RU52651U1 - SWITCHED ISOTOPE NEUTRON GENERATOR - Google Patents

Abstract

Field of technology: the utility model relates to the field of experimental nuclear physics, specifically to isotopic neutron sources, and can also be used in applications of nuclear technologies, such as the control of the presence of fissile materials, neutron activation analysis, and the search for explosive and narcotic substances.

Essence: in contrast to the known switched isotopic neutron generator containing an alpha particle source and a target made of a material capable of (α, n) reactions, mounted relative to each other with the ability to control the flow of alpha particles to the target, in the proposed receiving surface the target is fixed with a gap opposite the emitting surface of the source of alpha particles, geometrically similar to the receiving surface of the target, the ability to control the flow of alpha particles to the target is provided by izhnoyu-shutter screen, made of material completely absorbing alpha particles, and the set to be positioned in the gap between the target and the source of alpha particles and it is latched position. Also, a controlled neutron source - KING can be enclosed in a sealed enclosure. Specifically, it can be made in such a way that the shutter screen is equipped with an element of soft magnetic material, which is connected to an electromagnetic drive that allows you to fix the position of the screen outside the gap, and an elastic element that allows you to fix the position of the screen inside the gap, while the housing must be made of non-magnetic material.

Technical result: The technical result is to ensure the constancy (a fixed value) of the neutron yield of the switched source from on to on with safe use.

Description

1. The technical field and the intended area of use.

The proposed utility model relates to the field of experimental nuclear physics, specifically to isotopic neutron sources, and can be used in nuclear technology applications, such as fissile material monitoring, neutron activation analysis, and the search for explosives and narcotic substances.

2. The prior art.

Existing neutron sources can be classified as follows:

- isotopic neutron sources

- sources based on charged particle accelerators

- sources of spontaneous fission

- nuclear reactors and nuclear explosions

They differ in the spectrum of radiation, the presence of other types of radiation, pulsed or stationary nature of the action. The use of one or another source in various applications of nuclear technology is determined by the specific requirements of the process.

From the point of view of the task, sources of type 1 and 2 are interesting, since they allow the creation of a switched source of neutrons. This type of sources includes devices - analogues and a prototype. The neutron source of spontaneous fission, in principle, cannot be performed in a switchable version, based on the physics of the formation of neutrons in it. A nuclear explosion is a single-action source, and nuclear reactors are significantly more complex devices than sources of types 1 and 2, both in terms of the technology of their creation and their management.

2.1. Analog devices.

Neutrons in isotopic neutron sources are obtained by irradiating some light nuclei with alpha particles or gamma rays of radioactive isotopes. Historically, these sources were the first neutron sources and are widely used at present.

Sources of type (α, n) are usually of two types / M.A. Bak, N.S. Shimanskaya. Neutron sources. M. Atomizdat. 1969, p.30 /:

- a homogeneous mixture with a target substance, on the nuclei of which a neutron formation reaction occurs,

- a flat source in which the alpha-emitter layer is placed on a flat substrate and is covered by a plate of the target substance.

These sources act continuously, emitting per unit time in a first approximation a constant number of neutrons, determined by the design of the source. The magnitude of the neutron yield from the source is its most important characteristic, providing quantitative results in the measurements.

Continuous isotope sources require cumbersome protection of personnel from the radiation emitted by them, including when they are not used, for example, during transportation.

In sources based on charged particle accelerators / M.A. Bak, N.S. Shimanskaya. Neutron sources. M. Atomizdat. 1969, p. 4 / Neutron production reactions are used as a result of the interaction of accelerated protons, deuterons or alpha particles with target nuclei. Charged particles are accelerated when passing through a chamber in which an accelerating electric field is created. When the accelerating voltage is turned off, the formation of neutrons ceases (the source is turned off). A high vacuum must be maintained in the chambers in order to eliminate particle scattering on the residual gas molecules.

In the on state, the number of emitted neutrons (output) is determined by the type of accelerated particles, the target material and the acceleration mode. As a rule, the neutron yield varies from inclusion to inclusion and is measured by a special measuring complex.

Accelerators require bulky vacuum equipment, a powerful energy source for high-voltage power supply and highly qualified staff for reliable and stable operation.

2.2. Prototype device.

The closest (prototype) is a turn-off radioactive neutron source described in US patent No. 4829191 (published May 9, 1989). This source is a source (generator) of neutrons controlled (switched) type.

The source contains two plates. The first has a section on which an alpha emitter is applied (a source of alpha particles), the second has a section on which a target material is deposited, where neutrons arise when alpha particles are absorbed, i.e. the target is made of a material capable of (α, n) reaction under the action of alpha particles emitted by the source. During the operation of the source, the flux of alpha particles incident on the target is controlled as follows. With relative rotation of the plates, the alpha-emitting section of the first plate is periodically aligned with the target section of the second plate, accompanied by the appearance of a neutron flux (“on” position). When the plates are removed from this position (mismatch between the source – target pair and the “off” position), neutron production ceases. The source is quite safe to use, thanks to a sealed enclosure.

The disadvantage of this device is the lack of fixing the geometric position of the plates relative to each other, and therefore the magnitude of the neutron yield in the “on” position is not fixed.

3. The essence of the utility model.

The task is to create a safe enough to use switched isotope neutron generator (KING) with a fixed value of the neutron yield from on to on, which is necessary when conducting quantitative measurements using a neutron source.

The technical result is to ensure the constancy (fixed value) of the neutron yield of the switched source from on to on with safe use.

This technical result is achieved due to the fact that, in contrast to the known switched isotope neutron generator containing a source of alpha particles and a target made of a material capable of (α, n) reaction,

mounted relative to each other with the ability to control the flow of alpha particles to the target, in the proposed receiving surface of the target is fixed with a gap opposite the emitting surface of the source of alpha particles, geometrically similar to the receiving surface of the target, the ability to control the flow of alpha particles to the target is provided by a movable shutter screen made of a material that completely absorbs alpha particles, and installed with the possibility of placement in the gap between the target and the source of the alpha part q and out of it with fixing the position.

Also, a controlled neutron source - KING can be enclosed in a sealed enclosure.

Specifically, it can be made in such a way that the shutter screen is equipped with an element of soft magnetic material, which is connected to an electromagnetic drive that allows you to fix the position of the screen outside the gap, and an elastic element that allows you to fix the position of the screen inside the gap, while the housing must be made of non-magnetic material.

That is, a fixed fastening with a gap relative to each other, facing geometrically similar surfaces of the alpha emitter (source of alpha particles) and the target, ensuring their location in a fixed geometry relative to each other, in combination with the introduction of a movable screen - a shutter having the possibility of placement in the gap and fixation in the positions inside and outside the gap, allows to ensure the invariance of the geometry of the neutron source and, therefore, the invariability of the KING neutron output from cheniya for inclusion.

The shutter screen is made of a material in which neutrons are not generated under the influence of alpha radiation. It must have a thickness greater than the maximum alpha particle path in the gate material to ensure complete absorption of alpha particles.

Placing an alpha emitter, a target and a shutter screen in a sealed enclosure prevents alpha radiation from entering the environment and thereby ensures the safety of the use of the source, including the ecological one.

When implementing a neutron source with an electromagnetic drive in the “off” position (shutter between the alpha emitter and target plates), the screen can be held (fixed) by one or more elastic elements, which eliminates the possibility of accidentally shifting the shutter screen to the “on” position when changing position source in space (e.g. during transportation). In the “on” position, the shutter screen is translated by the action of an electromagnet,

which, when turned on, moves an element of soft magnetic material, rigidly attached to the shutter screen. Such a source switching mechanism guarantees a fixed neutron exit from the source in the “on” position and the practical absence of neutron exit from the source in the “off” position and, therefore, the source is safe during transportation in any position. The implementation of the housing is sealed from non-magnetic material (for example, stainless steel) allows you to control the output of the source using an electromagnetic drive.

In addition, an electromagnetic drive in combination with an elastic element provides the possibility of the source functioning in quasi-pulse mode, which is essential for a number of applications.

4. The list of figures and graphic images.

Figure 1 shows a diagram of the generator in the context in the off position,

where 1 is the target

2 - alpha emitter (source of alpha particles),

3 - shutter screen located in the gap,

4 - element of soft magnetic material,

5 - elastic element

6 - electromagnet,

7 - generator housing.

Figure 2 shows the generator diagram in section in the "on" position, with the same notation (3 - shutter screen, placed outside the gap).

Figure 3 shows a table of alpha-emitting isotopes and their properties.

Figure 4 schematically shows the device demonstration layout in the on position.

Figure 5 shows a table of the results of measuring the neutron yield from the KING layout.

5. Information confirming the possibility of achieving a technical result.

As a target, the neutron yield with the highest neutron yield in the alpha-particle energy range is about 5.3 MeV Be and B. The neutron yield of the (α, n) -reaction with ²¹⁰ Po alpha alpha emitter (alpha-particle energy is about 5.3 MeV) is 70 n / 10 ⁶ α-particles and 24 n / 10 ⁶ α-particles, respectively / V.V. Frolov. Nuclear-physical methods for controlling fissile substances. M. Energoatomizdat. 1989 g /. It should be noted that a source with a Be target has an average neutron energy of 4.5 MeV and the accompanying gamma radiation with an energy of 4.43 MeV. A source with a B target has an average neutron energy of 2.7 MeV and much weaker gamma radiation / M.L. Bak, N.S.Shimanskaya. Neutron sources. M. Atomizdat. 1969 /.

Suitable for use in the source alpha-radioactive isotopes should have the following properties:

- the half-life is long enough to allow the use of the source for several years, and at the same time, short enough so that the alpha activity per gram is high enough to obtain the necessary neutron yield in the target used,

- low neutron emission of spontaneous fission but compared with the neutron exit from the source by the reaction (α, n),

- a low level of gamma radiation accompanying the decay,

- the absence of a noticeable contribution of unwanted radiation from daughter products,

- reasonable price per gram and availability for production.

Possible to use isotopes and their properties are shown in the table shown in figure 3 / V.V. Frolov. Nuclear-physical methods for controlling fissile substances. M. Energoatomizdat. 1989; K. L. Hertz, N. R. Hilton, J. C. Lund, J. M. Van Scyoc. Alfa-emitting radioisotopes for switchable neutron generators. Nuclear Instruments and Methods in Physics Research A 505, p.41-45, 2003./

The neutron yield for a thin layer of alpha emitter is calculated using the formula

The value of Ω is the effective solid angle of exit of particles from the emitter and their hit on the beryllium target. For a thin layer of alpha emitter, when the plates are placed close to each other, this factor is 1/2.

For oxides, it is necessary to take into account the formation of neutrons on oxygen, for ²³⁸ PuО ₂ - 1.3 * 10 ⁴ n / s * g, for ²⁴¹ AmО ₂ - 2.5 * 10 ³ n / s * g.

For 10 mg of ²³⁸ PuO ₂ and a Be target, the yield is about (1-2) * 10 ⁵ n / s, on oxygen 1.3 * 10 ² N / s and spontaneous fission neutrons - 26 n / s.

A possible isotope for use is also ²⁴³ Cm, but it is less accessible than those shown in the table.

The level of gamma radiation from these isotopes in the off state of the source is negligible, which ensures the safety of handling the source in the off state. In the “off” state, gamma radiation from a source with the ²³⁸ Pu isotope consists mainly of one very soft line with an energy of about 45 keV. A millimeter lead filter completely absorbs it, as does the characteristic x-ray radiation. In the ²⁴¹ Am gamma spectrum there are no lines with energies above 370 keV. The highest intensity lines are 60 keV and 26 keV with absolute yields of 36% and 2.5%, respectively. A lead filter with a thickness of 1 mm reduces the intensity of ²⁴¹ Am gamma radiation by about 50 times / M.A. Bak, N. S. Shimanskaya. Neutron sources. M. Atomizdat. 1969 /

The fixed magnitude of the neutron yield from inclusion to inclusion allows the source to be used in quantitative measurements as a reference source.

We studied a demo sample (mock) of a switched neutron generator with a calculated neutron yield <10 ⁴ n / s, which is sufficient for measuring the main characteristics of KING.

Figure 4 shows the device demonstration layout. The movement of the shutter screen can be provided in accordance with the diagrams in Figs. 1, 2. To simplify the design, the shutter electromagnetic drive was not manufactured; the particle flow was shut off by manually moving the shutter.

The KING demonstration sample is a cylindrical cassette (case) (7) made of organic glass with a diameter of 66 mm and a height of 40 mm. Inside the case are located:

- a beryllium target in the form of a disk with a diameter of 20 mm and a thickness of 0.13 mm (1)

- emitter of alpha particles (layer ²³⁸ PuO ₂ ) (2),

- and a 1.2 mm thick stainless steel shutter screen in the form of a strip 70 mm long and 32 mm wide (3)

The distance (gap) N between the emitter layer and the beryllium disk was chosen equal to H = 7.17 mm. As the emitter (source) of alpha particles in the layout, 4 thin layers of oxide of the radioactive isotope ²³⁸ PuO ₂ deposited on a metal disk of titanium or tantalum with a diameter of 24 mm and a thickness of 0.2 mm were used. The diameter of the “active” spot of the emitter and the target is 18 ± 0.2 mm. With emitter No. 2, measurements were also taken at a distance of H = 2.87 mm. In order to avoid spontaneous movement of the shutter, a lock screw is provided in the design of the sample, which fixes the position of the shutter inside and outside the gap (in the non-working and working condition of the sample).

The calculation of the neutron yield for the geometry of the experiment took into account the absorption of alpha-particle energy in the air gap and a corresponding decrease in the specific yield of neutrons from the target.

The calculated specific yield from the layout for the distance between the plates of 0.717 cm is 37.9 n / 10 ⁶ alpha particles, with a distance of 2.87 mm - 56.4 n / 10 ⁶ alpha particles with an effective solid angle of 0.146 and 0.272, respectively.

A summary table of measurement results (measured output and comparison with calculation) are given in the table in fng. 5

The total yield of neutrons generated at a fixed position of the movable shutter screen (3) outside the gap between the source of alpha particles (2) and the target (1), at a distance (gap) between the ²³⁸ Pu layer and beryllium equal to 11 = 7.17 mm, and the mass of plutonium 844 μg is (2.9 ± 0.1) · 10 ³ n / s; at H = 2.87 mm and a plutonium mass of 297 μg - about (3.2 ± 0.1) · 10 ³ n / s. Fixing the position of the screen in the gap led to the complete absorption of alpha particles in the material of the screen and, accordingly, to turning off the neutron generator.

For the KING prototype prepared in this work, in the plutonium mass range from 60 to 844 μg, the average neutron yield of the prototype at H = 7.17 mm was 3.7 * 10 ⁶ n / s · g ²³⁸ Pu, at H = 2.87 mm - 10.8 * 10 ⁶ n / st ²³⁸ Pu, which is in good agreement with the calculated values.

The background neutron yield does not exceed 1.5-2% of the neutron yield in the on state.

The measured dose rate of gamma radiation in the off position was from the bottom of the prototype: close to 130 μSv / h, at a distance of 20 cm - 5 μSv / h.

The neutron yield from the source from measurement to measurement practically does not change, the statistical error of the yield measurements does not exceed ~ 3%, which confirms the possibility of achieving a fixed yield.

It was shown that the results of measurements and calculations of the neutron yield of the KING mockup practically coincide, which confirms the validity of the assessment of the characteristics of the switched ²³⁸ Pu-Be neutron source.

Thus, a fixed relative position of the source of alpha particles and the target (facing towards the emitting and receiving surfaces with their geometric similarity), providing the ability to control the flow of alpha particles between them by introducing into the device a movable shutter screen installed with the possibility of its location in the gap and outside the gap with fixing these provisions, it was possible to create a neutron source with a fixed value of the neutron yield from inclusion to inclusion with the safety of its use Bani.

Claims (3)

1. Switched isotope neutron generator containing an alpha particle source and a target made of a material capable of (α, n) reactions, mounted relative to each other with the ability to control the flow of alpha particles to the target, characterized in that the receiving surface of the target is fixed with a gap opposite the emitting surface of the source of the alpha part, geometrically similar to the receiving surface of the target, the ability to control the flow of alpha particles to the target is provided by means of a movable shutter screen, made of a material that completely absorbs alpha particles, and is installed with the possibility of placement in the gap between the target and the source of alpha particles and out of it with a fixed position. 2. The switched isotopic neutron generator according to claim 1, characterized in that it is enclosed in a sealed enclosure. 3. The switched isotopic neutron generator according to claim 2, characterized in that the shutter screen is equipped with an element of soft magnetic material, which is connected to an electromagnetic drive that allows you to fix the position of the screen outside the gap, and an elastic element that allows you to fix the position of the screen inside the gap, the housing is made from non-magnetic material.