RU2207550C2 - Method of neutron radiography with use of fast neutrons, shield for neutron radiography, shield for neutron and x- ray radiography - Google Patents

Abstract

FIELD: study of materials by radiation methods. SUBSTANCE: in compliance with method point source of fast neutrons is employed as radiation source. Optical image is stored in each luminescent plate of set comprising shield. Then images are read separately during short time hundred times lesser than recording process and fed into photodetector. Shield for neutron radiography ( main shield ) is made up of set of alternating plates produced from storing energy-accumulating luminophor and water-carrying substance ( converter ). Shield for neutron radiography and X-ray radiography has additional set of plates fabricated from storing energy-accumulating luminophor and placed between source of fast neutron radiation and main shield, lead shield is mounted between main shield and additional set of plates. EFFECT: enhanced efficiency of utilization of fast neutrons, shortened exposure time, diminished effect of background signal, improved quality of generated images, raised productivity of process, enlarged life time of neutron generator and generation of additional image in X-ray radiation accompanying neutron radiation. 7 cl, 2 dwg

Description

 The invention relates to the study or analysis of objects by radiation methods, for example, using neutron or x-ray radiation.

 A known method of neutron radiography using fast neutrons, which is implemented in a neutron converter (see, for example, ed. St. USSR 699914, class G 01 N 23/09, 1978), containing a source of fast neutrons, a detector, a two-layer reflective screen made of hydrogen-free material.

 For the prototype, a method of neutron radiography using fast neutrons was chosen (see, for example, K.K. Schwartz, Z.A. Grant, T.K. Mehs, M.M. Grube "Thermoluminescent dosimetry", ed. "3inatne", Riga, 1968), based on the conversion of fast neutron radiation into optical radiation, in which the spatial distribution of the intensity of fast neutron radiation is judged by the distribution of its brightness.

 In the known method of radiography, fast neutrons emitted by the generator form recoil protons in a luminescent screen, which excite an optical glow of the screen, the spatial distribution of the brightness of which reflects the spatial distribution of the intensity of fast neutron radiation. Next, the optical radiation is transmitted to the input of the image amplifier using a rotary mirror and an input lens. The amplified image from the output of the image amplifier through the output lens is recorded on a CCD matrix (charge-coupled device). In this case, during the exposure of the luminescent screen in a neutron beam in the matrix, the useful electron charge due to the neutron beam is accumulated, as well as the intrinsic thermal noise of the matrix and signal are accumulated.

 The known method is characterized by a low efficiency of using generated neutrons (<1%), a significant effect of the intrinsic noise of the CCD matrix and background radiation, which reduces the quality of the radiographic image.

Known luminescent screens for neutron radiography (see, for example, Mikerov V., Waschkowski W., "A Two-dimesional detector for fast neutron fields imaging // Proceedings of the V-th International Seminar on Interaction of Neutrons with Nuclei, pp 269 -275, Dubna, May 14-17, 1997, and Mikerov V., Waschkowski W., "a Two-dimesional detector for fast neutron fields imaging // Proceedings of the 3 ^{rd International Topical Meeting on neutron Radiography (} Lucerne, Swizerland , March 16-19, 1998), which consist of a polymer matrix and a powder phosphor embedded in it.

 A screen was selected for the prototype (see, for example, J. Rant, M. Balasko, E. Kristof, A. Simonits and J. Stade "Fast Neutron Radiography Using Photoluminescent Imaging Plates" (Abstracts of the VI-th International Conference on Neutron Radiography 129, Osaka, Japan, May 17-21, 1999) containing photoluminescent plates with an energy storage memory phosphor in combination with metal activation foils, which allows fast neutrons to be detected in a wide range of primary neutron energies.

 However, the use of metal activation foils is accompanied by low (more than 100 times) efficiency in comparison with a hydrogen-containing neutrographic converter.

 The present invention is aimed at increasing the efficiency of using fast neutrons, reducing the exposure time, reducing the influence of the background signal, improving the quality of the obtained images, as well as increasing the productivity of the process and the resource of the neutron generator and obtaining an additional image in x-ray radiation associated with neutron radiation.

 This problem is solved by the fact that in the method of neutron radiography using fast neutrons, based on the conversion of fast neutron radiation into optical radiation, in which the spatial distribution of the intensity of fast neutron radiation is judged by the distribution of its brightness, a point source of fast neutrons is used as a radiation source, and the optical image is stored in each of the set of luminescent plates of the screen, and then separately read out for a short (hundreds of times smaller memory process) time into the photodetector, while in the screen for neutron radiography, which is a luminescent screen, the luminescent screen consists of an alternating set of plates made of powder phosphor, between which there are additionally inserted plates of a hydrogen-containing substance, for example, polyethylene or polypropylene, whose thickness is selected from the condition of the required spatial distribution of emitted fast neutrons, and the total thickness of the screen is determined from the conditions I have a small influence of secondarily scattered neutrons, in addition, the screen for neutron radiography can consist of a set of plates, each of which is a mixture of a hydrogen-containing substance and an energy-storage phosphor, the volume of the phosphor in the mixture being determined by the neutron spectrum and for neutrons of the order of 14 MeV is (0 , 1-0.15)% of the total mixture volume.

 The screen for neutron radiography can be made of a set of plates, each of which is a mixture of a hydrogen-containing substance, a memory energy-storage phosphor and an additional powder phosphor emitting into the absorption band of a memory energy-storage phosphor.

 A screen for neutron and x-ray radiography, comprising a screen for neutron radiography, further comprises a lead screen and an additional set of plates from the energy storage phosphor located between the fast neutron radiation source and the main screen, and a lead screen is installed between the main screen and the additional set of plates.

At the same time, a material with a general stoichiometric formula is used as a energy storage phosphor
Ba _1-x Sr _x Ga ₂ O ₄ E _U ⁺² Dy ³ ,
where x = 0.001 - 0.1, and the content of E _U ⁺² = Dy ³ = 1%, and the material of the composition is used as the initiating powder phosphor
ZnS • AgTmS ₂ ,
in which the content of the activating compound AgTmS ₂ is from 0.005 to 0.01 wt.%.

 Using a combination of a hydrogen-containing substance with an energy-storage memory phosphor made it possible to separate the process of converting a neutron image into an optical one and reading it into a photodetector.

 A set of such combined screens with a total thickness, the value of which is determined by the condition of a small influence of secondarily scattered neutrons and amounts to 10-30 mm, eliminates the effect of secondarily scattered neutrons on image quality.

 Another feature of the invention is the introduction of an additional set of plates and a lead screen, which allows you to additionally obtain an image in x-ray radiation associated with neutron radiation.

 In FIG. 1 is a functional diagram of a screen for neutron radiography; FIG. 2 - screens for neutron radiography and radiography. The screen for neutron radiography (see Fig. 1, 2) consists of a set of 1 alternating plates made of energy storage phosphor 2 and a hydrogen-containing substance (converter) 3, the figure also shows a point source 4 of neutron radiation, for example, a neutron generator, and object of study 5.

 The screen for neutron radiography and radiography (see Fig. 2) contains an additional set of plates 6 from a storage energy-storage phosphor and a lead screen 7.

 The essence of the proposed method of neutron radiography using fast neutrons is that fast neutrons emitted by the neutron generator 3 form 3 proton protons in hydrogen-containing plates (converters) that excite phosphorus in the plate 2 from the energy storage phosphor. Further, this excitation, transferred to the potential energy of electrons and holes, is stored in a energy-storage phosphor.

 To develop the image, the phosphor plates are heated to 350-370K or irradiated with a laser with λ = 633 nm.

 Thus, the image acquisition process in the proposed screen consists of the process of image accumulation in alternating plates from a storage or energy-accumulating phosphor and its subsequent reading in a photodetector or other device of the same purpose, for example, an inkjet laser reader.

 Since in the process of image accumulation the reader is turned off or is in another room, the background signal does not accumulate, which is a significant advantage of the invention.

 The reading process takes place in a very short time, hundreds of times smaller than the accumulation process, of the order of 1 min. For one of the phosphor plates, depending on the area of the plate, the neutron generator is in the off state, as a result of which the background signal does not accumulate in the plates due to fast neutrons, and there is practically no background signal at the output after processing caused by activation of the surrounding space, nodes detector and inherent noise of the reader.

 The screen for neutron radiography works as follows.

 To obtain the image, the well-known radiographic transfer method is used, in which the converter plate 3, located in a neutron field, is activated by fast neutrons. After irradiation, this plate is brought into contact with a plate 2 made of a material in which an optical image is formed under the action of radiation emerging from the converter 3.

 By this principle, a screen is constructed consisting of a set of plates. At the same time, in plates 3 of a hydrogen-containing substance placed in a neutron field created by a point source 4 of neutron radiation, recoil protons are formed, which excite the phosphor grains in plates 2 made of powder phosphor.

 By a neutron radiation point source 4 is meant a source whose radiating surface is much smaller than the distance from the neutron radiation source to the irradiated object 5 and is equal to about 0.1 mm for a plasma focus type source, while the distance to the irradiated object is about 10 cm.

 The visible image may accumulate in latent form in the plates 2 of the energy storage phosphor as a result of excitation of the storage phosphor by recoil protons arising in the hydrogen-containing substance.

 A screen for neutron radiography can be made of a set of plates, each of which is a mixture of a hydrogen-containing substance that initiates a powder phosphor, for example, blue color emission and a storage phosphor, while the volume of the phosphor in the mixture is determined by the neutron spectrum. In this case, the image in the energy-storage phosphor is accumulated as a result of its excitation by both recoil protons and blue quanta emitted by the initiating powder phosphor under the influence of their recoil protons.

 The physical principle of operation of such a screen does not change.

The functionality of the proposed screen can be expanded by introducing an additional set of 6 plates of the storage phosphor 2, located between the radiation source of fast neutrons 4 and the main set 1 of the plates of the storage phosphor and a hydrogen-containing substance. An absorbing lead screen 7, mounted between the main 1 and additional 6 sets of plates, protects the main set 1 of the plates from x-ray radiation, while an additional set 6 of the plates of the storage phosphor can be obtained x-ray image, and on the main - pure neutron diffraction, because weaker X-ray radiation accompanying neutron radiation does not pass through an absorbing lead screen at a lead plate thickness d _pb > d _lum ≈ 10 mm.

 The image is developed after the image is accumulated in the plates with a special device, for example, a photo-reading device.

 Using a set of combined screens can increase the efficiency of neutron use, since the image will accumulate in each of the screens almost independently due to the small total attenuation of the fast neutron beam. Moreover, each of the screens provides a high spatial resolution, determined by its thickness, the size of the neutron-emitting region and the distance from the screen to the generator.

 Computer processing of images of each of the screens allows you to reduce all these images to a single image.

Claims (7)

 1. The method of neutron radiography using fast neutrons, based on the conversion of fast neutron radiation into optical radiation, in which the distribution of its brightness judges the spatial distribution of the radiation intensity of fast neutrons, characterized in that a point source of fast neutrons is used as a radiation source, and an optical image is stored in each of the set of luminescent plates of the screen, and then separately read out for a short (hundreds of times less memory process anya) time in the photodetector.  2. Screen for neutron radiography, which is a luminescent screen, characterized in that the luminescent screen consists of an alternating set of plates made of powder phosphor, between which are additionally inserted plates of a hydrogen-containing substance, for example, polyethylene or polypropylene, the thickness of which is selected from the condition the required spatial distribution of emitted fast neutrons, and the total screen thickness is determined from the condition of a small thrones.  3. The screen for neutron radiography according to claim 2, characterized in that it consists of a set of plates, each of which is a mixture of a hydrogen-containing substance and an energy storage phosphor.  4. The screen for neutron radiography according to any one of claims 2 and 3, characterized in that the volume of the phosphor in the mixture is determined by the neutron spectrum and for neutrons energy of the order of 14 MeV is 0.1-0.15% of the total volume of the mixture.  5. The screen for neutron radiography according to claim 2, characterized in that it consists of a set of plates, each of which is a mixture of a hydrogen-containing substance, an energy storage phosphor and an additional powder phosphor emitting into the absorption band of the energy storage phosphor.  6. A screen for neutron and x-ray radiography, comprising a screen for neutron radiography, characterized in that it further comprises a lead screen and an additional set of plates from a storage energy storage phosphor located between the fast neutron radiation source and the main screen, and between the main screen and the additional set plates mounted lead screen. 7. Screen for neutron radiography according to any one of claims 2 to 6, characterized in that a material with the general stoichnometric formula is used as a storage energy-storage phosphor
Ba _1-x Sr _x Ga ₂ O ₄ E _U ⁺² Dy ³ ,
where x = 0.001 - 0.1, and the content of E _U ²⁺ = Dy ³ = 1%,
and as the initiating powder phosphor, a material of the composition ZnS • AgTmS _{2 is used} , in which the content of the activating compound AgTmS ₂ is from 0.005 to 0.01 wt.%.

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