RU2724133C1 - Method of detecting reactor antineutrinos - Google Patents

Abstract

FIELD: physics.SUBSTANCE: invention relates to methods of detecting antineutrinos by scintillation method. Essence of the invention is that antineutrinos are detected by reverse beta decay on protons, wherein in layers of segmented gadolinium-containing inorganic scintillator, alternating with layers of organic scintillator, scintillation photons are detected from annihilation of positron reverse beta-decay (instantaneous signal) generated during the reaction, as well as scintillation photons from the gamma-ray cascade emitted when neutrons are absorbed during the reaction of reverse beta decay (delayed signal).EFFECT: possibility of detecting reactor antineutrinos.5 cl, 1 tbl, 3 ex, 17 dwg

Description

Technical field

The invention relates to methods for detecting reactor antineutrinos by the scintillation method using a combination of inorganic scintillation materials containing gadolinium atoms and organic scintillation materials, which are proton targets for antineutrinos, for use in antineutrino detectors for remote monitoring systems of nuclear reactor technology.

State of the art

A known method of detecting reactor antineutrinos in the reverse beta decay reaction using liquid organic scintillators containing gadolinium atoms. An example of this type of detector is the SONGS1 facility installed near a 3.46 GW SONGS nuclear reactor in California, USA (Experimental results from an antineutrino detector for cooperative monitoring of nuclear reactors / NS Bowden [et al.] // Nuclear Instruments and Methods in Physics Research. Sect. A: Accelerators Spectrometers Detectors and Assoc. Equipment. - 2007. - Vol. 572. - P. 985-998). The detector is installed 24.5 m from the reactor zone, outside the concrete shield, which provides a significant reduction in the flux of reactor neutrons (the thickness of the protection exceeds 100 absorption lengths of neutrons with an energy of 10 MeV) and gamma rays. The detector is located at a depth of 10 m below the surface of the earth, which allows you to reject the soft component of cosmic rays and reduce the flux of space muons by 7 times.

Despite the relative simplicity and compactness of the SONGS1 detector, it has the following disadvantages:

1. Small granularity (only four cells, each of which works as independent and does not take into account the transfer of absorbed energy between cells).

2. Small volume of a working medium of low density (liquid scintillator).

The noted features lead to the fact that this detector has low antineutrino detection efficiency, a moderate signal / background ratio and low energy resolution. An additional drawback of reactor antineutrino detection systems based on interaction with liquid scintillators is that they are subject to increased requirements for the choice of container material and its tightness due to the possibility of leakage of liquid scintillators, their fire hazard and the possibility of degradation due to interaction with the container material.

In addition, large volumes of liquid scintillators experience a degradation of their scintillation properties over time due to radiochemical reactions during detector irradiation, as a result of which free radicals and active molecules can form new compounds that can lead to a decrease in the optical transparency of the liquid, and insoluble precipitates can deposited on the input windows of photodetectors, reducing their optical transmission. To a large extent, these processes will be significant when using large volumes of liquid scintillators, which is required to increase the efficiency of antineutrino registration.

A known method of detecting reactor antineutrinos by the reaction of reverse beta decay using Cherenkov radiation detectors. An example is the Angra Neutrino Experiment project (Using Neutrinos to Monitor Nuclear Reactors: the Angra Neutrino Experiment, Simulation and Detector Status / JC Anjos [et al.] // Nuclear and Particle Physics Proceedings. - 2015. - Vol. 267- 269. - P. 108-115). The detector’s working medium is a tank with pure water with the addition of gadolinium atoms of a natural isotopic composition, surrounded by tanks with pure water as an active protection against cosmic rays and the external gamma-ray decay of natural radionuclides. The detector is located at a distance of 30 m from the Angra II reactor (Brazil) with a design thermal capacity of 4 GW. The detector structure includes four subsystems.

The main disadvantages of the antineutrino detector based on Cherenkov detectors include:

1. The detector has a low detection efficiency of antineutrinos, and therefore, to achieve a reliable measurement of 95%, the theoretical estimated time for collecting statistics is at least 10 days.

2. The muon decay, incomplete clipping of background processes during mixed dual-band amplitude-time selection, and the non-zero probability of failure of external veto systems increases the frequency of correlated signals due to muons to ~ 0.1 pulses / s, which is higher than the antineutrino count rate . To minimize this contribution, it is necessary to carry out accurate background measurements during periods of shutdown of the reactor, or to perform background measurements with replacing the detector’s working environment with clean water without adding gadolinium.

3. The described system is characterized by a low photon yield of Cherenkov radiation — less than 1500 photons from each positron. The yield of Cherenkov radiation photons per unit path of a charged particle can be estimated as dN / dL = 490⋅sin ² (θ) photons / cm in the range 400–700 nm, which with a water refractive index of n = 1.333 and a maximum positron energy E ^{e +} _max = 5 -8 MeV (the boundary energy of the beta spectrum of positrons in reverse beta decay from reactor antineutrinos) will give the indicated value, because positrons of these energies have a range in water of up to 3-4 cm (Grupen, K. Detectors of elementary particles / K. Grupen; edited by L.M. Kurdadze, S.I. Eidelman; transl. from English: N.Yu. Eidelman, Yu.I. Eidelman .-- Novosibirsk: Sib.Chronograph, 1999 .-- 408 p.).

откуда следует ограничение на порог бета-фактора заряженной частицы: β≥1/n. Энергия частицы может быть представлена в виде

где γ - Лоренц-фактор, m₀ - масса покоя частицы, откуда получаем выражения для нижнего порога энергии частицы, необходимый для начала генерации черенковского излучения:

который дает указанную величину для позитронов в воде (n=1,333). Это повышает требуемое время накопления сигнала.4. The detector has a high energy threshold for generating Cherenkov radiation: E _min = 338 keV for positrons in water, which can be estimated based on the expression of the angle of the raster of the Cherenkov radiation cone

whence follows the restriction on the threshold of the beta factor of the charged particle: β≥1 / n. Particle energy can be represented as

where γ is the Lorentz factor, m ₀ is the rest mass of the particle, whence we obtain the expressions for the lower threshold of the particle energy, which is necessary to start the generation of Cherenkov radiation:

which gives the indicated value for positrons in water (n = 1.333). This increases the required signal accumulation time.

A known method of detecting reactor antineutrinos by the reverse beta decay reaction using scintillators containing the ⁶ Li isotope. An example of such a detector is the MiniCHANDLER installation, operating near the first NANGS nuclear power unit in Blacksburg (Virginia, USA). The detector is located 25 m from the center of the reactor with a thermal power of 2.94 GW (Observation of Reactor Antineutrinos with a Rapidly-Deployable Surface-Level Detector / A. Haghighat [et al.] [Electronic resource] // arXiv.org. 2018. Update date: 12/05/2018. URL: https: //arxiv.org/abs/1812.02163). A detector weighing 80 kg has a cubic geometry, consists of five layers of an organic scintillator EJ-260 based on polyvinyltoluene manufactured by Eljen Technology, each of which is a matrix of 8 × 8 cubes with dimensions of 62 × 62 × 62 mm ³ . To each horizontal column of 1 × 8 cubes in a layer, on one side, a PMT Amperex XP2202 is connected. The maximum of the EJ-260 scintillation emission spectrum falls at 490 nm, the characteristic decay time of the scintillation kinetics is 9.2 ns (https://eljentechnology.com/products/plastic-scintillators/ej-260-ej-262). The layers of the organic scintillator are separated by six EJ-426 plate-type thermal neutron detectors manufactured by Eljen Technology. EJ-426 plates have a thickness of 0.32-0.5 mm, they consist of microparticles of lithium fluoride LiF enriched in the Li ⁶ isotope up to 95% mixed with ZnS: Ag zinc sulfide scintillator microcrystals with a maximum emission spectrum of 450 nm (https: / /eljentechnology.com/products/neutron-detectors/ej-426).

Important disadvantages of systems based on Li-containing detectors are as follows:

1. When kinetic energy is transferred from heavy ionizing particles (alpha particles and tritons in the case of neutrons detected by reaction ⁶ Li (n, α) t), the scintillation yield is suppressed, as a result of which the combined energy of these particles is 4.8 MeV (E _α = 2.05 MeV, E _t = 2.75 MeV) in the scale of gamma-ray energies will be recorded by the inorganic scintillation materials proposed in the described solution in the range of 1.5-1.7 MeV, which corresponds to a typical ratio of the contribution from alpha particles relative to contribution of gamma rays α / γ = 0.3. This energy range falls on the gamma-ray region in the region of the 1461-keV emission line of the ⁴⁰ K isotope, which is ubiquitous in terrestrial rocks, including building materials.

Вероятность данной реакции равна 93%, что повышает гамма-фон внутри детектора. Линия 480 кэВ находится вблизи аннигиляционной линии 511 кэВ, по которой идет дополнительная селекция полезных событий.2. The use of an outer protective layer of plastic with the addition of boron atoms to protect against the flow of reactor thermal neutrons leads to the emission of gamma rays with an energy of 480 keV associated with the removal of the excited state of the ⁷ Li nucleus formed during the reaction

The probability of this reaction is 93%, which increases the gamma background inside the detector. The 480 keV line is located near the 511 keV annihilation line, along which there is an additional selection of useful events.

3. The low density of the organic scintillator leads to an increase in the scintillation emission region from annihilation gamma rays, which worsens and complicates the spatial selection of useful events.

4. The relatively low degree of detector segmentation leads to a deterioration in the spatial selection of useful events.

5. The need to enrich a neutron detector using the ⁶ Li isotope in order to increase the detection efficiency of thermal neutrons significantly increases its cost.

The closest in technical essence to the claimed invention (prototype) is a method for detecting reactor antineutrinos described in (Observation of Reactor Antineutrinos with a Rapidly-Deployable Surface-Level Detector / A. Haghighat [et al.] [Electronic resource] // arXiv. org. 2018. Update date: 12/05/2018. URL: https: //arxiv.org/abs/1812.02163 (accessed date: 08/28/2019).

The technical problem to which the claimed invention is directed is to increase the reliability of registration of reactor antineutrino scintillation method using a combination of inorganic scintillation materials containing gadolinium atoms and organic scintillation materials, which are proton targets for antineutrino, for use in antineutrino detectors for remote control systems reactor technology.

Disclosure of the invention

The technical result of the claimed invention is the ability to detect reactor antineutrinos.

To achieve this technical result, a method for detecting reactor antineutrinos is proposed, which consists in registering scintillation photons generated when gamma rays and neutrons get into the gamma ray scintillator based on an inorganic material containing gadolinium atoms, and a gamma ray scintillator based on organic material containing hydrogen atoms and located in close proximity to a scintillator based on inorganic material is used as a proton-containing target, a medium for slowing down positrons and neutrons, as well as active protection from the charged and neutron components of background cosmic radiation and, in so doing, A gamma ray scintillator based on an inorganic material containing gadolinium atoms determines the coordinate of the interaction of gamma quanta, the moment of interaction in time and the energy released as a result of the interaction, and separate useful events from background ones, registering the energy output Removal from annihilation gamma rays in the energy range of 400-3000 keV, from neutrons in the energy range 5000-8000 keV, instant coincidences from ionization losses of positron energy and / or annihilation gamma rays in the time window of 10 ^-8 s and delayed by time are recorded 10 ^-5 - 10 ^-6 s signals from neutrons and analyze spatial regions in a gamma ray scintillator based on inorganic material for compliance with pre-calculated computer models in which the indicated energy release was recorded instantly or in certain time intervals.

In addition, 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 single-crystal 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.%.

In addition, in inorganic scintillation materials containing gadolinium atoms, up to 60% of Gd atoms are replaced by Y atoms or lanthanides.

In addition, 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.

In addition, plastic scintillators or liquid scintillators that do not contain gadolinium are used as organic scintillation material containing hydrogen atoms.

A brief description of the essence of the method is that the registration of antineutrinos is carried out by the reverse beta decay reaction on protons, in which scintillation photons generated from the annihilation of the reverse beta reaction are recorded in layers of a segmented gadolinium-containing inorganic scintillator, alternating with layers of an organic scintillator the decay of positrons (instantaneous signal), as well as scintillation photons from the cascade of gamma rays emitted during the absorption of neutrons arising from the reverse beta decay reaction (delayed signal).

The main differences of our proposed method from the prototype are as follows:

- registration of signals from positrons (instantaneous signal) generated during the reverse beta decay reaction in the layers of an organic scintillator, as well as signals thermalized in layers of an organic scintillator of neutrons (delayed signal) born in the same reaction, is performed in a high-density segmented inorganic scintillator containing gadolinium atoms of natural isotopic composition;

- the layers of the organic scintillator play a role only as a thermalizing neutron medium, as well as active protection against random coincidence signals arising under the influence of cosmic radiation;

- as a result of neutron capture by gadolinium atoms, cascade gamma rays with a total energy of up to 8 MeV are generated, the registration of which in the layers of a segmented gadolinium-containing inorganic scintillator forms a delayed signal;

- the use of the coincidence mode by different segments of the inorganic scintillator when registering an instant signal within the boundaries of the effective time window of coincidences, within which registration of the instant signal will be monitored, can significantly suppress the count rate of random background events;

- instantaneous and delayed signals localize the areas inside the detector, separated both by time and spatially, according to which the selection of useful events is performed taking into account spatial, temporal and amplitude selection, as well as taking into account the signals of veto organic scintillators that cut off the background of the charged and neutron components of the space background radiation;

- additional selection of useful events is carried out on the basis of the analysis of the mutual spatial orientation of the localization regions of the instantaneous and delayed signals in the detector. The basis of this approach is the fact of anisotropic emission of neutrons towards the antineutrino from the reactor core during the reverse beta decay reaction.

According to the proposed method, the registration of antineutrinos is carried out through their reaction with the protons of an organic scintillator, during which a positron and a neutron are born. After thermalization in an organic scintillator, the positron annihilates with the emission of two 511 keV gamma rays emitting at an angle of 180 °. The emitted annihilation gamma-quanta and positron energy losses during its thermalization are recorded in segments of an inorganic scintillator containing gadolinium atoms, forming an instant signal. The neutron emitted during the reverse beta decay reaction is thermalized in the organic scintillator layer and, due to drift, enters the segments of the inorganic scintillator in which it is captured by the gadolinium nucleus. When interacting with neutrons, the nuclei of gadolinium atoms emit a set of gamma rays in a wide energy range with a total energy of about 8 MeV, due to which a delayed signal is formed in segments of the inorganic scintillator. Scintillations are recorded by photodetectors - for example, semiconductor photodetectors directly connected to detector segments, or scintillation light from detector segments is transmitted to optical fibers passing through the segments, which are connected to photodetectors outside the detector’s sensitive volume - by a photoelectric multiplier or semiconductor photodetectors.

Based on the information about the coordinates of the annihilation gamma quanta entering the segments of the inorganic gamma ray scintillator, the region of space in the detector in which the positron was generated is determined, and based on the coordinates of the gamma-ray cascade caused by neutron capture by the gadolinium nucleus, the region is determined the space of the detector in which the neutron was captured. An analysis of the mutual spatial arrangement of these regions in the detector volume, together with the other indicated selection methods, makes it possible to identify the fact of registration of antineutrinos. An additional trigger is formed on the basis of spatial selection of the localization regions of instantaneous and delayed events: positrons generated during the reverse beta decay reaction are emitted almost isotropically relative to the direction of motion of the antineutrinos, while neutrons fly out predominantly in the direction of motion of the primary antineutrinos.

When registering scintillation pulses of these two processes, 4 types of signal selection are performed simultaneously:

- amplitude selection of events: registration of 511 keV simultaneous annihilation gamma-quanta in segments of layers of an inorganic scintillator containing gadolinium in a specific energy window, with a separate trigger for exceeding the threshold of total energy released in all segments of the inorganic energy scintillator more than 3 MeV, optimally more than 5 MeV MeV;

- spatial selection of events: by analyzing the information and registering the signal in different segments of the inorganic scintillator, the coordinates of the interaction of gamma quanta are determined; as useful signals, only signals generated in a certain spatial region are selected;

- temporary selection: registration is realized in the mode of delayed matches between instantaneous coincidence signals from ionization losses of positron energy and / or annihilation gamma-quanta and a delayed signal from cascade gamma-quanta born after neutron capture by the gadolinium nucleus. The delay of the neutron signal is due to the processes of thermalization of the neutron in the layers of the organic scintillator with subsequent drift to the segments of the inorganic scintillator containing gadolinium. Additional time selection is carried out by registering instant signals within the time window of the order of 10 ns, and delayed matches within the time window of 2-18 μs from the moment of registration of instant signals;

- an additional veto is formed due to the matching signals generated by the layers of the organic scintillator when cosmic ray particles pass through them.

Natural gadolinium has one of the highest thermal neutron capture cross sections - 46,000 barns (ENDF, https://www-nds.iaea.org/exfor/endf.htm), which significantly increases the efficiency of neutron detection and improves the localization of the neutron absorption region after they thermalization in scintillators with a high content of gadolinium. In the case of using only a hydrogen-containing scintillator, the neutron capture time increases to 100 μs with the emission of a cascade of gamma rays with a total energy of 2.2 MeV (Experimental results from an antineutrino detector for cooperative monitoring of nuclear reactors / NS Bowden [et al.] / / Nuclear Instruments and Methods in Physics Research. Sect. A: Accelerators Spectrometers Detectors and Assoc. Equipment. - 2007. - Vol. 572. - P. 985-998.), Which leads to the formation of an integrated delayed signal in the energy range of natural gamma background (up to 3 MeV), which, in turn, reduces the reliability of the selection of useful events. The use of a solid-state inorganic scintillator with the addition of gadolinium can significantly increase both the detection efficiency of gamma quanta (due to the high density and high magnitude of the effective charge of the scintillator substance) and thermalized neutrons (due to the content of Gd atoms), which allows the creation of highly sensitive antineutrino detectors of smaller volumes than analogs based on gadolinium-containing liquid scintillators. Segmentation of the gamma ray scintillator increases the spatial resolution of the detected events and additional rejection of background events. In addition, the use of non-hygroscopic inorganic scintillators simplifies the design of the detector compared to using liquid scintillators.

Using the multi-stage system for selecting useful events described above can significantly reduce the influence of background events of any nature: the intrinsic radioactivity of the materials of detectors and photodetectors, charged particles and fast neutrons of cosmic rays, external background slowed neutrons from a reactor, and also background gamma rays from the decay of natural radioisotopes.

Brief Description of the Drawings

In FIG. 1 is a schematic drawing of a detector section, where:

1 - layers of organic scintillator;

2 - segments of an inorganic scintillator containing gadolinium atoms;

3 - photodetectors.

In FIG. Figure 2 shows the energy spectrum of positrons emitted during the interaction of reactor antineutrinos with protons by the reverse beta decay reaction.

In FIG. Figure 3 shows the energy spectrum of neutrons emitted during the interaction of reactor antineutrinos with protons by the reaction of inverse beta decay.

In FIG. Figure 4 shows the calculated (simulation in GEANT4) time spectra of thermalization of neutrons with an energy of 1 keV - 5 MeV in polyvinyltoluene (PVT), as well as neutrons with a spectrum corresponding to the inverse beta decay in polyvinyltoluene.

In FIG. Figure 5 shows the calculated (simulation in GEANT4) distribution of thermalization depths of neutrons with an energy of 1 keV - 5 MeV in polyvinyltoluene (PVT), as well as neutrons with a spectrum corresponding to the inverse beta decay in polyvinyltoluene.

In FIG. Figure 6 shows the calculated (simulation in GEANT4) distribution of the positron thermalization depths with energy corresponding to the reverse beta decay reaction in the Gd ₃ Al ₂ Ga ₃ O ₁₂ scintillator: Ce (GAGG) and HTP.

In FIG. 7 shows the beta spectrum of positrons emitted by a source of ²² Na.

In FIG. Figure 8 shows the measured gamma spectra from a ²² Na source recorded in a BGO crystal with and without an aluminum absorber between the source and the crystal.

In FIG. 9 shows the measured background spectrum obtained from a GAGG crystal of 10 × 10 × 10 mm ³ . Energy calibration was carried out using gamma rays of 662 keV from ¹³⁷ Cs.

In FIG. 10 shows the distribution of absorbed energy in 10 mm thick GAGG and HTP layers upon neutron absorption with an emission spectrum corresponding to inverse beta decay.

In FIG. Figure 11 shows the dependence of the maximum neutron emission angle relative to the direction of the primary antineutrino during the reverse beta decay reaction.

In FIG. Figure 12 shows the dependences of the positron registration probability by the coincidence method for its annihilation, the thickness of the layers of the HTP and GAGG is 10 mm each.

In FIG. 13 shows the distribution of absorbed energy in GAGG layers with a thickness of 10 mm and an HTP with a thickness of 100 mm upon absorption of a neutron with an emission spectrum corresponding to inverse beta decay.

In FIG. Figure 14 shows the distribution of absorbed energy in 10 mm thick GAGG and 5 mm thick HTP layers during neutron absorption with an emission spectrum corresponding to inverse beta decay.

In FIG. 15 is a schematic drawing of a dual-detection detector cell, where:

4 - scintillator;

5 - photodetector.

In FIG. Figure 16 shows the dependences of the positron registration probability by the method of coincidence by its annihilation, the thickness of the PVT layers is 100 mm, and GAGG is 10 mm.

In FIG. Figure 17 shows the dependences of the positron registration probability by the method of coincidence by its annihilation, the thickness of the PVT layers is 5 mm, and GAGG is 10 mm.

The implementation and examples of the invention

The registration method includes the following operations:

1) Preparation of a measuring circuit (detector), consisting of continuous plates of an organic scintillator, between which segmented layers of an inorganic scintillator 1, which contain gadolinium atoms, are placed, and equipment that allows the light emitted by scintillators to be detected using any of the known devices, for example , using a photomultiplier, a silicon semiconductor device, or in any other way, and this equipment detects the light emitted in each plate of the organic scintillator and in each segment of each layer of the inorganic scintillator through separate independent channels. In FIG. 1 shows a schematic drawing of a detector section of the proposed design.

2) Registration of signals of all channels of the measuring circuit.

3) Signal processing, including:

A) temporary selection;

B) spatial selection;

B) amplitude selection;

D) the use of additional filtering, based on the "veto" rule, based on the signal of registration of cosmic rays with an organic scintillator.

The following is a description of the implementation and selection of signal processing parameters and geometric characteristics of the detector, allowing to achieve the technical result.

As an inorganic scintillation material containing gadolinium atoms, can be used 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 atoms Ce and / or Tb and / or Eu, which are introduced into the composition of the material, replacing one or more elements in an amount of up to 2 at. % Other inorganic scintillators that include Gd can also be used. In particular, these may be materials from the series described above in which up to 60% of Gd atoms are replaced by Y atoms or lanthanides. Also, as an inorganic scintillation material containing gadolinium atoms, scintillation glass containing Gd, Si oxides can be used, and Ce and / or Tb atoms as an activator.

When assessing the required spatial parameters of the detector elements, one should take into account the energy spectra of positrons and neutrons generated during the interaction of reactor antineutrinos with protons. In FIG. Figures 2 and 3 show the energy spectra of positrons and neutrons, respectively, emitted during the interaction of reactor antineutrinos with protons by the reaction of inverse beta decay. The positron spectrum is a standard spectrum of beta radiation with a boundary energy of about 8 MeV and an average energy of about 2 MeV. The most probable neutron energy is units of keV. These spectra were used to calculate the thermalization parameters of neutrons and positrons in the scintillation materials of the proposed detector.

Implementation of temporary selection of useful events.

In FIG. Figure 4 shows the calculated (simulation in GEANT4) (Geant4 - a simulation toolkit / S. Agostinelli [et al.] // Nuclear Instruments and Methods in Phys. Research. Sect. A: Accelerators Spectrometers Detectors and Associated Equipment. - 2003. - Vol. 506. - P. 250-303) temporal thermalization spectra of neutrons with an energy of 1 keV - 5 MeV, as well as neutrons with a spectrum corresponding to the inverse beta decay in polyvinyltoluene (HTP). The calculation results show that 90% of the neutrons emitted during interactions of reactor antineutrinos with protons of an organic gamma ray spectrometer are thermalized in no more than 18 μs from the moment of emission, the most likely thermalization time is about 3 μs, which allows you to set a time window for recording the delayed signal with boundaries from 2-3 μs to 18-20 μs from the moment of registration of instantaneous signals Thermalization of positrons in the material of inorganic and organic scintillators occurs for times up to several tens of picos seconds (simulation in GEANT4 shows that 90% of the positrons are thermalized in the HTP in 70 ps, and in GAGG in ~ 5 ps). Taking into account the characteristic times of increase in the intensity of scintillation emission in organic and inorganic scintillators (up to 1 ns), as well as the delay in collecting scintillation photons from the cell volume of an inorganic scintillator with a volume of several cubic centimeters, you can set the preferred value of the effective coincidence window, within which registration of an instant signal, in 10 ns. The use of a narrow time window of coincidences for recording instantaneous signals allows one to significantly suppress the counting rate of random background events: the suppression factor will be (18 μs - 2 μs) / 0.01 μs = 1600.

Implementation of spatial selection of useful events.

Spatial selection allows you to suppress the count rate of random coincidences with background signals: the localization of the energy regions of the instantaneous and delayed signals form the corresponding spheres of space inside the detector, the volumes of which are significantly less than the total volume of the detector. Then the degree of suppression of the count rate of random coincidences of background events will be equal to the ratio of the volume of the detector to the volume of these spheres.

In FIG. Figure 5 shows the calculated (simulation in GEANT4) distribution of thermalization depths of neutrons with an energy of 1 keV - 5 MeV in the HTP, as well as neutrons with a spectrum corresponding to the inverse beta decay in the HTP. The simulation results show that 90% of the neutrons emitted during the interaction of reactor antineutrinos with protons of an organic gamma ray spectrometer are thermalized in an HTP layer 10-12 cm thick, and the average thermalization depth of such neutrons is about 5 cm. The obtained values set the boundaries of the thermalization region neutrons in an inorganic scintillator.

Before the act of annihilation, the formed positrons also experience thermalization, with the majority of positrons annihilating after thermalization, because annihilation cross section σ _{γγ is} inversely proportional to the square of the positron kinetic energy: σ _γγ ~ 1 / Е ² (Electron-positron pairs in physics and astrophysics: From heavy nuclei to black holes / R. Ruffini, G. Vereshchagin, S.-S. Xue / / Physics Reports. - 2010 .-- Vol. 487. - P. 1-140).

In FIG. Figure 6 shows the calculated (simulation in GEANT4) distributions of the positron thermalization depths with energy corresponding to the reverse beta decay reaction in GAGG and HTP. The simulation results show that 90% of the positrons generated during the interaction of reactor antineutrinos with protons are thermalized in an HTP with a thickness of 15 mm, and the average thermalization depth in an HTP is about 6 mm. For GAGG, 90% are thermalized in a layer up to 2 mm thick, and the average thermalization depth corresponds to a GAGG thickness of 0.35 mm. During the registration of antineutrinos, part of the positrons generated may not completely thermalize in the thickness of the organic scintillator, but complete thermalization in the inorganic scintillator layer, in which the annihilation gamma quantum will be recorded. In such cases, when an instantaneous signal is generated, the rest of the kinetic energy of the positron, which it will release in the inorganic scintillator layer, will be added to the energy of the annihilation gamma quantum. To take this fact into account when forming the energy gate of the coincidence pattern of registration of annihilation gamma rays and / or ionization losses of positron energy, it is necessary to expand the right window border to 3000 keV.

To check the effect of the residual positron energy on the recorded signal of annihilation gamma rays, an experimental measurement of gamma spectra from a ²² Na positron source was performed. A ²² Na source emits positrons with a boundary energy of 545.6 keV and an average energy of 180 keV. In FIG. 7 shows the beta spectrum of positrons emitted by a source of ²² Na. To measure the influence of the residual positron energy on the recorded spectrum of annihilation gamma-quanta, a Bi ₄ Ge ₃ O ₁₂ (BGO) 10 × 10 × 10 mm ³ bismuth orthogermanate crystal was used, which was placed on the entrance window of the Photonis XP2020 PMT through an immersion lubricant, which was used Vaseline oil. The crystal was wrapped with a diffuse white reflector, which was used as a fluoroplastic film. The OSGI-R type ²² Na source was located on the upper face of the BGO crystal; the position of the source was centered relative to the upper face. The thickness of the reflector on the side faces of the crystal was 0.45 mm, on the upper side - 0.15 mm. The measurements were carried out both using an aluminum absorber 1.2 mm thick, placed between the positron source and the BGO crystal, and without an absorber. When using an aluminum absorber, positrons with an energy of 545.6 keV are thermalized in aluminum at an average depth of ~ 0.6 mm. The thickness of the absorber was chosen from the point of view of balance: maximizing the fraction of thermalized positrons and minimizing the probability of Compton scattering of the annihilation gamma ray in aluminum. In FIG. Figure 8 shows the obtained gamma spectra from a ²² Na source recorded in a BGO crystal with and without an aluminum absorber between the source and the crystal. As can be seen from the obtained spectra, part of the kinetic energy of the positrons is transferred to the scintillator, which leads to a shift in the peak of total absorption of 511 keV annihilation quanta, as well as an increase in its width due to the spread of the kinetic energy of positrons in accordance with their beta spectrum.

/ P. Vogel, J.F. Beacom // Phys.Rev. D. - 1999. - Vol. 60. - P. 053003). Тогда фактор подавления фоновых процессов может быть оценен как отношение телесного угла полной сферы (4π) к телесному углу, в который вылетает нейтрон: площадь сферического сегмента с вершиной при угле θ и радиусом сферы R равен S₁=2πR²(1-cosθ), площадь всей сферы S_all=4πR², степень подавления есть отношение K=S_all/S₁=2/(1-cosθ). Исходя из фиг. 11, для граничной энергии антинейтрино, при которой уже возможна реакция обратного бета-распада, максимальный угол вылета нейтрона составляет ~15°, что дает K=60, а для максимальной энергии антинейтрино 10 МэВ - K=4.Additional selection of events is carried out on the basis of spatial selection of areas of localization of instantaneous and delayed events. In FIG. Figure 11 shows the dependence of the maximum neutron emission angle relative to the direction of the primary antineutrino during the beta-decay reaction, according to which the maximum neutron emission angle relative to 10 MeV reactor antineutrinos during the proton beta-decay reaction does not exceed 60 ° (Angular distribution of the reaction

/ P. Vogel, JF Beacom // Phys. Rev. D. - 1999. - Vol. 60. - P. 053003). Then the suppression factor of background processes can be estimated as the ratio of the solid angle of the full sphere (4π) to the solid angle into which the neutron flies: the area of the spherical segment with the vertex at angle θ and the radius of the sphere R is S ₁ = 2πR ² (1-cosθ), the area of the entire sphere is S _all = 4πR ² , the degree of suppression is the ratio K = S _all / S ₁ = 2 / (1-cosθ). Based on FIG. 11, for the boundary antineutrino energy at which a reverse beta decay reaction is already possible, the maximum neutron emission angle is ~ 15 °, which gives K = 60, and for the maximum antineutrino energy 10 MeV, K = 4.

The implementation of the amplitude selection of useful events.

To suppress the natural gamma background recorded by an inorganic scintillator, it is proposed to use amplitude selection of instantaneous signals from ionization losses of positron energy and / or annihilation gamma quanta: registration of simultaneous pulses in adjacent layers of the inorganic scintillator in coincidence mode. To implement the coincidence mode, it is proposed to register synchronous instantaneous signals in the energy window with an upper threshold of up to 3000 keV. The width of the energy window, within which amplitude selection will be performed, affects the degree of suppression of the count rate of random coincidences.

To assess the degree of suppression of the natural gamma background due to such amplitude selection, measurements were made of the count rate of background pulses in the windows 100-3000 keV, 200-3000 keV, 400-3000 keV and under the whole spectrum in the window 30-3000 keV. For background measurements, we used a 10 × 10 × 10 mm ³ GAGG crystal sample wrapped in a fluoroplastic film, which served as a diffuse white reflector. The measurements were carried out using a Photonis XP2020 PMT; the crystal was placed in the center of the PMT entrance window through an immersion lubricant, which was used as liquid paraffin. In FIG. 9 shows the measured background spectrum obtained from a GAGG crystal. Energy calibration of the energy scale was carried out using 662 keV gamma rays from a ¹³⁷ Cs source of the OSGI-R type. Based on the data obtained, the following degrees of suppression of the count rate of background random matches are established:

- In the window 100-3000 keV - 1.52 times.

- In the window of 200-3000 keV - in 3.76 times.

- In the window of 400-3000 keV - 11.07 times.

It is proposed to use a trigger for the amount of energy absorbed in all segments of the inorganic scintillator of the energy of cascade gamma rays emitted by Gd nuclei after absorption of thermalized neutrons as an additional amplitude selection. To estimate the trigger formation threshold, computer simulation was carried out in the GEANT4 software package of the processes of energy transfer from cascade gamma rays to inorganic scintillator layers. The geometry of the model was a nested ball segments GAGG and HTP with a segment thickness of 10 mm. At the center of the geometry was a neutron source with an emission spectrum corresponding to neutrons generated during the reverse beta decay reaction in the interaction of reactor antineutrinos with protons (see Fig. 3). In the simulation, for each embedded GAGG and HTP layer, the energy absorbed in the layer was determined. In FIG. Figure 10 shows the simulation of the distribution of absorbed energy in a sequence of alternating 10 mm thick GAGG and HTP layers during neutron absorption with an emission spectrum corresponding to inverse beta decay. According to the results of calculations, about 88% of the total energy of cascade gamma rays (total energy is about 8 MeV) is absorbed in the GAGG layers, the rest of the energy is absorbed in the layers of the organic scintillator. Thus, when using scintillator layer thicknesses of 10 mm, the necessary trigger formation threshold for the total absorbed energy of the delayed event should be from 5 MeV to 10 MeV (above the boundary energy of the cascade quanta of the delayed events). The indicated energy window lies outside the boundary of the background gamma-ray spectrum (about 3 MeV), which makes it possible to further suppress the influence of background radiation.

To suppress the count rate of random coincidences with background signals during registration of annihilation gamma-quanta, they are recorded in segments of the inorganic scintillator in the coincidence mode. At the same time, the positron itself can give additional energy release in the segment before annihilation, if on the way to the inorganic scintillator segment it has not lost its kinetic energy. In FIG. Figure 12 shows the calculated (simulation in GEANT4) dependences of the probability of detecting a positron by the method of coincidence by its annihilation at a thickness of 10 mm and 10 mm of PVT and GAGG layers. The obtained dependences were calculated for spectral windows of 100-3000 keV, 200-3000 keV, 400-3000 keV. When choosing the width of the energy window for using the coincidence mode of the positron registration by its annihilation, it is necessary to proceed from optimization of the ratio of two competing processes:

1) reducing the width of the window helps to suppress the count rate of background events;

2) reducing the width of the window reduces the likelihood of registering an instant signal. The choice of the window width is carried out according to the analysis of the influence of the linear dimensions of the detector elements on the energy release during the formation of instantaneous signals.

An additional veto serves to reject the signals of particles of their cosmic radiation, primarily muons. The selection is formed by the coincident signals generated by the layers of the organic scintillator when cosmic ray particles pass through them. The coincidence of signals in two or more layers of an organic scintillator means the passage of cosmic radiation particles with high penetrating power through the layers, such a signal is rejected.

Example 1

An example of the design of the device for implementing the proposed method consists of segmented layers of a GAGG-based detector in the form of plates in which the sizes of the segments are 30 × 30 × 10 mm ³ , and a photodetector based on a semiconductor device is placed at the end of 30 × 10 mm ² segments. The inorganic scintillator layers are separated by 100 mm thick layers of an organic scintillator. In FIG. 13 shows the calculated (simulation in GEANT4) distribution of absorbed energy in 10 mm thick GAGG and 100 mm thick HTP layers upon neutron absorption with an emission spectrum corresponding to inverse beta decay (see FIG. 3). According to the calculation results, the fraction of the total energy release of cascade gamma rays leaving their energy in the GAGG layers upon absorption of the neutron from the reverse beta decay reaction will be 48.7% of the total energy release equal to 5-8 MeV. A decrease in the amplitude of the delayed signals will negatively affect the selection of useful events, because the background effect of natural gamma-emitting radionuclides will increase. In FIG. Figure 16 shows the dependences of the positron registration probability by the method of coincidence by its annihilation, the thickness of the PVT layers is 100 mm, and GAGG is 10 mm. The increased thickness of the HTP leads to an increase in Compton scattering and absorption of annihilation gamma rays, which negatively affects the probability of their registration by the coincidence method in GAGG.

Example 2

An example of the design of the device for implementing the proposed method consists of segmented layers of a GAGG-based detector in the form of plates in which the sizes of the segments are 30 × 30 × 10 mm ³ , and a photodetector based on a semiconductor device is placed at the end of 30 × 10 mm ² segments. The inorganic scintillator layers are separated by 5 mm thick layers of an organic scintillator. In FIG. Figure 14 shows the calculated (simulation in GEANT4) distribution of absorbed energy in 10 mm thick GAGG and 5 mm thick HTP layers during neutron absorption with an emission spectrum corresponding to inverse beta decay. According to the calculation results, the fraction of the total energy release of cascade gamma rays in the GAGG upon absorption of a neutron from the reverse beta decay reaction will be 93.1% of the total energy release of cascade gamma rays in the GAGG layers, equal to 5-8 MeV. In FIG. Figure 17 shows the dependences of the positron registration probability by the method of coincidence by its annihilation, the thickness of the PVT layers is 5 mm, and GAGG is 10 mm. An increase in the share of GAGG in the detector volume increases the probability of detecting positrons, but a decrease in the HTP thickness worsens the conditions for thermalization of neutrons, which, in turn, worsens the localization of the delayed signal.

Example 3

An example of the design of the device for implementing the proposed method consists of segmented detector layers based on a gadolinium-containing scintillator in the form of plates with dimensions of 30 × 10 × 500 mm, with photodetectors based on a semiconductor device placed at the ends. A schematic drawing of one segment is shown in FIG. 15. The segments of the inorganic scintillator with a double registration path form layers separated by continuous layers of an organic scintillator based on polyvinyl toluene (PVT). Spatial selection of events is carried out in segments of the inorganic scintillator by calculating the time delay of arrival of scintillation photons at photodetectors placed on the side faces, from which the longitudinal coordinate of the initial energy release region in the segment is calculated. Time resolution of GAGG crystals in coincidence mode for 511 keV annihilation gamma rays Δt <250 ps (Effect of Mg ²⁺ ions co-doping on timing performance and radiationtolerance of Cerium doped Gd ₃ Al ₂ Ga ₃ O ₁₂ crystals / MT Lucchini [et al.] // Nuclear Instruments and Methods in Physics Research. Sect. A: Accelerators Spectrometers Detectors and Assoc. Equipment. - 2016. - Vol. 816. - P. 176-183), (A PET detector prototype based on digital SiPMs and GAGG scintillators / FR Schneider [et al.] // Phys. Med. Biol. - 2015. - Vol. 60. - P. 1667-1679) and, taking into account the speed of light propagation v = c / n in the GAGG crystal at the exponent refraction n = 1.85 for the maximum of the scintillation emission spectrum, we can estimate the spatial resolution ΔL of signal formation as ΔL = Δt * c / n = 40 mm.

Claims (5)

1. The method of detecting reactor antineutrinos, which consists in registering scintillation photons generated when the annihilation gamma rays and neutrons get into the gamma ray scintillator based on an inorganic material containing gadolinium atoms, and a gamma ray scintillator based on an organic material containing organic material containing hydrogen atoms and located in close proximity to a scintillator based on inorganic material are used as a proton-containing target, a medium for slowing down positrons and neutrons, as well as active protection from the charged and neutron components of background cosmic radiation, characterized in that in gamma ray a scintillator based on an inorganic material containing gadolinium atoms determines the coordinate of the interaction of gamma quanta, the moment of interaction in time and the energy released as a result of the interaction, and separate useful events from background ones, registering energy release from annihilation gamma quanta in e in the energetic range of 400-3000 keV, from neutrons in the energy range of 5000-8000 keV, instant coincidences from ionization losses of positron energy and / or annihilation gamma-quanta in a time window of 10 ^-8 s and delayed by 10 ^-5 - 10 ^{-6 are recorded} c signals from neutrons and analyze for compliance with pre-calculated computer models the spatial regions in the gamma ray scintillator based on inorganic material in which the indicated energy release was recorded instantly or at certain time intervals. 2. 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-crystal 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.%. 3. The method according to p. 2, characterized in that in inorganic scintillation materials containing gadolinium atoms, replace up to 60% of the Gd atoms with Y atoms or lanthanides. 4. 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. 5. The method according to p. 1, characterized in that as an organic scintillation material containing hydrogen atoms, plastic scintillators or liquid scintillators are used that do not contain gadolinium.

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