WO2009119378A1 - 中性子検出用シンチレーターおよび中性子検出装置 - Google Patents
中性子検出用シンチレーターおよび中性子検出装置 Download PDFInfo
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- WO2009119378A1 WO2009119378A1 PCT/JP2009/055122 JP2009055122W WO2009119378A1 WO 2009119378 A1 WO2009119378 A1 WO 2009119378A1 JP 2009055122 W JP2009055122 W JP 2009055122W WO 2009119378 A1 WO2009119378 A1 WO 2009119378A1
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- C—CHEMISTRY; METALLURGY
- C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
- C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
- C09K11/00—Luminescent, e.g. electroluminescent, chemiluminescent materials
- C09K11/08—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
- G01T3/00—Measuring neutron radiation
- G01T3/06—Measuring neutron radiation with scintillation detectors
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- C—CHEMISTRY; METALLURGY
- C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
- C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
- C09K11/00—Luminescent, e.g. electroluminescent, chemiluminescent materials
- C09K11/08—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials
- C09K11/64—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials containing aluminium
- C09K11/644—Halogenides
- C09K11/645—Halogenides with alkali or alkaline earth metals
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- G—PHYSICS
- G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
- G21K—TECHNIQUES FOR HANDLING PARTICLES OR IONISING RADIATION NOT OTHERWISE PROVIDED FOR; IRRADIATION DEVICES; GAMMA RAY OR X-RAY MICROSCOPES
- G21K4/00—Conversion screens for the conversion of the spatial distribution of X-rays or particle radiation into visible images, e.g. fluoroscopic screens
Definitions
- the present invention relates to a neutron detection scintillator used for neutron detection, and more particularly to a neutron detection scintillator made of a metal fluoride crystal containing a lanthanoid.
- a scintillator is a substance that absorbs radiation and emits fluorescence when it is exposed to radiation such as ⁇ rays, ⁇ rays, ⁇ rays, X rays, neutron rays, etc. Used in combination with radiation detection, it has various application fields such as medical fields such as tomography, industrial fields such as non-destructive inspection, security fields such as personal belonging inspection, and academic fields such as high energy physics. . As this scintillator, there are various types of scintillators depending on the type of radiation and purpose of use, such as inorganic crystals such as Bi 4 Ge 3 O 12 , Gd 2 SiO 5 : Ce, PbWO 4 , CsI, KI, anthracene, etc.
- liquid crystal scintillators and gas scintillators as polymer materials such as polystyrene and polyvinyltoluene containing organic crystals and organic phosphors, and special ones.
- neutrons are targeted for detection, neutrons are detected using a nuclear reaction that quickly converts energy into charged particles because neutrons have a strong force to penetrate without any interaction in the material. .
- Typical characteristics required for scintillators include large light intensity (emission intensity), high radiation stopping power (detection efficiency), fast fluorescence decay (fast response), etc.
- ⁇ -rays are likely to be generated due to a radiation capture reaction between the neutrons and the absorbing material, so that discrimination ability from these ⁇ -rays is required.
- the present inventors have attempted to apply a cerium-doped LiCaAlF 6 crystal or the like as a scintillator for detecting neutrons with simulated scintillation characteristics under ultraviolet excitation light emission or ⁇ -ray irradiation. Evaluation has been conducted.
- the crystal generates ⁇ -rays (primary mechanism) as secondary radiation by the reaction between incident neutrons and 6 Li (primary mechanism), and then emits ultraviolet rays of about 290 nm due to electron transition of Ce ions by the ⁇ -rays (secondary). Light emission occurs by a two-stage mechanism called “next mechanism”.
- the neutron beam itself has not been irradiated in the evaluation so far, the true characteristics as a scintillator for neutron detection are evaluated, such as the evaluation of the primary mechanism, that is, the evaluation of the efficiency of ⁇ -ray generation is not made at all. It was not reached.
- the optimal composition as a scintillator for neutron detection has not yet been identified (see Non-Patent Document 2).
- the 6 Li content (described later) in the crystal used in the above-mentioned known technology is only about 0.73 atom / nm 3 , and it is shown that the neutron detection efficiency is insufficient at this level. The inventor confirmed himself.
- a neutron detection scintillator is made of a substance that absorbs the neutron and emits fluorescence when it hits the neutron.
- Non-Patent Document 1 CWE van Eijk et al, “LiBaF 3 , a thermal neutron scintillator with optimal n-ganma discrimination” Nuclar Instruments and Methods in Physics Research A 374 (1996) 197-201.
- Non-Patent Document 2 Kentaro Fukuda, Kenji Aoki, Akira Yoshikawa, Yoshio Fukuda, Proceedings of the 66th Japan Society of Applied Physics, NO.1, P.211 (2005)
- a neutron detection scintillator suitable for use in a neutron scintillation detection apparatus having high sensitivity to neutron radiation and low background noise derived from ⁇ rays, that is, low sensitivity to ⁇ rays.
- the present inventors produced various metal fluoride crystals and evaluated the performance as a neutron detection scintillator by a neutron irradiation experiment.
- the metal fluoride crystals having a 6 Li content and effective atomic number of the specified range, sensitivity to neutron rays is high and can be a background noise is less high-performance scintillator for neutron detection derived from ⁇ rays
- the present invention has been completed.
- a metal fluoride crystal containing lithium, a metal element having a valence of II or more as a constituent element, and containing 6 Li of 1.1 to 20 atoms (atom / nm 3 ) per unit volume, and an effective atomic number is provided.
- a scintillator for detecting neutrons characterized in that the scintillator comprises 10 to 40 and is composed of the metal fluoride crystal containing a lanthanoid.
- the neutron detection scintillator (1) containing 2.9 to 20 atoms (atom / nm 3 ) 6 Li per unit volume, and / or (2)
- the effective atomic number is 10 to 30 and / or (3)
- the lanthanoid is preferably cerium, praseodymium or europium.
- a neutron detection apparatus comprising the neutron detection scintillator and a photodetector that detects light emitted from the neutron detection scintillator and converts it into an electrical signal.
- the photodetector is a photomultiplier tube.
- a raw material mixture composed of lithium fluoride with a Li content of 20% or more, fluoride of a metal element with a valence of II or higher, and a lanthanoid fluoride is melted to form a raw material melt.
- a method for producing a metal fluoride crystal for a scintillator for neutron detection, characterized by growing a single crystal, is provided.
- the above-mentioned production method is based on lithium calcium aluminum fluoride ⁇ Lithium Calcium Aluminum Fluoride, M: LiCaAlF 6 (M is a lanthanoid) ⁇ crystal or lanthanoid-containing lithium yttrium fluoride ⁇ Lithium Yttrium Fluoride, M: LiYF 4 ( M is preferably used for the production of lanthanoid) ⁇ crystals.
- the scintillator of the present invention has high sensitivity to neutron rays and low background noise derived from ⁇ rays. Furthermore, it is excellent also in the permeability
- Such a scintillator is useful as a scintillator for neutron detection, and can be suitably used in industrial fields such as various nondestructive inspections and security fields such as personal belongings inspections.
- the scintillator for neutron detection of the present invention is a metal fluoride crystal containing lithium, a metal element having a valence of II or more, and fluorine, and is made of a metal fluoride crystal containing a lanthanoid.
- the metal fluoride crystal is a metal fluoride crystal containing lithium, has a specific 6 Li content and effective atomic number, and the greatest feature in that it contains a lanthanoid.
- the term “metal fluoride crystal” refers to a crystal containing all the above components
- matrix metal fluoride crystal refers to a crystal composed of components other than lanthanoids.
- 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 this 6 Li to generate ⁇ rays that are secondary radiation. 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, can be appropriately adjusted by adjusting the 6 Li content such as lithium fluoride (LiF) is used as the lithium material.
- the 6 Li content is an element ratio of 6 Li isotopes to all lithium elements, and is about 7.6% for natural lithium.
- 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.
- the content of 6 Li needs to be 1.1 atom / nm 3 or more.
- the 6 Li content can be achieved by selecting the type of metal fluoride crystal without using a lithium raw material with a specially increased 6 Li content, so that a neutron detection scintillator can be provided at a low cost.
- This sensitivity to neutrons can be expressed by the peak area in the pulse height distribution spectrum shown in FIG. 1, and the area (cps) is referred to as neutron detection efficiency. The larger this value, the higher the sensitivity to neutrons.
- the upper limit of the 6 Li content is 20 atoms / nm 3 .
- the 6 Li content exceeding 20 atoms / nm 3 it is necessary to use a large amount of a special lithium raw material in which 6 Li is concentrated at a high concentration in advance, so that the manufacturing cost becomes extremely high.
- the selection of the type of metal fluoride crystal is significantly limited.
- the 6 Li content can be obtained by obtaining the density of the scintillator, the mass fraction of the Li element in the scintillator, and the 6 Li content of the lithium raw material in advance and substituting them into the following formula [1].
- 6 Li content ⁇ ⁇ W ⁇ C / (700-C) ⁇ A ⁇ 10 ⁇ 23 [1] (Wherein ⁇ is the density of the scintillator [g / cm 3 ], W is the mass fraction of Li element in the scintillator [% by mass], C is the 6 Li content [%] of the raw material, and A is the Avogadro number [6 .02 ⁇ 10 23 ].)
- the base metal fluoride crystal in the present invention needs to contain a metal element having a valence of II or higher as a constituent element.
- the lanthanoid is contained in the metal fluoride crystal by replacing a part of the metal element having a valence of II or higher in the crystal.
- the metal element having a valence of II or higher any metal element capable of forming a fluoride crystal containing Li may be used.
- a typical metal element or transition metal element having an atomic number of 56 or less, particularly 40 or less is preferable.
- Typical examples include II-valent metal elements such as magnesium, calcium, strontium, and barium, III-valent metal elements such as aluminum, scandium, and yttrium, and IV-valent metal elements such as zirconium.
- a matrix metal fluoride crystal containing a metal element selected from II to IV metal elements is preferably employed from the viewpoint of ease of production.
- One or more metal elements having a valence of II or more contained in the base metal fluoride crystal may be used.
- the base metal fluoride crystal may contain an I-valent metal element other than lithium, such as sodium, potassium, and rubidium.
- the effective atomic number defined by the following formula [2] must be 10 to 40.
- the background noise derived from ⁇ rays can be sufficiently reduced, and a neutron detection scintillator capable of measuring neutron rays without being affected by ⁇ rays. Can do.
- the effective atomic number is particularly preferably 10 to 30.
- the effective atomic number is an index defined by the following formula [2], which affects the background noise derived from ⁇ rays, and the background noise is reduced as the effective atomic number is smaller. The phenomenon is seen.
- Effective atomic number ( ⁇ W i Z i 4 ) 1/4 [2] (In the formula, W i and Z i are the mass fraction and atomic number of the i-th element among the elements constituting the scintillator, respectively.)
- Such an effective atomic number is specific to the chemical composition of the scintillator for neutron detection, as is clear from the above formula [2]. Therefore, it can be arbitrarily adjusted by selecting the kind of metal fluoride crystal and the kind and content of lanthanoid to be contained in the metal fluoride crystal.
- the neutron detection scintillator of the present invention is characterized by containing a lanthanoid as a luminescent center.
- the lanthanoid is not particularly limited, and cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), Holmium (Ho), erbium (Er), thulium (Tm) or ytterbium (Yb) can be appropriately selected and used according to the desired emission wavelength, emission intensity and emission lifetime.
- the content of the lanthanoid contained in the metal fluoride crystal is not particularly limited, but a preferable range is 0.005 to 5 mol with respect to 100 mol of lithium constituting the metal fluoride crystal.
- the lanthanoid content generally improves the emission intensity as the content increases. However, if the content is too high, the emission intensity decreases due to concentration quenching, and the effective atomic number increases. The resulting background noise tends to increase.
- the lanthanoid contained in the crystal is partly present between the crystal lattices and partly replaced by some of the atoms constituting the crystal lattice, such as calcium, strontium, or yttrium. It is guessed.
- the lanthanoid segregation phenomenon in the fluoride crystal may be observed. Even when such a segregation phenomenon is observed, if the effective segregation coefficient (k) is obtained in advance and the content of the lanthanoid in the raw material is adjusted based on the following equation [3], the expected content of A metal fluoride crystal containing a lanthanoid can be easily obtained.
- C s kC 0 (1-g) k ⁇ 1 [3] ⁇ Containing lanthanides (M) in the formula, C s is the content of the lanthanoid (M) in the metal fluoride crystal [mol% (M / Li) ], k is the effective segregation coefficient, in C 0 is the raw material Amount [mol% (M / Li)], g represents solidification rate. ⁇
- the effective segregation coefficient a value described in literature (for example, Growth of Ce-doped LiCaAlF 6 and LiSrAlF 6 single crystals by the Czochralski technique under CF 4 atmosphere) may be adopted.
- the effective segregation coefficient of Ce is 0.02.
- the effective segregation coefficients of Pr and Eu are both 0.02, and the base metal fluoride crystal is LiYF 4 .
- the effective segregation coefficient of Ce is 1.
- a lanthanoid having a 5d-4f transition as a light emission principle, and in particular, a neutron containing Ce or Pr.
- the scintillator for detection has an emission decay time constant of about 20 to 50 ns and has an excellent high-speed response, and thus is preferably used for detecting neutron beams in the nuclear field, for example.
- Eu as the lanthanoid, a neutron scintillator having the maximum light emission intensity can be obtained, and can be suitably used in applications where high-speed response is not required, for example, transmission image photography using neutron beams.
- Matrix metal fluoride crystal of the present invention can satisfy the above 6 Li content and effective atomic number, and if lanthanoid the crystals can be incorporated as an ion is not particularly limited. Considering the fact that both a high 6 Li content and a low effective atomic number can be achieved, and that high quality crystals can be produced relatively easily, lithium calcium aluminum fluoride crystals (LiCaAlF 6 ), lithium strontium aluminum fluoride Base metal fluoride crystals such as (Lithium Strontium Aluminum Fluoride, LiSrAlF 6 ) and lithium yttrium fluoride (LiYF 4 ) are most suitable.
- LiCaAlF 6 lithium calcium aluminum fluoride crystals
- LiSrAlF 6 lithium strontium aluminum fluoride
- LiYF 4 lithium yttrium fluoride
- the metal fluoride crystal of the present invention is a single crystal or a polycrystal, but is preferably a single crystal for the following reason.
- a neutron scintillator with high emission intensity can be obtained without causing a loss due to non-radiative transition caused by lattice defects or dissipation of scintillation light at a crystal grain boundary.
- each crystal has a unique crystal system, LiCaAlF 6 and LiSrAlF 6 are hexagonal, and LiYF 4 is tetragonal.
- the metal fluoride crystal containing a lanthanoid used in the present invention is a colorless or slightly colored transparent crystal, 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 metal fluoride crystal is not particularly limited, and can be produced by a known crystal production method, but is preferably produced by a micro pull-down method or a Czochralski method.
- a metal fluoride crystal excellent in quality such as transparency can be produced.
- the crystal can be directly manufactured in a specific shape and can be manufactured in a short time.
- the Czochralski method a large crystal having a diameter of several inches can be manufactured at low cost.
- a predetermined amount of raw material is filled into a crucible 5 having a hole at the bottom.
- the shape of the hole provided at the bottom of the crucible is not particularly limited, but is preferably a cylindrical shape having a diameter of 0.5 to 5 mm and a length of 0 to 2 mm.
- the purity of the raw material 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 metal fluoride 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 raw materials lithium fluoride (LiF), calcium fluoride (CaF 2 ), aluminum fluoride (AlF 3 ), yttrium fluoride (YF 3 ), cerium fluoride (CeF 3 ), praseodymium fluoride (PrF 3 ) Metal fluorides such as europium fluoride (EuF 3 ) are used.
- the mixing ratio of these raw materials is preferably determined based on the phase equilibrium. That is, if the target metal fluoride crystal melts by coincidence, the mixing ratio is the chemical composition stoichiometric ratio. If the target metal fluoride crystal melts by mismatch, the mixing ratio is adjusted so that the target metal fluoride crystal becomes the primary crystal. Is preferred. Further, when a segregation phenomenon is observed when the lanthanoid is contained, it is preferable to determine the mixing ratio of the raw materials using the effective segregation coefficient as described above. Note that the phase equilibrium of the target metal fluoride crystal may be obtained in advance by an experiment using a thermal analysis method, or existing literatures and databases may be referred to.
- the crucible 5 filled with the raw materials, the after heater 1, the heater 2, the heat insulating material 3, and the stage 4 are set as shown in FIG.
- the inside of the chamber 6 is evacuated to 1.0 ⁇ 10 ⁇ 3 Pa or less using a vacuum exhaust device, 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.
- a solid scavenger such as zinc fluoride or a gas scavenger such as tetrafluoromethane in order to avoid adverse effects due to moisture that cannot be removed even by the gas replacement operation.
- 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.
- the raw material is heated and melted by the high frequency coil 7 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.
- the melted raw material melt is drawn out from the hole at the bottom of the crucible, and the manufacture of crystals is started.
- the pulling speed is not particularly limited, but is preferably in the range of 0.5 to 50 mm / hr.
- the metal fluoride crystal of the present invention may be subjected to an annealing operation after the crystal is produced by the micro-pulling-down method or the like for the purpose of removing crystal defects caused by deficiency of fluorine atoms or thermal strain.
- the obtained metal fluoride crystal has good workability and can be easily processed into a desired shape and used.
- 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 neutron detection scintillator comprising the metal fluoride crystal of the present invention can be combined with a photodetector such as a photomultiplier tube to provide an efficient neutron detector. That is, the light (scintillation light) emitted from the neutron detection scintillator (9) by the irradiation of the neutron beam is converted into an electrical signal by the photodetector (10), so that the presence / absence and intensity of the neutron beam is converted into an electrical signal. Can be caught.
- a photodetector such as a photomultiplier tube
- the scintillation light emitted from the scintillator of the present invention is usually light having a wavelength of 250 to 400 nm, and a photomultiplier tube, a silicon photodiode, or the like can be suitably used as a photodetector that can detect light in this region.
- a metal fluoride crystal block is bonded to the photocathode of a photomultiplier tube with optical grease or the like, and a high voltage is applied to the photomultiplier tube to output from the photomultiplier tube.
- a method of observing electrical signals For the purpose of analyzing the intensity of the neutron beam and the like using the electrical signal output from the photomultiplier tube, an amplifier, a multi-wave height analyzer, etc. may be provided after the photomultiplier tube.
- a photodetector a position sensitive type photodetector in which detectors having a sensitive area of several mm square are arranged in an array is used, and the individual detectors of the position sensitive type photodetector are used.
- a position-sensitive neutron beam detection apparatus can be configured by bonding a metal fluoride crystal having a size equivalent to that of the sensitive region.
- Example 1 A LiCaAlF 6 crystal containing Ce as a lanthanoid was manufactured using a crystal manufacturing apparatus using a micro-pulling-down method shown in FIG.
- raw materials lithium fluoride, calcium fluoride, aluminum fluoride and cerium fluoride having a purity of 99.99% or more were used.
- lithium fluoride, 6 Li content was used for 50%.
- the after heater 1, the heater 2, the heat insulating material 3, the stage 4, and the crucible 5 are made of high-purity carbon, and the shape of the hole provided at the bottom of the crucible is a circle having a diameter of 2.2 mm and a length of 0.5 mm. It was columnar.
- a high-frequency current is applied to the high-frequency coil 7, the raw material is heated and melted by induction heating, and a W-Re wire provided at the tip of the pull-down rod 8 is inserted into the hole at the bottom of the crucible 5 to melt the raw material melt.
- a W-Re wire provided at the tip of the pull-down rod 8 is inserted into the hole at the bottom of the crucible 5 to melt the raw material melt.
- While adjusting the output of the high frequency it was continuously pulled down at a speed of 3 mm / hr for 15 hours to obtain a LiCaAlF 6 crystal containing cerium as a lanthanoid.
- the crystal had a diameter of 2.2 mm and a length of 45 mm, and was a good quality crystal free of white turbidity and cracks.
- the metal fluoride crystal is cut into a length of 15 mm by a wire saw equipped with a diamond wire, and then subjected to grinding and mirror polishing to be processed into a shape having a length of 15 mm, a width of 2 mm, and a thickness of 1 mm.
- a neutron detection scintillator was obtained.
- the density of the scintillator, the mass fraction of the Li element in the scintillator, and the 6 Li content of the lithium raw material are 3.0 g / cm 3 , 3.7 mass%, and 50%, respectively. Therefore, the formula [1] Accordingly, the 6 Li content was 5.1 atom / nm 3 .
- the composition of the neutron scintillator was Ce 0.0004 LiCaAlF 6 , and the effective atomic number calculated from the formula [2] using the composition was 15.
- the performance as a neutron detection scintillator was evaluated by the following method. First, a scintillator for neutron detection was adhered to the photocathode of a photomultiplier tube (H7416 manufactured by Hamamatsu Photonics) with optical grease, and then light-shielded with a light-shielding sheet to prevent external light from entering. A 252 Cf neutron source having a radioactivity of 40 MBq was installed at a position 110 mm from the scintillator, and the scintillator was irradiated with a neutron beam.
- a scintillator for neutron detection was adhered to the photocathode of a photomultiplier tube (H7416 manufactured by Hamamatsu Photonics) with optical grease, and then light-shielded with a light-shielding sheet to prevent external light from entering.
- a 252 Cf neutron source having a radioactivity of 40 MBq was installed at a position 110 mm from the scin
- the scintillation light was converted into an electric signal through a photomultiplier tube to which a high voltage of 1000 V was applied.
- 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
- the waveform is the decay time constant of the scintillation light.
- the created wave height distribution spectrum is shown in FIG.
- 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.
- the peak value of the spectrum is expressed as a relative value.
- the vertical axis represents the frequency of the electric signal indicating each peak value, and here, the frequency is represented by the frequency per second (cps).
- cps the frequency per second
- the neutron detection efficiency represented by the peak area in the same wave height distribution spectrum is 1.3 cps, and it was shown that the neutron scintillator of the present invention has excellent sensitivity to neutrons. Furthermore, it can be seen that the baseline of the same wave height distribution spectrum is constant at around 0 cps, and the influence of background noise derived from ⁇ rays is minimal.
- the electrical signal output from the photomultiplier tube was input to an oscilloscope (Tektronix TDS3052B), and the signal waveform was analyzed to obtain the emission decay time constant of the neutron detection scintillator of the present invention.
- the obtained decay time constant was 24 ns, and it was shown that the scintillator for neutron detection of the present invention has excellent time response characteristics.
- Example 2 to 5 A metal fluoride crystal of the present invention was obtained in the same manner as in Example 1 except that the types and amounts of the raw materials were as shown in Table 1. Note Examples 1, 3 and using 6 Li content of 50% lithium fluoride in 4, Example 6 Li content in 2 50% lithium fluoride and 6 Li content 7.6% used as a mixture of a lithium fluoride eq, in example 5 6 Li content using the 7.6% lithium fluoride.
- the density of the metal fluoride crystal is shown the mass fraction of the Li elements in the metal fluoride crystal, and 6 Li content of the lithium material, and the 6 Li content calculated from these in Table 2.
- Table 2 also shows the composition of the metal fluoride crystal and the effective atomic number calculated therefrom.
- the performance of the neutron detection scintillator was evaluated by the same method as in Example 1. As an index of the emission intensity of the scintillator, a peak value region in which a signal based on scintillation light was obtained in a pulse height distribution spectrum created using the scintillator of each example was obtained. The peak value was a relative value where the peak value at the peak of the peak distribution spectrum in Example 1 was 1.
- Example 6 A metal fluoride crystal of the present invention was obtained in the same manner as in Example 1 except that the types and amounts of the raw materials were as shown in Table 1. In the present embodiment 6 Li content using 50% lithium fluoride. The density of the metal fluoride crystal is shown the mass fraction of the Li elements in the metal fluoride crystal, and 6 Li content of the lithium material, and the 6 Li content calculated from these in Table 2. Table 2 also shows the composition of the metal fluoride crystal and the effective atomic number calculated therefrom. The performance of the scintillator for neutron detection was evaluated by the same method as in Example 1 except that R7600 manufactured by Hamamatsu Photonics was used as the photomultiplier tube. The created wave height distribution spectrum is shown in FIG.
- the solid line is a spectrum prepared using the scintillator of the present invention
- the broken line is a spectrum prepared by irradiating only a photomultiplier tube without using the scintillator of the present invention
- the crest value of this example cannot be directly compared with the crest values of other examples and comparative examples because the photomultiplier tubes used are different.
- the wave height distribution spectrum as indicated by the shaded area, a clear increase in signal due to scintillation light is observed in the region where the wave height value is 0.7 to 1.4, and the neutron scintillator of the present invention has sufficient light emission. It can be seen that it has strength.
- the area of the shaded area was determined in the same manner as in Example 1. Further, the decay time constant of light emission was measured in the same manner as in Example 1. The respective results, together 6 Li content of the scintillator, the effective atomic number, and the lanthanides which contain, shown in Table 3.
- the scintillator for neutron detection of the present invention has sufficient neutron detection efficiency for neutrons
- the example 1 in which 6 Li content is 2 to 20 atom / nm 3 , 2, 3, and 4 were shown to have excellent neutron detection efficiency.
- the decay time constant of emission is 22 to 45 ns, and particularly excellent time response. It was shown to have properties.
- Example 6 containing Pr as a lanthanoid the decay time constant of light emission was 18 ns, and it was shown that the time response characteristic was superior to Ce.
- Example 3 in which the lanthanoid is Eu was shown to have particularly excellent emission intensity.
- FIG. 1 also shows the wave height distribution spectrum of LiCaAlF 6 crystal containing Eu as the lanthanoid obtained in Example 3.
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Abstract
Description
このシンチレーターとしては、放射線の種類や使用目的に応じてさまざまな種類のシンチレーターがあり、Bi4Ge3O12、Gd2SiO5:Ce、PbWO4、CsI、KIなどの無機結晶、アントラセンなどの有機結晶、有機蛍光体を含有させたポリスチレンやポリビニルトルエンなどの高分子体、特殊なものとして液体シンチレーターや気体シンチレーターがある。中性子線を検出対象とする場合は、中性子が物質中で何の相互作用もせずに透過する力が強いため、一般にエネルギーをもった荷電粒子に速やかに変換する核反応を利用して検出される。
従って、現在のところ、全ての特性を満たすような理想的な中性子検出用のシンチレーターというのは存在しない。なお、本願発明においては、中性子が当たったときに当該中性子を吸収して蛍光を発する物質からなるものを中性子検出用シンチレーターと称する。
非特許文献1: C.W.E. van Eijk et al、“LiBaF3, a thermal neutron scintillator with optimal n-ganma discrimination” Nuclar Instruments and Methods in Physics Research A 374 (1996) 197-201.
非特許文献2: 福田 健太郎、青木 謙治、吉川 彰、福田 承生、第66回応用物理学会学術講演会講演予稿集、NO.1,P.211(2005)
構成元素として、リチウム、II価以上の金属元素、およびフッ素を含む金属フッ化物結晶であって、単位体積当たり1.1~20原子(atom/nm3)の6Liを含み、有効原子番号が10~40であり、ランタノイドを含む前記金属フッ化物結晶からなることを特徴とする中性子検出用シンチレーターが提供される。
上記中性子検出用シンチレーターにおいて、
(1)単位体積当たり2.9~20原子(atom/nm3)の6Liを含むこと、および/または、
(2)有効原子番号が、10~30であること、および/または、
(3)ランタノイドが、セリウム、プラセオジムまたはユウロピウムであることが好適である。
上記中性子検出用シンチレーターと、当該中性子検出用シンチレーターから発する光を検知して電気信号に変換する光検出器とを具備する中性子検出装置が提供される。
上記中性子検出装置において、光検出器が光電子増倍管であることが好適である。
6Li含有率が20%以上のフッ化リチウム、II価以上の金属元素のフッ化物及びランタノイドのフッ化物からなる原料混合物を溶融して原料融液とし、該原料融液から融液成長法により単結晶体を成長させることを特徴とする中性子検出用シンチレーター用金属フッ化物結晶の製造方法が提供される。
上記製造方法は、ランタノイドを含有するフッ化リチウムカルシウムアルミニウム{Lithium Calcium Aluminium Fluoride、M:LiCaAlF6(Mはランタノイド)}結晶あるいはランタノイドを含有するフッ化リチウムイットリウム{Lithium Yttrium Fluoride、M:LiYF4(Mはランタノイド)}結晶の製造に好適に採用される。
2 ヒーター
3 断熱材
4 ステージ
5 坩堝
6 チャンバー
7 高周波コイル
8 引き下げロッド
9 中性子検出用シンチレーター
10 光検出器
6Li含有量=ρ×W×C/(700-C)×A×10-23 〔1〕
(式中、ρはシンチレーターの密度[g/cm3]、Wはシンチレーター中のLi元素の質量分率[質量%]、Cは原料の6Li含有率[%]、Aはアボガドロ数[6.02×1023]を示す。)
有効原子番号=(ΣWiZi 4)1/4 〔2〕
(式中、Wi及びZiは、それぞれシンチレーターを構成する元素のうちのi番目の元素の質量分率及び原子番号である。)
結晶中に含有される当該ランタノイドは、一部は結晶格子間に、一部は結晶格子を構成する元素、例えば、カルシウム、ストロンチウム、或いはイットリウムなどの原子の一部と置き換わって存在しているものと推察される。
Cs=kC0(1-g)k-1 〔3〕
{式中、Csは金属フッ化物結晶中でのランタノイド(M)の含有量[mol%(M/Li)]、kは実効偏析係数、C0は原料中でのランタノイド(M)の含有量[mol%(M/Li)]、gは固化率を表す。}
原料としては、フッ化リチウム(LiF)、フッ化カルシウム(CaF2)、フッ化アルミニウム(AlF3)、フッ化イットリウム(YF3)、フッ化セリウム(CeF3)、フッ化プラセオジム(PrF3)、フッ化ユウロピウム(EuF3)等の金属フッ化物が用いられる。
更に、光検出器として、数mm角の有感領域を有する検出部をアレイ状に配列してなる位置敏感型光検出器を用い、当該位置敏感型光検出器の個々の検出部に対して、その有感領域と同等の大きさの金属フッ化物結晶を接合することによって、位置敏感型の中性子線検出装置を構成することができる。
図3に示すマイクロ引下げ法による結晶製造装置を用いて、ランタノイドとしてCeを含有するLiCaAlF6結晶を製造した。原料としては、純度が99.99%以上のフッ化リチウム、フッ化カルシウム、フッ化アルミニウム及びフッ化セリウムを用いた。なお、フッ化リチウムは、6Li含有率が50%のものを用いた。アフターヒーター1、ヒーター2、断熱材3、ステージ4、及び坩堝5は、高純度カーボン製のものを使用し、坩堝底部に設けた孔の形状は直径2.2mm、長さ0.5mmの円柱状とした。
高周波コイル7に高周波電流を印加し、誘導加熱によって原料を加熱して溶融せしめ、引き下げロッド8の先端に設けたW-Reワイヤーを、坩堝5底部の孔上記孔に挿入し、原料の融液を上記孔より引き下げ、結晶化を開始した。高周波の出力を調整しながら、3mm/hrの速度で連続的に15時間引き下げ、ランタノイドとしてセリウムを含有するLiCaAlF6結晶を得た。該結晶は直径が2.2mm、長さが45mmであり、白濁やクラックの無い良質な結晶であった。
まず光電子増倍管(浜松ホトニクス社製 H7416)の光電面に、中性子検出用シンチレーターを光学グリースで接着した後、外部からの光が入らないように遮光シートで遮光した。
40MBqの放射能を有する252Cf中性子線源を、シンチレーターから110mmの位置に設置し、シンチレーターに中性子線を照射した。次いで、シンチレーターより発せられたシンチレーション光を計測するため、1000Vの高電圧を印加した光電子増倍管を介して、シンチレーション光を電気信号に変換した。ここで、光電子増倍管より出力される電気信号は、シンチレーション光を反映したパルス状の信号であり、パルスの波高がシンチレーション光の発光強度を表し、また、その波形はシンチレーション光の減衰時定数に基づいた減衰曲線を呈する。このようにして光電子増倍管から出力された電気信号を整形増幅器で整形、増幅した後、多重波高分析器に入力して解析し、波高分布スペクトルを作成した。
また、同波高分布スペクトルにおけるピーク面積にて表される中性子検出効率は1.3cpsであり、本発明の中性子用シンチレーターは中性子に対する優れた感度を有することが示された。さらに、同波高分布スペクトルのベースラインは0cps付近で一定であり、γ線に由来するバックグラウンドノイズの影響が極微であることが分かる。
原料の種類及び量を、表1に示すとおりとした以外は、実施例1と同様にして本発明の金属フッ化物結晶を得た。なお実施例1、3および4においては6Li含有率が50%のフッ化リチウムを用い、実施例2においては6Li含有率が50%のフッ化リチウムと6Li含有率が7.6%のフッ化リチウムとを等量混合して用い、実施例5においては6Li含有率が7.6%のフッ化リチウムを用いた。
中性子検出用シンチレーターの性能を実施例1と同様の方法によって評価した。シンチレーターの発光強度の指標として、各実施例のシンチレーターを用いて作成された波高分布スペクトルにおいて、シンチレーション光に基づく信号が得られた波高値の領域を求めた。なお、該波高値は、実施例1における波高分布スペクトルのピークにおける波高値を1とした相対値とした。また、中性子検出効率の指標として、上記波高分布スペクトルにおけるピークの面積を求めた。また、発光の減衰時定数を実施例1と同様にして測定した。それぞれの結果を、各シンチレーターの6Li含有量、有効原子番号、及び含有するランタノイドと併せて、表3に示す。
原料の種類及び量を、表1に示すとおりとした以外は、実施例1と同様にして本発明の金属フッ化物結晶を得た。なお本実施例においては6Li含有率が50%のフッ化リチウムを用いた。
得られた金属フッ化物結晶の密度、金属フッ化物結晶中のLi元素の質量分率、及びリチウム原料の6Li含有率、ならびにこれらより算出された6Li含有量を表2に示す。また、金属フッ化物結晶の組成、及びこれより算出された有効原子番号を表2に併せて示す。
光電子増倍管として、浜松ホトニクス社製 R7600を用いた以外は、実施例1と同様の方法によって中性子検出用シンチレーターの性能を評価した。作成した波高分布スペクトルを図2に示す。なお、当該スペクトルにおいて、実線は本発明のシンチレーターを用いて作成したスペクトルであり、破線は本発明のシンチレーターを用いることなく、放射線を光電子増倍管のみに照射して作製したスペクトルである(なお、本実施例の波高値は、用いた光電子増倍管が異なるため、他の実施例及び比較例の波高値との直接比較はできない)。当該波高分布スペクトルにおいて、斜線部に示すように、波高値が0.7~1.4の領域において、シンチレーション光による明らかな信号の増大が見られており、本発明の中性子シンチレーターが充分な発光強度を有することが分かる。
中性子検出効率の指標として、前記斜線部の面積を実施例1と同様にして求めた。また、発光の減衰時定数を実施例1と同様にして測定した。それぞれの結果を、シンチレーターの6Li含有量、有効原子番号、及び含有するランタノイドと併せて、表3に示す。
原料の種類及び量を、表1に示すとおりとした以外は、実施例1と同様にして、6Li含有率が従来知られているレベルの0.73atom/nm3の金属フッ化物結晶を作成し、かつその性能を評価した。これら比較例においては、原料として6Li含有率が7.6%のフッ化リチウムを用いた。組成及び有効原子番号を表2に、その評価結果を表3に示す。
Claims (9)
- 構成元素として、リチウム、II価以上の金属元素、およびフッ素を含む金属フッ化物結晶であって、単位体積当たり1.1~20原子(atom/nm3)の6Liを含み、有効原子番号が10~40であり、ランタノイドを含む前記金属フッ化物結晶からなることを特徴とする中性子検出用シンチレーター。
- 単位体積当たり2.9~20原子(atom/nm3)の6Liを含むことを特徴とする請求項1に記載の中性子検出用シンチレーター。
- 有効原子番号が、10~30であることを特徴とする請求項1に記載の中性子検出用シンチレーター。
- ランタノイドが、セリウム又はプラセオジムであることを特徴とする請求項1に記載の中性子検出用シンチレーター。
- ランタノイドが、ユウロピウムであることを特徴とする請求項1に記載の中性子検出用シンチレーター。
- 請求項1に記載の中性子検出用シンチレーターと、当該中性子検出用シンチレーターから発する光を検知して電気信号に変換する光検出器とを具備することを特徴とする中性子検出装置。
- 光検出器が、光電子増倍管であることを特徴とする請求項6に記載の中性子検出装置。
- 6Li含有率が20%以上のフッ化リチウム、II価以上の金属元素のフッ化物及びランタノイドのフッ化物からなる原料混合物を溶融して原料融液とし、該原料融液から融液成長法により単結晶体を成長させることを特徴とする中性子検出用シンチレーター用金属フッ化物結晶の製造方法。
- 中性子検出用シンチレーター用金属フッ化物結晶が、ランタノイドを含有するフッ化リチウムカルシウムアルミニウム結晶(M:LiCaAlF6、Mはランタノイド)あるいはランタノイドを含有するフッ化リチウムイットリウム結晶(M:LiYF4、Mはランタノイド)であることを特徴とする請求項8に記載の中性子検出用シンチレーター用金属フッ化物結晶の製造方法。
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Also Published As
Publication number | Publication date |
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JPWO2009119378A1 (ja) | 2011-07-21 |
EP2256177A1 (en) | 2010-12-01 |
RU2494416C2 (ru) | 2013-09-27 |
KR20100125326A (ko) | 2010-11-30 |
KR101538194B1 (ko) | 2015-07-20 |
EP2256177A4 (en) | 2014-10-22 |
US20100314550A1 (en) | 2010-12-16 |
US8044367B2 (en) | 2011-10-25 |
CN101945974A (zh) | 2011-01-12 |
CA2717341C (en) | 2016-05-31 |
RU2010143421A (ru) | 2012-04-27 |
CA2717341A1 (en) | 2009-10-01 |
EP2256177B1 (en) | 2016-01-13 |
JP5378356B2 (ja) | 2013-12-25 |
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