RU2711345C1 - High-frequency electron paramagnetic resonance spectrometer - Google Patents

Abstract

FIELD: physics.SUBSTANCE: invention can be used to create a high-frequency spectrometer of electron paramagnetic resonance. Essence of the invention lies in the fact that the high-frequency electron paramagnetic resonance spectrometer includes a microwave unit, a cryogenic system, a superconducting electromagnet, control unit for superconducting electromagnet, microwave insert for specimen from nonmagnetic material, recording system and computer, wherein superconducting electromagnet and microwave insert for sample are located inside cryogenic system, and the output of the microwave unit is connected to the first input of the recording system, the input / output of the microwave unit is connected through an adapter to the input / output of the microwave insert for the sample, the input / output of the recording system is connected to the first input / output of the computer, output of superconducting electromagnet control unit is connected to superconducting electromagnet, superconducting electromagnet control unit input is connected to computer output, inlet / outlet of cryogenic system is connected to second input / output of computer, microwave insert for sample is made with possibility of rotation around longitudinal axis in form of waveguide of circular cross-section with diameter of 3–5 mm, closed on end face by transverse solid partition.EFFECT: possibility of simplifying its design while maintaining sensitivity.1 cl, 4 dwg

Description

The invention relates to techniques for spectroscopy of electron paramagnetic resonance (EPR) and may find application in studies of condensed materials and nanostructures in physics, chemistry, biology, geology and other fields. The main focus in modern EPR studies is to increase the sensitivity and spectral resolution of spectrometers by increasing the operating frequency.

An increase in the operating frequency leads to a significant increase in the sensitivity, the minimum recorded concentration of spins (for small samples):

where f is the frequency of the EPR spectrometer, Hz;

N _min - the number of spins in the sample, pcs.

The operating frequency is related to the magnetic field for the simplest system with a spin S = 1/2 ratio:

where: S is a dimensionless quantity equal to the back of the paramagnetic system under consideration; B - constant magnetic field, T; ge is a dimensionless quantity called the g-factor and characterizes the gyromagnetic ratio for the electronic magnetic moment of the used spin system; in the simplest case, for an unpaired electron qe = 2.00; βе = 9.2740⋅10 ^-24 - Bohr magneton, J / T;

h = 6.62606896⋅10 ^-34 - Planck constant, Js.

The spectral resolution is determined by the ability to record small changes in the g-factor Δg, which can be written as:

where ΔB is the change in the position of the EPR line in the magnetic field (shift of the EPR line) with a change in the g factor Δg, and, as follows from formula (3), this change is proportional to the magnitude of the magnetic field B, which, as follows from formula (2), in proportion to frequency:

раза. Увеличение рабочей частоты спектрометра ЭПР также приводит к достижению более высоких больцмановских факторов, играющих определяющую роль во многих физических спин-зависимых процессах, включая динамическую поляризацию ядер. Увеличение рабочей частоты спектрометра позволяет исследовать спиновые системы с большими начальными расщеплениями, которые не доступны для измерений в стандартном 3 см диапазоне ЭПР.Thus, an increase in the operating frequency of the spectrometer from the traditional frequency of 9.4 GHz to 94 GHz leads to an increase in the resolution of the spectrometer by a factor of 10, and a subsequent increase in frequency to 130 GHz leads to a further increase in the resolution of the spectrometer by another 1.4 times. In this case, the sensitivity in the first case increased by (10) ^9/2 ≈30000 times, and in the second

times. An increase in the operating frequency of the EPR spectrometer also leads to higher Boltzmann factors, which play a decisive role in many physical spin-dependent processes, including dynamic polarization of nuclei. An increase in the operating frequency of the spectrometer makes it possible to study spin systems with large initial splitting, which are not available for measurements in the standard 3 cm EPR range.

Known high-frequency electron paramagnetic resonance spectrometer (see patent RU 2411529, IPC G01R 33/60, G01N 24/10, published 02/10/2011). The spectrometer contains a microwave generator 90-100 GHz, a microwave power transmission system for the sample, a resonator equipped with a piston and a sample holder, a microwave signal detector, a synchronous detector, a magnetic field modulation generator, modulation coils, a magnetic field sweep, superconducting a magnet, a cryogenic system for maintaining the temperature of liquid helium, equipped with an optical window, and a control unit. The microwave power transport system to the sample is made in the form of a first 3 mm waveguide, a first horn antenna, at least one dielectric lens, a second horn antenna facing the horn of the first horn antenna, and a second 3 mm waveguide in series. The cryogenic system contains a superconducting magnet, modulation coils, a second horn antenna, a second 3 mm waveguide and resonator. The second horn antenna is installed against the optical window of the cryogenic system and through the second 3 mm waveguide is connected to the resonator through the communication hole. The first horn antenna and at least one dielectric lens are mounted outside the cryogenic system against its optical window.

The disadvantage of the spectrometer is the large loss of microwave power due to the use of the open channel and the aperture of the windows in the cryogenic system.

Known high-frequency electron paramagnetic resonance spectrometer (see HJ.van der Meer, JAJM Disselhorst, J. Allgeier, J. Schmidt and W. Th. Wenckebach, Meas. Sci. Technol., 1, pp. 396-400 (1990), JAJM Disselhorst, HJ van der Meer, OG Poluektov, and J. Schmidt, J. Magn. Reson., Ser. A 115, pp. 183-188, 1995). The spectrometer includes a microwave generator of 3 mm range with a frequency of 94.9 GHz, a system for transporting microwave power to a sample in the form of a combination of waveguides of 3 mm, 8 mm and 3 cm ranges, a cryogenic system with a liquid helium temperature of 2 K, a superconducting magnet. In the device, the EPR signal is recorded by the electronic spin echo signal in the microwave channel using a microwave radiation receiver.

The disadvantage of the spectrometer is a long microwave path (more than two meters), including waveguide systems of three ranges (3 cm, 8 mm, 3 mm) with corresponding transitions between waveguides, leading to microwave power losses and the appearance of numerous reflections at the boundaries of waveguide systems. The presence of a waveguide system inevitably leads to significant heat losses and causes additional difficulties in manufacturing a thermal gate in the form of an additional section of the waveguide from a material with low thermal conductivity

A high-frequency electron paramagnetic resonance spectrometer is known (see Letters in ZhTF 43, issue 8, p. 63-7, 2017), which coincides with the present decision by the largest number of essential features and adopted as a prototype. The spectrometer prototype includes a microwave unit, for example, consisting of a highly stable solid-state generator of fixed frequency U = 7.23 GHz, a superheterodyne receiver, a pulse shaper and a circulator, a cryogenic system, a superconducting electromagnet, a control unit for a superconducting electromagnet, a microwave insert for a sample of non-magnetic material, a system registration and computer. The output of the microwave unit is connected to the input of the registration system, the input / output of the microwave unit is connected to the input / output of the microwave insert for the sample through an adapter, the input / output of the registration system is connected to the first input / output of the computer, the output of the control unit of the superconducting electromagnet is connected to the superconducting electromagnet , the input / output of the control unit of the superconducting electromagnet is connected to the second input / output of the computer, the input / output of the cryogenic system is connected to the third input / output of the com yutera. The microwave insert for the sample includes a microwave resonator with the sample, which is placed in an isolated volume of the cryostat. The design of tunable single-mode cylindrical resonators H011 has been developed, which for different ranges differ only in size. Communication is carried out along the side wall, and the frequency is tuned by moving the top cover. To reduce losses, the transportation of microwave power from the microwave unit to the resonator for all operating ranges is carried out along 8-mm waveguides using transition sections to the waveguide of the corresponding range.

The disadvantages of the prototype spectrometer include the complex design of the microwave insert for the sample, including a resonator, which limits the size of the test sample to a diameter of less than 0.5 mm, which complicates the orientation of the sample in a magnetic field and requires highly qualified personnel to configure. The need to transport microwave power from the microwave unit using 8-mm waveguides and waveguide transitions between the ranges also complicates the design of the spectrometer and leads to the appearance of numerous reflections in the microwave system.

The objective of this technical solution is to develop a high-frequency electron paramagnetic resonance spectrometer that would simplify its design while maintaining the sensitivity of the spectrometer.

The problem is solved in that the high-frequency electron paramagnetic resonance spectrometer includes a microwave unit, a cryogenic system, a superconducting electromagnet, a superconducting electromagnet control unit, a microwave insert for a sample of non-magnetic material, a recording system and a computer, while the superconducting electromagnet and microwave insert for the sample are located inside cryogenic system, and the output of the microwave unit is connected to the first input of the registration system, input / output micro The main unit is connected through the adapter to the input / output of the microwave insert for the sample, the input / output of the registration system is connected to the first input / output of the computer, the output of the superconducting electromagnet control unit is connected to the superconducting electromagnet, the input of the superconducting electromagnet control unit is connected to the computer output, input / output the cryogenic system is connected to the second input / output of the computer. New in this technical solution is that the microwave insert for the sample is made to rotate around its longitudinal axis in the form of a circular waveguide with a diameter of 3-5 mm, closed at the end of the transverse solid partition.

A high-frequency electron paramagnetic resonance spectrometer may include a low-frequency generator and modulation coils, while the first output of the low-frequency generator is connected to modulation coils located outside the circular waveguide, and the second output of the low-frequency generator is connected to the second input of the registration system.

The use of a microwave insert for a sample made in the form of a circular cross-sectional waveguide with a diameter of 3-5 mm, closed at the end of a transverse solid partition, allows using microwave modes similar to the distribution of microwave magnetic and electric fields in a cylindrical cavity of 3 mm and 2 mm ranges without the need for a single-mode resonator . The latter excludes the operation for tuning the resonator, which allows us to solve several problems that arise when using a single-mode resonator. The size of the sample is limited only by the diameter of the circular waveguide, that is, there is no need to reduce the size of the sample by about an order of magnitude for working with the resonator. There is no need to use quartz tubes for the sample, which eliminates spurious EPR signals. The design of the microwave insert for the sample is simplified due to the fact that the resonator tuning system and its matching with the waveguide system are removed. To a large extent, due to the fact that the number of compounds in the waveguide system is reduced, leading to undesirable reflected signals and due to the increase in sample size, the sensitivity of the EPR spectrometer related to this technical solution is preserved. Such a spectrometer is not susceptible to vibrations generated during operation of a closed-cycle cryostat, for example, which is part of a cryogenic system. The circular waveguide shape simplifies the rotation of the microwave insert for the sample around the longitudinal axis, which makes it possible to set the orientation of the sample in a magnetic field.

This EPR spectrometer is illustrated by drawings, where

in FIG. 1 shows a block diagram of a real high-frequency electron paramagnetic resonance spectrometer operating in a pulsed mode;

In FIG. 2 shows a block diagram of a real high-frequency electron paramagnetic resonance spectrometer operating in a continuous mode;

in FIG. 3 shows the EPR spectrum recorded on a prototype spectrometer;

in FIG. 4 shows the EPR spectrum recorded on the spectrometer described in this technical solution.

A real high-frequency electron paramagnetic resonance spectrometer (see Fig. 1) includes a microwave unit (MB) 1, consisting, for example, of a highly stable solid-state generator of a fixed frequency f ₁ = 7.23 GHz, a superheterodyne receiver, a pulse shaper, and a circulator (not shown in the drawing) , a cryogenic system (CS) 2, including, for example, a closed-loop cryostat (not shown in the drawing), a superconducting electromagnet 3, a control unit (PCB) 4 by a superconducting electromagnet 3, a microwave insert 5 for the sample 6 of non-magnetic material, registration system (CP) 7 and computer (K) 8. In this case, the output of the microwave unit (MB) 1 is connected to the first input of the registration system (CP) 7, the input / output of the microwave unit (MB) 1 is connected to the input / output of the microwave insert 5 for sample 6 through the adapter 9, the input / output of the registration system (CP) 7 is connected to the first input / output of the computer (K) 8, the output of the control unit (BOOM) 4 by the superconducting electromagnet 3 is connected to the superconducting electromagnet 3, the input control unit (BOOM) 4 superconducting electromagnet 3 nen with the output of the computer (K) 8, the input / output of the cryogenic system (KS) 2 is connected to the second input / output of the computer (K) 8. The superconducting electromagnet 3 and microwave insert 5 for sample 6 are located inside the cryogenic system (KS) 2. Microwave the insert 5 for the sample 4 is made with the possibility of rotation around the longitudinal axis in the form of a waveguide 10 of circular cross section with a diameter of 3-5 mm, closed at the end of the transverse solid partition 11.

A high-frequency electron paramagnetic resonance spectrometer (see FIG. 2) may comprise a low-frequency generator (GN) 12 and modulation coils 13, while the first low-frequency generator (GN) 12 output is connected to modulation coils 13 located outside the circular waveguide 10, and the second output of the generator (GN) 12 low frequency connected to the second input of the registration system (CP) 7.

A real high-frequency electron paramagnetic resonance spectrometer, the block diagram of which is shown in Figure 1, operates in the pulsed mode as follows. The microwave power from the microwave unit (MB) 1 of the range 94 GHz or 130 GHz through the microwave insert 5 for sample 6 is supplied to sample 6. The rotation of the microwave insert 5 for sample 6 around the longitudinal axis makes it possible to set the orientation of sample 6 in the magnetic field created by the superconducting electromagnet 3. The rotation of the insert 5 is possible, for example, due to the fact that it is made of a waveguide 10 of circular cross section with a diameter of 3-5 mm, consisting of a fixed part and a part made for rotation and closed at the end peppermint solid partition 11. Parts of the waveguide 10 in this case are connected via a rotating joint. A cryogenic system (CS) 2, including, for example, a closed-loop cryostat, and a superconducting electromagnet 3 and a microwave insert 5 for sample 6, placed in the working volume of the cryostat, allow creating and developing a magnetic field on sample 6 to fulfill resonance conditions, as well as set the required temperature measurements. The high-frequency signal reflected from sample 6 is fed back through the microwave insert 5 for sample 6 to the input / output of the microwave unit (MB) 1, where it is detected and fed to the registration system (CP) 7 and then to computer (K) 8.

A high-frequency electron paramagnetic resonance spectrometer, the block diagram of which is shown in Figure 2, can operate in the continuous mode as follows. Modulation coils 13 connected to a low-frequency generator (GN) 12 create an additional oscillating magnetic field at a low frequency, which makes it possible to use synchronous detection to measure the EPR signal. Example. The inventive device is illustrated by the example of a prototype EPR spectrometer operating at fixed operating frequencies of 94 GHz and 130 GHz. The prototype device includes a microwave unit, a cryogenic system including a closed cycle cryostat in which the temperature is regulated in the range from 1.5 to 300K, a Helmholtz system superconducting electromagnet, a superconducting electromagnet control unit, a microwave insert for a non-magnetic stainless steel specimen, a recording system, and a computer. Microwave units were assembled in which the driving frequency was 7.23 GHz and the frequency conversion was used with a multiplication factor of N = 13 to obtain a frequency of 94 GHz and N = 18 to obtain a frequency of 130 GHz. A microwave insert for the sample and a superconducting electromagnet are located inside a closed-loop cryostat. The microwave insert for the sample is connected to the microwave unit through a rectangular waveguide / circular waveguide adapter. The insert is made of a circular waveguide with a diameter of 5 mm, consisting of a fixed part and a part made with the possibility of rotation and closed at the end by a transverse solid partition. Parts of the waveguide are connected through a rotating joint.

Figure 3 shows the EPR spectrum of a TiO ₂ : Fe crystal recorded on a prototype spectrometer at a frequency of 94 GHz using a resonator. The spectrum was recorded at a temperature of 20 K. Figure 4 shows the EPR spectrum of the same TiO ₂ : Fe crystal, at the same temperature, in a similar orientation, recorded on a spectrometer declared in this technical solution at a frequency of 94 GHz. The signal-to-noise ratio shows that the sensitivity of the spectrometer stated in this technical solution is not inferior to the resonator circuit of the prototype spectrometer.

Claims (2)

1. A high-frequency electron paramagnetic resonance spectrometer including a microwave unit, a cryogenic system, a superconducting electromagnet, a superconducting electromagnet control unit, a microwave insert for a sample of non-magnetic material, a recording system and a computer, while the superconducting electromagnet and microwave insert for the sample are located inside the cryogenic system, and the output of the microwave unit is connected to the first input of the registration system, the input / output of the microwave unit is connected via a sink with an input / output of the microwave insert for the sample, the input / output of the registration system is connected to the first input / output of the computer, the output of the superconducting electromagnet control unit is connected to the superconducting electromagnet, the input of the superconducting electromagnet control unit is connected to the computer output, the input / output of the cryogenic system is connected to the second input / output of the computer, characterized in that the microwave insert for the sample is made to rotate around the longitudinal axis in the form of a round waveguide sections with a diameter of 3-5 mm, closed at the end by a transverse solid partition. 2. The spectrometer according to claim 1, characterized in that it comprises a low-frequency generator and modulation coils, wherein the first output of the low-frequency generator is connected to modulation coils located outside the circular waveguide, and the second output of the low-frequency generator is connected to the second input of the registration system .

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