JPS6231480B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a linear particle accelerator, and particularly to control of an accelerated particle (electron) beam. As shown in Figure 1, this type of linear particle accelerator uses high power of several thousand MHz microwaves from a microwave generator MG such as a magnetron.
A strong electric field is created inside a 2 m long accelerating tube AT kept in vacuum, and the electron beam emitted from the electron gun EG is incident with an energy of several tens of K _e V.
The electrons are accelerated to near the speed of light by riding on the wave crest of the strong electric field, and a high-energy electron beam is extracted into the air through an output line TW of a light metal such as titanium (Ti) of about 0.1 mm, or The electron beam collides with a heavy metal target such as tungsten (W) to extract X-rays. The microwave generator MG and the electron gun EG are driven by a pulse voltage of several tens of KV from a high-voltage pulse power supply PS. When this linear particle accelerator is used for medical purposes (radiotherapy equipment), the intensity distribution of the electron beam is made uniform by a flattening filter or the like, and the radiation is stabilized before use. In other words, the intensity distribution of the electron beam derived from the accelerator tube is distributed in a mountain shape with the radiation axis as the peak point, so it is corrected to have a flat intensity distribution through a filter, but the trajectory of the electron beam is If it deviates from the original radiation axis, the intensity distribution transmitted through the filter cannot be made uniform. Therefore, as shown in Fig. 1, two pairs of radiation detectors RD ₁₁ , RD ₁₂ , RD ₂₁ , RD ₂₂ are installed in the irradiation field after the thin window TW, which is the output end of the accelerator tube AT.
), its detectors RD ₁₁ , RD ₁₂ , RD ₂₁ , RD ₂₂
The opposing output signals of the
In addition to SA ₁ and SA ₂ , the output signals of each difference are given to the excitation current control circuits EC ₁ and EC ₂ , and the steering coil
There is a known system that controls SC ₁ and SC ₂ to correct the trajectory of the electron beam emitted from the electron gun EG into the accelerator tube AT, thereby making the intensity distribution of the output radiation uniform. There is. However, in the above method, feedback is provided to the steering coil based on the average intensity distribution in a predetermined area after the electron beam emitted from the accelerator tube has expanded, so the radiation axis of the electron beam is It was difficult to correct the trajectory to the position. That is, the radiation detectors RD ₁₁ , RD ₁₂ , RD ₂₁ , RD ₂₂ are
As shown in the figure, they are arranged so as to be inscribed in a circular irradiation field Q and to face each other with the radiation axis P in between. Since the difference in detection signals between the opposing detectors is extracted, even if there is a difference in the dose rate at each location along the longitudinal direction of each detector, the overall dose rate If there is no difference between the output signal proportional to and the output signal of the opposing detector, the intensity distribution of the electron beam is considered to be uniform, and the trajectory of the electron beam cannot be optimized. Further, in this type of device, the magnitude of the electron flow, that is, the output dose is detected, and feedback is applied so that a set amount is achieved. As a method to detect the size of the electron flow,
A known method is to arrange an annular magnetic material to surround the electron flow and detect the magnitude of the electron flow by the voltage induced in a coil wound around the annular magnetic material. It could not be detected. This invention has been made in view of the above circumstances, and involves detecting the trajectory and size of the electron flow using the same detection means, and feeding back each detection signal to each control means to obtain the optimally accelerated particles (electron beam). ) is intended to provide a linear particle accelerator that can obtain The configuration of an embodiment of the present invention will be described below with reference to FIGS. 3 and 4. Note that the same parts as in FIG. 1 are given the same reference numerals, and the explanation will be simplified. In FIG. 3, OD is a detector whose one end is connected to the output end of the accelerating tube AT, the other end is provided with an output window TW made of titanium (Ti), etc., and four wires are stretched inside. That is, as shown in FIGS. 4a and 4b, two pairs of wires are formed between both end surfaces of a cylindrical body having a length l, separated by a distance d from the central axis, and forming an X-axis direction and a Y-axis direction. ω ₁ , ω
₂ , ω ₃ and ω ₄ , one end of which is connected to the input terminals of differential amplifiers SA ₁ and SA ₂ for each pair. Each output terminal of differential amplifier SA ₁ and SA ₂ is
Steering coil for controlling the trajectory of the electron beam between the electron gun EG and the input end of the accelerator tube AT
It is connected to the control input terminals of the excitation current control circuits EC ₁ and EC ₂ of SC ₁ and SC ₂ . Further, each output terminal of the wires ω ₁ to ω ₄ is connected to an input terminal of an operational amplifier OA, and an output terminal of the operational amplifier OA is connected to a control input terminal of a high-voltage pulse power supply PS. Next, we will explain the operation of the above structure in Figures 5 to 8.
This will be explained with reference to the figures. That is, the electron beam emitted by the electron gun EG is accelerated by the accelerator tube AT, passes through the detector OD, and then is emitted into the air through the window TW. In this case, in the detector OD, a current I due to the electron beam flows as shown in _{FIG .}
is a voltage proportional to the amount of change in current I.
e ₁ and e ₂ are induced. In other words, an electromagnetic field is generated around the current I, and if conductive objects (ω ₁ and ω ₃ or ω ₂ and ω ₄ ) are placed at two locations to block the direction of this electromagnetic field, a voltage will be generated between them. induced. Therefore, the two dielectrics (ω ₁ and ω ₃ or ω ₂ and ω ₄ ) are connected to a differential amplifier SA ₁
Alternatively, it can be connected to the input of SA ^2, etc., and the potential difference between the two can be determined. This differential amplifier SA ₁ or SA ₂
A circuit connecting wires ω ₁ and ω ₃ or ω ₂ and ω ₄ is called a second circuit C ₂ , and a current path emitted from the electron gun EG and passing through the detector OD is called a first circuit C ₁ . Then, the voltage e induced in the second circuit _C2 by the change in current I over time is e=-MdI/dt (1). Here M is mutual inductance,
According to Neumann's formula, M=μ/4π∫ _C1 ∫ _C2 cosθdS ₁ dS ₂ /r...
…(2) becomes. Here, dS ₁ and dS ₂ are minute portions from a certain point of both line elements in the first and second circuits C ₁ and C ₂ , r is the distance between the two points, and θ is the angle between them. μ is the magnetic permeability of the medium. Therefore, the lengths of both lines to a certain point are S ₁ and S ₂
As r=√( ₁ − ₂ ) ² + ² , the angle θ is parallel to the direction of the current I path wires ω ₁ to ω _4.
If we calculate the mutual inductance M by substituting cosθ=1 into the equation (2) above, we get If l≫d, then M=μl/2π(log2l/d−1). Therefore, the mutual inductance M/l per unit length is M/l=μ/2π(log2l/d=2μs log2l/d
×10 ^-7 ...(4) where μs is the magnetic permeability of the medium in vacuum. Therefore, by substituting this equation (4) into the above equation (1), l=-2μs log2l/d×10 ^-7 dI/dt……(
5) becomes. Therefore, the wires ω ₁ , ω ₂ ,
Voltages e _{1 ,} e ₂ , e ₃ , and e ₄ are induced at ω 3 and ω ₄ , respectively. In this case, if the wires ω ₁ to ω ₄ are all separated by d from the trajectory of the electron flow, the voltages e ₁ , e ₂ , e ₃ , and e ₄ induced in each will be equivalent, but if the trajectory of the current is If the distances from each of the wires ω ₁ to ω ₄ are different, voltages corresponding to the distances are induced in each wire. Therefore, if the output voltages e ₁ to e ₄ induced in each wire ω ₁ to ω ₄ are added to the operational amplifier OA, we get
A signal proportional to the magnitude of the electron flow is obtained, which is further compared with a set standard value, and its deviation value is obtained as the output of the arithmetic amplifier OA. This output signal is applied as a control signal to the high-voltage pulse power supply PS to control the emission of thermoelectrons from the electron gun EG, and the magnitude of the electron flow is corrected so that the deviation value is eliminated. That is, as shown in FIG. 6, the heater H (filament) of the electron gun EG heats the cathode K by the heater power source E _f , thermionic electrons are emitted from the cathode K, and the anode voltage E is increased between the cathode K and the anode A. Since _p is applied, thermionic electrons are drawn toward the anode and are eventually emitted toward the entrance of the accelerating tube AT. At this time, if an appropriate impact voltage E _b is applied between the cathode K and the heater H, the thermoelectrons from the heater H are accelerated by the impact voltage E _b and collide with the cathode K surface. Its collision voltage E _b
Since the amount of thermionic electrons emitted from the cathode K can be changed by changing the
By controlling the impact voltage E _b using the output signal of the OA, the anode voltage I _p , that is, the magnitude of the accelerated electron flow, can be controlled to an appropriate value (set value). FIG. 7 shows I _p -E _p characteristic curves in the electron gun EG when the impact voltage is changed from E _b1 to _{E b3} . Furthermore, the respective output voltages e ₁ and e ₃ of the wires ω ₁ and ω ₃ arranged in the X direction are applied to the first differential amplifier SA ₁ , and the respective output voltages of the wires ω ₂ and ω ₄ arranged in the Y direction are applied to the first differential amplifier SA 1. If voltages e ₂ and e ₄ are applied to the second differential amplifier SA ₂ ,
A signal corresponding to the X-Y coordinates of the orbital position of the electron flow appears at the output terminals of the cooperative differential amplifiers SA ₁ and SA ₂ ,
The current is applied to the excitation current control circuits EC ₁ and EC ₂ and controls the current flowing through the steering coils SC ₁ and SC ₂ to correct the orbital position of the electron flow so that both the X-Y coordinates become "0". In this case, as shown in FIG. 8, it is assumed that a magnetic field with a magnetic flux density of 3B is created in a region of length b by the steering coils SC ₁ and SC ₂ , and the electron flow from the electron gun is V ₀ (V ) The electron flow incident on the steering coils SC ₁ and SC ₂ with a voltage corresponding to the speed of y
_s is given by the following formula. R: radius of curvature

[Formula] L: Steering coil SC ₁ , SC ₂ to output window
Length to TW Here, magnetic flux density B is steering coil SC ₁ ,
Since it can be selected arbitrarily depending on the current flowing through SC ₂ ,
The magnitude of the displacement of the electron flow y _s can be controlled. That is, in order to keep the trajectory of the electron flow emitted from the detector constant, the signal detected by the detector is fed back, thereby applying appropriate voltages to the steering coils SC ₁ and SC ₂ . For example, as shown in Fig. 8, if the trajectory of the electron stream _ys deviates upward in the drawing, this is detected by a detector, and a magnetic field B is applied to the trajectory of the electron stream from the back of the page to the front using a steering coil. to return to the desired trajectory. As explained above, according to the present invention, before the electron flow accelerated in the acceleration tube is released into the air, the strength and orbital position of the electron flow are simultaneously detected in the acceleration tube, and each detection signal is detected. By providing feedback to each control means, it is possible to accurately optimize the strength of the electron flow and the orbital position. It should be noted that the present invention is not limited to the above-mentioned embodiment, and can of course be implemented in the following modified manner, for example. That is, in the above embodiment, the detector
The detection signal of the size of the electron flow by OD is sent to the electron gun.
Feedback is applied to the heating circuit of heater H of the EG, but the intensity of the electron flow can also be controlled by changing the focal length of the electron flow from the electron gun EG by feedback to the magnetic field lens near the entrance of the electron beam accelerator tube AT. In addition, the intensity of the electron flow can be changed by using a grid control type electron gun EG and feeding back the detection signal to the grid, or any other place where the intensity of the electron flow can be changed. Of course, the intended purpose can be achieved. Furthermore, in the above embodiment, the detector OD
The position detection signal of the trajectory of the electron flow is fed back to the steering cols SC ₁ and SC ₂ , but in medical applications where the electron flow is bent in the vertical direction, it is Of course, the desired purpose can be achieved at any location where the trajectory of the electron flow can be varied, such as by optimizing the trajectory of the electron flow.

[Brief explanation of the drawing]

Figure 1 is a configuration diagram for explaining an example of a control method for optimizing the electron flow trajectory in a conventional linear particle accelerator, and Figure 2 is a configuration diagram for explaining the radiation detector in Figure 1. , FIG. 3 is a block diagram for explaining an embodiment of a control method for optimizing the size and trajectory of electron flow in the linear particle accelerator of the present invention, and FIGS. Figures illustrating the details of the detector OD, a is a front view, b is a side sectional view, FIG. 5 is an explanatory diagram for explaining the operation of FIGS. 3 and 4, and FIG. An explanatory diagram showing one embodiment of the control of the electron gun EG, FIG. 7 is a diagram showing I _p -E _p characteristics for explaining the operation of FIG. 6, and FIG. 8 is a diagram showing an example of the steering coil in FIG. FIG. 3 is a diagram for explaining the operation. EG...electron gun, MG...microwave generator,
AT...Acceleration tube, PS...High voltage pulse power supply, SC ₁ ,
SC ₂ ...steering coil, OD...detector,
SA ₁ , SA ₂ ... Differential amplifier, OA ... Operational amplifier, EC ₁ , EC ₂ ... Excitation current control circuit.

Claims (1)

[Claims] 1. In a linear particle accelerator in which charged particles emitted from a charged particle generator are introduced into an acceleration tube and are linearly accelerated within the acceleration tube by a strong electric field of microwaves to obtain high-energy accelerated particles, the above-mentioned a detection section connected to the output end of the acceleration tube to detect the loaded particles, and at least two pairs of wires parallel to the trajectory of the accelerated particles inside the detection section, and an induced signal to the wires. The strength and orbital position of the accelerated particle are detected by a voltage, the strength of the accelerated particle is controlled based on the detection signal of the strength, and the orbital position of the accelerated particle is controlled based on the detection signal of the orbital position. A linear particle accelerator characterized by control.