JPH0213736B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to simultaneous neutron and gamma ray radiography utilizing a single radiation source and apparatus used therefor. More specifically, the present invention relates to the use of Californium-252 (hereinafter referred to as " ²⁵² Cf") as a source for simultaneous neutron and gamma ray radiography, and to a simultaneous neutron and gamma ray radiography apparatus using ²⁵² Cf. . As a conventional non-destructive radiographic testing method for structures, etc., there is a radiographic testing method that utilizes the transmission phenomenon of X-rays, gamma rays, or neutrons. The neutron-based radiation transmission test method is easy to test for specific substances such as hydrogen-containing materials hidden behind metal materials, so it is being put into practical use as a complementary test method to X-ray and gamma ray transmission tests. In recent years, industrial materials have diversified, and as seen in pyrotechnics such as gunpowder relays, there are many structural parts made of metal and non-metallic materials, and non-destructive inspection of the inside of these parts requires There is an increasing need to use neutron transmission tests in addition to radiation or gamma ray transmission tests. In this case, two different types of radiation sources are required, two test operations are required on the same sample, and testing under the same geometric conditions is difficult. Furthermore, when interpreting neutron images and X-ray or gamma-ray images obtained under different conditions, care must be taken to take into account differences in geometric conditions. The present invention eliminates the above drawbacks by using a single source that emits both neutrons and gamma rays, and superimposes the neutron and gamma ray images in a single operation under the same geometric conditions. The present invention provides so-called simultaneous neutron and gamma ray radiography that simultaneously captures images on two detectors, and its apparatus, thereby making it possible to improve the accuracy and simplify the radiographic examination method. Neutron sources for neutron radiography include nuclear reactors, accelerators, and isotope neutron sources, but among these, those that use isotopes do not have the same strong neutron flux intensity as nuclear reactors, but they can be handled without maintenance. Because it is easy to use and can be moved, it is expected that it will be put to practical use in the space and aviation fields, where gamma ray transmission tests are difficult to test, for equipment parts that require high reliability and integrity. There is. Isotopic neutron sources that have become available in recent years
²⁵² Cf is produced per gram per second by spontaneous fission.
It emits 2.3×10 ¹² neutrons and is a small and powerful neutron source that can be moved, making it suitable as a source for neutron radiography in the field. In addition, ^{the 252} Cf source simultaneously
10 ¹³ gamma rays are emitted, and the energy distribution of the gamma rays is widely distributed from zero to several Mev, but generally there are many low energy components,
On average, it is considered to be approximately equivalent to the gamma ray energy (0.662 Mev) of the ¹³⁷ Cs ray source, which is widely used as a source for gamma ray radiography. By the way, the gamma rays emitted by ²⁵² Cf have not been used at all until now, and were instead treated as an interfering element. The present inventors focused on the gamma rays emitted by ²⁵² Cf, and completed the present invention by using a ²⁵² Cf ray source as a source for simultaneous neutron and gamma ray radiography. Therefore, the main object of the present invention is to provide ²⁵² Cf for use as a source for simultaneous neutron and gamma radiography. Yet another object of the present invention is to provide a simultaneous neutron and gamma radiography apparatus that utilizes ²⁵² Cf as a radiation source. A further object of the invention is to provide a ²⁵² Cf irradiation device for use in simultaneous neutron and gamma radiography equipment. Yet another object of the invention is to provide an image detector for use in simultaneous neutron and gamma radiography equipment. Hereinafter, the configuration of the present invention will be explained based on the attached drawings. FIG. 1 is a conceptual diagram of the simultaneous neutron and gamma ray radiography apparatus of the present invention. In FIG. 1, 1 is an object, 3 is an irradiation device incorporating ^{a 252} Cf radiation source 2 and equipped with a collimator 4 , and 5 is an image detector composed of a gamma ray image detector 5A and a neutron image detector 5B . FIG. 3 is a longitudinal section showing one embodiment of the irradiation device used in the radiography apparatus of the present invention. In FIG. 3, 6 is ^{a 252} Cf radiation source, 7 is a moderator made of paraffin or high-density polyethylene, 8 is _a lead shielding material, 9 is a paraffin neutron shielding material containing _B2O3 , and 10 is a beam collimator. The beam collimator is made of cadmium, indium, and lead, and is replaceable, so that the spread of the neutron and gamma ray beams can be adjusted depending on the size of the object. The inventors prototyped a beam collimator with a length of 400 mm and a base diameter of 20 mm. In this case, the beam collimator correction factor (L/
D) is 20. It is desirable that L/D be as large as possible in order to avoid geometric staleness of the radiograph. Additionally, we used an extended collimator with a length of 500 mm to improve the geometry. Simultaneous neutron and gamma radiography was performed at a distance of 900 mm from the base of the collimator and L/D = 45. Devices for detecting neutron images include those that combine a neutron scintillator and X-ray film, or those that simply use a foil that is easily activated by neutrons. In the former, a neutron image can be obtained on an X-ray film using the light generated when neutrons that have passed through an object hit a scintillator. The latter is made by bringing the foil activated by neutrons into close contact with X-ray film.
After all, a neutron image can be obtained on film. The scintillator and foil are nearly insensitive to gamma radiation. Other devices that detect neutron images include damage caused by charged particles to an insulating solid (particle tracks) by combining a substance that generates charged particles with neutrons and an insulating solid.
Possible methods include enlarging the image using chemical etching to obtain a neutron image, or converting a neutron image into an optical image using a combination of a neutron scintillator and fiber plate, and observing it two-dimensionally. . On the other hand, as in the case of general gamma ray radiography, a device for detecting gamma ray images uses a metal foil intensifying screen, such as a combination of a lead foil intensifying screen with a large atomic number and density, and an X-ray film. It is completely insensitive to neutrons. Simultaneous radiography of neutrons and gamma rays is possible by stacking the detectors for neutron images and gamma rays as described above in the same cassette and using a ²⁵² Cf radiation source. That also became clear. FIG. 4 is a diagram showing the configuration of the image detector used in the radiography apparatus of the present invention and the process of image formation. In FIG. 4, the image detector 5 has a gamma ray detector 5A and a neutron image detector 5B disposed therein. The gamma ray image detector 5A is, for example, an X-ray film sandwiched between two pieces of lead foil, and the neutron image detector 5B is an X-ray film and a scintillator made by mixing lithium fluoride, zinc sulfide, and organic glass. A combination of films is used. As shown in the cross section of the object 1 seen from the image detector 5 side shown in FIG . Assuming that the metal material substance 1B is hidden inside, according to the method of the present invention, the gamma ray image detector 5A in the image detector 5 will receive a gamma ray image as shown in FIG. A neutron image as shown in FIG. 2B is simultaneously obtained on the detector 5B . From the two images of FIG. 2A and FIG. 2B, the presence of the metal material substance 1B hidden in the hydrogen-containing substance 1A and the relative positional relationship between the two can be clarified. Although the structure of the present invention is as described above, experiments were conducted using the apparatus of the present invention, and the results will be described as examples. EXAMPLE γ-Ray Spectrum FIG. 5 shows the energy spectrum of γ-rays measured using a 45 mm×45 mml NaI scintillator at a radiographic position 900 mm from the base of the collimator. The energy spectrum extends to several MeV without any observable peaks. The γ-rays originate from ²⁵² Cf and some from neutrons trapped in the materials of construction of the device. It can be seen that the effective γ-ray energy is approximately equal to the energy of ¹³⁷ Cs γ-ray. Effect of source position on neutron:γ ratio In direct irradiation thermal neutron radiography using an NE426 scintillator, the beam neutron:γ ratio (n/γ) is 5 × 10 ⁴ n cm _× ^-2 mR ^-1 It is. On the other hand, the intensity of the γ-rays must be high enough to obtain a γ-ray radiograph at the same time. These contradictory requirements must be reconciled to obtain optimal beam conditions. The influence of the vertical position of the source in the irradiator on the n/γ ratio was investigated. The results are shown in FIG. It can be seen that the thermal neutron flux density at a distance of 400 mm from the collimator base gradually decreases as the vertical distance from the collimator axis to the source increases, while the γ-ray dose rate decreases rapidly. . Effect of radiographic position on the neutron:γ ratio Figure 7 shows the effect of radiographic position on the n/γ ratio. These data were obtained with the source placed 30 mm vertically from the collimator axis. It can be seen that the γ-ray dose rate decreases approximately in inverse proportion to the square of the distance from the collimator base, while the relative change in the thermal neutron flux density is smaller than that of the γ-ray dose rate. These are thought to correspond to differences in the physical size of the effective radiation source. The depth range of the source is larger for thermal neutrons than for γ-rays. Therefore, the n/γ ratio is significantly improved by increasing the distance from the collimator base. The value at the 900mm position is 1.7×10 ⁵ ncm ^-2 mR ^-1 when the source is 30mm above the collimator axis.
and 8.8×10 ⁴ n・ at the collimator axis.
cm ^-2 mR ^-1 . Image characteristics of γ-ray radiography Simultaneous neutron and γ-ray radiography was performed at a position 900 mm from the base of the collimator to reduce geometric unsharpness and fog due to γ-ray background. Fuji−100X for neutron images
Linear film (high contrast and medium sensitivity) was used in combination with a NE426 scintillator. Meanwhile lead foil (thickness 0.03 and 0.1mm)
Fuji-150 X-ray film (medium contrast and high sensitivity) sandwiched in a sandwich pattern was used for gamma-ray images. When the ²⁵² Cf source was placed 30 mm above the collimator axis, the neutron image characteristics were good, but the gamma-ray radiograph was unsatisfactory due to insufficient irradiation. Satisfactory neutron and gamma-ray radiographs were obtained with the source on the collimator axis, however the neutron radiographs were somewhat adversely affected by gamma-ray fog. FIG. 8 shows the relationship between steel thickness and photographic density in a γ-ray radiograph obtained under the latter conditions. Photographic density increases with irradiation time. Contrast per thickness of steel is in the density range 1.5-2.0
It is 0.04/mm. The relationship between the detection sensitivity of the γ-ray radiograph and the steel thickness is shown in FIG. Detection sensitivity is improved by increasing film density.
For steel thicknesses of 5 mm or less, the value is 5%. This value is different from the normal − using a ¹³⁷ Csγ− source.
It is almost the same as the value obtained with line radiography. By looking at the results of the above examples,
By following the present invention, simultaneous gamma ray and neutron radiography is possible under the same test conditions, the relative positional relationship between the provided gamma ray image and neutron image is clear, and more detailed information can be obtained by a single operation. It can be seen that internal information of the object can be obtained and inspection accuracy is further improved. According to the present invention, radiographic inspection of structures that require a high degree of reliability, such as rockets and nuclear reactor parts, can be carried out simply and with high precision, greatly contributing to quality assurance of the parts.

[Brief explanation of drawings]

FIG. 1 is a conceptual diagram showing the apparatus of the present invention. FIG. 2 is a schematic diagram showing a gamma ray image and a neutron image obtained by the image detector of the present invention. FIG. 3 is a sectional view showing an example of an irradiation device used in the apparatus of the present invention. FIG. 4 is a diagram showing the configuration of an image detector used in the apparatus of the present invention and the process of image formation. FIG. 5 is a graph showing the energy spectrum of γ-rays. FIG. 6 is a graph showing the influence of source position on the neutron:γ ratio. 7th
The figure is a graph showing the effect of radiographic position on the neutron:γ ratio. FIG. 8 is a graph showing the relationship between steel thickness and photographic density among the image characteristics of a γ-ray radiograph. FIG. 9 is a graph similarly showing the relationship between steel thickness and detection sensitivity.

Claims (1)

[Scope of Claims] 1. An irradiation device containing californium-252 as a neutron and gamma ray source, placed on one side facing the subject, and an irradiation device placed facing the other side, which captures images of neutrons and gamma rays transmitted through the subject. In a simultaneous neutron and gamma ray radiography device that is composed of an image detector that simultaneously detects a neutron image and a gamma ray image under the same geometric conditions, the irradiation device includes a californium-252 radiation source, a californium-252 radiation source, and a It consists of a moderator surrounding the radiation source, lead shielding material and neutron shielding material placed around it, and a beam collimator, and an image detector sandwiched between two metal foil intensifying screens. The device is a cassette containing a gamma ray image detector made of an X-ray film and a neutron image detector made of a scintillator and an X-ray film stacked in parallel. 2. The device according to claim 1, wherein the moderator is high-density polyethylene or paraffin. 3. The device according to claim 1, wherein the neutron shielding material is _paraffin containing _B2O3 . 4. The device according to claim 1, wherein the beam collimator is made of cadmium, indium and lead and is replaceable. 5. The device according to claim 1, wherein the metal foil intensifying screen is a lead foil. 6. The device according to claim 1, wherein the scintillator comprises lithium fluoride, zinc sulfide and organic glass.