RU100270U1 - PHOTO-NUCLEAR DEVICE WITH COMBINED GAMMA-NEUTRON DETECTOR FOR DETECTION OF NUCLEAR MATERIALS - Google Patents

Abstract

 1. A photonuclear device with a combined gamma-neutron detector for detecting nuclear materials, including a bremsstrahlung source with a maximum energy greater than the threshold of photonuclear reactions, in which neutrons and gamma-quanta emitted from nuclear material are recorded, sequentially located on the side of the incidence of gamma-neutron radiation thermal neutron absorber layer, epithermal neutron absorber layer, fast neutron moderator layer, gas-filled thermal neutron counters, distinguishing The device is additionally equipped with a layer of an organic scintillator, a photoelectron multiplier, reflective and light-conducting means for optical communication of the organic scintillator with a photoelectron multiplier made of a material with a high density of hydrogen atoms; placed behind gas-filled thermal neutron counters in the direction of gamma-neutron radiation. ! 2. The device according to claim 1, characterized in that the thermal neutron absorber layer is made of cadmium with a thickness of 0.5-1.5 mm ! 3. The device according to claim 1, characterized in that the epithermal neutron absorber layer is made of 4-6 mm borated polyethylene with a boron content of 4-6 wt.%. ! 4. The device according to claim 1, characterized in that the fast neutron moderator layer is made of polyethylene 8-12 mm thick. ! 5. The device according to claim 1, characterized in that as gas-filled thermal neutron counters, SNM-67 type counters are used to record delayed neutrons. ! 6. The device according to claim 1, characterized in that the organic scintillator is made on the basis of

Description

The solution relates to the field of analysis of materials by radiation methods, measurement of secondary emissions from exposure to bremsstrahlung and can be used to detect nuclear materials, including fissile materials, shielded by the contents of the analyzed objects (large sea containers, cars, wagons, buildings, etc. ) without opening them.

A device [1] is known for detecting nuclear materials by the photonuclear method, based on measuring the neutron yield after irradiation of the test object with a pulsed flux of bremsstrahlung. The physical premise of the photonuclear method is that nuclear materials have a relatively low threshold for photonuclear reactions (<8 MeV), while for the most common chemical elements that are part of structural materials or contained in the soil, this threshold is much higher (> 10 MeV). This photonuclear device consists of a bremsstrahlung source with a maximum energy greater than the threshold of photonuclear reactions, a neutron detector, and electronic equipment for recording the amplitude and time of registration of pulses from a neutron detector. The presence of nuclear materials is determined by the increased neutron yield after the passage of the bremsstrahlung flux. The disadvantage of this device is that when screening nuclear materials with hydrogen-containing materials, the detection sensitivity of nuclear materials is significantly reduced. This is due to the fact that neutrons are slowed down and absorbed upon collision with hydrogen nuclei, due to which the flux of neutrons arriving at the neutron detector decreases.

Closest to the proposed device is a photonuclear device [2], which contains a bremsstrahlung source with a maximum energy greater than the threshold of photonuclear reactions, in which neutrons and gamma rays emitted from nuclear material are detected. The presence of nuclear materials is determined by the increased yield of neutrons and gamma rays after passing through the bremsstrahlung stream. During the radiation capture of neutrons by hydrogen nuclei in a hydrogen-containing substance that screens nuclear material, gamma rays with an energy of 2.2246 MeV are formed. Since these gamma rays appear during neutron capture, they are an indirect sign of the presence of increased neutron yield and the presence of nuclear materials. Thus, the registration of gamma rays ensures the detection of nuclear materials in hydrogen-containing media. To register gamma rays in a photonuclear device, Geiger-Muller gas-discharge counters are used. As a neutron detector, gas-filled thermal neutron counters are used, located behind a layer of a thermal and epithermal neutron absorber and a fast neutron moderator.

Thermal and epithermal neutrons are mainly background (non-nuclear material), therefore, it is undesirable to detect them. Cadmium is used as a thermal neutron absorber, borated polyethylene is used as an epithermal neutron absorber, and polyethylene is a fast neutron moderator.

The disadvantage of this device is the low sensitivity of detection of nuclear materials in hydrogen-containing environments due to the low efficiency of registration of gas-discharge Geiger-Muller counters. The use of other, more efficient gamma-ray detectors (for example, scintillation) is possible, however, this will shield one type of radiation with detectors of another type of radiation, which will reduce the sensitivity of the device. For example, when scintillation gamma-ray detectors are placed in front of neutron detectors, the neutron flux will weaken and the detection sensitivity of nuclear materials determined by the neutron yield will decrease. The arrangement of neutron and gamma-ray detectors in such a way that they do not overlap (next to each other in the same plane) will also lead to a decrease in the detection sensitivity of nuclear materials by reducing the area of the neutron detector and gamma-detector with a constant total area of the detectors.

To increase the sensitivity of detection of nuclear materials in hydrogen-containing media in a photonuclear device containing a bremsstrahlung source with a maximum energy greater than the threshold of photonuclear reactions, in which neutrons and gamma-rays emitted from nuclear material are detected, neutrons and gamma-quanta are recorded by a combined gamma-neutron a detector consisting (from the side of the incident gamma-neutron radiation) of a layer of a thermal neutron absorber, an epithermal neutron absorber layer, Loy moderator of fast neutrons, thermal neutrons gas-meter, an organic scintillator layer, recording gamma rays. In addition, the organic scintillator layer simultaneously performs the function of a neutron reflector to increase the efficiency of neutron detection.

Thus, the technical results of the claimed proposal is to increase the detection sensitivity of nuclear materials surrounded by a hydrogen-containing medium by increasing the efficiency of neutron detection.

The specified technical result is implemented using the set of essential features below.

A photonuclear device with a combined gamma-neutron detector for detecting nuclear materials, including a bremsstrahlung source with a maximum energy greater than the threshold of photonuclear reactions, in which neutrons and gamma-quanta emitted from nuclear material are recorded, sequentially located on the side of the incidence of gamma-neutron radiation,

thermal neutron absorber layer,

epithermal neutron absorber absorber,

fast neutron moderator layer,

gas filled thermal neutron counters,

moreover, the device is further provided

made of a material with a high density of hydrogen atoms, placed behind gas-filled thermal neutron counters in the direction of gamma-neutron radiation, an organic scintillator layer, a photoelectron multiplier, reflective and light-conducting means for optical coupling of the organic scintillator with a photoelectron multiplier,

wherein,

- the thermal neutron absorber layer is made of cadmium 0.5-1.5 mm thick .;

- the epithermal neutron absorber layer is made of 4-6 mm borated polyethylene with a boron content of 4-6 weight percent;

- the fast neutron moderator layer is made of polyethylene 8-12 mm thick .;

- as gas-filled counters of thermal neutrons, counters of the SNM-67 type were used to record delayed neutrons;

- organic scintillator is made on the basis of scintillating stilbene with a thickness of 70-90 mm .;

- reflective means made in the form of a coating of Mylar foil;

- light guide means are made of plexiglass;

- used a photomultiplier tube type FEU-182.

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 in the physical protection systems of nuclear facilities, in the customs control systems of containers and vehicles to identify nuclear materials in them. The proposed device allows to determine the presence of nuclear materials in controlled facilities without violating the integrity of the facility. Therefore, the proposed device is industrially applicable and socially acceptable.

Figure 1 presents a block diagram of a device for implementing the proposed device, where the positions indicated:

1 - electronic accelerator,

2 - converter

3 - filter

4 - collimator,

5 - combined gamma-neutron detector,

6 - container

7 - bremsstrahlung,

8 - nuclear material,

9 is a data acquisition system for signals from a detector.

Figure 2 shows the geometry of the combined gamma-neutron detector for detecting gamma rays and neutrons in the proposed solution, where the positions are:

10 - recorded radiation,

11 - layer of thermal neutron absorber,

12 is a layer of an absorber of epithermal neutrons and a moderator of fast neutrons,

13 - layer moderator of fast neutrons,

14 - gas-filled neutron counters,

15 - organic scintillator,

16 - reflective coating

17 - light guide

18 - photomultiplier tube.

The inventive device operates as follows. The electron beam leaves the accelerator 1 and is converted by the converter 2 into bremsstrahlung 3. The filter 3 delays the remaining electrons, and only the bremsstrahlung is emitted in the direction of the container 6 with nuclear material 8. The collimator 4 forms a sharply directed beam of bremsstrahlung. The detector 5 registers gamma rays and neutrons from nuclear material 8, the signals from the detectors are recorded by the data acquisition system 9.

With a specific implementation of the method:

- the electron accelerator 1 creates a pulsed beam with an energy of 12 MeV, a pulsed current of 100 mA, a pulse duration of 1 μs;

- Converter 2 with a thickness of 0.8 mm converts the electron beam into a stream of bremsstrahlung with a maximum energy of 12 MeV;

- behind converter 2, a filter 3 of an aluminum plate 20 mm thick is installed;

- behind the filter 3 is installed a collimator 4 of copper 200 mm thick with a rectangular hole with a cross section of 40 × 40 mm along the propagation axis of the bremsstrahlung stream;

- nuclear material 8 is a cube or ball weighing 1 kg, consisting of uranium-235 with an enrichment of 92%, located in the center of the container 6;

- the container 6 with dimensions of 2400 × 2400 × 6000 mm ^{3 is} completely filled with a substance with an average density of 0.5 g / cm ³ ;

Combined gamma-neutron detector 5 consists of

- a cadmium layer 11 1 mm thick for absorption of thermal neutrons;

- a layer of boron polyethylene 12 with a thickness of 5 mm with a boron content of 5 weight percent to absorb epithermal neutrons and slow down fast neutrons;

- a layer of polyethylene 13 with a thickness of 10 mm to slow fast neutrons;

- gas-filled counters of 14 neutrons of the SNM-67 type for recording delayed neutrons;

- organic scintillator 15 based on scintillating stilbene with a thickness of 80 mm,

- reflective coating 16 of Mylar foil,

- fiber 17, the geometry of which is selected on the basis of recommendations from [3], made of plexiglass,

- photomultiplier tube 18 type FEU-182.

The numerical simulation of the sounding of a container 6 with nuclear material 8 by bremsstrahlung and the registration of neutrons and gamma rays emerging from the container was calculated using the GEANT 3.21 program [4]. When processing the results of calculations, the effect (neutrons and gamma rays emitted from nuclear material) and the background (neutrons and gamma rays of a different nature) were determined. When passing through a substance filling a container, part of the neutrons is scattered and absorbed, and the loss of neutrons released from the nuclear material in the case of filling the container with hydrogen-containing substance is several orders of magnitude higher than in other options for filling the container. This is due to the large cross section of elastic scattering (thermalization) processes and the absorption of neutrons by hydrogen nuclei. Background neutrons are emitted mainly when the bremsstrahlung interacts with the ¹³ C isotope. These factors lead to very low values of the effect and the effect / background ratio, which makes it very difficult to detect nuclear materials by detecting neutrons in containers that are heavily shielded by water, paper, oil products or other hydrogen-containing substances.

After the end of the flash of bremsstrahlung, in addition to neutrons, gamma quanta exit the container. These gamma-rays are of a purely secondary nature and mainly appear as a result of radiation capture of thermal neutrons by the nuclei of the substance filling the container (capture gamma-quanta), thus being an indirect sign of neutron radiation. The source of thermal neutrons is fast neutrons that occur during thermalization in a hydrogen-containing medium of neutrons released as a result of photonuclear reactions.

The intensity of capture gamma radiation depends on the contents of the container. If the container is filled with inorganic materials, for example, heavy or light metal, the output of capture gamma radiation is small compared to the output of neutrons. This is due to both a low neutron interaction cross section and a strong attenuation of gamma radiation in the container. The output of capturing gamma rays of the effect and the effect / background ratio are maximum when there are hydrogen-containing substances in the container.

Capture gamma-rays are emitted for a significantly longer time (with a characteristic decay constant of several ms) compared to the neutron response (several hundred microseconds), since the source of the capture gamma-quanta is decelerated neutrons. This allows the use of temporal selection of events to discriminate signals caused by neutrons in a gamma detector.

The results of calculating the efficiency of registration with a combined neutron and gamma-ray detector for the spectrum of neutrons and gamma-rays emerging from a container filled with iron, aluminum and a hydrogen-containing substance - paper, are shown in Table 1.

Table 1. Registration efficiency with a combined neutron and gamma-ray detector. Particle type Container filling option Aluminum Iron Paper Instant effect neutrons 0.209 0.211 0.147 Instant neutrons background 0.209 0.206 0.194 Capturing gamma-ray effect 0.35 0.34 0.37 Capture gamma rays background 0.31 0.31 0.33

The number of registered particles of the effect N _e after one pulse of bremsstrahlung is proportional to the mass of nuclear material M:

where n _e is the number of registered particles of the effect with the mass of nuclear material m ₀ = 1 kg With a small mass of nuclear material, the number of detected background particles does not depend on the mass of nuclear material.

The sensitivity of the nuclear material detection device corresponds to the minimum detectable mass of nuclear material M _min and can be determined from the solution of equation [5]:

where α is the confidence interval, n _f is the number of detected background particles. From relations (1) and (2) it follows that

With a confidence interval of α> 2 (for a detection probability of more than 95%) 8n _f / α ² >> 1 (according to the calculated data presented in Table 1) and expression (3) can be represented as:

.

The number of detected particles of effect n _e and background n _{f is} proportional to the efficiency of the neutron detector when registering the effect ε _e and background ε _f :

where K _e and K _f are constants, depending on the parameters of the photonuclear installation, the contents of the container, but not depending on the type of detector. From equations (4) - (6) it follows that:

,

those. minimum detectable mass

.

Assuming that the efficiency of recording the effect is close to the efficiency of recording the background ε _e ≈ε _f ≈ε, relation (8) can be rewritten as:

.

Let us evaluate the decrease in the efficiency of gamma-ray detection by a combined gamma-neutron detector due to the absorption of gamma-quanta by a cadmium layer of a cadmium layer 11 with a thickness of l ₁ = 1 mm, a borated polyethylene layer of 12 with a thickness of l ₂ = 5 mm, a polyethylene layer of 13 with a thickness of l ₂ = 5 mm . According to [7], the gamma-ray flux reduction coefficient is:

,

where ρ ₁ , ρ ₂ and ρ ₃ - the density of cadmium, borated polyethylene and polyethylene equal to 8.65, 0.92 and 1.0; µ ₁ , µ ₂ and µ ₃ are the mass attenuation coefficients of gamma rays in cadmium, borated polyethylene and polyethylene [8] (it is assumed that, due to the low concentration of boron, the mass attenuation coefficients of gamma rays in boron polyethylene and polyethylene are equal). Table 2 gives the values of μ ₁ , μ ₂ and μ ₃ and the coefficient of decrease in the detection efficiency of gamma quanta k _γ calculated by expression (11) for various gamma ray energies less than 2.2246 MeV.

The prototype device uses an LND 719 counter [2]. According to the specification of this counter [6], its sensitivity ξ is: 1 count at a dose rate of gamma radiation ( ⁶⁰ Co) of 1.25 · 10 ^-5 R / h or 3.5 · 10 ^-9 R / s. The dose rate of 1 P corresponds to the fluence F of the gamma-ray flux 2 10 ⁹ 1 / cm ² [8]. According to the specification of the LND 719 counter, its effective diameter is 15 mm and its effective length is 228 mm, therefore, the effective area of the S _GM counter with a unidirectional gamma-ray flux is 34 cm ² It follows that the efficiency of registering the LND 719 gamma-quanta isotope ⁶⁰ Co (1.17 and 1.33 MeV) is equal to

.

For other gamma-ray energies, the registration efficiency of a counter of a similar type changes by no more than 10 times [9]. The efficiency of registering gamma rays with a combined detector is given in Table 1. A comparison using expression (9) of the proposed device and the prototype device shows that the sensitivity (minimum weight) of the proposed device is less than at least 3 times less compared to the prototype device, even taking into account a decrease in the efficiency of gamma-ray registration due to absorption in layers 11, 12 and 13 of the combined gamma detector.

Table 2. The values of the coefficient k _γ reduce the efficiency of registration of gamma rays for different energies of gamma rays due to absorption in the detector material. Coefficient k _γ The energy of gamma rays, MeV Mass coefficient of attenuation of gamma rays, cm ² / g Cadmium Borated polyethylene Polyethylene 0.897703 2 0.0414 0,0506 0,0506 0,857371 one 0,0583 0,0726 0,0726 0,801647 0.5 0.0925 0,099 0,099

SOURCES OF INFORMATION TAKEN INTO ACCOUNT:

1. Mastny G.F .: Detection of subsurface fissionable nuclear contamination through the application of photonuclear techniques United States Patent 5495106.

2. Jones J.L., Blackburn B.W., Haskell K.J. et al. Pulsed Photonuclear Assessment (PPA) Technology Enhancement Study. Report INEEL / EXT-06-11175, Idaho National Engineering and Environmental Laboratory, Idaho, USA, 2006, 38 pp.

3. Tsirlin Yu.A., Sysoeva EP, Globus M.E. Optimization of detection of gamma radiation by scintillation crystals, M .: Energoatomizdat. 1991, 153 p.

4. GEANT3.21 Detector Description and Simulation Tool, Manual, CERN Program Library, CERN Geneva, Switzerland, 1993.

5. I. Goldanskiy, A. V. Kutsenko, M. I. Podgoretsky. Statistics of readings during registration of nuclear particles. - M .: Fizmatgiz, 1959, 411 p.

6. Thin wall beta-gamma detector specifications. http://www.lndinc.com/products/pdf/397/

7. N.G. Gusev, V.P. Mashkovich, A.P. Suvorov. Protection against ionizing radiation: Textbook for universities. In 2 t. T.1. The physical basis of radiation protection. Energoatomizdat, Moscow: 1989, 461 pp.

8. J.H. Hubbell and S. M. Seltzer. Tables of X-Ray Mass Attenuation Coefficients and Mass Energy-Absorption Coefficients (version 1.4, 2004). NISTIR 5632, National Institute of Standards and Technology, Gaithersburg, MD, USA, http://physics.nist.gov/PhysRefData/XrayMassCoef/verhist.shtml

9. V. Price. Registration of nuclear radiation. InLit, Moscow: 1960, 464 pp.

Claims (9)

1. A photonuclear device with a combined gamma-neutron detector for detecting nuclear materials, including a bremsstrahlung source with a maximum energy greater than the threshold of photonuclear reactions, in which neutrons and gamma-quanta emitted from nuclear material are recorded, sequentially located on the side of the incidence of gamma-neutron radiation thermal neutron absorber layer, epithermal neutron absorber layer, fast neutron moderator layer, gas-filled thermal neutron counters, distinguishing The device is additionally equipped with a layer of an organic scintillator, a photoelectron multiplier, reflective and light-conducting means for optical communication of the organic scintillator with a photoelectron multiplier made of a material with a high density of hydrogen atoms; placed behind gas-filled thermal neutron counters in the direction of gamma-neutron radiation. 2. The device according to claim 1, characterized in that the thermal neutron absorber layer is made of cadmium with a thickness of 0.5-1.5 mm 3. The device according to claim 1, characterized in that the epithermal neutron absorber layer is made of 4-6 mm borated polyethylene with a boron content of 4-6 wt.%. 4. The device according to claim 1, characterized in that the fast neutron moderator layer is made of polyethylene 8-12 mm thick. 5. The device according to claim 1, characterized in that as gas-filled thermal neutron counters, SNM-67 type counters are used to record delayed neutrons. 6. The device according to claim 1, characterized in that the organic scintillator is made on the basis of scintillating stilbene with a thickness of 70-90 mm 7. The device according to claim 1, characterized in that the reflective means is made in the form of a coating of Mylar foil. 8. The device according to claim 1, characterized in that the light guide means are made of plexiglass. 9. The device according to claim 1, characterized in that the photomultiplier tube type FEU-182 is used.