RU2816244C1 - Position-sensitive detector of thermal and cold neutrons from compact analysed sample - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to detection and measurement of nuclear radiation flux. Position-sensitive detector of radiation of thermal and cold neutrons from a compact test sample contains spacer inserts having a wedge shape and arranged so that the substrates are inclined relative to each other at an angle β, equal to the angle of the diverging neutron flux from the compact test sample falling into each chamber, determined by the formula where 2h is the width of the chambers on the side of the flow entering the chamber; R is the distance from the sample to the middle of the first anode wire, which is determined from the side of the flow entering the chamber. Detector itself acquires a fan-shaped structure and is placed so that neutrons fall on the surface of the cathode at the inlet of each chamber at the same small angle α, which is set from 1 to 5 degrees.

EFFECT: high efficiency of detecting the flow of scattered thermal and cold neutrons.

3 cl, 6 dwg

Description

The invention relates to the field of registration and measurement of the flux of nuclear radiation, namely to the registration of the flux of thermal and cold neutrons scattered on a compact sample, using a gas neutron detector with a converter made of a thin solid layer based on the boron isotope ¹⁰ B. The invention is intended to measure the density value neutron flux depending on the scattering angle and can be used for a wide range of instruments used in studying the nature of the scattering of thermal and cold neutrons in various substances, as well as for other applied and fundamental studies of neutron scattering, where it is necessary to cover a wide angle of neutron scattering from the compact samples being studied , the characteristic dimensions of which are much smaller than the distance from the sample to the detector.

The invention relates to the class of thermal and cold neutron detectors, which are gas multi-wire proportional chambers consisting of electrodes (anode and cathode), placed in a sealed housing, filled with a gas mixture. As a rule, in such detectors, thin wires are used as an anode, placed in a row with a certain pitch in one plane, and cathodes are conductive electrodes placed at a certain distance on both sides of the anode plane on a flat substrate. The substance of the neutron converter (converter of neutrons into charged particles registered in the gas chamber) is either the working gas filling the chamber, containing gas from the helium-3 isotope ( ^3He ) or a gaseous compound containing the boron-10 isotope ( ^10B ), or a thin layer of solid material containing the isotope boron-10 ( ^10B ) deposited on the surface of the cathode. To determine the coordinates of the neutron capture (conversion) event, anode wires and conductive parallel strips are used. To do this, strips are formed on one of the cathodes, or the solid-state converter layer is formed in the form of strips. Anode wires and strips are installed perpendicular to each other and oriented perpendicular to the neutron beam radiation axis. Position-sensitive gas detectors are manufactured using this principle.

Due to the shortage of ³ He associated with its artificial origin, many leading scientific centers are intensively developing new detectors based on the earth-available ¹⁰ V converter made of solid boron carbide, enriched in the isotope ¹⁰ B, - ¹⁰ B ₄ C. Capture reaction products The neutrons are particles of ⁴ He and ⁷ Li, scattering in almost opposite directions. There is a peculiarity of using a solid-state coating of the cathode with boron carbide. The thicker the coating layer containing the converter, the greater the number of neutrons that interact with it. In this case, ⁴ He and ⁷ Li particles, formed deep in the thickness of the converter, can, having experienced large losses, leave the layer with energies close to the detection threshold, or be absorbed by the converter material. The path length of the ⁴ He particle in the converter material ¹⁰ B ₄ C is approximately 3.4 microns, and ⁷ Li - 1.7 microns - (C. et al. “B4C thin films for neutron detection,” Journal of Applied Physics, vol. 111, no. 10, 2012) [1]. Optimal thickness of one layer, providing maximum detection efficiency, for wavelength is within 2.5 microns. In this case, the efficiency of the layer for thermal neutrons reaches no more than 5-8%, while the efficiency of detectors built on converter ³ He cannot reach 60-95%. One of the solutions to increase efficiency is to tilt the layer with the converter relative to the direction of neutron movement so that the plane of the converter layer is at a small angle a to the direction of incidence of the neutrons. Due to the tilt, the path length of neutrons in the near-surface layer of the converter increases, which leads to an increase in the amount of interaction of neutrons with ¹⁰ B nuclei at a shallow depth of the converter layer, and therefore to a greater yield of decay particles from the converter layer and greater registration efficiency, in comparison with perpendicular incidence of neutrons .

The effective neutron path length in the converter (effective thickness) d _eff is determined by the formula d _eff = d/sinα, where d is the physical thickness of the converter. For angles α=6°, 5° and 2°, the effective thickness d _eff becomes greater than the physical thickness d by 9.57, 11.47 and 28.65 times, respectively.

Neutron studies of a substance in a dense or, as they say, condensed state, record the field of thermal or cold neutrons scattered after interaction with the sample under study. To do this, neutrons formed in the form of a high-density beam are directed at the sample. Neutrons are scattered by the nuclei of the sample, carrying information about its structure. Neutrons, after interacting with the sample, form a divergent flow. Therefore, to obtain more complete information, it is necessary to cover as large a solid scattering angle as possible.

Among the detectors built on the principle of inclined geometry of a converting layer with boron carbide, designed to register a diverging neutron flux, one can single out a two-coordinate position-sensitive detector of thermal and cold neutrons according to the utility model of the Russian Federation No. 174185 U1 dated 10/06/2017 [2]. The detector consists of an input window placed perpendicular to the incident neutron flux, a sealed housing filled with a mixture of working gases 80% Ar + 20% CO ₂ at atmospheric blowing pressure, electrode systems (cathode and anode) that make up two detecting layers placed one behind the other . Each detecting layer consists of identical cells of proportional gas chambers placed in a row with a certain pitch. The cells of the gas chambers consist of an anode in the form of thin tungsten wires installed in a row, forming an anode plane, directed parallel to the input window of the detector. The anode plane is located in the middle between the cathodes, which are parallel aluminum plates with a converter deposited on their surface in the form of a thin film of boron carbide ¹⁰ B ₄ C, enriched with the ¹⁰ B isotope. The plates with the converter coated on both sides with a continuous film alternate with the plates with a converter applied on both sides to a previously passivated (to obtain an electrically insulating layer) surface in the form of strips directed perpendicular to the direction of the anode wires. The plates of the first detecting layer of cells are inclined at a small angle α (in the example α=6°) relative to the normal to the input window of the detector. The plates of the second detection layer are rotated at an angle -α opposite to the first layer so that a chevron structure is formed. This design makes it possible to register a divergent neutron flux by providing identical conditions over the entire area of the input window for the passage of the detecting layers by a divergent neutron flux incident with a flux divergence angle ϕ less than the inclination angle α.

The disadvantage of the detector is the low efficiency of neutron detection. Firstly, the design of the detector is based on the principle of neutrons passing through aluminum plates (four plates) with ¹⁰ B ₄ C converter layers deposited on them on both sides. Despite the fact that aluminum is a material with low losses of neutrons passing through it due to scattering , but placing the plates at a small angle α=6° increases its effective thickness by 9.6 times and enhances neutron scattering. In the example described, the thickness of each of the four plates is 0.5 mm. The total effective thickness of aluminum in the neutron path increases to more than 19 mm. To increase the efficiency of neutron detection, it is necessary to reduce the angle a, but at the same time, the effective thickness of the aluminum substrate rapidly increases, which similarly increases the loss of neutrons due to scattering. In this case, part of the scattered neutrons is recorded by the detector, distorting the true distribution of the scattered intensity of the neutron flux in space. Secondly, part of the incident neutron flux enters the end of the cathode plate and is scattered before entering the converter layer, which can be considered as shielding. With a cell pitch of 5 mm, the shielding effect is 0.5/5=0.1 or 10% for each detection layer.

The closest solution to the present invention, taken as a prototype, is the detector according to the patent of the Russian Federation No. 2797497C1 dated 06.06.2023 [3].

The prototype detector of thermal and cold neutrons, built on the principle of the incidence of a neutron flux at a small angle on a converting layer of boron carbide enriched with the ¹⁰ B isotope, consists of identical, parallel-standing units combined into a single sealed housing filled with a working gas at atmospheric pressure. multiwire proportional chambers enclosed between flat parallel substrates. The substrates are placed with a certain pitch, which determines the width of the chamber. On one side of the substrate, a cathode is formed containing a layer of boron carbide enriched with the ¹⁰ B isotope. On the second side of the substrate, a strip system is formed, which is electrically isolated from the cathode and substrate current-conducting strips of the same width, installed in a row with a certain pitch and oriented in a direction equal to small angle α with the normal to the plane of the detector input angle (hereinafter referred to as the normal). Between the substrates in each chamber, thin anode wires are installed perpendicular to the normal, located at a certain pitch in a row. The substrate is aluminum foil or a polymer film made of radiation-resistant material, which is attached on two opposite sides to the substrate holders in such a way that it represents a plane. The ends of the anode wires are attached to similar anode holders. Between the anode holders and substrate holders there are spacer inserts that define the width of the chambers, as well as upper and lower wedge-like inserts. The upper wedge-shaped inserts are installed above the upper chamber, and the lower wedge-shaped inserts are installed between the base and the lower chamber. Wedge-shaped inserts set the angle of inclination of the cameras from 1° to 5°; anode holders, substrate holders, spacers and wedge-shaped inserts are connected to the fastening elements. The fastening elements are designed to regulate the tension of the substrates and anode wires.

The boron carbide converter layer is chosen to be of such thickness that the neutrons incident on it are completely absorbed by it. This is the main difference between the prototype design and the analogue described above. This design principle excludes the registration of neutrons rescattered on the detector design elements. The use of foil as a substrate reduces the effect of neutron screening by the end of the substrate. In the prototype, when the angle α of neutron incidence on the cathode is reduced to 2°, the achievable neutron detection efficiency reaches 67% at wavelength (Kolesnikov A.G., Zalikhanov B.Zh., Bodnarchuk V.I., Kurilkin A.K., Gorbunov N.V. “Multi-foil” neutron detector based on thin-layer ¹⁰ B coatings // Innovative scientific research. 2023. No. 4-1(28), pp. 65-85. URL: https://ip-iournal.ru/) [4].

In neutron research centers, as a rule, the sample under study is placed in the path of a beam of slow neutrons and neutrons scattered on the sample into a solid angle of 4π are recorded. To obtain maximum information, it is necessary to cover the largest possible solid angle of the neutron flux diverging from the sample. In the case of a single detector with a small flat entrance window, large solid angle coverage is achieved using measurements with sequential movement of the detector. Or by placing identical detectors close to each other in a circle at the same distance from the sample under study. In this case, the dimensions of the sample compared to the distance to the detector are such that the sample can be represented as a point sample, hereinafter called a compact test sample. In this case, the registration of neutrons scattered on the sample under study can be interpreted as the registration of the radiation of neutrons diverging from one point. The angle ϕ of the diverging flux of neutron radiation from a compact sample entering a detector having an entrance window of 200×200 mm and located at a distance of 2, 5 and 20 meters is ≥5.7°, ≥2° and ≥0.5°, respectively . In work [4], the design of the detector according to the patent [3] was simulated using the Geant4 software package, which implements the Monte Carlo method, and it was shown that reducing the angle a of neutron incidence on the converter from 5° to 2° increases the efficiency of detecting neutrons at a wavelength from 42% to 67%, or 1.6 times and then from 2° to 1° - from 67% to 82%, or another 1.2 times. Such a high and nonlinear dependence of efficiency on the angle a of neutron incidence, which increases with decreasing angle a, when placing a prototype detector with an input window of 200x200 at a distance of 2 to 5 meters from the compact sample, leads to an unequal angle of incidence of neutrons on the converter of each detector chamber - prototype, which increases from α to α+ϕ along the input window of the detector, which in turn leads to unequal detection efficiency of neutrons of the detector, decreasing along the input window. To reduce the influence of the angle ϕ on efficiency, it is necessary to increase the angle of incidence α, which leads to a decrease in maximum efficiency. When placing the prototype detector at a distance of 20 meters, the angle α of neutron incidence on the converter of each chamber increases along the input window by 0.5°, which noticeably reduces the efficiency at the calculated incidence angle q=2°, and ranges from 67% to 57% along the input window. Increasing the angle α to a value greater than 5° reduces the effect of the angle ϕ on efficiency, but leads to a decrease in the maximum achievable efficiency of the detector, ranging from 42% to 40% along the input window.

The disadvantage of the prototype detector is a decrease in the efficiency of recording the flux of scattered thermal and cold neutrons diverging from the compact sample under study, the magnitude of which decreases as the distance from the sample to the detector decreases.

The technical objective of the invention is to obtain high efficiency in recording the flux of scattered thermal and cold neutrons diverging from a compact test sample.

The technical result is achieved due to the fact that the spacer inserts have a wedge-shaped shape and are located so that the substrates are inclined relative to each other at an angle β equal to the angle of the diverging neutron flux from the compact sample under study (hereinafter referred to as the sample) falling into each chamber (hereinafter referred to as the included flow into the chamber), determined by the formula:

Where

2h is the width of the chambers from the side of the flow entering the chamber;

R is the distance from the sample to the middle of the first anode wire, determined from the side of the flow entering the chamber. In this case, the detector itself acquires a fan-shaped design and is placed so that neutrons fall on the surface of the cathode at the entrance to each chamber at the same small angle α, set from 1 to 5 degrees. As an embodiment, each anode plane divides the chamber containing it into equal parts; in addition, spring elements are additionally installed between the fastening elements and the anode holders, as well as between the fastening elements and the substrate holders.

The proposed set of features provides a technical effect by forming a wedge-shaped chamber and assembling a fan-shaped detector structure from them. This detector design records the entire neutron flux diverging from the sample under the same conditions for all chambers. In this case, the sample is located at a given fixed distance R. High efficiency is ensured by the fact that at the entrance to all chambers neutrons fall on the cathode surface at the same small angle a from 1° to 5° and by the fact that thin substrates made of metal foil or polymer films are used , the tension stability of which is provided by spring elements. High flatness, maintained when high voltage is applied between the cathode and the anode wires, makes it possible to reduce the width of the gas chamber and the pitch (distance) between the anode wires, which leads to an increase in the counting rate and an improvement in the spatial and temporal resolution of the thermal and cold neutron detector. Description of the figures.

Fig. 1. Neutron detector (housing not shown)

1 - substrate;

2 - cathode;

4 - anode wires;

5 - substrate holder;

6 - panel of strip lamellas;

7 - strip lamellas;

8 - spacer insert;

9 - spring elements;

11 - anode holder;

12 - anode socket;

13 - anode lamellas;

14 - screed (fasteners);

15 - stand (fasteners);

16 - tensioner of spring elements;

17 - detector base;

18 - compact test sample (or sample);

19 - neutron flux emitted by sample 18;

ϕ is the divergence angle of the neutron flux incident on the detector from sample 18.

Fig. 2. Arrangement of the cathode formed on the substrate.

1 - substrate;

2 - cathode;

3 - strips;

5 - substrate holder;

6 - panel of strip lamellas;

7 - strip lamellas;

8 - spacer insert;

9 - spring elements;

10 - mounting holes.

Fig. 3. Design of a strip system formed on a substrate.

1 - substrate;

2 - cathode;

3 - strips;

5 - substrate holder;

6 - panel of strip lamellas;

7 - strip lamellas;

8 - spacer insert;

9 - spring elements;

10 - mounting holes.

Fig. 4. Anode structure.

4 - anode wires;

8 - spacer insert;

9 - spring elements;

10 - mounting holes

11 - anode holder;

12 - anode socket;

13 - anode lamellas.

Fig. 5. Cross-sectional diagram of the detector.

1 - substrate;

2 - cathode;

3 - strips;

4 - anode wires;

n is the direction of neutron movement;

2h - chamber width;

L - cathode length;

b is the size of the cathode sensitive area;

β is the angle of inclination of the substrates relative to each other in each chamber;

ϕ is the divergence angle of the neutron flux incident on the detector from the sample;

α is the angle of incidence of neutrons on the cathode surface at the entrance to the chambers (maximum angle of incidence of the neutron flux on the cathode surface);

α-β is the angle of incidence of neutrons on the cathode surface in a place as far as possible from the sample (minimum angle of incidence of neutrons on the cathode surface).

Fig. 6. Cross-sectional diagram of one of the chambers, explaining the operation of the detector. The central area has been cut out. The arrows on the right are electrical signals taken from the anode wires 4 and from the strips 3.

1 - substrate;

2 - cathode;

3 - strips;

4 - anode wires

n is the direction of neutron movement;

α is the angle of incidence of neutrons on the cathode surface at the entrance to the chambers (maximum angle of incidence of the neutron flux on the cathode surface);

β is the angle of inclination of the substrates relative to each other in each chamber;

α-β is the angle of incidence of neutrons on the cathode surface in a place as far as possible from the sample (minimum angle of incidence of neutrons on the cathode surface);

- the angle of inclination of the substrate relative to the anode plane dividing the chamber formed by the substrates in half;

d is the physical thickness of the cathode from the ¹⁰ B ₄ C layer;

d _eff - effective neutron path length in the ¹⁰ B ₄ C layer;

d ₁ is the depth of the neutron capture site in the ¹⁰ B ₄ C layer;

h - distance anode wires - cathode and anode wires - strips;

h+Δh is the maximum value of the distance anode wires - cathode and anode wires - strips, increased due to the fact that the substrates are deployed;

s is the distance between the anode wires;

Li is a circle of radius equal to the particle path length ⁷ Li;

He is a circle of radius equal to the path length of a ⁴ He particle.

In the proposed detector design, shown in the diagram of Fig. 5, due to the placement of the substrates at a small angle β relative to each other, the width of each chamber increases linearly in the direction of the neutron flow, that is, there is a linear increase in the distance anode wires - cathode and anode wires - strips from h to h+Δh. This leads to a weakening of the field strength along the length of the chamber, reducing the gas gain in the chamber, which introduces an error in the detection of neutrons. The assessment of this influence is illustrated by the figure in Fig. 5 and figure fig. 6. The area of registration of the divergent flow is determined by the size b of the sensitive zone of the detector. The incident neutron flux has a divergence angle ϕ, determined by the expression For example, for a sensitive area of 200×200 mm ² with a distance R to the sample equal to 2 m, 5 m and 15 m, the angle y is 5.7°, 2.3° and 0.8°, respectively. With a chamber width of 2h=4 mm, neutrons with divergence angles of 0.11°, 0.05° and 0.016°, respectively, which are equal to the substrate tilt angle β, will enter each chamber. In this case, the angle of incidence of neutrons on the cathode along its length L decreases from angle α to angle α-β. For a detector installed at a distance to the sample R=2 m at an angle α=2°, the minimum angle is α-β=1.89°, which affects the registration efficiency, increasing it from 67% to 68.5% [4], that is gives a maximum increase in efficiency of no more than 2.2%. The length of the cathode is For such a length, tilting the substrates at an angle β=0.11° leads to an increase in the chamber width by 2Δh=L sin β=0.2 mm. The spread of the anode-cathode distance from the average value is about 5%, that is Changing the anode-cathode distance linearly changes the gas gain. Thus, at a distance of R=2 m to the sample, the additional measurement error is ±5%. With a further increase in the distance R to the sample, the additional error rapidly decreases and does not need to be taken into account. When the distance to the sample is R≤2 m, it is advisable to take into account the influence of the chamber geometry, especially during channel recording from anode wires. Since the change in efficiency in the first approximation can be considered linear, we will apply the coefficient obtained during calibration, which makes it possible to quite simply take into account the influence of the tilt of the substrates.

The design of the cathode is illustrated in Fig. 2. As a substrate 1, either metal, for example, aluminum foil with a thickness of 14 to 50 microns, or a polymer film with a thickness of 10 to 30 microns made of a radiation-resistant material, for example polyimide or lavsan, is used. When high voltage (up to 4000 V) is applied to the cathode, the substrate 1 is affected by the electrostatic forces of mutual attraction between the substrate 1 and the anode wires 4. To prevent deflection of the substrate 1, it is pulled into a plane with great force. This tension is provided by substrate holders 5, to which the opposite sides of the substrate 1 are attached. As one of the options, a 2 mm thick stainless steel plate is taken as substrate holders 5, having holes 10 at the ends for attaching a tie 14, designed to tie the components of the detector into a single assembly. To avoid breakdown of the high voltage supplied to the cathode, the substrate holder plate 5 is covered with a layer of electrically insulating material, for example, covered with a polyimide or lavsan film 400 microns thick. To obtain cathodes of the same width, a mandrel is used in which a substrate 1 stretched with the same force is glued to the plates of the substrate holders 5, placed at the same fixed distance. For this, an auxiliary frame is used, larger than the required size of the substrate, onto which with a certain force stretch the substrate 1 with a cathode 2 and strips 3 pre-formed on it. The function of the cathode 2 is performed by a layer of boron carbide ¹⁰ B ₄ C, which is applied to one side of the substrate 1 using vacuum plasma sputtering of a magnetron target from boron carbide B ₄ C, enriched in the isotope ¹⁰ B to a value of at least 95%. The thickness d of the boron carbide layer ¹⁰ B ₄ C depends on the angle α of neutron incidence on the converter layer chosen during design. For example, for α=2° the thickness d=3.5 µm is used. The thickness is selected in accordance with the expression d _eff = d/sinα. The effective length d _eff of the neutron path in the ¹⁰ B ₄ C layer increases compared to the physical thickness d by 28.65 times and becomes d _eff = 100.3 μm. Calculations performed in [4] showed that for a thickness d=3.5 µm, no more than 10% of neutrons with a wavelength of and less than 1% of neutrons with wavelength 2 . In the case of using substrates made of polymer films, a layer of boron carbide is applied to the polymer substrate. The strip system is formed on the back side of the substrate 1. Strips 3 from a thin layer of aluminum are obtained, for example, by spraying aluminum in a vacuum through a mask onto the substrate 1 made of aluminum foil, previously coated with a layer of dielectric necessary for electrical isolation of strips 3 from the aluminum substrate 1. B As a dielectric, for example, aluminum oxide Al ₂ O ₃ is used, which is obtained by passivating the substrate before applying boron carbide ¹⁰ B ₄ C. The thickness of the layer of aluminum strips 3 should be sufficient to create conductive conductors and is, for example, about 70-100 nm. The width of the strips is 4 mm, the gap between the strips is 0.2-0.5 mm. In the case of using polymer films as a substrate 1, strips 3 are applied without an insulating layer, directly onto the substrate. A flexible panel 6 with lamellas 7 strips is placed on the side opposite to the sample under study. The flexible socket 6 is glued to the substrate 1. Conductors (not shown in the figure) made of thin wire are attached to the strips 3 with conductive glue, the second end of which is soldered to the lamellas of the 7 strips.

When assembling the anode, pre-tensioned anode wires 4 with a thickness of 15 to 50 microns are attached with glue to the anode holders 11, made similarly to the substrate holders 5. For this, a mandrel is also used, to which anode wires 4 are attached with a pitch of s = 2 mm, tensioned with a force depending on the thickness of the anode wire 4. Wires 4 with a thickness of 20 microns are tensioned with a force of 70 g. Sockets 12 are attached to each plate of the holder 11 with 13 lamellas. The number of 13 lamellas on socket 12 corresponds to half the number of anode wires 4, which allows even 4 wires to be soldered to 13 lamellas on one side, and 4 odd wires to 13 lamellas on the opposite side.

The detector is assembled in the following sequence. The racks 15 are attached to the base 17. Then spring elements 9 are attached to the substrate holders 5 and the anode wire holders 11. Next, an assembly is assembled from sequentially installed substrate 1 on holders 5 and anode wires 4 on holders I, starting and ending with substrate 1, with installation between them spacer frames 8. The substrates 1 on the holders 5 are installed so that in the assembled detector the lamellas 7 are facing in the opposite direction from the sample and the beam from the sample hits the cathode 2 of the substrate 1. The anode wires 4 on the holders 11 are installed so that the anode wires 4 divided in half the space formed between the substrates 1. To form the assembly, tie rods 14 are inserted into the holes 10 in the holders 5 and 11. The assembly is installed between the posts 15 and the spring elements are connected to the tensioners 16. After this, each substrate 1 is tensioned and the anode wires 4 are tensioned using regulating the stretching of the corresponding spring elements 9 to a given force by displacing the corresponding tensioners 16 of the spring elements. The displacement of the tensioners 16 is carried out, for example, by rotating the screws included in the design of the tensioners 16. The tensioners 16 in their design have a locking element, for example, shown in the figure in FIG. 1 nut that does not allow tension to loosen. After all the settings have been completed, the assembled assembly is compressed into a single unit using ties 14. The installation of the detector in the position with the calculated angle a relative to the sample is checked, for example, using a laser pointer and adjusted with additional adjusting devices (not shown in the figure).

The operating principle of the detector is illustrated by a cross-sectional diagram of one of the chambers shown in Fig. 6. For better viewing, the central sectional area is cut out. The diagram is shown for a chamber width of 2h=4 mm. Anode wires 4 divide the width of the chamber in half, so the distance anode wires 4 - cathode 2 and the distance anode wires 4 - strips 3 is h = 2 mm. In this case, the pitch between the anode wires 4 is s=2 mm. The thickness of the boron carbide layer is ¹⁰ B ₄ C d=3.5 µm. In the diagram, the thickness of the boron carbide layer, the thickness of the substrate and the thickness of the strips are shown enlarged and not to scale. The detected neutron n enters cathode 2, consisting of a layer of boron carbide ¹⁰ B ₄ C, at a minimum angle α-β=1.89°. Neutron capture by the ¹⁰ B isotope occurs at a depth d ₁ . During its decay, two particles ⁷ Li and ⁴ He are formed, scattering equally likely in opposite directions in any direction of the solid angle 4π. The path length of particles in the converter material made of boron carbide ¹⁰ B ₄ C is for ⁷ Li - 1.7 μm, for ⁴ He - 3.4 μm (G. Albani, et al "Evolution in boron-based GEM detectors for diffraction measurements: from planar to 3D converters" Meas. Sci. Technol. 27 (2016) 115902 (9p) [5], G. Nowak et al. "In-kind detector activity @ HZG for the ESS detector group", https://studylib .net/doc/9502615/am-cld---ess-indico [6]). In fig. 6 circles of different radii conventionally show the possible range of these particles in the ¹⁰ B ₄ C material. From a depth d ₁ <1.7 μm, either a ⁷ Li particle or a ⁴ He particle emerges into the gaseous environment of the chamber, depending on the direction of expansion. Due to the equally probable dispersion of particles, the number of registered particles is proportional to the value of the solid angle with the vertex at the neutron capture point, limited by the line of intersection of the travel radii with the surface of the boron carbide layer, which is larger the smaller the depth d ₁ . If the scattering particles do not end up in this solid angle, then the neutron will not be detected. If neutron capture occurs at a depth d ₁ >1.7 μm, then the detection of a neutron due to registration of a ⁷ Li particle will not occur, since this particle will not come out of the layer of ¹⁰ B ₄ C material. Registration of neutrons in a gas chamber is carried out due to the fact that ⁷ Li and ⁴ He particles have high initial energies, amounting to 1.47 MeV for ⁴ He (in 6% of cases 1.77 MeV) and for ⁷ Li 0.84 MeV (in 6% of cases 1.01 MeV) . The ionization potential of the working gas argon is 15 eV. Particles that enter the gas ionize the gas and form a track of ion-electron pairs along the path of movement. In the figure fig. Figure 6 conventionally shows the path of the particle and the electron-ion pairs formed. Electrons, as light and mobile particles, are directed to the nearest anode wires 4, which have a diameter of about 20 microns. Due to such a small diameter, a high electric field strength is formed near the anode wires 4, in which electrons, receiving energy during acceleration, produce secondary ionization (gas amplification). As a result, an electron avalanche occurs near the anode wire 4. The avalanche electrons go to the anode wires 4 that created them, and the ions drift towards the cathode 2 and strips 3. An electrical signal is read from each anode wire 4 (shown by an arrow), which determines the Y coordinate of the neutron capture point (taking into account the correction for h). The ion cloud induces a 3 charge pulse on the strips, the magnitude of which is proportional to the width m of the strip (m=4 mm). The pulse is read independently from each strip (shown by an arrow) and determines the X coordinate of the neutron capture point. On the one hand, reducing the strip width m improves the accuracy of determining the X coordinate. On the other hand, the signal amplitude is proportional to the strip area. Therefore, searching for the center of gravity of pulses from strips using mathematical processing can improve the resolution along the X coordinate.

High flatness, maintained when extremely high voltage is applied between the cathode and anode wires, makes it possible to create chambers up to 2h≤3 mm wide, operating stably at voltages up to 4000 V in the plasma gas discharge mode (B.Zh. Zalikhanov “Features of an electron avalanche in a large gas amplification", Letters to ECHAYA. 2006. Vol. 3, No. 2 (131), pp. 81-100 [7]). The time resolution and maximum load of the detector are determined by the cell of the gas chamber, i.e. the distance h between the anode wire and the cathode and the distance s between the anode wires, namely the product s⋅h. In [7], the duration of the anode signal at the base was measured in the mode of such a high gas amplification, which was ≈ 5 ns. To prevent products from neutron capture from escaping into the adjacent cell of the chamber, the thickness of the converter is chosen to be d=3.5 µm. In this case, the effective “thickness” of the converter along the path of neutrons is d _eff = 100 μm, which ensures the capture of incident neutrons by more than 95% [4].

Claims (7)

1. A position-sensitive detector of radiation of thermal and cold neutrons from a compact test sample, which is a sealed housing that includes identical gas multi-wire proportional chambers (hereinafter referred to as chambers), enclosed between thin substrates made of radiation-resistant material, fixed on two opposite sides to substrate holders so that a plane is formed; the cathode in each chamber is formed on one side of the substrate from a material enriched with the ¹⁰ V isotope; the anode is made of anode wires installed in a row with a certain pitch, placed perpendicular to the direction of neutron radiation entering the chamber, the ends of the wires are fixed to the anode holders so that they form an anode plane dividing the chamber into two parts; Between the anode holders and the substrate holders there are spacer inserts that set the width of the chambers, characterized in that the spacer inserts are wedge-shaped and are located so that the substrates are inclined relative to each other at an angle β equal to the angle of the diverging neutron flux from the compact sample under study (hereinafter referred to as the sample ), entering each chamber (hereinafter the flow entering the chamber), determined by the formula: Where 2h is the width of the chambers from the side of the flow entering the chamber; R is the distance from the sample to the middle of the first anode wire, determined from the side of the flow entering each chamber; in this case, the detector itself acquires a fan-shaped design and is placed so that neutrons fall on the surface of the cathode at the entrance to each chamber at the same small angle a from 1 to 5 degrees; 2. A position-sensitive detector of radiation of thermal and cold neutrons from a compact test sample according to claim 1, characterized in that each anode plane divides the chamber containing it into equal parts; 3. A position-sensitive detector of thermal and cold neutron radiation from a compact test sample according to claim 1, characterized in that the substrate holders and anode holders are equipped with spring elements.