RU2143711C1 - Detector for registration of ionizing radiation - Google Patents

Abstract

FIELD: dosimetry of fast and thermal neutrons and gamma-radiation. SUBSTANCE: invention is designed for use in complexes and systems of radiation monitoring. Given detector is manufactured in the form three scintillators in multiple-series connection. External neutron scintillator is made from organic hydrogen-carrying substance sensitive to fast neutrons based on plastic material (CH)_n or stilbene (scintillator with well) and scintillation crystal Nal-Tl placed in well of external scintillator in standard container sensitive to gamma radiation. Internal scintillator is based on ⁶Li-soda-lime glass activated with cerium sensitive to thermal neutrons placed in one case with photoelectric multiplier. Electronic signal processing unit incorporates circuit of time selection of scintillation pulses coming from neutron-sensitive scintillators and from gamma-sensitive scintillator. Detector also includes spectrometric analyzer to process scintillation pulses coming from scintillation crystal Nal-Tl. Detector provides for counting of neutrons and for spectrometric analysis of gamma-quanta. EFFECT: higher capability of separate registration of fast and thermal neutrons. 2 cl, 1 dwg

Description

 The device relates to the field of dosimetry of fast and thermal neutrons and gamma radiation: especially to the field of dosimetry of thermal neutrons, it is suitable for use in complexes and radiation monitoring systems designed to detect fissile materials (uranium, plutonium, California and products from them), for radiation inspection of nuclear submarines to be disassembled to solve the problems of Gosatomnadzor, customs control, for dosimetric and nuclear safety services of nuclear fuel processing enterprises.

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

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 has low sensitivity (efficiency) when detecting neutrons on the gamma background, since, along with neutrons, the detector simultaneously records gamma quanta, the signals from which are difficult to separate, since they have the same spectral and temporal parameters. In addition, the known detector [1] is not suitable for spectrometry of gamma radiation.

 The known selective neutron detector according to the patent [2] 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 detecting gamma radiation in the patent [2] is not stipulated, and, in addition, for a sensor sensitive at the same time to charged particles and neutrons, the registration efficiency cannot be high.

 The well-known detector [3] of several radiations includes two scintillation sensors with green and red glows, one of which is sensitive to high-energy radiation, and the other to low-energy, and an electronic optical recording unit that emits signals from sensors using filters (green and red) and registering them using photodiodes. Such a detector has limited applications; according to [3], it is suitable for detecting x-rays with two different energies, but it is not suitable for detecting neutrons and at the same time gamma-ray spectrometry.

Known all-wave neutron detector [4], the sensor of which consists of ³ He counters sensitive to thermal neutrons, however, such a detector is not suitable for the simultaneous detection of neutrons and gamma radiation, is not suitable for spectrometry of gamma radiation.

 A known detector [5], 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 the scintillation pulse 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 not suitable for gamma-ray spectrometry.

 A known epithermal neutron detector [6], which contains a thermal neutron sensor, protection against thermal neutrons surrounding this sensor; epithermal neutron moderator, which penetrate the shield so that these 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 count rate that can be obtained when the counter completely fills the inner diameter of the thermal neutron shield. However, the known detector [6] does not allow detecting gamma radiation and, accordingly, does not allow spectrometry of gamma radiation.

 A known detector [7], similar to the detector [5], for detecting ionizing radiation according to the US patent. 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 [7] is not suitable for spectrometry of gamma radiation.

Of all the known detectors for detecting ionizing radiation, the detector described in the patent [8] is the closest to the claimed one. The known detector [8] comprises 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-scin illyatora Bi ₄ Ge ₃ O ₁₂ scintillator by fast neutrons fiber. However, the known detector [8], being sensitive to fast neutrons, is not suitable for detecting thermal neutrons. In addition, the device [8], which uses Bi ₄ Ge ₃ O ₁₂ bismuth orthogermanate as the α, β, γ scintillator, cannot provide a spectrometric mode with high energy resolution, since 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%.

The proposed device provides registration of both fast and thermal neutrons in the counting mode, as well as the 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 three parallel-in-series connected scintillators: an external neutron scintillator 1 made of fast-neutron-sensitive organic hydrogen-containing substance based on plastic (CH) _n or a stilbene (scintillator with a well) placed in it (in the well of an external scintillator) of a scintillation a NaI-Tl 2 crystal in a standard container sensitive to gamma radiation and an internal scintillator based on ⁶ cerium activated Li-silicate glass 3 sensitive to te neutrons, and a photomultiplier placed in a single housing 5, and the electronic signal processing unit 6 includes a scheme for temporal selection of scintillation pulses from neutron-sensitive scintillators (1 and 3) and from a gamma-sensitive scintillator (2), as well as a spectrometric analyzer for processing scintillation pulses from a scintillation crystal NaI-Tl.

The essence of the invention lies in the fact that an external neutron scintillator (made of a hydrogen-containing substance selectively sensitive to fast neutrons and detecting them by the light flashes created by them) is simultaneously a moderator of fast neutrons, and the thickness of the external scintillator 1 is chosen so that the neutrons are slowed down to thermal energies. Thermal neutrons are detected by an internal glass scintillator 3 containing the ⁶ Li isotope. They are recorded 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 3 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 device operates in the fields of neutron and accompanying it 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 (CH) _n ), light flashes arise 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 optical contacts and an internal glass scintillator, which in this case plays the role of a light guide (with minimal optical loss), to the photomultiplier photomultiplier tube, generating at the output its electrical pulses of 2-3 ns duration. Fast neutrons passing through an external neutron scintillator and incurring radiation losses in the form of light flashes (which is why they are recorded) lose their energy mainly not due to these light flashes, but due to collision with the nuclei of hydrogen, which is part of the scintillation material, and slow down. The thickness of the moderator is chosen so that fast neutrons before they enter the internal glass scintillator are slowed down to thermal energies. Thermal neutrons enter the internal scintillator containing a ⁶ Li radionuclide at a concentration of up to 10 ²¹ cm ^-3 , sufficient to ensure high detection efficiency as a result of the nuclear reaction ⁶ Li + n _ → α ( $\genfrac{}{}{0ex}{}{\text{4}}{\text{2}}$ He) + ³ H, flowing on ⁶ Li nuclei and having a cross section of more than 900 bar. For comparison, we point out that gas-discharge ³ He counters (reaction (n, p) with a cross section of approximately 4000 barn), which are usually used for detecting thermal neutrons, have lower overall neutron detection efficiency than ⁶ Li-glass due to the low density of ³ He nuclei in gas volume meter. Alpha particles of the reaction ⁶ Li (n, α) ³ H cause light flashes in the inner silicate glass with a radiation wavelength of 395 - 400 nm and a duration of 60 ns. These light flashes enter the photomultiplier photomultiplier through an optical contact, creating 60 ns of electrical pulses at its output. The thickness of the inner glass scintillator is chosen small: 0.8 - 1 mm to reduce the possible appearance of noise gamma scintillations in the inner glass scintillator. Their possible appearance does not, however, reduce the signal-to-noise ratio created by the internal scintillator, since these noise gamma-quanta are easily discriminated against by the electronic information processing unit.

 The gamma rays emitted by the radiation source to be detected and identified easily penetrate the hydrogen-containing material of the external scintillator and are recorded using the NaI-Tl heavy crystal contained 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 (τ = 250 ns) pass through optical contacts and an internal glass scintillator, which plays the role of a light guide for gamma scintillations, to the photomultiplier of the PMT, creating its output electric pulses of 250 ns duration.

 Thus, when neutron and gamma radiation interacts with the sensor’s sensitive elements (scintillators), three groups of signals with different durations will arrive at the electronic information processing unit with a PMT. 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 makes it possible to determine the energy spectrum of the detected gamma radiation, weak additional signals from gamma rays arising in an external hydrogen-containing scintillator with a scintillation duration of 3 ns are easily discriminated. Thus, the device provides a neutron count and spectrometric analysis of gamma rays.

 An additional advantage of the proposed device is the possibility of both separate registration of fast and thermal neutrons, and the determination of the total number of fast and thermal neutrons from the neutron sources of complex spectral composition to be detected, including the corresponding fission spectrum of fissile materials.

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

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

 3. The detector of several emissions. Application EPU (EP) N 0311503, G 01 T 1/00, 1/20, 1989.

 4. Ivanov V.I. Dosimetry course. Moscow, Energoatomizdat, 1988, 399 p.

 5. Plastic scintillation detector SPS-T4A. Sukhumi. A leaflet of the Sukhumi Institute of Physics and Technology, 1990.

 6. The epithermal neutron detector. U.S. Patent 4,242,253b G 01 T 3/00, 1980.

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

 8. A detector for detecting ionizing radiation. RF patent N 2088952. Publ. 08/27/97, Bull. N 24.

Claims (3)

1. A detector for detecting ionizing radiation, neutrons and gamma rays, comprising a sensor and an electronic information processing unit, including a temporal selection of scintillation pulses, characterized in that for detecting fast and slow neutrons against a background of simultaneously detected accompanying gamma radiation, the sensor is made in the form three parallel-series connected scintillators: an external neutron scintillator made of a hydrogen-containing substance sensitive to fast neutrons based on ve plastics (CH) _n or stilbene; a gamma-sensitive Nal-Tl scintillator located in a well of an external scintillator; an internal glass scintillator sensitive to thermal neutrons, and a photomultiplier placed in a single housing, and the electronic signal processing unit additionally includes a spectrometric analyzer of scintillation pulses coming from a Nal-Tl scintillation crystal.  2. The detector according to claim 1, characterized in that the thickness of the external scintillator from the hydrogen-containing material is selected sufficient so that fast neutrons passing through the scintillator are slowed down to thermal energies. 3. The detector according to claim 1, characterized in that cerium activated lithium silicate glass containing ⁶ Li isotope in an amount of up to 10 ²² cm ^-3 is sufficient for effective detection of thermal neutrons as an internal scintillator sensitive to thermal neutrons.