JPS6170484A - Radioactivating foil - Google Patents

Abstract

PURPOSE:To make possible the simultaneous measurement of neutron energy spectra over a wide range in a short period without spoiling the ease of a radioactivating foil method by using the foil in which plural nuclides are combined to one in the form of an alloy or mixture. CONSTITUTION:For example, gold, nickel, aluminum, titanium and alloy are selected as the nuclides to constitute the foil and are mixed at a suitable ratio. The mixture is melted at a high temp. to manufacture the alloy which is then molded to a disk shape. Such disk shape alloy is used. When the foil is subjected to irradiation of neutrons, the foil is made radioactive by the reaction of the neutrons in the energy range different with the nuclides and atomic nuclei and the foil releases gamma rays. Since just one foil is required for the measurement, the time for the measurement is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to neutron flux measurement, and particularly to an activation foil suitable for measurement of neutron energy spectra.

[Background of the invention]

In addition to activation foils, prior art related to neutron flux measurement (Japanese Patent Application Laid-open No. 70481/1983) discloses that a nuclear fission ionization chamber is useful. In addition, there is also a book titled ``Nuclear Reactor Engineering Course 1 = Fundamentals of Nuclear Engineering'' (Baifukan) by Tomitabe Ishimori, which shows that there are self-power detectors, neutron thermocouples, and seedling detectors in radiation measurement. has been done. However, except for self-powered detectors, these detectors are designed to measure neutrons with a specific energy;
Measurements of neutron fluxes with b' extending from a few eV to a few +MeV are not considered. This is due to the following reasons. Fission ionization chambers measure neutron flux from the ionizing radiation emitted by fissile material coated in the ionization chamber and fission induced in a neutron beam surrounding. Therefore, the measured neutrons are limited only to the thermal region (several eV or less) where the absorption cross section of uranium-235, which is a fissile material, is large. Neutron thermocouples also utilize fissile material, so
The same limitations as for fission ionization chambers arise.

In addition, these detectors, excluding the activation foil method, send electrical signals through cables, so they are good for determining dynamic changes in neutron flux, but they are not suitable for determining the distribution of neutron flux over a wide area. Not yet.

In other words, a long cable is required to cover the neutron flux over a wide area. In addition, since humans cannot be present in the space during high neutron flux measurements, it is necessary to either install the detector at a predetermined position or set up a runway and move it by operating it from the outside. It is not possible to obtain the spatial distribution of neutron flux without using the method described above. Therefore, cost problems arise due to the increase in the number of detectors and the installation of runways.

The activated foil method does not require cables and is easy to manufacture, thus avoiding the above cost problems.

In the foil activation method, a material made of an element that interacts with neutrons and emits gamma rays is made into a foil shape, and after being irradiated with neutrons, the material is taken out and used for use in syntelephone detectors, semiconductor detectors, etc. This is a method of determining neutron flux by measuring gamma rays emitted with measurement equipment. The principle is as follows.

When the foil-like material is composed of a single nuclide, the rate at which radionuclides with a decay constant λ are generated after time Ttl- after the start of irradiation is: dN/dT=ΣφV−λN ・・・・・・・・・・・・...
...0) is given. However, N is the number of radionuclides, Σ is the macroscopic activation cross section of the foil, ■ is the volume of the foil, φ
is the neutron flux. Since N=0 when T=U, (
1) Formula becomes the following formula.

N (T)=ΣφV(1-exp(-λT)/λ...
...(2) Since the radioactivity A of the activated foil is equal to Nλ, A (T) = Σφv(i-exp(-λT)
) ・−・−・・・・(3) Irradiation time T're
XI) If the length is long enough so that (-λT) can be ignored, the radioactivity of the activated foil will be saturated, and when irradiated with a constant neutron flux, the maximum radioactivity at that neutron flux, that is, the saturated radioactivity. is obtained.

A(oo)=ΣφV ・・・・・・・・・・・・・・・
・・・・・・・・・・・・・・・(4) Activate foil for time T
If it is removed from the neutron flux field after being irradiated for a period of time, decay will continue to occur, so the radioactivity A(t) when time t has elapsed after removal is as follows.

A(t)=ΣφV(1-exp(-λT))X eXI
) (-λt)・・・・・・・・・・・・(5) The value of A(t) is obtained by a gamma ray detector,
is obtained depending on the experimental conditions. Also, Σ and λ can be found from literature. Therefore, if the only unknown quantity is φ in equation (5), then the value of φ can be found. Also, since the range of neutron energy at which certain nuclides cause nuclear reactions differs,
By taking advantage of this property and using several nuclides, it is also possible to obtain the energy spectrum.

The activation foil used in this activation foil method conventionally has a flat plate-like structure made of highly pure metal (Figure 6).
However, this does not take into account the following two points.

First, when determining the energy spectrum of neutron flux, multiple nuclei i are required, and activation foils must be prepared for each nuclide, making the work including measurement after neutron irradiation. It becomes complicated.

Second, it does not take into account how to support the activation foil in high radiation fields. Especially when multiple activation foils are required as mentioned above, the activation foils react with neutrons,
In order to prevent the distortion of the neutron flux distribution caused by absorption of this from affecting other activation foils, the direction in which the neutron flux is incident when these activation foils are combined into one is determined. Each must face the source. In order to arrange the activation foil in this way, there is a conventional method of fixing it in a single holder.

When making this holder, polymer products such as vinyl and polyethylene cannot be used in high radiation fields because their properties change during neutron irradiation, so metals and alloys such as steel and stainless steel can be used instead. desirable. However, such a material has poor workability due to its strength, and requires an amount of work of z5 to make the holder and to take out and put in the activation foil. FIG. 7 shows an example in which an aluminum net is used as the holder. The advantage of using a net is that the amount of material used to make the holder is so small that the neutron absorption of the holder itself can be almost ignored. This holder is made of two pieces of net' sewn together with wire to form a bag, and the radioactive foil is placed inside this bag. The activation foil is fixed by tying the net with rings on all sides as shown in FIG. In this example, by the time the finished product is finished, we will need to: ■ Cut the net into appropriately thick pieces, ■ Load the activated foil between the two nets, ■ sew the net together, and ■ attach the activated foil using a ring. Fixing work is required. When it comes to measuring neutrons in a wide range of spaces, it is necessary to prepare as many as 1000 pieces of activation foil as shown in Figure 7, so the amount of work is quite high.

[Purpose of the invention]

An object of the present invention is to provide an activation foil that enables simultaneous measurement of a wide range of neutron energy spectra without sacrificing the simplicity of the conventional activation foil method.

[Summary of the invention]

The activation foil method involves reacting a certain nuclide with neutrons having an energy of several tens of MeV from the thermal energy range. The neutron energy spectrum is determined by using two factors: emitting gamma rays with . In order to obtain a wide range of neutron energy spectra, the present invention does not use multiple nuclides as separate foils as in the past, but instead combines them in the form of an alloy or mixture, and does not use a holder. This allows neutron energy spectra to be measured independently.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIG.

Figure 1 shows that nickel, aluminum, titanium, gold, and iron are selected as the nuclides constituting the activation foil, mixed in appropriate proportions, melted at high temperature, and made into an alloy metal. This is an overview of what has been molded into a shape. When this activation foil is irradiated with neutrons, the neutrons and atomic nuclei in different energy ranges react with each other depending on the nuclear beam, becoming radioactive and emitting gamma rays.

In the example shown in Figure 1, the nuclide used is gold.

Nickel, aluminum, titanium, and iron each require neutron energy to activate in the thermal energy range, 11.7 M e V.

2.1 M e V , 1.1 M e V ,
2. ．． It is around 9 M e V.

The gamma rays generated from each nuclear reaction are 0.4118
Men, 0.843 MeV, 0.067 MeV.

1.314 Me V, 0.847 Me V't'
Al (7) Te, can be identified by energy. For example, to find the radioactivity At of gold, 0.4j18MeV
All you have to do is count the 7'/7-r lines.

The neutron energy corresponding to the neutron flux obtained from this weir corresponds to the thermal energy region in which gold becomes radioactive. Similarly, if the same method is applied to other nuclides, one ton of neutron flux corresponding to different energies can be obtained at four energy range locations. If these are connected appropriately, a rough neutron energy spectrum from the thermal region to 12 MeV can be obtained.

The composition ratio of nuclides is determined by considering the activation cross section and decay constant of each nuclide. This is to prevent the counting rate from becoming extremely low due to nuclides. The gamma win/loss per unit time emitted from the activated foil that has been irradiated for a sufficiently long time can be obtained by (5) differentiating the metal, which is calculated by the following equation.

dA/dt=-λΣφVeXP(-λt)...
...(6) According to equation (6), this 1-axis is proportional to λΣφ, so changing the value of λΣ according to the pulse height distribution of the neutron flux makes the counting rate of gamma rays emitted from all nuclides constant. It is possible to secure more than the value of . For example, if φ is constant for all neutron energies, λΣ is constant for all nuclides. If σ is the microscopic activation cross section and nt atomic density, Σ=σ×n ・・・・・・・・・
・・・・・・・・・・・・・・・・・・・・・・・・(
7) Therefore, σInlλ, = a, n2λ2 =... ...φ・Danore....
+(8) holds true. From equation (8), n is proportional to l/(σλ). σ and λ are obtained from the literature and are gold in the example in Figure 4.

σ is 98 for nickel, aluminum, titanium, and iron.
．． 8.0.18.0.0? , 0.09.0.110(
unit burn), and tn2/λ is Z6946 days, 9
9 minutes, 9.46 minutes, 1.83 days, and 2.576 hours. From this, the n ratio, that is, the composition ratio is determined, and the ratio is 1
:14: 3.4 near 46: 3.7.

Also, if we apply an appropriate weight to this ratio, the energy side becomes
The neutron flux biased towards the low energy side is also efficient < 1ll
J can be determined.

FIG. 2 shows an example in which the material of the activated foil is powdered and sintered as a mixing means. It is made by mixing fine particles of elements or compounds containing gold, iron, aluminum, titanium, and nickel nuclides, and baking them at a temperature below their melting point to form a foil. This method is easy to manufacture because it does not require high-temperature melting.

FIG. 3 shows an example in which the powder of the activated foil material was mixed and the entire can was made as a mixing means. In this case, there is no need to raise the temperature, and materials with low melting points such as sulfur and phosphorus can also be used. Use materials with low neutron absorption for can making materials.

FIG. 4 shows an example of an activation foil having a two-layer structure: a layer consisting of a nuclide with a large activation cross section and a layer consisting of a nuclide with a small activation cross section. Nuclides such as gold and i/dium that undergo a nuclear reaction with thermal neutrons and become radioactive have a high cross section, ranging from several dozen to 100 burns. On the other hand, the activation cross section of nuclides that react with neutrons of IMeV or higher is low, several millimeters
Many of them cost a few hundred mitzbans. Therefore, two foils, one for thermal neutron measurement and one for fast neutron measurement, are glued together.

Since the foil for measuring thermal neutron flux can be thin, it can also be made by plating or vacuum deposition on the foil for fast neutrons. This two-layer structure allows nuclides with a high activation cross section to escape, thereby reducing the effect of changing the neutron flux distribution within the activation foil.

The upper example of FIG. 5 shows an example in which a hole is made at the end of the activated foil of the invention. Although material deterioration due to radiation is not a major problem for the activated foil itself, there are major restrictions in selecting the material for fixing it.

For example, if a chemical metal such as an adhesive is used to fix the activated foil, its properties may change due to radiation and the metal may become useless. Therefore, in such a case, the activation foil can be fixed by tying it to a structure near the measurement point with a wire or the like to avoid such danger.

The holes in the activated foil in the upper example of FIG. 5 are for passing the wire through.

In addition, in the lower example of Figure 5, if there is no structure near the measurement point,
An example is shown in which a pattern is provided on the activated foil of the invention, and this is inserted into a prepared base and fixed. By the above method, it is possible to fix the activated foil while completely avoiding material degradation due to radiation.

〔Effect of the invention〕

1. Regarding measurement of neutron energy spectrum According to the present invention, only one activation foil is required instead of a plurality of foils required for neutron spectrum measurement. The neutron flux determined by the activation foil is approximately 10 M e from the thermal energy region.
This also applies to V, but this is due to the fact that the neutron energy that is activated differs depending on the nuclide, and in order to obtain the neutron energy spectrum, it was necessary to prepare activation foils for each nuclide. However, what is needed is the type of atomic nucleus that causes a nuclear reaction with neutrons, so if the necessary nuclides are selected and mixed in the form of an alloy or powder, the activation foil that previously required two or more can be reduced to one. Only one piece of foil is needed.

2. With the spread of mate channel analyzers, it has become possible to simultaneously measure gamma rays emitted from multiple nuclides contained in this activation foil. That is, when five nuclides are required in neutron energy spectrum measurement, for example, the gamma rays emitted from the activation foil in which the nuclides are mixed can be measured once, instead of five times in the conventional method. In this way, the measurement required for each activated foil only needs to be carried out once, which has the effect of significantly shortening the measurement time.

3, 1! Regarding the durability of the i-irradiation foil, for nuclides with a large absorption cross section, it is necessary to use a very thin foil in order to reduce the distortion of the neutron flux.

According to the present invention, when such nuclides are mixed with other nuclides, by reducing the atomic number density and uniformly distributing it throughout the foil, an effect equivalent to that of making a thin foil can be obtained. be able to. For example, gold has a large activation cross section of 96 milliburn for low-energy neutrons, so when making it into a thin layer, the thickness is from 2 millionths of a centimeter to 8 centimeters. There is a need. With such thin foil, even the slightest force can destroy it. Therefore, by mixing it with other nuclides so that it contains the same number of atoms as thick as this, it is possible to create a activation foil that has the same effect as a single-nuclide activation foil and has sufficient strength. This is because even though the atomic number density of each nuclide is low, the activation foil made by mixing them has a considerable thickness.

4. Regarding disturbance of neutron flux by activation foil, nuclides that react with thermal neutrons and become activated are 10
Although it is extremely large, at around 0 burn, the activation cross section of nuclides that cause nuclear reactions with neutrons in other energy ranges is from several milliburns to several hundred milliburns. In order to obtain the neutron energy spectrum, the latter nuclides are often the main ones. According to the present invention, in order to obtain a neutron energy spectrum, an activation foil is created by mixing various nuclei. Among these nuclides, if the number of atoms of a nuclide with an extremely large activation cross section is reduced, Since the cross-section of the remaining nuclides is very small, the absorption cross-section of the entire activation foil becomes small. This shows that the disturbance in the spatial distribution of neutron flux due to the presence of the activation foil is very small, and even if the shape of the activation foil is made quite freely for a total of only a few burns, this advantage is will not be lost. Therefore, this activation foil is effective in determining the neutron flux with small errors.

5. Self-shielding of activation foils Due to the presence of activation foils, distortions in the spatial distribution of neutron flux can also occur inside the activation foils. When the activation foil is made of a nuclide with a large absorption cross section, the amount of neutrons absorbed while being transported within the activation foil is large, so that the neutrons exit from the side of the activation foil where the neutrons enter. The value of neutron flux differs depending on the side. Therefore, the distribution of the amount of activation within the activation foil becomes uneven. According to the present invention, the activation foil is made by mixing several nuclides, but since the nuclide gold, which has a small absorption cross section, is mainly used, neutron absorption within the activation foil is small and there is almost no self-shielding effect. Therefore, since the amount of radiation within the activation foil is evenly distributed, there is an effect that a neutron flux with less error can be obtained from this.

6. Applicability According to the present invention, since the nuclide and atomic density of the activation foil can be changed arbitrarily, activation foils suitable for various purposes can be obtained. If we increase the number of nuclides in the mixture, we can understand the detailed shape of the neutron energy spectrum, but 1
As the number of atoms per nuclide decreases, the counting rate of emitted gamma rays decreases, making measurement virtually impossible. If the energy range of the neutron spectrum that you want to find is known in advance, you can create an activation foil by selecting a nuclide that will stimulate a nuclear reaction with neutrons in that energy range. This makes it possible to efficiently obtain the desired energy spectrum.

7. Regarding compatibility with conventional activation foils According to the present invention, except for the fact that activation foils are manufactured by mixing multiple nuclides, they are basically compatible with activation foils made of a single nuclide. are the same. In other words, when multiple nuclides are mixed, it is the same as stacking up multiple activations made of different single nuclides. Therefore, if the shapes are made the same, equipment for measuring emitted gamma rays can be used as is.

8. Treatment after use According to the present invention, by mixing various nuclides, the number of activation foils that would otherwise be required can be reduced to one. In this way, since the number of radioactive foils used is reduced, the amount of radioactive waste generated after use can be reduced.

[Brief explanation of drawings]

Figure 1 shows the structure of the activation layer made by mixing various nuclides into an alloy. Figure 2 shows the structure of the activation foil made by sintering the mixed powder. Figure 3 Figure 1 shows the shape of the activation foil made by packing the mixed powder into a gold holder. Figure 4 is a perspective view of the activation foil made by dividing the nuclide with a large activation cross section and the nuclide with a small activation cross section into two layers. Section 5 (a) is a perspective view showing the state in which there are holes on all sides to fully fix the activated foil, and FIG. 5 (b) is a perspective view showing the activated foil with a handle. (C) is a perspective view showing the state in which the activation foil is inserted into a hollow rod-shaped base, and Figure 6 is a structural diagram of the activation foil, which is the main body of a conventional neutron foil director. , Figure 7 is 2
FIG. 8 is a detailed structural diagram of FIG. 7, showing a state in which the activated foil is contained in a bag made by sewing together two pieces of net wire. l...Holder, 5...Hole, 6...Handle, 7...
Base, 8...Net, 9...Wire, lO...Radiation foil, ll...Front; T net, 12...Back side net, 13...Ring.

Claims (1)

[Claims] 1. In a member containing an atomic nucleus that interacts with neutrons in a limited energy range and emits gamma rays,
Activated foil is characterized by a mixture of many types of nuclides with such properties.