JP2016045124A - Estimation method for depth distribution of radioactive substance in soil - Google Patents
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本発明は、土壌中の放射性物質深度分布推定法に関する。 The present invention relates to a method for estimating a radioactive material depth distribution in soil.
放射性物質により汚染された地域では土壌の除染が必要であり、汚染土の量やその保管、処理が問題となっている。また、汚染地域の土壌の除染は、宅地が優先され、その後田畑や山林なども除染することになるが、効果的・効率的に除染するには、除染深さを確認する必要がある。
土壌中の放射性物質の深さ方向の分布の測定には、実際に土をサンプリングして測定する方法が従来とられている（特許文献１参照）。しかしながら、この土をサンプリングして測定する方法では、サンプリング測定に非常に手間がかかり、測定結果が得られるまで時間がかかるという問題があった。また、測定に要するコストも高く、さらに、サンプリングした土の測定後の処分についても考える必要があった。
In areas contaminated with radioactive materials, soil decontamination is necessary, and the amount of contaminated soil, its storage and treatment are problematic. In addition, residential land is given priority for decontamination of soil in contaminated areas, and fields and forests are then decontaminated. However, in order to effectively and efficiently decontaminate, it is necessary to check the decontamination depth. There is.
For measuring the distribution of radioactive materials in the soil in the depth direction, a method of actually sampling and measuring the soil has been used (see Patent Document 1). However, the method of sampling and measuring this soil has a problem that it takes much time for sampling measurement and it takes time until a measurement result is obtained. Moreover, the cost required for the measurement is high, and it is also necessary to consider the disposal of the sampled soil after the measurement.
そこで、本発明の解決しようとする課題は、実際に土をサンプリングすることなく、土壌中の放射性物質の深さ方向の分布を推定できる推定法を提供することにある。 Therefore, the problem to be solved by the present invention is to provide an estimation method capable of estimating the distribution of radioactive materials in the soil in the depth direction without actually sampling the soil.
上記課題を解決するために、本発明は、予めシミュレーション計算により、土壌中のある深度にのみ放射性物質が分布する場合（以下これを「不均一分布」という）と、土壌中のある深度まで一様に放射性物質が分布する場合（以下これを「一様分布」という）について、地表面に届く光子の低エネルギー、中間エネルギー、高エネルギーの３成分の光子数の比率をもとめ、一方の軸を光子数の比率「低エネルギー／中間エネルギー」とし、他方の軸を光子数の比率「（高エネルギー＋中間エネルギー）／低エネルギー」としたグラフに不均一分布の検量線と、一様分布の検量線をそれぞれの検量線に沿って深度数値を付して作成しておき、次に、被測定対象の土壌の地表面に光子スペクトロメータを設置し土壌からくる放射線を計測して得られた光スペクトルから、低エネルギー、中間エネルギー、高エネルギーの３成分の光子数の比率として「低エネルギー／中間エネルギー」および「（高エネルギー＋中間エネルギー）／低エネルギー」を求め、予めシミュレーション計算により求めていた前記不均一分布の検量線と前記一様分布の検量線のどちらにマッチングするかを求めることにより土壌中の放射性物質の深度分布を推定することを特徴とする放射性物質深度分布推定法。 In order to solve the above-mentioned problems, the present invention is based on a simulation calculation in which a radioactive substance is distributed only at a certain depth in the soil (hereinafter referred to as “non-uniform distribution”), and when the radioactive material is distributed to a certain depth in the soil. When the radioactive material is distributed like this (hereinafter referred to as “uniform distribution”), the ratio of the number of three photons of low energy, intermediate energy, and high energy of the photons that reach the ground surface is determined. A graph with a photon number ratio of “low energy / intermediate energy” and a photon number ratio of “(high energy + intermediate energy) / low energy” on the other axis. Lines are created with depth values along each calibration curve, and then a photon spectrometer is installed on the surface of the soil to be measured to measure the radiation coming from the soil. From the measured light spectrum, “low energy / intermediate energy” and “(high energy + intermediate energy) / low energy” are obtained as the ratio of the number of three-component photons of low energy, intermediate energy, and high energy, and simulation calculations are performed in advance. Radioactive material depth distribution estimation method, wherein the depth distribution of radioactive material in soil is estimated by determining whether the non-uniform distribution calibration curve or the uniform distribution calibration curve is matched .
本発明の推定法によれば、汚染土壌をサンプリングする必要がないので従来必要であったサンプリングのための時間と手間がかからない。
また、本発明の推定法によれば、光子スペクトロメータを地表面に設置して計測するだけでよく、現場でのデータ取得が可能となることから、効率的・効果的な除染作業が行え、除染土の削減によりその保管に要する費用なども大幅に削減できる。
According to the estimation method of the present invention, since it is not necessary to sample the contaminated soil, it does not take time and labor for sampling, which is conventionally required.
In addition, according to the estimation method of the present invention, it is only necessary to install and measure a photon spectrometer on the ground surface, and data can be acquired on-site, so that efficient and effective decontamination work can be performed. The cost of storage can be greatly reduced by reducing the decontamination soil.
本発明者等は、シミュレーション計算により、汚染土壌中の放射性物質の深度分布によって、土表面で計測される光子スペクトルの形状が変化することに着目し、この深度分布の違いによる光子スペクトル形状をデータベース化することによって、迅速に、また非破壊で土壌中の放射性物質の深度分布を推定するものである。
すなわち、本発明では、光子スペクトロメータを地表面に設置して土壌からくる放射線を計測することにより、その放射性物質の深度分布、およびその放射能濃度を推定するものであり、非破壊での測定により、またその場で結果が得られるため、従来のサンプリングによる手法にくらべて効率的な測定が可能となる。
土壌中の放射性物質が浅い位置では、放射性物質から直接放出されるγ線成分が検出器に届く確率が高くなる。一方深い位置にある放射性物質では、同様にγ線が放出されるが、地表面にある検出器に届くまでに土と相互作用して、持っているエネルギーを失い中間、また低エネルギー光子となる。土壌はこのように散乱体として働くが、さらに吸収体としても作用する。すなわち深部になればなるほど、低エネルギー成分は吸収されてしまい、地表に到達するのは中間エネルギーの光子が多くなる。この３成分の比率を考慮することによって、どの深さまで放射性物質が分布しているか、またその量について推定することができる。
なお、この３成分の比率だけでは、土壌中の分布の仕方によっては、正しく推定できない可能性があるので、あらかじめ分布を仮定してデータセットを準備しておく必要がある
The present inventors pay attention to the fact that the shape of the photon spectrum measured on the soil surface changes according to the depth distribution of the radioactive material in the contaminated soil, and the photon spectrum shape due to the difference in the depth distribution is stored in the database. It is possible to estimate the depth distribution of radioactive materials in soil quickly and non-destructively.
That is, in the present invention, the photon spectrometer is installed on the ground surface and the radiation coming from the soil is measured to estimate the depth distribution of the radioactive material and the radioactivity concentration thereof, and the measurement is performed nondestructively. Therefore, since the result can be obtained on the spot, the measurement can be performed more efficiently than the conventional sampling method.
At a position where the radioactive material in the soil is shallow, there is a high probability that a γ-ray component directly emitted from the radioactive material reaches the detector. On the other hand, radioactive materials at deeper positions emit gamma rays in the same way, but interact with the soil before reaching the detector on the ground surface, losing their energy and becoming intermediate and low energy photons. . The soil thus acts as a scatterer, but also acts as an absorber. In other words, the deeper the part, the lower energy components are absorbed, and more intermediate energy photons reach the earth's surface. By considering the ratio of these three components, it is possible to estimate to what depth the radioactive material is distributed and the amount thereof.
Note that it may not be possible to correctly estimate the ratio of these three components depending on the distribution method in the soil, so it is necessary to prepare a data set assuming the distribution in advance.
図１は、本発明の測定原理を説明するための図であり、地表に設置した光子スペクトロメータでの測定結果をシミュレーション計算により示したものであり、横軸が光子エネルギー［ＭｅＶ］、縦軸が光子数［＃／ｃｍ2］（単位平方ｃｍ当たりの個数）である。シミュレーション計算にはモンテカルロ法による光子輸送コードを用い、地中に分布した放射性物質から放出されるγ線を模擬して地上まで到達する光子を計算した。土壌としてＳｉＯ2（３９.８３％），ＣａＯ（１.６５％），Ａｌ2Ｏ3（２８.６７％），Ｈ2Ｏ（１６.８％），Ｋ2Ｏ（０.２４），ＭｇＯ（６.０５％），Ｆｅ2Ｏ3（１７.７％），ＴｉＯ2（１.７２％），Ｐ2Ｏ3（０.１％）としている。ガンマ線源としてＣｓ−１３７を想定し、０.６６２ＭｅＶの光子が発生するものとした。白抜き丸のプロット○は、放射性物質が深さ５ｃｍにのみ分布している場合の光子スペクトルを示し、白抜き四角のプロット□は、放射性物質が深さ２０ｃｍのところまで一様に分布している場合の光子スペクトルを示している。そして、光子エネルギーの大きさにより、図１のごとく低エネルギー（図では約０.２２［ＭｅＶ］以下）、中間エネルギー（図では、約０.２２［ＭｅＶ］〜約０.５［ＭｅＶ］）、高エネルギー（図では約０.５［ＭｅＶ］以上）の３成分に分けてその比率を考える。
未除染地域の土壌では土壌中の放射性物質はある深さにのみ分布（以下これを「不均一分布」と表記することがある）し、耕作地などでは土壌を撹拌しているため土壌中の放射性物質はある深さまで一様に分布（以下これを「一様分布」と表記することがある）すると考えられる。そこでこの２パターンを考慮したデータを予め準備しておき、マッチングにより分布の深さを推定することができる。
すなわち、予めシミュレーション計算により、土壌中の放射性物質が深さ５ｃｍ、１０ｃｍ、１５ｃｍ、２０ｃｍ、２５ｃｍに不均一分布している場合についてそれぞれ図１に示した光スペクトルのグラフを得、低エネルギー、中間エネルギー、高エネルギーの３成分の光子数の比率として「低エネルギー／中間エネルギー」および「（高エネルギー＋中間エネルギー）／低エネルギー」を求め、横軸に比率「低エネルギー／中間エネルギー」を、縦軸に比率「（高エネルギー＋中間エネルギー）／低エネルギー」を取ったグラフにプロットしたものが図２の白抜き四角□の破線で示したグラフである。同様に、土壌中の放射性物質が深さ５ｃｍ、１０ｃｍ、１５ｃｍ、２０ｃｍ、２５ｃｍまで一様分布している場合についてそれぞれ図１に示した光スペクトルのグラフを得、低エネルギー、中間エネルギー、高エネルギーの３成分の光子数の比率として「低エネルギー／中間エネルギー」および「（高エネルギー＋中間エネルギー）／低エネルギー」を求め、横軸に比率「低エネルギー／中間エネルギー」を、縦軸に比率「（高エネルギー＋中間エネルギー）／低エネルギー」を取ったグラフにプロットしたものが図２の白抜き丸○の実線で示したグラフである。
上記の説明では、不均一分布と一様分布について、それぞれ５ｃｍ刻みでシミュレーション計算を行うとしたが、これに限定されるものではない。
FIG. 1 is a diagram for explaining the measurement principle of the present invention, in which a measurement result with a photon spectrometer installed on the ground surface is shown by simulation calculation, where the horizontal axis represents photon energy [MeV] and the vertical axis. Is the number of photons [# / cm 2 ] (number per unit square cm). The photon transport code by Monte Carlo method was used for the simulation calculation, and the photons reaching the ground were calculated by simulating γ-rays emitted from radioactive materials distributed in the ground. Soil as SiO 2 (39.83%), CaO (1.65%), Al 2 O 3 (28.67%), H 2 O (16.8%), K 2 O (0.24), MgO (6.05%), Fe 2 O 3 (17.7%), TiO 2 (1.72%), P 2 O 3 (0.1%). Assuming that Cs-137 is used as a gamma ray source, a photon of 0.662 MeV is generated. The white circle plot ◯ shows the photon spectrum when the radioactive material is distributed only at a depth of 5 cm, and the white square plot □ shows that the radioactive material is uniformly distributed up to a depth of 20 cm. The photon spectrum is shown. Then, depending on the magnitude of the photon energy, low energy (about 0.22 [MeV] or less in the figure) and intermediate energy (about 0.22 [MeV] to about 0.5 [MeV] in the figure) as shown in FIG. The ratio is divided into three components of high energy (approximately 0.5 [MeV] or more in the figure).
In undecontaminated soil, the radioactive material in the soil is distributed only to a certain depth (hereinafter this may be referred to as “non-uniform distribution”). The radioactive material is considered to be uniformly distributed up to a certain depth (hereinafter sometimes referred to as “uniform distribution”). Therefore, data considering these two patterns is prepared in advance, and the depth of distribution can be estimated by matching.
That is, by the simulation calculation in advance, the graph of the optical spectrum shown in FIG. 1 is obtained for each case where the radioactive material in the soil is unevenly distributed at a depth of 5 cm, 10 cm, 15 cm, 20 cm, and 25 cm. “Low energy / intermediate energy” and “(high energy + intermediate energy) / low energy” are calculated as the ratio of the number of photons of the three components of energy and high energy, and the ratio “low energy / intermediate energy” is plotted on the horizontal axis. A graph plotted with a ratio “(high energy + intermediate energy) / low energy” on the axis is a graph indicated by a broken square □ in FIG. Similarly, when the radioactive material in the soil is uniformly distributed to a depth of 5 cm, 10 cm, 15 cm, 20 cm, and 25 cm, the optical spectrum graph shown in FIG. 1 is obtained, and the low energy, intermediate energy, and high energy are obtained. “Low energy / intermediate energy” and “(high energy + intermediate energy) / low energy” are obtained as the ratio of the number of photons of the three components, the ratio “low energy / intermediate energy” is plotted on the horizontal axis, and the ratio “ What is plotted in a graph of “(high energy + intermediate energy) / low energy” is a graph indicated by a solid line with white circles in FIG.
In the above description, the simulation calculation is performed in increments of 5 cm for the non-uniform distribution and the uniform distribution, but the present invention is not limited to this.
本発明の推定法では、予めシミュレーション計算より求めた、図２の実線グラフ（白抜き丸○）に深度数値を付したものを一様分布の検量線として用い、破線グラフ（白抜き四角□）に深度数値を付したものを不均一分布の検量線として用いる。次に測定現場では、被測定対象の土壌の地表面に光子スペクトロメータを設置し土壌からくる放射線を計測して得られた光スペクトルから、低エネルギー、中間エネルギー、高エネルギーの３成分の光子数の比率として「低エネルギー／中間エネルギー」および「（高エネルギー＋中間エネルギー）／低エネルギー」を求め、予めシミュレーション計算により求めていた２つの検量線のどちらにマッチングするかを求めることにより土壌中放射性物質の深度分布を推定する。 In the estimation method of the present invention, a solid line graph (white circle) in FIG. 2 obtained by simulation calculation in advance is used as a uniform distribution calibration curve, and a broken line graph (white square □) The value with the depth value added to is used as a calibration curve for non-uniform distribution. Next, at the measurement site, a photon spectrometer is installed on the surface of the soil to be measured, and the number of photons of three components of low energy, intermediate energy, and high energy is obtained from the light spectrum obtained by measuring the radiation coming from the soil. "Low energy / intermediate energy" and "(high energy + intermediate energy) / low energy" as the ratio of the soil, and by deciding which of the two calibration curves obtained by simulation calculation in advance matches the radioactivity in the soil Estimate the depth distribution of the material.
本発明により放射性物質による土壌の除染状況が現場で且つ非破壊で迅速に推定できるので、土壌の汚染域の推定や、除染の事業計画等に利用可能である。 According to the present invention, the decontamination status of soil with radioactive materials can be estimated quickly on site and in a non-destructive manner. Therefore, the present invention can be used for estimation of contaminated areas of soil, business plans for decontamination, and the like.
Claims (1)
次に、被測定対象の土壌の地表面に光子スペクトロメータを設置し土壌からくる放射線を計測して得られた光スペクトルから、低エネルギー、中間エネルギー、高エネルギーの３成分の光子数の比率として「低エネルギー／中間エネルギー」および「（高エネルギー＋中間エネルギー）／低エネルギー」を求め、予めシミュレーション計算により求めていた前記不均一分布の検量線と前記一様分布の検量線のどちらにマッチングするかを求めることにより土壌中の放射性物質の深度分布を推定することを特徴とする放射性物質深度分布推定法。 According to simulation calculations, radioactive materials are distributed only at a certain depth in the soil (hereinafter referred to as “non-uniform distribution”), and radioactive materials are distributed uniformly to a certain depth in the soil (hereinafter referred to as this). For the “uniform distribution”), the ratio of the number of photons of three components, low energy, intermediate energy, and high energy, reaching the ground surface is obtained, and one axis is the ratio of the number of photons “low energy / intermediate energy”. , The graph with the other axis as the photon number ratio “(high energy + intermediate energy) / low energy” shows the nonuniform distribution calibration curve and the uniform distribution calibration curve along the respective calibration curves. To create
Next, from the light spectrum obtained by installing a photon spectrometer on the surface of the soil to be measured and measuring the radiation coming from the soil, the ratio of the number of photons of the three components of low energy, intermediate energy, and high energy “Low energy / intermediate energy” and “(high energy + intermediate energy) / low energy” are obtained and matched to either the calibration curve of the non-uniform distribution or the calibration curve of the uniform distribution obtained in advance by simulation calculation. A method for estimating a depth distribution of a radioactive material, wherein the depth distribution of the radioactive material in the soil is estimated by obtaining the above.
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