WO2011115179A1 - Scintillator for neutron detection, neutron detector, and neutron imaging apparatus - Google Patents

Abstract

Provided are: a scintillator for neutron detection, which is highly sensitive to neutron beams and which produces little background noise assignable to γ-ray; and a neutron detector and a neutron imaging apparatus that are made using the same. A scintillator for neutron detection, which consists of a LiCaAlF₆ crystal that contains 0.04 to 0.16mol%, preferably 0.05 to 0.09mol%, of Ce and 1.1 to 10 ⁶Li atoms per unit volume (in atom/nm³); and a neutron detector and a neutron imaging apparatus which are made using the scintillator.

Description

Neutron detection scintillator, neutron detector and neutron imaging device

The present invention relates to a scintillator (Neutron Scintillator) for neutron detection for use in the detection of neutron radiation (Neutron), detail lithium fluoride calcium aluminum which contains cerium (Ce) is (LiCaAlF ₆₎ scintillator for neutron detection of crystalline, And a neutron detector or neutron imaging apparatus using the crystal.

A scintillator is a substance that absorbs the radiation and emits fluorescence when irradiated with radiation such as α-rays, β-rays, γ-rays, X-rays, and neutrons, and is a photomultiplier tube (PMT). It is used for radiation detection by combining with a photodetector. It has various application fields such as medical field such as tomography, industrial field such as non-destructive inspection, security field such as inventory inspection, academic field such as high energy physics.
As this scintillator, there are various types of scintillators depending on the type of radiation and purpose of use, and inorganic such as bismuth germanium oxide (Bi ₄ Ge ₃ O ₁₂ ) and cerium-containing gadolinium silicon oxide (Gd ₂ SiO ₅ : Ce). Crystals; organic crystals such as anthracene; high molecular weight materials such as polystyrene and polyvinyltoluene containing organic phosphors; liquid scintillators; gas scintillators and the like.

Conventional neutron detection has used neutron detectors using ³ He gas, but due to the rising price of rare ³ He gas, replacement with alternative technology is required. A neutron beam detector using a solid neutron detection scintillator is one of the promising alternatives.
Typical characteristics required of scintillators include high light emission (Light Yield), high radiation stopping power, and fast fluorescence decay. In particular, in scintillators that detect neutrons, neutrons and absorbing substances Since a radiation capture reaction occurs between the γ-ray and γ-rays easily, discrimination ability from the γ-rays is required.
The amount of luminescence in the present invention is a technical term used in the scintillator field, and the total number of photons in a single luminescence of the scintillator by radiation excitation is divided by the energy of the radiation of the excitation source. Indicates the value. The unit is, for example, photons / MeV for γ-ray and α-ray excitation, and photons / neutron for neutron beam excitation.

As a solid scintillator for detecting neutrons, a ⁶ Li glass scintillator has been used as a material having no deliquescence and high-speed response. However, it is expensive due to the complicated manufacturing process, and there is a limit to enlargement. It was. In contrast, a neutron detection scintillator made of a fluoride crystal has an advantage that a large scintillator can be manufactured at a low cost. For example, a scintillator made of lithium barium fluoride (LiBaF ₃ ) crystal has been proposed. However, since the scintillator is highly sensitive to γ rays and has a large background noise derived from γ rays, special measures have to be taken when used as a scintillator for neutron detection (see Non-Patent Document 1).

In order to solve this problem, the present inventors performed evaluation by irradiating a neutron beam for some crystals containing LiCaAlF ₆ containing Ce in order to try application as a scintillator for detecting neutrons. As a result, 1.1 to 20 atoms (atom / nm ³ ) ⁶ Li per unit volume was contained in the fluoride single crystal, and the Ce content in the LiCaAlF ₆ crystal was 0.005 to 5 to 100 mol of Li. It has been found that by making it a mole, it has particularly good characteristics as a scintillator for neutron detection (see Patent Document 1).

However, as for the discrimination ability between neutron rays and γ rays of the LiCaAlF ₆ crystal containing Ce, only the effective atomic number is calculated and the effect of γ ray noise during neutron irradiation is only considered. Therefore, an experiment in which a sufficient dose of γ rays was intentionally irradiated has not been conducted, and the α / γ ratio of the light emission amount has not been investigated so far.
The α / γ ratio of the luminescence amount is a value obtained by dividing the luminescence amount at the time of α-ray excitation by the luminescence amount at the time of γ-ray excitation, and affects the discrimination ability between neutrons and γ rays. This is because CeCa-containing LiCaAlF ₆ crystals generate secondary radiation (primary mechanism) due to the nuclear reaction between incident neutrons and ⁶ Li, followed by electronic transition of Ce ions by this alpha radiation. The light emission is caused by a two-stage mechanism called ultraviolet light emission (secondary mechanism) of about 290 to 310 nm. For this reason, since light emission is finally caused by excitation by α rays, the ratio of the light emission amount at the time of α ray excitation and the light emission amount at the time of γ ray excitation affects the discrimination ability.

There is no generally accepted theoretical formula for predicting the discrimination ability, and it is very difficult to accurately predict the result without experimental examination. Therefore, it was difficult to predict in advance that the discrimination ability of the LiCaAlF ₆ crystal containing Ce was further improved at a specific Ce content.
Further, neutron imaging devices have been studied mainly using a neutron source having a radioactivity that is extremely harmful to a human body such as a nuclear reactor so far, and is not highly versatile. It is expected to spread more application range, consider an example of radioactivity less neutron source (e.g. ^{252 Cf} sealed source of) the imaging possible neutron ray imaging apparatus is in a relatively small situations, ³ the He gas system The neutron imaging apparatus using the solid neutron detection scintillator with good discrimination ability, which is an alternative candidate, is industrially valuable. In the present invention, a substance made of a substance that emits fluorescence when a neutron beam collides is referred to as a neutron detection scintillator.

International Publication No. 2009/119378 Pamphlet

C. W. E. Van Eijk et al “LiBaF3, a thermal neutron scintillator with optimal n-γ discriminating” Nuclar Instruments and Methods in Physics19Research19.

To provide a neutron detection scintillator suitable for use in a neutron scintillation detection apparatus that has high sensitivity to neutron rays and low background noise derived from γ rays, that is, low sensitivity to γ rays. Objective.

The inventors of the present invention prepared CeCa-containing LiCaAlF ₆ crystals with various compositions and evaluated the discriminating ability between neutron rays and γ rays. The amount of luminescence was measured and compared. As a result, it was found that good discrimination ability was obtained when a specific amount of Ce was contained.
Furthermore, it has been found that the crystal of the present invention can be operated as a neutron detector by combining with a photomultiplier tube, and can be operated as a neutron imaging device by combining with a position sensitive photomultiplier tube (position sensitive PMT). It came to complete.

That is, the present invention relates to a scintillator for neutron detection comprising a LiCaAlF ₆ single crystal containing 0.04 to 0.16 mol% of Ce and containing 1.1 to 10 atoms (atom / nm ³ ) of ⁶ Li per unit volume. It is.
Another invention is a neutron beam detector characterized by combining the scintillator and a photomultiplier tube.
Furthermore, another invention is a neutron beam imaging apparatus characterized by combining the scintillator and a position sensitive photomultiplier tube.

The LiCaAlF ₆ crystal containing Ce of the present invention can be a scintillator for detecting neutrons with less background noise derived from γ rays than before. This scintillator is useful as a neutron detector that can be used for applications such as determining the presence or absence of neutrons in the environment by combining a photomultiplier tube. Combining the scintillator and a position-sensitive photomultiplier tube The neutron imager can be suitably used in non-destructive inspection using the imager.

It is the schematic of the crystal manufacturing apparatus by the micro pulling-down method used for manufacture of the scintillator of this invention. It is a wave height distribution spectrum figure at the time of ²⁴¹ Am, ¹³⁷ Cs irradiation of the scintillator for neutron detection of Example 1. FIG. It is a wave height distribution spectrum figure at the time of ²⁴¹ Am and ¹³⁷ Cs irradiation of the scintillator for neutron detection of Example 2. FIG. It is a wave height distribution spectrum figure at the time of ²⁴¹ Am, ¹³⁷ Cs irradiation of the scintillator for neutron detection of Example 3. FIG. It is a wave height distribution spectrum figure at the time of ²⁴¹ Am and ¹³⁷ Cs irradiation of the scintillator for neutron detection of the reference example 1. FIG. It is the schematic which shows the neutron beam detector of this invention. It is a wave height distribution spectrum figure at the time of ²⁵² Cf irradiation by the neutron beam detector of this invention. It is the schematic which shows the neutron beam imaging device of this invention. It is a schematic diagram for demonstrating the operation | movement test method of the neutron beam imaging device of this invention. It is a neutron imaging image by ²⁵² Cf irradiation by the neutron beam imaging device of this invention and Example 5. FIG. It is a neutron imaging figure by ²⁵² Cf irradiation by the neutron beam imaging device of the present invention and Example 6.

The scintillator for neutron detection of the present invention is a LiCaAlF ₆ crystal containing Ce, and has a specific feature of having a specific Ce content and a ⁶ Li content.
In the present invention, the ⁶ Li content refers to the number of Li elements contained per 1 nm ³ of the scintillator. The incident neutron causes a nuclear reaction with the ⁶ Li to generate α rays. Therefore, the ⁶ Li content affects the sensitivity to neutron beams, and the sensitivity to neutron beams increases as the ⁶ Li content increases.

Such ⁶ Li content, select the chemical composition of the scintillator for neutron detection, also, it can be appropriately adjusted by adjusting the ⁶ Li content of LiF or the like used as the Li raw material. Here, the ⁶ Li content is the elemental ratio of ⁶ Li isotopes with respect to the total Li elements, and is about 7.6% for natural Li. As a method of adjusting the ⁶ Li content is as a starting material a general purpose material having a natural isotopic ratio, a method of adjusting by concentrating the ⁶ Li isotope to the desired ⁶ Li content or advance ⁶ Li is There is a method in which a concentrated raw material concentrated to a desired ⁶ Li content or more is prepared, and the concentrated raw material and the general-purpose raw material are mixed and adjusted.

In the present invention, the content of ⁶ Li needs to be 1.1 atom / nm ³ or more. By setting the ⁶ Li content to 1.1 atom / nm ³ or more, sufficient sensitivity to neutron radiation is obtained. This content can be achieved by selecting the type of metal fluoride crystal without using a Li raw material with a specially increased ⁶ Li content, so that a neutron detection scintillator can be provided at low cost. To further increase the sensitivity to neutron rays, it is particularly preferable to the ⁶ Li content 2.9atom / nm ³ or more.
On the other hand, the upper limit of the ⁶ Li content is 10 atoms / nm ³ . The ⁶ Li content in the LiCaAlF ₆ crystal containing Ce is about 10 atom / nm ³ at the maximum in the calculation, and a ⁶ Li content higher than this cannot be obtained.

The above ⁶ Li content can be determined by the following equation (1).
⁶ Li content = A × C × ρ × 10 ⁻²¹ / M [1]
(Wherein ρ is the density [g / cm ³ ] of the LiCaAlF ₆ crystal containing Ce, M is the molecular weight [g / mol], C is the ⁶ Li content in the Li element [%], and A is the Avogadro number [6.02 × 10 ²³ ] is shown.)

When the scintillator is irradiated with α rays as the secondary radiation, Ce of the scintillator is excited and emits light. The light emission is light emission due to the electron orbital transition of Ce, and in particular, a scintillator with a short fluorescence lifetime and thus excellent high-speed response can be obtained.

The range of the Ce content contained in the LiCaAlF ₆ crystal is the greatest feature of the present invention, and is 0.04 to 0.16 mol% with respect to LiCaAlF ₆ . Originally, when the Ce content increases, the effective atomic number increases, and therefore background noise derived from γ rays tends to increase (Patent Document 1). However, in a LiCaAlF ₆ crystal containing Ce, The higher the Ce content, the lower the amount of light emitted at the time of γ-ray incidence relative to the amount of light emitted at the time of neutron beam incidence, and thus the ability to discriminate between a signal from neutron rays and noise from gamma rays. In particular, when the Ce content is 0.04 mol% or more with respect to LiCaAlF ₆ , it is preferable because good discrimination ability is obtained. In particular, by setting the content to 0.05 mol% or more, as a scintillator for detecting LiCaAlF ₆ neutrons containing Ce, an unprecedented high discrimination ability can be obtained. However, if it exceeds 0.16 mol% with respect to LiCaAlF ₆ , white turbidity and cracking occur and it becomes difficult to grow a single crystal. Therefore, it is preferably 0.16 mol% or less.
The existence state of Ce contained in the crystal is not certain, but it is presumed that it exists in place of part of the elements Ca and Al atoms constituting the crystal lattices or between the crystal lattices.

When Ce is contained in the LiCaAlF ₆ crystal manufacturing process described later, a segregation phenomenon of Ce in the crystal may be observed. Even when such a segregation phenomenon is observed, the effective segregation coefficient (k) is obtained in advance, and if the Ce content in the raw material is adjusted based on the following formula [2], Ce having a desired content can be obtained. It is possible to easily obtain the LiCaAlF ₆ crystal contained.
C _s = kC ₀ (1-g) ^k−1 [2]
{Wherein, _{C s} is Ce content of the metal fluoride crystal [mol% (Ce / Ca) ], k is the effective segregation coefficient, _{C 0} is Ce content in the raw material [mol% (Ce / Ca) ] , G represents the solidification rate. }
As the effective segregation coefficient, a value described in literature (for example, Growth of Ce-doped LiCaAlF6 and LiSrAlF6 single crystals by the Czochralski technique under CF4 atmosphere) can be adopted. However, the effective segregation coefficient varies depending on the growth method, and according to the study by the present inventors, the effective segregation coefficient of Ce with respect to LiCaAlF ₆ is 0.02 in the Czochralski method, and is 0.2 in the micro pull-down method. 04.
The Ce content in the actual crystal can be examined by a general elemental analysis technique such as ICP mass spectrometry or ICP atomic emission spectrometry.

The crystal of the present invention is a single crystal or a polycrystal, but by using a single crystal, emission intensity does not occur without causing loss due to non-radiative transition caused by lattice defects or scintillation light dissipation at grain boundaries. A single crystal is preferable because a neutron scintillator with a high neutron can be obtained.
The crystal of the present invention is a transparent crystal that is colorless or slightly colored, and has excellent scintillation light transmission. In addition, it has good chemical stability, and in normal use, no performance degradation is observed in a short period of time. Furthermore, mechanical strength and workability are also good, and it is easy to process and use it in a desired shape.

The method for producing the crystal of the present invention is not particularly limited and can be produced by a known crystal production method, but is produced by the Czochralski method or the micro-pulling-down method. It is preferable. By producing by the Czochralski method or the micro pull-down method, a LiCaAlF ₆ crystal containing Ce having excellent quality such as transparency can be produced. According to the micro-pulling down method, the crystal can be directly manufactured in a specific shape and can be manufactured in a short time. On the other hand, according to the Czochralski method, a large crystal having a diameter of several inches can be manufactured at low cost.

Hereinafter, a general method for producing a metal fluoride crystal by the Czochralski method will be described.
First, a predetermined amount of raw material is filled in a crucible 1. The purity of the following raw materials is not particularly limited, but is preferably 99.99% or more. By using such a high-purity mixed raw material, the purity of the obtained crystal can be increased, and thus characteristics such as emission intensity are improved. The raw material may be a powdery or granular raw material, or may be used after being sintered or melted and solidified in advance.
As a raw material, a metal fluoride of lithium fluoride (LiF), calcium fluoride (CaF ₂ ), aluminum fluoride (AlF ₃ ), or cerium fluoride (CeF ₃ ) is used.

When producing a LiCaAlF ₆ crystal containing Ce by the melt growth method such as the Czochralski method or the micro pull-down method, the mixing ratio of these raw materials is 1: 1 for LiF, CaF ₂ and AlF _3. Weigh to a molar ratio of 1. However, since LiF and AlF ₃ are easy to volatilize, they may be weighed by about 1 to 10%. Since the volatilization amount is completely different depending on crystal growth conditions (temperature, atmosphere, and process), it is desirable to determine the weighing value by examining the volatilization amount of LiF and AlF ₃ in advance. In addition, when holding for a long time at a temperature at which volatilization tends to occur during crystal growth, LiF and AlF ₃ may need to be weighed more than 10%. The volatilization of CaF ₂ and CeF ₃ hardly poses a problem under normal LiCaAlF ₆ crystal growth conditions.

The amount of CeF ₃ determines the mixing ratio of the raw materials using the effective segregation coefficient as described above in consideration of the segregation phenomenon of Ce. At that time, the mixing ratio of the raw materials is calculated so that Ce is contained in the range of 0.04 to 0.16 mol% with respect to LiCaAlF ₆ . Crystals having good discrimination ability can be obtained by adjusting the content to 0.04% mol or more, and crystals having no cloudiness or cracks can be produced by adjusting the content to 0.16 mol% or less.

Next, the crucible 1, the heater 2, the heat insulating material 3, and the movable stage 4 filled with the raw materials are set as shown in FIG. On the crucible 1, another crucible with a hole at the bottom may be installed, and fixed to the heater 2 or the like, and hung with a double crucible structure. The seed crystal 5 is attached to the tip of the automatic diameter control device 6.
The seed crystal may be a high melting point metal such as platinum, but the LiCaAlF ₆ single crystal or a single crystal having a crystal structure close to it has a crystallinity of the grown crystal. It tends to be good. For example, a LiCaAlF ₆ single crystal having a rectangular parallelepiped shape having a size of about 6 × 6 × 30 mm ³ and cut, ground, and polished so that a side of 30 mm is along the c-axis direction can be used.
The automatic diameter control device measures the total weight of the seed crystal and the grown crystal, adjusts the growth rate of the seed crystal from the information, and can control the diameter of the crystal to be grown. . As the apparatus, a load cell for a pulling apparatus that is commercially available for crystal growth of the Chocrasky method can be used.

Next, the inside of the chamber 7 is evacuated to 1.0 × 10 ⁻³ Pa or less using an evacuation apparatus, and then an inert gas such as high purity argon is introduced into the chamber to perform gas replacement. The pressure in the chamber after gas replacement is not particularly limited, but atmospheric pressure is common. By this gas replacement operation, the water adhering to the raw material or the chamber can be removed, and the deterioration of the crystal derived from the water can be prevented.
In order to avoid the adverse effects of moisture that cannot be removed even by the gas replacement operation, it is preferable to use a solid scavenger such as zinc fluoride or a gas scavenger such as tetrafluoromethane. When using a solid scavenger, a method of mixing in the raw material in advance is preferable, and when using a gas scavenger, a method of mixing with the above inert gas and introducing it into the chamber is preferable.

After performing the gas replacement operation, the raw material is heated and melted by the high frequency coil 8 and the heater 2. The heating method is not particularly limited, and for example, a resistance heating type carbon heater or the like can be appropriately used instead of the configuration of the high frequency coil and the heater.
Next, the melted raw material melt is brought into contact with the seed crystal. After adjusting the heater output so that the temperature at which the portion in contact with the seed crystal is solidified, the crystal is pulled up while automatically adjusting the pulling speed under the control of the automatic diameter control device 6. During the growth, the movable stage 4 may be appropriately moved in the vertical direction in order to adjust the liquid level. Containing Ce by continuously pulling up while adjusting the output of the high-frequency coil appropriately, separating from the liquid surface when the desired length is reached, and cooling for a sufficient amount of time so that cracks do not occur in the grown crystal A single crystal of LiCaAlF ₆ can be obtained.

Annealing treatment may be performed on the grown crystal for the purpose of removing crystal defects caused by fluorine atom defects or thermal strain.
The obtained Ce-containing LiCaAlF ₆ crystal has good workability and can be easily processed into a desired shape. For processing, a known cutting machine such as a blade saw or wire saw, a grinding machine, or a polishing machine can be used without any limitation.

The scintillator of the present invention can be combined with a photomultiplier tube to form a neutron detector. That is, the light (scintillation light) emitted from the neutron detection scintillator by the irradiation of the neutron beam is converted into an electric signal by a photomultiplier tube, so that the presence and intensity of the neutron beam can be grasped as an electric signal. it can. The scintillation light emitted from the scintillator of the present invention is light having a wavelength of about 290 to 310 nm, and a photomultiplier tube capable of detecting light in this region can be preferably used. Specific examples of such photomultiplier tubes include R7600U and H7416 manufactured by Hamamatsu Photonics.

Specifically, for example, a LiCaAlF ₆ crystal block containing Ce is bonded to the light entrance window of a photomultiplier tube with optical grease or the like, and a high voltage is applied to the photomultiplier tube. Thus, there is a method of observing an electric signal output from the photomultiplier tube. For the purpose of analyzing the intensity of the neutron beam using the electrical signal output from the photomultiplier tube, a shaping amplifier, a multi-channel analyzer, etc. are provided after the photomultiplier tube. May be provided.
Further, as a photodetector, a position sensitive photomultiplier tube in which detectors having a sensitive area of several mm square are arranged in an array is used, and Ce having a size covering a part or all of the light incident window. By joining LiCaAlF ₆ crystals containing neutrons, the neutron beam imaging apparatus of the present invention can be obtained.

As the position sensitive photomultiplier tube, one capable of detecting light having a wavelength of about 290 to 310 nm, which is scintillation light emitted from the scintillator of the present invention (for example, XP85012 manufactured by PHOTONIS) is used. Optical grease or the like may be used for joining the light incident window and the crystal. The crystal may have any shape, and can be a scintillator array in which plate-like, block-like, or quadrangular prism-like crystals are regularly arranged.
Further, the surface other than the incident surface of the neutron beam may be covered with a cadmium plate, a LiF block or the like so that the neutron beam does not enter from the surroundings.
The signal output from the position-sensitive photomultiplier tube can be read out using any reading device, and may be controlled using a control program on a control personal computer.

Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited to these examples. In addition, not all combinations of features described in the embodiments are essential to the solution means of the present invention.

Examples 1 to 3
[Manufacture of neutron detection scintillators]
Hereinafter, the production method will be described with respect to Example 1, but Examples 2 and 3 were produced in the same manner except that the raw material weighing values were different.
A LiCaAlF ₆ crystal containing Ce was manufactured using the crystal manufacturing apparatus by the Czochralski method shown in FIG. As a raw material, high purity fluoride powder of LiF, CaF ₂ , AlF ₃ , and CeF ₃ having a purity of 99.99% or more was used. Incidentally, LiF ^is, 6 Li content was used for 50%. The crucible 1, the heater 2, and the heat insulating material 3 were made of high purity carbon.

First, as shown in Table 1, each material was weighed and mixed raw materials obtained by well mixing were filled in the crucible 1. The crucible 1 filled with the raw material was placed on the movable stage 4, and the heater 2 and the heat insulating material 3 were sequentially set around the crucible 1. Next, a LiCaAlF ₆ single crystal was cut, ground and polished so that a side of 30 mm along the c-axis direction with a 6 × 6 × 30 mm ³ rectangular parallelepiped shape was used as a seed crystal 5 and attached to the tip of an automatic diameter controller. .
The inside of the chamber 6 is evacuated to 5.0 × 10 ⁻⁴ Pa by using an evacuation device comprising an oil rotary pump and an oil diffusion pump, and then a tetrafluoromethane-argon mixed gas is brought into the chamber 7 to atmospheric pressure. The gas was replaced.

A high frequency current was applied to the high frequency coil 8, and the raw material was heated and melted by induction heating. The seed crystal 5 was moved and brought into contact with the liquid surface of the melted raw material melt. After adjusting the heater output so that the temperature at which the portion in contact with the seed crystal is solidified, the crystal was pulled up under the control of the automatic diameter controller 6 while automatically adjusting the pulling speed with a diameter of 55 mm as a target.
During the growth, the movable stage 4 is appropriately moved in order to adjust the liquid level to be constant, and continuously lifted while appropriately adjusting the output of the high frequency coil, and separated from the liquid level when the length becomes about 80 mm. By cooling over 48 hours, a LiCaAlF ₆ crystal containing Ce having a diameter of 55 mm and a length of about 80 mm was obtained.

The obtained crystal was cut with a wire saw equipped with a diamond wire, ground and mirror-polished, and processed into a shape having a length of 7 mm, a width of 2 mm, and a thickness of 1 mm to obtain a neutron scintillator of the present invention. In the neutron detection scintillators of Examples 1 to 3, the Ce content was about 0.08, 0.06, and 0.04 mol%, respectively, with respect to LiCaAlF _{6 according} to the formula [2].
The ⁶ Li content of the Li raw material of the neutron detection scintillators of Examples 1 to 3 was 50%. Therefore, from the formula [1], the ⁶ Li content was 5.1 atom / nm ³ .
As the neutron detection scintillator of Reference Example 1, a commercially available ⁶ Li glass scintillator GS20 (manufactured by Saint-Gobain; length 7 mm, width 2 mm, thickness 1 mm) was used.

[Comparison of light emission during excitation of α-rays and γ-rays]
In order to investigate the neutron ray and γ ray discrimination ability of the neutron detection scintillators of Examples 1 to 3 and Reference Example 1, the amount of luminescence upon excitation of α ray and γ ray was evaluated by the following method.
First, a 7 mm long and 2 mm wide surface of a scintillator for neutron detection was bonded to the light incident window of a photomultiplier tube (R7600U manufactured by Hamamatsu Photonics) with optical grease.
Next, in the case of evaluation by α-ray, ²⁴¹ Am having a radioactivity of 4 MBq was arranged in the immediate vicinity of the scintillator, and was shielded by a light-shielding sheet so that external light did not enter when irradiated with α-ray. In the case of the evaluation by γ-rays, ¹³⁷ Cs having a radioactivity of 1 kBq was placed at a position of about 30 mm from the scintillator after being shielded by a light-shielding sheet so that light from the outside did not enter, and γ-rays were irradiated.

In order to measure the scintillation light emitted from the scintillator, the scintillation light was converted into an electric signal through a photomultiplier tube to which a high voltage of 800 V was applied. Here, the electrical signal output from the photomultiplier tube is a pulse signal reflecting the scintillation light, the pulse height represents the emission intensity of the scintillation light, and the waveform is the decay time constant of the scintillation light. Presents an attenuation curve based on The electric signal output from the photomultiplier tube was shaped and amplified by a shaping amplifier in this way, and then input to a multi-wave height analyzer for analysis to create a pulse height spectrum.

FIGS. 2, 3, 4 and 5 show the wave height distribution spectra obtained for the neutron detection scintillators of Examples 1, 2, 3 and Reference Example 1, respectively. The horizontal axis of the wave height distribution spectrum represents the wave height value of the electric signal, that is, the emission intensity of the scintillation light. Further, the vertical axis represents the frequency of the electrical signal indicating each peak value, and here, the frequency is represented by the number of times the electrical signal is measured (counts).

In the ⁶ Li glass scintillator of Reference Example 1, although a photoelectric absorption peak was observed in the vicinity of the peak value of 0.5 in FIG. 5 by excitation with ¹³⁷ Cs, the implementation was made of a LiCaAlF ₆ crystal having an effective atomic number lower than that of the ⁶ Li glass scintillator. In the neutron detection scintillators of Examples 1 to 3, the photoelectric absorption peak was not observed, and the amount of luminescence at the time of γ-ray excitation could not be determined accurately, so it was difficult to calculate the α / γ ratio. Therefore, the peak value at the time of ²⁴¹ Am α-ray excitation was set to 1, and the maximum value of the peak values in the wave height distribution at the time of ¹³⁷ Cs γ-ray excitation was compared. The lower this value, the lower the crest value of γ-ray noise. Therefore, γ-ray noise can be removed by setting a threshold and removing low crest value signals and selecting high crest value signals. Becomes easy.

The maximum values of the crest values of the neutron detection scintillators of Examples 1, 2, 3 and Reference Example 1 when excited with ¹³⁷ Cs by γ-ray were 0.34, 0.44, 0.56, and 0.58, respectively. became. From Examples 1, 2, and 3, it can be seen that the CeCa-containing LiCaAlF ₆ single crystal decreases as the Ce content increases, making it easier to remove γ-ray noise. Further, any of Examples 1 to 3 is a neutron detection scintillator having a value that is lower than the value of 0.58 of the ⁶ Li glass scintillator of Reference Example 1 and having good discrimination ability. Among them, especially when the Ce content is 0.06 and 0.08 mol%, the discrimination ability is good, and if it is 0.05 mol% or more, it is clearly superior to Reference Example 1 ⁶ Li glass scintillator. Discrimination ability is obtained.

Example 4
[Production of neutron detector]
Using the neutron detection scintillator of Example 2, a neutron detector comprising a combination of a neutron detection scintillator made of a LiCaAlF ₆ crystal containing Ce and a photomultiplier tube was produced as follows.
FIG. 6 shows the configuration of the neutron beam detector of the present invention. R7600U made by Hamamatsu Photonics Co. is used for the photomultiplier tube 9, and the 7 mm long and 2 mm wide surface of the neutron detection scintillator of Example 2 is used as the neutron detection scintillator with respect to the light incident window of the photomultiplier tube And bonded with optical grease.

After the neutron detector is covered with a black vinyl sheet shading material 10 so that outside light does not enter the incident window, a neutron beam from a ²⁵² Cf sealed radiation source having a radioactivity of 3.7 MBq or less is about 40 mm. Irradiated with a thermal block through a polyethylene block of a thickness of.
In order to measure the scintillation light emitted from the scintillator, a high voltage of 800 V was applied to the photomultiplier tube 9 from the power supply line to convert the scintillation light into an electric signal and output from the signal output line. Here, the electrical signal output from the photomultiplier tube is a pulse signal reflecting the scintillation light, the pulse height represents the emission intensity of the scintillation light, and the waveform is the decay time constant of the scintillation light. Presents an attenuation curve based on In this way, the electric signal output from the photomultiplier tube was shaped and amplified by the shaping amplifier, and then input to the multiple wave height analyzer for analysis to create a wave height distribution spectrum.

The obtained wave height distribution spectrum is shown in FIG. From FIG. 7, a clear peak indicating that a neutron beam has been detected can be confirmed, and it can be seen that the device is operating as a neutron beam detector. The horizontal axis of the wave height distribution spectrum represents the wave height value of the electric signal, that is, the emission intensity of the scintillation light. Here, the wave height distribution spectrum is shown as a relative value where the peak value of the wave height distribution spectrum is 1. Further, the vertical axis represents the frequency of the electrical signal indicating each peak value, and here, the frequency is represented by the number of times the electrical signal is measured (counts).

Examples 5 and 6
[Production of neutron imaging system]
A LiCaAlF ₆ crystal containing Ce was used as a neutron detection scintillator, and a neutron imaging apparatus combined with a position sensitive photomultiplier tube was produced as follows.
FIG. 8 shows the configuration of the neutron imaging apparatus of the present invention. As the neutron detection scintillator unit 11, the neutron beam imaging devices of Examples 5 and 6 processed into different shapes were used. The crystal used as the base of the neutron detection scintillator unit 11 is obtained by cutting the crystal used for the production of the neutron detection scintillator of Example 2 with a wire saw equipped with a diamond wire, grinding and mirror polishing. Using.

As the neutron detection scintillator of the neutron beam imaging apparatus of Example 5, a scintillator plate processed into a shape having a diameter of 55 mm and a thickness of 2 mm was used.
As the neutron detection scintillator of the neutron imaging apparatus of Example 6, 400 pieces processed to 2 × 2 × 4 mm ³ were prepared, and 20 × 20 2 × 2 mm ² surfaces were arranged in rows and columns in an array form. A scintillator array was used in which crystals were joined using barium sulfate having a high reflectance as a joining material.
The scintillator plate and the scintillator array were bonded to the light incident window of the position sensitive photomultiplier tube 12 using optical grease, thereby obtaining the neutron beam imaging apparatus of Examples 5 and 6. As the position-sensitive photomultiplier tube 12, XP85012 made by PHOTONIS of 64 channels was used.

The position sensitive photomultiplier tube 12 was connected to the head amplifier unit 13 and then covered with a black vinyl sheet light shielding material 10 so that light from the outside did not enter the light incident window. For the head amplifier unit 13, an 80190 type multi-anode photomultiplier head amplifier unit manufactured by Clear Pulse is used, and connected to a personal computer from the signal line via an interface device (80190 type PCIF manufactured by Clear Pulse). It was operated using the company's control program so that the peak values of each of the 64 channels could be acquired every hour.

Using the neutron imaging apparatus of Examples 5 and 6 in which signals can be read in this way, neutron transmission images were taken. Hereinafter, a description will be given according to the schematic diagram shown in FIG.
In Examples 5 and 6, neutron beams from a ²⁵² Cf sealed radiation source 14 having a radioactivity of 3.7 MBq or less are passed through a polyethylene block 15 having a thickness of about 40 mm with respect to the neutron detection scintillator unit 11 of FIG. And irradiated with thermal neutrons.

A lead block 16 having a thickness of 50 mm was installed between Examples 5 and 6 and the polyethylene block 15 for the purpose of removing γ rays. Further, in order to prevent the neutron beam from being incident from an unexpected direction, the periphery of Examples 5 and 6 was covered with a LiF block 17 that hardly transmits the neutron beam except for the surface irradiated with the neutron beam.
At this time, in order to confirm that a neutron transmission image can be taken, a T-shaped cadmium plate 18 having a thickness of 4 mm was placed on the surface irradiated with the neutron beam of Examples 5 and 6. Since cadmium absorbs neutrons well, when irradiated with neutrons, the portion of the T-shaped cadmium plate 18 does not transmit neutrons and a T-shaped shadow should be imaged.

Next, a voltage of 2200 V was applied to operate the position sensitive photomultiplier tube 12 and the head amplifier unit 13. Data was recorded when a peak value signal exceeding the value set as the threshold value was output from any of the 64 channels. The threshold value was set for the purpose of acquiring as little as possible a signal due to the background noise of the device that occurs even when the neutron detection scintillator unit 11 does not emit light.
Note that the integration time of one peak value is 5 microseconds, and the 64 channel signals obtained during that time are called events. The recorded data were the serial number of the event and the information of the peak value of each of the 64 channels (8 × 8) in the event.

In order to further select only neutron radiation events from the obtained data, the total value of the peak values of 64 channels for each event was calculated, and a histogram of the total value (frequency distribution diagram) was created. In the histogram, a peak was observed in a portion where the total value was high. It was considered that the event of the total value corresponding to the peak portion was caused by the neutron beam and was adopted as an event for imaging described later.

For the purpose of imaging the adopted data, the following calculation processing was performed.
Arbitrary points were selected from the four corners of the 64 channels, and 8 × 8 coordinates (X _A , Y _A ) were determined. Next, the charge centroid (X _D , Y _D ) for each event was calculated from the coordinate value thus determined and the measured peak value.
The charge centroid are coordinates corresponding to the center of gravity of the group of electrons generated by the photoelectric surface of the position-sensitive photomultiplier tube (photocathode), in each occurrence of the 64 channels, with a value and its position of X _A the sum of all 64 channels of a value obtained by multiplying the peak value, divided by the sum of the peak values of all 64 channels is X _D. The value of Y _D is obtained by the same calculation with respect to Y _A. The calculated values of X _D and Y _D were each further multiplied by 8 and rounded off to convert to 64 × 64 coordinates.

For each event, for the coordinates obtained by performing the above calculation, the number of times the coordinates were derived by the calculation was tabulated. The X-axis and Y-axis were used as position information (unit: mm), and the figure expressed by the color shading was drawn with the places where the number of times was large and the places where the number was small being white and black, respectively.
Neutron transmission images captured by the neutron beam imaging apparatuses of Examples 5 and 6 are shown in FIGS. In any case, the shadow of the T-shaped cadmium plate can be imaged, and in the neutron beam imaging apparatus of Example 6, the shadow of the junction of the scintillator array (not reacting to the neutron beam) can also be imaged. .
10 and 11, it can be seen that Examples 5 and 6 are highly sensitive neutron imaging devices capable of imaging with a sealed radiation source having a relatively low activity.

DESCRIPTION OF SYMBOLS 1 Crucible 2 Heater 3 Heat insulating material 4 Movable stage 5 Seed crystal 6 Automatic diameter control device 7 Chamber 8 High frequency coil 9 Photomultiplier tube 10 Shading material 11 Neutron detection scintillator part 12 Position sensitive photomultiplier tube 13 Head amplifier unit 14 ²⁵² Cf sealed radiation source 15 Polyethylene block 16 Lead block 17 LiF block 18 Cadmium plate

Claims (4)

A scintillator for neutron detection, comprising a LiCaAlF ₆ single crystal containing 0.04 to 0.16 mol% of Ce and containing 1.1 to 10 atoms (atom / nm ³ ) of ⁶ Li per unit volume. 2. The scintillator for neutron detection according to claim 1, containing 0.05 to 0.09 mol% of Ce. A neutron beam detector comprising a combination of the neutron detection scintillator according to claim 1 and a photomultiplier tube. A neutron beam imaging apparatus comprising a combination of the neutron detection scintillator according to claim 1 and a position sensitive photomultiplier tube.

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