JP5183115B2 - X-ray generator - Google Patents

Description

  The present invention relates to an X-ray generator that generates X-rays by electrons that circulate while drawing a circular orbit in an accelerator.

  As a conventional X-ray generator using a circular accelerator, there is an apparatus using an accelerator based on the principle of betatron acceleration (abbreviated as betatron accelerator). For example, it is disclosed in Non-Patent Document 1. One of the important performances for evaluating an X-ray generator using a betatron accelerator is the intensity of X-rays that can be generated. Assuming that the betatron accelerators are the same size, the parameters that determine the X-ray intensity include the amount of beam generated from the electron gun, the beam injection efficiency to the accelerator, the beam survival rate in the accelerator, and the target irradiation. There is a beam utilization efficiency that is a ratio of an electron beam that generates X-rays, and an X-ray extraction efficiency that is a ratio of taking out and using the generated X-rays. Among them, the beam generation amount from the electron gun and the beam incidence efficiency to the accelerator are the most important parameters that determine the base of the X-ray generation amount regardless of other efficiencies.

  As the electron gun in the conventional betatron accelerator, a Kerst type electron gun is used as described in Non-Patent Document 1. This is composed of a cathode that also serves as a filament for high-temperature heating, a grid electrode that is installed in the immediate vicinity of the cathode, and an anode electrode that is installed on the outermost side and accelerates an electron beam from the cathode.

“Experimental Physics Lecture 28 Accelerator”, first edition 3 prints, Kyoritsu Publishing Co., Ltd., September 10, 1988, p. 547-564 JP 2004-296302 A

If the cathode area is the same, the amount of beam generated from the electron gun increases as the temperature increases. On the other hand, in the electron gun of the conventional apparatus, it is necessary to keep the beam current constant in order to stabilize the X-ray intensity, and the beam current is limited by the grid electrode. In this configuration, the beam current value to be taken out is controlled in a direction to be smaller than that in the case where no grid is provided, so that there is a problem that the beam current from the electron gun is smaller than in the case of the bipolar tube.
As another example, an electron gun as shown in Patent Document 1 has also been proposed, but this also uses a grid for beam current stabilization, as in the above-mentioned document, and is compared with the case of a bipolar tube. As a result, the beam current from the electron gun is reduced.

  When the beam from the electron gun is incident on the accelerator, the beam circulates in a generally circular orbit and is accelerated. At this time, the orbiting beam is incident on the structure constituting the electron gun, particularly the anode. Loss. In reality, the magnetic field is controlled so that the beam trajectory after one lap takes the inner trajectory as much as possible and does not collide with the anode.

  On the other hand, in the conventional electron gun, there is a grid between the anode and the cathode, and the potential of the grid is set to a value close to the cathode potential as compared with the potential difference between the anode and the cathode. Therefore, since a high potential difference is generated between the grid and the anode, it is necessary to provide a sufficient distance between the grid and the anode. Since the distance between the grid and the cathode is added to this, the incidence efficiency becomes lower as a result of the larger ratio between the cathode and the anode and the loss of the incident beam at the anode than in the case of the bipolar tube. There is a problem. Thus, in the conventional X-ray generator, since the beam performance of the electron gun could not be utilized to the maximum, there was a problem that the generated X-ray dose was small.

  The present invention has been made to solve the above problems, and an object of the present invention is to obtain an X-ray generator that generates high-intensity X-rays in X-ray intensity stability.

  An X-ray generator according to the present invention includes an electron gun and an electron gun power source for generating an electron beam, a vacuum chamber for holding a circular orbit of the electron beam in a vacuum, and an electromagnet for generating a magnetic field acting on the electron beam by an exciting coil. An X-ray generator comprising: an electromagnet power source that drives the exciting coil; and a target that generates an X-ray by interaction with an electron beam that is installed on a circular orbit of the electron beam and collides with the electron gun. And the electron gun power source includes: a cathode that emits electrons; an anode that accelerates electrons at a high potential with respect to the cathode; a power source that applies a voltage to the anode; and a cathode heating unit that emits thermal electrons from the cathode. The electron beam current emitted from the electron gun or the intensity of X-rays generated by the electron beam is measured and measured. A monitor for obtaining a value, and a controller for controlling the electron gun power source based on the measurement value measured by the monitor, and controlling the average current per unit time of the generated electron beam by the controller, The strength is made constant.

Further, the X-ray generator according to the present invention generates an electron gun and an electron gun power source for generating an electron beam, a vacuum chamber for holding a circular orbit of the electron beam in a vacuum, and a magnetic field acting on the electron beam by an exciting coil. An electromagnet to be driven, an electromagnet power source for driving the exciting coil,
An X-ray generator comprising: a target installed on a circular orbit of an electron beam and generating an X-ray by interaction with the colliding electron beam, wherein the electron gun and the electron gun power source are cathodes that emit electrons And an anode for accelerating electrons at a high potential with respect to the cathode, a power source for applying a voltage to the anode, and cathode heating means for emitting thermoelectrons from the cathode, and emitting from the electron gun. A monitor that measures the average intensity per unit time of X-rays generated by the electron beam to be measured and obtains a measurement value, and controls the X-ray generation time based on the measurement value measured by the monitor to obtain a predetermined X-ray intensity It ’s what you get.

According to the X-ray generator of the present invention, an electron gun having a cathode and an anode and using a bipolar tube that does not include a grid electrode for controlling the current emitted from the cathode is used. In addition to realizing an increase in beam incidence efficiency, a monitor that measures the electron beam current or the intensity of X-rays generated by the electron beam and obtains a measurement value against the instability of the electron beam current value, which is a drawback of the diode. provided, so that the measured value is constant, by controlling the average current per unit of generated electron beam time, the X-ray intensity is stabilized, the X-ray generator for generating X-rays of high intensity Can be obtained .

In addition, according to the X-ray generator of the present invention, an electron beam current can be increased by using, as an electron gun, a bipolar tube that has a cathode and an anode and does not include a grid electrode that controls the current emitted from the cathode. The average intensity per unit time of X-rays generated by the electron beam emitted from the electron gun is reduced against the instability of the electron beam current value, which is a drawback of the bipolar tube. Can be measured to obtain a measured value, and based on the measured value, the X-ray generation time can be controlled to obtain a predetermined X-ray intensity, and the X-ray intensity can be stabilized to generate high-intensity X-rays. An X-ray generator can be obtained .

Embodiment 1 FIG.
FIG. 1 is a block diagram showing an X-ray generator using an electron beam induction accelerator according to Embodiment 1 of the present invention. FIG. 2 is a diagram showing a configuration of an electromagnet portion of the X-ray generator in the first embodiment. As shown in FIGS. 1 and 2, the X-ray generator includes an electron gun 2 and an electron gun power source 3 (pulse power source, heater power source) that generate the electron beam 1, and a vacuum that keeps the orbit of the electron beam 1 in a vacuum. A chamber 4, an electromagnet 5 for generating a magnetic field acting on the electron beam 1, a main excitation coil 6 for exciting the electromagnet 5, a control coil 7 for exciting the electromagnet 5 to control the trajectory of the electron beam 1, a yoke 13 for the electromagnet 5, An electromagnet power source 8 for driving the main excitation coil 6 and the control coil 7, a target 9 that generates X-rays 12 by interaction with the colliding electron beam 1 installed on the orbit of the electron beam 1, and generated X-rays An X-ray monitor 10 that measures the intensity of 12 and a controller 11 that controls the electron gun power source 3 based on the measured X-ray intensity.

  FIG. 3 is a diagram showing the configuration of the electron gun unit in the first embodiment. FIG. 4 is a diagram showing the configuration of the electron gun power source 3 in the first embodiment. The electron gun 2 and the electron gun power source 3 are composed of a cathode 21 coated with an oxide 23 at a position where electrons are emitted, an anode 22 having a high potential with respect to the cathode 21 when accelerating electrons, and a high voltage across the anode 22. The electron gun 2 includes a pulse power supply 31 for applying a pulse voltage, a heater 24 for heating the cathode 21 to emit thermoelectrons from the cathode 21, and a heater power supply 32 having a stabilization circuit 14. It is a bipolar tube electron gun that does not include a so-called grid electrode that controls the current emitted from the cathode 21.

  The cathode 21 is made of a high melting point metal such as tungsten or molybdenum, and the oxide applied on a part of the cathode 21 uses a low work function material suitable for electron emission, such as BaO or SrO. . The controller (controller) 11 includes an electronic circuit that generates an analog voltage signal or a digital signal that varies inversely with the input of the X-ray intensity detected by the X-ray monitor 10, and this signal is a pulse power source for the electron gun power source 3. 31 is input.

  In FIG. 4, 15 is a capacitor, 16 is a high voltage FET switch, and has a diode 18. Reference numeral 17 denotes a resistor. The heater 24 of the cathode 21 is heated by a heater power source 32 so that an electron beam can be emitted from the cathode 21. The capacitor 15 is charged by the high voltage power supply 25, and when the high voltage FET switch 16 is turned on by TTL control by the control signal of the controller 11, a negative high voltage is applied to the cathode 21 side. That is, the anode 22 has a higher voltage than the cathode 21, and the electron beam is accelerated toward the anode 22 to emit electronic beer. When the high voltage FET switch 16 is turned off under TTL control by the control signal of the controller 11, the charge on the cathode 21 side flows through the resistor 17, and the negative high voltage on the cathode 21 side is eliminated. The current flowing through the cathode side 21 includes a constant current from the heater power supply 32, a current when the high-voltage power supply 25 is applied (ON time), and a current when the high-voltage power supply 25 is OFF (current opposite to the ON current). From these, the electron beam current can be measured.

  Next, the operation will be described. FIG. 5 is a schematic diagram for explaining an electron trajectory in the first embodiment. X-rays 12 are generated when the electron beam 1 from the electron gun cathode 21 circulates in the vacuum chamber 4 and then collides with the target 9. When the temperature of the cathode 21 of the electron gun 2 is raised by the heater power source 32 and the energy of the thermoelectron begins to exceed the work function of the material, thermionic emission starts. Since it is applied, thermal electrons are emitted only from the oxide 23 portion. The emitted thermoelectrons are accelerated by a pulse voltage applied to the anode, pass through a gap provided in the anode 22, and are emitted from the electron gun 2 to become an electron beam 1.

  Next, the electron beam 1 is accelerated by the action of the magnetic field generated by the electromagnet 5 and circulates in the vacuum chamber 4. At the timing immediately after extraction, the electromagnet power source 8 is controlled so that the diameter of the orbit is reduced. 1 is controlled so as to suppress a loss of collision with the anode 22. After that, the beam acceleration is continued, and after accelerating to a predetermined beam energy, the orbital diameter is controlled to be reduced again at the timing when it collides with the target 9, and collides with the target 9 provided in the inner part of the orbit. . As a result, X-rays 12 that are electromagnetic waves corresponding to the beam energy and the amount of electron beam current that collides with the target 9 are generated.

  This is taken out from an X-ray extraction window provided in the vacuum chamber 4 and used for X-ray imaging or the like. A part of the extracted X-ray is taken into the X-ray monitor 10 and a change in X-ray intensity is measured. This measurement result is input to the controller 11, and when the X-ray intensity is increased from a predetermined value, an analog voltage signal that varies inversely is generated, and the high voltage is supplied to the pulse power supply 31 of the electron gun power supply 3. Feedback control is performed to turn on / off the voltage FET switch 16 to lengthen the electron beam pulse generation period, and to shorten it in the opposite case. Thereby, the X-ray irradiation amount in the X-ray irradiation for one time is kept constant and stabilized.

  Further, the average intensity per unit time of X-rays generated by the electron beam emitted from the electron gun is measured by the X-ray monitor 10 to obtain a measurement value, and the controller 11 controls the X-ray generation time based on this measurement value. Thus, a predetermined X-ray intensity may be obtained. The X-ray generation time is controlled by the duration of predetermined ON / OFF control of the high voltage FET switch 16.

  FIG. 6 is a diagram showing a comparison of pulse trains when the repetition frequency f in the first embodiment is 1500 Hz and 1000 Hz. t indicates time. At 1500 Hz, the number of pulses per unit time increases, and therefore the generated X-ray dose increases. Thereby, the average current per unit time of the generated electron beam is controlled to make the X-ray intensity constant.

Next, the reason why a large beam current can be obtained will be described. In the first embodiment, the performance and performance of the electron gun 2 can be maximized by the configuration and operation as described above. The current value of electrons emitted from the electron gun cathode 21 is generally given by the following equation.
J = A T ² exp (−φ / (kT))
Here, J is the current density, T is the absolute temperature, k is the Boltzmann constant, and φ is the work function. Now, the maximum temperature T = 1273K determined by the cathode material,
When the work function of the oxide cathode is φ = 1.6 [eV] and the coefficient A = 15 [A / (cm ² K ² )],
^{J = 11.3 [A / cm 2} ] = 0.113 [A / mm 2]
Is calculated.

  This value is the upper limit of the current density obtained with this material at this temperature and is called the saturation current density. On the other hand, in the electron gun used in the space charge limiting region using the grid as in the conventional example, the space charge limitation over the region I shown in FIG. 11 is controlled in order to control the amount of current by the value of the electric field. Since the grid voltage [V] of region II is applied and used, only a current density lower than the saturation current density (saturation current density of cathode temperatures T1 and T2, that is, anode current [A]) can be obtained. In the electron gun of the embodiment, there is no grid, the current can be used up to the saturation current density, and a higher current density can be obtained. From another point of view, if the current density setting is the same as the conventional one, the electron gun can be further downsized. In addition, FIG. 11 is explanatory drawing which shows operation | movement of the grid of the electron gun of the conventional X-ray generator.

  After the electron beam 1 from the electron gun 2 is incident on the accelerator, a part of it collides with the anode 22 and becomes an incident loss. This is achieved by designing the distance between the cathode 21 and the anode 22 to be closest. Can be minimized. In this embodiment, since there is no grid electrode in the triode in the conventional apparatus, the distance between the cathode and the anode can be made close to the discharge withstand voltage with respect to the voltage between the cathode and the anode. The incident efficiency can be increased.

  The bipolar tube structure electron gun of this embodiment can achieve the above-described effects, but has a problem that the beam current value fluctuates due to a subtle temperature change or the like. On the other hand, in the case of an X-ray generator, it is necessary to finally control the integral value of the X-ray irradiation intensity during the irradiation of about one second, that is, the X-ray irradiation amount to be controlled to be constant. In the first embodiment, as a simpler means than the beam current control, the generation period of the electron beam pulse is controlled to control the X-ray irradiation amount constant.

  As described above, according to this embodiment, while maintaining the stability of the X-ray irradiation amount, the electron beam current from the electron gun is increased, the electron beam incident efficiency to the accelerator is improved, and the generated X-ray dose is reduced. Since it can be increased, if it is used for medical examinations, non-destructive examinations, etc., the imaging time can be shortened, and the burden on the patient can be reduced and the throughput of the product examination can be improved.

In the first embodiment, the heater and the heater power source are used for cathode overheating. However, the cathode itself may be energized to be overheated. The same effect can be obtained even if other heating means such as electron impact is applied to the cathode.
Further, in the first embodiment, the measurement value is obtained by measuring the intensity of the X-rays generated by the electron beam, but the measurement value is obtained by measuring the electron beam current emitted from the electron gun. And a controller for controlling the electron gun power source based on the above, and the controller may control the average current per unit time of the generated electron beam so as to make the X-ray intensity constant.

Embodiment 2. FIG.
The X-ray generator of the second embodiment is substantially the same as the configuration of the first embodiment, but the electron gun power source 3 to which a signal from the controller 11 is input is generated based on the control input signal. Rather than changing the period of the beam pulse, the pulse period is constant and the time width for generating the pulse train is changed. The pulse width is usually about several microseconds, and this is generated as a pulse train of several kHz to several tens kHz during one irradiation time of several ms to several seconds. Therefore, the X-ray irradiation dose can be controlled to a predetermined value by controlling this one irradiation time. FIG. 7 is a diagram showing a pulse waveform when the irradiation time width w is 8 ms and 4 ms when the pulse repetition frequency in the second embodiment is constant at 1000 Hz. The number of pulses per unit time increases in proportion to the irradiation time width, and therefore the amount of X-ray generation increases.

  In the second embodiment, the X-ray intensity is stabilized by the pulse train generation time width w, that is, the average current per unit time of the electron beam generation pulse is controlled, so that the pulse cycle control of the first embodiment is performed. Compared to the above, the circuit configuration is simplified and the cost can be reduced.

Embodiment 3 FIG.
The X-ray generator of the third embodiment is generally the same as the configuration of the second embodiment, except that the operation of the electron gun power supply controls the pulse time width of the generated electron beam pulse. . The control of the X-ray irradiation dose is the same as that of the second embodiment in that it is performed by controlling the irradiation time width of one time. FIG. 8 is a diagram showing a waveform in which the electron gun voltage is changed in accordance with the irradiation time width in the third embodiment. When the irradiation time width is changed, the X-ray dose per unit time can be changed and controlled.

  In the third embodiment, the average current per unit time of the generated electron beam is controlled by the measurement value measured by the monitor, that is, the pulse time width of the generated electron beam pulse is controlled to keep the X-ray intensity constant. Therefore, there is an effect that an electron beam with a larger current can be obtained and a high-intensity X-ray generator can be obtained.

Embodiment 4 FIG.
The X-ray generator of the fourth embodiment is generally the same as the configuration of the first embodiment, except that an oxide is not applied to the cathode 21 body, but another low work function material 23 is incorporated. Yes. 9A and 9B are diagrams showing an example of a structure in which another low work function material according to Embodiment 4 is incorporated. FIG. 9A is a side view of the cathode 21 and FIG. 9B is a front view of the cathode 21. As the material to be incorporated, those used for an electron beam processing machine such as LaB ₆ are used. Although the operation is the same as in the first embodiment, the effect of the fourth embodiment is that, in the case of oxide coating, after the processing such as aging is performed by placing the electron gun in a vacuum once, it is returned to the atmosphere. However, in the actual embodiment 4, since it can be returned to the atmosphere and repaired and reused, there is an effect of reducing the running cost of the apparatus.

Embodiment 5 FIG.
The X-ray generation apparatus of the fifth embodiment is generally the same as the configuration of the first embodiment, but the electron beam extraction position on the surface of the cathode 21 main body where the oxide 23 is applied or the low work function material 23 is embedded. This structure is characterized by a concave shape. FIG. 10 is a cross-sectional view showing the cathode structure in the fifth embodiment. Although the operation is almost the same as that of the first embodiment, the concave surface structure serves to converge the trajectory of the electron beam immediately after emission, so that the beam divergence is suppressed as shown in FIG. It is possible to reduce the loss of collision. As a result, an X-ray generator with high X-ray intensity can be obtained.

It is a block diagram which shows the X-ray generator which uses the electron beam induction accelerator in Embodiment 1 of this invention. 3 is a diagram showing a configuration of an electromagnet part of the X-ray generator in Embodiment 1. FIG. 3 is a diagram showing a configuration of an electron gun unit in the first embodiment. FIG. FIG. 3 is a diagram showing a configuration of an electron gun power supply in the first embodiment. 4 is a schematic diagram illustrating an electron trajectory in the first embodiment. FIG. It is a figure which shows the comparison of the pulse train in case repetition frequency f in Embodiment 1 is 1500 Hz and 1000 Hz. It is a figure which shows a pulse waveform when irradiation time width w is 8 ms and 4 ms when the repetition frequency of the pulse in Embodiment 2 is made constant at 1000 Hz. It is a figure which shows the waveform which changed the electron gun voltage corresponding to the irradiation time width in Embodiment 3. It is a figure which shows the structure at the time of incorporating the other low work function material in Embodiment 4, (a) is a side view of a cathode, (b) is a front view of a cathode. FIG. 6 is a cross-sectional view showing a cathode structure in a fifth embodiment. It is explanatory drawing which shows operation | movement of the grid of the electron gun of the conventional X-ray generator.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electron beam 2 Electron gun 3 Electron gun power supply 4 Vacuum chamber 5 Electromagnet 6 Main excitation coil 7 Control coil 8 Electromagnet power supply 9 Target 10 X-ray monitor 11 Controller 12 X-ray 13 Yoke 14 Stabilization circuit 15 Capacitor 16 High voltage FET switch 17 Resistor 18 Diode 21 Cathode 22 Anode 23 Oxide 24 Heater 25 High-voltage power supply 31 Pulse power supply 32 Heater power supply

Claims (7)

An electron gun for generating an electron beam and an electron gun power source;
A vacuum chamber for holding the orbit of the electron beam in a vacuum; and
An electromagnet that generates a magnetic field acting on the electron beam by means of an exciting coil;
An electromagnet power source for driving the exciting coil;
In an X-ray generator comprising: a target installed on an orbit of an electron beam and a target that generates X-rays by interaction with the colliding electron beam
The electron gun and the electron gun power source are a cathode that emits electrons, an anode that accelerates electrons at a high potential with respect to the cathode, a power source that applies a voltage to the anode, and a thermal electron that is emitted from the cathode. A cathode tube including a cathode heating means,
A monitor that measures the electron beam current emitted from the electron gun or the intensity of X-rays generated by the electron beam to obtain a measurement value; and a controller that controls the electron gun power source based on the measurement value measured by the monitor; With
An X-ray generator that controls the average current per unit time of the generated electron beam by the controller so that the X-ray intensity is constant.   The generated electron beam is an electron beam pulse, and the period of the generated electron beam pulse is controlled to control the average current per unit time of the electron beam so that the X-ray intensity is constant. X-ray generator.   The generated electron beam is an electron beam pulse, and the generation time width of the pulse train of the generated electron beam pulse is controlled to control the average current per unit time of the electron beam so that the X-ray intensity is constant. Item 2. The X-ray generator according to Item 1.   The generated electron beam is an electron beam pulse, and the pulse current width of the generated electron beam pulse is controlled to control the average current per unit time of the electron beam so that the X-ray intensity is constant. The X-ray generator described. An electron gun for generating an electron beam and an electron gun power source;
A vacuum chamber for holding the orbit of the electron beam in a vacuum; and
An electromagnet that generates a magnetic field acting on the electron beam by means of an exciting coil;
An electromagnet power source for driving the exciting coil;
In an X-ray generator comprising: a target installed on an orbit of an electron beam and a target that generates X-rays by interaction with the colliding electron beam
The electron gun and the electron gun power source are a cathode that emits electrons, an anode that accelerates electrons at a high potential with respect to the cathode, a power source that applies a voltage to the anode, and a thermal electron that is emitted from the cathode. A cathode tube including a cathode heating means,
A monitor that measures the average intensity per unit time of X-rays generated by the electron beam emitted from the electron gun and obtains a measurement value, and controls the X-ray generation time based on the measurement value measured by the monitor to determine a predetermined value. X-ray generator which can obtain the X-ray intensity. The X-ray generator according to any one of claims 1 to 5, wherein LaB ₆ is attached to an electron beam generating portion of the cathode of the electron gun.   The X-ray generator according to any one of claims 1 to 6, wherein the cathode of the electron gun has a concave shape in an electron beam generating portion.

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