JP4563928B2 - Beam position monitor - Google Patents

Description

  The present invention relates to a beam position monitor for measuring the position and profile of a charged particle or X-ray beam.

  A conventional strip type beam position monitor is configured by combining a plurality of electrode substrates in a container, and one of the electrode substrates is used as a first electrode having slits on both sides, and the first electrode is used as an electrode substrate. The first electrode and the second electrode are held at a predetermined distance by being sandwiched between two second electrodes, and a voltage is applied to the second electrode so that each slit from the first electrode A position detection unit for extracting an ionizing current is provided, and a pair of position detection units are provided in directions orthogonal to each other to detect the position of the beam (for example, see Patent Document 1).

JP 2002-006051 A

  In this beam position monitor, the magnitude of the current signal from the strip electrode corresponding to the center of the beam is large, and the magnitude of the current signal corresponding to the end is small, so that the beam shape is accurately grasped. It is necessary to increase the dynamic range of the A / D converter for reading the strip electrode signal. However, if the allowable input range is set so that the current signal corresponding to the center of the beam does not saturate, the error of the center of gravity calculation increases because the size of the current signal corresponding to the end becomes small and the error increases. There is a problem of end.

  An object of the present invention is to provide a beam position monitor having an improved S / N ratio when measuring the center position of a beam using a current signal from a strip electrode.

The beam position monitor according to the present invention comprises a plurality of strips in which gas ions or electrons that are ionized when a charged particle or X-ray beam passes are arranged in parallel in one axial direction of a plane perpendicular to the beam passing direction. In the beam position monitor for measuring the central position of the beam in the uniaxial direction from the magnitude of the current flowing through the strip electrode and the position information of the strip electrode, the strip electrodes are arranged at both ends in the uniaxial direction. In addition, at least four strip electrodes having a width twice as large as the width of the strip electrode disposed in the central portion in the uniaxial direction are included.

  The effect of the beam position monitor according to the present invention is that a plurality of strip-shaped strip electrodes arranged at positions corresponding to positions where gas ions or electrons ionized by passage of charged particles or X-ray beams have passed. The center position of the beam is calculated from the position information of the strip electrode and the magnitude of the current flowing through the strip electrode. However, the width of the strip electrode is narrow in the central area, and in both end areas. Since it is wide, a large current flows from the strip electrodes in both end regions where the distribution of gas ions or electrons is small, and the S / N ratio is improved.

Embodiment 1 FIG.
1 is a schematic sectional view of a parallel plate type beam position monitor according to Embodiment 1 of the present invention.
A parallel plate type beam position monitor 1 according to the first embodiment of the present invention has one high-voltage electrode 2 and two signal electrodes 3a and 3b sandwiching the high-voltage electrode 2.
Further, the beam position monitor 1 separates the insulating support member 5 that supports the outer peripheral edge portions of the high-voltage electrode 2 and the signal electrodes 3a and 3b from the insulating support member 5 at a predetermined interval, and connects the signal electrodes 3a and 3b. It has a spacer 6 that is separated from the high-voltage electrode 2 by a predetermined distance, and a frame 7 that stacks and fixes the insulating support member 5 and the spacer 6.
In the central part of the frame 7 through which the charged particle beam or X-ray (hereinafter abbreviated as a beam) passes, a window with a thin film is applied to reduce the phenomenon that the beam collides with the substance and is scattered. 8 is provided. Air is sealed between the high-voltage electrode 2 and the signal electrodes 3a and 3b. In addition, you may enclose the gas adjusted for beam position monitoring instead of air.

FIG. 2 is a diagram illustrating the arrangement of the high-voltage electrode 2 and the signal electrodes 3a and 3b according to the first embodiment. In the following description, the direction in which the beam passes is referred to as the Z axis, and the two orthogonal axes of the plane orthogonal to the Z axis are referred to as the X axis and the Y axis.
The high voltage electrode 2 and the signal electrodes 3a and 3b are orthogonal to the Z axis.
The high voltage electrode 2 is formed by forming a metal layer having a thickness of several μm to several tens of μm on the surface of an insulating film such as a metal foil or polyimide, and is maintained at a potential of about 1 kV to 10 kV.
The signal electrodes 3a and 3b are arranged at positions facing the high voltage electrode 2 and spaced apart by a predetermined interval.
As shown in FIG. 2, one signal electrode 3a is formed by forming a plurality of strip-like strip electrodes 10a and 10b on one surface of an insulating film. The high-voltage electrode 2 and the strip electrodes 10a and 10b Arranged to face each other. The strip electrodes 10a and 10b are maintained at substantially zero potential.
The beam position monitor 1 according to the first embodiment is configured by sandwiching one high-voltage electrode 2 between two signal electrodes 3a and 3b, but is not limited thereto. For example, a structure in which one signal electrode 3 is arranged at the center and both sides thereof are sandwiched between two high-voltage electrodes 2 may be adopted. In this case, strip electrodes 10 a and 10 b are formed on both surfaces of the signal electrode 3.

FIG. 3 is a plan view of the surface of the signal electrode 3a on which the strip electrodes 10a and 10b are formed.
As shown in FIG. 3, the signal electrode 3 a has strip-like strip electrodes 10 a and 10 b each having a long side in the Y-axis direction and a short side in the X-axis direction on the surface of the insulating film 12. The length of the short side of the strip electrode 10b formed in the end region B in the X-axis direction (hereinafter referred to as the width of the strip electrode) is the length of the strip electrode 10a formed in the center region A in the X-axis direction. It is twice the width. However, the width of the strip electrodes 10a and 10b is a value including the insulating space between the strip electrodes 10a and 10b.

The strip electrodes 10 a and 10 b are insulated from each other and connected to the connector 14 via the signal line 13. Since the strip electrodes 10a and 10b are maintained at substantially zero potential, an electric field is generated between the high voltage electrode 2 and the signal electrode 3a in the Z-axis direction perpendicular to the high voltage electrode 2 and the signal electrode 3a.
In FIG. 1, when a beam is incident from the left-hand direction, air is ionized at the position where the beam has passed, and positively charged gas ions and negatively charged electrons are generated. The gas ions and electrons move toward the high voltage electrode 2 and the signal electrode 3a according to the polarity of the voltage applied to the high voltage electrode 2. For example, when a positive voltage is applied to the high voltage electrode 2, electrons move toward the high voltage electrode 2 and gas ions move toward the signal electrode 3a. When the gas ions reach the signal electrode 3a, a current flows through the strip electrodes 10a and 10b at the corresponding positions. At this time, since the electric field between the high-voltage electrode 2 and the signal electrode 3a is substantially perpendicular to these electrodes and parallel to the beam axis (Z-axis), the magnitude of the current flowing through each strip electrode 10a, 10b. This distribution reflects the distribution of the beam spread in the X-axis direction. That is, when the strip electrodes 10a and 10b are formed as shown in FIG. 3, the distribution of the beam spread in the X-axis direction can be known from the magnitude of the current signal from the strip electrodes 10a and 10b.

  The other signal electrode 3b is formed by rotating the strip electrodes 10a and 10b formed on one signal electrode 3a by 90 degrees, and the distribution of the beam spread in the Y-axis direction can be known. From the signals of the signal electrodes 3a and 3b, the two-dimensional distribution in the direction perpendicular to the beam traveling direction can be known.

FIG. 4 is a diagram illustrating a state of the signal processing system connected to the strip electrodes 10a and 10b according to the first embodiment.
A preamplifier 16 is connected to each strip electrode 10a, 10b, and a current signal is converted into a voltage signal and amplified. An A / D converter 17 is connected to the preamplifier 16, and the voltage signal is converted into a digital voltage signal and sent to the arithmetic circuit 18. The arithmetic circuit 18 calculates the center position of the beam from the position information of the strip electrodes 10a and 10b input in advance and the magnitude of the current signal at the strip electrodes 10a and 10b input from the A / D converter 17.

  As a calculation method of the center position of the beam, for example, a method of calculating the center of gravity using the magnitude of the current signal and the position information of the strip electrodes 10a and 10b, or a function assuming that the distribution of the beam spread follows a Gaussian distribution. There is a method of approximation. When performing arithmetic processing at high speed, a method of calculating the center of gravity is used. In particular, when used in a medical accelerator, it is necessary to confirm that the beam always passes through the center position of the beam position monitor. For example, when the beam position monitor is used in a cancer treatment apparatus using particle beam spot scanning, it is necessary to detect an abnormality in the center position of the beam between several tens of microseconds to several hundreds of microseconds, and thus high-speed processing is required. . Also, high-speed processing is required to configure a high-speed correction circuit that detects the beam position variation and outputs an output correction signal to the position correction electromagnet upstream of the beam transport system.

FIG. 5 shows the result of simulating the magnitude of a current signal from a strip electrode having the same width in the conventional signal electrode. The horizontal axis is the distance from the center of the signal electrode to each strip electrode on the X axis. The position of the strip electrode is represented by the center of the width of the strip electrode. In the simulation of the magnitude of the current signal, the beam is incident so that the center of the beam is positioned at the center of the signal electrode.
The magnitude of the current signal from the strip electrode becomes the largest when the strip electrode is located at the center of the beam, that is, the position closest to the center of the signal electrode. At this time, the range of the current signal that is allowed to be input to the preamplifier 16 and the A / D converter 17 needs to be set with a margin with respect to the largest current signal. For the strip electrode at a certain position, the magnitude of the current signal is relatively small with respect to the range of the input signal, so that the sensitivity to the current signal is lowered.

FIG. 6 is a simulation result of the magnitude of the current signal when the beam is incident so that the center of the beam is positioned at the center of the signal electrode 3a according to the first embodiment.
The ten strip electrodes 10a in the central region A have a width of 1 mm, so that the current signal has the same magnitude as in FIG. 5, but the strip electrode 10b in the outer end region B has a width of strip. Since it is 2 mm, which is twice that of the electrode 10a, the magnitude of the current signal is also twice the magnitude of the current signal from the strip electrode at the corresponding position in FIG.

The boundary position where the widths of the two types of strip electrodes 10a and 10b change is the strip electrode 10a in which the magnitude of the current signal of the strip electrode 10b in the end region B closest to the center region A is located at the beam center position. The position was almost the same as the magnitude of the current signal. That is, the boundary position is in the vicinity of the half width of the beam spread distribution.
The range of the magnitude of the current signal allowed to be input to the A / D converter 17 may be the same as when all the strip electrodes have the same width, and the current signal is applied to the strip electrode 10b in the end region B. Since the size of is doubled, the signal sensitivity is increased.

FIG. 7 shows a simulation result regarding the magnitude of the current signal when the center of the beam is moved in the X-axis direction by half 0.5 mm of the width 1 mm of the strip electrode 10a with respect to the center of the signal electrode 3a according to the first embodiment. It is.
In this case, the change in the magnitude of the current signal from the strip electrode 10b close to the central region A in the strip electrode 10b in the end region B is large, and the signal sensitivity is high with respect to the beam position variation.

  Next, the beam center position is calculated by calculating the center of gravity from the magnitude of the current signal obtained from the strip electrodes 10a and 10b. When obtaining the center position of the beam by calculating the center of gravity using the current signal magnitudes of the n strip electrodes 10a and 10b, the position of the i-th strip electrode 10a and 10b (the position in the arrangement direction of the strip electrodes) is denoted by xi, Assuming that the magnitude of the current signal obtained from the strip electrodes 10a and 10b is Si, the beam center position xc is obtained by the equation (1). However, i is an integer of 1 to n, and A is a calibration coefficient.

FIG. 8 shows the center of gravity calculation position obtained when the center of the beam is mechanically moved from the center of the signal electrode 3a according to the first embodiment. FIG. 8 shows the results of calculating the center of gravity when the number of strip electrodes is 10, 14, and 24 for all signal electrodes having the same width of the strip electrodes as Comparative Examples 1 to 3.
For the signal electrode 3a according to the first embodiment, the center of gravity was calculated using current signals from ten strip electrodes 10a having a width of 1 mm and two strip electrodes 10b having two widths on both sides. Further, the magnitudes of current signals from 10 strip electrodes having a width of 1 mm as Comparative Example 1, 14 strip electrodes having a width of 1 mm as Comparative Example 2, and 24 strip electrodes having a width of 1 mm as Comparative Example 3 are used. The center of gravity is calculated. The center of gravity is calculated with all correction coefficients A being 1.

  As can be seen from FIG. 8, in the comparative example 3, the center-of-gravity calculation position indicates a substantially correct beam center position moved mechanically, but in other cases, the center position of the beam center and the center of gravity moved mechanically at the respective ratios. There is a deviation from the calculation position. This deviation can be corrected by appropriately selecting the calibration coefficient A. The correction coefficient A is the largest in Comparative Example 1 with a small number of strip electrodes, but the correction coefficient A is smaller in the case of Embodiment 1 than in the case of Comparative Example 2 even with the same 14 strip electrodes. That is, it can be seen that a correction coefficient close to 1 is sufficient. When the correction coefficient is close to 1, the gravity center calculation error due to the correction coefficient error can be reduced, and more accurate position measurement can be performed.

  Next, a centroid calculation error of the beam center position xc obtained by the centroid calculation will be considered. The center-of-gravity calculation error of the beam center position xc is mainly caused by noise in the preamplifier 16 and the transmission system. This noise is constant for each strip electrode 10a, 10b regardless of the size of the strip electrode 10a, 10b. If the magnitude of the noise is ε with respect to the magnitude Si of the current signal obtained from each strip electrode 10a, 10b, the barycentric calculation error Δxc of the center position xc of the beam can be obtained by equation (2).

  An error in measurement of the center of gravity position related to the arrangement of the strip electrodes can be evaluated by Expression (3).

FIG. 9 is a diagram showing a change in the ratio between the gravity center calculation error and the noise due to the difference in the arrangement of the strip electrodes.
For the case where the beam size σ was changed in the range of 3 mm to 9 mm, the gravity center calculation error was evaluated based on this equation (3).
As can be seen from FIG. 9, in the case of Comparative Example 3, the centroid calculation error increases as the beam size σ decreases, and the centroid position calculation error generally increases as the beam size σ ranges from 3 mm to 9 mm. This is because more data at the beam end where the signal from the strip electrode is small is used in the calculation.
In Comparative Example 1, the centroid calculation error increases as the beam size σ increases. Further, in Comparative Example 2, the center of gravity calculation error is generally smaller than in Comparative Example 1 and Comparative Example 3.
On the other hand, in the first embodiment, the centroid calculation error is smaller than that in the comparative example 2, and the centroid calculation error is generally stable and small. The reason for this is that the width of the strip electrode in the end region B is increased and the current signal is increased.
When the beam size σ is in the range of 3 mm to 9 mm, a high S / N ratio can be obtained by adopting the strip electrode configuration as in the first embodiment.

  When the boundary position where the widths of the strip electrodes 10a and 10b change is set to 5 mm, it has been found that measurement with a high S / N ratio can be performed when the beam size σ is in the range of 3 mm to 9 mm. The boundary position where the width of the strip electrodes 10a and 10b changes when Ymm can be obtained. That is, W satisfying Expression (4) is obtained, and ± Wmm from the center of the signal electrode 3a is set as a boundary position where the width of the strip electrodes 10a and 10b is changed. Then, the width of the strip electrodes 10a and 10b is changed between the inside and outside of the boundary position ± W mm from the center of the signal electrode 3a. By setting the width of the strip electrode 10b in the end region B to twice the width of the strip electrode 10a in the central region A, the beam position monitor 1 having a high S / N ratio can be provided.

Further, the number of strip electrodes 10b in the end region B is sufficient if it is four, but it may be set so as to cover the range in which the beam can be moved in consideration of convenience in beam adjustment.
The approximate width of the strip electrode 10a in the central region A can be determined from the beam size σ, the required position sensitivity Δxc, and the noise level ε in the electrical system based on the equation (2).

  In addition, the signal electrode 3a according to the first embodiment has been described as being a flat plate type, but the same effect can be obtained when a wire type is used. In the wire type beam position monitor, the strip electrode is composed of a metal wire having a diameter of 10 μm to several hundreds of μm, and by arranging the metal wire at the center position of the strip electrode, it is possible to make the configuration almost the same as the flat plate type. it can. That is, the width of the strip electrode can be replaced with the interval between the metal wires. In this case, the insulating film 12 is not necessary, and the amount of material through which the beam passes can be reduced.

  Such a beam position monitor 1 includes a plurality of strip-shaped strip electrodes 10a arranged at positions corresponding to positions where gas ions or electrons ionized by passing charged particles or X-ray beams pass, and positions where the beams have passed. The current flows through the current collected by 10b, and the center position of the beam is calculated from the position information of the strip electrodes 10a and 10b and the magnitude of the current flowing through the strip electrodes 10a and 10b. The width of the strip electrodes 10a and 10b is the central portion. Since the width of the strip electrode 10b is wide in both end regions where the distribution of gas ions or electrons is small, a larger current flows in the both end regions. The S / N ratio is improved. Further, the accuracy of beam position estimation is improved in the centroid calculation.

  The width of the strip electrode closest to the boundary outside the boundary is twice the width of the strip electrode closest to the center of the signal electrode, with the position separated from the center of the signal electrode by the half width or dispersion of the beam spread distribution as the boundary. Therefore, by adjusting the gain of the preamplifier to the magnitude of the current signal from the strip electrode closest to the center of the signal electrode, the center position of the beam is not saturated without saturating the current signal from the strip electrode closest to the outer boundary. It can be used for calculation.

Embodiment 2. FIG.
FIG. 10 is a diagram showing a state of connection between the signal electrode 3B and the current signal processing system according to Embodiment 2 of the present invention.
The beam position monitor according to the second embodiment of the present invention is different from the beam position monitor 1 according to the first embodiment in the signal electrode 3B and the current signal processing system. The same reference numerals are added and description thereof is omitted.
In the signal electrode 3B according to the second embodiment, all the strip electrodes 10 have the same width. The strip electrode 10 formed in the central region A of the signal electrode 3B is connected to separate preamplifiers 16. On the other hand, the strip electrodes 10 formed in the end region B are grouped so as to form a pair with adjacent strip electrodes 10, and the pair of strip electrodes 10 are connected to the same preamplifier 16.
If the connection can be switched by a dip switch (not shown) or the like, the boundary position for changing the effective strip electrode width can be changed in accordance with the use form and the beam condition.

  In such a beam position monitor, even when the widths of the plurality of strip electrodes 10 of the signal electrode 3B are all equal, the strip electrodes 10 in the end region are combined together and connected to the same preamplifier 16, so that In both end regions where the distribution of ions or electrons is small, current signals from the two strip electrodes 10 are combined to improve the S / N ratio.

Embodiment 3 FIG.
FIG. 11 is a plan view of a signal electrode 3C on which strip electrodes 10a to 10f according to Embodiment 3 of the present invention are formed. FIG. 12 is a simulation result of the magnitude of the current signal when the beam is incident so that the center of the beam is positioned at the center of the signal electrode 3C according to the third embodiment.
The beam position monitor according to the third embodiment of the present invention is different from the beam position monitor 1 according to the first embodiment in that the signal electrode 3C is the same as that of the first embodiment. Description is omitted.
Since the signal electrode 3C according to the third embodiment is different from the signal electrode 3a according to the first embodiment and the strip electrodes 10c to 10f in the central region A as shown in FIG. Similar parts are denoted by the same reference numerals, and description thereof is omitted.

Of the strip electrodes 10a, 10c to 10f in the central region A of the signal electrode 3C according to the third embodiment, the strip electrode 10a has the same width as the strip electrode 10a of the signal electrode 3a according to the first embodiment. The strip electrodes 10c to 10f are different from the strip electrode 10a according to the first embodiment.
The width of the strip electrode 10f is determined so that the magnitude of the current signal from the strip electrode 10f is larger than the magnitude of the current signal from the adjacent strip electrode 1b. Similarly, the width of the strip electrode 10e is determined so that the magnitude of the current signal from the strip electrode 10e is larger than the magnitude of the current signal from the adjacent strip electrode 10f. Furthermore, the width of the strip electrode 10d is determined so that the magnitude of the current signal from the strip electrode 10d is larger than the magnitude of the current signal from the adjacent strip electrode 10e. Further, the width of the strip electrode 10c is such that the magnitude of the current signal from the strip electrode 10c is larger than the magnitude of the current signal from the adjacent strip electrode 10d and smaller than the magnitude of the current signal from the strip electrode 10a. Can be decided.
When the beam is incident so that the center of the beam is positioned at the center of the signal electrode 3C on which the strip electrodes 10a to 10f are determined in this way, as shown in FIG. The magnitude of the current signal decreases monotonously from the beam center toward the outside.

FIG. 13 shows the result of calculation of the center of gravity obtained when the center of the beam is mechanically moved from the center of the signal electrode 3C according to the third embodiment. In the third embodiment, ten strip electrodes 10a, 10c to 10f in the central region and two strip electrodes 10b in the end region are used in the gravity center calculation.
In the centroid position calculation using the signal electrode 3C of the third embodiment, the sensitivity of the centroid calculation value with respect to the beam position is lower than when the signal electrode 3a of the first embodiment is used. As compared with Comparative Example 1 in which the widths of the two are equal, the sensitivity is increased.
Further, as can be seen by comparing FIG. 6 and FIG. 11, the average value of the magnitude of the current signal from the 12 strip electrodes including the strip electrode in the central region is the same as that of the third embodiment. The S / N ratio in the calculation of the center of gravity is improved by about 10%.

It is a schematic sectional drawing of the parallel plate type beam position monitor concerning Embodiment 1 of this invention. FIG. 3 is a diagram illustrating a state of arrangement of high-voltage electrodes and signal electrodes according to the first embodiment. It is a top view of the surface in which the strip electrode of a signal electrode is formed. FIG. 3 is a diagram illustrating a state of a signal processing system connected to a strip electrode according to the first embodiment. It is the result of having simulated the magnitude | size of the current signal from the strip electrode where all the width | variety of the conventional signal electrode is equal. It is a simulation result of the magnitude | size of an electric current signal when a beam injects so that the center of a beam may be located in the center of the signal electrode concerning Embodiment 1. FIG. It is a simulation result regarding the magnitude | size of an electric current signal when the center of a beam moves to the X-axis direction only 0.5 mm of the width | variety 1mm of a strip electrode with respect to the center of the signal electrode concerning Embodiment 1. FIG. It is a gravity center calculation result calculated | required when the center of the beam moved mechanically from the center of the signal electrode concerning Embodiment 1. FIG. It is a figure which shows the change of the measurement error of the gravity center position by the difference in arrangement | positioning of a strip electrode. It is a figure which shows the mode of the connection of the signal electrode concerning Embodiment 2 of this invention, and a current signal processing system. It is a top view of the signal electrode in which the strip electrode concerning Embodiment 3 of this invention was formed. It is a simulation result of the magnitude | size of an electric current signal when a beam injects so that the center of a beam may be located in the center of the signal electrode concerning Embodiment 3. FIG. It is a gravity center position calculation result calculated | required when the center of the beam moved from the center of the signal electrode concerning Embodiment 3 mechanically.

Explanation of symbols

  1 Beam position monitor, 2 High voltage electrode, 3 Signal electrode, 5 Insulating support member, 6 Spacer, 7 Frame, 8 Window, 10 Strip electrode, 12 Insulating film, 13 Signal line, 14 Connector, 16 Preamplifier, 17 A / D converter , 18 Arithmetic circuit.

Claims (3)

The magnitude of current flowing through a plurality of strip-shaped strip electrodes in which gas ions or electrons ionized when a charged particle or X-ray beam passes and arranged in parallel in a uniaxial direction in a plane perpendicular to the beam passing direction In a beam position monitor that measures the center position of the beam in the uniaxial direction from the position information of the strip electrode,
The plurality of strip electrodes include at least four strip electrodes disposed at both ends in the uniaxial direction and having a width twice as large as the width of the strip electrode disposed in the central portion in the uniaxial direction. A beam position monitor characterized by   The width of the strip electrode arranged outside the boundary that is separated from the center in the uniaxial direction by the half width or dispersion of the beam spread distribution is twice the width of the strip electrode arranged inside the boundary. The beam position monitor according to claim 1, wherein there is a beam position monitor. The width of the strip electrode arranged outside the boundary that is separated from the center in the uniaxial direction by the half width or dispersion of the spread distribution of the beam is 2 of the width of the strip electrode arranged at the position closest to the center. Is double
2. The beam position monitor according to claim 1, wherein the width of the strip electrode arranged on the inner side of the boundary is set so that the magnitude of the flowing current increases as the distance from the center is closer.

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