JPH09197050A - Radiation detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a detector with a large area which reduces back ground counts due to cosmic rays and detects γ-ray or X-ray by layering by turns plastic scintillator emitting light by the detection of radiation and converter and absorber made of a material over a specific atomic number. SOLUTION: A detection part is assembled by layering in turn plastic scintillator 1 and converter and absorber 2. The scintillator 1 is a radiation detecting material made mainly of polystyrene and the like and can be replaced by other detection material having a function similarly generating fluorescent light by radiation. The converter and absorber 2 has functions to convert γ-ray (or X-ray) to secondary electrons and exhaust to the scintillator 1 and to absorb the secondary electrons transmitted through the scintillator 1, which is desired to be a material with an atomic number over 13 considering to facilitate treatment. The light from the scintillator 1 is processed with a signal processing circuit 5 by way of light transmission path 3 and a photoelectro converter element 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radiation detector, and more particularly to a technique suitable for energy discrimination and increasing the area in γ-ray or X-ray intensity measurement.

[0002]

2. Description of the Related Art A plastic scintillator is a radiation detecting material mainly composed of polystyrene or the like, and when radiation enters it, atoms or molecules constituting the scintillator enter an excited state, and when the excited state shifts to a lower energy state. Then, the energy difference is emitted as light (fluorescence). The light is guided directly or via a light transmission path to a photoelectric conversion element such as a photomultiplier tube or a photodiode, and is extracted as an electric signal. In the detection of γ-rays and X-rays (hereinafter, unless otherwise specified, radiation refers to γ-rays and X-rays in the present invention), radiation is incident on a substance to form atoms or atoms of the substance. Interacts with the electrons to generate secondary electrons with energy. When the secondary electrons move through the substance, the atoms or molecules constituting the substance are set to an excited state. In the plastic scintillator, fluorescence is generated by the action of secondary electrons. The energetic secondary electrons lose their energy as they move through the substance, and eventually stop and are trapped by ionized atoms in the vicinity. The migration distance of secondary electrons is shorter than that of γ-rays or X-rays having the same energy, and the electron having energy of 1 MeV is about 1.5 mm when the substance is aluminum.

The interaction between radiation and a substance is more likely to occur as the substance has a larger atomic number and a higher density. Therefore, since the density of the plastic scintillator is as low as about 1, the plate having a large atomic number and a high density (hereinafter, referred to as a converter) is arranged on the surface of the plastic scintillator to increase the trapping rate of radiation. .
However, when the converter becomes thicker, the secondary electrons generated by internal interaction do not reach the plastic scintillator that absorbs the fluorescent light and does not generate an electric signal, so it is important to select the material and thickness of the converter. Is. A large area radiation measurement is required for a survey of contamination such as floors and walls of radiation handling facilities and nuclear facilities, and sites after dismantling of nuclear facilities. The survey meter for γ (X) rays is an ionization chamber type, GM counter type, NaI (T
l) Scintillation type, semiconductor type, etc. (hereinafter,
Called the first prior art). These instruments are portable and useful for partial pollution surveys. On the other hand, the plastic scintillator is effective as a large-area radiation detector, but has a problem of background due to cosmic rays because the sensitive part is wide. As an example of the technique for removing the background, there is the technique shown in FIG. This technology is
The detection layer 1 using a plastic scintillator or the like is provided on both sides of the absorbing material 2, and the thickness of the absorbing layer 2 is set to be equal to or more than the thickness capable of absorbing the radiation to be measured. Since cosmic rays have strong penetrating power, they also pass through the absorption layer 2. Therefore, if there is a signal from both layers at a certain event (the event in which a detection signal is generated by detecting radiation is referred to as an event below), it is determined as an event due to cosmic rays and the event is removed. Remove background from cosmic rays. With this technique, the background caused by cosmic rays can be removed (hereinafter referred to as the second conventional technique).

[0004]

However, the first conventional technique has a problem that a large number of measurements must be performed by moving the measurement points in order to randomly inspect a large area. Further, the second conventional technique has a problem that it is difficult to discriminate the energy of γ rays or X rays to be measured.

The present invention has been made in view of the above problems, and a first object of the present invention is to reduce the background caused by cosmic rays and detect a large area for detecting γ rays or X rays. The purpose is to provide a container.

A second object of the present invention is, in addition to the first object, to provide a large-area detector capable of discriminating energy of γ-rays or X-rays.

[0007]

In order to achieve the first object, according to the first concept of the present invention, a radiation detector is
A plastic scintillator which emits light upon detection of radiation and a laminated part in which plates mainly made of a substance having an atomic number larger than 13 are alternately laminated, an optical transmission line for transmitting the light emitted by the plastic scintillator, and the optical transmission line. It becomes possible by comprising a photoelectric conversion element that converts the emitted light into an electric signal and a signal processing circuit that processes the electric signal from the photoelectric conversion element.

To achieve the second object, according to the second concept of the present invention, the signal processing circuit is provided with a circuit for discriminating the pulse height of the signal from the photoelectric conversion element, and a radiation from the discriminated signal. This is possible by providing a circuit for counting the frequency of the number of consecutive layers in the detected plastic scintillator.

According to the first concept of the present invention, a plastic scintillator for detecting radiation is provided in a plurality of layers (detection layers).
By alternately stacking layers mainly composed of a substance having an atomic number of more than 13 (converter layer), the radiation selectively interacts in the converter layer, and secondary electrons are discharged to the detection layer. Can be detected efficiently. At this time, a large-area detector can be easily constructed by increasing the area of the plastic scintillator. In the signal processing circuit,
Background signals due to cosmic rays are reduced by removing signals from photoelectric conversion elements corresponding to the detection layers, for example, when there are signals from all layers, as cosmic ray events, despite the large area detector. be able to.

According to the second concept of the present invention, the generation of the signal can be easily detected by providing the circuit for discriminating the pulse height of the signal from the photoelectric conversion element. Further, the higher the energy of radiation, the higher the average energy of secondary electrons generated, and the number of layers (continuous layers) that generate a signal at each event corresponding to the energy of radiation increases. Therefore,
By counting the number of consecutive layers in the detection layer in which radiation is detected from the pulse height discriminated signal, it is possible to discriminate the energy of the incident radiation.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1) FIGS. 1 to 4 are explanatory views showing an embodiment of the present invention, and FIG. 5 is an explanatory view showing a conventional technique.

FIG. 1 shows the configuration of an embodiment of the present invention. The detection part is assembled by alternately stacking the plastic scintillator 1 and the conversion and absorption material 2. A plastic scintillator is a radiation detection material whose main material is polystyrene. The present invention is not limited to plastic scintillators, but other detection materials (NaI (Tl), CsI (Tl), Ba, etc.) having a similar function of generating fluorescence with respect to radiation are also included.
A single crystal scintillator such as F, CeF, CWO, BGO, GSO, LSO, or a liquid scintillator such as anthracene) can also be used. The conversion and absorption material 2 is γ
It has a function of converting (X) rays into secondary electrons and discharging them to the plastic scintillator 1, and absorbing secondary electrons that have passed through the plastic scintillator 1. The converter and absorber 2 needs to be denser than the plastic scintillator because it has the purpose of selectively reacting radiation. Considering that the material is easy to handle, a material after aluminum is considered, and it is preferable that the material mainly has an atomic number larger than 13. The light from the plastic scintillator 1 is guided to the photoelectric conversion element 4 via the optical transmission line 3. Both ends of the optical transmission line 3 are optically connected to the side surface of the plastic scintillator 1 and the light receiving surface of the photoelectric conversion element 4. The optical transmission line 3 is made of a light-transmissive material, and has a shape such that the light from the plastic scintillator 1 is guided to the photoelectric conversion element 4 by internal reflection and internal propagation, and gradually narrows toward the other end. Also, fibrous materials such as optical fibers can be used. The signal from the photoelectric conversion element 4 is processed by the signal processing circuit 5.

The data from the signal processing circuit 5 is processed by a computer (not shown) to display / record / measure the measurement results.
Other functions such as transmission can be added.

FIG. 2 shows an explanatory view of another embodiment of the present invention. The function of each unit is the same as in FIG. In this example,
It is characterized in that the thickness of each layer of the plastic scintillator 1 is made equal to the thickness of each layer of the conversion and absorption material 2. The effect will be described below. For example, the thickness of the combination of the detection layer (plastic scintillator) and the converter layer (conversion and absorption material) is set to ⅕ of the moving distance of secondary electrons. Corresponding to this thickness, the basic unit of detection is five sets of the detection layer and the converter layer. γ (X) rays have a higher penetrating power than secondary electrons, and it is difficult to specify in which part of the stack the reaction occurs. Even in such a case,
By making the thicknesses of the respective layers equal, wherever the reaction occurs, the same detection can be performed by counting from the layers in which the reaction has taken place and taking five sets of layers. That is, the capture rate of γ (X) rays is increased and the measurement efficiency is improved.

FIG. 3 shows an explanatory view of another embodiment of the present invention. In this embodiment, a scintillation fiber 3a having a wavelength conversion function is used for the optical transmission line 3. By winding the scintillation fiber 3a around the plastic scintillator 1 and optically connecting it with an optical adhesive (not shown) or the like, the light emitted from the plastic scintillator 1 can be efficiently taken and guided to the photoelectric conversion element 4. Since the scintillation fiber 3a has the attenuation of light, when the distance between the plastic scintillator 1 and the photoelectric conversion element 4 is large, the optical fiber 3b for optical transmission is used on the way.

FIG. 4 shows an explanatory view of another embodiment of the present invention. Since the plastic scintillator 1 is attenuated when the light propagates through the inside, if the light is incident on the photoelectric conversion element 4 when the light is incident on the photoelectric conversion element 4 if the area is made wider by a single plate, it cannot be detected. Therefore, in order to cover a larger area, it is effective to divide the block into a plurality of blocks.

An embodiment of data processing for discriminating energy is shown in equation 1.

[0019]

[Equation 1]

The energy discrimination of the present invention aims at setting a plurality of energy ranges and determining the intensity of radiation in each energy range. Here, for example, a case of discriminating three energies (ranges) will be described. Let A _i be the true intensity of the radiation having the i-th energy. Further, the response function of the detector is f (E _i , j), where E _i is the i-th energy and j is the number of layers in which a signal is generated in successive layers at each event. The number of generated layer signal continuous layer in each event count value in the case of j and N _j. N
_j can be represented by the determinant of Equation 1 from the true intensity A _i and the detector response function f (E _i , j). A _i can be calculated from the value of N _j by obtaining the inverse matrix of the response function in advance by measuring or calculating it.

[0021]

According to the present invention, it is possible to realize a large-area detector in which the background due to cosmic rays is reduced, and it is possible to inspect contamination over a wide area.

Further, since the energy of incident radiation can be discriminated, the radiation intensity can be measured by dividing the radiation by energy in a wide area contamination inspection, and the contamination nuclide and its intensity can be determined.

[Brief description of drawings]

FIG. 1 is an explanatory view showing one embodiment of the present invention.

FIG. 2 is an explanatory view showing a second embodiment of the present invention.

FIG. 3 is an explanatory view showing one embodiment of the present invention.

FIG. 4 is an explanatory diagram showing an embodiment of the present invention.

FIG. 5 is an explanatory diagram showing the principle of a conventional technique.

[Explanation of symbols]

1 ... Plastic scintillator, 2 ... Conversion and absorption material, 3 ... Optical transmission line, 3a ... Scintillation fiber,
3b ... Optical fiber, 4 ... Photoelectric conversion element, 5 ... Signal processing circuit.

Claims (5)

[Claims] 1. A laminated portion in which a plastic scintillator which emits light upon detection of radiation and a plate mainly made of a substance having an atomic number larger than 13 are alternately laminated, an optical transmission line which transmits the light emitted by the plastic scintillator, and A radiation detector comprising: a photoelectric conversion element that converts light that has passed through an optical transmission path into an electric signal; and a signal processing circuit that processes the electric signal from the photoelectric conversion element. 2. The radiation detector according to claim 1, wherein the layers of the plastic scintillator have the same thickness, and the layers of the plate mainly made of a substance having an atomic number greater than 13 have the same thickness. . 3. The signal processing circuit comprises a circuit for discriminating a pulse height of a signal from the photoelectric conversion element, and a circuit for counting the number of consecutive layers in the plastic scintillator detecting radiation from the signal discriminated by the pulse height. The radiation detector according to claim 1, which is provided with the radiation detector. 4. The radiation detector according to claim 1, further comprising a computer that processes data from the signal processing circuit. 5. The computer counts the value of the number of consecutive layers detected by the plastic scintillator detecting radiation in the signal processing circuit, the energy of the radiation and the count corresponding to the number of consecutive layers in the plastic scintillator. The radiation detector according to claim 4, wherein the radiation intensity is calculated from the product of the inverse matrices of the response functions of the detectors, which are composed of probabilities.