RU2300121C1 - Mode of detection of direction to the source of fast neutrons - Google Patents

Abstract

FIELD: the invention is used for detection of direction to the source of fast neutrons.

SUBSTANCE: the essence is in using a fiber-optical screen -transformation out of longitudinal fibers with a phosphor mask marked in a chess order on one of its end-face surfaces for registration of thermal neutrons, the changing of quantity of photons from one side of the fiber-optical screen-transformation to another is registered and according to calibration values of a light signal received under different angles of dropping of a beam of penetrating radiation or the plane on which the source of fast neutrons is defined according to the value a signal's gradient between opposite sides of the fiber-optical screen-transformer and for definition of direction to the source it is necessary to turn the fiber-optical screen-transformer so that its longitudinal axle be perpendicular to early found plane and repeatedly measure the direction of the gradient indicating in this case to the source of radiation. At that according to the value of the ratio of signals from thermal neutrons and fast neutrons identification of the source of radiation is carried out.

EFFECT: definition of the direction to the source of fast neutrons, identification of the source of radiation, exposure of nuclear substances and articles from them camouflaged in neutron retarding mediums.

2 cl, 6 dwg

Description

The invention relates to the field of research and / or analysis of materials by determining their physical properties, specifically to the detection of radioactive materials.

A known method of analysis of multicomponent materials, according to which the test sample is placed between the source of γ-radiation and the detector. Next, γ-quanta are recorded, the detector pulses are amplified and sent through a counter to a computing device (controller), after processing, the information is displayed. UK patent No. 2088050, G01N 23/08, 1998

The disadvantage of the invention is the need to use a radiation source and a detector with high energy resolution.

A known method for detecting contraband based on a polyenergy source of γ-radiation and spectrometric estimates of γ-radiation. Patent of the Russian Federation №2161299, IPC: G01N 23/08, 2000

The disadvantage of the invention is the difficulty in determining the composition of the hidden material, since the albedo (reflection index) of a wide beam of γ-radiation does not allow us to relate the intensity of the detected γ-radiation to the atomic number or density of the reflecting substance and, therefore, does not allow us to identify the nature of the hidden bookmark. A disadvantage of the invention is the need to use a radiation source and a high energy resolution detector.

A known method of determining penetrating radiation and nuclear-physical parameters and elemental composition of the assembly containing fissile material. Proton pulses of 5-50 ns duration irradiate the assembly, the temporal distribution of gamma rays is recorded for 200-2000 ns after passing through the object under study an irradiation pulse that causes the emission of delayed gamma rays in fissile materials. For each registered gamma-ray, its energy is measured, and the registration time is stored. Patent of the Russian Federation No. 2130653, G21C 17/06, Bull. No. 14, 1999

The invention is based on a comparison method with calibration data when registering delayed gamma rays and is applicable only in complex stationary installations.

A method is known in which heavy charged particles with an energy of 5-6 MeV in the form of short pulses and a MHz repetition rate are converted to receive a weakly directed fast neutron flux, a pencil-type neutron beam is formed, directed to an inspected object, and moved along the surface of the object. The substance of an object (bookmark, contraband) is activated by microvolumes, characteristic gamma-quanta of the elements that make up this substance are recorded. U.S. Patent No. 5076993, IPC: G21G 1/06, 1991

In this method, time-of-flight analysis is used to display the contents of an object on monitors to construct a three-dimensional image of the internal contents of an object.

The method is not applicable for the inspection of nuclear materials hidden in the environment of neutron-slowing element elements (such as water, oil, alcohol, etc.) or covered with protective shields from substances of large atomic number Z. The method is not applicable in the production and field conditions.

A known method of detecting penetrating radiation in the form of a neutron flux using a fiber block of layers of polymer scintillating optical fibers, based on the creation of recoil protons in the fiber material, the conversion of recoil proton energy into light radiation and registration of radiation by a position-sensitive photodetector. Patent of the Russian Federation No. 2119178, IPC: G01T 3/06, Bull. No. 26, 1998. Prototype.

The disadvantages of the prototype are that this method allows you to register only fast neutrons and does not allow to identify radiation, determine the direction of radiation. This method is not applicable for neutron energy, the path of recoil protons from which is less than the fiber cross section.

The present invention eliminates the disadvantages of analogues and prototype.

The technical result of the invention is the determination of the direction to the source of fast neutrons and the identification of the radiation source, the identification of nuclear substances camouflaged in neutron-slowing media and their products.

The technical result is achieved by the fact that in the method of detecting a source of fast neutrons using polymer scintillating optical fibers, based on the deceleration of fast neutrons, the production of recoil protons in a fiber material, the conversion of recoil proton energy into light radiation and the detection of radiation by a position-sensitive photodetector, fiber optical screen transducer of longitudinal fibers staggered on one of its end surfaces with a phosphor mask for registration of thermal neutrons, register the change in the number of photons from one side of the fiber-optic screen of the transducer to the other and the calibration values of the light signal obtained at different angles of incidence of the penetrating radiation beam or the value of the signal gradient between the opposite sides of the VEP, determine the plane in which the source of fast neutrons, and to determine the direction to the source, it is necessary to rotate the WEPP so that its longitudinal axis is perpendicular to that found previously plane and re-measure the direction of the gradient, indicating in this case the radiation source. The magnitude of the ratio of signals from thermal neutrons and fast neutrons carry out the identification of the radiation source.

The invention is illustrated in figures 1-6.

Figure 1 schematically shows the recorder of ionizing radiation in the form of a block of fiber optic screen-converter (VOEP), where

1 - position-sensitive photodetector,

2 - assembly of polystyrene scintillating hydrogen-containing fiber, made in the form of a cylinder (truncated cone, truncated pyramid),

3 - mask for recording thermal neutrons in the form of a phosphor layer, staggered on one of the end surfaces of the tow 2

4 - a beam of ionizing radiation, such as fast neutrons, φ-angle between the beam and the normal to the axis of the screen unit. The azimuthal displacement system, made, for example, based on a ball joint, is not shown in the figures.

Figure 2 shows the spatial distribution of photons from fast neutrons with an energy of 14 MeV, namely, the dependence of the signal on the distance to the central plane of the WEP at various incidence angles φ.

Figure 3 shows the spatial distribution of photons at the VEP exit, due to fast neutrons of various spectra, when the primary neutron beam is incident perpendicular to the fiber axis (φ = 0, the neutron beam falls to the right).

Figure 4 shows the spatial distribution of photons on the surface of the VEP due to thermal neutrons when a primary neutron beam of a different spectrum is incident perpendicular to the fiber axis (φ = 0, the neutron beam falls on the right).

Figure 5 shows the spatial distribution of the ratio of photons from thermal and fast neutrons at various angles φ (the neutron beam falls on the right) for various sources of fast neutrons.

Figure 6 schematically in two stages presents the determination of the direction to the neutron source.

A method for detecting penetrating radiation is to obtain two-dimensional images of the spatial distributions of fast and thermal neutrons using VOEP, a fiber bundle 2 of which with a mask 3 and a position-sensitive photodetector 1 serves simultaneously as a moderator of fast neutrons, a detector of both fast and thermal neutrons. For this, the tow 2 is made of fibers scintillating under the action of fast neutrons. A luminescent mask 3 is applied to the end of the bundle 2 to detect thermal neutrons resulting from the deceleration of fast neutrons or emitted by an external source 4.

Fast neutrons (beam of ionizing radiation 4) are slowed down in a bundle 2 of polystyrene scintillating fibers and, when neutrons are scattered elastically on hydrogen nuclei, lose their energy, produce recoil protons, which cause scintillations. Light from scintillation flashes propagates through the fibers of the tow 2 to its ends. To increase the amount of light (photons) collected, the fibers are coated with a reflective sheath.

The luminescent mask 3 for thermal neutrons is deposited in the form of a phosphor layer (light composition) Li ⁶ F + ZnS in a checkerboard pattern on one of the end surfaces of the bundle 2. This layer can also be made in the form of a transparent plate superimposed on the end of the bundle 2 and containing a phosphor, staggered.

Thermal neutrons formed in the bundle 2 as a result of deceleration escape outward through the surface of the bundle 2. To register them on the ends of the fibers, with a total area of several millimeters to several hundred millimeters, a phosphor is emitted (for example, in a checkerboard pattern) that emits light when the heat is captured neutron. This light is captured by the corresponding fibers and transferred to a position-sensitive photodetector 1. Since fibers with a phosphor deposited on them also detect fast neutrons, the signal only from thermal neutrons is determined by the difference in signals in adjacent areas of mask 3, designed to detect fast and thermal neutrons. To increase the signal from fast neutrons, sections of the surface of the mask 3, not covered with a phosphor, are coated with reflective material.

The detection of a source of thermal or fast neutrons occurs when the signal recorded by the position-sensitive photodetector 1 exceeds the level of the background signal taking into account its standard deviation (signal plus two or three standard deviations).

The direction to the source is determined by the direction of the fastest growth of the two-dimensional signal of fast neutrons and is found as a vector between the regions of the WEP used for detecting fast neutrons, symmetric about the axis of the WEP, for which the value of the function

F (θ) = ln (S ₁ / S ₂ ) / r _1-2

maximally, where:

S ₁ and S ₂ - averaged signals from fast neutrons in cells arranged symmetrically relative to the center of the image;

r _1-2 is the distance between the centers of the selected cells.

A schematic definition of the direction to the neutron source is shown in Fig.6.

The direction of the fastest growth of the signals recorded using the position-sensitive photodetector 1 at the VOEP output is the projection of the direction to the source on the VEP basis. The orientation of this vector makes it possible to determine the angle φ between the plane in which the source of fast neutrons lies and the X axis (step 1). To determine the direction to the source, it is necessary to turn the VOEP so that its longitudinal axis is perpendicular to the previously found plane (step 2).

The direction of the fastest signal growth determined from the second measurement makes it possible to determine the azimuthal angle θ and, thereby, determine the direction to the source of fast neutrons.

From the graphs in figure 2 it is seen that with a normal incidence of 14 MeV neutrons, the number of photons decreases from one edge of the VEP to another approximately 2.2 times. At lower neutron energies, this dependence is sharper. The number of photons decreases from one edge of the WEP to the other by 5.4 times for neutrons with an energy of 3 MeV and 7.9 times for the fission spectrum.

From the above dependencies in figure 2, 3, 4 shows that the direction to the source is detected in the direction corresponding to the largest ratio of the number of photons from fast neutrons on opposite sides of the WEP.

The direction to the source is in the plane defined by the axis of the VEP and the line corresponding to the largest decay of the signal between the opposite sides of the VEP. When the VOEP is rotated so that its axis becomes perpendicular to this plane, the newly obtained line corresponding to the largest decay of the signal directly indicates the source.

The spectrum of incident neutrons is identified by the following criteria:

- according to the spatial distribution of the optical signal from fast neutrons in VOEP (figure 3);

- according to the spatial distribution of the optical signal from thermal neutrons formed as a result of thermalization of fast neutrons in VOEP and flowing through one of its end surfaces (figure 4);

- the spatial distribution of the ratio of optical signals from thermal and fast neutrons (figure 5).

VOEP is made of ⌀15 cm polystyrene fibers and 10 cm high. A phosphor is staggered on one of the end surfaces of the VEPP, designed to detect thermal neutrons emanating from the VEPP, for example, Li ⁶ F + ZnS: Ag.

Fast neutrons are recorded with polystyrene fibers passing between the ends of the VOEP.

Figure 3 shows a uniform decay of the signal from fast neutrons with distance. The magnitude of the signal is determined by the product of the number of generated recoil protons and their average energy. With a decrease in neutron energy, the number of recoil protons increases due to an increase in the scattering cross section. However, this effect is much smaller than the effect associated with a decrease in the energy average over the spectrum of recoil protons. The signal decreases with decreasing neutron energy due to a decrease in their number and average energy, despite an increase in the proton production cross section for recoil. The difference in cross sections is manifested in the magnitude of the ratio of signals at the lateral boundaries of the VEP. The value of the ratio decreases with increasing primary neutron energy.

The ratio for primary neutrons with an energy of 14 MeV is approximately 2.2. For neutrons with an energy of 3 MeV, the ratio is 5.4. For neutrons in the fission spectrum of Pu ^{239, the} ratio is 7.9.

In Fig. 3, the spatial distribution of photons at the VOEP output is due to fast neutrons, when the primary neutron beam is incident perpendicular to the fiber axis (φ = 0, the neutron beam is incident on the right).

The signal from thermal neutrons is formed by fast neutrons that are slowed down in the WEP. The magnitude of the signal decreases with increasing primary neutron energy due to the fact that neutrons of higher energies are thermalized less efficiently. To identify the source, two-dimensional distributions of thermal and fast neutron signals previously measured for various sources of fast neutrons are used and the value of K _n , which is the ratio of signals from thermal and fast neutrons summed over the entire area of the position-sensitive photodetector 1. Table 1 shows the maximum and minimum K _n values with varying angles φ and θ

Neutron sources K _n (φ = -90 °) K _n (φ = 90 °) 14 MeV 1,2 1.4 3 MeV 10,2 8.8 ²⁵² C _f 19.5 18.6 Ra-be 5.8 6.8 ²⁴⁰ Pu 22.9 21.9 ²³⁵ U 22.9 21.6 ²³⁹ Pu 21.7 207

The table shows that the considered neutron sources can be divided into four groups according to the value of K _n :

- 14 MeV neutron generator;

- 3 MeV neutron generator;

- isotopic sources based on Be9 (α, n) C12 reactions;

- source of spontaneous fission neutrons.

The difference in the value of K _n between these groups exceeds 50%, which is significantly larger than the change in K _n with a change in the orientation of the EPEC relative to the neutron source. In the group of fission neutron sources, a ²⁵² Cf source is distinguished, K _n for which is approximately 20% lower than for fission neutrons of other isotopes.

The type of source is established by determining K _n at an arbitrary position of the VEP and comparing it with the value of K _n from the data bank. In the case of ambiguous determination of the type of source at an arbitrary position of the VEP, an additional measurement is carried out in which the VEP is set so that the neutron beam falls on the end of the VEP with a thermal neutron detector. With this orientation VOEP decreases the range of possible values, the coefficient K _n for this type of source. To increase the reliability of source identification, two-dimensional distributions of signals from fast and thermal neutrons measured in this position are compared with previously measured for various types of sources. As a criterion, the ratio of the values of the signals at the edges of the VEP is used.

The spatial distribution of thermal neutrons (figure 4) has a bell-shaped appearance. The distance to the incidence surface, at which the maximum of the thermal neutron flux is observed, is the smaller, the lower the energy of the primary neutron. The distribution maximum is observed at a distance of 1-2 cm from the central plane of the VEP. The maximum position is used as an additional criterion in establishing the type of source. To identify the source of fast neutrons, based on the spatial dependences presented in figures 3 and 4, carry out calibration measurements for a specific type of photodetector with various types of sources.

The best criterion for identifying a source of fast neutrons is the ratio of signals from thermal neutrons recorded by a Li ⁶ F + ZnS layer and fast neutrons (Fig. 5). The magnitude of this ratio is practically independent of the position of the source.

The magnitude of the signal ratio differs, by approximately an order of magnitude, in the transition from primary neutrons with an energy of 14 MeV to neutrons with an energy of 3 MeV and in the transition from neutrons with an energy of 3 MeV to neutrons in the fission spectrum.

The difference between the ratio for primary neutrons with an energy of 3 MeV and primary neutrons in the fission spectrum, despite the relatively small difference in the average neutron energies, is caused by the presence of a significant part of neutrons with an energy of less than 3 MeV in the fission spectrum.

Identification of the radiation source, based on measuring the ratio of optical signals: from thermal and fast neutrons, allows you to do without angular displacements VOEP.

Thus, the behavior of the ratio of the number of photons from fast and thermal neutrons solves the problem of identifying the spectrum of primary neutrons.

The direction of the gradient of the optical signal recorded with the matrix photodetector at the output of the VOEP allows us to determine the plane in which the source of fast neutrons is located.

In any case, this plane for a unidirectional beam passes through the VOEP axis. To determine the direction to the source, it is enough to turn the WEPP around the gradient direction by 90 degrees and conduct an additional measurement. The direction of the gradient determined from the second dimension directly indicates the source of fast neutrons.

Claims (2)

1. A method for detecting the direction of a fast neutron source using polymer scintillating optical fibers, based on the slowing down of fast neutrons, the production of recoil protons in a fiber material, the conversion of recoil proton energy into light radiation, and the detection of radiation by a position-sensitive photodetector, characterized in that they use fiber -optical screen-converter (VOEP) made of longitudinal fibers staggered on one of its end surfaces with a phosphor mask for p thermal neutron scattering, register the change in the number of photons from one side of the fiber-optic screen of the transducer to the other and using the calibration values of the light signal obtained at different angles of incidence of the penetrating radiation beam or the value of the signal gradient between the opposite sides of the WEP, determine the plane in which the source lies fast neutrons, and to determine the direction to the source, it is necessary to rotate the WEPP so that its longitudinal axis is perpendicular to the previously found plane sharpness, and re-measure the direction of the gradient, indicating in this case the radiation source. 2. The method of detecting penetrating radiation according to claim 1, characterized in that according to the magnitude of the ratio of signals from thermal neutrons and fast neutrons, the radiation source is identified.

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