WO2015125418A1

WO2015125418A1 - Electron beam irradiator and irradiation system with emission detection

Info

Publication number: WO2015125418A1
Application number: PCT/JP2015/000380
Authority: WO
Inventors: Kaveh Bakhtari
Original assignee: Hitachi Zosen Corporation
Priority date: 2014-02-19
Filing date: 2015-01-29
Publication date: 2015-08-27
Also published as: JP6628728B2; JP2017506335A

Abstract

An electron beam irradiator 7 of the present invention includes: an vacuum chamber 1 having an interior in a vacuum state; an electron beam generator 6 provided in the vacuum chamber 1 to generate electron beams; a window structure 5 including a transmission window for emitting electron beams generated by the electron beam generator 6 to the outside of the vacuum chamber 1 and a thin portion that covers the transmission window to keep the vacuum state in the vacuum chamber and allows transmission of electron beams; an insulating separator 4 that separates the window structure 5 and the vacuum chamber 1; an insulating cover 3 shaped like a layer covering the outer surface of the window structure 5; and a conductive layer 2 that is provided over the outer surfaces of the insulating separator 4 and the insulating cover 3 and is grounded.

Description

ELECTRON BEAM IRRADIATOR AND IRRADIATION SYSTEM WITH EMISSION DETECTION

The present invention relates to an electron beam irradiator and electron beam irradiation system that provide on-line beam current and dose measurement, enabling process control for electron beam irradiation.
The present invention creates a novel design for the electron beam window that facilitates functionalities of both electron transmission and beam measurement. At a system level, besides the monitoring application for the beam measurement, this invention offers methods to control the irradiation process.

Conventionally known electron beam irradiators are used for applications such as sterilization and surface treatment on an irradiated body, e.g., a container (for example, Patent Literature 1). Such an electron beam irradiator includes a vacuum chamber internally placed in a vacuum state, an electron beam generator provided in the vacuum chamber to generate electron beams, and a window structure. The window structure includes a transmission window through which electron beams generated by the electron beam generator are emitted to the outside of the vacuum chamber, and a thin portion that covers the transmission window to keep the vacuum state in the vacuum chamber and allows transmission of electron beams. The thin portion includes, for example, metallic membranes such as a titanium membrane. The vacuum chamber is made of, for example, stainless steel.

Electron beams generated from the electron beam generator are transmitted through the window structure to the outside of the electron beam irradiator (vacuum chamber). When electron beams generated from the electron beam generator pass through the window structure, some of the electron beams (electrons) are absorbed by the window structure. The electron beams are outputted in proportion to the quantity of electrons absorbed by the window structure. Thus, the intensity of the electron beams generated from the electron beam generator may be recognized by detecting the quantity of the electrons absorbed into the window structure. If properly isolated, the quantity of the absorbed electrons would be detected as an electrical current flowing to the window, hereby referred to as current. Such apparatus encompasses the principle of this patent.

Patent Literature 1 : Japanese Patent Laid-Open No. 2011-26000

However, the window structure and the vacuum chamber are conductive and typically grounded, therefore not allowing any current detection. Even, in the case of floating and not grounding the structure, the signal is not representative of the absorbed electrons. The reason is that the current values detected in the system include current components flowing from the vacuum chamber as well as the window structure current components based on electrons absorbed by the window structure. Moreover, the current values detected in the window structure include charged particle components from the plasma, that are a result of charge particle interaction with the window structure in response to plasma generation near the outer surface of the window structure based on the ambient conditions for the irradiation process. Thus, a current value determined based on electrons absorbed by the window structure will hardly represent the primary beam current.

In order to solve the problem, an object of the present invention is to provide an electron beam irradiator that is capable of precisely detecting a current value based on electrons absorbed by a window structure when electron beams generated from an electron beam generator pass through the window structure, which is used both for monitoring of an electron beam irradiator output and/or process control for an electron beam irradiation system

The present invention is configured as follows:
(1) An electron beam irradiator including: a vacuum chamber having an interior in a vacuum state; an electron beam generator that is provided in the vacuum chamber and generates electron beams; a window structure including a transmission window for emitting electron beams generated by the electron beam generator to the outside of the vacuum chamber, and a thin portion that covers the transmission window to keep the vacuum state in the vacuum chamber and allows transmission of electron beams; an insulating separator that separates the window structure and the vacuum chamber; and an insulating cover in the form of a layer covering the outer surface of the window structure.
(2) The electron beam irradiator according to (1) further includes a conductive layer that is provided over the outer surface of the insulating member and is grounded.
(3) The electron beam irradiator according to (1), wherein the window structure has a cooling path that allows passage of a cooling medium for temperature control.

(4) An electron beam irradiation system including: the electron beam irradiator according to (1); and a monitoring controller that detects the electrical current value extracted from the window structure and adjusts, according to the detected current value, the intensity of an electron beam emitted from the electron beam generator.
(5) An electron beam irradiation system including: the electron beam irradiator according to (2); and a monitoring controller that detects the current value extracted from the conductive layer and adjusts, according to the detected current value, at least one of residence time, temperature and humidity of an external atmosphere outside of the window structure.

(6) An electron beam irradiation system including: the electron beam irradiator according to (3); a monitoring controller that detect the electrical current value of the window structure; and a flow controller that adjusts the flow rate of a cooling medium according to the current value detected by the monitoring controller.

Advantageous Effect of Invention

The present invention can provide an electron beam irradiator that is capable of accurately sensing the quantity of the electrons leaving the emitter by recognizing a current value based on electrons absorbed by a window structure when electron beams generated from an electron beam generator pass through the window structure. Furthermore, the present invention can provide an electron beam irradiation system including the electron beam irradiator.

FIG. 1 is a structural diagram schematically showing an electron beam irradiator system including an electron beam irradiator according to an embodiment of the present invention.

FIG. 2 shows an example of the electron beam irradiator used for the electron beam irradiation system shown in FIG. 1.

FIG. 3 is an enlarged cross-sectional view of an end portion 36 of a nozzle part 31b shown in FIG. 2.

Referring to FIG. 1, an embodiment of an electron beam irradiation system including an electron beam irradiator according to the present invention will be described below with reference to FIG. 1. The present invention is not limited to this embodiment.
As shown in FIG. 1, an electron beam irradiation system includes an electron beam irradiator 7. The electron beam irradiator 7 is used for emitting electron beams to, for example, the outer surface of a container. The electron beam irradiator 7 includes a vacuum chamber 1 having an interior in a vacuum state, an electron beam generator 6 that is provided in the vacuum chamber 1 and generates electron beams, and a window structure 5. The vacuum chamber 1 is made of a conductive material, e.g., stainless steel.

The window structure 5 is provided in the vacuum chamber 1. The window structure 5 includes a transmission window for emitting electron beams generated by the electron beam generator 6 to the outside of the vacuum chamber 1, and a thin portion that covers the transmission window to keep the vacuum state in the vacuum chamber 1 and allows transmission of electron beams. The window structure 5 is made of conductive materials such as titanium and copper.

The window structure 5 includes, for example, a grid made of a material such as copper with a plurality of openings and a metallic membrane (e.g., titanium membrane) that is supported on the surface of the grid so as to cover the openings of the grid. The transmission window is composed of the openings. The thin portion includes the metallic membrane.
The window structure 5 may include only the thin portion without the grid.
The window structure 5 may have small-thickness and large-thickness portions that are formed by patterning with laser. The thin portion has a small-thickness portion. The transmission window includes concave portions formed by large-thickness and small-thickness portions.
The thin portion may be provided over the window structure 5 or constitute a part of the window structure 5.

The electron beam irradiator 7 further includes an insulating member that separates the window structure 5 and the vacuum chamber 1 and is shaped like a layer covering the outer surface of the window structure 5 (a surface exposed to an external atmosphere from the window structure 5). The insulating member includes an insulating separator 4 that separates the window structure 5 and the vacuum chamber 1 and an insulating cover 3 that is a layer covering the outer surface (the surface exposed to an external atmosphere from the window structure 5) of the window structure 5. The surface of the window structure 5 is covered with insulating materials (the insulating separator 4 and the insulating cover 3), except for portions irradiated with electron beams 16 from the electron beam generator 6.
The insulating separator 4 can interrupt current flowing from the vacuum chamber 1 to the window structure 5. The insulating separator 4 is made of, for example, Al₂O₃ or SiO₂. The insulating separator 4 may be made of a ceramic that is usable as an insulating material. However, the material of the insulating separator 4 is not limited to these materials. Furthermore, the insulating separator 4 may be a laminar insulating member formed by an evaporation method or a block insulating member. The insulating separator 4 is joined to the vacuum chamber 1 and the window structure 5 by, for example, a brazing method. The joining method is not particularly limited as long as the insulating separator 4 can be joined to the vacuum chamber 1 and the window structure 5.

The electron beam irradiator 7 includes a conductive layer 2 that is provided over the outer surface of the insulating member (a surface where the insulating members, that is, the insulating separator 4 and the insulating cover 3 are exposed to an external atmosphere) and is grounded with the insulating member. This can prevent plasma generated near the outer surface of the window structure 5 from passing plasma current through the window structure 5.
More specifically, the insulating cover 3 can prevent the window structure 5 from absorbing electrons generated by plasma occurring near the outer surface of the window structure 5. Moreover, the conductive layer 2 covers the outer surface of the insulating cover 3 and is grounded, thereby releasing, from the conductive layer 2 through a path 15, electrons generated by plasma occurring near the outer surface of the window structure 5. This can suppress plasma current that passes through the window structure 5 due to charge stored (charge-up phenomenon) in the insulating cover 3 by plasma. The plasma is generated near the outer surface of the window structure 5 by electron beam irradiation.

The plasma near the outer surface of the window structure 5 is generated by interaction between electrons (primary electrons), which are emitted out of the vacuum chamber 1, and an external atmosphere. Near the outer surface of the window structure 5, charged species such as secondary electrons or ions are generated by scattering of electrons emitted out of the vacuum chamber 1.

According to the present invention, the window structure 5 is electrically insulated both in the vacuum chamber 1 and an external atmosphere. This can directly detect a current value with precision based on electrons absorbed by the window structure 5 when the electron beams 16 generated from the electron beam generator 6 pass through the window structure 5.
According to the present invention, the intensity of an electron beam used for sterilization and surface treatment can be precisely recognized based on data about electron beams before electron beam emission out of the vacuum chamber 1. Moreover, the data can be processed as data about all the electron beams 16 that reach the window structure 5. Hence, according to the present invention, measured values do not vary depending on the location of measurement or the state of an external atmosphere (e.g., a plasma state) if electron beams are directly measured after being emitted to the external atmosphere of the vacuum chamber 1. Detected current values are applicable to an electron beam irradiator having a similar design.

For example, the insulating cover 3 includes a SiO₂ layer. The insulating cover 3 may be made of a ceramic that is usable as an insulating material. The insulating cover 3 disposed on the outer surface of the window structure 5 is obtained by, for example, forming a layer of an insulating material on the outer surface of the window structure 5 according to an evaporation method. Alternatively, the insulating cover 3 disposed on the outer surface of the window structure 5 is obtained by, for example, bringing the insulating cover 3 into contact with the outer surface of the window structure 5. The insulating cover 3 is a thin layer (e.g., a layer having a nano-level thickness) that allows transmission of most electron beams. The thickness of the insulating cover 3 may be properly determined so as to reduce the quantity of electrons absorbed by the insulating cover 3 to a negligible level.

The conductive layer 2 is composed of a metallic compound. The conductive layer 2 is formed on the insulating cover 3 by, for example, a deposition method. It is preferable to use a deposition method because thin layer can be easily formed. The method is not particularly limited as long as the insulating cover 3 can be joined to the conductive layer 2. The conductive layer 2 is grounded via the path 15. The path 15 has an ammeter 15a that detects plasma current passing through the conductive layer 2. The state of an external atmosphere (e.g., the degree of plasma generation, a temperature, and a humidity of the external atmosphere) near the outer surface of the window structure 5 can be recognized according to plasma current detected by the ammeter 15a.
The conductive layer 2 is a thin layer (e.g., a layer having a nano-level thickness) that allows transmission of most electron beams. The thickness of the conductive layer 2 may be properly determined so as to reduce the quantity of electrons absorbed by the conductive layer 2 to a negligible level.

The vacuum chamber 1 has side walls in a dual structure. A space between two walls forms a cooling path (not shown) for passage of a cooling medium. The space is divided into two areas by walls. One of the areas serves as a supply path for supplying a cooling medium to the window structure 5 while the other area serves as a discharge path for discharging a cooling medium having passed through the window structure 5. The window structure 5 has cooling paths (not shown) that allow passage of a cooling medium for temperature control. One of the cooling paths of the window structure 5 is connected to the supply path via a supply path provided in the insulating separator 4. The other cooling path of the window structure 5 is connected to the discharge path via a discharge path (not shown) provided in the insulating separator 4. The cooling medium is a liquid such as water or gas such as air that is usable as a cooling medium. The cooling path is, for example, a cooling path described in Japanese Patent Application No. 2013-093145.
In the present embodiment, for the provision of the cooling paths, the vacuum chamber 1 has the side walls in a dual structure. If the cooling paths are not provided on the sides of the vacuum chamber 1, a single wall may be provided on the side of the vacuum chamber 1.

The electron generator 6 includes a thermionic-emission electron source (cathode) 19 and a gun shell 18 that accommodates the electron source 19. The electron source 19 may be a field-emission type instead of the thermionic-emission type. A filament is used for the electron source 19. The electron source 19 may use a disk cathode instead of the filament cathode. However, the cathode used for the electron source 19 is not limited to the filament or disk cathode. Electric power is supplied to the electron source 19 through

leads

10 and 11. In order to keep a necessary electric field around the electron source 19, the two leads 10 and 11 are floated to an acceleration voltage. The lead 11 is in contact with the gun shell 18 at a node 17. The lead 10 is connected to the electron source 19 through a hole of the gun shell 18. The gun shell 18 acts as an electron lens that converges electron beams generated from the electron source 19 into a beam and adjusts the path of the beam.

A high voltage source 9 forms a high potential difference between the gun shell 18 and the window structure 5 and forms a high-voltage electric field between the gun shell 18 and the window structure 5. Thus, electrons generated from the electron source 19 are accelerated to the window structure 5 (transmission window) and then are emitted to the outside of the vacuum chamber 1 from the window structure 5. The vacuum chamber 1 is grounded via a path 20 while the window structure 5 is in contact with electrically insulating materials, for example, an insulating member and an insulating layer. The high voltage source 9 is grounded via a path 12.
The electron beams 16 generated by the electron beam generator 6 pass through the window structure 5 (transmission window), are transmitted through the thin portion (not shown), the insulating cover 3, and the conductive layer 2 and then are emitted as an electron beam 14 to the outside from the vacuum chamber 1 of the electron beam irradiator 7. At this point, some of the electron beams 16 are absorbed by the window structure 5, the insulating cover 3, and the conductive layer 2.

In this case, the electron beam 14 emitted to the outside of the electron beam irradiator 7 and used to perform sterilization or surface treatment has a beam current i_T expressed by the following equation:
i_T = i_B - iw + E_r
where i_B is the beam current of the electron beam 16 generated from the electron generator 6, iw is current passing through the window structure 5 in response to electrons absorbed by the window structure 5 when the electron beams 16 generated from the electron beam generator 6 pass through the window structure 5, and E_r is an error determined based on electrons absorbed by the insulating cover 3 and the conductive layer 2 and electrons that do not reach the window structure 5 when electron beams pass through the cover 3 and the conductive layer 2.
The insulating cover 3 and the conductive layer 2 are properly reduced in thickness so as to transmit most electrons through the insulating cover 3 and the conductive layer 2 after passing through the window structure 5. Moreover, the output conditions of electron beams are properly set such that most electrons generated from the electron beam generator 6 can reach the window structure 5. Thus, the error E_r can be reduced to a negligible level. In the case of the negligible error E_r, the current iw is accurately detected so as to correctly recognize the current value i_T.

The electron beam irradiation system according to the present invention further includes a monitoring controller 8 that detects the current value iw passing through the window structure 5 and adjusts, according to the detected current value iw, the intensity of electron beams generated from the electron beam generator 6. The monitoring controller 8 is provided in the high voltage source 9. Power supplied from the

leads

10 and 11 to the electron source 19 can be accurately adjusted by the monitoring control unit 8 in response to a signal for the current iw passing through the window structure 5. Furthermore, a signal for current ip passing through the conductive layer 2 is transmitted to the monitoring control unit 8 via a path connecting the ammeter 15a and the monitoring control unit 8. Thus, the state of an external atmosphere can be reflected in the control of the monitoring control unit 8. For example, the electron beam irradiator 7 and the external atmosphere are surrounded by a casing (not shown). In this case, a plasma products including ozone or nitric acid are retained in the external atmosphere outside of the window structure 5, and cause a corrosion of electron beam irradiator 7. Therefore, it is preferable to provide ventilation gas 24 between the electron beam irradiator 7 and the casing (i.e. into the external atmosphere outside of the window structure 5), and to adjust the state such as residence time, temperature and humidity of the external atmosphere in order to prevent the corrosion. In particular, the monitoring controller 8 is connected to an external atmosphere controller 25 which provides the ventilation gas 24 into the external atmosphere, and is connected to the ammeter 15a which reflects the state of the external atmosphere. The external atmosphere controller 25 is configured to control at least one of ventilation speed, temperature and humidity of the ventilation gas 24. Therefore, the monitoring controller 8 can adjust at least one of residence time, temperature and humidity of the external atmosphere via the external atmosphere controller 25 according to the current value detected by the ammeter 15a. By this configuration, it is possible to prevent the corrosion of the electron beam irradiator 7.
If the error E_r is negligible, the current iw passing through the window structure 5 is subtracted from the beam current i_B of electron beams generated from the electron beam generator 6, obtaining the current value i_T of an electron beam emitted to the outside from the electron beam irradiator (vacuum chamber 1). This can accurately recognize the intensity of an electron beam emitted to an irradiated body.

For example, the signal for the current iw passing through the window structure 5 is transmitted to the monitoring control unit 8 through a path 13 in FIG. 1. The current iw passing through the window structure 5 can be continuously monitored online in the monitoring control unit 8. In online control using the monitoring controller 8 in the system of FIG. 1, a first step is performed to detect the current iw passing through the window structure 5 in response to the transmission of electron beams through the window structure 5, and a second step is performed to adjust power supplied to the electron source 19 according to inputted data on the shape of the irradiated body in the monitoring controller 8 and a current value detected in the first step. The first and second steps can be repeatedly performed.

Power (the intensity of electron beams) supplied to the electron source 19 from the monitoring controller 8 is controlled based on data on the shape of the irradiated body, thereby accurately adjusting an amount of irradiation to the surface of the irradiated body having an irregular shape. A dose uniformity ratio (DUR) can be optimized while the amount of electron-beam irradiation does not exceed a maximum permissible dose, achieving efficient sterilization or surface treatment.

According to the present invention that achieves simple online control using the monitoring controller 8, unlike in the related art, optimization of an irradiation dose to a container having an irregular shape does not need complicated adjustments, e.g., an adjustment to the orientation of the transmission window relative to the container or an adjustment to the relative speed of travel of the container in relations to the emitter window.

The electron beam irradiation system according to the present invention further includes a flow controller 22 (mass flow controller) that adjusts the flow rate of a cooling medium passing through the window structure 5 according to the current value iw detected by the monitoring controller 8, in addition to the monitoring controller 8 that detects the current value iw passing through the window structure 5.
More specifically, the signal for the current iw passing through the window structure 5 is inputted to the monitoring controller 8 through the path 13. A predetermined signal is outputted from the monitoring controller 8 based on the signal for the inputted current iw. The signal outputted from the monitoring controller 8 is inputted to the path controller 22 through a path 21. Based on the signal inputted to the flow controller 22, the flow controller 22 adjusts the flow rate of a cooling medium passing through the window structure 5 so as to cool the window structure 5 to a predetermined temperature.
The system including the monitoring controller 8 and the flow controller 22 can adjust the flow rate of a cooling medium so as to keep the window structure 5 to have a constant temperature. This can suppress deterioration of the window structure that is caused by a change of a heat load to the window structure 5 (a temperature change of the window structure 5), thereby increasing the life of the window structure 5.

The electron beam irradiator 7 used in the electron beam sterilization system according to the present embodiment may be replaced with an electron beam irradiator 37 having a nozzle part. For example, the electron beam irradiator 37 is used for irradiating the inner surface of a container with electron beams.
As shown in FIGS. 2 and 3, the electron beam irradiator 37 includes an vacuum chamber 31 having an interior in a vacuum state, an electron beam generator (not shown) provided in the vacuum chamber 31 to generate electron beams, and a window structure 35 including a transmission window for emitting electron beams generated by the electron beam generator to the outside of the vacuum chamber 31 and a thin portion that covers the transmission window to keep the vacuum state in the vacuum chamber and allows transmission of electron beams.

The vacuum chamber 31 includes a body part 31a that accommodates the electron beam generator and a nozzle part 31b extended from the body part 31a. The nozzle part 31 has such a shape as to be insertable into a container (such a length and a diameter as to be insertable into the container). The window structure 35 is provided on an end portion 36 of the nozzle part 31b.
The electron beam irradiator 37 further includes an insulating separator 34 that separates the window structure 35 and the vacuum chamber 31, an insulating cover 33 that covers the outer surface of the window structure 35, and a conductive layer 32 that is provided over the outer surfaces of the insulating member 34 and the insulating cover 33 and is grounded.

Claims

An electron beam irradiator comprising:
a vacuum chamber having an interior in a vacuum state;
an electron beam generator that is provided in the vacuum chamber and generates electron beams;
a window structure including a transmission window for emitting electron beams generated by the electron beam generator to outside of the vacuum chamber, and a thin portion that covers the transmission window to keep the vacuum state in the vacuum chamber and allows transmission of electron beams;
an insulating separator that separates the window structure and the vacuum chamber; and
an insulating cover in a form of a layer covering an outer surface of the window structure.
The electron beam irradiator according to claim 1, further comprising a conductive layer that is provided over an outer surface of the insulating member and is grounded.
The electron beam irradiator according to claim 1, wherein the window structure has a cooling path that allows passage of a cooling medium for temperature control.
An electron beam irradiation system comprising:
the electron beam irradiator according to claim 1; and
a monitoring controller that detects an electrical current value extracted from the window structure and adjusts, according to the detected current value, an intensity of an electron beam emitted from the electron beam generator.
An electron beam sterilization irradiation system comprising:
the electron beam irradiator according to claim 2; and
a monitoring controller that detects an electrical current value of extracted from the conductive layer and adjusts, according to the detected current value, at least one of residence time, temperature and humidity of an external atmosphere outside of the window structure.
An electron beam irradiation system comprising:
the electron beam irradiator according to claim 3;
a monitoring controller that detects an electrical current value of the window structure; and
a flow controller that adjusts a flow rate of a cooling medium according to the current value detected by the monitoring controller.