JP7132787B2 - Neutron spectrum measurement device and neutron spectrum measurement method - Google Patents

Description

The present invention relates to a neutron spectrum measuring apparatus and method.

An inspection technique for nondestructively seeing through a substance using a neutron beam is known, and is called neutron radiography testing (NRT) or neutron imaging. Neutron imaging is mainly used to inspect substances containing elements with small atomic numbers that are difficult to inspect with X-rays. For example, water containing hydrogen (H), oil or resin, lithium (Li) and boron ( It can be used for inspection of substances containing B). While the transmittance of X-rays depends on the atomic number and density of a substance, the scattering and absorption characteristics of neutron beams vary according to the difference in the atomic nuclei of the elements that make up the substance. Therefore, neutron imaging can also distinguish between the isotopes that make up the material under examination.

Since the mode of interaction between neutrons and matter also changes depending on the neutron energy, it is required to appropriately adjust and measure the energy (spectrum) of the neutron beam used for inspection. As a method for measuring the neutron spectrum, for example, a method of measuring neutrons with different energy values by changing the thickness of the moderator surrounding the detector (for example, Patent Documents 1 and 2), the time of flight of the neutron pulse (TOF; Time Of Flight) is known to identify energy (for example, Patent Document 3).

Registered Utility Model No. 3116543 Japanese Unexamined Patent Application Publication No. 2011-53218 Japanese Patent Publication No. 2014-534442

When trying to directly measure the spectrum of a stationary neutron source that is continuously output over time, the TOF method cannot be applied as it is. In addition, in order to measure neutron spectra over the thermal neutron region (approximately 10 ⁻² to 10 ⁻¹ eV) and the epithermal neutron region (approximately 10 ⁰ to 10 ³ eV or more), the thickness of the moderator should be several centimeters to several tens of cm, which leads to an increase in the size of the measuring apparatus and a decrease in measurement position accuracy. In addition, since general neutron counters simultaneously measure high-dose gamma rays generated in a neutron irradiation environment, it is necessary to correct or calibrate the measurement results with high accuracy in consideration of the effects of gamma rays. .

One exemplary object of one aspect of the present invention is to provide a technique for simply measuring a neutron spectrum.

A neutron spectrum measuring device according to one aspect of the present invention includes a converter unit having a plurality of types of metal foil converters each containing different types of metal elements that are activated in response to the energy of a neutron beam to be measured; a filter unit having a plurality of types of metal filters each containing the same type of metal element as the metal foil converter, and arranged on the neutron beam incident side of the converter unit. The plurality of types of metal filters are arranged at positions different from each other in a plan view viewed from the direction of incidence of neutron beams, and the positions of the plurality of types of metal foil converters overlap at least partially with the corresponding metal filters in the direction of incidence of neutron beams. placed in

Another aspect of the present invention is a neutron spectrum measurement method using a neutron spectrum measurement device. This method consists of irradiating a converter unit with a neutron beam through a filter unit to activate multiple types of metal foil converters, and radiation emitted from each of the multiple types of activated metal foil converters. transferring the dimensional intensity distribution to a transfer plate; measuring an image corresponding to the two-dimensional intensity distribution of the radiation transferred to the transfer plate to generate image data; analyzing the image data; calculating neutron flux at a plurality of energy values corresponding to each of the foil converters.

It should be noted that arbitrary combinations of the above-described constituent elements and mutually replacing the constituent elements and expressions of the present invention in methods, devices, systems, etc. are also effective as aspects of the present invention.

ADVANTAGE OF THE INVENTION According to this invention, a neutron spectrum can be measured simply.

3 is a cross-sectional view schematically showing the configuration of an irradiation unit according to the embodiment; FIG. FIGS. 2(a)-(c) are graphs showing examples of neutron spectra absorbed by an Au foil converter. Fig. 2 is a graph showing an example of neutron spectra absorbed by an In foil converter through an In filter and a Cd filter; FIGS. 4(a) and 4(b) are graphs showing examples of neutron spectra absorbed by the Dy foil converter. 1 is a graph showing an example of neutron spectra absorbed by an In foil converter through an Au filter and a Cd filter; 6A and 6B are sectional views schematically showing the configuration of the transfer unit. 4 is a graph showing an example of changes in radioactivity of a metal foil converter over time in an irradiation process and a transfer process; FIG. 4 is a cross-sectional view schematically showing the configuration of an irradiation unit using a step wedge; 9 is a diagram schematically showing an example of image data acquired using the irradiation unit of FIG. 8. FIG. FIG. 4 is a cross-sectional view schematically showing the configuration of an irradiation unit using multiple kinds of metal foils; FIG. 11 is a top view schematically showing the configuration of the converter unit of FIG. 10; FIG. 11 is a top view schematically showing the configuration of the filter unit of FIG. 10; FIG. 11 is a cross-sectional view schematically showing the configuration of the step wedge of FIG. 10; It is a cross-sectional view schematically showing the configuration of an irradiation unit according to a modification. FIG. 3 is a top view schematically showing the configuration of an irradiation unit according to an example; FIG. 4 is a diagram showing an example of a transferred image of a Dy foil converter; It is a figure which shows an example of the transfer image of an In foil converter. FIG. 4 is a diagram showing an example of a transfer image of an Au foil converter; FIG. 4 is a diagram showing an example of a transferred image of a Mn foil converter; It is a figure which shows an example of the transfer image of Er foil converter. 4 is a graph showing an example of neutron spectrum measurement results and corresponding simulation results; It is a figure which shows typically the structure of a neutron spectrum measuring apparatus. It is a flowchart which shows the flow of a neutron spectrum measuring method.

The present embodiment relates to a technique for measuring a neutron spectrum, which is the energy distribution of neutron beams. The present embodiment can be applied, for example, to an inspection method using neutron beams, which is also called neutron imaging. An inspection technique for non-destructively seeing through a substance is generally called radiography. Radiography techniques using X-rays are generally called roentgenography, and are widely used in medical and industrial applications. When neutrons are used as the radiation source, it is particularly called "neutron radiography" or "neutron imaging".

X-rays have the property that the larger the atomic number Z of the substance to be irradiated and the higher the density of the substance to be irradiated, the more difficult it is to penetrate. In the case of X-rays, if the densities are about the same, elements with similar atomic numbers Z have almost the same transmittance, so it is difficult to distinguish substances with similar atomic numbers. In addition, X-rays can transmit differently depending on the energy of the absorption edge depending on the orbital of the electron shell such as the K shell or L shell of the atom, but in general, the higher the energy of the X-rays, the higher the transmittance. It is in.

On the other hand, the mode of interaction between neutrons and matter generally does not directly depend on the atomic number, but changes according to the configuration of the nucleus. Therefore, even if the element is the same, the ratio of reaction with neutrons and absorption changes depending on the difference in the isotope, and the isotope of the element that constitutes the substance can be distinguished according to the difference in characteristics. A neutron cross section (unit burn (b): 1b = 10 ^-24 cm ² ) is used as an index of interaction between neutrons and matter. The neutron cross section multiplied by the atomic number density ρ is called the macroscopic cross section Σ(cm ⁻¹ ) and corresponds to the linear attenuation coefficient of X-rays. Neutron scattering has two main characteristics: coherent scattering and incoherent scattering. Coherent scattering is dominant in many elements. An element with a large coherent cross section is mainly scattered by diffraction, and a Bragg edge appears due to the difference in neutron energy. Incoherent scattering is scattering with atomic motion, and the cross-sectional area increases in inverse proportion to the neutron velocity v, which is equivalent to the neutron energy. This is called the "1/v rule" and corresponds to the 1/v rule for elements such as the boron isotope ¹⁰ B and the lithium isotope ⁶ Li. In addition to these scattering cross-sections, there are absorption cross-sections due to resonance capture that appear in the neutron energy region, which varies from element to element.

Neutron imaging is mainly used to inspect substances containing elements with small atomic numbers that are difficult to inspect with X-rays. For example, water containing hydrogen (H), oil or resin, lithium (Li) and boron ( It can be used for inspection of substances containing B). These examinations often use neutron beams in the thermal neutron region (for example, about 10 ⁻³ to 10 ⁻¹ eV) with low neutron energy. When using energy in the thermal neutron region, high-energy neutrons are moderated by a material called a moderator (mainly hydrogen-containing water, polyethylene, etc.). Examples of neutron sources include nuclear reactor neutron sources, RI neutron sources using radioisotopes (RI) such as californium (Cf), americium (Am), and beryllium (Be), and accelerators using accelerators such as cyclotrons. A neutron source and the like can be mentioned. In addition, there are stationary neutron sources that output neutron beams continuously over time and pulse neutron sources that output neutron beams in pulses.

In general, neutrons emitted from a neutron source contain neutrons of various energies. Among them, only neutrons with specific energies are extracted and used for non-destructive stress measurement. For example, an angle-dispersive diffractometer is known, which uses a stationary neutron source and a monochromator crystal device to extract a neutron beam of a single wavelength, irradiates a measurement sample, and detects the diffracted angle. An energy dispersive diffractometer that measures wavelength (energy) determined by Bragg's diffraction conditions is also known, and mainly uses a time-of-flight method (TOF method) with a pulsed neutron source. Also, the isotope is specified by measuring the resonance absorption peak peculiar to the element (isotope) using the TOF method.

Thus, in neutron imaging technology, it is desirable to be able to adjust the neutron energy according to the purpose of use, and it is desirable to be able to accurately measure the energy distribution (spectrum) of the neutron beam output from the neutron source. With a pulsed neutron source, the neutron spectrum can be measured by the TOF method described above. On the other hand, in the case of a stationary neutron source, a method is known in which neutrons with different energy values are measured by changing the thickness of the moderator placed in front of the detector. However, in order to measure neutron spectra over the thermal neutron region (approximately 10 ⁻² to 10 ⁻¹ eV) and the epithermal neutron region (approximately 10 ⁰ to 10 ³ eV or more), the maximum thickness of the moderator is several cm to It needs to be about several tens of centimeters, which leads to an increase in the size of the measuring device. In addition, general neutron counters simultaneously measure high-dose gamma rays generated in a neutron irradiation environment, so the measurement results must be corrected or calibrated with high accuracy in consideration of the effects of gamma rays. .

In addition, a method of analytically deriving the neutron spectrum of the irradiation field by simulating the irradiation environment using the Monte Carlo method is also used. However, in order to obtain appropriate calculation results, it is necessary to reproduce the actual arrangement of buildings and equipment in the irradiation field with high accuracy, and to calculate the effects of scattered radiation under the irradiation environment. , the amount of data required for analysis is enormous. In addition, it may take several days to several weeks for one calculation, so it cannot be said that it is a method that can be used casually to obtain neutron spectra.

Therefore, the present embodiment proposes a technique for simply measuring a neutron spectrum. A neutron spectrum measuring apparatus according to the present embodiment includes a converter unit having a plurality of types of metal foil converters each containing a plurality of types of metal foil converters each containing a different type of metal element that is activated in response to the energy of a neutron beam to be measured, and a corresponding a filter unit having a plurality of types of metal filters each containing the same type of metal element as the metal foil converter, and arranged on the neutron beam incident side of the converter unit. The plurality of types of metal filters are arranged at positions different from each other in a plan view viewed from the direction of incidence of neutron beams, and the positions of the plurality of types of metal foil converters overlap at least partially with the corresponding metal filters in the direction of incidence of neutron beams. placed in

In this embodiment, a metal filter composed of the same kind of metal element and a metal foil converter are used in combination, and the metal foil converter absorbs the neutron beam that has passed through the metal filter. The metal foil converter is activated by absorption of neutron beams and acquires radioactivity with a strength corresponding to the amount of neutron beam absorption. The activated metal foil converter is tightly fixed to a transfer plate, such as an imaging plate, and an image corresponding to the radioactivity intensity distribution on the surface of the metal foil converter is developed on the transfer plate. After that, by reading out the image developed on the transfer plate and analyzing the generated image data, the neutron flux absorbed by the metal foil converter can be calculated. At this time, by selecting a metal element that resonates and absorbs at a specific energy, the neutron flux at the resonance energy can be obtained with high accuracy. Furthermore, by using a plurality of types of metal elements having different energy characteristics such as different peak positions of resonance energies, neutron fluxes at a plurality of different energies can be obtained. Thereby, the energy distribution of neutrons can be roughly obtained.

DETAILED DESCRIPTION OF THE INVENTION Embodiments for carrying out the present invention will be described in detail below. The configuration described below is an example and does not limit the scope of the present invention. Also, in the description of the drawings, the same elements are denoted by the same reference numerals, and overlapping descriptions are omitted as appropriate. In addition, in the drawings referred to in the following description, the size and thickness of each constituent member are for convenience of description, and do not necessarily represent actual dimensions and ratios.

(First embodiment)
FIG. 1 is a cross-sectional view schematically showing the configuration of a measuring unit 50 according to an embodiment. The measurement unit 50 comprises a metal foil converter 52 , a thermal neutron filter 54 and a metal filter 56 . In FIG. 1, only one type of metal foil converter 52 and metal filter 56 are shown for the sake of simple explanation of the measurement method according to the present embodiment.

The metal foil converter 52 and the metal filter 56 are composed of the same kind of metal element, such as gold ( ¹⁹⁷ Au). That is, the metal foil converter 52 is an Au foil converter and the metal filter 56 is an Au filter. The thermal neutron filter 54 is a metal plate made of, for example, cadmium (Cd) and having a uniform thickness, and is a so-called Cd filter. The thickness of the metal foil converter 52 is, for example, 0.01 mm or more and 0.3 mm or less. The thickness of the metal filter 56 is, for example, 0.1 mm or more and less than 3 mm. The thickness of the Cd filter 54 is 0.1 mm or more and less than 3 mm, for example 0.5 mm or 1 mm.

The metal foil converter 52 has three measurement areas 52a, 52b, 52c. The thermal neutron filter 54 and the metal filter 56 are not provided above the first measurement region 52a, and the neutron beam NB to be measured is directly irradiated onto the first measurement region 52a. Only the thermal neutron filter 54 is provided above the second measurement area 52b, and the neutron beams NB passing through the thermal neutron filter 54 enter the second measurement area 52b. A thermal neutron filter 54 and a metal filter 56 are superimposed on the third measurement region 52c, and the neutron beams NB that have passed through both the thermal neutron filter 54 and the metal filter 56 enter the third measurement region 52c.

FIGS. 2(a)-(c) are graphs showing examples of neutron spectra absorbed by an Au foil converter. FIG. 2(a) is a neutron spectrum corresponding to the first measurement region 52a of the Au foil converter 52 of FIG. The dashed line in FIG. 2(a) is the spectrum of the neutron beam NB incident on the first measurement region 52a of the Au foil converter 52, and the solid line is the spectrum of neutrons absorbed in the first measurement region 52a. The neutron beam to be measured contains neutrons of various energies. In the illustrated example, the neutron flux in the thermal neutron region E1 has a moderate peak of about 10 ⁻⁶ , and the neutron flux in the epithermal neutron region E2 is approximately constant at about 10 ⁻⁷ . As indicated by the solid line, the spectrum of neutrons absorbed in the first measurement region 52a of the Au foil converter 52 has a resonance absorption peak E _Au at a position of approximately 5 eV (4.906 eV). The first measurement region 52a of the Au foil converter 52 also absorbs neutrons in the thermal neutron region E1 to some extent. The total amount of neutrons absorbed by the first measurement region 52a of the Au foil converter 52 corresponds to the integrated value of the entire region indicated by the solid line spectrum. In the example of FIG. 2A, the integrated value of neutron absorption in the thermal neutron region E1 and the integrated value of neutron absorption in the epithermal neutron region E2 are approximately the same.

FIG. 2(b) is a neutron spectrum corresponding to the second measurement region 52b of the Au foil converter 52 of FIG. The dashed line in FIG. 2(b) is the spectrum of the neutron beam NB that passes through the thermal neutron filter 54 and enters the second measurement region 52b of the Au foil converter 52, and the solid line is the neutron absorbed in the second measurement region 52b. A spectrum is shown. In FIG. 2(b), the neutron flux in the thermal neutron region E1 is shielded by the thermal neutron filter 54 unlike FIG. 2(a). As a result, only the epithermal neutron region E2 is absorbed in the second measurement region 52b of the Au foil converter 52, and most of the absorbed neutrons have the energy value of the resonance peak E _Au .

FIG. 2(c) is a neutron spectrum corresponding to the third measurement region 52c of the Au foil converter 52 of FIG. The dashed line in FIG. 2(c) is the spectrum of the neutron beam NB passing through the Au filter 56 and the thermal neutron filter 54 and entering the third measurement region 52c of the Au foil converter 52, and the solid line is the third measurement region 52c. Absorbed neutron spectra are shown. In FIG. 2(c), the metal filter 56 further shields the neutron flux of the resonance peak E _Au from the broken line spectrum in FIG. 2(b). As a result, the third measurement region 52c of the Au foil converter 52 absorbs neutrons of energy other than the portion of the resonance peak E _Au in the epithermal neutron region E2.

Here, when the absorption spectra indicated by the solid lines in FIGS. 2(b) and 2(c) are compared, the only substantial difference is the presence or absence of absorption of the neutron flux having the energy of the resonance peak E _Au . Therefore, by subtracting the neutron absorption of the third measurement region 52c of FIG. 2(c) from the neutron absorption of the second measurement region 52b of FIG. 2(b), only the neutron absorption of the resonance peak E _Au can be obtained. can be done. As a result, the neutron flux at the resonance peak E _Au energy (approximately 5 eV) of the Au foil converter 52 can be obtained. The neutron absorption amount of the Au foil converter 52 can be obtained indirectly by measuring the radioactivity intensity after radioactivity of Au due to neutron absorption. More specifically, the neutron absorption of the Au foil converter 52 can be calculated by measuring the gamma ray intensity in the (n, γ) reaction in which Au atoms capture neutrons and emit gamma rays. Details will be described separately later.

The neutron flux at the resonance peak E _Au of the Au foil converter 52 can be obtained without using the thermal neutron filter 54 . Specifically, of the measurement area of the Au foil converter 52, the neutron absorption amount of the area shielded by the metal filter 56 and the area not shielded by the metal filter 56 are measured, and by comparing the two, the resonance peak E _Au neutron absorption amount can also be asked for. However, in this case, since the ratio of neutron absorption in the thermal neutron region E1 is large, the ratio of change in neutron absorption due to the presence or absence of absorption in the resonance peak E _Au becomes small, and the signal-to-noise ratio (S /N ratio). Therefore, by using the thermal neutron filter 54, the S/N ratio of the measurement at the resonance peak E _Au can be increased, and the neutron flux at the resonance peak E _Au can be calculated more accurately.

The above approach is also applicable to combinations of metal foil converters and metal filters containing other types of metal elements with different resonant absorption peaks. FIG. 3 is a graph showing an example of a neutron spectrum absorbed by an In foil converter through an In filter and a Cd filter. That is, in FIG. 3, indium ( ¹¹⁵ In) is used instead of gold (Au) as the metal element forming the metal foil converter 52 and the metal filter 56 . As shown, indium ( ¹¹⁵ In) has a resonance absorption peak E _In at a position of about 1.5 eV (1.457 eV). Therefore, by using In as a combination of the metal foil converter 52 and the metal filter 56, a neutron flux with an energy (about 1.5 eV) different from that of Au (about 5 eV) can be obtained.

Metal elements having resonance absorption peaks in the epithermal neutron region E2 include gold ( ¹⁹⁷ Au), indium ( ¹¹⁵ In), manganese ( ⁵⁵ Mn), cobalt ( ⁵⁹ Co), copper ( ⁶³ Cu), erbium ( ¹⁷⁰ Er) and the like. Manganese ( ⁵⁵ Mn) has a resonance peak of about 300 eV (337 eV) and cobalt ( ⁵⁹ Co) has a resonance peak of about 100 eV (132 eV). Copper ( ⁶³ Cu) has a resonance peak of about 600 eV (580 eV) and erbium ( ¹⁷⁰ Er) has a resonance peak of 100 eV. An alloy containing these metal elements can also be used. For example, a manganin alloy containing manganese (86% copper, 12% manganese, and 2% nickel (Ni)) can be used instead of using manganese simple metal.

Among the above metal elements, cobalt (Co) has a longer half-life than other elements, so it may not be suitable for relatively short-time measurement by the transfer method. On the other hand, erbium ( ¹⁷⁰ Er), which has a resonance peak close to that of cobalt, has an isotope abundance of 14.93% and a half-life of 7.516 hours. The abundance of ¹⁶⁸ Er, which is another isotope, is 26.78% and the half-life is 9.40 days. Both ¹⁷⁰ Er and ¹⁶⁸ Er are beta decays, but ¹⁷⁰ Er, which has a short half-life, is thought to dominate in the processes of activation and transcription. The physical property values of the metal elements exemplified in this specification are obtained from the general-purpose standard nuclear data library (JENDL-4.0, https://wwwndc.jaea. go.jp/jendl/j40/J40_J.html).

The above technique is also applicable to combinations of metal foil converters and metal films containing metal elements with reaction characteristics different from resonance absorption, for example, dysprosium (Dy) can be used. FIGS. 4(a) and 4(b) are graphs showing examples of neutron spectra absorbed by the Dy foil converter. The solid line in FIG. 4(a) shows the neutron absorption spectrum of the Dy foil converter without the thermal neutron filter 54 (Cd filter). As illustrated, the Dy foil absorbs neutrons over a thermal neutron region E1 and a partial region E3 (approximately 0.4 eV to 1 eV) of the epithermal neutron region E2. The solid line in FIG. 4(b) shows the neutron absorption spectrum of the Dy foil converter with the thermal neutron filter 54 (Cd filter). As shown in the figure, by combining a Cd filter, the thermal neutron region E1 can be shielded, and only neutrons in a partial region E3 of about 0.4 eV to 1 eV can be absorbed by the Dy foil converter. Therefore, by combining a Dy foil converter and a Dy filter (and a Cd filter), neutron flux in the energy range of about 0.4 eV to 1 eV can be measured.

In the measuring method according to the present embodiment, it is meaningless to combine metal foil converters and metal filters having different reaction characteristics. This is because the neutron flux at a specific energy can be measured with high accuracy only by combining a metal foil converter and a metal filter made of the same type of metal element that reacts to a specific energy in the same way. FIG. 5 shows neutron absorption spectra when metal foil converters of different kinds of metal elements and metal filters are combined, and shows a case where an In foil converter is combined with an Au filter and a Cd filter. As shown in the figure, since the positions of the resonance peaks E _Au and E _In are different between Au and In, it can be seen that the neutron absorption spectrum of the In foil converter is not substantially affected by the Au filter.

Next, a method for measuring the radioactivity intensity of the activated metal foil converter 52 will be described. 6A and 6B are sectional views schematically showing the configuration of the transfer unit 14. FIG. As shown in FIG. 6A, the transfer unit 14 includes a transfer plate 42, a cushioning material 44, and a shielding container 46. As shown in FIG. The transfer unit 14 is a cassette for developing and imaging the radioactivity intensity distribution of the activated metal foil converter 52 on the transfer plate 42 .

Transfer plate 42 is an imaging plate (IP) for producing images corresponding to radiation intensity distributions in response to radiation (eg, beta and gamma rays). As the transfer plate 42, for example, one using a stimulable phosphor can be used. The photostimulable phosphor emits fluorescence in response to radiation, and has a memory function (optical memory function) in which the site that reacted to the radiation emits light again when irradiated with reading light afterward. As a result, it is possible to obtain a high-sensitivity image and obtain image data with a wide dynamic range as compared with the case of using a general non-stimulating phosphor. In addition, the neutron beam irradiation environment and the transfer environment to the imaging plate can be separated, and the high-dose gamma rays generated in the neutron beam irradiation environment can be prevented from adversely affecting the transfer process.

The activated metal foil converter 52 is accommodated inside the shielding container 46 with one side thereof in close contact with the transfer plate 42 . The cushioning material 44 is a spongy resin member or rubber member, and presses the transfer plate 42 against the metal foil converter 52 . This ensures that there is no gap between the transfer plate 42 and the metal foil converter 52 and that the relative positions of the transfer plate 42 and the metal foil converter 52 do not change during the transfer process. The shielding container 46 shields light from the outside so that the inside of the container becomes a dark room during the transfer process.

FIG. 6B shows a modification of the transfer unit 14. FIG. 6B differs from the configuration of FIG. 6A in that transfer plates 42a and 42b are arranged on both sides of the metal foil converter 52, respectively. Since the thickness of the metal foil converter 52 is as thin as, for example, about 0.01 mm to 0.3 mm, both the surface on which the neutron beams NB are incident and the opposite back surface are activated with a similar two-dimensional distribution. Therefore, the image transferred to the first transfer plate 42a corresponding to the front surface of the metal foil converter 52 and the image transferred to the second transfer plate 42b corresponding to the back surface of the metal foil converter 52 can be used together. can. As a result, by accumulating the transfer results on both sides, the sensitivity of the developed image can be increased compared to the case where only one side of the metal foil converter 52 is transferred.

ここで、ｆ：照射中性子線の中性子束（ｎ／ｃｍ^２ｓ）、σ：金属箔の放射化断面積（ｃｍ^２）、θ：金属箔中の同位体存在割合、ρ：金属箔の密度（ｇ／ｃｍ^３）、Ｖ：金属箔の体積（ｃｍ^３）、ｔ：中性子線の照射時間（ｓ）、Ｔ：生成核の半減期（ｓ）、Ｍ：金属箔の原子量、Ｓ：金属箔の表面積（ｃｍ^２）である。式（１）より、照射時間ｔが長くなると、放射平衡に漸近していくことが分かる。 FIG. 7 is a graph showing an example of the time change of the radioactivity intensity of the metal foil converter 52 in the irradiation process and the transfer process, showing an example of the Dy foil converter. When using dysprosium (Dy) for the metal foil, the main radionuclide produced by reaction with neutrons is ¹⁶⁴ Dy(n,γ) ¹⁶⁵ Dy. The vertical axis of the graph is the radioactivity surface density (Bq/cm ² ) of the metal foil converter 52, and the horizontal axis of the graph is the elapsed time (minutes). _The period up to time t1 is an irradiation step in which the measurement unit 50 is continuously irradiated with the neutron beam NB, and the radioactivity intensity increases with the passage of time. The radioactive surface density As(t) generated by irradiating the metal foil converter 52 with thermal neutrons for t seconds can be described by the following equation (1).

Here, f: neutron flux of irradiated neutron beam (n/cm ² s), σ: activation cross section of metal foil (cm ² ), θ: abundance ratio of isotope in metal foil, ρ: density of metal foil. (g/cm ³ ), V: volume of metal foil (cm ³ ), t: neutron beam irradiation time (s), T: half-life of generated nuclei (s), M: atomic weight of metal foil, S: metal It is the surface area of the foil (cm ² ). From equation (1), it can be seen that as the irradiation time t increases, the radiation equilibrium is asymptotically approached.

After time t1 in FIG. ₇ , the neutron beam irradiation is stopped. Times t ₁ to t ₂ are preparatory steps for housing the activated metal foil converter 52 in the transfer unit 14 . Time t ₂ to t ₃ is a transfer step for transferring the radioactivity intensity distribution of the metal foil converter 52 to the transfer plate 42 . When the neutron beam irradiation is stopped, the radioactive nuclides decay without generating new radionuclides, so the radioactivity intensity attenuates over time. The radioactivity surface density Ad(d) when d seconds have passed since the irradiation stop (time t ₁ ) can be described by the following equation (2).

In the transfer process from time t ₂ to t ₃ , radiation emitted from the metal foil converter 52 is accumulated in the transfer plate 42 . The integrated radioactivity intensity B accumulated on the transfer plate 42 corresponds to the shaded area in the graph of FIG. 7, and corresponds to the integrated value of the radioactivity surface density Ad ₍ d) from time t2 to _time t3. In order to increase the measurement sensitivity of the radioactivity intensity, it is preferable to maximize the value of the integrated radioactivity intensity B accumulated in the transfer plate 42 . In order to maximize the integrated radioactivity intensity B, the neutron beam NB is irradiated for a time longer than five times the _half -life T of the generated nuclei, and the time for the preparation process from _time t1 to t2 is shortened as much as possible to reduce the time to half. It is preferable to irradiate the transfer plate 42 with radiation from the metal foil converter 52 for a period of five times the period T or more.

The radioactivity intensity distribution transferred to the transfer plate 42 is obtained, for example, by scanning and irradiating the transfer plate 42 with a laser beam for reading, and detecting the amount of fluorescence at the laser irradiation position with a detector such as a photodiode or a photomultiplier tube. can be imaged as a digital signal (image data). For example, a reader (scanner) dedicated to imaging plates can be used as a reading device. Since the pixel value of the image data obtained in this way corresponds to the cumulative radioactivity intensity B described above, the time values of the irradiation process, the preparation process, and the transfer process, and the physical property values of the metal foil converter 52 are used in the formula By back-calculating (1) and equation (2), the neutron flux f with which the metal foil converter 52 is irradiated can be obtained.

As shown in FIG. 1, when the measurement regions 52a to 52c of the metal foil converter 52 are irradiated with neutron beams under different conditions, the neutron flux irradiated to each measurement region 52a to 52c is calculated and compared. By doing so, the neutron flux in the energy range of the resonance absorption peak can also be obtained. At this time, as shown in FIGS. 6(a) and 6(b), the metal foil converters 52 having the respective measurement regions 52a to 52c are brought into close contact with the same transfer plate 42, so that the time values of the preparation process and the transfer process can be adjusted respectively. The measurement areas 52a-52c can be the same. As a result, the parameters necessary for calculating the neutron flux can be shared, and the neutron flux at the resonance absorption peak can be obtained with higher accuracy. In addition, since the irradiation process and the transfer process are separated, the radioactivity intensity distribution of the metal foil converter 52 can be transferred to the transfer plate 42 in an environment less affected by gamma rays, and noise contained in the image data can be suppressed.

(Second embodiment)
FIG. 8 is a cross-sectional view schematically showing the configuration of a measurement unit 60 using step wedges. The measurement unit 60 comprises a metal foil converter 62 , a thermal neutron filter 64 and a metal filter 66 . This embodiment differs from the above-described embodiments in that a step wedge having a stepped thickness t _a to t _e is used as the metal filter 66 . By using the step wedge, the neutron flux corresponding to the difference in thickness of the metal filter 66 can be obtained, and the neutron flux at the resonance absorption peak can be obtained with higher accuracy. The following description focuses on differences from the above-described embodiment.

The metal filter 66 is a five-stage step wedge and has a first portion 66a, a second portion 66b, a third portion 66c, a fourth portion 66d and a fifth portion 66e having different thicknesses. The thicknesses t _a to t _e of the respective portions 66a to 66e are set, for example, within a range of 0.1 mm or more and less than 3 mm. _For example, the thickness ta of the first portion 66a is 0.1 mm, the thickness _tb of the second portion 66b is 0.2 mm, the thickness _tc of the third portion is 0.5 mm, and the thickness tb of the third portion 66b is 0.5 mm. The thickness td of the fourth portion _66d is 1.0 mm, and the thickness te of the fifth portion _66e is 2.0 mm. Note that the number of step wedges is not limited to five, and two, three, or four step wedges may be used, or six or more step wedges may be used. Further, the thickness of each step of the step wedge is not limited to that described above, and the thickness may be changed linearly at equal intervals, or may be changed exponentially or exponentially in a non-linear manner.

Metal foil converter 62 has a plurality of measurement areas 62a, 62b, 62c, 62d, 62e. The neutron beam NB that has passed through the first portion 66a of the metal filter 66 is incident on the first measurement region 62a. Similarly, second measurement region 66b, third measurement region 66c, fourth measurement region 66d and fifth measurement region 66e are provided with second portion 66b, third portion 66c, fourth portion 66d and second portion 66b of metal filter 66, respectively. A neutron beam NB that has passed through the fifth portion 66e is incident. A thermal neutron filter 64 such as a Cd filter is arranged between the metal foil converter 62 and the metal filter 66 . Therefore, the neutron beams NB that have passed through the thermal neutron filter 64 are incident on the respective measurement regions 62a to 62e of the metal foil converter 62. As shown in FIG.

The metal foil converter 62 and the metal filter 66 are made of gold ( ¹⁹⁷ Au), for example, and have a resonance absorption peak of about 5 eV, as in the above embodiments. Therefore, in the neutron absorption spectrum of the metal foil converter 62, the neutron flux of the resonance peak E _Au is at least partially removed by the metal filter 66, as in FIG. 2(c). In this embodiment, since the thickness of the metal filter 66 differs depending on the measurement region, the amount of neutron absorption of the resonance peak E _Au by the metal filter 66 differs. Specifically, in the first measurement region 62a where the thickness of the metal filter 66 is small, since the amount of absorption by the metal filter 66 is small, the neutron flux of the resonance peak E _Au absorbed by the first measurement region 62a is relatively large, The spectral groove at the resonance peak E _Au becomes shallower. On the other hand, in the fifth measurement region _62e where the thickness of the metal filter 66 is large, the amount of absorption by the metal filter 66 is large. The spectral groove in _Au deepens.

FIG. 9 is a diagram schematically showing an example of image data acquired using the measurement unit 60 of FIG. 8, and schematically showing an image 68 transferred to the transfer plate 42. As shown in FIG. Transfer image 68 has a plurality of areas 68a, 68b, 68c, 68d, 68e corresponding to each measurement area 62a-62e of metal foil converter 62. FIG. The pixel values of the areas 68a to 68e of the transfer image 68 are proportional to the magnitude of the neutron flux absorbed by the measurement areas 62a to 62e of the metal foil converter 62, and each has a blackening density that varies stepwise. The first region 68a corresponds to the first measurement region 62a having a relatively large neutron absorption amount, so it becomes a relatively black (dark) region. On the other hand, the fifth region 68e corresponds to the fifth measurement region 62e having a relatively small amount of neutron absorption, and thus becomes a relatively white (thin) region. In this way, as a result of measurement using the measurement unit 60, a transferred image 68 having a plurality of areas 68a to 68e with different blackening densities (light and shade) is obtained.

Note that the background area 68f outside the areas 68a to 68e is white for the sake of clarity of the drawing, but may actually be black. For example, if the metal filter 66 were not present on the region outside the measurement regions 62a-62e of the metal foil converter 62, the neutron flux incident on that region would be greater than the measurement region. As a result, the background region 68f of the transferred image 68 becomes a black (dark) portion so as to correspond to a relatively large amount of neutron absorption. Note that if a filter were provided to shield most of the neutron beams NB incident outside the measurement area of the metal foil converter 62, the background area 68f of the transferred image 68 would be a white (thin) area. .

In this embodiment, the thickness of the step wedge used as the metal filter 66 is known. Therefore, the absorption rate (or attenuation rate) of the neutron flux at the resonance peak E _Au by each portion 66a to 66e of the step wedge can be obtained by calculation. Therefore, the resonance peak E Neutron flux in _Au can be determined. In the present embodiment, since measured values based on a step wedge with a plurality of steps (for example, five steps) can be used, the resonance peak is more pronounced than in the case of comparing the measured values with and without the metal filter 56 as in the embodiment of FIG. Neutron flux in E _Au can be obtained with high accuracy.

(Third Embodiment)
FIG. 10 is a cross-sectional view schematically showing the configuration of a measurement unit 12 using multiple kinds of metal foils. The measurement unit 12 has a converter unit 20 and a filter unit 30 . The filter unit 30 is arranged on the neutron beam NB incidence side of the converter unit 20 so as to block the neutron beam NB entering the converter unit 20 . In the drawings, the incident direction of the neutron beam NB is the z direction (+z direction), and two directions orthogonal to the incident direction are the x direction and the y direction.

The converter unit 20 includes multiple types of metal foil converters 21 to 26 and a thermal neutron shield plate 28 . The filter unit 30 includes multiple types of metal filters 31 to 35, a support plate 38, and a thermal neutron filter 40. FIG. In this embodiment, by using a plurality of types of metal elements having different reaction characteristics with neutrons, neutron fluxes of a plurality of energies can be measured simultaneously. The following description focuses on differences from the above-described embodiment.

FIG. 11 is a top view schematically showing the configuration of converter unit 20 of FIG. A plurality of types of metal foil converters 21 to 26 are arranged side by side in the x direction on a thermal neutron shielding plate 28 . A plurality of types of metal foil converters 21 to 26 are arranged at different positions so as not to overlap each other in a plan view (xy plane) viewed from the incident direction of the neutron beam NB. The plurality of types of metal foil converters 21 to 26 are, for example, rectangular long in the y direction as shown, and are arranged at intervals in the x direction. The illustrated shape and arrangement of the metal foil converters 21 to 26 are merely examples, and other shapes and arrangements may be used. For example, a plurality of types of metal foil converters 21-26 may be arranged in a two-dimensional array. The thermal neutron shielding plate 28 is provided to remove thermal neutrons (noise components) scattered from the opposite side (back side) of the incident direction of the neutron beam NB and returning to the metal foil converters 21-26. An aluminum (Al) plate for supporting the thermal neutron shielding plate 28 may be further provided.

The plurality of types of metal foil converters 21 to 26 contain different types of metal elements, and react with different neutron energies to activate them. Examples of metal elements contained in each of the plurality of types of metal foil converters 21 to 26 include dysprosium (Dy), gold ( ¹⁹⁷ Au), indium ( ¹¹⁵ In), manganese ( ⁵⁵ Mn), cobalt ( ⁵⁹ Co), and copper. Either ( ⁶³ Cu) or erbium ( ¹⁷⁰ Er) can be selected. As mentioned above, dysprosium (Dy) can be used to measure the neutron flux in the thermal neutron region E1. In addition, gold ( ¹⁹⁷ Au), indium ( ¹¹⁵ In), manganese ( ⁵⁵ Mn), cobalt ( ⁵⁹ Co), copper ( ⁶³ Cu) and erbium ( ¹⁷⁰ Er) have specific resonance absorption energies in the epithermal neutron region E2. can be used to measure the neutron flux of

Aluminum (Al) or magnesium (Mg) can also be used to measure high-energy neutrons of about 1 MeV or higher. When aluminum ( ²⁷ Al) is irradiated with high-energy neutrons, an (n, α) reaction that captures neutrons and emits alpha rays occurs, generating activated sodium ( ²⁴ Na). Also, when magnesium ( ²⁴ Mg) is irradiated with high-energy neutrons, an (n, p) reaction occurs in which neutrons are captured and protons are emitted, generating activated sodium ( ²⁴ Na). Radioactive sodium ( ²⁴ Na) is stabilized to magnesium ( ²⁴ Mg) by beta decay. Neutron flux can be measured. The reaction cross-sections of Al and Mg are smaller than the cross-sections of resonance absorption of Au or the like, and the advantage of combining a metal filter is small, so a metal foil converter may be used alone.

Although it is not particularly limited which metal element is used for each of the plurality of types of metal foil converters 21 to 26, as an example, the first metal foil converter 21 is Dy foil and the second metal foil converter 22 is In foil. , the third metal foil converter 23 is Au foil, the fourth metal foil converter 24 is Mn foil (or manganin foil), the fifth metal foil converter 25 is Er foil, and the sixth metal foil converter 26 is Al foil. be able to.

12 is a top view schematically showing the configuration of the filter unit 30 of FIG. 10. FIG. A plurality of types of metal filters 31 to 35 are arranged side by side in the x direction on a support plate 38 . The support plate 38 is preferably made of a material having a small absorption cross-section in the thermal neutron region E1 and the epithermal neutron region E2. For example, aluminum (Al) or magnesium (Mg) can be used. A plurality of types of metal filters 31 to 35 are arranged at positions overlapping the corresponding metal foil converters 21 to 25 in the neutron beam incident direction (z direction). In the illustrated example, the positions and sizes in the x and y directions of the corresponding metal foil converters 21 to 25 and the metal filters 31 to 35 are exactly the same, but they are aligned with each other in at least one of the x and y directions. They may be overlapped so as to be shifted from each other. That is, the corresponding metal foil converters 21-25 and metal filters 31-35 need only partially overlap in the z-direction. In the illustrated example, a metal filter corresponding to the sixth metal foil converter (for example, Al foil) 26 is not provided, but a sixth metal filter corresponding to the sixth metal foil converter 26 may be further arranged.

Each of the plurality of types of metal filters 31-35 is configured as a step wedge. FIG. 13 is a cross-sectional view schematically showing the configuration of the step wedge of FIG. 12, showing a CC line cross-section of FIG. The first metal filter 31 has a plurality of portions 31a-31j whose thickness changes stepwise. Each stage of the step wedge is aligned in the x-direction. The first to fifth portions 31a to 31e located in the left half of the step wedge (the upper half in FIG. 12) are arranged at positions not overlapping the thermal neutron filter 40. As shown in FIG. As a result, the neutron beams NB passing through the first to fifth portions 31a to 31e of the first metal filter 31 and entering the first metal foil converter 21 include the neutron flux in the thermal neutron region E1. On the other hand, the sixth to tenth portions 31f to 31j located in the right half of the step wedge (lower half in FIG. 12) are arranged at positions overlapping the thermal neutron filter . As a result, the neutron flux in the thermal neutron region E1 is excluded from the neutron beams NB that pass through the sixth to tenth portions 31f to 31j of the first metal filter 31 and enter the first metal foil converter 21. Therefore, by using the measurement unit 12 of the present embodiment, data including neutrons in the thermal neutron region E1 and data excluding the neutrons in the thermal neutron region E1 can be obtained simultaneously.

Each of the plurality of types of metal filters 31-35 is composed of the same type of metal element as the corresponding metal foil converters 21-25. Thus, to give an example of the above, the first metal filter 31 is a Dy step wedge, the second metal filter 32 is an In step wedge, the third metal filter 33 is an Au step wedge, and the fourth metal filter 34 is a Mn (or manganin) step wedge and the fifth metal filter 35 is an Er step wedge. Also in the present embodiment, the thickness of the step wedge can be appropriately set within a range of, for example, 0.1 mm or more and less than 3 mm.

After irradiating the measuring unit 12 with the neutron beam NB to be measured, the activated metal foil converters 21 to 26 are brought into close contact with the transfer plate 42 using the transfer unit 14 . At this time, by transferring all of the metal foil converters 21 to 26 of a plurality of types to the same transfer plate 42, the time value of the preparation process for calculating the neutron flux and the transfer process can be shared. It should be noted that each of the plurality of types of metal foil converters 21 to 26 may be separated and transferred to separate transfer plates. In this case, it is preferable to calculate the neutron flux absorbed by each metal foil converter using the time values of the preparation and transfer steps on separate transfer plates. After that, by analyzing the image data acquired through the transfer plate, the neutron flux of the energy corresponding to each of the plurality of types of metal foil converters 21 to 26 can be calculated.

For example, by selecting Dy, In, Au, Mn, Er, and Al as metal elements of the metal foil converters 21 to 26 of a plurality of types, thermal neutron region (~0.4 eV), epithermal neutron region (0.4 eV), 4 eV to 1 eV, Dy), neutron flux at resonance absorption energies of 1.5 eV (In), 5 eV (Au), 100 eV (Er), and 337 eV (Mn), and high energy neutron flux of about 1 MeV or higher (Al) can be obtained. A simple neutron energy distribution (spectrum) can be obtained by graphing the neutron flux at these multiple energy values.

FIG. 14 is a cross-sectional view schematically showing the configuration of a measurement unit 12 according to a modification. This modification has the same configuration as the measurement unit 12 shown in FIGS. It differs from the above-described embodiment in that it is arranged.

The converter unit 20 comprises a Dy foil converter 21 , an In foil converter 22 , an Au foil converter 23 , a Mn foil (or manganin foil) converter 24 , an Er foil converter 25 and an Al foil converter 26 . The plurality of types of metal foil converters 21 to 26 are arranged so as to overlap all of the plurality of types of metal filters (or step wedges) 31 to 35 of the filter unit 30 in the incident direction of the neutron beams NB.

It is preferable that the stacking order of the plurality of types of metal foil converters 21 to 26 is appropriately set according to the types of metal elements contained. For example, the Al foil converter 26, which has a small reaction cross section in both the thermal neutron region E1 and the epithermal neutron region E2, has less influence on the other metal foil converters, so the incidence of the neutron beam NB is lower than that of the other metal foil converters. It is arranged on the near side (-z direction side) of the direction. On the other hand, the Dy foil converter 21, which has a large reaction cross section in both the thermal neutron region E1 and the epithermal neutron region E2, has a higher incidence of neutron beams NB than the other metal foil converters in order to avoid affecting the other metal foil converters. It is arranged on the back side of the direction (+z direction side).

The stacking order of In, Au, Mn, and Er foil converters having other resonant absorption peaks is not so strict, but metal elements with relatively smaller absorption cross sections can be placed closer to the neutron beam incident direction. preferable. In addition, since Au foil is commonly used as a reference for neutron measurement, the Au foil converter 23 is arranged in a direction of incidence of neutron beams compared to other metal foils in order to reduce measurement errors in the Au foil. It is preferable to arrange it on the near side. For this reason, in the illustrated example, the metal foil converters are laminated in the order of Al foil 26, Au foil 23, Er foil 25, In foil 22, Mn foil 24, and Dy foil 21 from the neutron beam incident side. .

Some of the metal foil converters 21 to 26 of the plurality of types may be arranged only at positions overlapping the corresponding metal filters. For example, since the Mn foil 24 and the Er foil 25 are more expensive than other metal foils, it is preferable to reduce the area of the metal foil as much as possible from the economical point of view. Therefore, some of the metal foils such as the Mn foil 24 and the Er foil 25 may be arranged only in the portions where the corresponding metal filters 34 and 35 are located, as in the embodiment of FIG. 10 described above. On the other hand, other metal foils (eg, Dy foil 21, In foil 22, Au foil 23, Al foil 26) may be arranged so as to overlap all of the plurality of metal filters 31-35.

Also in this modification, the neutron spectrum can be calculated in the same manner as in the above-described embodiment by using the radioactivity intensity of the portion where the metal foil converter and the metal filter of the same kind of metal element overlap. In addition, in this modification, the data of the overlapping portion of the metal foil converter and the metal filter (for example, the Au foil converter 23 and the In filter 32) composed of different types of metal elements can be obtained, but the above-mentioned neutron spectrum is calculated. This data acquisition is not required for the purposes of

(Example)
Next, an example used for actual measurement will be described. FIG. 15 is a top view schematically showing the configuration of the measurement unit 12 according to the example. This embodiment has a configuration similar to the modification of FIG. 14 described above. The filter unit 30 has a Dy filter 31, an In filter 32, an Au filter 33, a Mn (manganin) filter 34, an Er filter 35 and a Co filter 36, which are step wedges. In this embodiment, erbium (Er) and cobalt (Co) are used together to measure the neutron flux around 100 eV. A Cd filter 40 is positioned to cover only the lower half of the step wedge.

Various indicators are arranged on the support plate 38 of the filter unit 30 . Specifically, a character mark 72, a beam purity indicator 73 and a sensitivity indicator 74 conforming to the American Society of Samples and Materials (ASTM) standards, a circle mark 75, a cross mark 76, and the like are arranged. Furthermore, a pen point 78 of a fountain pen is arranged as a sample containing gold (Au).

A converter unit 20 similar to that shown in FIG. 14 is arranged below the filter unit 30 . Converter unit 20 includes Dy foil converter 21 , In foil converter 22 , Au foil converter 23 , Mn (manganin) foil converter 24 , Er foil converter 25 and thermal neutron shield plate (Cd plate) 28 . The Al foil converter 26 is not included in this embodiment. Also, the Er foil converter 25 is arranged only in a limited area where the Er filter 35 or Co filter 36 and the Cd filter 40 overlap. Other metal foil converters 21 - 24 are sized and positioned to be positioned throughout filter unit 30 . The metal foil converters 21 to 25 are arranged in the order of Er foil, Au foil, In foil, Mn foil, and Dy foil in the incident direction of the neutron beam to be measured.

In this example, a cyclotron accelerator neutron source was used, the current amount was set to 16 μA, and the measurement unit 12 was continuously irradiated with neutron beams NB for 7 hours (420 minutes). The neutron flux f in the thermal neutron region E1 of the irradiated neutron beam is approximately 1.5×10 ⁵ (n/cm ² /s). After the irradiation, the preparatory step of setting the metal foil converters 21 to 25 in the transfer unit 14 was 24 minutes, and the transfer time to the transfer plate 42 was 17 hours (1020 minutes). The image transferred to the transfer plate 42 was converted into image data using a dedicated scanner (reader).

FIG. 16 is a diagram showing an example of a transferred image of the Dy foil converter 21. As shown in FIG. As shown, various metal filters (step wedges) and indicators included in the filter unit 30 can be imaged with high spatial resolution. In particular, it has been confirmed that the 50 μm gap indicated by the ASTM standard indicator is visible.

Since the upper half area without the thermal neutron filter 40 absorbs the neutron flux of the thermal neutron area E1, the blackening density is higher than the lower half area with the thermal neutron filter 40. Also, it can be seen that the blackening density of the portion corresponding to the Dy filter 31 among the portions overlapping with the plurality of types of metal filters 31 to 36 is relatively low. This is because the Dy filter 31 made of the same metal element as the Dy foil converter 21 shields the neutron flux in the energy range with high measurement sensitivity (that is, the absorption cross section is high). By analyzing the pixel values of the portion corresponding to the step wedge of the Dy filter 31 in the illustrated transfer image of the Dy foil converter 21, the neutron flux in the energy range in which Dy reacts can be calculated.

FIG. 17 is a diagram showing an example of a transferred image of the In foil converter 22. As shown in FIG. Similar to the transferred image of the Dy foil converter 21 in FIG. 16, the configuration of the filter unit 30 can be imaged with high spatial resolution. In the transferred image of the In foil converter 22, the blackening density of the overlapping portion of the In filter 32 and the thermal neutron filter 40 is remarkably low. This is because the filter unit 30 shields both the thermal neutron region E1 and the resonance absorption peak E _In in which the measurement sensitivity of the In foil converter 22 is relatively high. In the illustrated transferred image of the In foil converter 22, the neutron flux at the energy (approximately 1 eV) corresponding to In resonance absorption can be calculated by analyzing the pixel values of the portion corresponding to the step wedge of the In filter 32.

FIG. 18 is a diagram showing an example of a transferred image of the Au foil converter 23. As shown in FIG. Similar to the transfer image of the In foil converter 22 in FIG. 17, the blackening density of the overlapping portion of the Au filter 33 and the thermal neutron filter 40 is remarkably low. In addition, the pen tip 78 containing gold (Au) also has a low blackening density, and it can be seen that the sample containing gold can be imaged with high definition. In the illustrated transfer image of the Au foil converter 23, by analyzing the pixel values of the portion corresponding to the step wedge of the Au filter 33, the neutron flux of the energy (approximately 5 eV) corresponding to the resonance absorption of Au can be calculated.

FIG. 19 is a diagram showing an example of a transferred image of the Mn (manganin) foil converter 24. As shown in FIG. Similar to the transfer image of the In foil converter 22 in FIG. 17, the blackening density of the portion where the Mn (manganin) filter 34 and the thermal neutron filter 40 overlap is remarkably low. In the illustrated transfer image of the Mn foil converter 24, by analyzing the pixel values of the portion corresponding to the step wedge of the Mn filter 34, the neutron flux of the energy (approximately 300 eV) corresponding to the resonance absorption of Mn can be calculated.

FIG. 20 is a diagram showing an example of a transferred image of the Er foil converter 25. As shown in FIG. Since the Er foil converter 25 is arranged only in a partial area of the measurement unit 12, the transfer image is shown at the position of the Er foil converter 25 so that the positional relationship with the entire measurement unit 12 can be easily understood. As shown in the figure, it can be seen that the blackening density of the portion overlapping with the Er filter 35 and the Co filter 36 is remarkably low. Although not clearly shown in the illustrated image, the difference due to the thickness of the step wedge can also be identified in the pixel values of the acquired image. Therefore, by analyzing the pixel values of the portion of the Er filter 35 or Co filter 36 corresponding to the step wedge, the neutron flux of energy (approximately 100 eV) corresponding to the resonance absorption of Er or Co can be calculated.

FIG. 21 is a graph showing an example of neutron spectrum measurement results and corresponding simulation results. The thick solid line in the graph is the measurement result of the neutron flux calculated using the transfer images of FIGS. 16 to 20 described above. The thin solid line and dotted line in the graph are the results of simulation analysis of the irradiation field of neutron beams. For the simulation, PHITS (Particle and Heavy Ion Transport code System, https://phits.jaea.go.jp/index.html, Tatsuhiko Sato, Yosuke Iwamoto, Shintaro Hashimoto, Tatsuhiko Ogawa, Takuya Furuta, Shin-ichiro Abe, Takeshi Kai, Pi-En Tsai, Norihiro Matsuda, Hiroshi Iwase, Nobuhiro Shigyo, Lembit Sihver and Koji Niita Features of Particle and Heavy Ion Transport code System (PHITS) version 3.02, J. Nucl. Sci. Technol. (2018)) was used. As shown in the figure, it can be seen that the measurement results according to this example are within the range of the standard deviation of the simulation results. Therefore, according to this example, it can be seen that the tendency of the neutron energy distribution can be obtained by a relatively simple measuring method.

FIG. 22 is a diagram schematically showing the configuration of the neutron spectrum measurement device 10. As shown in FIG. The neutron spectrum measurement device 10 includes a measurement unit 12 , a transfer unit 14 , a reading device 16 and an analysis device 18 . Measuring unit 12 is measuring unit 12, 50 or 60 in the embodiments described above. The transfer unit 14 is a cassette for transferring the radioactivity intensity of the activated metal foil converter contained in the measurement unit 12 to the transfer plate 42 . The reading device 16 is a reader or scanner for reading the image transferred onto the transfer plate 42 . The analysis device 18 analyzes the image data generated by the reading device 16 and calculates the neutron flux for each energy. The analysis device 18 can be realized by, for example, a general-purpose computer and analysis software installed on the general-purpose computer.

FIG. 23 is a flow chart showing the flow of the neutron spectrum measurement method. First, the measurement unit 12 is irradiated with a neutron beam to be measured (S10). As a result, the converter unit 20 is irradiated with neutron beams through the filter unit 30 of the measurement unit 12, and the metal foil converters 21 to 26 included in the converter unit 20 are activated. Next, the activated metal foil converters 21 to 26 are set in the transfer unit 14 (S12). Thereby, the two-dimensional intensity distribution of the radiation emitted from each of the activated metal foil converters is transferred to the transfer plate 42 . The transfer process is continued until a predetermined transfer time elapses (N of S14), and after the predetermined transfer time elapses (Y of S14), the transfer plate 42 is removed from the transfer unit 14, and the image transferred to the transfer plate 42 is transferred. is read using the reader 16 (S16). Thereby, image data corresponding to the two-dimensional intensity distribution of the radiation transferred to the transfer plate 42 is generated. Subsequently, the generated image data is analyzed to calculate the neutron flux (S18). This makes it possible to calculate the neutron flux at a plurality of energy values corresponding to each of the plurality of types of metal foil converters 21 to 26, and obtain a simple energy distribution (spectrum) of neutrons.

According to the present embodiment, measurement with a high spatial resolution (for example, 50 μm) is possible, so even if step wedges of a plurality of types of metal elements are arranged two-dimensionally, a relatively small measuring unit 12 can be realized. . Furthermore, by arranging a plurality of small measurement units 12, neutron spectra at different positions can be measured simultaneously. For example, when the beam diameter of the neutron beam NB is large, a two-dimensional distribution of the neutron flux within the beam can be obtained by arranging a plurality of measurement units 12 within the range of the beam diameter.

The present invention has been described above based on the embodiments. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments, and that various design changes and modifications are possible, and that such modifications are within the scope of the present invention. It is about

In the above-described embodiments, the case of using a stimulable phosphor as the transfer plate was shown, but other types of phosphors may be used, or a combination of a lead-encapsulated intensifying screen and a photosensitive film may be used. good.

Although the irradiation process and the transfer process are separated in the above-described embodiment, image data may be acquired simultaneously in the irradiation process in a modified example. For example, by attaching a phosphor sheet to a metal foil converter, image data corresponding to the radioactivity intensity distribution can be generated by imaging the phosphor sheet that emits light during neutron beam irradiation with a CCD or CMOS sensor. good too.

A member used for another measuring method may be additionally arranged in the measuring unit of the above-described embodiments. For example, instead of metal foil, a block-shaped metal member may be arranged, and radiation emitted from the activated metal block may be measured using a radiation counter such as a germanium (Ge) semiconductor detector. As an example, a gold (Au) block and an aluminum (Al) or magnesium (Mg) block may be provided in a measurement unit and gamma rays emitted from each activated block may be measured. Thereby, a high-energy neutron flux of about 1 MeV or more may be calculated using the measurement result of the Au block as a reference standard.

DESCRIPTION OF SYMBOLS 10... Neutron spectrum measuring apparatus, 12... Measurement unit, 14... Transfer unit, 16... Reader, 18... Analyzer, 20... Converter unit, 21 to 26... Metal foil converter, 30... Filter unit, 31 to 35... Metal Filter 40 Thermal neutron filter 42 Transfer plate 50 Measurement unit 52 Metal foil converter 54 Thermal neutron filter 56 Metal filter 60 Measurement unit 62 Metal foil converter 64 Thermal neutron Filter, 66... Metal filter, NB... Neutron beam.

Claims (10)

a converter unit having a plurality of types of metal foil converters each containing different types of metal elements that are activated in response to the energy of a neutron beam to be measured;
a filter unit having a plurality of types of metal filters each containing the same type of metal element as the corresponding metal foil converter, and arranged on the neutron beam incident side of the converter unit;
The plurality of types of metal filters are arranged at mutually different positions in a plan view viewed from the incident direction of the neutron beam,
The plurality of types of metal foil converters are arranged at positions at least partially overlapping the corresponding metal filters in the incident direction of the neutron beam ,
The converter unit is separable from the filter unit, and the intensity of radiation emitted from each of the plurality of types of activated metal foil converters is measured while being separated from the filter unit. and a neutron spectrum measurement device. 2. The neutron spectrum measurement apparatus according to claim 1, wherein said plurality of types of metal foil converters are positioned and sized so as to at least partially overlap with the direction of incidence of said neutron beam. The plurality of types of metal foil converters are
a Dy foil converter containing dysprosium (Dy);
an Au foil converter containing gold (Au);
and a metal foil converter containing any one metal element selected from the group comprising indium (In), manganese (Mn), cobalt (Co), erbium (Er) and copper (Cu). 3. The neutron spectrometer according to claim 1 or 2. The Dy foil converter is arranged on the far side in the direction of incidence of the neutron beams relative to the other metal foil converters, and the Au foil converter is arranged on the front side in the direction of incidence of the neutron beams relative to the other metal foil converters. The neutron spectrum measurement device according to claim 3, characterized in that The plurality of kinds of metal foil converters are metal foils containing a first metal element selected from a group comprising indium (In), manganese (Mn), cobalt (Co), erbium (Er) and copper (Cu). 5. A metal foil converter according to any one of claims 1 to 4, comprising a converter and a metal foil converter containing a second metal element different from said first metal element selected from said group. neutron spectrometer. a converter unit having a plurality of types of metal foil converters each containing different types of metal elements that are activated in response to the energy of a neutron beam to be measured;
a filter unit having a plurality of types of metal filters each containing the same type of metal element as the corresponding metal foil converter, and arranged on the neutron beam incident side of the converter unit;
The plurality of types of metal filters are arranged at mutually different positions in a plan view viewed from the incident direction of the neutron beam,
The plurality of types of metal foil converters are arranged at positions at least partially overlapping the corresponding metal filters in the incident direction of the neutron beam,
The neutron spectrum measuring apparatus, wherein the plurality of types of metal filters are step wedges whose thickness in the direction of incidence of the neutron beam changes stepwise. The neutron spectrum measuring apparatus according to any one of claims 1 to 6, wherein the thickness of the plurality of kinds of metal filters in the direction of incidence of the neutron beam is 0.1 mm or more and less than 3 mm. a converter unit having a plurality of types of metal foil converters each containing different types of metal elements that are activated in response to the energy of a neutron beam to be measured;
a filter unit having a plurality of types of metal filters each containing the same type of metal element as the corresponding metal foil converter, and arranged on the neutron beam incident side of the converter unit;
The plurality of types of metal filters are arranged at mutually different positions in a plan view viewed from the incident direction of the neutron beam,
The plurality of types of metal foil converters are arranged at positions at least partially overlapping the corresponding metal filters in the incident direction of the neutron beam,
The neutron spectrum measuring apparatus, wherein the filter unit further comprises a thermal neutron filter arranged at a position at least partially overlapping with at least one of the plurality of types of metal filters in the incident direction of the neutron beam. The neutron spectrum measurement according to any one of claims 1 to 8, wherein the thickness of the plurality of types of metal foil converters in the direction of incidence of the neutron beam is 0.01 mm or more and 0.3 mm or less. Device. A neutron spectrum measurement method using the neutron spectrum measurement device according to any one of claims 1 to 9,
irradiating the converter unit with a neutron beam through the filter unit to activate the plurality of types of metal foil converters;
transferring a two-dimensional intensity distribution of radiation emitted from each of the plurality of types of activated metal foil converters onto a transfer plate in a state in which the converter unit is separated from the filter unit ;
measuring an image corresponding to a two-dimensional intensity distribution of radiation transferred to the transfer plate to generate image data;
Analyzing the image data, and calculating neutron flux at a plurality of energy values respectively corresponding to the plurality of types of metal foil converters.

Images