JPH04290985A - Neutron detector - Google Patents

Abstract

PURPOSE:To obtain a neutron detector of a high neutron detecting sensitivity and high responsiveness and suitable for real-time photographing with a simple structure. CONSTITUTION:A neutron detector is made up by integrally combining a scintillator 1 which emits scintillation rays in accordance with the intensity of incident neutrons with a semiconductor photodetecting element for converting incident rays emitted from the scintillator 1 into an electric signal. The scintillator 1 is made up of a sintered body in which Ga as a neutron absorbing element and at least one kind of element selected out of Pr, Tb, and Eu as an activator are included.

Description

[Detailed description of the invention]

[Object of the invention]

0002

[Industrial Field of Application] The present invention relates to a neutron detector, which has a particularly high detection sensitivity for thermal neutrons, makes it possible to take transmission images in real time with a fast response in neutron radiography, etc., and has a simplified structure. Regarding neutron detectors that can be used.

0003

[Prior Art] Neutron radiography is a method of obtaining structural or compositional information of an object as a two-dimensional image by detecting the intensity distribution of thermal neutron beams that are attenuated after passing through the object. It is effective for inspecting hydrogen-containing compounds and composite materials consisting of metals and light element substances that are difficult to inspect with X-rays or gamma rays, and is an effective inspection method in a wide range of fields such as plant equipment, aircraft, and automobile parts. It's being used.

[0004] Conventionally, two-dimensional images as described above are produced using a neutron converter that converts transmitted neutron beams into visible light, etc.
It was made of photographic film that is sensitive to the intensity of visible light. In other words, a photographic film method is generally used that combines a photographic film with a neutron converter made of a material that has a large neutron absorption cross section and contains elements that easily cause nuclear reactions with neutrons, or a mixture of these materials and a phosphor. It had been.

In this case, the materials constituting the neutron converter include, for example, Gd metal foil or Gd oxide fine particles, which have the largest neutron absorption cross section, and ZnS, as disclosed in Japanese Patent Application Laid-Open No. 62-277587. A mixture with Ag phosphor, a mixture of 6LiF and ZnS:Ag phosphor, or Gd2O2S:Tb phosphor is used.

[0006] Also, in a neutron detector for environmental measurement, 6L
A combination of a luminescent material such as an i-glass scintillator, a LiI:Eu scintillator, or a liquid scintillator filled with boron or cadmium and a photodetector such as a photomultiplier tube is generally used.

[0007] Furthermore, in the field of medical image diagnosis, where radiography is most often applied, X-ray photography has traditionally been the mainstream, but in recent years, X-ray imaging plates that can obtain images with higher resolution have been introduced. X-ray digital radiography is attracting attention.

Furthermore, the photographic film method is still the mainstream of neutron radiography used in the field of non-destructive testing of industrial products, as mentioned above. However, digitizing image data makes data processing, transmission, storage, etc. easier, so neutron radiography also uses
As with X-ray digital radiography, there is a strong demand for digitization, and a neutron detector that can capture image signals digitally is required.

[0009] Furthermore, it is natural that real-time (simultaneous) observation is required for equipment that uses neutron radiography, such as hazardous material detection equipment at airports and material manufacturing process monitoring equipment that monitors the injection molding status of ceramic products. be done. In this case, it is essential that the neutron detector built into the device has high sensitivity and fast response.

0010

However, as long as the conventional method uses photographic film, real-time observation is virtually impossible because the developing process of the photographic film takes a certain amount of time.

[0011] In order to solve the above problems and realize real-time observation in a systematic manner, for example,
As disclosed in Publication No. 587 and "Inspection Equipment News" No. 704, a method is also adopted in which a dark box is provided and the fluorescent screen portion of the photographic film is photographed using an imaging tube. However, this method has the disadvantage that the capacity of the device becomes extremely large, and that depending on the arrangement of the image pickup tube, graphic distortion may occur, resulting in low image accuracy.

On the other hand, as a conventional neutron detector for environmental measurement, a combination of a LiI:Eu scintillator and a photomultiplier tube as described above is widely known. However, in order to apply this neutron detector to neutron radiography, it is necessary to equip it with a large number of photomultiplier tubes, which results in an increase in the size of the device and a problem in that manufacturing costs skyrocket. .

On the other hand, in environmental radiation measurements and neutron diffraction experiments, a function that can measure the two-dimensional distribution of emitted thermal neutrons is often required. However, conventional thermal neutron detectors have low spatial resolution. For example, even a detector that combines a scintillator and a position-detecting photomultiplier tube has a low spatial resolution of about 3 mm. Therefore, detectors with higher resolution are required.

[0014] In addition to the detector, if the components thereof are stable and the structure is simple, the manufacturing cost of the device will be lower. However, in conventional neutron detectors, the converter portion is often unstable or difficult to mold, resulting in a complicated detector structure. For example, since the LiI:Eu scintillator described above has strong hygroscopicity, the outer surface of the scintillator needs to be airtightly covered with plastic or the like. Furthermore, if the scintillator is made of a liquid substance, it must be housed in a container, making the structure complicated and making it difficult to downsize.
There are many issues to be solved when using it as a component of a neutron detector.

The present invention has been made to solve the above problems, and has a higher thermal neutron detection sensitivity than conventional neutron detectors, a fast response, suitable for real-time imaging, and a simple structure. The purpose of this research is to provide a neutron detector. [Structure of the invention]

[0016]

[Means for Solving the Problems] In order to achieve the above object, the present inventors changed the materials constituting the scintillator and measured their characteristics such as sensitivity, and also developed a detection element for detecting scintillator light. By changing the combination of , the overall structure was simplified, and the neutron detector according to the present invention was completed.

That is, the neutron detector according to the present invention integrates a scintillator that emits scintillation light depending on the intensity of an incident neutron beam and a semiconductor photodetector element that converts the emitted light from the scintillator into an electrical signal. The scintillator is characterized in that it is composed of a sintered body containing Gd as a neutron absorbing element and at least one element selected from Pr, Tb and Eu as an activator. .

[0018]

[Operation] Thermal neutrons incident from the front of the scintillator are
The neutrons are efficiently absorbed by a scintillator containing Gd, which has a large neutron absorption cross section, and converted into energy. and,
These absorbed energy activate the activators Tb and Pr.
, Eu emits scintillator light, this scintillator light is absorbed by the semiconductor photodetection element, converted into an electrical signal corresponding to the intensity of the incident neutron beam, and output.

According to the neutron detector having the above structure, Gd having excellent neutron absorption characteristics and at least one selected from Pr, Tb and Eu having excellent light emission characteristics are used as activators.
It uses a scintillator made of a sintered body containing a seed element, and is combined with a semiconductor photodetector element that has high detection sensitivity and a simple structure, so the overall structure of the detector is It is possible to obtain a neutron detector that is simple and has excellent sensitivity characteristics.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, an embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a perspective view showing an embodiment of a neutron detector according to the present invention.

[0021] That is, the neutron detector according to this embodiment is as follows:
It consists of a scintillator 1 that emits scintillation light according to the intensity of an incident neutron beam, and a semiconductor photodetector element 2 that converts the emitted light from this scintillator 1 into an electrical signal. It is composed of a sintered body containing Gd as an absorbing element and at least one element selected from Pr, Tb, and Eu as an activator.

The scintillator 1 used in the above-mentioned neutron detector is, for example, a multi-channel scintillator formed by sequentially adhering a plurality of scintillator elements 1a formed into strips through a separator plate 3, as shown in FIG. Consists of. The separator plate 3 is provided to prevent crosstalk of γ-rays (rebound γ-rays due to Gd(n.γ) reaction) between adjacent scintillator elements 1a, 1a, and has a high γ-ray absorption rate. It is made of a thin plate of heavy metal, such as lead or tungsten. Further, both surfaces of each separator plate 3 are coated with a reflective agent for efficiently reflecting scintillator light generated by each scintillator element 1a.

Each scintillator element 1a has, for example, a compositional formula of (Gd1-x Mx)2O2S or (Gd1-xMx)2O3 (where M is Pr,
It is at least one element selected from Tb and Eu, and x is a value satisfying 0.0001≦x≦0.1 in terms of atomic ratio. ) is preferably made of a sintered body. Gd has a large neutron absorption cross section and absorbs neutron beams into the scintillator element 1a with high efficiency. Also Pr, T
b and Eu both act as activators that efficiently emit scintillator light in response to absorbed neutron beam energy.

[0024] Furthermore, in the composition of the scintillator, G
The content x of Pr, Tb, and Eu with respect to d is 0 in atomic ratio.
．． It is necessary to set it in the range of 0001 to 0.1. That is, when the content of the M element (Pr, Tb, Eu) is less than 0.0001 in atomic ratio, the luminescence sensitivity decreases. On the other hand, as the concentration of these activators M increases, the emission intensity increases, but if the content x exceeds 0.1, the emission intensity similarly decreases due to concentration quenching. Therefore, the content x of the M element is set within the above range. Especially when the M element is Pr, its content is 0.0001
A range of 0.02 to 0.02 is desirable, and in the case of Tb and Eu, a range of 0.001 to 0.1 is preferred.

Further, the thickness t of the scintillator 1 is 1
It is preferable to set it within the range of ~3 mm. When the thickness t of the scintillator 1 is less than 1 mm, it becomes difficult to sufficiently absorb γ-rays and internal conversion electrons within the scintillator 1. On the other hand, when the thickness t exceeds 3 mm, the light transmittance of the scintillator 1 deteriorates, the scintillator light extraction efficiency decreases, and as a result, the electrical output from the semiconductor photodetector element described later decreases, and the neutron detection sensitivity decreases. will decrease. Therefore, the thickness t is preferably in the range of 1 to 3 mm.

Further, each of the scintillator elements 1a is produced by hot isostatic pressing (HI) a raw material mixture having the above compositional formula.
P) It is manufactured from a ceramic sintered body that is formed by pressure molding in a heated state.

The neutron detector of this embodiment is composed of the scintillator 1 made of the above-mentioned light-transmitting ceramic and a semiconductor photodetector element that detects the scintillator light from the scintillator 1 and converts it into an electric signal. For example, a Si photodiode 4 is used as the semiconductor photodetector element.

As shown in FIG. 1, the photodiode 4 is in optical contact with the scintillator 1 by being integrally bonded to the surface of the scintillator 1 on the side opposite to the neutron incident surface.

FIG. 3 is a perspective view showing an example of the configuration of a multi-channel photodiode 4 as a semiconductor photodetecting element, in which a plurality of strip-shaped photodiode elements 6 are integrally arranged on one semiconductor substrate 5. established and configured. Each photodiode element 6 is formed with a terminal 7 for signal extraction, and each terminal 7 and a wiring terminal printed on an insulating substrate 8 are electrically connected by wire bonding (not shown). .

For example, the channels of the multi-channel scintillator 1 configured as shown in FIG. 2 and the multi-channel photodiode 4 configured as shown in FIG. A multi-channel neutron detector 9 as shown in FIG. 1 is manufactured by adhesively bonding them together using a transparent adhesive such as a glass adhesive.

As described above, according to the neutron detector of this embodiment, which is composed of the scintillator 1 and the photodiode 4 as a semiconductor photodetecting element, the neutron beam is irradiated onto the front surface of the scintillator 1 (the upper surface in FIG. 1). . Since the scintillator 1 contains the Gd element, which has a particularly large neutron absorption cross section, as a component, most of the incident neutron beam is absorbed by the scintillator 1. The absorbed neutron beam is converted into gamma rays and internally converted electrons by the (n, gamma) reaction. As in this embodiment, the thickness of the scintillator 1 is
By setting the thickness to approximately 3 mm, most of the gamma rays absorbed within the scintillator 1 are low energy gamma rays of 1 MeV or less, and the internal conversion electrons are also at a low energy level of approximately 70 KeV, and are sufficiently absorbed within the scintillator 1. be done. And by these absorbed energies, the activator Tb
, Pr, and Eu emit scintillation light. The emitted scintillation light is detected by a photodiode 4 that is in optical contact with the scintillator 1, and is converted into an electrical signal there. In this way, an electrical signal corresponding to the intensity of the neutron beam incident on the neutron detector 9 is output. The electrical signals are transmitted through each terminal 7 to a signal processing circuit (not shown) and reconstructed as image data.

According to the neutron detector 9 according to this embodiment,
The scintillator 1 is made of a sintered body containing Gd, which has excellent neutron absorption properties, and at least one element selected from Pr, Tb, and Eu, which have excellent luminescence properties, as an activator. To obtain a neutron detector which has a simple structure as a whole and has excellent sensitivity characteristics since it is configured by integrally combining a photodiode 4 as a semiconductor photodetecting element with high detection sensitivity and a simple structure. Can be done.

The multi-channel neutron detector 9 shown in FIG. 1 has multiple channels formed by arranging scintillator elements 1a and photodiode elements 6 one-dimensionally (in one direction). However, the invention is not limited to the embodiment shown in FIG. That is, by arranging the scintillator element 1a and the photodiode element 6 two-dimensionally (in two directions), a multi-channel neutron detector may be obtained. If this detector is used in neutron radiography, it is possible to obtain a two-dimensional image of the neutron beam that has passed through the object.

Next, effects based on the composition of the scintillator of the neutron detector according to the present invention will be explained in more detail based on the following examples and comparative examples.

Example 1 As Example 1, the composition formula (Gd0.999 Pr0.0
A raw material mixture containing 01)2O2S was subjected to hot isostatic pressing (HIP) to prepare a large number of scintillator elements each having a thickness of 1 mm. In addition, a large number of separator plates were prepared by processing thin lead plates having the same height as the scintillator elements. Both sides of the separator plate are coated with white paint as a reflective agent to efficiently reflect scintillation light generated by the scintillator elements. On the other hand, a multi-channel Si photodiode as shown in FIG. A scintillator was adhesively bonded onto the photodiode so that the channels coincided with each other, and a multichannel neutron detector (Example) as shown in FIG. 1 was manufactured.

Examples 2 to 18 Neutron detection of Examples 2 to 18 having the same dimensions using the same manufacturing method as Example 1 except that the composition and thickness of the scintillator were prepared as shown in Table 1. I created a vessel.

Comparative Example 1 As Comparative Example 1, a LiI:Eu emitter, which has been used in the scintillator of a conventional neutron detector, was adjusted to a thickness of 1 mm, and was integrally bonded onto a multi-channel photodiode shown in FIG. A multichannel neutron detector having the same dimensions as in Example 1 was prepared.

Comparative Example 2 As Comparative Example 2, a material used as a neutron converter material in a conventional neutron detector for neutron radiography was used.
A solid mixture of LiF powder and ZnS:Ag phosphor in a binder was molded to a thickness of 1 mm, and was integrally bonded onto the multichannel photodiode shown in FIG. 3 to form the same dimensions as in Example 1. A multi-channel neutron detector was manufactured.

Comparative Example 3 As Comparative Example 3, a material used as a neutron converter material in a conventional neutron detector for neutron radiography was used.
A solid material made by mixing Gd2O3 powder and ZnS:Ag phosphor in a binder is molded to a thickness of 1 mm, and is integrally bonded onto the multichannel photodiode shown in Fig. 3 to form a multichannel neutron detector. was manufactured.

Each of the neutron detectors of Examples 1 to 18 and Comparative Examples 1 to 3 prepared as described above was irradiated with a neutron beam of a predetermined intensity generated from a small cyclotron for a certain period of time. The dose of irradiated thermal neutron flux is 1.1×106
n/cm2·sec, and the irradiation time was 5 minutes. The electrical signal values output from each photodiode were then measured to evaluate the sensitivity of each neutron detector. The sensitivity of each detector is expressed as a relative ratio, with the electrical signal value output from the photodiode of the neutron detector shown in Comparative Example 1 being a reference value of 100. In addition, in order to compare the chemical stability of the scintillator, we left the scintillator in the air and observed its changes over time, and found that the LiI:Eu used in Comparative Example 1 deliquesced after one day and was extremely unstable. On the other hand, those of Examples 1 to 18 were stable even after several months.

The above measurement and evaluation results are shown in Table 1 below.

[0042]

[Table 1]

As is clear from the results shown in Table 1, the neutron detectors shown in Examples 1 to 18 are all the same as those in Comparative Examples 1 to 18.
It was confirmed that the detector has a sensitivity equal to or higher than that of the conventional detector shown in No. 3, and most of the detectors have a sensitivity that is 2 to 6 times better.

In terms of chemical stability, since the LiI:Eu scintillator element of Comparative Example 1 has hygroscopicity, the luminescence properties and sensitivity deteriorate over time, whereas the scintillator element of Example 1 Since all of the scintillator elements of the detectors according to items 1 to 18 have extremely excellent stability, there is no change in sensitivity over time and the reliability of neutron beam measurement operations can be greatly improved.

As described above, according to the neutron detector according to each embodiment, Gd2 O2 S:M or Gd2 O3:
M (where M is at least one element selected from Pr, Tb, and Eu) contains Gd, which has a particularly large neutron absorption cross section and excellent neutron absorption ability, and also has excellent light emission characteristics. It is equipped with a scintillator made of translucent ceramics containing Tb, Eu, and Pr as active agents, and a semiconductor photodetecting element with a simple structure and high photodetecting ability, such as a photodiode. This significantly improves the sensitivity and simplifies the structure compared to conventional neutron detectors. Therefore, it is extremely useful as a neutron detector that can measure dose in real time with high-speed response, such as a neutron detector for environmental monitoring or a neutron detector for neutron digital radiography.

[0046]

As explained above, the neutron detector of the present invention uses Gd, which has excellent neutron absorption properties, and at least one selected from Pr, Tb, and Eu, which have excellent luminescence properties, as an activator. It uses a scintillator made of a sintered body containing elements, and is combined with a semiconductor photodetector element that has high detection sensitivity and a simple structure, so the overall structure of the detector is simple. Therefore, a neutron detector with excellent sensitivity characteristics can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing an embodiment of a neutron detector according to the present invention.

FIG. 2 is a perspective view showing an example of the configuration of a scintillator used in the neutron detector shown in FIG. 1;

FIG. 3 is a perspective view showing a configuration example of a semiconductor photodetecting element used in the neutron detector shown in FIG. 1;

[Explanation of symbols]

1.Scintillator 1a scintillator element 3 Separator plate 4 Photodiode 5 Semiconductor substrate 6 Photodiode element 7 Terminal 8 Insulating substrate 9 Neutron detector

Claims (1)

[Claims] 1. A scintillator that emits scintillation light according to the intensity of an incident neutron beam, and a semiconductor photodetection element that converts the emitted light from the scintillator into an electrical signal, the scintillator comprising:
Gd as a neutron absorbing element and Pr, T as an activator
A neutron detector comprising a sintered body containing at least one element selected from B and Eu.