Resonator For Gyromagnetic-resonance Spectrometer

Abstract

In order to avoid disturbing the tuning of a resonator, formed by an essentially capacitive line and an essentially inductive Lecher line connected in series, when the sample is introduced into the resonator, the capacitive line is a coaxial line section, the inner conductor of which forms a shield against the electric field due to this line. Alternately the first capacitive line also is Lecher line but a metal tube protects the space containing the sample from the electric field due to this line.

Description

The present invention relates to an improvement in resonators used in gyromagnetic-resonance spectrometers.

Spectrometers of this kind comprise a resonator tuned to a frequency f.sub.o and excited on this same frequency; the sample is arranged so as to present a close inductive coupling with the resonator and is also subjected to the influence of a d.c. magnetic field H.sub.o, which is at right angles to the alternating magnetic field created in the resonator, and the value of which is varied slowly in order to show in succession the various resonances (nuclear magnetic resonances or electronic paramagnetic resonances) of the sample for the frequency f.sub.o.

The error detection circuit is coupled to this resonator. It comprises, for example, a bridge connection coupled to both the high-frequency generator and the resonator.

The qualities required from the resonator as regards the sensitivity of the spectrometer are, on the one hand, a high quality factor Q and, on the other, a high coefficient .eta., the latter expressing the concentration, in the volume occupied by the sample, of the lines of force of the electromagnetic field created by the resonator under the influence of energisation by the high-frequency generator.

Furthermore, it is desirable that the resonator should occupy a relatively small volume for reasons of bulk.

A resonator made up of two series-connected Lecher lines is known.

The conductors of the first line are shaped in cross section like arcs of concentric circles, the angular value of which is just below 180.degree. , and, apart from two gaps, define laterally a cylindrical volume. The conductors of the second line are arranged symmetrically over the extension lengthwise of the cylindrical surface defining this cylindrical volume, but their cross sections are shaped like arcs having a smaller angular value; this second line is short-circuited at its end. The first line, which is essentially capacitive, has a characteristic impedance Z.sub.1 ; the second line, which is essentially inductive, has a higher characteristic impedance Z.sub.2. The assembly simulates a quarter-wavelength line short-circuited at one end and open at the other, although the capacitive and inductive impedances are localized in two distinct parts of the resonator. The axis of the tube containing the sample is arranged along the axis of the aforesaid cylindrical volume, which will be referred to as the axis of the resonator , and the part of the sample to be analyzed is on a level with the second line where the alternating magnetic field of the cavity is very strong. The resonator is subjected to a d.c. magnetic field parallel to the axis of the resonator.

By a line which is essentially capacitive is meant, in the specification and claims, a line whose distributed inductance is negligible whereas its distributed capacitance is high. In the same way, by a line which is essentially inductive is meant, in the specification and claims, a line whose distributed capacitance is negligible whereas its distributed inductance is high.

In the quarter-wavelength line simulated by the assembly the essentially inductive line is in the vicinity of a short-circuit, i.e., of a current antinode. Therefore the magnetic field radiated there will be not only the maximum one for this line, but also a high one, which would not be the case for the maximum magnetic field radiated at a current antinode of an essentially capacitive line.

This resonator has advantages, but two drawbacks :

1. In order to avoid a deterioration of the homogeneity of the magnetic field at the level of the second line, it is necessary that the tube carrying the sample should penetrate inside the cylindrical volume of the high-capacity part of the resonator, which results in a substantial modification of the tuning of the resonator through a change in the dielectric medium (a change which is a function of the nature of the sample). 2. The aforementioned coefficient .eta. increases with the ratio Z.sub.2 /Z.sub.1 . The increase of Z.sub.2 is limited by the fact that it is necessary to ensure considerable homogeneity of the magnetic field in the cylindrical volume with high induction; for this reason, the impedance Z.sub.1 of the resonator of the prior art is too high.

The aim of the present invention is to overcome these drawbacks.

According to the invention, there is provided a resonator for gyromagnetic resonance spectrometers comprising a cylindrical volume for receiving a sample, said cylindrical volume comprising in the axial direction first and second parts; said resonator comprising serially connected first and second lines, said first line being essentially capacitive, and said second line being an essentially inductive Lecher line having a higher characteristic impedance than said first line; said first line having two conductors arranged outside and at the level of said first part; said second line having two conductors arranged outside and at the level of said second part; and said first part being shielded, at least in the vicinity of said second part, from the electric field due to said first line.

The invention will be better understood and other characteristics thereof will appear in the light of the following description and the appended drawing, in which :

FIG. 1 shows a first embodiment of the invention; and

FIG. 2 shows a second embodiment of the invention.

FIG. 1 shows a first embodiment of the resonator according to the invention with its possible method of connection with a measuring bridge.

The resonator comprises an insulating support consisting of a tube made of pyrex or silica, for example, on the outer wall of which the conductors of the two lines are formed by metal coating or by sticking on metal foil, silver foil, for example.

For the sake of simplicity, it will be assumed that a metal coating process is concerned here.

For clarity of the drawing, only the (metallized) upper edge of the tube T is shown in the drawing. For the same reason, the relative scales have not been observed; an example to size will be given below.

The first transmission line is here a coaxial line, the two conductors 11 and 12 of which are respectively formed by a metal coating of the internal and external walls of the tube T over a certain length. Since the two conductors are very close to each other, this line is essentially capacitive and therefore has a very low impedance.

The second line is a Lecher line consisting of two conductors, the cross sections of which are arcs the angular value of which is not critical, but should be small enough for this line to be essentially inductive and of high impedance in relation to the impedance of the preceding line. One of the conductors 21 is formed on the internal wall of the tube and the other 22 on its external wall for connections respectively with the conductors 11 and 12.

The short-circuiting of the resonator at the top end is effected here by a short section of coaxial line, whose conductors 31 and 32 are at one end of the line, connected respectively to the conductors 21 and 22, line 31 - 32 being short-circuited at its other end. This terminal coaxial section makes it possible to limit better lengthwise the inductive part of the resonator and to minimise the radiation of energy.

Towards the bottom, the conductor 11 of the coaxial line 11 - 12 is extended in comparison with the conductor 12 as far as the base of the tube T and this extension is surrounded over a certain length by metal coating 20 , separate from the conductor 12 and formed on the external wall of the insulating tube and constituting, together with the conductor 11, a coupling capacity.

This capacitive coupling may be eliminated if the conductor 20 is extended as far as the base of the tube T and onto the lower edge of the tube T , in order to connect it electrically with the internal conductor 11 .

In the external conductor 12 , in proximity to the open end of the line 11 - 12 , there is an area 13 , which is insulated by means of a peripheral gap in the metal coating and which provides a close capacitive coupling with the internal conductor 11.

On the area 13 a conductor 15 is soldered at 14 , which is coupled by a variable capacitor 16 to the internal conductor of a coaxial cable 50 coupling the resonator to the measuring bridge.

A second wire 18 is soldered at 17 to the body of the conductor 12 and connected to ground, to the external conductor of the cable 50 and to the first electrode of a variable capacitor 19 , the second electrode of which is connected to one end of a conductor, whose second end is soldered at 26 to the conducting ring 20 .

The capacitor 16 makes it possible to adapt the resonator to the cable 50 .

The variable capacitor 19 makes it possible to perfect the tuning obtained by means of the dimensions of the resonator and the capacity 11 - 20.

The drawing also shows a tube carrying the sample 10 , arranged coaxially with the coaxial lines of the resonator. The tube, which is supported at its upper part by standard methods, is shown only in part.

In this embodiment, the inner conductor 11 of the coaxial cable 11 - 12 forms a shield, which prevents the tuning frequency of the resonator from being modified by the insertion of the sample.

The dimensions can be calculated approximately to obtain a given tuning frequency, the simplest way being to determine them precisely from experience. Moreover, all else being equal, the tuning frequency increases if the lengths of the lines are reduced.

By way of example, for a tuning frequency of 240 MHz and a tube of synthetic silica, in which the thickness of the wall is 0.5 mm , the dimensions are as follows :

length of first line 45 mm

length of second line 6 mm

length of short-circuited line 15 mm

radius (external) of coaxial lines 3.5 mm

angular value of cross sections of conductors of second line : 50.degree. .

FIG. 2 shows an embodiment of the invention, may be less perfect the first, but having the advantage of affording the possibility of being easily obtained from a resonator with two Lecher lines.

The resonator likewise comprises an insulating tube T' , which has only been shown by the section on a level with the separation between the two lines.

The first Lecher line comprises two conductors 111 and 112, whose cross sections are arcs with an angular value around 180.degree. , both being formed by metal coating of the external wall of the tube.

For technical reasons, the conductor 112 is extended by a closed ring 113 , insulated from the conductor 111 by a horizontal gap in the metal coating of the external wall.

The second Lecher line is of the same type of as the Lecher line in FIG. 1 , but its conductors 101 and 102 are here both formed on the external wall of the insulating tube and, in this example, have in cross section a smaller angular value . The terminal short-circuit is effected by a conducting ring 103.

Here, the shield is effected by a conducting tube 130 formed by internal metal coating of the tube T' , covering this surface at least at the level of the upper part of the first Lecher line (more precisely so as to surround the lower part of the specimen tube when it is positioned for analysis in the resonator) and preferably extending as far as the base of the resonator.

This shield may have a constant potential, although, as shown by experience, it may also be left floating.

The Figure also shows a tube 110 carrying the sample.

The connections of the resonator with the two conductors of the coaxial cable 150, ensuring the coupling of the resonator with the measuring bridge, are made by means of leads 115 and 188, soldered respectively at one end to the conductor 111 and to the cylindrical part 113 , extending the conductor 112. The wire 118 is grounded and connected to the wire 115 through a variable capacitor 119, which is a tuning capacitor. An impedance matching capacitor 116 is also introduced into the connection 115 between the terminal 131 and the inner conductor of the cable 150.

This arrangement also ensures that there is a space which is substantially free of electric field in the high-capacity part of the resonator which could be disturbed by the presence of the sample.

Furthermore, the presence of the shield 130 at a short distance from the conductors 111 and 112 results in a reduction of the impedance Z.sub.1 of the first line.

Claims

What is claimed is :

1. A resonator for gyromagnetic resonance spectrometers comprising a cylindrical volume for receiving a sample, said cylindrical volume comprising in the axial direction first and hollow second parts; said resonator comprising serially connected first and second lines, said first line being essentially capacitive, and said second line being an essentially inductive Lecher line having a higher characteristic impedance than said first line; said first line having two conductors arranged outside and at the level of said first part; said second line having two conductors arranged outside and at the level of said second part; said first part being shielded, at least in the vicinity of said second part, from the electric field due to said first line.

2. A resonator according to claim 1 , wherein said first line is a coaxial line having an inner and an outer conductor, the gap separating the two conductors of the coaxial line being small in relation to their radii.

3. A resonator according to claim 2 , wherein the outer conductor of said coaxial line comprises a gap, peripherally insulating a small portion of this conductor from the remainder thereof, and wherein the output of said resonator consists of two wires, one being linked to said small portion and the other to said remainder.

4. A resonator according to claim 2 , further comprising a third line, which is a coaxial line having two ends, and which is connected at one end to said second line and short-circuited at its other end.

5. A resonator according to claim 1 , wherein said first line is a Lecher line, whose two conductors have cross-sections in the form of arcs of circles with angular values little below 180.degree., said resonator further comprising a cylindrical metal tube located inside the cylindrical space substantially defined by the two conductors of said first line.