RU2570588C2 - Neutron detector - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: invention relates to measuring devices of neutron radiation by means of scintillation detectors. A neutron detector includes a housing, in which an arrangement is made for a composite scintillator, spectrum displacement fibres, the absorption spectrum of which is located in the flashing spectrum area of the composite oscillator, and at least one photoreceiver to which end faces of spectrum displacement fibres are optically connected, with that, the composite oscillator is made in the form of individual granules that are located at least in one layer around the spectrum displacement fibres.

EFFECT: improving neutron recording efficiency.

6 cl, 7 dwg

Description

The invention relates to devices for measuring neutron radiation using scintillation detectors. The main field of application of the proposed neutron detector is the registration of fast and thermal neutrons in mixed gamma and neutron radiation fields for nuclear safety purposes, the study of extensive air showers, and the rejection of electron and hadron showers at research space stations. These applications of the neutron detector require the creation of detectors with a large sensitive surface, high speed and low sensitivity to gamma radiation. Therefore, the known single-crystal neutron detectors cannot be used to solve the problems posed.

1. The prior art.

Modern technologies in principle make it possible to create thermal neutron detectors with a large sensitive surface based on counters containing ³ He and BF ₃ gases. However, such detectors have a low speed due to the slow drift of electrons, which are caused by ionization of the gas by charged particles resulting from nuclear reactions during the interaction of thermal neutrons with the ³ He and ¹⁰ B isotopes. Another disadvantage of these gas-discharge detectors is a ³ He deficiency caused by a reduction in nuclear production weapons [1].

Known neutron detector, in the cylindrical body of which is placed a cylindrical fiberglass made of organic glass with a ratio of diameter to length from 1: 8 to 1:24 [2]. A layer of a composite scintillator made in the form of a mixture of ZnS (Ag) with the ¹⁰ B isotope is deposited on the side surface of the fiber. The scintillation light is collected by photomultipliers optically connected to the ends of the fiber. The disadvantage of this detector is the low detection efficiency of thermal neutrons due to the thin layer of a composite scintillator, which has a low transparency to its own radiation.

Known neutron detector, containing a casing in the form of a parallelepiped, in which two scintillation screens are placed made of a composite scintillator by pressing a mixture of ZnS (Ag) with LiF enriched in ⁶ Li isotope with an enrichment degree of up to 95% and polymethyl methacrylate [3].

The scintillation light is collected from the surfaces of the screens using a spectroscopic plate, at the ends of which photomultipliers are installed. The indicated detector design does not allow to efficiently collect light radiation overexposed by the plate due to inefficient use of the photocathode photocathode area.

The closest technical solution adopted for the prototype is a neutron detector containing a parallelepiped housing, which houses a scintillation screen made of a composite scintillator by pressing a mixture of ZnS (Ag) with LiF enriched in ⁶ Li isotope with an enrichment degree of up to 95%, and polymethyl methacrylate [4]. The ends of the fibers are optically coupled to two photomultipliers. The use of the ZnS (Ag) polycrystalline scintillator is due to its unique scintillation characteristics: high conversion efficiency, reaching 20-25%, high detection efficiency of α radiation (α / β ~ 1), high light output of up to 160,000 photons / MeV. The scintillation light is collected from the surfaces of the screen using spectroscopic fibers placed close to each other on each surface as shown.

Figure 1 presents the design of the specified neutron detector, which contains: a housing 1, a screen of a composite scintillator 2, spectroscopic optical fibers 3.

Figure 2 shows the emission spectrum of ZnS (Ag) with a maximum wavelength of 460 nm.

Figure 3 presents the absorption spectra 1 and emission 2 spectroscopic fiber Y-8 company Kuraray [5].

As can be seen from the graphs presented in figures 2 and 3, the absorption spectrum of the spectroscopic fibers is in the region of the emission spectrum of the composite scintillator, in this case ZnS (Ag). Scintillation light, incident on a spectroscopic fiber, is re-emitted into the longer wavelength region and is recorded by photomultipliers. The use of flexible spectroscopic fibers makes it possible to efficiently collect scintillation light from a large surface onto the photocathodes of photomultipliers.

Due to the small diameter of the spectroscopic fiber, equal to 1-2 mm, the effective use of the photomultiplier surface is achieved by bringing the fibers into a bundle. However, the disadvantage of this neutron detector is its lack of registration efficiency due to inefficient light collection by spectroscopic fibers.

The objective of the invention is to provide a neutron detector with high detection efficiency by providing conditions for the efficient collection of scintillation light.

2. Disclosure of the invention.

This problem is solved due to the fact that a neutron detector containing a housing containing a composite scintillator, spectroscopic fibers, the absorption spectrum of which is located in the emission spectrum of the composite scintillator, and at least one photodetector, to which the ends of the spectroscopic fibers are optically connected , characterized in that the composite scintillator is made in the form of individual granules, which are located in at least one layer around the spectroscopic fibers.

The technical result provided by the given set of essential features is to increase the registration efficiency by collecting scintillation light by the entire lateral surface of the spectroscopic fibers, and not just its part, as it was in the technical solution adopted for the prototype. At the same time, the light of scintillations undergoes less attenuation during its movement from the place of origin to the surface of the spectroscopic fiber, since individual granules are located in at least one layer around the spectroscopic fibers.

An additional positive effect of the invention is to increase the efficiency of neutron registration in comparison with the prototype while reducing the number of spectroscopic fibers.

The proposed technical solution has an inventive step, since a causal relationship between the purpose of the invention and its essential features is new and unknown from the prior art. Patent studies did not reveal sources of information discrediting the novelty of the aggregate of essential features of the claimed technical solution.

3. A brief description of the drawings.

The invention is illustrated in the drawing, where figure 4 shows a schematic diagram of a neutron detector, which contains a housing 1, a composite scintillator 2, spectroscopic fibers 3, the ends of which are optically connected to at least one photodetector 4.

In FIG. 5 shows a cross section AA of the proposed neutron detector: case 1, composite scintillator 2, spectroscopic fibers 3.

Figure 6 shows a schematic diagram of a neutron detector, which additionally contains a light concentrator in the form of a monolithic focon - 5, the remaining designations are the same as in figure 4.

Depending on the composition of the composite scintillator, a neutron detector can detect both fast and thermal neutrons. When performing a composite scintillator of single-crystal granules of anthracene, stilbene, n-terphenyl, Gd ₂ SiO ₅ (Ce), Gd ₂ Si ₂ O ₇ (Ce), CdWO ₄ , ZnWO ₄ , Bi ₄ Ge ₃ O ₁₂ , LiI (Eu) , NaI (Tl), CsI (Tl), CsI (Na), BaF ₂ , LaBr ₃ (Ce), Lu ₂ SiO ₅ (Ce), ZnS (Ag, Te), ZnSe (Te, Al, Ga, О) , the detector registers fast neutrons by recoil protons arising from the interaction of fast neutrons with hydrogen atoms in spectroscopic fibers, which in turn cause scintillation bursts in the above scintillation granules.

Another mechanism for detecting fast neutrons is inelastic scattering of fast neutrons by the nuclei of elements that make up scintillation granules [6]. When performing a composite scintillator from single-crystal granules Gd ₂ SiO ₅ (Ce), Gd ₂ Si ₂ O ₇ (Ce), CdWO ₄ , as well as from granules LiI (Eu), ZnS (Ag, Te), ZnSe (Te, Al , Ga, O), additionally containing ⁶ Li, ¹⁰ V, and ²³⁵ U isotopes, the detector detects thermal neutrons based on the induced nuclear reactions with the formation of charged particles and gamma rays:

n + ¹⁵⁵ Gd → Gd * → electrons of internal conversion + Е _γ

n + ¹⁵⁷ Gd → Gd * → internal conversion electrons + Е _γ

n + ¹¹³ Cd → ¹¹⁴ Cd + E _γ

n + ⁶ Li → ⁴ He + ³ H + 4.79 MeV

n + ¹⁰ B → ⁷ Li * + ⁴ He + 2.31 MeV + E _γ (0.48 MeV)

→ ⁷ Li * + ⁴ He + 2.78 MeV

n + ²³⁵ U → charged fission fragments.

Charged particles and gamma radiation due to ionization processes occurring in scintillation granules excite atoms and molecules. Returning to the ground state, atoms emit photons. Photons falling into a spectroscopic fiber, in turn, cause it to glow in the longer wavelength range. Due to the effect of total internal reflection, light propagates through a spectroscopic fiber and is detected by photodetectors optically connected to its ends. As a photodetector, as a rule, a vacuum photoelectronic multiplier is used. To increase the efficiency of detecting light emerging from a fiber whose spectrum is shifted to the region of longer waves compared with the spectrum of scintillation photons, it is proposed to use a solid-state silicon photomultiplier [7]. The spectral sensitivity of a silicon photomultiplier is significantly higher than the spectral sensitivity of a vacuum photoelectron multiplier in the wavelength range (500-1000 nm) of emission of spectroscopic fibers.

A neutron detector with a large sensitive surface contains a large number of spectroscopic fibers, the total cross-sectional area of which, as a rule, exceeds the area of industrially produced silicon photomultipliers. To coordinate the total cross-sectional area of the spectroscopic fibers with the sensitive surface of silicon photomultipliers, it is proposed to additionally use light concentrators made in the form of a focon [8].

4. The implementation of the invention.

The experimental sample of the neutron detector contained 9 spectroscopic fibers BCF-91A (Saint-Gobain) 1000 mm long with an emission spectrum in the region of 490 nm. The total area of the sensitive surface of the neutron detector was 120 cm ² . The ends of the fibers with a diameter of 2 mm were optically coupled to two Ketek silicon photomultipliers with a sensitive surface of 6 × 6 mm ² . The composite scintillator was made of ZnS (Ag) granules containing the ¹⁰ V isotope. Figure 7 shows the amplitude spectrum of pulses obtained by irradiating the detector with a thermal neutron flux when using ZnS (Ag) granules containing the ¹⁰ V isotope as the composite scintillator. By measuring the neutron flux emitted by a Pu-Be source surrounded by a 50 mm thick polyethylene layer, a thermal neutron detection efficiency of about 90% was obtained when the fibers were arranged in 3 layers with 3 fibers in each layer.

Information sources

1. A review of neutron detection technology alternatives to helium-3 for safeguards applications. A.P. Simpson, S. Jones, M.J. Clapham and S.A. McElhaney. Pajarito Scientific Corporation, Santa Fe, NM, 87505, USA.

2. Bridgeport Instruments, LLC.

http://www.bridgeportinstruments.com/products/ncounter/ncounter.html

3. The development of a scalable he-3 free neutron detection technology and its potential use in nuclear security and physical protection applications. Matthew Dallimore, Calvin Giles *, David Ramsden * and Geraint S. Dermody. Symetrica Inc. 1 Clock Tower Place, Suite 130, Maynard MA 01754.

4. Full Scale Coated Fiber Neutron Detector Measurements. RT Kouzes, JH Ely, LE Erikson, WJ Kernan, DC Stromswold, ML Woodring, March 17, 2010. PNNL-19264 (prototype).

5. AZIMUT PHOTONICS. KURARAY scintillation optical fiber.

http://www.azimp.ru/catalogue/111/259/.

6. Method of detection of fast neutrons US 8058624 B2. Priority date: May 5, 2008 Date of publication: November 15, 2011

7. AZIMUT PHOTONICS. Silicon photomultipliers.

http://www.azimp.ru/catalogue/silicon-photomultipliers2/.

8. Physical Encyclopedia - Fokon.

http://enc-dic.com/enc_physics/Fokon-2720.html.

Claims (6)

1. A neutron detector comprising a housing in which a composite scintillator, spectroscopic fibers are placed, the absorption spectrum of which is in the region of the emission spectrum of the composite scintillator, and at least one photodetector to which the ends of the spectroscopic fibers are optically connected, characterized in that the composite the scintillator is made in the form of individual granules, which are located in at least one layer around the spectroscopic fibers. 2. The neutron detector according to claim 1, characterized in that the composite scintillator is made of single crystal granules of anthracene, stilbene, n-terphenyl, Gd ₂ SiO ₅ (Ce), Gd ₂ Si ₂ O ₇ (Ce), CdWO ₄ , ZnWO ₄ , Bi ₄ Ge ₃ O ₁₂ , LiI (Eu), NaI (Tl), CsI (Tl), CsI (Na), BaF ₂ , LaBr ₃ (Ce), Lu ₂ SiO ₅ (Ce), ZnS (Ag, Te ), ZnSe (Te, Al, Ga, O). 3. The neutron detector according to claim 1, characterized in that the composite scintillator further comprises isotopes ⁶ Li, ¹⁰ V and ²³⁵ U. 4. The neutron detector according to claim 1, characterized in that a silicon photomultiplier is used as a photodetector. 5. The neutron detector according to claim 1, characterized in that it further comprises at least one light concentrator, the input of which is optically connected to the ends of the spectroscopic fibers, and the output is optically connected to a photodetector. 6. The neutron detector according to claim 5, characterized in that the light concentrator is made in the form of a focon.

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