JPH08201596A - Neutron reflector - Google Patents

Abstract

(57) [Summary] [Object] An object of the present invention is to obtain a neutron reflecting mirror having a high reflectance in a neutron reflecting mirror composed of a multilayer film that reflects neutrons such as a neutron supermirror and a neutron monochromator. To aim. According to the present invention, titanium nitride and nickel nitride are alternately formed on a substrate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a neutron reflecting mirror such as a neutron supermirror that reflects neutrons in a wide wavelength range and a neutron monochromator that reflects only neutrons of a specific wavelength.

[0002]

2. Description of the Prior Art Wave dynamic properties of neutrons (λ = 0.1 to 1n)
As an optical element using m), a high-refractive index material (for example, Mn, Ti, V, Si) and a low-refractive index material (for example, Ni, Fe) are used for neutrons, and they are composed of alternating layers. Neutron reflection in neutron supermirrors with non-uniform thickness multilayer film formed on the substrate, and neutron monochromator with uniform thickness multilayer film consisting of alternating layers of high refractive index material and low refractive index material The mirror is known. These neutron reflectors are used in neutron diffractometers and neutron conduits.

In particular, a neutron reflecting mirror in a neutron conduit was manufactured by forming a thin film of nickel (Ni) in the past, but nowadays, two kinds of materials having different refractive indexes are used. Has changed to a neutron supermirror consisting of non-uniform thickness alternating layers. By the way, as an example of a conventional neutron supermirror, titanium (Ti) is used as a high refractive index material and nickel (N) is used as a low refractive index material.
FIG. 3 shows a neutron supermirror that is a non-uniform thickness multilayer film obtained by alternately depositing a total of 254 layers of these substances using i). And the reflectance of the neutron supermirror is shown in FIG. The incident condition of neutrons in FIG. 4 is when incident from the formed multilayer film at an angle of 0.01 rad.

In FIG. 4, the vertical axis represents reflectance and the horizontal axis represents neutron wavelength. Further, the curve shown by a thin line is a theoretical reflectance curve in the neutron supermirror, and the curve shown by a thick line is the actually measured value of the reflectance shown by the neutron supermirror obtained by actual fabrication. It is a curve shown. As shown in FIG. 4, the reflectance of the neutron supermirror actually manufactured has a lower reflectance in the middle portion of the wavelength band having a high reflectance (hereinafter, referred to as a high reflection wavelength range).

By the way, there is a neutron conduit as a product using this neutron supermirror. This is for guiding neutrons from a certain neutron source to various experimental equipments with almost no loss. This is a quadrangular prism made by bonding four neutron supermirrors inside on a flat glass substrate. The neutrons extracted from the neutron source repeat total reflection by the supermirror inside the neutron conduit. While being guided to a predetermined device. Therefore, the total length of the completed neutron conduit is usually 30 m or more. For this reason, neutron supermirrors formed on many flat glass substrates are necessary for producing neutron conduits.

Therefore, in order to reduce the loss of the neutron conduit, it is necessary for the neutron supermirror used for the neutron conduit to have a high reflectivity and no decrease in the reflectivity in the middle part of the high reflection wavelength band. Therefore, it is an urgent task to eliminate the decrease in reflectance in the middle part of the high reflection wavelength band in order to fabricate a neutron supermirror used for a neutron conduit. By the way, there is a neutron monochromator that reflects only neutrons of a certain wavelength, but this is similar to the neutron supermirror, in which high-refractive index materials and low-refractive index materials are alternately deposited, and each film thickness is the same. Is. This neutron monochromator also theoretically improves the reflectance by increasing the number of deposited films, but the actually manufactured neutron monochromator has a reflectance of more than a certain number of films. Does not improve. At present, there is a demand for a neutron monochromator having a high reflectance, and the need for it is increasing.

[0007]

The conventional neutron supermirror has a problem that the reflectivity is remarkably lowered in the middle portion of the high reflection wavelength band. Further, in the conventional neutron monochromator, as described above, even if the number of multi-layer film layers is increased, the reflectance is not improved at a certain number of film layers. The shorter the wavelength of these reflected neutrons,
This becomes remarkable, and there is a problem that a sufficient reflectance cannot be obtained.

Therefore, an object of the present invention is to obtain a neutron reflecting mirror having a high reflectance in a neutron reflecting mirror composed of a multilayer film for reflecting neutrons such as a neutron supermirror and a neutron monochromator.

[0009]

In order to solve the problems, according to the present invention, titanium nitride and nickel nitride are alternately formed on a substrate (the invention of claim 1). Regarding the composition ratio of titanium nitride, titanium is 1 and nitrogen is 0.1 or more and 1 or more.
With respect to the composition ratio of nickel nitride, the content of nitrogen is 0.1 to 1 inclusive with respect to 1 of nickel (the invention of claim 2).

Further, according to the present invention, a method for manufacturing a neutron reflecting mirror in which two or more kinds of substances having different refractive indexes are arranged in a vacuum container and the two or more kinds of substances are alternately evaporated to form a film on a substrate. In the above, an exhaust step of exhausting the inside of the vacuum container to a desired degree of vacuum, a reaction gas introducing step of introducing nitrogen gas into the vacuum container, and an evaporation step of alternately evaporating two or more kinds of substances are to be performed. (Invention of Claim 3).

The inventors have found that the above problems can be solved by the above means, and have reached the present invention.

[0012]

Based on the principle of neutron reflection, the inventors of the present invention have numerically calculated that the reflectance of 100% can be achieved by stacking a sufficient number of films if the substance forming the multilayer film does not absorb neutrons. I'm confirming. However, the neutron supermirror actually manufactured had a neutron reflectance lower than the calculated value in the middle part of the high reflection wavelength region.
Therefore, as a result of pursuing the cause of the low reflectance, the present inventors have concluded that the cause is the incompleteness of the formed multilayer film. An ideal neutron supermirror is one in which a multilayer film with a perfectly smooth interface is formed on a perfectly smooth substrate, and the film is completed from a substance with an ideal crystal structure. Is assumed. From this, the imperfections are considered to be the cause of the very small multi-layer interface roughness and the defects present in each layer in the multi-layer film decreasing the reflectance of the neutron supermirror. The present inventors observed the cross section of the multilayer film by a transmission electron microscope.

As a result of observing the cross section of the multilayer film of the conventional neutron supermirror as a sample, the present inventors have found the following. One of the main causes of the increase in the interface roughness is that the metal film is composed of innumerable polycrystalline grains. Due to the phenomenon, the interface roughness increases as the number of laminated layers increases. Met. Further, in the titanium film or the nickel film, crystal grains are formed during the film formation process, and these crystal grains grow. When the crystal grains are formed in this way, and the individual crystal grains grow larger, these crystal grains do not necessarily grow in the film thickness direction. Micro unevenness occurs on the surface of the film. This appears as interface roughness. Then, the multi-layer interface roughness is increased by stacking multiple layers while maintaining the roughness.

Therefore, from the above, it is concluded that the phenomenon that the reflectance decreases in the wavelength band that reflects neutrons occurs based on the above consideration, and as a remedy for this, the present inventors have succeeded in improving the Ti film and the Ni film. We have come to think that suppressing the growth of crystal grains can improve the decrease in reflectance in the middle part of the high reflection wavelength band. The inventors of the present invention believe that the properties of a metal in a bulk being easily crystallized and a nitride being hard to be crystallized can be utilized for a thin film substance in order to refine the crystal grains in the film, and therefore, the Ti film and the Ni film can be used. We tried to suppress the crystal grain growth by nitriding.

Actually, when a nitride of Ti and a nitride of Ni were formed into a film to manufacture a neutron supermirror, and the reflectance of the neutron supermirror was measured, an unexpected improvement was obtained. At present, little is known about the process of nitriding and growing a metal film. However, even from the results of the present invention, the present inventor has considered that the island-like structure that grows while being nitrided is much smaller than that of the metal film, and as a result, a layer having a small roughness is formed. Therefore, if the Ti film and the Ni film are nitrided,
It is considered that the initial formation process of the i film was different, and the multilayer film in which the interface roughness was suppressed was laminated, and a more ideal neutron supermirror was formed.

It is obvious that the above is effective for increasing the saturation reflectance even for a neutron monochromator which reflects only neutrons of a specific wavelength. Further, the present inventor, when forming a Ti nitride film or a Ni nitride film, uses a reactive physical film formation method. These include, among others, the reactive vacuum deposition method, the reactive sputtering method and the reactive ion plating method. In these methods, a Ti or Ni substance (whether it is a pure substance or a nitride substance) is used.
Is melted and vaporized by resistance heating, electron beam, or plasma, or vaporized by a sputtering method. Then, the evaporated Ti or Ni is blown into a nitrogen gas, and a Ti nitride film or a Ni nitride film is formed while reacting with the nitrogen gas. The present invention uses this method to
A titanium nitride film and a nickel nitride film for a neutron supermirror were formed, and titanium nitride and nickel nitride were repeatedly formed until the desired number of films was obtained.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention is not limited to this.

[0018]

FIG. 1 is a schematic sectional view of a neutron supermirror which is an embodiment of the present invention. Float glass is used for the substrate 1, titanium nitride (TiN _x ) is used as the high refractive index material, and nickel nitride (Ni) is used as the low refractive index material for the multilayer film 2.
Using N _y), to form an alternating film of TiN _x and NiN _y. These film thicknesses are not equal.

The film structure of TiN _x and NiN _y is as follows: substrate / TiN _x / NiN _y / ... / TiN _x / NiN _y
/ Air. The thickness distribution tj of the multilayer film is

[0020]

[Equation 1]

It was calculated according to By the way, j shown in the equation (1) is the order of film arrangement when the order of films is counted from air to the substrate. As you can see from this number 1,
The thickness of the TiN _x layer is about 5 nm at the minimum and 25 nm at the maximum.
Film thickness is formed, and the film thickness gradually decreases from the layer close to air toward the substrate.

Regarding the thickness of the NiN _y layer, the minimum is about 6
nm to a maximum film thickness of 70 nm.
Similar to _x , the film thickness decreases in order. These TiN
_The total number of layers of the _x film and the NiN _y film was 254 in total. The critical wavelength of total reflection of the neutron supermirror designed as described above is 0.23 nm when the incident angle of neutrons is 0.01 radian. By the way, since the critical wavelength of total reflection of a neutron mirror made of a Ni single layer film is 0.6 nm under the same conditions, the neutron supermirror of this example can reflect neutrons in a shorter wavelength range. I understand that.

The wavelength of incident neutrons is 0.6 nm.
In that case, the critical angle of total reflection of the neutron supermirror of this example is 0.026 radians. As a comparative example, it can be seen that the neutron mirror made of a Ni single-layer film has a critical angle for total reflection of 0.01 radian, so that neutrons can be incident at a high angle. Next, the method for forming the neutron supermirror in this embodiment will be described.

First, a float glass substrate is placed in a vacuum chamber for vacuum vapor deposition, and Ti and Ni as vapor deposition substances are also placed. Next, in order to create a high vacuum in the vacuum chamber, various vacuum pumps are driven to reduce the atmospheric pressure to the ultimate vacuum level. After reducing the atmospheric pressure to the ultimate vacuum level,
Nitrogen gas (N ₂ gas) is introduced into the vacuum chamber until the film forming vacuum is reached. After introducing the nitrogen gas to a film forming vacuum degree, the electron gun is driven to melt and evaporate Ti as a vapor deposition material and vapor deposit it on the float glass substrate until a desired film thickness is obtained. At this time, Ti combines with nitrogen gas to form Ti.
It becomes N _x and is deposited on the float glass substrate. TiN
_{When x} is deposited to the desired film thickness, the melting of Ti of the evaporation material is stopped, and then the melting of Ni of the other evaporation material is started. Then, vapor deposition is performed until a desired film thickness is obtained.
At this time, Ni becomes NiN _y by combining with the nitrogen gas introduced into the vacuum chamber, as in Ti vapor deposition. Then, when NiN _y is evaporated to a desired film thickness,
Stop melting Ni. Then, the melting of Ti is started again. By repeatedly melting Ti or Ni as the vapor deposition material in this manner, vapor deposition is performed up to a desired number of films to manufacture a neutron supermirror.

By the way, the film forming conditions for manufacturing the neutron supermirror of this example were as follows. Ultimate vacuum 3 × 10 ^-7 Torr Deposition vacuum 5 × 10 ^-5 Torr (Introduction of nitrogen gas) Substrate heating No heating (temperature rise due to radiant heat from evaporation source) Vapor deposition material Titanium (Ti) (purity 99. 9% or more) Nickel metal (Ni) (purity 99.9% or more) Evaporation source method Electron impact heating method Under the above film forming conditions, nitrogen gas was introduced to form a film T
The iN _x layer and the NiN _y layer were lower nitrides.
As a result of evaluating the stoichiometric ratio of the Ti layer and the Ni layer by XPS (X-ray photoelectron spectroscopy analysis), the value of x of TiN _x is 0.3 to 0.5, and the value of y of NiN _y is 0. 15 to 0.3
Met. A sufficient reflectance was obtained within the range of this value.

FIG. 2 shows the reflectivity of the neutron supermirror according to the present embodiment, which was designed, and the measured neutron reflectivity of the neutron supermirror formed according to the present embodiment.
Comparing with FIG. 4, which shows the reflectance with the conventional neutron supermirror, it can be seen that the reduction of the reflectance in the middle part of the high reflection wavelength band, which has been a problem, is remarkably improved.

Next, the incident angle of neutrons on the neutron supermirror was measured at a critical wavelength when fixed at 0.01 radian measured from the mirror surface. When neutrons are incident at this angle, the critical wavelength in the case of a neutron mirror formed of only a Ni film is 0.6 nm, but the neutron supermirror in this example has a wavelength range shorter than 0.6 nm. Can also reflect neutrons, and the critical wavelength of reflection of the neutron supermirror in this example is 0.23n.
It became m.

From the above, the critical angle of reflection of the neutron supermirror in this example is 2.6 times the critical angle of the neutron mirror formed of the Ni monolayer film. I understand. As described above, in the present example, it was possible to manufacture a neutron supermirror in which the reflectance does not decrease in the wavelength region longer than the critical wavelength. If a neutron conduit is manufactured using this neutron super mirror,
A neutron conduit with low neutron loss is obtained.

Further, also with respect to the neutron monochromator using TiN _x and NiN _y , a high saturated reflectance can be obtained, and a monochromator with less loss can be obtained. Further, in this example, nitrogen, which is less toxic than halogen gas, was used as the reaction gas. Therefore, it was possible to safely obtain a thin film having a high refractive index against neutrons and a thin film having a low refractive index and a substance having a flat interface between the thin films.

[0030]

As described above, titanium nitride (TiN _x ) is used as the high refractive index material and nickel nitride (N) is used as the low refractive index material.
A neutron supermirror having a non-uniform thickness multilayer film formed by alternately depositing a high-refractive index material and a low-refractive index material using iN _y ) has a lower reflectance in the middle of the high reflection wavelength band. Was eliminated, and sufficient reflectance could be secured in the wavelength region longer than the critical wavelength.

[Brief description of drawings]

[FIG. 1]: Cross-sectional view of a neutron supermirror in an example

[FIG. 2]: Neutron reflectance characteristics of the neutron supermirror in the example

[Fig.3]: Cross section of conventional neutron supermirror

[Figure 4]: Neutron reflectivity characteristics of conventional neutron supermirror

[Explanation of symbols]

1 Float glass substrate 2 254 layers of titanium nitride (TiN _x ) / nickel nitride (NiN _y ) neutron supermirror 3 254 layers of titanium (Ti) / nickel (Ni) neutron supermirror

Claims (3)

[Claims] 1. A neutron reflector, wherein titanium nitride and nickel nitride are alternately formed on a substrate. 2. The titanium nitride has a composition ratio of 1: 1.
2. Nitrogen is 0.1 or more and 1 or less, and the composition ratio of the nickel nitride is such that the nickel is 1 and the nitrogen is 0.1 or more and 1 or less. mirror. 3. A method for manufacturing a neutron reflecting mirror, wherein two or more kinds of substances having different refractive indexes are arranged in a vacuum container, and the two or more kinds of substances are alternately evaporated to form a film on a substrate. An exhausting step of exhausting the inside of the container to a desired degree of vacuum; a reaction gas introducing step of introducing nitrogen gas into the vacuum container; and an evaporating step of alternately evaporating the two or more kinds of substances. Method for manufacturing neutron reflector.