RU2189057C2 - Scintillation detector of neutron and gamma radiation - Google Patents

Abstract

FIELD: radiation monitoring. SUBSTANCE: scintillation detector can be used in custom houses, units of State Atomic Supervision, in systems of special radiation monitoring of nuclear submarines subject to stripping, in detection and identification of fissionable materials ( uranium, plutonium, californium ) and articles from them, and materials related to class of radioactive substances-gamma radiation sources. Scintillation detector of neutron and gamma radiation includes sensor coming in the form of two scintillators connected in parallel: external neutron scintillator based on plastic material (CH)_n or stilbene ( scintillator with well ) placed into jacket of boroncarrying (boron nitride or carbide ) material and internal scintillation crystal Nal-T1 located in well of external scintillator in standard container and sensitive to gamma radiation, photoelectron multiplier positioned in jacket of boron nitride or carbide ( natural mixture of isotopes of boron or isotope $\genfrac{}{}{0ex}{}{\text{10}}{\text{5}}$ B ) sensitive to thermal neutrons placed in single case and unit of electron processing of signals. EFFECT: provision for counting neutrons of wide spectrum of energy, both fast and thermal neutrons, with raised efficiency. 1 cl, 1 dwg, 1 tbl

Description

 The invention relates to the field of remote simultaneous detection of sources of neutron and gamma radiation, especially neutron sources against a background of gamma radiation, and is intended for use in dosimetric customs practice to solve the problems of Gosatomnadzor, for radiation monitoring systems and special radiation technical control systems for nuclear submarine inspection to be disassembled for the detection and identification of fissile materials (DM) (uranium, plutonium, California and products from them) and m materials under belonging to the class of radioactive substances (RS) - gamma-ray sources.

 Known nuclear radiation detectors contain, as a rule, a sensor and an electronic signal processing unit [1-7].

Known scintillation gamma-ray detectors, including a sensor (detection unit) and an electronic signal processing unit [1, 2], in which the sensor contains a scintillation inorganic crystal and a photodetector (photoelectron multiplier or photodiode). The scintillation crystal (one of the alkali halide crystals or one of the oxide-based crystals) usually has a cylindrical shape with a diameter of 10 to 150 mm and a height of 10 to 150 mm, or it has a cubic shape of 10 • 10 • 10 mm ³ when photodiode registration . However, detectors with such a sensor are not suitable for detecting neutron radiation, since the sensor does not contain elements sensitive to neutrons.

Known scintillation detector of beta, gamma and neutron radiation according to the patent [3], containing the sensor is a scintillation crystal, for example Lu ₂ SiO ₅ -Ce or stilbene, a spectrum shifter (in the form of a thin scintillating film or crystal) and a silicon PIN photodiode, as well as an electronic signal processing unit. However, such a detector is not intended for simultaneous efficient registration of neutron and gamma radiation. If a Lu ₂ SiO ₅ -Ce crystal is used as the scintillation in the sensor, the latter will only detect gamma rays and will not detect neutrons. If the sensor contains stilbene as a scintillation crystal, the latter will mainly detect neutrons. Since in standard detectors with diode registration, due to the small size of the diodes, the standard size of the scintillation crystals is small (usually a cube of 10 • 10 • 10 mm ³ ), the detection efficiency of neutrons and, especially, gamma rays will be small for remote measurements, not higher than 1% . In addition, the detector according to the patent [3] is unsuitable for detecting slow and thermal neutrons; it is capable of detecting only fast neutrons by recoil protons.

Known neutron detector of scintillation type with a sensor based on ⁶ LiI-Eu crystals containing ⁶ Li isotope [1]. However, such a detector is hygroscopic and has a very long scintillation time (1400 ns), which does not allow for a high loading capacity of the detector, and most importantly, such a detector only detects thermal neutrons, but does not detect fast neutrons, it has low sensitivity (efficiency) for neutron detection against the gamma background, since, along with thermal neutrons, the detector simultaneously detects gamma quanta, the signals from which are difficult to separate because they have the same spectral e and timing. In addition, the known detector [1] is not suitable for spectrometry of gamma radiation.

 The known selective neutron detector according to the patent [4] contains two sensors, one of which is sensitive to charged particles and neutrons, while the other is sensitive only to charged particles; the number of detected neutrons is determined by the difference signal from these sensors, allocated using the difference circuit of the electronic unit. However, the possibility of using such a detector for registration and spectrometry of gamma radiation in the patent [4] is not specified.

 The well-known multi-radiation detector [5] has a sensor that includes two scintillation crystals with green and red glows, one of which is sensitive to high-energy radiation, and the other to low-energy, and two photodiodes that emit signals using light filters (green and red) and detect them using the electronic signal processing unit. Such a detector has limited applications; according to [5], it is suitable for detecting x-rays with two different energies, but it is unsuitable for detecting neutrons and simultaneously for gamma-ray spectrometry.

Known all-wave neutron detector [6], the sensor of which consists of ³ He counters sensitive to thermal neutrons. Its sensitivity to fast neutrons is low: no more than 10%, it registers them after deceleration. Such a detector is unsuitable for the simultaneous detection of neutrons and gamma radiation, unsuitable for spectrometry of gamma radiation.

 A known detector [7], the sensor of which is a plastic scintillation detector SPS-T4A, designed to detect gamma radiation and fast neutrons. The detector has the following characteristics: the duration of a scintimpulse generated by a neutron or gamma ray is 8.5 ns; light output UESV (according to GOST 23077-78) when excited by electrons with an energy of 662 keV - 0.29; maximum luminescence spectrum 490 nm, diameter and height up to 50 mm. However, such a detector is unsuitable for gamma-ray spectrometry and unsuitable for detecting thermal neutrons.

 A known epithermal neutron detector [8], which contains a thermal neutron sensor, protection against thermal neutrons surrounding this sensor; epithermal neutron moderator, which penetrate the shield so that already decelerated neutrons are more easily absorbed by the counter. The thickness of the moderator and the ratio of the diameter of the counter to the outer diameter of the moderator are such that the maximum counting speed can be obtained when the counter completely fills the inner diameter of the thermal neutron shield. However, the known detector [8] does not allow detecting gamma radiation and, accordingly, does not allow spectrometry of gamma radiation.

 A known detector, similar to the detector [7], for detecting ionizing radiation according to US patent [9]. The detector comprises a sensor, in particular a single-chip scintillation sensor that is sensitive to neutrons and gamma rays at the same time, and an electronic signal processing unit that includes an electronic selection circuit for separating signals (pulses) generated by neutrons and gamma rays. However, any single-chip sensor is not optimal for the simultaneous detection of neutrons and gamma rays, since it does not have a sufficiently high sensitivity, selectivity, and the necessary functionality. The known detector [9] is unsuitable for gamma-ray spectrometry.

Known detector [10], it contains a sensor and an electronic signal processing unit; the sensor is made in the form of a Bi ₄ Ge ₃ O ₁₂ scintillation crystal connected in series, sensitive to proton, x-ray, as well as α, β, γ radiation, and a fiber made of an organic scintillating substance based on stilbene or plastic (CH) _n sensitive to fast neutrons, a photomultiplier that converts light flashes (scintillations) into electrical signals, and the electronic signal processing unit includes a circuit for temporal selection of scintillation pulses coming from α-, β-, γ-s intillyatora Bi ₄ Ge ₃ O _12, and from the optical fiber scintillating by fast neutrons. However, the known detector [10], being sensitive to fast neutrons, is unsuitable for detecting thermal neutrons. In addition, the device [10], in which Bi ₄ Ge ₃ O ₁₂ bismuth orthogermanate is used as an α, β, γ scintillator, cannot provide a spectrometric mode with high energy resolution. The energy resolution of these crystals is usually 15–20%, while in NaI-Tl crystals it is 2-3 times better and amounts to 6–8%.

 Known neutron detectors [11], using the neutron capture reaction (n, γ), accompanied by the emission of 3 or 4 γ-quanta with a total energy of approximately 4-8 MeV. In particular, a (n, γ) -detector based on a liquid scintillator is known, in which plates of an absorbing substance such as cadmium are placed [11]. Cadmium absorbs slow neutrons and emits γ-quanta (reaction (n, γ)), which cause light flashes in a liquid scintillator. A (n, γ) detector with a NaI-Tl scintillator surrounded by a silver cover is also known (silver has a large resonance peak for the (n, γ) reaction) [11]. Silver effectively absorbs neutrons of resonant energies due to the (n, γ) reaction and emits γ-quanta, which are detected by a NaI-Tl scintillation crystal. However, such detectors are very expensive due to the high cost of cadmium and silver, and a NaI-Tl based detector with a silver cover cannot detect fast neutrons.

Closest to the claimed device is the patent [12], which contains a sensor and an electronic signal processing unit. The sensor is made in the form of three parallel-series-connected scintillators: an external neutron scintillator made of an organic hydrogen-sensitive substance based on plastic (SN) _n or stilbene (scintillator with a well) sensitive to fast neutrons, and a scintillation detector (located in the well of an external scintillator) NaI-Tl crystal in a standard container, sensitive to gamma radiation, and internal scintillator based on ⁶ Li-silicate glass doped with cerium sensitive t pilaf neutrons, and a photomultiplier tube placed in a single body, and the electronic signal processing unit includes circuitry internal stsintiimpulsov neytronochuvstvitelnyh time selection of the scintillator and the gamma sensitive scintillators, and spectroscopic analyzer for processing stsintiimpulsov from the scintillation crystal NaI-Tl. An external neutron scintillator (made of a hydrogen-containing substance that is selectively sensitive to fast neutrons and detects them from light flashes created by them) is simultaneously a moderator of fast neutrons, and the thickness of the external scintillator is chosen so that the neutrons slow down to thermal energies. Thermal neutrons are detected by an internal glass scintillator containing the ⁶ Li isotope due to the nuclear reaction ⁶ Li (n, α) ³ H: the α-particle resulting from this reaction causes light flashes inside the scintillator. The internal neutron scintillator is made of a material (glass) that is transparent to light flashes coming to the photoelectron multiplier from an external neutron scintillator and from a NaI-Tl crystal located in the well. For these flashes, he plays the role of a fiber. The NaI-Tl crystal (uterine scintillator) is used to register gamma rays. The known device operates in the fields of neutron and accompanying it and requiring accounting and spectrometric analysis of gamma radiation. When neutron and gamma radiation passes through the sensitive elements (scintillators) of the sensor with PMT, three groups of signals with different durations will arrive at the electronic information processing unit. One group of signals associated with fast neutrons will have a short duration of no more than 3-5 ns, the second group of signals associated with the registration of thermal neutrons will have a duration of ~ 60 ns, and finally, the third group of signals due to gamma rays has duration ~ 250 ns. These signal groups are separated by a time selection circuit. A selected group of signals with a duration of 250 ns is fed to a spectrometric block analyzer, which allows one to determine the energy spectrum of the detected gamma radiation. Weak additional signals from gamma rays arising in a hydrogen-containing scintillator with a duration of 3 ns are easily discriminated. Thus, the known device provides neutron counting and spectrometric analysis of gamma rays.

However, the known device according to the patent [12] has insufficiently high operational reliability due to the difficulty of providing high-quality optical contacts of three parallel-series scintillators and has a high cost determined by the cost of the sensor component responsible for detecting thermal neutrons, i.e. the cost of glass with isotope ⁶ Li (up to 50 US dollars per gram of glass). In addition, for lithium nuclei, the interaction cross section σ for thermal energy neutrons and the resonance integrals of interaction I are relatively small (in comparison with those, for example, for ¹⁰ V nuclei, see table [13]). For a natural mixture of lithium isotopes (also containing the ⁶ Li isotope) in the neutron absorption reaction σ = 70.5 • 10 ^-24 cm ² (70.5 barn), and I = 32 • 10 ^-24 cm ² (32 barn). Directly for the isotope ⁶ Li, σ = 940 barn (reaction (n, α)), and I = 425.5 barn. That is, the known detector [12] does not provide high detection efficiency of thermal neutrons.

The proposed device provides higher detection efficiency of both fast and thermal neutrons in the counting mode, as well as registration of gamma radiation in a spectrometric mode with high energy resolution. A block diagram of the inventive device is shown in the drawing. The inventive device comprises a sensor and an electronic signal processing unit. The sensor is made in the form of two scintillators connected in parallel: an external neutron scintillator 1 of an organic hydrogen-sensitive substance based on plastic (SN) _n sensitive to fast neutrons or stilbene (a scintillator with a well) with a cover made of boron-containing (nitride or boron carbide) material 2 placed in it (in the well of the internal scintillator) of a gamma-radiation-sensitive scintillation crystal NaI-Tl 3 in a standard container, and in a case of thermal neutrons sensitive 4 of nitride or carbide boron (natural mixture of boron isotopes or isotope ¹⁰ ₅ V) and a photomultiplier 5, placed in a single body 6, block the electronic signal processing 7 includes circuitry stsintiimpulsov time selection from organic external scintillator 1 and the inner inorganic scintillator 3, and spectrometric analyzer for processing scintillation pulses from gamma scintillation crystal NaI-Tl.

The essence of the invention lies in the fact that the proposed device, along with the registration of fast neutrons (organic scintillator 1), provides efficient registration of slow neutrons by the reaction (n, α, γ) using covers made of boron-containing material. Primary thermal neutrons from a detected DM source are detected by the reaction (n, α, γ) using a case 2 made of boron-containing material. Fast neutrons are detected by an external neutron scintillator made of a hydrogen-containing substance that is selectively sensitive to fast neutrons and registers them with the help of light bursts created by recoil protons. Not all fast neutrons (usually not more than 10%) are detected by a hydrogen-containing scintillator. So that they do not "get out of the game," a hydrogen-containing substance is used as a moderator of fast neutrons. Moreover, the dimensions of the external scintillator 1 are chosen such that the neutrons are slowed down by them to thermal energies. Next, thermal neutrons are absorbed by the boron nuclei, which are part of the sheath 4, containing boron in the form of a natural mixture of isotopes or in the form of a ¹⁰ V isotope, and enter the reaction (n, α, γ): γ-quanta resulting from this reaction fall into the sensitive gamma radiation scintillator NaI-Tl and cause light flashes inside the scintillator. A significant novelty of the invention lies in the fact that the internal "uterine" scintillator NaI-Tl carries information about both the gamma component and the neutron component of the radiation from the DM source. A crystal scintillator registers several types of gamma quanta: it registers external gamma quanta corresponding to the measured gamma field, and internal gamma quanta generated by thermal neutrons ((n, α, γ) reaction) in an inner case made of boron-containing material. The thermal neutrons of the primary radiation are detected after they are absorbed in an outer case of boron-containing material by the reaction (n, α, γ), followed by registration of the γ-quantum generated in this reaction with both an external plastic or stilbene scintillator and an internal NaI-Tl scintillator.

а затем через ~10^-13 с
(2) $\genfrac{}{}{0ex}{}{\text{7}}{\text{3}}$ Li $\genfrac{}{}{0ex}{}{\text{*}}{\text{(возб.сост.)}}$ _→ $\genfrac{}{}{0ex}{}{\text{7}}{\text{3}}$ Li_{(осн.сост.)}
с испусканием γ-кванта с энергией 0,48 МэВ.The device operates in the fields of neutron (fast and thermal neutrons) and its attendant and requiring accounting and spectrometric analysis of gamma radiation as follows. Under the action of fast neutrons falling into the volume of an external scintillator (from stilbene or transparent plastic (SN) _n ), light flashes appear in it with a radiation wavelength of 400-420 nm with a duration of up to 2-3 ns. These light flashes created by fast neutrons pass through an optical contact to the photomultiplier photomultiplier, creating electrical pulses of 2-3 ns duration at its output. Fast neutrons passing through an external neutron scintillator, due to collisions with hydrogen nuclei and subsequent ionization of the medium, suffer radiation losses partially in the form of light flashes (due to which they are recorded). However, they lose their energy mainly not due to these light flashes, but due to collisions with the nuclei of hydrogen, which is part of the scintillation material, while they slow down, but do not drop out of the game, but are recorded as thermal neutrons by the reaction (n , α, γ). The thickness of the external scintillator, which simultaneously plays the role of a moderator of fast neutrons, is chosen such that fast neutrons, before falling into the cover (made of boron-containing material) surrounding the internal NaI-Tl crystal, are slowed down to thermal energies. Thermal neutrons enter the cover 4 having in its composition in the form of a natural isotopic mixture of boron nucleus (always contains a significant percentage of isotope ¹⁰ B) or in the form of an isotope ¹⁰ C. The cores ¹⁰ provided (see. Table) high efficiency of thermal neutron capture cross section interactions σ = 767 barn for a natural mixture of isotopes (which is 10 times higher than in the well-known detector [11], where thermal neutrons were detected due to the reaction (n, α) by ⁶ Li nuclei). The reaction (n, α, γ) on ¹⁰ V nuclei proceeds in two stages. At the first stage (1), due to the reaction (n, α), ⁷ Li nuclei and an α-particle are formed, with some of the ⁷ Li nuclei being formed in the excited state [14]. The α-particle is absorbed in the cover without registration consequences (the cover is opaque and scintillation does not occur under the action of the α-particle in it). The second stage (2) of the reaction (n, α, γ) consists in the fact that the excited ⁷ Li nucleus goes into the ground state with the emission of a γ quantum with an energy of ~ 0.48 MeV [14]. This γ-quantum enters the NaI-Tl scintillator and is recorded in it due to light flashes (scintillations) produced by it. In the formula representation, the reaction (n, α, γ) has the form

and then after ~ 10 ^-13 s
(2) $\genfrac{}{}{0ex}{}{\text{7}}{\text{3}}$ Li $\genfrac{}{}{0ex}{}{\text{*}}{\text{(exc. status)}}$ _ → $\genfrac{}{}{0ex}{}{\text{7}}{\text{3}}$ Li _{(main condition)}
with the emission of a γ-ray with an energy of 0.48 MeV.

 γ-quanta have a 4π distribution (i.e., they are emitted spherically symmetrically). Such gamma rays arrive not only in the NaI-Tl crystal, but also in an external organic scintillator. However, the efficiency of their registration with an external organic scintillator due to its small atomic number is much lower than that for the NaI-Tl scintillator, and they are easily discriminated in the analysis.

Thermal neutrons present in the primary neutron flux from fissile materials entering the detector interact directly with the boron-containing material of the outer sheath 2 (they ultimately interact with ¹⁰ V nuclei). The above reaction (n, α, γ) occurs with the release of γ-quanta with an energy of 0.48 MeV. These γ-quanta are recorded mainly by the internal “uterine” inorganic scintillator and partially (less likely) by the external organic scintillator, creating light flashes in them, which are converted into electrical impulses using a photomultiplier and are analyzed using the spectrometric scheme of the electronic signal processing unit, which allows calculate the thermal neutron flux density in the primary neutron beam created by fissile materials to be detected.

 Primary γ-quanta (of the hard part of the spectrum of 0.5-3 MeV) emitted by a radiation source (DM or PB) to be detected and identified easily penetrate through the thin walls of the detector body, through the outer case and through the hydrogen-containing material of the external scintillator and are recorded mainly with the help of the NaI-Tl heavy crystal sensor, which is located in the well ("womb") of the external scintillator. Gamma rays cause light flashes in a NaI-Tl crystal with a wavelength of 410 nm and a duration of τ = 250 ns. These light flashes through an optical contact enter the photocathode of the PMT, creating electric pulses with a duration of 250 ns at its output. Primary γ-quanta, especially γ-quanta of the soft region of the spectrum of 0.4-0.5 MeV, are partially recorded by an external organic scintillator. However, the electric pulses generated by the latter, with a duration of 3-5 ns, have a several times smaller amplitude (their number is small), unlike those generated by a NaI-Tl crystal.

 Thus, when neutron and gamma radiation interacts with the sensitive elements of the sensor, several basic groups of signals of different duration and amplitude are fed to the electronic signal processing unit with a PMT. One group of signals created by fast neutrons has a short duration, not higher than 3-5 ns, and a sufficiently high amplitude, these are signals from an organic scintillator. The efficiency of detecting fast neutrons with a plastic or stilbene crystal depends on their size (the larger the size, the higher the efficiency, up to certain limits) and varies between 5-20% and higher. That is, a significant fraction of fast neutrons is not detected by an organic scintillator. However, unregistered fast neutrons are not “dropped out of the game”, they are slowed down by a hydrogen-containing substance, and are prepared for further registration by the reaction (n, α, γ) on boron nuclei. In this case, gamma quanta are mainly recorded by a NaI-Tl crystal, which forms the second group of signals with a duration of 250 ns, which is easily emitted by the spectrometric path in the “energy window” corresponding to an energy of 0.48 MeV. Such processing of the analyzed gamma-neutron field increases the efficiency of neutron detection up to 30% and higher (with a sufficiently large size plastic detector). Signals created by primary thermal neutrons from the source, after their interaction with boron nuclei in the case 2 surrounding the organic scintillator, also fall into the same group of signals, but of smaller amplitude in a slightly shifted “window” of 0.48 MeV.

 The third group of signals with a duration of 250 ns is created by an inorganic NaI-Tl scintillator that registers primary gamma radiation from sources (DM or PB). This group of signals has a wide range of amplitudes. The spectrometric mode of the electronic signal processing unit makes it possible to analyze the amplitudes of the pulses and identify the sources of γ-radiation.

 The fourth group of signals with a duration of 3-5 ns corresponds to both primary and secondary (due to the (n, α, γ) reaction) gamma rays that caused light flashes in an organic scintillator. If necessary, this group of signals is easily discriminated.

 Thus, the proposed device provides a neutron count of a wide range of energies, both fast and thermal, with increased efficiency, provides registration and spectrometric analysis of gamma rays.

 An additional advantage of the proposed device is the low cost of heat-sensitive neutron materials - boron: boron is hundreds of times cheaper than silver, cadmium or lithium-7 isotope, previously used in neutron detectors [11].

LITERATURE
1. Akimov Yu.K. Scintillation methods for detecting high-energy particles. Ed. Moscow State University, Moscow, 1963.

 2. A device for measuring neutrons and gamma rays. U.S. Patent 4,483,808, G 01 T 3/06, 1984.

 3. RF patent 2142147, G 01 T 1/20 from 09.24.1997.

 4. Selective neutron detector. U.S. Patent 3,688,118, G 01 T 1/00, 1/20, 1972.

 5. Multiple radiation detector. Application EPP (EP) 0311503, G 01 T 1/00, 1/20, 1989.

 6. Ivanov V.I. Dosimetry course. - M.: Energoatomizdat, 1988, 399 p.

 7. Plastic scintillation detector SPS-T4A Sukhumi. Leaflet of the Sukhumi Institute of Physics and Technology, 1990 (copy attached).

 8. The epithermal neutron detector. U.S. Patent 4,241,253, G 01 T 3/00, 1980.

 9. A device for measuring neutrons and gamma rays. U.S. Patent 4,482,808, G 01 T 3/06, 1984.

 10. A detector for detecting ionizing radiation. RF patent 2088952, publ. from 08.27.97, Bull. 24.

 11. Price V. Registration of nuclear radiation. IIL. M., 1960, 464 p.

 12. RF patent 2143711, G 01 T 1/20, 3/00 dated 12/27/1999.

 13. Mashkovich V.P., Kudryavtseva L.V. Protection against ionizing radiation. -M. : Energoatomizdat, 1995.494 s.

 14. Knoll G.F. Radiation Detection and Measurement. John Wiley and Sons. - N-Y. p. 483.

Claims (2)

1. A neutron and gamma radiation scintillation detector, comprising a sensor including an external neutron scintillator located in a single housing made of a hydrogen-sensitive substance based on plastic (CH) _n or stilbene sensitive to fast neutrons, and an internal gamma-sensitive NaI-Tl scintillator located in the well of an external scintillator, a photoelectronic multiplier, and an electronic signal processing unit, including a temporary selection circuit and a spectrometric analyzer of scintillation pulses, about Leach in that the sensor further comprises two cover of a boron-containing material providing the reaction (n, α, γ), wherein the first outer cover includes an organic scintillator, and the second case covers the internal container scintillator NaI-Tl located in the well and the external scintillator.  2. The detector according to claim 1, characterized in that boron nitride or carbide is used as the boron-containing material for the covers, while the thickness of the covers from the boron-containing material is selected sufficient to completely absorb thermal neutrons.

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