driehuys, G. D. Cates, E. Miron, K. Sauer, D. K.
Walter, and W. Happer. High-volume production of laser-polarized
.sup.129Xe. Appl. Phys. Lett., 69:1668-1670, 1996.
[0156] [5A] I. C. Ruset, S. Ketel, and F. W. Hersman. Optical
Pumping System Design for Large Production of Hyperpolarized
.sup.129Xe. Phys. Rev. Lett., 96: 053002, 2006.
[0157] [6A] F. W. Hersman. Xemed webpage. http://xemedllc.com/.
[0158] [7A] N. N. Kuzma, B. Patton, K. Raman, and W. Happer. Fast
nuclear spin relaxation in hyperpolarized solid .sup.129 xe. Phys.
Rev. Lett., 88(14):147602, March 2002.
[0159] [8A] W. E. Groth, Konrad Beyerle, Erich Nann, and K. H.
Welge. Enrichment of uranium isotopes by the gas centrifuge method.
In Peaceful Uses of Atomic Energy, page 439, 1958.
[0160] [9A] Ralph van Wissen, Micheal Golmbok, and J. J. H.
Brouwers. Separation of carbon dioxide and methan in continuous
contercurrent gas centrifuges. Chem. Eng. Sci., 60:4397-4407,
2005.
[0161] [10A] Michael Golombok and Les Chewter. Centrfugal
separation for cleaning well gas streams. Ind. Eng. Chem. Res.,
43:1743-1739, 2004.
[0162] [11A] Steven R. Auvil and Bruce W. Wilkinson. The steady and
unsteady state analysis of a simple gas centrifuge. AIChE J.,
22(3):564-568, 1976.
[0163] [12A] X. Zeng, Z. Wu, T. Call, E. Miron, D. Schreiber, and
W. Happer. Experimental determination of the rate constants for
spin exchange between optically pumped K, Rb, and Cs atoms and
.sup.129Xe nuclei in alkali-metal-noble-gas van der Waals
molecules. Phys. Rev. A, 31: 260-278, 1985.
[0164] [13A] B. Driehuys, G. D. Cates, and W. Happer. Surface
Relaxation Mechanisms of Laser-Polarized .sup.129Xe. Phys. Rev.
Lett, 74:4943-4946, 1995.
* * * * *
References