RU2663683C1 - Method for registration of neutrons and device for its implementation - Google Patents

Abstract

FIELD: physics.SUBSTANCE: group of inventions relates to area of neutron registration by scintillation method using inorganic scintillation material. Essence of the inventions lies in the fact that neutron registration method comprises stages on which photons of scintillation are registered, which are formed in contact with neutrons in gamma ray scintillator based on inorganic material containing neutron absorbing components, and determine characteristics of neutron radiation, using inorganic scintillation material containing gadolinium atoms, photons of scintillations from gamma quanta are registered in energy range of 40–1,300 keV and make benchmarking analysis of line intensity and / or group of line intensity in measured spectrum of emitted gamma quanta in at least two energy ranges and according to the results of this analysis a conclusion about magnitude and spectral characteristics of registered neutron radiation is made.EFFECT: expanding of energy range and increase in sensitivity of neutron detector.10 cl, 9 dwg, 1 tbl

Description

Technical field

The invention relates to methods for detecting neutrons by the scintillation method using inorganic scintillation material obtained by one of the known methods, for example, by growing or drawing from a melt, producing glass from a melt, producing glass ceramics by heat treatment of glass, for use in ionizing radiation detectors in inspection technology fissile material detection devices, research, etc.

State of the art

Non-destructive testing equipment, detection of fissile materials and safety systems uses a wide range of physical methods for recording various types of ionizing radiation. Among them, the registration of neutrons is a separate task, since neutrons have a small cross section for interaction with the electron shells of atoms, as a result of which a significant modification of the methods for detecting other types of ionizing radiation is required. The neutron detection efficiency is determined by the cross section of the interaction of neutrons with the nuclei of various elements in the selected detector material.

The technique for detecting thermal and epithermal neutrons is based on interaction with isotopes of light elements having a high capture cross section, such as ³ He, ⁶ Li, and ¹⁰ V. The concentration of interacting nuclei in the working substance of the detector determines the sensitivity of the method.

Gas neutron counters are highly sensitive in the field of thermal and epithermal (epithermal) neutrons [Radiation detection and measurement. Glenn F. Knoll. Third edition. John Wiley & Sons Inc. New York, Chichester, Weinheim, Brisbane, Toronto, Singapore, 2000, 816 pp], but the increase in gas density in the meter is above 12-15 atm. leads to a significant decrease in its electrical properties, which leads to a deterioration in consumer properties.

The registration of neutrons using scintillation materials (scintillators) is based on the absorption of neutrons by nuclei with a high neutron capture cross section and the subsequent registration of secondary radiation (gamma, alpha, high-energy ions) with a scintillator. The sensitivity of solid-state detectors, for example, lithium silicate glass or glass ceramics containing lithium ions enriched in the ⁶ Li isotope in the matrix, is significantly higher than that of gas meters, due to a higher concentration of absorbing nuclei in the detector material. These materials have scintillation properties, and are widely used in scintillation detectors [Radiation detection and measurement. Glenn F. Knoll. Third edition. John Wiley & Sons Inc. New York, Chichester, Weinheim, Brisbane, Toronto, Singapore, 2000, 816 pp]. Among their most important characteristics, the output of scintillations is a necessary condition for the effectiveness of diagnostic equipment, as well as a condition for increasing the performance of any measurements using ionizing radiation [P. Lecoq, A. Gektin, V. Korzhik, Inorganic Scintillators for Detecting Systems, Springer, 2017, P. 408]. The disadvantage of lithium inorganic compounds is the small yield of scintillations.

A common drawback of all light nuclei that capture neutrons is the monotonous dependence of the absorption cross section of neutrons on their velocity v, which decreases according to the 1 / v law, which makes direct registration of fast neutrons ineffective.

It is possible to use fast neutron moderators based on materials containing light nuclei (graphite, plastics) with the subsequent registration of slow neutrons, however, this design significantly increases the dimensions of the detectors and reduces sensitivity due to scattering of neutron radiation in the moderator material.

A known method of detecting fast neutrons using ionization fission chambers. As nuclei interacting with neutrons, the nuclei of heavy elements U-236, U-238, Th-232 are used, sprayed onto walls or other internal structural elements of ionization chambers. In such chambers, gas ionization is caused by fission fragments. The disadvantage of ionization fission chambers is the small amount of absorbing substance in the chamber, as well as the use of isotopes of uranium and thorium, which requires preliminary separation of isotopes and, as a result, makes such detectors expensive.

A known method of detecting fast neutrons by recoil nuclei. Fast neutrons during elastic scattering on the nuclei of the medium transfer some of their energy to them, therefore recoil nuclei are detected. In scintillators containing light nuclei, for example, organic stilbene crystals ["Production of Stilbene for Fast Neutron Detection," Inrad Optics white paper (2012), http: //bit.1y/XnJzfw], recoil protons cause ionization and can be detected by scintillation outbreaks. When registering fast neutrons, recoil protons can leave the detector element based on a light organic crystal, thereby reducing the sensitivity of registration of fast neutrons. The same drawback is inherent in the detector elements based on organic plastics used for registration of recoil protons.

A known method of detecting thermal neutrons using composite scintillation materials consisting of particles of inorganic scintillation material containing gadolinium atoms bonded with organic glue [Galunov NZ, Grinyov BV, Karavaeva NV et al. (2011) Gd-bearing composite scintillators as the new thermal neutron detectors. IEEE Trans Nucl Sci 58 (1): 339-346]. The natural mixture of gadolinium isotopes has the largest thermal neutron absorption cross section of all elements due to the presence in the natural mixture of isotopes with a capture cross section of 259,000 barn ( ¹⁵⁷ Gd). This method allows you to register fast neutrons in soft lines in the spectrum of gamma rays emitted as a result of the interaction of gadolinium atoms and fast neutrons, 33 and 77 keV. The disadvantage of this method is the low detection efficiency of gamma rays of higher energies, in the range from 100-1300 keV, emitted as a result of the interaction of gadolinium atoms and fast neutrons, as well as a low scintillation yield due to the opacity of the composite, which reduces the efficiency of the device.

A known method of detecting predominantly thermal neutrons using an inorganic single-crystal scintillation material Gd ₂ SiO ₅ : Ce (GSO: Ce) surrounded by .. plastic scintillators acting as a spectrometer for the complete absorption of gamma rays in conjunction with GSO: Ce [PL Reeder / Neutron detection using GSO scintillator // Nucl. Instrum. and Meth. in Phys. Res. A., Vol. 340 (2), 21 Feb. 1994, pp. 371-378]. Since plastic scintillators are characterized by low absorption efficiency of high-energy gamma rays, the proposed total absorption spectrometer will either be ineffective or will require very large volumes of plastic scintillator; however, a thick layer of plastic scintillator inevitably scatters and absorbs part of the neutron radiation, thus reducing the efficiency of the neutron detector.

As a prototype, a method for detecting predominantly thermal neutrons was selected [RF Patent 2501040], namely, that the neutron is captured in a gamma ray scintillator based on an inorganic material containing neutron-absorbing components, then the light emitted from the gamma ray scintillator is measured, from it determine the total energy loss of gamma radiation after neutron capture, and classify the event as neutron capture, when the measured total energy loss is higher than 2.614 MeV. The neutron-absorbing components contained in the inorganic scintillation material, according to the method described in the prototype, must release the energy communicated to the excited nuclei after neutron capture, mainly through gamma radiation. The method is implemented on the basis of a device including a gamma ray scintillator, a light detector optically connected to it, and an evaluation device that is capable of comparing the magnitude of the photodetector signal with some predetermined values. The method for processing measurement data described in this method for detecting neutrons, which consists in comparing a certain value of the total energy loss with a threshold value of 2.614 MeV, allows one to separate the detected neutron from other types of radiation, but does not allow to obtain any information about the spectral characteristics (energy) of the detected neutron radiation.

Disclosure of invention

The technical result of the claimed invention is the possibility of detecting neutrons in a wide energy range with high sensitivity, providing the ability to obtain information about the energy of detected neutrons, in the case of neutrons emitted by a monochromatic source, or to create signatures of non-monochromatic sources of complex spectral composition, and to distinguish such sources by their signatures.

To achieve this technical result, a neutron detection method is proposed in which scintillation photons generated when neutrons enter the gamma ray scintillator based on an inorganic material containing neutron-absorbing components are recorded and neutron radiation characteristics are determined, using inorganic scintillation material containing gadolinium atoms , scintillation photons from gamma rays are recorded in the energy range of 40-1300 keV and a comparative Aliz intensity of the lines and / or trunk groups in the measured emission spectrum of gamma rays in at least two energy ranges, and the results of this analysis concluded on the magnitude and spectral characteristics of the detected neutron radiation. Besides:

- scintillation photons from gamma rays are recorded in the energy range of 40-600 keV

- compare the intensities in at least two lines or groups of lines selected in the energy range of 40-100 keV, 100-200 keV, and line 511 keV

- as inorganic scintillation material containing gadolinium atoms, use materials from the series: Gd ₂ SiO ₅ , Gd ₂ Si ₂ O ₇ , GdBr ₃ , Gd ₃ Al ₂ Ga ₃ O ₁₂ in the form of single-crystal or polycrystalline material containing as activator Ce atoms and / or Tb and / or Eu atoms, which are introduced into the composition of the material, replacing one or more elements in an amount of up to 2 at. %

- in the above inorganic scintillation materials containing gadolinium atoms replace up to 60% of Gd atoms with Y atoms or lanthanides.

- as an inorganic scintillation material containing gadolinium atoms, scintillation glass containing Gd, Si oxides is used, and Ce and / or Tb atoms are used as an activator.

Also, to achieve the indicated technical result, a neutron registration device is proposed comprising a gamma ray scintillator based on an inorganic scintillation material optically connected to a photodetector connected to an analysis device in which the inorganic scintillation material contains gadolinium atoms, the gamma ray scintillator and photodetector being surrounded by a screen from an inorganic substance that absorbs background gamma radiation.

Besides:

- as an inorganic scintillation material containing gadolinium atoms, use materials selected from the series: Gd ₂ SiO ₅ , Gd ₂ Si ₂ O ₇ , GdBr ₃ , Gd ₃ Al ₂ Ga ₃ O ₁₂ in the form of single-crystal or polycrystalline material containing as an activator, Ce, and / or Tb, and / or Eu atoms, which are introduced into the material by replacing one or more elements in an amount of up to 2 at. %

- in the above inorganic scintillation materials containing gadolinium atoms replace up to 60% of Gd atoms with Y atoms or lanthanides.

- as the screen absorbing background gamma radiation from an inorganic substance, an outer lead layer with a thickness of at least 1 mm and an inner layer of copper with a thickness of at least 1 mm are used.

According to the proposed method, neutrons are detected by absorbing them in an inorganic scintillation material containing gadolinium atoms Gd in a crystalline or amorphous matrix. When interacting with neutrons, the nuclei of gadolinium atoms emit a set of gamma rays in a wide energy range, causing scintillations in inorganic scintillation material. The most pronounced (having the highest yield, intense) lines in the spectrum of emitted gamma rays lie in the energy range from about 40 keV to about 1300 keV. Scintillations are detected by a photodetector, for example a photoelectron multiplier or a semiconductor photodetector. The volume and parameters of the scintillation material are selected sufficient for the efficient registration of gamma rays with energies up to 200 keV, preferably up to 600 keV or up to 1300 keV. Since, as a result of the interaction of neutrons with scintillation material, gamma rays that directly appear in the scintillation material of the detector are recorded, the proposed method significantly increases the sensitivity of neutron detection compared to known methods when interacting with neutrons to obtain gamma quanta is carried out in one material, and registration radiated gamma rays in another spatially spaced scintillation material. Compared with the known methods, when the interaction with neutrons to obtain gamma rays and their registration is carried out in one material, including - in comparison with the prototype, it is not a measurement of the integral value - the total loss of radiation energy in the material - but a measurement of the line intensity or groups of lines in more than one energy range; comparing the intensities of these various lines or groups of lines allows, in addition to the intensity of the recorded neutron radiation, to obtain information about its spectral characteristics: it allows you to determine the energy of high-energy monochromatic neutron sources in the energy range from no more than 100 keV to at least 10 MeV, or create signatures of non-monochromatic neutron sources of complex spectral composition, and distinguish such sources by their signatures.

The described method can be implemented using a neutron registration device, which includes a scintillation element of an inorganic scintillation material containing gadolinium atoms, and a photo detector surrounded by an inorganic substance absorbing background gamma radiation and connected to an analysis device based on a multi-channel amplitude analyzer, located off screen.

The combination of the above essential features leads to the fact that the sensitivity and information content of neutron detection are increased, which expand the range of applications of diagnostic methods, remote screening and detection using neutron detection.

Brief Description of the Drawings

In FIG. Figure 1 shows the calculated percentages of absorbed neutrons in a metal gadolinium line of natural isotopic composition of various thicknesses in the range of neutron energies of 0.0253 eV - 15 MeV.

In FIG. Figure 2 shows the spectrum of individual energies of the emitted gamma rays in a metal gadolinium with a thickness of 2 mm when irradiated with neutrons by an Am-Be source.

In FIG. Figure 3 shows the spectrum of the Am-Be source in the energy range 0-12 MeV used in the simulation, ISO 8529-2.

In FIG. Figure 4 shows the dependence of the area under the gamma lines in the energy range of 40-100 keV on the neutron energy in the spectrum of individual gamma-ray energies emitted in a metal gadolinium 2 mm thick when irradiated with monoenergetic neutrons (simulation in GEANT4).

In FIG. Figure 5 shows the dependence of the area under gamma lines in the energy range of 100-200 keV on the neutron energy in the spectrum of individual gamma-ray energies emitted in a metal gadolinium 2 mm thick when irradiated with monoenergetic neutrons (simulation in GEANT4).

In FIG. Figure 6 shows the dependence of the area of an individual gamma line with a maximum at an energy of 511 keV on the neutron energy in the spectrum of individual gamma-ray energies emitted in a metal gadolinium 2 mm thick when irradiated with monoenergetic neutrons (simulation in GEANT4).

In FIG. 7 shows the percentages of absorbed neutrons calculated in GEANT4 in various boron layers of a natural isotopic composition.

In FIG. Figure 8 shows the spectra of an Am-Be source measured at a thickness of boron absorber measured with a Gd ₃ Al ₂ Ga ₃ O ₁₂ : Ce detector with dimensions 18 × 18 × 7 mm ³ .

In FIG. 9 shows one embodiment of a device for implementing the proposed method. 1 - gamma ray scintillator, 2 - photodetector, for example, a photoelectronic multiplier;

3 and 4 — layers of the screen for absorbing background radiation; 3 — a layer of metallic copper; 4 - a layer of metallic lead.

The device also includes a signal converter based on a multichannel amplitude analyzer containing, for example, an analog-to-digital converter (not shown in the figure) connected to the photodetector 2 and located outside the screen. A registered neutron source (not shown in the figure) can be located on either side of the device.

The implementation and examples of implementation of the invention

The neutron detection method includes the following operations: 1) Preparation of a measuring circuit (detector), consisting of a scintillator 1, which contains gadolinium atoms, and equipment that allows you to measure the light emitted by the scintillator by any of the known devices, for example, using photoelectron a multiplier, a silicon semiconductor device, or in any other way.

The proposed method for detecting neutrons in a wide energy range is based on the features of the process of neutron capture by gadolinium nuclei. As a result of neutron capture by gadolinium nuclei, gamma rays are emitted, and the spectrum of emitted gamma rays depends on the energy of the absorbed neutron radiation.

In addition, the nucleus of the gadolinium atom has a high absorption cross section not only for thermal, but also for fast neutrons. In FIG. Figure 1 shows the percentages of absorbed neutrons calculated on the basis of absorption cross sections in a metal gadolinium line of natural isotopic composition of various thicknesses in the neutron energy range 0.0253 eV - 15 MeV. The resonance region is filled with color from maximum to minimum. In the region of fast neutrons, an increase in the absorption efficiency begins with an energy of about 50 keV.

In addition, the gadolinium atom has a large atomic number, and its presence in the matrix as a matrix-forming element allows us to increase the effective charge of the compound and, as a result, increase the efficiency of gamma-ray registration.

Thus, the use of a substance based on gadolinium as a sensitive scintillation material makes it possible to simultaneously provide a high neutron capture cross section in a wide energy range and high detection efficiency of secondary gamma rays.

The following are examples of scintillation materials containing gadolinium atoms, which can be used to implement the proposed method.

The table shows examples of scintillation materials containing gadolinium atoms, and their scintillation parameters are compared. The materials shown in lines 2-6 are examples of materials that can be used to implement the inventive method for detecting neutrons.

It can be seen that the scintillation material, based on an organic matrix of a plastic scintillator filled with gadolinium (line 1), is significantly inferior to inorganic scintillation materials in terms of characteristics. It has a significantly lower density, and, consequently, the efficiency of absorption of gamma radiation. Inorganic scintillation materials based on gadolinium have a significantly higher scintillation yield (for crystalline scintillators, lines 3-6), a high effective charge and photoabsorption coefficient of gamma quanta, and the concentration of gadolinium atoms in them is 2 orders of magnitude higher than the concentration of these atoms when introduced into plastic in an amount of 3 weight. % [VB Brudaninh, VI Bregadze at al. Element - loaded organic scintillators for neutron and neutrino physics, Letters in POP 6 (109) (2001) 69-77]. Among inorganic scintillators, GdBr ₃ : Ce and Gd ₃ Al ₂ Ga ₃ O ₁₂ : Ce have the best combination of parameters, however, GdBr ₃ : Ce is a hygroscopic material and requires special measures in the production of scintillation elements. Although the scintillation glass based on the Ba-Gd-Si-Ce composition (line 2) has a lower scintillation yield, comparable to the output of a material based on a plastic matrix, it has high transparency and is produced according to standard glass production technology, and therefore it can be produced in the form of large volumes necessary for the complete absorption of gamma rays in the energy range up to 1300 keV and higher. In the above examples of materials, the activator atom - Ce - can be introduced into the material, replacing any of the matrix elements or several such elements. In addition, scintillation materials activated by other known activators, such as Tb or Eu, can be used to implement the method.

The indicated scintillation materials do not limit the range of application of gadolinium-based materials in the invention, but outline a general approach to their selection and give the most suitable of those actually used.

To implement the inventive method, other scintillators containing gadolinium atoms in a sufficient amount to provide neutron detection efficiency sufficient for measurement purposes, as well as possessing other scintillator characteristics suitable for measurement purposes, can be used.

2) For the used detector based on gadolinium-containing scintillation material, the following parameters are determined:

F (E _n ) is the function of the dependence of the detector response in the energy ranges of 40-100 keV (F _40-100 ), 100-200 keV (F _100-200 ), the line area is 511 keV (F ₅₁₁ ), on the neutron energy. F (E _n ) can be calculated by the Monte Carlo method, or determined experimentally, or calculated, and then supplemented by experimental data for non-monochromatic neutron sources.

- функцию зависимости чувствительности к гамма - квантам (ослабления, поглощения) гадолиний-содержащего материала (в том числе сцинтилляционного) от энергии гамма - квантов для заданной величины энергии (гамма-линии), или усредненная в требуемом диапазоне. Может быть рассчитана, например, в GEANT4, или в [https://physics.nist.gov/PhysRefData/Xcom/html/xcom1.html], или определена экспериментально. Определяют численный параметр для каждого из указанных выше энергетических диапазонов: λ_40-100, λ_100-200, λ₅₁₁.

- a function of the dependence of sensitivity to gamma quanta (attenuation, absorption) of gadolinium-containing material (including scintillation) on the energy of gamma quanta for a given amount of energy (gamma line), or averaged over the required range. It can be calculated, for example, in GEANT4, or in [https://physics.nist.gov/PhysRefData/Xcom/html/xcom1.html], or determined experimentally. A numerical parameter is determined for each of the above energy ranges: λ _40-100 , λ _100-200 , λ ₅₁₁ .

3) Then measure the detected neutron radiation by placing the detector in the region of the measured neutron radiation, or by placing the neutron source in the sensitivity region of the detector (this can be implemented by any of the known methods). The intensity of the recorded signals (the amount of light emitted by the scintillator) of the response to gamma rays in the energy ranges of 40-100 keV (S _40-100 ), 100-200 keV (S _100-200 ), as well as the line intensity with a maximum at 511 keV ( S ₅₁₁ ).

Preferred options for determining signal intensities are recording the amplitude spectrum of the recorded neutron radiation, measuring the amount of light emitted during registration by the gadolinium-containing scintillation material of gamma quanta emitted during the interaction of neutrons with the nuclei of gadolinium atoms and the subsequent calculation of the integral spectral intensity of line groups in the energy ranges 40 -100 keV (S _40-100 ), 100-200 keV (S _100-200 ) and line intensity with a maximum at 511 keV (S ₅₁₁ ). This can be done by Gaussian approximation of the shape of the lines and linear interpolation of the background stand under them, or in any other way.

Another example of a method for determining the intensity of recorded signals S _40-100 , S _100-200 , S ₅₁₁ is the direct accumulation of signals in these energy ranges without intermediate recording of the amplitude spectrum and calculation of the area under the spectral lines.

The intensity of the recorded signals in the energy ranges: 40-100 keV (S _40-100 ), 100-200 keV (S _100-200 ), 511 keV (S ₅₁₁ ) can be determined by any other known method.

When nuclei of gadolinium atoms interact with neutrons, gamma-rays with a characteristic spectral composition are emitted. In FIG. 2 shows the simulated in the GEANT4 software package [S. Agostinelli et al. / Geant4 - a simulation toolkit // Nucl. Instrum. and Meth. in Phys. Res. A., Vol. 506 (2003), pp. 250-303] using statistical Monte Carlo methods [N. Metropolis, S. Ulam / The Monte Carlo Method // Journal of the American Statistical Association, Vol. 44, No. 247 (1949), pp. 335-341], the spectrum of the individual energies of the emitted gamma rays in a metal gadolinium 2 mm thick (a thickness sufficient to completely thermalize the positrons in the gadolinium resulting from the reactions of neutrons with gadolinium and, as a result, the maximum possible output of the gamma line from the annihilation gamma rays) when irradiated with neutrons from an Am-Be source with a wide and known spectrum of emitted neutrons, which is shown in FIG. 3. When simulating the spectrum of gamma rays resulting from neutron reactions of the Am-Be source in the gadolinium, it was smoothed so that the numerous lines in the spectrum, especially in the soft part, merged into groups as it happens in the real spectrum measured by a low-resolution scintillation detector.

In the spectrum of gamma radiation generated from the interaction of neutrons with gadolinium, one can distinguish the most pronounced soft lines and groups of lines that have energies of 511 keV (monoenergetic line from annihilation gamma rays), about 190 keV (group of lines) and about 90 keV (group of lines )

The intensities of each of these groups of lines depend on the neutron energy with which the gadolinium nuclei interact. In FIG. 4-6. dependences of the area under gamma lines in the energy ranges of 40-100 keV (S _40-100 ), 100-200 keV (S _100-200 ) and the area of the 511 keV line (S ₅₁₁ ) versus neutron energy are shown. The dependences were obtained by processing the spectra of emitted gamma rays simulated in the GEANT4 software package in 2 mm thick metal gadolinium when it was irradiated with equal fluxes of monochromatic neutrons of the following energies: 0.1 MeV, 0.3 MeV, 0.5 MeV, 1.0 MeV, 3.0 MeV, 5.0 MeV, 10.0 MeV. When modeling the spectra, it was smoothed in such a way that the numerous lines in the spectrum, especially in its soft part, were transmitted separately and were well approximated by the Gauss function. The area of lines and groups of lines in the indicated energy ranges was determined by a Gaussian approximation of the shape of the lines and linear interpolation of the background stand under them. It is fundamentally important that the form of the given dependences differs significantly in the entire range of neutron energies.

Thus, the ratio of the integrated intensities of the indicated groups (spectral ranges) of the lines can be used to determine the spectral characteristics of the detected neutron radiation.

4) Then, a calculated determination of the energy (for monochromatic neutron radiation) or the average energy (for non-monochromatic neutron radiation) of the detected neutrons is carried out, or the values obtained in relative units are compared with the values previously measured for sources of interest for distinguishing.

To determine the energy (for monochromatic radiation of neutrons) or the average energy (for non-monochromatic radiation of neutrons) of the detected neutrons, a system of at least two of any equations from the set (1), (2), (3) is solved numerically:

where Ф _n is the neutron flux (neutron radiation intensity) or the activity of a neutron source (accurate to a calibration coefficient); E _n is the energy or average energy of the source; S _40-100 , S _100-200 , S ₅₁₁ - intensities of the signals recorded by the detector, signals in the energy ranges 40-100 keV, 100-200 keV, line area 511 keV, respectively; F _40-100 ( _En ), F _100-200 ( _En ), F ₅₁₁ ( _En ) are functions of the dependence of the detector response in the energy ranges of 40-100 keV, 100-200 keV, the line area of 511 keV and neutron energy; λ _40-100 , λ _100-200 , λ ₅₁₁ - sensitivity coefficients of the detector to gamma quanta in the energy ranges 40-100 keV, 100-200 keV, 511 keV; relative to two unknown parameters - Ф _n and E _n .

If necessary, depending on the range of neutron energies, the set of the above equations can be expanded to include equations similar to (1), (2), (3) for any expressed lines or groups of lines in the gamma-ray energy range up to about 1300 keV.

In addition, a library of experimental data of values S _40-100 , S _100-200 , S ₅₁₁ (as well as for other energy ranges) for sources intended for detection can be recorded. Then, when registering neutron radiation from an unknown source, the measured values of S _40-100 , S _100-200 , S ₅₁₁ can be compared with the values from the library, the coincidence of the values will mean identification of the source.

As an example of the implementation of the method, a device is presented that consists of a scintillation detector 1 based on a gadolinium-containing scintillator with dimensions that provide a high degree of thermalization in the used positron scintillator resulting from the interaction of neutrons with gadolinium atoms, photodetector 2 based on an electrovacuum or semiconductor device, external protection - a two-layer screen from background gamma radiation 3, 4 with a thickness sufficient to effectively suppress natural ha MMA background in the selected analyzed range of energies generated by neutrons in the gadolinium of gamma quanta, as well as a signal converter based on a multi-channel amplitude analyzer containing an analog-to-digital converter (not shown in the figure) located behind the screen and connected to photodetector 2.

As the scintillator, for example, the following materials activated with lanthanides can be used: scintillation glass based on the composition Ba-Gd-Si; gadolinium orthosilicate Gd ₂ SiO ₅ or a mixed orthosilicate of gadolinium and another element from the group of lanthanides (Gd, Ln) ₂ SiO ₅ ; gadolinium pyrosilicate Gd ₂ Si ₂ O or a mixed pyrosilicate of gadolinium and another element from the group of lanthanides (Gd, Ln) ₂ Si ₂ O ₇ ; gadolinium bromide GdBr ₃ or a mixed bromide of gadolinium and another element from the group of lanthanides (Gd, Ln) Br ₃ , a complex oxide with a garnet structure containing gadolinium Gd ₃ Al ₂ Ga ₃ O: Ce, as well as other inorganic scintillators containing gadolinium.

External protection against background radiation should effectively absorb gamma radiation in the energy range of 40-600 keV, and should be selected based on the planned conditions for the use of the device and the requirements for its technical characteristics, in particular, for weight and size characteristics. For example, the screen may consist of a layer of lead 4, and in this case, in a preferred embodiment, include an additional layer 3 of internal copper or copper-containing protection from characteristic lines of lead with energies <100 keV falling within the analyzed range of neutron-generated gamma-ray gadolinium lines quanta, with an equivalent thickness of metallic copper of at least 2-3 mm. Or, protection based on rare earth compositions may be used. Also, for protection, any other known materials for protection against gamma radiation can be used. The device operates as follows:

The outer layer of protection absorbs background gamma radiation, as a result of which the scintillator selectively detects neutron radiation, according to the claimed method. This improves the accuracy of the claimed method of detecting neutrons.

Example 1

A Gd ₃ Al ₂ Ga ₃ O ₁₂ : Ce crystal with dimensions 18 × 18 × 7 mm ^{3 was} used to make a scintillation detector with light sensing using a photoelectron multiplier. An Am-Be source was used as a neutron source, placed in a shaft 10 cm in diameter surrounded by a boron-based neutron absorber. The distance from the source to the detector was about 20 cm. A lead disk with a diameter of 8 cm and a thickness of 2.6 was installed in the immediate vicinity of the source cm to weaken the influence on the detector gamma rays emitted by the source and the neutron absorber surrounding the mine. In the immediate vicinity of the detector, a second lead disk with a diameter of 8 cm and a thickness of 1.4 cm was installed to reduce the influence on the detector readings of gamma rays emitted by a neutron absorber based on boric acid H ₃ BO ₃ , a plastic container with which was installed between lead disks. With the same “live” set time, two amplitude spectra of gamma rays recorded using a Gd ₃ Al ₂ Ga ₃ O ₁₂ : Ce detector were recorded, in one case the thickness of the boric acid layer in the container was 4.5 cm, and in the other - 10.5 cm. In this way, neutron emission with different spectral characteristics was simulated - the relative fraction of fast neutrons in it is the higher, the greater the thickness of the boron absorber.

In FIG. Figure 7 shows the percentages of absorbed neutrons calculated in GEANT4 in various boron layers of a natural isotopic composition.

In FIG. Figure 8 shows the spectra of the Am-Be source measured at a Gd ₃ Al ₂ Ga ₃ O ₁₂ : Ce detector with dimensions 18 × 18 × 7 mm at various boron absorber thicknesses. Processing these spectra gives the following result: an increase in the thickness of the absorber from 4.5 cm to 10.5 cm led to a decrease in the area of S40-100 by more than 4 times, while S ₅₁₁ decreased by only 2 times. This example demonstrates the possibility of solving the system of equations (1) and (3).

Thus, the proposed method makes it possible to distinguish between detectable neutron radiation with different spectral characteristics from the registered gamma-ray spectra.

Example 2

An example of the design of a device for implementing the proposed method consists of a scintillation detector based on a gadolinium-containing scintillation crystal Gd ₃ Al ₂ Ga ₃ O ₁₂ : Ce (GAGG: Ce) with dimensions in each dimension of at least 1 mm, a photo detector based on an electrovacuum or semiconductor device , external lead protection against background gamma radiation with a thickness sufficient to effectively suppress the natural gamma background in the selected analyzed range of energies generated by neutrons in the gadolinium line of gamma rays, and internal copper or copper-containing protection against characteristic lines of lead with energies <100 keV falling into the analyzed range of neutron-generated gadgets of gamma rays with an equivalent thickness of metallic copper of at least 2-3 mm,

Thus, since as a result of the interaction of neutrons with scintillation material, gamma rays that directly appear in the scintillation material of the detector are recorded, the proposed technical solution significantly increases the sensitivity of neutron detection compared to known methods when interacting with neutrons to obtain gamma rays is carried out in one material, and registration of emitted gamma rays in another, spatially separated, scintillation mat iale. In addition, the sensitivity of neutron detection is increased by taking into account a greater number of expressed lines or groups of lines in the spectrum of emitted gamma rays, and a comparison of the intensities of these various lines or groups of lines allows, in addition to the intensity of the recorded neutron radiation, to obtain information about its spectral characteristics.

Claims (10)

1. A method for detecting neutrons, in which the scintillation photons generated when neutrons enter the gamma ray scintillator based on an inorganic material containing neutron-absorbing components are recorded, and neutron radiation characteristics are determined, characterized in that they use inorganic scintillation material containing gadolinium atoms, photons scintillations from gamma rays are recorded in the energy range of 40-1300 keV and a comparative analysis of the intensity of lines and / or groups of lines is carried out the measured emission spectrum of gamma rays in at least two energy ranges, and the results of this analysis concluded on the magnitude and spectral characteristics of the detected neutron radiation. 2. The method according to p. 1, characterized in that the scintillation photons from gamma rays are recorded in the energy range of 40-600 keV. 3. The method according to p. 1, characterized in that the intensities are compared in at least two lines or groups of lines selected in the energy range of 40-100 keV, 100-200 keV, and the line 511 keV. 4. The method according to p. 1, characterized in that as an inorganic scintillation material containing gadolinium atoms, use materials from the series: Gd ₂ SiO ₅ , Gd ₂ Si ₂ O ₇ , GdBr ₃ , Gd ₃ Al ₂ Ga ₃ O ₁₂ in the form of a single-crystalline or polycrystalline material containing Ce, and / or Tb, and / or Eu atoms as an activator, which are introduced into the composition of the material, replacing one or more elements in an amount of up to 2 at. % 5. The method according to p. 4, characterized in that in inorganic scintillation materials containing gadolinium atoms, replace up to 60% of the Gd atoms with Y atoms or lanthanides. 6. The method according to p. 1, characterized in that as an inorganic scintillation material containing gadolinium atoms, scintillation glass containing Gd, Si oxides is used, and Ce and / or Tb atoms are used as an activator. 7. A device for detecting neutrons for implementing the method according to claims. 1-6, comprising a gamma ray scintillator based on an inorganic scintillation material optically coupled to a photodetector connected to an analyzer, the inorganic scintillation material contains gadolinium atoms, the gamma ray scintillator and photodetector being surrounded by a background-absorbing gamma ray-absorbing screen . 8. The device according to p. 7, characterized in that as an inorganic scintillation material containing gadolinium atoms, use materials selected from the series: Gd ₂ SiO ₅ , Gd ₂ Si ₂ O ₇ , GdBr ₃ , Gd ₃ Al ₂ Ga ₃ O ₁₂ in the form of a single-crystalline or polycrystalline material containing, as an activator, Ce, and / or Tb, and / or Eu atoms, which are introduced into the composition of the material, replacing one or more elements in an amount of up to 2 at. % 9. The device according to p. 8, characterized in that in inorganic scintillation materials containing gadolinium atoms, replace up to 60% of the Gd atoms with Y atoms or lanthanides. 10. The device according to p. 7, characterized in that the outer layer of lead with a thickness of at least 1 mm and the inner layer of copper with a thickness of at least 1 mm are used as the screen absorbing background gamma radiation from the inorganic substance.

Images