RU101846U1 - DEVICE FOR FORMING MONO-ENERGY NEUTRON BEAMS OF LOW ENERGIES - Google Patents

Abstract

 A device for generating monoenergetic neutron beams of low energy, including an ultracold neutron confinement chamber with inlet and outlet openings, a means for interacting with neutrons mounted inside the confinement chamber, comprising a polyethylene element, characterized in that it is further provided with an output connected to the outlet of the confinement chamber neutrons by a channel having an exit window of aluminum foil, the means of interaction with neutrons is made in the form a polyethylene disk-absorber, the bottom of the ultracold neutron confinement chamber is made with artificial macro-roughness of the surface, the walls of the ultracold neutron confinement chamber and the walls of the neutron-emitting channel are coated with a liquid fluoropolymer.

Description

The proposed technical solution, a utility model, is intended for the formation of monoenergetic neutron beams from ultracold neutron gas (UCN), in particular, for the formation of monoenergetic neutron beams by acceleration of neutrons in the gravitational field when they fall in a vertical neutron guide with liquid mirror walls. The device can be effectively used on any UCN source with a wide energy spectrum. The need for monoenergetic low-energy neutron beams is high and is due to actively developing research on the use of cold and ultracold neutrons for nuclear physics and in neutron-physical studies of solids.

The task of obtaining low-energy monochromatic neutron beams was usually solved by the use of expensive time-of-flight selectors with mechanical breakers, which were installed on horizontal or vertical neutron leads of reactors. Devices of this type are described, for example, in [1, 2]. A disadvantage of devices of this type is the relatively low energy resolution of the emitted beams. So the spectrometer described in [2] has an energy resolution of about 20% for neutrons with a speed in the range of (1-100) ms. ^-one

A device [3] is known for producing monoenergetic beams in the UCN energy range (neutron velocity from 0 to 7 ms ^-1 ) based on an interference filter that extracts neutrons from an isotropic UCN flux with a neutron velocity component specified by the filter parameters. Thus, the interference filter passes neutrons not with a given energy, but with a certain component of the neutron velocity towards the filter surface. Therefore, the use of a filter requires additional collimation of the selected UCN flow to produce a monoenergetic beam, which leads to a loss of intensity. In addition, to change the neutron energy, it is necessary to manufacture a new filter, which is possible only for a limited set of neutron energies. Moreover, the formation of beams by interference filters is real only in a limited range of UCN energies. (scale 10 ^-7 eV.)

Also, the technical solution SU 1083793, IPC G01T 3/00 "DEVICE FOR DETERMINING THE DISTRIBUTION OF THE FLOW OF ULTRA-COLD NEUTRONS ALONG A NEUTROGEN" is known.

A device for determining the distribution of ultracold neutron flux along a neutron guide, containing a neutron guide and a neutron detector, characterized in that, in order to improve the accuracy of determining the distribution of the neutron flux, it is equipped with a cylinder made of ferromagnetic material with high boundary energy, which is placed in the neutron guide, a disk made of a hydrogen-containing material with zero boundary energy and attached to the end of the cylinder, a magnet equipped with a means of movement, placed near the cylinder form therewith a closed magnetic circuit, wherein the neutron detector located outside the neutron movable relative thereto. Moreover, the specified disk is made of polyethylene.

However, the proposed solution does not form a monoenergetic line, not all neutrons with energies higher than mgh (where m is the neutron mass, g is the gravitational acceleration, h is the height) have time to reach the absorber and leave the chamber, since absorption requires the vertical velocity component the neutron after some collision was sufficient to rise to a height h. Accordingly, the production of monoenergetic neutron beams, both of the ultracold range and of higher speeds (the range of very cold neutrons with energies greater than 2.0 × 10 ^-7 eV) is not ensured. The device does not work in this range

Closest to the claimed device is the solution SU 1074260 IPC G01T 3/00, G01T 1/36, "DEVICE FOR DETERMINING THE ENERGY DISTRIBUTION OF ULTRA-COLD NEUTRONS ACCUMULATED IN THE VESSEL".

The device comprises a storage vessel equipped with inlet and outlet nozzles with shutters and a shutter shutter, an ultracold neutron detector connected to the output nozzle, and a rod inserted into the vessel, the device being equipped with a thermal neutron detector placed inside the storage vessel and enclosed in a container on which an ultracold neutron heater is installed, while the container is connected to the rod,

moreover, a polyethylene layer is used as a heater for ultracold neutrons.

However, the known solution has the following disadvantages.

The device does not form a monoenergetic line, not all neutrons with energies higher than mgh have time to reach the absorber and leave the chamber, since absorption requires that the vertical component of the neutron velocity after some collision is sufficient to rise to a height h. Therefore, the formed spectrum has an admixture of neutrons with an energy higher than mgh. To reduce this impurity, it is necessary to increase the residence time of neutrons in the volume of the device. However, an increase in the residence time leads to a greater absorption of neutrons upon impacts on the walls and a decrease in the intensity of the resulting flux.

Modern sources of UCNs in reactors and accelerators provide a spectrum of UCN flux in the range (0-2 × 10 ^-7 ) eV. The device provides the formation of spectra only in this energy range. However, for a large number of experiments, monoenergetic neutron beams, both of the ultracold range and higher speeds (the range of very cold neutrons with energies greater than 2 × 10 ^-7 eV) are needed. The device does not work in this range.

The technical task of the proposed solution is to form a monoenergetic line so that all neutrons with energies higher than mgh have time to reach the absorber and leave the chamber, receive monoenergetic neutron beams of both the ultracold range and higher speeds (the range of very cold neutrons with energies greater than 2.0 × 10 ^{- 7} eV).

The specified technical problem is provided by using the proposed combination of essential features.

A device for forming monoenergetic neutron beams of low energy, including an ultracold neutron confinement chamber with inlet and outlet openings, a means for interacting with neutrons mounted inside the confinement chamber, including a polyethylene element, moreover, it is additionally equipped with a channel that outputs neutrons to the confinement chamber having an exit window of Al-foil, the means of interaction with neutrons is made in the form of a polyethylene disk the absorber, the bottom of the ultracold neutron confinement chamber is made with artificial macro-roughness of the surface, the walls of the ultracold neutron confinement chamber and the walls of the neutron-emitting channel are coated with a liquid fluoropolymer.

Description of the proposed solution

These disadvantages are absent in the proposed device for the formation of monoenergetic beams of UCNs or very cold neutrons by the method of accelerating neutrons of ultra-low energies in the Earth's gravitational field.

The device is illustrated graphically.

Figure 1 presents a diagram of a device for the formation of monoenergetic neutron beams by acceleration of neutrons in a gravitational field when they fall in a vertical channel with liquid mirror walls.

In figure 1, the positions indicated :.

1 - pipe source UCN;

2 - input neutron guide;

3 - input adjustable hole;

4 - UCN retention chamber;

5 - stock disk absorber;

6 - disk-absorber;

7 - output adjustable hole;

8 - a layer of spherical balls to create macro-roughness of the bottom of the chamber;

9 - vertical channel;

10 - an output window of aluminum foil.

As shown in figure 1, the device consists of an input neutron guide 2 connected to the pipe 1 of the UCN source, the UCN retention chamber 4 with the input 3 and output 7 adjustable holes, a movable polyethylene disk-absorber 6 and vertical (in the working position, when using the device ) channel 9 for accelerating neutrons in a gravitational field, equipped with an output window 10 of aluminum foil. The device operates as follows. UCNs with a wide energy spectrum come in the form of a neutron gas through an inlet 3 into a neutron confinement chamber 4. Moving in the confinement chamber, neutrons experience many collisions with its walls before they enter the outlet 7. Therefore, in the confinement chamber, after multiple reflections from the walls and a change in direction of movement, only neutrons with a small energy in the range from 0 to ΔЕ <mgΔH remain where ΔH is the height of the absorber disk relative to the bottom of the containment chamber. These neutrons cannot reach the disk. Higher-energy neutrons, falling on a polyethylene disk, inelastically scatter in it, become thermal and leave the system. By changing the position of the disk in height ΔH, it is possible to change the width ΔE of the starting spectrum of neutrons entering the vertical accelerating channel through the outlet.

Figure 2 on a larger scale presents a diagram of the confinement chamber to form the starting UCN spectrum.

Curved lines in figure 2 denote the parabolic trajectories of neutrons. The camera is equipped with two diaphragms at the inlet and outlet nozzles. By changing the aperture openings, the input and output UCN flow can be changed. The surface of the containment chamber is coated with a liquid fluoropolymer in the form of a thin viscous layer. The fluoropolymer provides a low coefficient of neutron absorption in collisions with the walls of the chamber (probability of about 3 × 10 ^-5 in a single collision), as a result of which neutrons remain in the volume for a long time before being absorbed into the disk or out of it through the openings of the diaphragms. A layer with macro roughness of the surface is created on the lower surface of the chamber. The layer is spherical balls (iron or lead shot) coated with a viscous fluoropolymer. The layer is designed for continuous and random changes in the direction of the velocity of neutrons when reflected from the bottom of the chamber. In this case, the probability that the neutron energy will be concentrated in the vertical component of its velocity increases significantly. The arrangement of the layer is an essential feature of the device, since with a high specularity of the chamber walls, the reflected neutron does not change the vertical velocity component. This effect leads to the fact that neutrons with an energy higher than ΔЕ, but insufficient for the vertical component of the velocity to rise to the disk, can get into the outlet. Macro roughness of the bottom of the chamber suppresses this effect. By changing the inlet and outlet of the chamber, it is possible to achieve the desired degree of purification of the UCN output stream from neutrons with an energy greater than ΔЕ.

In the accelerator channel, neutrons falling in the Earth’s gravitational field and following the path determined by the channel length H receive additional kinetic energy mgH and exit the output pipe 7 through a separation foil with a narrow beam with energy E = mgH and energy blur ΔE. << mgH. The angular divergence of the beam decreases with increasing channel length, since the vertical component of the neutron velocity increases with incidence, and the horizontal component is preserved. For every 1 meter of channel length, a neutron increases its energy by 10 ^-7 eV.

A key element of the shaper is a vertical accelerator channel with walls with high specularity. In the proposed device, the vertical accelerating channel can be made of arbitrary material without a special and expensive procedure for processing its inner surface (anode-mechanical polishing, etc.) to the desired class of surface cleanliness. The degree of roughness of the material on the inner surface of the channel itself does not matter. The mirroring of its walls is ensured by the fact that the inner surface of the channel is covered with a liquid fluoropolymer, for example, YL VAC18 / 8. This polymer is one of the class of fluoro-substituted liquid fluoropolymers with a low saturated vapor pressure (10 ^-8 Torr), a low reflection neutron absorption coefficient (3 × 10 ^-5 ) and a sufficiently large boundary energy (1.05 × 10 ^-7 eV) . Viscous fluoropolymer when applied to the surface of any metal creates a smooth mirror layer, ensuring the equality of the angles of incidence and reflection of neutrons upon impacts on the channel walls. The fluoropolymer is easily applied to the surface of the metal.

The mirroring of the channel walls ensures the preservation of the horizontal component of the neutron velocity during the fall. The latter is very important, since in diffuse reflection a neutron that has gained a lot of energy can leave the channel. This occurs when a significant portion of the neutron energy is redistributed into the horizontal component of the neutron velocity. If in this case the velocity component normal to the surface at the point of incidence exceeds the boundary velocity of the material of the channel walls, but the neutron is absorbed in the wall. When the probability of diffuse reflection is high, the incident neutron flux is greatly attenuated along the length of the channel. Due to its high surface tension, the smooth layer of liquid polymer is stable and ensures that the horizontal components of the neutron velocity are preserved during each collision with the wall. In this case, the neutron does not leave the channel space during the fall. The increase in neutron energy occurs only due to an increase in the vertical component of the neutron velocity. The neutron energy at the exit from such an accelerating channel is determined only by its length. So, with ΔН = 10 cm and a channel length of H = 5 m, the energy of the output beam will be 5 × 10 ^-7 eV, the relative energy resolution is 2%, and the neutron velocity is 10 ± 0.15 m / s.

An example of technical implementation.

The described device was repeatedly used with accelerating channels of various lengths at the Russian Research Center Kurchatov Institute. The sources of UCN were the IR-8 reactors and the high-flow reactor of the Laue-Langevin Institute. Monoenergetic neutron beams formed using the described device were used to study the diffusion propagation of cold neutrons in inhomogeneous media, measure the total cross sections, inelastic scattering cross sections, and other neutron physical problems.

So, to measure the inelastic scattering cross sections at the UCN source of the IR-8 reactor, this device was made in the form of a sectioned vertical neutron guide with a diameter of 8 cm made of stainless steel with a total length of 4 m. The channel walls were coated with YL VAC18 / 8 fluoropolymer. The chamber for the formation of the UCN start spectrum, made of stainless steel and also coated with YL VAC18 / 8, had a diameter of 30 cm and made it possible to form the initial UCN start spectrum with energies from zero to (5-10) × 10 ^-9 eV. A polyethylene disk with a diameter of 28 cm was used in the chamber. To create macro roughness, a lead fraction of 3 mm in diameter was used at the bottom of the chamber. The device allowed to obtain at different lengths of a vertical neutron guide (0.5 m, 1.0 m, 1.5 m, 2.0 m, 3.0 m, 4.0 m) energy beams with a blur of 5 × 10 ^-9 eV and energy, respectively (0.5 × 10 ^-7 eV, 1.0 × 10 ^-7 eV, 1.5.10 ^-7 eV, 2.0 × 10 ^-7 eV, 3.0 × 10 ^-7 eV, 4.0 × 10 ^-7 eV).

The proposed device can be used on existing sources of UCN research reactors, where research is carried out with ultracold and cold neutrons to study the processes of their interaction with matter: PNPI (Gatchina), JINR (Dubna), NIST (USA), PSI (Switzerland) ), ILL (France).

Information sources

[1] A. Steyerl. Nucl. Instr. and Meth. 101, 295 (1972).

[2] A.A. Antonov et al. Brief Communications on Physics. FIAN., No. 10, (1977), p.10.

[3] A.I. Frank and other communications of JINR, R3-2004-207, Dubna, 2004

Claims (1)

A device for generating monoenergetic neutron beams of low energy, including an ultracold neutron confinement chamber with inlet and outlet openings, a means for interacting with neutrons mounted inside the confinement chamber, comprising a polyethylene element, characterized in that it is further provided with an output connected to the outlet of the confinement chamber neutrons by a channel having an exit window of aluminum foil, the means of interaction with neutrons is made in the form a polyethylene disk-absorber, the bottom of the ultracold neutron confinement chamber is made with artificial macro-roughness of the surface, the walls of the ultracold neutron confinement chamber and the walls of the neutron-emitting channel are coated with a liquid fluoropolymer.