U.S. patent number 3,757,204 [Application Number 05/229,788] was granted by the patent office on 1973-09-04 for cavity resonator structure for an epr spectrometer employing dielectric material for improving rf electric and magnetic field uniformity along the sample.
This patent grant is currently assigned to Varian Associates. Invention is credited to James Stewart Hyde.
United States Patent |
3,757,204 |
Hyde |
September 4, 1973 |
CAVITY RESONATOR STRUCTURE FOR AN EPR SPECTROMETER EMPLOYING
DIELECTRIC MATERIAL FOR IMPROVING RF ELECTRIC AND MAGNETIC FIELD
UNIFORMITY ALONG THE SAMPLE
Abstract
In an electron paramagnetic resonance cavity resonator having a
line sample therein, a dielectric material positioned near the
sample and along at least a portion of its length, and extending
into the RF electric field region near the sample, changes the
gradient of the electric field intensity and, as a result, the RF
magnetic field intensity along the line sample so as to make both
the RF electric and magnetic field intensities more uniform along
the sample length. The dielectric material may take the form of a
shaped sleeve surrounding the sample, or two shorter sleeves
extending over the sample from either end and spaced apart at the
center region, or parallel plates on either side of the sample,
Inventors: |
Hyde; James Stewart (Menlo
Park, CA) |
Assignee: |
Varian Associates (Palo Alto,
CA)
|
Family
ID: |
22862663 |
Appl.
No.: |
05/229,788 |
Filed: |
February 28, 1972 |
Current U.S.
Class: |
324/321;
333/227 |
Current CPC
Class: |
G01R
33/345 (20130101) |
Current International
Class: |
G01R
33/34 (20060101); G01R 33/345 (20060101); G01n
027/78 () |
Field of
Search: |
;324/.5R,.5AH,.5AC,58.5C,58C ;333/24.1,24G,83 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lynch; Michael J.
Claims
What is claimed is:
1. Apparatus for use in testing samples in an electron paramagnetic
resonsance spectrometer system comprising
a cavity resonator having an aperture therethrough to receive an
elongated sample cell and to apply a radio frequency magnetic field
to said sample cell along a substantial portion of its length to
produce electron paramagnetic resonance therein, and
a dielectric material positioned in the electric field near said
sample cell aperture for controlling the relative intensity of the
radio frequency magnetic field applied to a cell along the sample
cell length wherein a greater thickness of dielectric material is
present at regions of the sample cell length on opposite sides of
the central region of the cell length than at the central
region.
2. Apparatus as claimed in claim 1 wherein said cavity resonator
operates in the rectangular TE.sub.102 mode.
3. Apparatus as claimed in claim 1 wherein said cavity resonator
operates in the cylindrical TE.sub.011 mode.
4. Apparatus as claimed in claim 1 wherein said cavity resonator
operates in the cylindrical TM.sub.110 mode.
5. Apparatus as claimed in claim 1 wherein said dielectric material
has a concave shape with the thin section near the central
region.
6. Apparatus as claimed in claim 5 wherein said dielectric material
is a hollow cylindrical member surrounding the elongated sample
cell and with a concave outer surface.
7. Apparatus as claimed in claim 1 wherein said dielectric material
comprises at least two sections extending near the two end regions
of the sample cell removed from the central region.
8. Apparatus as claimed in claim 7 wherein each of said sections
comprises a hollow cylinder surrounding the sample cell.
9. Apparatus as claimed in claim 1 wherein said dielectric material
at each side region comprises a pair of plates extending parallel
to the sample cell and being positioned on opposite sides of the
cell.
10. Apparatus as claimed in claim 9 wherein the inner ends of the
plates near the central region of the sample cell are thicker than
the remaining portion of the plates extending away from the central
region.
11. A cavity resonator for an EPR spectrometer including, means for
sustaining electric and magnetic field intensity waves in said
resonator, said resonator including an aperture defining an axis
therethrough for receiving an elongated sample cell, said magnetic
field intensity being non-uniform along said sample cell axis,
the improvement comprising:
means for converting said non-uniform magnetic field intensity
along said sample cell axis into a more nearly uniform magnetic
field intensity along the said sample cell axis, said means
comprising a dielectric material being interposed between said
sample cell and at least a pair of cavity walls, said dielectric
material includes a surface wherein the cross section of the
surface through said axis includes elongated elements parallel to
said axis.
12. A cavity resonator of claim 11 wherein said means comprising a
dielectric material is configured with respect to the cavity so as
to have less effect upon the cavity magnetic field at the region of
maximum magnetic field than in the zones removed from the region of
maximum magnetic field, said dielectric material being thinner at
said region of maximum magnetic field.
Description
BACKGROUND OF THE INVENTION
The two most widely used cavity resonators in electron paramagnetic
resonance spectrometers are the rectangular TE.sub.102 mode and the
cylindrical TE.sub.011 mode cavities, while the cylindrical
TM.sub.110 mode cavity is used to a much lesser extent. These
cavity resonators form one arm of a microwave bridge and are
positioned in the magnetic field gap of the polarizing magnet with
the elongated sample cell extending down into the cavity along a
line vertically through the center of the cavity and parallel to
the magnet pole faces. Typical forms of such cavity resonators are
shown in U.S. Pat. No. 3,122,703 issued on Feb. 25, 1964 to R.C.
Rempel et al entitled "Gyromagnetic Resonance Method and Apparatus"
and U.S. Pat. No. 3,197,692 issued June 27, 1965 to J.S. Hyde
entitled "Gyromagnetic Resonance Spectroscopy".
The radio frequency power delivered to the cavity resonator at the
electron resonance frequency, for example 9.5 GHz, produces the
driving RF magnetic field along the sample length, this RF magnetic
field intensity varying as sin (X.pi./L), where L is the length of
the cavity along the sample and x is the position of a point along
the sample length measured from the bottom of the cavity, for the
rectangular TE.sub.102 mode cavity and the cylindrical TE.sub.011
mode; a somewhat similar function exists for the RF magnetic field
intensity in a cylindrical TM.sub.110 mode cavity. The RF magnetic
field intensity is therefore stronger at the central region of the
sample length than at the two regions on either end of the central
region. As the RF power incident on the cavity increases, that part
of the sample at the central region (x = L/2) experiences microwave
power saturation first, and, if the power continues to increase,
the parts of the sample removed from the central region
saturate.
Several problems arise because the RF magnetic and electric field
intensities are not constant along the line sample extending
through the cavity. Measurement of relaxation parameters, for
example the spin lattice relaxation time (often called T.sub.1) and
the spin-spin relaxation time (often called T.sub.2), is performed
by varying the power incident on the cavity, and, if there is a
spatial variation of the RF magentic field intensity along the
sample, this presents a difficult problem in making accurate
measurements. Also, dielectric heating of the sample may occur, and
the change in temperature of the sample may change the resonance
characteristics of the sample. Diffusion tends to minimize thermal
gradients, but significant temperature variation may still occur
along the length of the sample if the RF electric field intensity
varies along this length, which makes it difficult to study the
resonance characteristics with precision. Additionally, in a
specific situation involving nitroxide radical spin labels in
water, when the sample is inserted into the cavity the temperature
dependence of the dielectric constant of the water causes an
undesirable variation in time of the balance of the microwave
bridge because of sample heating. The sample is, in addition,
relatively easy to saturate with the RF magnetic field. In order to
obtain the largest magnetic resonance signal from the sample and at
the same time minimize the variation in bridge balance after the
sample is inserted into the microwave cavity, it is desirable to
have both RF electric and RF magnetic field intensities more nearly
constant along the sample length.
BRIEF SUMMARY OF THE PRESENT INVENTION
In the EPR cavity resonator of the present invention, a dielectric
material is positioned in the RF electric field near the sample so
as to change the RF electric and magnetic field intensities and to
make them more uniform along the sample length, so as to minimize
spatial variation of the RF magnetic field intensity along the
sample length, minimize thermal gradients along the sample, and/or
minimize bridge inbalance while optimizing the magnetic resonance
signal output.
In one embodiment, the dielectric material is in the form of a
cylinder surrounding the sample and extending into the electric
field region in the cavity, the wall of the cylinder being concave
with its thinnest portion in the central region of the sample. In a
variation of this embodiment, the cylinder is replaced with a pair
of dielectric plates, one on each side of the sample, the plates
being concave.
In another embodiment, the dielectric is in the form of two hollow
cylindrical members extending over the sample regions on either
side of the central region, with the central region clear of the
dielectric. In a variation of this embodiment, each separate
cylinder is replaced by a pair of plates positioned on opposite
sides of the sample and in planes normal to the broad sides of the
cavity resonator.
DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 are perspective diagramatic views of a rectangular
TE.sub.102 mode EPR cavity resonator and a cylindrical TE.sub.011
mode EPR cavity resonator, respectively, incorporating one
embodiment of the present invention.
FIG. 3 is a plot of RF magnetic field obtained with and without the
use of the present invention, illustrating the increase in
uniformity of the RF magnetic field along the sample with the
present invention.
FIG. 4 is a diagramatic view of another rectangular cavity
resonator incorporating another embodiment of the present
invention.
FIGS. 5 and 6 illustrate still another embodiment of the present
invention.
FIG. 7 is another structural form of the invention as utilized in a
cylindrical TM.sub.110 mode cavity resonator.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, there is shown a typical form of
rectangular TE.sub.102 mode cavity resonator which is placed in the
gap of a strong magnet in an EPR spectrometer such that the two
broad sidewalls 11 and 12 are parallel to the magnetic pole faces
and normal to the direction of the magnetic field lines in the gap.
Means are provided (not shown) for supplying RF power to the cavity
resonator at the magnetic resonance frequency of the electrons in
the sample held in the cylindrical sample cell 13 extending
vertically in the center of the resonator.
The electric field lines E in the cavity resonator extend normal to
the two sidewalls 11 and 12 in a sinusoidal function with zero
electric field strength at the center of the cavity where the
sample is positioned, and maximum electric fields, in opposite and
alternating directions, midway between the sample 13 and the two
end walls 14 and 15. The RF magnetic field lines extend in an
alternating manner parallel to the elongated sample, with the
maximum field strength in the center of the cavity at the sample
position.
The RF magnetic field intensity along the sample varies
approximately as sin (x.pi./L ), where L is the length of the
cavity along the sample, as illustrated by the solid line curve A
in FIG. 3. The ordinate in FIG. 3 represents distance from the
center of the sample to either end of the sample, and the abscissa
represents RF field intensity in arbitrary units. It can be seen
that the RF field intensity is maximum at the center region of the
sample and decreases in the regions along the sample on either side
of the center region.
As the RF power applied to the cavity resonator is increased, the
sample in the center region experiences microwave power saturation
first, followed by saturation at the upper and lower regions as the
power continues to increase. Undesirable heating of the sample
results from sample dielectric loss, and is concentrated in the
center region. It is therefore desirable to make the RF magnetic
field intensity more uniform along the central region so that
saturation can occur over an increased sample length at the same
power incident on the cavity, and it is desirable to reduce the
spatial variation of the RF electric field intensity along the
sample length so that the sample temperature will be more
uniform.
It is known from earlier experiments, discussed for example in the
Varian Associates Technical Information Bulletin, Fall, 1965, pages
13 and 14, that the RF magnetic field intensity at a sample
position can be increased by placing a sleeve of dielectric
material, such as quartz, about the sample. However, a simple
increase or decrease of RF magnetic field in the present instance
will not solve the problem caused by the RF magnetic field
concentration at the cavity sample center; what is desired is
control over the RF field distribution in the cavity to obtain more
RF magnetic field intensity uniformity over greater sample
length.
In the embodiment of the present invention shown in FIG. 1, a
sleeve 16 of dielectric material, such as quartz, having a concave
outer surface is placed over the sample, the walls of the sleeve
extending into the region of the electric field lines E on either
side of the sample. This dielectric material in the electric field
changes the gradients of the electric field intensity which in turn
give rise to changes in the intensity of the RF magnetic field. The
thinner walled section of the dielectric material at the center
region of the sample causes very little change in the RF magnetic
field intensity at that region whereas the thicker walled section
of the sleeve in the regions on either side of the center sample
region result in an increase in the intensity of the RF magnetic
field in these regions, in comparison to the uncompensated
resonator. As a consequence of the increase in the intensity of the
RF magnetic field on either side of the central region, the RF
magnetic field intensity becomes more uniform over the sample. This
result is illustrated by the dotted line B in FIG. 3.
Spatial variation of the RF magnetic field intensity with changing
incident power is reduced. More uniform sample temperature along
the length of the sample exists due to the more uniform RF electric
field intensities along the sample. Saturation of the sample will
also occur more uniformly along the length of the sample. As a
result of the more uniform RF electric and magnetic field
intensities along the sample length, more sensitivity is obtained
in experiments where the sample is near saturation and more
faithful representation of the line shape in the region near
saturation is obtained, more uniform sample temperature occurs, and
more reliable measurements of saturation characteristics of lines
results.
A similar form of dielectric sleeve for controlling the relative
intensity and orientation of the RF electromagnetic field vectors
along the sample length in a cylindrical TE.sub.011 mode EPR cavity
resonator 11 is illustrated in FIG. 2.
Referring to FIG. 4, the dielectric cylinder of FIG. 1 is replaced
with a structure approximating the cylinder and comprising a pair
of dielectric plates 17, 18 extending on either side of the sample
and in the electric field. The outer surfaces of the two plates are
concave so that the material is thinner at its center region than
at the areas on either side of the center. This results in a more
uniform RF magnetic field as represented by curve B in FIG. 3.
In another embodiment of the invention, as shown in FIG. 5, two
cylindrical sleeve section 19 and 21 with uniform wall thickness
may be employed to make the RF magnetic field intensity more
uniform along the sample. The sleeves extend over the sample from
either end thereof, leaving the central region free from dielectric
material. Further control over the field uniformity may be
accomplished by increasing the thickness of the sleeve wall
slightly at the inner ends of the sleeves, as by annular dielectric
ring portions 22 affixed thereto as shown in FIG. 6.
Referring now to FIG. 7, there is shown a diagrammatic view of a
cylindrical TM.sub.110 mode cavity resonator with still another
embodiment of the invention utilized therein. A typical form of
such resonator for X-band has about a 1.5 inch diameter and is 0.17
inch wide. Two pairs of plates 23, 24 and 25, 26 of dielectric
material extend from the wall of the resonator along either side of
the elongated sample 13 located in the center of the cavity. The
plates extend about one-third of the way into the cavity from each
side, leaving the middle third of the length along the sample free
of dielectric. With a sample cell diameter of about 0.16 inch, good
results were obtained with quartz plates about 0.04 inch thick and
spaced about 0.20 inch from the axis through the sample.
Although the sizes and shapes of the dielectric material can be
selected to accomplish the desired results, it has been found that
if the dielectric extends too far into the RF electric field, there
is a tendency for the change in the RF magnetic field intensity to
be reversed in direction.
In the various embodiments, the dielectric material may be quartz,
which has a low loss and a high dielectric constant or it may be of
other suitable material such as polyethylene, teflon or
corundum.
* * * * *