DD275316B5 - Resonator arrangement for measuring EPR signals - Google Patents

Description

For this 2 pages drawings

Field of application of the invention

The wire-ring resonator is used in EPR spectroscopy, in particular in EPR tomography in the frequency range from 500 MHz to TOGHz. It can be used as a layer-selective resonator and as a surface coil.

The use as a slice-selective resonator causes the excitation of the spins in a cylindrical disc whose diameter corresponds to the inner diameter of the wire ring. The axial field extent determines the spatial resolution in the axial direction. By an axial displacement of the sample, this can be examined by the disk.

A second application is to use as a surface coil for specimens whose dimensions are much larger than the wire ring diameter.

Spins in the surface regions can be excited in accordance with the axial field distribution and the penetration depth of the microwaves, which depends on the sample material. The use of field gradients in the axial direction allows the measurement of spatial dependencies perpendicular to the surface. The low quality of the wire ring resonator allows for individual measurements in free-running AFC and the use of pulse techniques in ESE and STE. This is especially true for large wire ring diameters for low frequencies in the L band.

Characteristic of the known state of the art

In the analytical measuring technique, a number of methods are known with which material properties are examined by the fact that magnetic dipole transitions are excited in samples of these materials. The best known methods of this kind are nuclear magnetic resonance and electron spin resonance.

To generate these resonance phenomena, it is necessary to simultaneously expose the sample to be examined to a high-frequency field and to a constant-magnetic field directed perpendicular thereto. The sample is introduced into a resonator and positioned there at a position where magnetic field lines propagate with high density. A common problem with such investigations is to impart the highest possible sensitivity to the equipment used. The achievable sensitivity is proportional to the product of the quality of the resonator used and the fill factor.

The use of surface coils in NMR and EPR is known (Nishikawa, Fuju, Berliner: Journal of Magnetic Resonance, 1985, pp. 62, 79-86). Specified was a surface coil with a diameter of 7 mm and a length of the parallel feed lines of 41 mm, which is fixedly connected to a coaxial line and is coupled via a coaxial short-circuiting slide.

The disadvantages are that the arrangement is only possible for a defined loop diameter at a fixed predetermined frequency and no critical coupling with coaxial short-circuiting slide.

In order not to have to rotate the sample about two axes in measurements of the spatial distribution of paramagnetic centers, a slice-selective excitation would be favorable, which allows a two-dimensional measurement of the layer of a sample. This requires resonators that allow the excitation of a very narrow layer. Such resonators have not previously been used. The resonators used hitherto and also described in the literature (Ono, Ogata, Chemistry Letters, 1986, pp. 491-494, Ogata, Ono, Fujisawa: Chemistry Letters, 1986, pp. 1681-1684) are unsuitable for a slice-selective excitation.

Also known is the solution according to DE 2 91 7 794 A1. Here it is shown that in EPR spectroscopy only cavity resonators with a high quality can be used, due to a high microwave radiation in the GHz range. However, only samples that are smaller than the dimensions of the resonator and that are adapted to the highly inhomogeneous field distribution in the resonator can be examined. This results in fill factors η

/ sample B ₁ dV ..... _л ". , ,

η = -, which are less than 1%.

J resonator bi dV

The measured signal intensity is proportional to the product of square of quality and fill factor

S ~ Q ² η, so that the low quality of the coils is compensated by a filling factor close to 1.

Object of the invention

The aim of the invention is to expediently carry out sensitive measurements of EPR signals in biological samples in slice-selective regions as well as in the surface region of larger organisms.

Explanation of the essence of the invention

The invention has for its object to develop a resonator having a microwave distribution, with which the spins can be excited in a narrow cylindrical disc and the diameter or the diameter of the disc can be varied over a wide range at a constant frequency and the ensures the best possible broadband coupling.

According to the invention this is achieved by a resonant arrangement consisting of a wire ring and two parallel outlets of defined length, the Länget and the distance a two parallel wire lines of diameter d of the wire ring are determined, and that the energy supply via a parallel wire line of the same distance , which is severed to n Чг and overlaps with a coming from the supply line parallel wire line with a distance a over a length ü without touching the lines, and that depends on the overlap length ü and the distance h between the parallel wire lines the degree of coupling , Depending on the embodiment, the parallel wire lines can be directly overlapped or trombone similar pushed into each other. The wire ring resonator according to the invention has the following advantages and possibilities due to its novel microwave coupling with respect to the surface coil: The coupling to the microwave line is achieved by the overlapping of the parallel wire line ends of the wire ring resonator and of the microwave energy feeding coaxial cable. The optimization of the coupling is achieved by:

- displacement of the supply lines,

- mechanically, by means of a screw transformer,

- electronically, by means of varactor diode.

The quality Q is 4-5 times higher (450-500) due to the critical coupling, and the maximum resonator diameter could thus be increased to 0 20 mm (for L-band).

embodiment

The invention will be explained in more detail with reference to an embodiment. Show:

Fig. 1 a: wire ring resonator with microwave coupling in a spatial representation;

Fig. 1 b: wire ring resonator with microwave coupling in plan view;

Fig. 2: wire ring resonator in trombone-like arrangement;

3: wire ring resonator with screw transformer;

Fig. 4: wire ring resonator with varactor diode.

The wire ring resonator shown in FIG. 1 consists of the parallel wire piece 4 with the length I and the spacing a and the wire ring 5 with the diameter d. For reasons of coupling, the length I is increased by η · Чg (η = 0,1, 2 ...) 3. The size of the diameter d determines the inductive component of the resonator, while the length I and the distance a form the capacitive component. At constant d and constant a, an increase of I leads to a reduction of the resonance frequency f _o .

The dimensions of the wire ring resonator must satisfy the following condition: d; I; a; <λ The equation for calculating the wire ring resonator is:

fo

2a 1 2

πεμ ₀ · (5 a + 2I)

For example, for a resonance frequency f _o = 1.9 GHz and a loop diameter d = 10 mm, a loop length l = 2 mm results. For V2 = 79mm, the total length required for coupling is I + η · Чg = 81 mm.

For coupling to the microwave line and for mechanical stabilization, the wire ring resonator is fitted in a special holder made of a material with a low microwave attenuation. The also fitted in this holder, parallel wire ends 2 of the power supply coaxial cable 1 are arranged so that the extended by η Чг parallel wire pieces of the wire ring resonator and the coaxial cable overlap and have on the overlap length ü to each other the distance h. For isolation and to ensure a defined distance h for an optimal coupling condition, a dielectric film with a defined thickness is placed between the parallel wire ends.

By varying the overlap length ü and the distance h, the wire ring resonator can be adapted to the feed line.

FIGS. 2, 3 and 4 show further coupling or tuning possibilities of the resonator.

Claims (6)

A resonator arrangement for measuring EPR signals in a sample, characterized in that said resonant structure consists of a wire ring (5) open on one side, the diameter of which is smaller than the wavelength of the exciting microwave, and a parallel wire line (3), (4) whose length is also smaller than the wavelength used, the dimensions of the wire ring (5) and the parallel conductor piece (4) determining the resonant frequency, and in that for the purpose of coupling this parallel conductor piece (4) is galvanically connected to a second, unidirectional open parallelepiped (3) whose line length is η times half the wavelength (n ·V2; η = 0,1, 2 ...), said second parallel line piece (3) being at the open side with a third, open on one side, parallel lead piece (2), which is directly connected to the supplying coaxial line (1) is electrically connected via the microwave field and there ß the spatial assignment of the two open parallel-line ends is a measure of the coupling between the resonator and the supply cable (1). 2. Resonator arrangement according to claim 1, characterized in that overlap both open parallel conductor ends (2), (3) to a constant length ü and that by a change in the distance h, between the two parallel lines, the coupling is varied. 3. resonator arrangement according to claim 1, characterized in that for a variable coupling with a constant distance between the lines, the overlap length ü is varied. 4. resonator arrangement according to claim 3, characterized in that for the purpose of coupling the open line ends are pushed into one another. 5. resonator assembly according to claim 1, characterized in that the parallel wire lines are overlapped and the Ankopplungsänderung by means of a screw transformer, which is at a distance of V2 removed from the wire ring and the incoming parallel wire line is approximated, is achieved. A resonator arrangement according to claim 1, characterized in that the parallel wire lines are overlapped, and that between the two ends of the parallel wire line to be fed is a varactor diode which is distant from the wire ring at a distance of V2, with which the coupling can be varied electronically ,