RU137122U1 - DEVICE FOR ANALYSIS OF MATERIALS BY MEANS OF LABELED NEUTRONS - Google Patents

Abstract

Labeled neutron material analysis device, including a deuterium-tritium neutron generator with a built-in multi-pixel alpha detector, an object of research installed between the neutron generator and gamma detectors, inorganic scintillator gamma detectors, data acquisition equipment connected to a computer and recording under the condition the operation of the gamma detector and alpha detector in a given time window, the magnitude of the amplitude of the signal from the gamma detector, the time of appearance of the signal from the gamma detector from regarding the signal from the alpha detector, the number of the triggered gamma detector, the number of the triggered pixel of the alpha detector, characterized in that the inorganic scintillators of the gamma detectors contain oxygen, the gamma detectors are installed in such a way that labeled neutrons fall on each of the gamma detectors, and the alpha detector is located so that alpha particles accompanying neutrons that fall on the gamma detector arrive at least one pixel (calibration pixel) of the alpha detector, data acquisition equipment with it is injected with a means of measuring the spectrum of signals from gamma detectors provided that the signals in the gamma detector coincide with the signals recorded by the alpha-detector calibration pixel in a given time window, with the ability to automatically calibrate the energy scale of the gamma detector simultaneously with recording events when the target is irradiated with tagged neutrons using the obtained spectrum of signals from gamma detectors and the presence of peaks in it, arising from inelastic scattering of fast neutrons by oxygen nuclei

Description

The solution relates to the field of analysis of materials by radiation methods, measurement of secondary emission during irradiation with fast neutrons and can be used to detect and identify organic substances, including explosives.

One of the main problems of using neutron activation analysis is the high level of background loading of gamma detectors during registration of information radiation. The source of the background is gamma-rays emitted during the interaction of neutrons with a gamma-ray detector, plant elements and surrounding objects, decay of the resulting isotopes, etc. The method of tagged neutrons [Bogolyubov EP, Karetnikov MD, Klimov AI, Korotkoy SA, Kozlov KN, Meleshko E significantly reduces the level of recorded background signals due to spatial and temporal selection of events) .A., Ostashev I.E., Khasaev T.O., Yakovlev G.V. Control and measuring module for experiments with labeled neutrons // Instruments and experimental equipment, No. 5, 2006, S. 67-73.].

Solution ([Balygin K.A., Karetnikov M.D., Klimov A.I., Kozlov K.N., Meleshko E.A., Ostashev I.E., Yakovlev G.V. Studies of the characteristics of detecting equipment for nanosecond method of labeled neutrons // Instruments and experimental equipment, No. 2, 2009, P. 122-132.]) can be considered as the closest to the proposed device in this application, contains a deuterium-tritium neutron generator with a built-in multi-pixel alpha detector, gamma - detectors based on inorganic scintillator;

The labeled neutrons produced by the neutron generator pass through the inspected object and cause the emission of gamma rays, the data acquisition equipment, subject to the operation of the gamma detector and alpha detector in a given time window, records the magnitude of the signal amplitude from the gamma detector, the time of appearance of the signal from the gamma detector relative to the signal from the alpha detector, the number of the triggered gamma detector, the number of the triggered pixel of the alpha detector;

Using the values of the amplitude of the signal from the gamma detector, the time of appearance of the signal from the gamma detector relative to the signal from the alpha detector in a given time window, the number of the triggered gamma detector, the pixel number of the alpha detector, the gamma quantum energy and location coordinates are determined for each event its emission during the irradiation of the investigated object with labeled neutrons. This device allows you to increase the effect / background ratio by 2-4 orders of magnitude [Balygin K.A., Karetnikov M.D., Klimov A.I., Kozlov K.N., Meleshko E.A., Ostashev I.E., Yakovlev G.V. Studies of the characteristics of detecting equipment for the nanosecond method of labeled neutrons // Instruments and experimental equipment, No. 2, 2009, S. 122-132.] In comparison with similar devices without using the method of labeled neutrons due to the fact that events (effect) are mainly recorded, labeled neutrons, and signals from gamma detectors that occur when background neutrons and gamma quanta hit it are not recorded, since they are not accompanied by the operation of the gamma detector and alpha detector at a given time window. Gamma detectors are installed so that labeled neutrons do not fall into them. The disadvantage of this device is an uncontrolled change in the calibration dependence for gamma detectors during operation, for example, when the temperature and load of the detector change. This increases the error in measuring the energy of gamma rays.

To reduce the error of measuring gamma-ray energy in a device based on the tagged neutron method, containing a deuterium-tritium neutron generator with a built-in multi-pixel alpha detector, gamma detectors based on an inorganic scintillator, labeled neutrons produced by a neutron generator pass through the inspected object and cause emission gamma quanta, data acquisition equipment, subject to the operation of the gamma detector and alpha detector in a given time window recording the magnitude of the amplitude the signal from the gamma detector, the time of appearance of the signal from the gamma detector relative to the signal from the alpha detector, the number of the triggered gamma detector, the number of the triggered pixel of the alpha detector, gamma detectors with a high oxygen content are used to register gamma quanta, the object of study is set between the neutron generator and gamma detectors, gamma detectors are installed so that labeled neutrons fall on each of the gamma detectors, and the alpha detector is located so that the alpha particles corresponding to neutrons that fall on the gamma-ray detector came at least one pixel (calibration pixel) of the alpha-detector, the distance S between the studied object and gamma-ray detectors along the flux of labeled neutrons is at least S = 3v · τ, where τ - time resolution for measuring alpha-gamma coincidences, v - speed of labeled neutrons, provided that the signals in the gamma-detector coincide with the signals recorded by the calibration pixel alpha-detector in a given time window, the spectrum of signals with gamma tectors, according to this spectrum, using the peaks arising from inelastic scattering of fast neutrons by the oxygen nuclei contained in gamma detectors, they automatically calibrate the energy scale of the gamma detector at the same time as events are recorded when the target is irradiated with tagged neutrons, gamma detectors made on the basis of an inorganic scintillator with a high oxygen content, for example, of bismuth germanate Bi ₄ Ge ₃ O ₁₂ (BGO) or lutetium orthosilicate Lu _1.8 Y _0.2 SiO ₅ (Ce) (LYSO).

Thus, the technical result of the claimed proposal is to reduce the error in measuring the energy of gamma rays by automatically calibrating the energy scale of the gamma detector.

Providing the specified technical result is possible with the implementation of the following set of essential features.

Labeled neutron material analysis device, including a deuterium-tritium neutron generator with a built-in multi-pixel alpha detector, an object of research between the neutron generator and gamma detectors, inorganic scintillator gamma detectors, data acquisition equipment,

subject to the operation of the gamma detector and alpha detector in a given time window, recording the magnitude of the signal amplitude from the gamma detector, the time of appearance of the signal from the gamma detector relative to the signal from the alpha detector, the number of the triggered gamma detector, the number of the triggered pixel of the alpha detector, moreover

inorganic scintillators of gamma detectors contain oxygen,

gamma detectors are installed so that labeled neutrons fall on each of the gamma detectors,

and the alpha detector is located in such a way that alpha particles accompanying neutrons that fall on the gamma detector arrive at least one pixel (calibration pixel) of the alpha detector,

The data acquisition equipment is equipped with a means of measuring the spectrum of signals from gamma detectors, provided that the signals in the gamma detector coincide with the signals recorded by the alpha-detector calibration pixel in a given time window, with the ability to automatically calibrate the energy scale of the gamma detector

simultaneously with the registration of events during irradiation of an object under study with labeled neutrons using the obtained spectrum of signals from gamma detectors and the presence of peaks in it arising from inelastic scattering of fast neutrons by the nuclei of oxygen contained in gamma detectors, the distance S between the object under study and gamma detectors along the flux of tagged neutrons is at least S = 3v · τ, where τ is the time resolution in measuring alpha-gamma coincidences, v is the speed of tagged neutrons.

The proposal for this application is illustrated by the following illustrative materials.

FIG. 1 - Scheme of the proposed device.

Fig 2 is a block diagram of an experimental model for implementing the proposed device.

FIG. 3 - Pixel numbering of the alpha detector (side view of the tritium target 2).

FIG. 4 - The spectrum of gamma rays corresponding to the neutron transit time of the investigated object 7.

FIG. 5 - Gamma-ray spectrum corresponding to the neutron transit time of the gamma-ray detector 8.

The positions in the illustrations indicate:

1 - neutron generator,

2 - a beam of deuterons,

3 - tritium target,

4 - neutrons,

5 - alpha particles,

6 - position-sensitive (multi-pixel) alpha detector,

7 - the investigated object,

8 - gamma detectors,

9 - data acquisition system,

10 - computer.

The ion source of neutron generator 1 creates a deuteron beam 2 incident on the tritium target 3. In the reaction of deuterium with tritium T (d, n) He ⁴ , fast neutrons 4 and alpha particles 5 (He ⁴ ) are formed, with the initial energy and direction of motion neutron and accompanying alpha particles are uniquely connected and determined by the laws of conservation of energy and momentum in a given nuclear reaction:

where p _d , p _n , p _α - momentum of the deuteron, neutron and α-particles, respectively; θ _α-d is the angle between the directions of the alpha particle and deuteron, θ _nd is the angle between the directions of the neutron and deuteron, Q is the reaction energy T (d, n) He ⁴ ≈17.6 MeV.

A position-sensitive (multi-pixel) alpha detector 6 is built into the neutron generator, which records the number (coordinates) R _{α of the} triggered pixel and the alpha particle registration time t _α , which allows, by introducing a correction for the alpha particle velocity (~ 1.3 cm / ns ), determine the time of departure and the direction of motion of the alpha particle. Using these data, using relations (1), one can determine the time of departure, the direction of motion, and the energy (speed) of the neutron, i.e. “Tag” a neutron with an accompanying registered alpha particle.

The energy of tagged neutrons is about 14 MeV (velocity 5 · 10 ⁷ m / s). When a neutron with such energy passes through an organic substance located in the studied object 7, one of the most probable reactions of the interaction of a neutron with a substance is the inelastic scattering of a neutron (n, n ', γ) on the nuclei of carbon, nitrogen and oxygen with the emission of gamma quanta . Gamma-quanta are recorded by gamma-detectors 8, and the gamma-quantum registration time t _γ is measured. Given the known neutron velocity, the distance L from the registration of an alpha particle and gamma quantum Δt = tγ-t _α can be used to determine the distance L from the target 3 of the neutron generator to the point of emission of the gamma quantum as a result of inelastic scattering of the labeled neutron in the object under study. Knowing the distance L and the direction of motion of the labeled neutron, it is possible to determine the spatial coordinates of the place where the gamma-ray emitted during inelastic scattering of the labeled neutron in the object under study.

The energy of the gamma ray carries information about the scattering core. In the case of inelastic scattering of fast neutrons, gamma radiation occurs, the spectrum of which is signature, i.e. unique to various chemical elements. The table shows the gamma-ray line energies emitted during inelastic neutron scattering by the nuclei of nitrogen, oxygen and carbon and used to identify these elements.

Table Chemical element Gamma-ray energy, MeV Section, barn Carbon 4.44 185 Oxygen 2.74 52.7 3.76 48.5 6.12 149.3 6.91 50.30 7.12 57.12 Nitrogen 2.313 49.2 3.685 29.8 5.106 37.6

The data acquisition system 9 registers events in coincidence mode, i.e. subject to the simultaneous operation of the gamma detector and alpha detector in a given time window. Information about the event is transmitted to computer 10 and includes four parameters: a) the magnitude of the signal Q from the gamma detector, b) the time Δt of the signal from the gamma detector relative to the signal from the alpha detector in a given time window, c) the number of the triggered gamma detector , d) the number of the triggered pixel of the alpha detector. A small part (1-2%) of the total flow of alpha particles from the target enters the alpha detector. Since background signals from a gamma detector (which are not accompanied by a signal from an alpha detector in a given time window) are not recorded, the tagged neutron method can significantly increase the signal / background ratio.

In the method of labeled neutrons for detecting gamma rays, it is most efficient to use gamma detectors based on inorganic scintillators (NaI, LYSO, BaF ₂ , Bi ₄ Ge ₃ O ₁₂ , Lu _1.8 Y _0.2 SiO ₅ (Ce), coupled to photoelectron multipliers (PMTs) ). The energy of the gamma quantum is determined by measuring the value of signal Q from the gamma detector. As such a signal, the charge collected on the PMT anode, the maximum amplitude of the current on the PMT anode, etc. To determine the energy of gamma quanta W by measuring signal 6 gamma-d calibration required tectors by irradiation with gamma-quanta with known energy, for example, resulting from the radioactive decay of radioactive isotopes (for example, Co-60 or Cs-137) or with inelastic neutron scattering by some substances, accompanied by the emission of gamma-quanta of several lines, for example, carbon , nitrogen, oxygen, aluminum (see Table). When using gamma detectors based on inorganic scintillators, the Q signal linearly depends on the absorbed energy W [Yu.K. Akimov. Nuclear radiation detectors based on inorganic scintillators. Particle Physics and Atomic Nucleus, vol. 25, no. 1, 1994, p. 229-284.], And calibration is performed on two lines of the spectrum of gamma rays W ₁ and W ₂ corresponding to the signal Q ₁ and Q ₂ , respectively, and the energy of the gamma quantum is found from the relation:

where the calibration coefficients a and b are determined from the equations:

If Q = 0 at W = 0, then the calibration is performed on one line of the spectrum of gamma rays W ₁ corresponding to the signal Q ₁ and the calibration coefficients are equal to:

When changing influencing factors, for example, temperature or detector load, the calibration coefficient b in the calibration dependence (2) changes uncontrollably, which leads to an increase in the error in determining the energy of the gamma quantum.

In the prior art, it is not known a means of the same purpose as the claimed utility model, which is inherent in all the essential features given in the independent claim of the utility model formula, including the purpose of the application, therefore, the proposed device is new.

The proposed device can be used in industry for measuring the content of oxygen, nitrogen, carbon in a material, in particular, in explosive detection systems. The proposed device allows measurements in controlled objects without violating the integrity of the object.

Therefore, the proposed device is industrially applicable and socially acceptable.

Additional explanations for illustrations.

A block diagram of an experimental model for implementing the proposed device is shown in FIG. 2.

The pixel numbering of the alpha detector (side view of the tritium target 2) is shown in FIG. 3. The object under investigation is a cube with a side of 0.1 m of graphite.

Neutron generator 1 with a built-in multi-pixel alpha detector 6 emits tagged neutrons, when a tagged neutron is emitted, its time of departure and direction of motion is detected by an alpha detector. The studied object 7 is placed between the neutron generator 1 and the gamma detector 8 based on a Bi ₄ Ge ₃ O ₁₂ crystal, and the distance between the studied object 7 and the gamma detector 8 is 0.45 m (the time resolution of the system when using gamma detectors based on crystal Bi ₄ Ge ₃ O ₁₂ is 3 · 10 ^-9 s [Balygin K.A., Karetnikov M.D., Klimov A.I., Kozlov K.N., Meleshko E.A., Ostashev I.E. , Yakovlev GV Investigation of the characteristics of detecting equipment for the nanosecond method of labeled neutrons // Instruments and experimental equipment, No. 2, 2009, S. 122-132.]). Part of the labeled neutrons emitted by neutron generator 1 passes through the studied object 7. In this case, gamma-quanta are emitted as a result of inelastic scattering of neutrons by the nuclei of the studied object 7. If the gamma-detector 8 and alpha-detector 6 are triggered in a given time window, the collection system 9 records events (alpha-gamma coincidence) When registering each event, four parameters are recorded: the magnitude of the signal amplitude from the gamma detector, the time of appearance of the signal from the gamma detector relative to the signal from a of the alpha detector in the specified time window, the number of the triggered gamma detector, and the pixel number of the alpha detector. These parameters determine the energy of the gamma quantum and the coordinates of the place of its emission when the object under study is irradiated with tagged neutrons to conduct an elemental analysis of the object under study. A part of the labeled neutrons, the accompanying alpha particle of which is detected by pixel No. 2 (Fig. 3), falls on the gamma-ray detector 8 and initiates inelastic neutron scattering reactions with the emission of gamma-quanta on the oxygen nuclei that are part of the gamma-ray detector 8, which are detected gamma-detector 8. When the gamma-detector 8 and alpha-detector 6 are triggered, the data acquisition system 9 registers events in a given time window. From the spectrum of the magnitudes of the signals from the gamma detector 8, the computer 10 determines the magnitude of the signal from the gamma detector 6 (Q ₁ and Q ₂ ) corresponding to the energies of the two peaks (W ₁ = 3.76 MeV and W ₂ = 6.12 MeV) of the inelastic spectrum neutron scattering on oxygen nuclei (Table 1), which is part of the working body of the gamma detector 8.

In FIG. 4 shows a gamma-ray spectrum corresponding to the neutron transit time of the studied object 7, if the signals of pixel No. 5 (detecting alpha particles accompanying the neutrons passing through the studied object 7) of the alpha detector 6 and gamma detector 8 coincide in a given time window, where the characteristic spectrum of gamma rays of inelastic scattering by carbon (material of object 7) is visible. In FIG. 5 shows the gamma-ray spectrum corresponding to the neutron transit time of the gamma-detector 8, when the signals of the pixel No. 2 of the alpha detector (calibration pixel) and the gamma-detector 8, where the oxygen lines of 3.76 MeV and 6 are clearly visible, coincide in a given time window , 13 MeV, which is part of the gamma detector 5.

Under the condition of the operation of the gamma detector and alpha detector in a given time window, the data acquisition system 9 registers events. From the spectrum of the magnitude of the signals from the gamma detector 6, the computer 10 determines the magnitude of the signal from the gamma detector 6 (Q ₁ and Q ₂ ) corresponding to the energies of the two peaks (W ₁ = 3.76 MeV and W ₂ = 6.12 MeV) of the inelastic spectrum neutron scattering on oxygen nuclei (Table 1), which is part of the working fluid of gamma-detector 8. Using ratios (3) and (4), we determine the coefficients of the adjusted calibration dependence (2), which relates the signal value from the gamma-detector 8 Q and the energy gamma-ray W, and the calibration dependence is automatically changed.

The measurement error ΔW of the gamma-ray energy can be represented as the sum of the random δ and systematic θ error

Since the signals from the gamma detector recorded in the composition of events for conducting elemental analysis of the studied object and automatic calibration were obtained at the same temperature and loading of the gamma detector, the coefficient b in relation (2) is the same for them. Therefore, the systematic error in measuring the energy of gamma rays due to a change in the temperature of the scintillator or the detector load is absent (θ) in the inventive device, and the error ΔW is

where ε is the energy resolution of the gamma detector, measured by the full peak width at half maximum (FWHM) in the energy spectrum. For example, the energy of gamma rays emitted during inelastic scattering of fast neutrons by carbon nuclei is 4.44 MeV. The energy resolution of the gamma detector on this line is ε = 4%, therefore, ΔW = 0.076 MeV.

The temperature coefficient of the Bi ₄ Ge ₃ O ₁₂ k scintillator is 0.011 / deg [4. Gironneta J., Mikhailikc VB, Krausc H., de Marcillaca P., Coron N. Scintillation studies of Bi4Ge3O12 (BGO) down to a temperature of 6 K // Nuclear Instruments and Methods in Physics Research, Section A, Volume 594, Issue 3, 2008, P. 358-361]. During operation, the temperature change ΔT of the scintillator due to heating from the side of the photomultiplier is 3 deg / C. In the absence of automatic calibration (in the prototype device), the systematic error in measuring the energy of the gamma quantum is determined by the expression

and for the 4.44 MeV line it is 0.132 MeV, and the total error (formula (6)) is 0.208 MeV, which is more than 3 times the error value achievable when using the inventive device.

Claims (1)

Labeled neutron material analysis device, including a deuterium-tritium neutron generator with a built-in multi-pixel alpha detector, an object of research installed between the neutron generator and gamma detectors, inorganic scintillator gamma detectors, data acquisition equipment connected to a computer and recording under the condition the operation of the gamma detector and alpha detector in a given time window, the magnitude of the amplitude of the signal from the gamma detector, the time of appearance of the signal from the gamma detector from regarding the signal from the alpha detector, the number of the triggered gamma detector, the number of the triggered pixel of the alpha detector, characterized in that the inorganic scintillators of the gamma detectors contain oxygen, the gamma detectors are installed in such a way that labeled neutrons fall on each of the gamma detectors, and the alpha detector is located so that alpha particles accompanying neutrons that fall on the gamma detector arrive at least one pixel (calibration pixel) of the alpha detector, data acquisition equipment with it is injected with a means of measuring the spectrum of signals from gamma detectors provided that the signals in the gamma detector coincide with the signals recorded by the alpha-detector calibration pixel in a given time window, with the ability to automatically calibrate the energy scale of the gamma detector simultaneously with recording events when the target is irradiated with tagged neutrons using the obtained spectrum of signals from gamma detectors and the presence of peaks in it, arising from inelastic scattering of fast neutrons by oxygen nuclei as contained in the gamma detectors, the distance S between the subject and the gamma detectors along tagged neutron flux is not less than S = 3v · τ, where τ - temporal resolution in the measurement of alpha-gamma coincidence, v-speed neutrons labeled.

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