RU2730392C1 - Neutron scintillation detector - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to detecting fast and thermal neutrons. Essence of the invention is that the neutron scintillation detector comprises a scintillation sensor assembled of alternating elements composed of plates in form of a plastic scintillator for detecting fast neutrons and gamma-quanta, with longitudinal channels made on their side surfaces, in which re-emitting fibers are laid, and separate layers of glass fiber, in the form of which there is a glass scintillator for detecting thermal neutrons and gamma-quanta, wherein to each alternating element separate photodetectors are connected, and thickness of alternating elements is selected provided that average path length of neutrons and gamma-quanta is greater than total thickness of plate and glass fiber layer.

EFFECT: high gamma radiation discrimination coefficient.

1 cl, 3 dwg

Description

The invention relates to the field of detection of fast and thermal neutrons in conditions of increased γ-background and can be used in stationary and portable devices for detecting fissile and radioactive materials by their own neutron radiation.

Known scintillation detectors of fast neutrons with discrimination of gamma quanta (Stolyarova EA Neutron spectrometers and their application in applied problems. M .: Atomizdat, 1969). Detectors based on an organic stilbene crystal allow discrimination of gamma quanta by pulse shape. However, their application is limited due to the small size of the crystal and insensitivity to thermal neutrons.

Known scintillation detector of fast and thermal neutrons (RF patent No. 2259571 G01T 1/20, 3/06, publ. 27.08.2005). The detector consists of a sensor and an electronic signal processing unit. The sensor consists of two scintillators: a plastic one, sensitive to fast neutrons, and a glass one containing lithium-6, sensitive to thermal neutrons. This detector does not allow for the separation of neutrons and gamma quanta and, thus, does not allow the detection of neutron radiation sources against the background of gamma radiation.

Known detectors (RF patent RU 2129289, G01T 1/167; publ. 20.04.1999), in which large-area plastic scintillators are used to register gamma radiation, and helium discharge neutron detectors are used to register neutrons. The disadvantage of this detector is the lack of separate registration of neutrons and gamma quanta.

Known scintillation neutron detector based on plastic and ⁶ Li-silicate glass, containing a sensor-scintiblock and an electronic signal processing unit, described in RF patent No. 2143711, IPC G01T 1/20, publ. 12/29/1999 under the name "Detector for registration of ionizing radiation."

The disadvantages of this technical solution is the impossibility of ensuring effective registration of both fast and thermal neutrons using compact photodiode devices or multichannel photomultiplier tubes, since the detector is designed to operate in combination with conventional single-window ones, i.e. single-channel photomultiplier tubes as photodetectors.

The closest and selected as a prototype is the detector described in RF patent No. 2300782, IPC G01T 1/20, publ. June 10, 2007 under the name "Scintillation neutron detector". Scintillation neutron detector containing a scintiblock sensor, which includes a plastic scintillator for recording fast neutrons and gamma quanta, with a reflective coating and longitudinal channels along the outer side surface, and a glass scintillator based on cerium ⁶ activated Li-silicate glass for detecting thermal neutrons and gamma quanta, made in the form of glass fibers, a photodetector in the form of a photodiode recorder or photomultiplier, and an electronic signal processing unit.

The disadvantages of the known scintillation detector include the inability to separate neutron and gamma radiation with the aim of subsequent discrimination of gamma radiation.

The objective of the claimed invention is to improve the operational capabilities of the scintillation neutron detector by increasing the sensitivity to neutron radiation and increasing the detection efficiency of both fast, thermal neutrons and gamma radiation, while ensuring a high discrimination factor of the accompanying gamma radiation (K _γ >> 10 ³ ).

The technical result, which makes it possible to solve the problem posed, is to ensure a high discrimination coefficient of gamma radiation due to the time discrimination of correlated signals from gamma quanta in the detector layers in addition to the usually used amplitude discrimination while maintaining the efficiency of registration of neutron radiation, due to the use of amplitude-time separation of signals from neutrons and gamma quanta while maintaining high efficiency of registration of neutron radiation.

This is achieved by the fact that in a scintillation neutron detector containing a scintiblock sensor, including a plastic scintillator for registering fast neutrons and gamma quanta, with a reflective coating and longitudinal channels on the outer side surface, a glass scintillator based on cerium ⁶ -activated Li-silicate glass for registration of thermal neutrons and gamma quanta, made in the form of glass fibers, a photodetector in the form of a photodiode recorder or a photomultiplier and an electronic signal processing unit according to the invention, a scintiblock sensor is assembled from alternating elements made up of plates, in the form of which a plastic scintillator is made for registration fast neutrons and gamma-quanta, with longitudinal channels made on their lateral surfaces, in which re-emitting fibers are laid, and separate layers of glass fiber, in the form of which a glass scintillator is made to register thermal neutrons and gamma-quanta, while m, separate photodetectors are connected to each alternating element, and the thickness of the alternating elements is selected on the condition that the average path length of neutrons and gamma quanta is greater than the total thickness of the plate and fiberglass layer.

An analysis of the state of the art, carried out by the applicant, including a search for patent and scientific and technical sources of information and the identification of sources containing information about analogues of the claimed invention, made it possible to establish that the applicant did not find an analogue characterized by features identical to all essential features of the claimed invention, but the definition from the list identified analogs of the prototype, as the closest analogue in terms of a set of features, made it possible to identify a set of significant in relation to the technical result perceived by the applicant of the distinctive features in the claimed object set forth in the claims.

Consequently, the claimed invention meets the requirement of "novelty" under the current legislation.

To check the compliance of the claimed invention with the condition of the inventive step, the applicant conducted an additional search for known solutions in order to identify features that match the distinctive features of the prototype of the claimed invention, the results of which show that the claimed invention does not follow explicitly for a specialist from the prior art.

Therefore, the claimed invention complies with the “inventive step” requirement.

The invention is illustrated by the following drawings.

FIG. 1 is a schematic diagram of a scintillation neutron detector.

FIG. 2 shows the layout of the elements in the scintillation neutron detector.

FIG. 3 shows a diagram of the registration of signals from a scintillation neutron detector by a device with amplitude-time discrimination of γ-radiation.

The following symbols have been introduced in the drawings:

1 - plates of a plastic scintillator C _i , where i = 1, 2 ... i + 1 (CH _1.1 , ρ = 1.06 g / cm ³ ) (C _i- plates 1);

2 - layers of the glass scintillator B _i , where i = 1, 2 ... i (Si _1.0 O _2.58 ⁶ Li _0.363 ⁷ Li _0.027. Ρ = 2.58 g / cm ³ (B _i - glass fiber layers 2) from scintillating glass fiber activated by an isotope ⁶ Li;

3 - re-emitting fiber;

4 - longitudinal channels in the plastic scintillator plates, in which the re-emitting fiber is laid;

5 - a line of separate photodetectors, to which scintillating fiberglass from each layer is connected;

6 - a line of individual photodetectors to which re-emitting fiber from each C-layer is connected.

For light insulation of C- and B-layers 1 and 2, all plates of the plastic scintillator in the scintiblock are covered with a thin film of reflective material.

The scintillation neutron detector contains a scintillation sensor made (Fig. 1) of alternating C _i , where i = 1, 2 ... i + 1 plates 1 (C _i- plates 1), in the form of which a plastic scintillator is made for registering fast neutrons and gamma quanta (СН _1.1 , ρ = 1.06 g / cm ³ ) and layers 2 of scintillating glass fiber B _i , where i = 1, 2 ... i (B _i layers 2 of glass fibers), activated by the ⁶ Li isotope (Si _1.0 O _2.58 ⁶ Li _0.363 ⁷ Li _0.027. Ρ = 2.58 g / cm ³ ), in the form of which a glass scintillator is made to register thermal neutrons and gamma quanta. In the longitudinal channels 4 C _i plates 1 (Fig. 2) of the plastic scintillator, made on their outer and inner side surfaces, re-emitting fibers 3 are laid (Fig. 2), on one side all re-emitting fibers 3 are collected in a bundle and their ends are connected to separate photodetectors 6 (Fig. 1). In the scintiblock sensor, each layer is connected to a separate photodetector 5 or 6 (Fig. 1). The scintillation neutron detector contains an electronic signal processing unit (Fig. 3). To implement the time dependence between the registration of fast neutrons and gamma-quanta in C _i- plates 1 and B _i- layers 2 of glass fiber of the scintiblock sensor, it is necessary that the thickness of alternating elements: plates 1 and B _i- layers 2 of glass fiber, be chosen in such a way, so that the average path length of neutrons and gamma quanta is greater than or equal to the total thickness of C _i plates 1 and B _i layers 2 of glass fiber, i.e. the condition R _e (R _n ) ≥ (Δ _B + Δ _C ) must be satisfied, where R _e is the average path length of fast charged particles formed during the interaction of the detected gamma radiation with C _i- plates 1, R _n is the average path length of a neutron , and Δ _C , Δ _B are the thicknesses of C _i -plates 1 and B _i- layers 2 of glass fiber, respectively.

The device works as follows. Neutrons and gamma quanta fall on the frontal surface of the scintillation neutron detector and, interacting with the substance of its working C _i plates 1 and B _i layers 2 of glass fiber, create a flux of fast protons, electrons (positrons), the energy E of which is converted in the detecting elements into light scintillations recorded by photodetectors 5 and 6 from each element of the scintiblock sensor (Fig. 3). In this case, the registration of neutrons and gamma quanta in B _i layers 2 of glass fiber is a secondary process with respect to the process of their primary interaction in C _i plates 1, i.e. each act of registration of a gamma quantum or neutron in B _i -layers 2 of glass fiber with a high degree of probability is preceded by an act of their registration in C _i- plates 1.

When registering gamma quanta, the signal in the B _i layers 2 of glass fiber is formed due to the registration of fast electrons (positrons) formed during the interaction of gamma quanta with the substance of the previous C _i plates 1.

When registering neutrons, the signal in B _i layers 2 of glass fiber arises as a result of the exothermic reaction ⁶ Li (n, α) T with an energy release of 4.8 MeV on thermal neutrons, which are formed during their preliminary deceleration due to their elastic collisions with hydrogen nuclei in C _i- plates 1, which leads to the appearance of recoil protons in C _i- plates 1 and the formation of a signal.

According to the calculations, the maximum absorbed energy of electrons in B _i -layers 2 of the glass fiber of the scintillation neutron detector in a wide range of energies of gamma quanta of the source E _γ ≈ (1 ÷ 10) MeV does not exceed the value of ε _m ~ 1.5 MeV, which is significantly less than the energy charged particles of the exothermic reaction ⁶ Li (n, α) T, which determine the registration of neutrons in these B _i layers 2 of glass fibers after their slowing down and thermalization in the C _i plates 1 of a plastic scintillator for registering fast neutrons and gamma quanta. The choice of the appropriate threshold for the registration of signals in the B _i -layers 2 of glass fiber allows you to realize the amplitude discrimination of gamma radiation and achieve the discrimination coefficient recorded in the B _i -layers 2 of the glass fiber gamma quanta, K _{n / γ} ~ 10 ³ (Fig. 3).

Thus, thermal neutrons are detected by B _i -glass fiber layers 2 with gamma-ray discrimination by choosing an amplitude discrimination threshold (Fig. 3). When registering neutrons, the amplitude discriminator from B _i -layers 2 of glass fiber transmits signals with an amplitude exceeding the threshold value A ₀ , corresponding to the registration of neutrons in it, and does not pass signals, with amplitudes A <A ₀ , corresponding to the registration of gamma quanta, (amplitude step n / γ - discrimination K _{n / γ} ~ 10 ³ ).

When registering fast neutrons, correlated signals appear in any of the C _i- plates 1 and B _i -layer 2 of the glass fiber of the scintiblock sensor with a time interval between them up to 200 μs, and during the correlated registration of gamma quanta signals appear in the adjacent C _i- plate 1 and B _{i -} layer 2 of glass fiber of the scintiblock sensor with a time interval of 0.2 μs between them. In the proposed device, this property is used to discriminate the correlated registration of gamma quanta in the time window Δτ _lz ~ 0.2 μs by eliminating the correlated signals by the anti-coincidence circuit. The anti-coincidence circuit is triggered from the signals of B _i -layers 2 of glass fiber coming from the output of the amplitude discriminator (Fig. 3).

Time discrimination of gamma quanta is carried out by the anti-coincidence circuit (Fig. 3) by registering the absence of the fact of coincidence in the time window with a duration of Δτ _CAC ~ 0.2 μs of the signal from the amplitude discriminator (Fig. 3) and the signal from C _i -plate 1, delayed by the delay line for a time Δτ _LZ ~ 0.2 μs (Fig. 3). The choice of the duration of the time window of the anti-co-ownership scheme, which is two orders of magnitude shorter than the time interval between the correlated signals from the C _i plate 1 and the B _i layer 2 of the glass fiber, arising during the registration of a fast neutron, makes it almost impossible for the anticoincidence scheme to make false exclusions of signals from fast neutrons recorded these layers (Fig. 3).

It follows that the use of amplitude and time discrimination of signals corresponding to correlated and uncorrelated registration of gamma quanta in the B-channel of the detector (Fig. 3), when conditions (1, 2) are met, will increase the coefficient of their discrimination to the value К _γ >> 10 ³ without a significant decrease in the efficiency of registration of neutrons in it in a wide energy range (from thermal to fast neutrons).

The enterprise has created an experimental sample of a scintillation neutron detector containing a scintiblock sensor assembled from 10 C _i plates 1 of a plastic scintillator. On each side of the C _i -plate 1, 6 longitudinal grooves 4 are made with a depth of 2.5 mm and a width of 1.1 mm. In the longitudinal grooves 4, re-emitting fiber 3 from the Kuraray brand Y11 (200) with a fiber diameter of 1 mm is placed. A bundle of re-emitting fibers 3 emerging from each C _i -plate 1 is connected to a separate PMT. Each re-emitting fiber 3 is laid in two longitudinal grooves 4 with the formation of a loop at one end of the C _i -plate 1. At the other end of the C _i -plate 1, the ends of the re-emitting fiber 3 are collected in a bundle and connected to a photodetector 6. A bundle of re-emitting fibers 3 emerging from each C _i -plate 1 is connected to a separate photodetector. Between the C _i plates 1 of the plastic scintillator, a neutron-sensitive scintillation fiber made of glass doped with lithium and activated with cerium is laid in one B _i -layer 2 of glass fiber. The main components of such glass are (in atomic%): silicon (25.21), oxygen (64.96) and lithium enriched with the ⁶ Li isotope ( ⁶ Li-9.14; ⁷ Li-0.69). The scintillation fiber diameter is 150 µm. Fiberglass B _i -layer 2 is laid with the formation of a loop at one end of C _i -plate 1. At the other end of C _i -plate 1 ends of glass fibers B _i -layer 2 are bundled and connected to a photodetector 5. Each B _i -layer 2 of glass fibers connected to a separate photodetector 5. FEU85 and multipixel avalanche photodiodes 6 made by SensL 60035 are used as photodetectors 5. The thickness of each C _i -plate 1 is 5 mm, the thickness of each B _i- layer 2 is 0.15 mm, which is less than the average path length charged particles and neutrons in the detector material.

Thus, the above information testifies to the fulfillment of the following set of conditions when using the invention: in a scintillation neutron detector intended for use in systems for detecting fissile and nuclear materials and measuring neutron fluxes, it has improved operational characteristics and capabilities for conducting neutron measurements compared to the prototype, which is expressed in the achievement of a higher level of discrimination of background gamma quanta due to the use of additional time discrimination of the corresponding correlated signals in the layers of the detector while maintaining a high efficiency of registration of neutron radiation.

For the claimed invention in the form as it is described in the claims, the possibility of its implementation using the above-described design solutions was confirmed, namely, a neutron scintillation detector with improved characteristics was obtained.

Therefore, the claimed invention meets the condition of "industrial applicability".

Claims (1)

Scintillation neutron detector containing a scintiblock sensor, which includes a plastic scintillator for recording fast neutrons and gamma quanta, with a reflective coating and longitudinal channels along the outer side surface, and a glass scintillator based on cerium ⁶ activated Li-silicate glass for detecting thermal neutrons and gamma quanta, made in the form of glass fibers, a photodetector in the form of a photodiode recorder or photomultiplier, and an electronic signal processing unit, characterized in that the scintiblock sensor is assembled from alternating elements made up of plates, in the form of which a plastic scintillator is made to register fast neutrons and gamma quanta, with longitudinal channels made on their lateral surfaces, in which the re-emitting fibers are laid, and separate layers of glass fiber, in the form of which a glass scintillator is made for recording thermal neutrons and gamma quanta, with each alternating separate photodetectors are connected to the element, and the thickness of the alternating elements is chosen on the condition that the average path length of neutrons and gamma quanta is greater than the total thickness of the plate and the layer of glass fiber.

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