RU2796123C1 - Wide-range annular time-of-flight neutron diffractometer with variable resolution - Google Patents

Abstract

FIELD: neutron diffraction.

SUBSTANCE: variable resolution, wide-range annular time-of-flight neutron diffractometer consists of several ring detectors composed of planar neutron scintillation detectors. Each such detector can move along the axis of the primary beam along guides. Each ring contains several flat neutron counters placed in the form of a regular polyhedron on the inner surface of the ring, while the number and size of the counters are sufficient to form a closed ring. Each counter can be rotated around its own long axis using an actuator or other electromechanical device, adjusting the angle of incidence of the scattered neutron beam.

EFFECT: possibility of adjusting the measurement ranges, increasing the resolution and increasing the efficiency of registration of thermal neutrons.

1 cl, 4 dwg

Description

The invention relates to the field of neutron diffraction, and can be used to study the crystalline and magnetic structure of materials by neutron diffraction using the time-of-flight technique. The invention is intended for use on pulsed neutron sources or on stationary sources with neutron flow interrupters.

For neutron diffraction, diffractometers with ring detectors are used.

Gas-filled counters (usually cylindrical) based on helium-3 or BF ₃ are widely used as detectors. Also, scintillation detectors with flat scintillators are used [Marin V.N., Sadykov R.A., Trunov D.N., Litvin BC, Aksenov S.N. / Ring neutron detector for a time-of-flight diffractometer, consisting of linear scintillation detectors based on silicon photomultipliers // Instruments and Experimental Technique. - 2018. - No. 1. pp. 5-12. DOI: 10.7868/S003281621801007X]. For the latter, the displacement of the sample relative to the plane of the ring will inevitably lead to a change in the efficiency of neutron detection, since By changing the angle of incidence, the range of the neutron in the scintillator material also changes, and, consequently, the probability of neutron capture. It is also not possible to set the required resolution.

As a rule, with time-of-flight diffraction, ring detectors remain stationary, and scanning is carried out by changing the neutron wavelength with time. The range of interplanar distances measured by the ring detector is determined by the range of wavelengths. To increase the range, a large number of ring detectors are used, which leads to an increase in the cost of the diffractometer. Moving ring detectors of the proposed diffractometer allow you to choose the angle of neutron scattering. Rotating neutron counters, which form a ring detector, make it possible to achieve the maximum efficiency of neutron detection at any scattering angle. In the existing ring detectors, as a rule, gas-filled counters filled with ^3He are used. In the proposed diffractometer, scintillation counters are used as neutron counters. In particular, can be used described in [Marin V.N., Sadykov R.A., Trunov D.N. physics. - 2015. - V. 41, No. 18. - S. 96-101.] counters containing solid-state photomultipliers. Unlike helium counters, which require a voltage of up to 1.5 kV, the proposed scintillation counters have a bias voltage of no more than 35 V, while in terms of neutron detection efficiency they are not inferior to helium counters.

From the existing prior art known neutron diffractometers, which use ring detectors. A known diffractometer has two annular sections of detectors for scattering angles of 87-93° and 35-43° [Kozlenko D.P., Kichanov S.E., Lukin E.V., Savenko B.N. "The DN-6 Neutron Diffractometer for High-Pressure Research at Half a Megabar Scale", Crystals, 8(8), 331 (2018). doi: 10.3390/cryst8080331], and each section contains 16 blocks, each of which contains six helium counters separated by collimators. Thus, each section forms 6 detection rings.

However, this technical solution has the following disadvantages:

- detection of scattered neutrons at fixed angles

- fixed resolution

- helium meters are used, requiring high voltage

Also known is a diffractometer containing 7270 PMT-based scintillation detectors [Day P., Enderby J., Williams W. et al. Scientific Reviews: GEM: The General Materials Diffractometer at ISIS-Multibank Capabilities for Studying Crystalline and Disordered Materials // Neutron News. - 2004. - Vol. 15-1. - P. 19-23]. The device has a large solid angle and a wide range of scattering angles (1.1°-169.3°).

However, this technical solution has the following disadvantages:

- a large number of detectors and, as a result, high manufacturing and maintenance costs.

- fixed resolution

- PMTs are used that require high voltage

Thus, the known technical solutions have limited functionality.

Also known is a diffractometer [Patent SU 1293594 A1 IPC G01N 23/20], which includes a block of ring detectors moving along guides along the beam axis past the sample, setting the scattering angle. The detectors are equipped with collimators which are rotated by means of a rocker so that they are always pointed at the sample. This device is designed for monochromatic beams, but can also be used for time-of-flight techniques. The disadvantages of this technical solution is the inability to simultaneously measure the diffraction spectra at large, small and close to 90° angles, as well as the inability to adjust the resolution. In addition, when using scintillation detectors, there is no way to change their efficiency.

The invention is based on the task of improving neutron time-of-flight diffractometers: the maximum measurement range with the maximum efficiency of neutron detection by flat scintillation detectors, as well as the ability to set the required resolution of the device.

The technical result that allows solving this problem is the ability to select the measurement range, while maintaining the maximum efficiency of neutron detection, as well as the ability to vary the resolution of the diffractometer.

This technical result is achieved by the fact that the ring detectors can move along the beam axis, while changing the diffraction angle, and the counters can be rotated around its axis, which makes it possible to obtain the maximum efficiency of neutron detection or to set the required resolution of the device. The essence of the invention is illustrated by drawings.

In FIG. 1 explanation for the rotation of the counter that affects the resolution, i.e. demonstrates possible orientations of the counter relative to the incident beam, where:

1 - light guide,

2 - scintillator,

3 - sample,

α1 - maximum solid angle and low resolution,

α2 - minimum solid angle and maximum resolution.

In FIG. 2 shows the principle of operation of the device, where:

2Θ ₁ , 2Θ ₂ , 2Θ ₃ , 2Θ ₄ - scattering angle,

3 - sample,

4 - neutron beam axis,

5 - guides,

6 - mount,

7 - holder,

8 - ring detector.

In FIG. 3 shows an example of placing the detector on the ring, where

8 - ring detector,

9 - rectangular scintillation neutron counters.

In FIG. 4 explanation of the principle of detector rotation and resolution change, where:

8 - ring detector,

9 - rectangular scintillation neutron counters,

X - axis (each counter rotates around its own axis).

Thermal neutron ring detectors, consisting of planar scintillation counters, can be moved by means of stepper motors along guides along the beam axis.

A wide-range ring time-of-flight neutron diffractometer with adjustable resolution consists of two guides (5) fixed on a common base, parallel to the neutron beam axis (4). Ring detectors (8) move along the guides with the help of independent stepper motors. Each ring contains protection from background neutrons from boron-containing material and rectangular neutron scintillation counters (9) on the inner side of the ring, which form a regular polyhedron. Each detector consists of two light guide layers (1) and one scintillator layer (2). The rings have different radii to ensure that the specimen always remains in line of sight, and have fasteners (6) of different lengths to fasten the rings on two rails. Each counter can rotate around its own axis (the X axis in Fig. 4) by changing its angle relative to the axis of the incident beam. All counters in one ring are rotated by the same angle using an electromechanical device. Also, a sample (3) is fixed on the beam axis using a holder (7). A wide-range ring time-of-flight neutron diffractometer with adjustable resolution operates as follows: a collimated pulsed neutron beam hits a sample - a powder or a polycrystal. As a result of diffraction, neutrons are scattered into a cone with an angle corresponding to the Wulf-Bragg conditions. At the moment when this cone crosses the ring detector, neutrons are registered. Signals from the counters (pulses corresponding to neutron detection) are recorded using time-digital converters, which make it possible to obtain a time-of-flight spectrum. Further, a diffraction pattern can be obtained from the time-of-flight spectra. Signals from each counter can be registered both separately and summed up (according to the “OR” scheme) from all counters of the ring. The latter method saves time channels - a digital converter, but is not applicable for large loads of detectors, and also does not allow observing the texture of samples.

The resolution of the diffractometer is determined by the relation:

,

,

where θ is the scattering angle,

- погрешность определения угла,

- error in determining the angle,

Δθ ₀ - neutron beam collimation,

Δθ _d - angular size of the detector,

Δθ _s - angular size of the sample,

in this case, Δθ _d and Δθ _s are inversely proportional to the distance from the sample to the detector,

Δλ is the error in determining the neutron wavelength,

λ is the wavelength of the neutron.

By moving the rings along the beam axis, one can change the scattering angle (for example, 2Θ ₁ , 2Θ ₂ , 2Θ ₃ , 2Θ ₄ in Fig. 2), and hence the measured range of interplanar distances. In addition, the distance from the sample to the detector changes, and hence the resolution of the instrument.

In particular, by placing the detector in front of the sample along the beam (small scattering angles), one can observe a magnetic structure, which, as a rule, has a period larger than the atomic one. By placing the detector behind the sample (large scattering angles), high-resolution diffraction patterns can be obtained, which follows from expression (1). Since, as a result of the movement of the rings, the angle of incidence of the scattered neutron beam on the detector plane also changes, and the efficiency of the detectors depends on this angle (see [D.A. Buchny, V.S. Litvin, D.N. Trunov, V.N. Marin , S.N. Aksenov, R.A. Sadykov / Simulation of the efficiency and resolution of neutron detection by a scintillation counter based on ZnS(Ag):6Li // Surface X-ray, synchrotron and neutron studies - 2022. - No. 12. - p. 38-47. DOI.10.31857/S1028096022120081]), the design provides for the possibility of rotating each neutron counter using actuators or other electromechanical device so that the counter plane is perpendicular to the scattered neutron beam, and therefore the neutron detection efficiency is maximum. In addition, by turning the counters, a higher resolution of the diffractometer can be achieved by reducing their angular size (see Fig. 2, Fig. 4). All electromechanical devices are controlled by a controller or computer, and the ratio of the positions of the detecting rings and the rotation angles of the counters is determined by software.

The main advantages of a wide-range ring time-of-flight neutron diffractometer with adjustable resolution are:

- the ability to vary the measured ranges of interplanar distances and resolution.

- automatic adjustment of detectors for maximum efficiency.

- no high voltage

Thus, the use of this technical solution significantly expands the functionality of temporary neutron diffractometers due to tunable measurement ranges and resolutions. In addition, a wide-range ring time-of-flight neutron diffractometer with adjustable resolution is characterized by low cost and ease of operation, since based on inexpensive scintillation neutron detectors and does not require high voltage.

Claims (1)

A wide-range annular time-of-flight neutron diffractometer with adjustable resolution, containing a plurality of detector rings made of a boron-containing material located perpendicular to the neutron beam and having on its inner radius a plurality of neutron scintillation detectors located perpendicular to the neutron beam, characterized in that the detector rings are able to move along the beam and have in their composition an electromechanical rotary device of scintillation neutron detectors capable of changing the angle around its own long axis of the detector.

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