RU2408902C1 - Two-dimensional detector - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: in a two-dimensional detector, between layers of mutually perpendicular polymer scintillating fibres made from spectrum-shifting material, there is a matrix converter screen with optical contact, where the said converter screen is in form of a cellular structure with light-reflecting walls and channels filled with transparent scintillator. Each fibre is connected to a photodetector directly or using optical fibre.

EFFECT: high efficiency of detecting ionising radiations of different types, wide energy range of detected radiations, high sensitivity of the surface of the detector.

2 cl, 2 dwg

Description

The invention relates to the field of registration of ionizing radiation and can be used for non-destructive testing of materials, products and objects by radiographic methods, as well as for the detection and identification of hazardous materials by both active and passive methods.

Known two-coordinate detectors containing a fiber screen converter, optical lenses, electron-optical amplifiers, a CCD matrix and microprocessor devices for recording radiographic images. The optical image arising under the action of ionizing radiation in the screen transducer is transferred using the lens to the image amplifier, and then using another lens to the CCD, where it is digitally recorded using a microprocessor device, then transferred to a computer for display, further processing and use. Mikerov V., Bogolubov E., Samosyuk V., Verushkin S., "Fast Neutron Imaging with CCD Detectors and Imaging Plates", International Workshop on Fast Neutron Detectors, University of Cape Town, South Africa, April 3-6, 2006, http://pos.sissa.it; MOR I., VARTSKY D., BAR D., et al., "High Spatial Resolution Fast-Neutron Imaging Detectors for Pulsed Fast-Neutron Transmission Spectroscopy", 2009 JINST 4 P05016.

These devices have limitations on the size of the sensitive surface, due to the fact that the optical image that appears in the screen-converter is transferred to the input window of the image amplifier, the size of which is much smaller than the screen size. Light losses that occur during image transfer are approximately inversely proportional to the square of the image reduction ratio. This leads to the fact that the efficiency of the detector depends on the size of the screen transducer. When radiographic images are obtained, for example, using fast neutrons, the effective area of the detector with a plastic scintillator currently does not exceed 20 × 20 cm. In addition, the CCD arrays used for recording the optical image have a low speed and usually operate in a storage mode. The increase in the effective area of the detector due to the combination of several transformer screens in one design leads to a corresponding increase in the number of optical, electron-optical and electronic devices and a corresponding increase in the cost of the entire device. The number of screens and other components of the detector quadratically depends on the sensitive area of the entire device. Another disadvantage of scintillation CCD detectors is the impossibility of removing its electron-optical and electronic devices at a safe distance from the radiation beam and, as a result, shortening their service life.

Known converter of ionizing radiation made of flat elements, and at least one of the elements is made in the form of a layer of powder phosphor. The converter contains spectroscopic elements in the form of tapes and optical fiber, at the ends of which photodetectors are installed, a layer of powder phosphor is deposited on the surface of the tape or introduced into its composition. Utility Model Patent of the Russian Federation No. 54438, IPC G01T 3/06, 2006

The disadvantages of the analogue are the low production efficiency of recoil protons and, as a consequence, the low conversion efficiency due to the fact that part of the volume of the converter is occupied by an inorganic powder phosphor.

Known neutron detector containing a fiber module, assembled from layers of polymer scintillating optical fibers, stacked alternately in two mutually perpendicular directions, and an electron-optical system for recording optical radiation emerging from the ends of these fibers, the electron-optical system contains photodetectors. US patent No. 4942302, IPC G01T 3/06, 1990. This device has low efficiency due to the small range of recoil protons in polymer scintillating fibers; it is intended for detecting only fast neutrons, the minimum energy of which is determined by the cross-section of the fiber, has limitations on the size of the sensitive surface, due to the need to interface the end surface of the device with photodetectors and the resulting loss of light.

Known neutron detector containing a fiber module, assembled from layers of polymer scintillating optical fibers, stacked alternately in two mutually perpendicular directions, and an electron-optical system for recording optical radiation emerging from the ends of these fibers. The ends of the fibers are located in the planes of the faces of the fiber parallelepiped formed by the layers of fibers, and the electron-optical system is made in the form of position-sensitive photodetectors that are optically paired with the corresponding faces of the fiber parallelepiped. The diameter of the fibers is equal to half the mean free path of the recoil proton in the fiber material. The electron-optical system contains local subsystems into which translucent plates are introduced for branching optical power to high-speed receivers.

Patent of the Russian Federation No. 2119178, IPC G01T 3/06, 1998. Prototype.

The prototype has low efficiency, because It does not provide two-coordinate registration of recoil protons with a range comparable to that of a single fiber; it is designed to detect only fast neutrons, the minimum energy of which is determined by the fiber cross-section. Typically, the fiber cross-section is about 1 mm, and polypropylene or polystyrene is used as the material. When using polystyrene fiber, only neutrons with an energy of more than 12 MeV begin to knock recoil protons with a range of more than 1 mm. When using electron-optical systems, there are restrictions on the size of the sensitive surface of the detector, due to the need to pair the faces of the fiber parallelepiped with the sensitive surface of the electron-optical system and the resulting light loss.

This invention eliminates the disadvantages of analogues and prototype.

The technical result of the invention is to increase the efficiency of registration of ionizing radiation: thermal and fast neutrons, x-rays and gamma rays, various types of charged particles; expanding the energy range of the recorded radiation, increasing the sensitive surface of the detector, reducing the cost of the device.

The technical result is achieved by the fact that in a two-coordinate detector containing a module of layers of polymer scintillating optical fibers arranged alternately in two mutually perpendicular directions, and an electron-optical system for recording optical radiation, between the layers of mutually perpendicular polymer scintillating fibers from a spectroscopic material is installed with an optical contact matrix screen-converter in the form of a honeycomb structure with light-reflecting walls and channels filled with light scintillator, and each fiber is connected to a photodetector.

Each fiber is connected to the photodetector using optical fiber.

The invention is illustrated in figures 1 and 2.

Figure 1 schematically shows a General view of the device, where 1 is a matrix screen converter, 2 is a spectroscopic fiber, 3 is a scintillation flash that has arisen in one of the cells of the matrix screen converter, 4 is the light output to photodetector devices.

Figure 2 shows a variant of the detector using photodiodes as a photodetector included in the coincidence circuit in order to reduce the influence of their own electronic noise, where 1 is a matrix screen converter (view from the source side), 2 are spectroscopic fibers, 5 are photodetectors ( photodiodes), 6 - analog amplifiers, 7 - discriminators, 8 - matching schemes.

The matrix screen converter 1 is a honeycomb structure with light reflecting walls and channels filled with a scintillator transparent to light. The scintillator material is determined by the type of radiation detected. To detect fast neutrons, plastic scintillators are used, for example, from polystyrene or polypropylene; for thermal neutrons - plastic scintillators with boron-10 additives or powder scintillators Li ⁶ F + ZnS: Ag and B ¹⁰ + ZnS: Ag. For registration of charged particles: protons, electrons, positrons, muons, as well as x-ray and gamma radiation, in addition to organic scintillators, transparent scintillators from inorganic materials can also be used, for example, from NaI: Tl, CsI: Tl, LaBr3: Ce LSO: Ce LaCl3: Ce, YAG: Ce, YAP: Ce, BGO, etc.

One example of a matrix transducer screen is a fiber optic scintillator. Compound scintillators can be used to effectively register various types of radiation.

Spectroscopic fiber 2 is a plastic scintillator with a diameter of fractions of a millimeter or more, coated on the outside with a single or double layer sheath of materials with a lower refractive index of light, usually polymethyl methacrylate. It serves to capture the photons that came to it from the outside, reradiate them and transport the reradiated photons to photodetectors 5 located at one end or both ends of the fiber.

Spectroscopic fibers 2 are either directly or by means of optical fiber brought into optical contact with photodetectors 5. To register photons emerging from spectroscopic fibers 2, two-axis PMTs or photodiodes are used.

The device operates as follows.

The radiation enters the transducer matrix screen 1 and causes a scintillation flash 3 in one or several of its cells. Light from the scintillation flash 3 propagates through the cell (s) in both directions to the ends of the transducer matrix screen 1, where it hits one or several spectroscopic fibers 2. In each of the spectroscopic fibers 2, which received light from a scintillation flash 3, the initial light is first absorbed (with a probability of more than 80%), and then reradiated in the corresponding range no wavelengths. Partially re-emitted light (about 5% of the total number of photons arising) reaches the ends of the fibers 4 and then to the photodetector 5, where it is recorded.

The X and Y coordinates of the cell of the matrix screen in which the scintillation flash 3 occurred is determined by the numbers of PMT cells or photodiodes from which the signal arrived almost simultaneously and which are in optical contact with mutually perpendicular spectroscopic fibers located on opposite sides of the transducer matrix screen 1. The degree of simultaneity is determined by the length of the spectroscopic fibers 2 and the speed of light propagation in the spectroscopic fibers 2. Thus, the selection of useful events carried out both in time and in belonging to opposite groups of spectroscopic fibers 2. The number of 5 photons reaching the photodetector is about 2 × 10 ^-3 of the total number of photons emitted during the scintillation flash 3. When using the photodetector 5 only at one end of the spectroscopic fiber 2 the opposite end of the spectroscopic fiber 2 can be coated with a reflective material. This almost doubles the number of photons incident on the given photodetector 5. The number of photons reaching the photodetector, unlike the prototype, does not depend on the area (volume) of the detector.

The number of photodetectors (in the case of PMTs is the total number of its cells) is proportional to the perimeter of the matrix screen converter 1, and not its area or the area of the end face, as in the case of the prototype. With a sensitive detector area of 1 m ² and a spectroscopic fiber cross section of 1 mm, only two two-coordinate photomultipliers from Photonis Planacon model 85011-011 are required, each of which contains 1024 photosensitive cells.

In the device of FIG. 2, photons from a scintillation flash 3, which has arisen in one of the cells of the matrix screen of the transducer 1, immediately fall into two adjacent spectroscopic fibers 2, where they are reradiated and transported to the photodiodes 5. The signal from the output of one of the photodiodes 5, which came first, after amplification by an analog amplifier 6 and discrimination by discriminator 7, it enters the input of coincidence circuit 8 and opens a time window. The arrival of a signal from the second photodiode 5 during this window causes a signal to appear at the output of the coincidence circuit 8.

Only the operation of two devices at once in a certain time interval leads to the registration of a scintillation flash 3. At the same time, the probability of false positives from a signal caused by the intrinsic electronic noise of photodiodes 5 is reduced.

The efficiency of the detector is determined by the thickness of the matrix screen converter 1 and, in addition to other physical restrictions common to all detectors, is limited only by the length of the light attenuation in its scintillator (for polymer scintillators it is several meters). The light collection efficiency is practically independent of the detector size up to the size determined by the attenuation length of light in a spectroscopic fiber, which is usually not less than 2 m. Calculations show that in the case of a matrix screen converter 1 made of polypropylene, the detection efficiency of fast neutrons with an energy of 2 MeV can be 25% and grows with increasing energy, for example, up to 55% for neutrons with an energy of 8 MeV.

Using fiber optics, the photodetector devices 5 are removed from the radiation beam and, thus, provide normal conditions for the operation of the electronic devices of the detector. In the case of registration of radiation at the level of the natural background, the photodetector 5 is advisable to cool in order to increase their sensitivity (reduce intrinsic electronic noise).

Claims (2)

1. A two-coordinate detector containing a module of layers of polymer scintillating optical fibers, stacked alternately in two mutually perpendicular directions, and an electron-optical system for recording optical radiation, characterized in that between the layers of mutually perpendicular polymer scintillating fibers from a spectroscopic material, a matrix is installed with an optical contact screen-transformer in the form of a honeycomb structure with light-reflecting walls and channels filled with light-transparent sci scintillators and fiber connected to each photodetector. 2. The two-coordinate detector according to claim 1, characterized in that each fiber is connected to a photodetector using optical fiber.

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