JP2010156671A - Ionization chamber sensor and dose distribution measurement system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ionization chamber sensor and a dose distribution measurement system, concurrently achieving high resolution and high precision in dose distribution measurement. <P>SOLUTION: An array chamber 1 is an ionization chamber sensor equipped with a plurality of sensing cells and is provided with: a common electrode 12 shared by the respective sensing cells; a plurality of divided electrodes 101a, 101b, ... 101z which face the common electrode 12 and are arranged for each sensing cell; a plurality of signal lines 103a, 103b, ... 103z connected to the respective divided electrodes; and a plurality of dummy signal lines 104a, 104b, ... 104z arranged in the vicinity of each of the signal lines. A calculation/display device 28 subtracts an integrated value of the dummy signal line from an integrated value of the signal line for each of the sensing cells and executes noise cancellation. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an ionization chamber detector capable of measuring a dose distribution of a radiation beam used for, for example, cancer radiotherapy, and a dose distribution measuring apparatus using the same.

  In radiation therapy for cancer, the dose distribution in a water phantom that simulates the human body before irradiating the patient with the beam in order to confirm the energy and shape of the radiation beam such as X-rays, electron beams, and particle beams used for treatment. Need to be measured. In order to adjust the radiation irradiation apparatus such as an accelerator and to confirm the beam energy distribution and shape which are different for each patient, it is necessary to measure the dose distribution on a daily basis as quality control of the radiation beam.

  As described in Patent Document 1, conventional absorbed dose distribution measurement is performed by using a water tank simulating a human body and an ionization chamber equipped with a driving device so that the position in water can be changed. By scanning the box, the dose distribution in water due to the irradiation of radiation is measured. Therefore, a great deal of time and effort are required even for a single dose distribution measurement. In addition, since confirmation by dose distribution measurement is required every time the beam condition is changed, there is a limit in improving the number of patients that can be treated per irradiation apparatus, that is, the operating rate of the treatment apparatus.

  In order to solve such problems, various types of radiation detectors and dose distribution measuring apparatuses have been proposed as apparatuses capable of measuring a dose distribution in a short time. For example, in addition to a detector in which semiconductor detectors are arranged in an array, a method for measuring the degree of blackening of a photographic film, etc., as in Patent Document 2, a high resolution of 1 mm or less is realized using a pixel-type electrode. Examples of radiation detectors have been proposed. Patent Document 3 proposes a structure of a flat electrode-intermediate layer (with ionization chamber air holes) -divided electrode in order to measure a beam profile. In addition to the lateral distribution, an energy correction plate having an energy attenuating portion is arranged on the upstream side in order to artificially measure the depth direction distribution.

Japanese Patent No. 3203211 JP 2008-180647 A Special table 2009-502263

  In dose distribution measurement for quality control of radiation beams for cancer treatment, it is necessary to obtain an output equivalent to human tissue. Semiconductor detectors and photographic films are not preferred because the elemental composition differs greatly from the composition of the human body. Therefore, as a quality control of the therapeutic irradiation apparatus, since the method of measuring the absorbed dose in the water phantom with an ionization chamber can obtain an output equivalent to human tissue, High reliability.

  In order to improve the operating rate of the treatment apparatus, it is necessary to perform dose distribution measurement using an ionization chamber in a short time. Measurement time can be increased by constructing a multi-cell detector in which ionization chambers are arranged in an array and performing simultaneous measurement. Further, high resolution can be realized by forming a fine electrode structure using a printed circuit board production technique as described in Patent Document 2.

  However, when trying to achieve both high speed and high resolution, the volume per ion chamber is inevitably reduced, and sufficient signal output cannot be obtained. That is, the signal-to-noise ratio cannot be increased, and it is difficult to ensure accuracy.

  An object of the present invention is to provide an ionization chamber detector capable of achieving both high resolution and high accuracy in dose distribution measurement, and a dose distribution measurement apparatus using the same.

  In order to achieve the above object, the present invention provides an ionization chamber detector having a plurality of detection cells, a common electrode shared by each detection cell, and each detection cell facing the common electrode. A plurality of divided electrodes, a plurality of signal lines connected to each divided electrode, and a plurality of dummy signal lines respectively disposed in the vicinity of each signal line.

  A dose distribution measuring apparatus according to the present invention includes the ionization chamber detector, a high-voltage power supply that supplies a predetermined voltage to the common electrode, a first charge integration circuit that integrates a signal output from the signal line, A second charge integration circuit that integrates a signal output from the dummy signal line, and a signal processing circuit that subtracts the output of the second charge integration circuit from the output of the first charge integration circuit.

  According to the present invention, by arranging a plurality of dummy signal lines in the vicinity of a plurality of signal lines connected to each divided electrode, among ions ionized by radiation irradiation, those that reach the divided electrode are normal signals. However, if the signal reaches the signal line, it becomes noise and is superimposed on the normal signal. On the other hand, the dummy signal line also generates a noise component similar to that of the signal line. Therefore, the noise superimposed on the signal line can be canceled by subtracting the noise generated in the dummy signal line from the signal (regular signal + noise) output from the signal line. As a result, the spatial resolution of the dose distribution that can be detected by the divided electrodes is increased, and highly accurate dose distribution measurement can be achieved.

It is the schematic which shows dose distribution measurement using a water phantom. It is a block diagram which shows the electrical structure of the drive processing apparatus shown in FIG. It is the schematic which shows the whole structure of an array chamber. It is a top view which shows an example of the conductor pattern containing a division | segmentation electrode. It is an equivalent circuit which shows a division | segmentation electrode and its periphery. It is a top view which shows the other example of the conductor pattern in a division | segmentation electrode substrate. FIG. 7 is a cross-sectional view taken along line A-A ′ in FIG. 6. It is a perspective view which shows only the conductive pattern part of a division | segmentation electrode substrate and a common electrode. It is a top view which shows the electroconductive pattern part of a common electrode part. FIG. 10 is a cross-sectional view of the ionization chamber detector along B-B ′ in FIG. 9. It is a figure which shows the wiring structure of an ionization chamber detector.

Embodiment 1 FIG.
FIG. 1 is a schematic diagram showing dose distribution measurement using a water phantom. Water 4 is stored in the water tank 3 to form a “water phantom” that is equivalent to a human body tissue. Before irradiating the patient, for example, a therapeutic radiation beam 5 such as an X-ray, an electron beam, or a particle beam is irradiated toward the water phantom, and the array chamber 1 positioned in the water is used. By measuring the absorbed dose and the dose distribution, it is possible to confirm whether the energy and beam profile of the radiation beam 5 are appropriate.

  The array chamber 1 has a multi-cell structure in which a large number of ion chamber detectors are arranged in an array. The drive processing device 2 performs voltage supply to the array chamber 1, signal processing from the array chamber 1, display of measured values, and the like.

  FIG. 2 is a block diagram showing an electrical configuration of the drive processing apparatus 2 shown in FIG. The array chamber 1 has a large number of ion chamber cells, and each cell is provided with a common electrode and a divided electrode facing each other. The common electrode is commonly connected to the high-voltage power supply 22, and each divided electrode is connected to an individual charge integrating circuit 23a, 23b,... Via a plurality of signal lines. For example, when 256 divided electrodes are provided, 256 charge integration circuits 23a, 23b,... Are used. By supplying a DC high voltage from the high-voltage power supply 22 to the common electrode, an electric field is formed between the common electrode and the divided electrode inside the cell.

  Regarding the operation of the ionization chamber, when radiation is incident on the inside of the cell, a part of gas (for example, air) existing in the cell is ionized by the ionization action of the radiation. At this time, when an electric field is applied between the electrodes, positive ions are attracted to the divided electrodes, while negative ions are attracted to the common electrode. When both ions move inside the cell, they are output as current. Therefore, the absorbed dose of radiation incident on each cell can be measured by integrating the current flowing within a predetermined time and converting it into a charge amount.

  In this embodiment, as will be described later, dummy signal lines are arranged in the vicinity of individual signal lines, and each dummy signal line is connected to an individual charge integration circuit 24a, 24b,. For example, when 256 dummy signal lines are provided, 256 charge integration circuits 24a, 24b... 24z are used.

  The outputs from the charge integration circuits 23a, 23b... And the outputs from the charge integration circuits 24a, 24b... 24z are selected in a time division manner by the multiplexer 25 and converted into digital signals by the analog / digital converter 26, respectively. The data is stored in a memory installed at 28. The controller 27 controls operations of individual circuits such as the multiplexer 25, the analog-digital converter 26, the arithmetic / display device 28, and the entire apparatus.

  The arithmetic / display device 28 has a function of executing signal processing, measurement value display, and the like. In particular, in this embodiment, the integration value of the dummy signal line is changed from the integration value of the signal line for each detection cell of the array chamber 1. Has a noise cancellation function.

  FIG. 3 is a schematic diagram showing the overall configuration of the array chamber 1. FIG. 4 is a plan view showing an example of a conductor pattern including divided electrodes. FIG. 5 is an equivalent circuit showing the divided electrode and its periphery.

  The array chamber 1 is a parallel plate ionization chamber in which a divided electrode substrate 11 and a common electrode 12 face each other at a predetermined interval. On the divided electrode substrate 11, as shown in FIG. 4, a plurality of divided electrodes 101a, 101b,... 101z corresponding to a plurality of detection cells are linearly arranged. For example, when 256 detection cells are provided, 256 divided electrodes 101a, 101b... 101z are used. Note that the number, size, and pitch of the divided electrodes are determined from the required beam size and the resolution required for measurement, and are partially omitted in FIG. The common electrode 12 is composed of a single electrode plate shared by each detection cell.

  A plurality of signal lines 103a, 103b,... 103z connected to the respective divided electrodes 101a, 101b,... 101z are arranged on the divided electrode substrate 11 and the signal extraction unit 13, respectively. A plurality of dummy signal lines 104a, 104b... 104z are respectively arranged. These signal lines and dummy signal lines are connected to the drive processing device 2 via the signal cable 14. A guard ring 102 surrounding the entire array of divided electrodes is disposed in the vicinity of each divided electrode 101a, 101b,... 101z.

  When a plurality of divided electrodes 101a, 101b ... 101z, a plurality of signal lines 103a, 103b ... 103z, a plurality of dummy signal lines 104a, 104b ... 104z, and a guard ring 102 are arranged on one side of the divided electrode substrate 11, electrical insulation is provided. By using the printed board 105 in which a metal foil is bonded to the upper surface of the board, patterning by a printing technique is possible. As a result, it is possible to easily manufacture an arrayed divided electrode having a highly accurate position, size, and area. Furthermore, even if one divided electrode is made smaller in order to increase the spatial resolution of measurement, the dimensional variation can be suppressed small, so that both high resolution and reduced sensitivity variation can be achieved. In addition, since the relative positions of the electrodes can be arranged with high accuracy, measurement with little measurement position error is possible.

  On the other hand, when one divided electrode is made small in order to increase the spatial resolution of measurement, the sensitive part volume of each detection cell becomes small. Then, the amount of generated charges due to the ionizing action of radiation is reduced, the signal-to-noise ratio is lowered, and the resistance to noise is lowered. In the present embodiment, signal errors are reduced and noise tolerance is improved by using the method described below.

  The main cause of noise is charge generation caused by exposure of the portions other than the divided electrodes 101a, 101b,... 101z, that is, the signal lines 103a, 103b,. It becomes an error factor. One means for solving this is to shield the signal line and the signal cable from radiation, but the problem is that the volume of the array chamber 1 increases due to the installation of the shielding material.

  In the present embodiment, as shown in FIG. 4, dummy signal lines 104a, 104b,... 104z are arranged on the printed circuit board 105 in the vicinity of the signal lines 103a, 103b,. With such a configuration, out of the ions ionized by radiation irradiation, those reaching the divided electrodes 101a, 101b,... 101z are detected as normal signals, but those reaching the signal lines 103a, 103b,. On the other hand, a noise component similar to that of the signal line is generated in the dummy signal lines 104a, 104b,. Therefore, the noise superimposed on the signal line can be canceled by subtracting the noise generated in the dummy signal line from the signal (regular signal + noise) output from the signal line. As a result, the spatial resolution of the dose distribution that can be detected by the divided electrodes is increased, and highly accurate dose distribution measurement can be achieved.

  In addition, by using printed circuit board patterning technology, signal lines and dummy signal lines can be formed very close to each other, so that most of noise and errors due to radiation exposure of signal lines can be removed. Accurate measurement is possible. In addition, the addition of the dummy signal line for the noise canceling function can be realized by a fine wiring structure using a printed circuit board, so that the entire apparatus can be reduced in size.

  Furthermore, it is preferable to arrange the guard ring 102 in the vicinity of each of the divided electrodes 101a, 101b. As shown in FIG. 5, the guard ring 102 is maintained at the same potential (eg, ground potential) as the divided electrodes 101a, 101b,. Therefore, among the leakage current straying in the printed circuit board 105, the current flowing into the divided electrodes 101a, 101b,... 101z is blocked by the guard ring 102, so that noise caused by the leakage current can be suppressed to a low level.

  Next, countermeasures for sensitivity variations during radiation irradiation will be described. The arithmetic / display device 28 stores in advance sensitivity calibration constants corresponding to fluctuations in the signal output from each signal line and fluctuations in the signal output from each dummy signal line when receiving radiation of the same intensity. Keep it. When the actual dose distribution is confirmed, sensitivity calibration processing (for example, addition / subtraction or multiplication / division) is performed on the integral value of the signal line and the integral value of the dummy signal line using a sensitivity calibration constant stored in advance. As a result, an output with corrected sensitivity variations can be obtained, and finally high-precision measurement can be performed.

  Thus, according to this embodiment, the absorbed dose distribution of the beam used for cancer radiotherapy can be measured in a short time with high accuracy.

Embodiment 2. FIG.
FIG. 6 is a plan view showing another example of the conductor pattern in the divided electrode substrate 11. FIG. 7 is a cross-sectional view taken along the line AA ′ in FIG. Since the present embodiment is the same as the first embodiment except for the configuration of the divided electrode substrate 11, the redundant description is omitted.

  On the divided electrode substrate 11, a plurality of divided electrodes 101a, 101b... 101z corresponding to a plurality of detection cells are linearly arranged. A single common electrode is arranged at a predetermined interval with respect to these divided electrodes, thereby forming an array chamber having a plurality of detection cells.

  As shown in FIG. 7, the divided electrode substrate 11 and the signal extraction unit 13 are configured by a multilayer printed board 105 having a plurality of electrical insulating layers and a plurality of wiring layers. A plurality of signal lines 103a, 103b,... 103z are arranged on the first wiring layer of the multilayer printed board 105, and a plurality of dummy signal lines 104a, 104b,... 104z are arranged on the second wiring layer. Conductive patterns 106 are formed on the uppermost surface and the lowermost layer of the multilayer printed board 105 so as to sandwich a plurality of signal lines and a plurality of dummy signal lines.

  As shown in the plan view of FIG. 6, the plurality of divided electrodes 101 a, 101 b... 101 z are formed on the uppermost surface of the multilayer printed board 105. A guard ring 102 surrounding the entire array of divided electrodes is also formed on the uppermost surface of the multilayer printed board 105. Individual divided electrodes are connected to individual signal lines located in the first wiring layer via via holes. The dummy signal lines are arranged so as to overlap the signal lines in plan view.

  With such a configuration, since the signal line and the dummy signal line are very close to each other, the effect of removing the common mode of noise due to radiation exposure of the signal line is enhanced, and more accurate measurement is possible. Further, by arranging the dummy signal lines in the inner layer of the multilayer wiring board, the entire area of the divided electrode substrate 11 can be reduced, and the entire apparatus can be downsized.

  Further, by arranging the signal line in the inner layer of the multilayer wiring board, the guard ring 102 can be arranged so as to completely surround the entire array of the divided electrodes. That is, it is possible to dispose the guard ring portion between the divided electrodes so as to surround each divided electrode. Therefore, the effect of interrupting the leakage current in the printed circuit board 105 is further improved, and the leakage current mixed in the signal line connected to each divided electrode can be suppressed to a very low level.

  Further, the conductive pattern 106 is arranged on the uppermost surface and the lowermost layer of the multilayer printed circuit board 105 so as to sandwich the plurality of signal lines and the plurality of dummy signal lines, and grounded to the ground potential, so that the conductive pattern 106 serves as a shielding material. Function. As a result, it is possible to shield electromagnetic waves arriving at the signal line and the dummy signal line and improve noise resistance.

  Thus, according to this embodiment, the absorbed dose distribution of the beam used for cancer radiotherapy can be measured in a short time with high accuracy.

Embodiment 3 FIG.
FIG. 8 is a perspective view showing only the conductive patterns of the divided electrode substrate and the common electrode according to the third embodiment. Since the third embodiment is the same as the first embodiment except for the shape of the conductive pattern, a duplicate description is omitted.

  In the present embodiment, the outer edge shape of the conductive pattern 107 of the common electrode and the outer edge shape of the guard ring 102 on the divided electrode substrate are substantially coincident with each other in plan view, that is, the shape of the conductive pattern 107 of the common electrode and The dimensions are preferably substantially the same as the outer edge of the guard ring 102 on the divided electrode substrate, and the position in the in-plane direction is preferably the same as the outer edge of the guard ring 102. With the configuration in which the outer peripheral portion of the conductive pattern 107 and the outer peripheral portion of the guard ring 102 substantially coincide with each other in the in-plane direction, the strength and direction of the electric field 108 formed between the common electrode and the divided electrode is uniform and Since they are parallel, the sensitivity characteristics of the divided electrodes 101a, 101b,... 101z are uniform, and the sensitivity variation is reduced. As a result, the absorbed dose distribution can be measured with high accuracy.

Embodiment 4 FIG.
FIG. 9 is a plan view showing a conductive pattern of the common electrode portion 12 of the fourth embodiment. FIG. 10 is a cross-sectional view taken along the line BB ′ of FIG. Since the internal configuration of the divided electrode substrate according to the present embodiment is the same as that of the first embodiment, duplicate description is omitted.

  Generally, in the ionization chamber detector, since the leakage current is a main cause of noise, it is desirable that the leakage current does not enter the signal line. A high voltage is applied between the common electrode and the divided electrode, and is insulated by air between the electrodes and a support material for the common electrode and the divided electrode. Since air is a good insulator, the leakage current, which is the main cause of noise, is predominantly the one that flows through the common electrode to which a high voltage is applied and the support material of the divided electrodes. Therefore, although the leakage current is reduced by optimizing the material selection and surface treatment of the support material, the leakage current cannot be suppressed completely.

  Therefore, in this embodiment, a structure in which the second guard ring 109 is disposed in the vicinity of the conductive pattern 107 of the common electrode is employed. Accordingly, the guard ring 109 on the common electrode side, the divided electrodes 101a, 101b,... 101z on the divided electrode substrate 11 side, and the guard ring 102 near the divided electrodes have the same potential (for example, ground potential). As shown in FIG. 10, most of the leakage current 111 flows from the conductive pattern 107 of the common electrode to the guard ring 109, and the leakage current flowing through the support member 110 between the common electrode and the divided electrode is substantially equal. To zero. As a result, the leakage current mixed in the signal line on the divided electrode side can be made almost zero, and highly accurate measurement can be performed.

  As shown in FIG. 10, the ionization chamber detector including the common electrode 12 and the divided electrode substrate 11 is preferably stored in an airtight manner in the case 112 so that external stress is not applied. The material of the case 112 is preferably an acrylic resin having a composition and density close to water. With such a structure, even when the ionization chamber detector is installed in water and measurement is performed, the common electrode 12 and the divided electrode substrate 11 are not easily affected by water pressure. Therefore, the deformation of the interelectrode space can be suppressed, and the volume of the sensitive part is kept constant. Thereby, measurement accuracy can be made high.

  Furthermore, it is preferable that a conductive film 113 such as aluminum or copper is formed on the inner surface of the case 112 at a position facing the common electrode 12 by coating or the like. With such a configuration, a local parallel electric field is generated between the common electrode 12 and the divided electrode 11 and between the common electrode 12 and the conductive film 113 on the inner surface of the case, but the electric field in other external regions is It can be kept low. As a result, since the electric field spreading outside the sensitive region between the common electrode 12 and the divided electrode 11 can be suppressed as low as possible, the sensitive volume can be limited between the common electrode 12 and the divided electrode 11, Measurement accuracy can be increased.

  As shown in FIG. 10, openings 114a and 114b are provided in the case 112, which are sealed with an O-ring 116 and lids 115a and 115b, and can be opened and closed as necessary, for example, by attaching and detaching screws 117. It is preferable to provide a structure. Thereby, since the air in the ionization chamber can be quickly replaced, fluctuations in measured values due to atmospheric pressure and humidity can be suppressed, and the measurement sensitivity can be stabilized.

Embodiment 5 FIG.
FIG. 11 is a diagram showing a wiring structure of the ionization chamber detector according to the fifth embodiment. A signal line 118 for reading the divided electrodes 101a, 101b,... 101z and a dummy signal line 119 arranged in the vicinity of the signal line are on the opposite side of the common electrode 107 along the direction intersecting the main surface of the divided electrode substrate. It is taken out and three-dimensionally wired to a portion other than the plane of the divided electrode substrate 11. The divided electrode signal line 118 and the dummy signal line 119 are connected to an external circuit by a twisted pair line, and a conductive shield 123 is wound on the outside to surround the twisted pair line. The shield 123 is connected to the guard ring 102 by a guard ring lead wire 120 and further grounded (not shown). On the other hand, the common electrode lead wire 121 for supplying a high voltage to the common electrode 107 is disposed outside the shield 123. These twisted pair wires and shields are put in a storage tube 124 so as to be integrated, and are connected to the ionization chamber main body by a connector 122.

  With such a configuration, since the sensitive volume can be defined only by the area between the divided electrode surface and the common electrode, the measurement accuracy is improved. This configuration is effective for improving accuracy and reducing variation particularly when the area of the divided electrode is small. Further, since the divided electrode signal line 118 and the dummy signal line 119 are separated from the common electrode lead wire 121 to which a high voltage is applied by the shield 123, the divided electrode signal line 118 and the dummy signal line 119 are shared with the common electrode. Electric field formation between the lead wires 121 can be prevented. As a result, the leakage current can be prevented from being mixed into the signal line, so that the measurement accuracy can be improved.

1 array chamber, 2 drive processing unit, 3 water phantom (water tank),
4 water, 5 radiation beam, 11 split electrode substrate, 12 common electrode,
13 Signal extraction part, 14 Signal cable, 22 High voltage power supply,
23a, 24a, 23b, 24b, 24z charge integration circuit, 25 multiplexer,
26 analog-digital converter, 27 controller, 28 calculation / display device,
101a, 101b, 101c, 101z split electrode, 102 guard ring,
103a, 103b, 103c signal lines,
104a, 104b, 104c dummy signal lines,
105 printed circuit board, 106 conductive pattern,
107 conductive pattern, 108 electric field, 109 guard ring,
110 support material, 111 leakage current, 112 case, 113 conductive film,
114a, 114b opening, 115a, 115b lid, 116 O-ring,
117 screws, 118 divided electrode signal lines, 119 dummy signal lines,
120 lead wire for guard ring, 121 lead wire for common electrode,
122 connector, 123 shield, 124 storage tube.

Claims (15)

An ionization chamber detector having a plurality of detection cells,
A common electrode shared by each detection cell;
A plurality of divided electrodes arranged for each detection cell so as to face the common electrode,
A plurality of signal lines connected to each divided electrode;
An ionization chamber detector comprising a plurality of dummy signal lines arranged in the vicinity of each signal line.   The ionization chamber detector according to claim 1, further comprising a guard ring disposed in the vicinity of each divided electrode.   The ionization chamber detector according to claim 2, wherein the plurality of divided electrodes, the plurality of signal lines, the plurality of dummy signal lines, and the guard ring include an electrically insulating substrate disposed on the first main surface. A multilayer wiring board having a plurality of electrical insulation layers and a plurality of wiring layers;
A plurality of divided electrodes and guard rings are arranged on the first main surface of the multilayer wiring board,
A plurality of signal lines are arranged on the first wiring layer of the multilayer wiring board,
3. The ionization chamber detector according to claim 2, wherein a plurality of dummy signal lines are arranged on the second wiring layer of the multilayer wiring board.   The ionization chamber detection according to claim 4, wherein a shielding material is formed on the first main surface and the second main surface of the multilayer wiring board so as to sandwich the plurality of signal lines and the plurality of dummy signal lines. vessel.   The ionization chamber detector according to claim 2, wherein the guard ring extends between the divided electrodes so as to surround the individual divided electrodes.   6. The ionization chamber detector according to claim 2, wherein an outer edge shape of the common electrode and an outer edge shape of the guard ring substantially coincide with each other in a plan view.   The ionization chamber detector according to claim 2, further comprising a second guard ring arranged in the vicinity of the common electrode.   The ionization chamber detector according to any one of claims 1 to 5, wherein the ionization chamber detector is hermetically housed inside the case.   The ionization chamber detector according to claim 9, wherein a conductive film is formed at least at a position facing the common electrode on the inner surface of the case.   The ionization chamber detector according to claim 9 or 10, wherein the case is provided with at least one openable / closable opening. A plurality of divided electrodes and guard rings are provided, further comprising a divided electrode substrate facing the common electrode,
3. The ionization chamber detector according to claim 1, wherein the signal line and the dummy signal line are taken out on the opposite side of the common electrode along a direction intersecting with the main surface of the divided electrode substrate. The signal line and the dummy signal line are connected to an external circuit by a twisted pair wire surrounded by a shield line,
The shield wire is connected to the guard ring,
The ionization chamber detector according to any one of claims 1 to 12, wherein a lead wire connected to the common electrode is disposed outside the shield wire. An ionization chamber detector according to any one of claims 1 to 13,
A high-voltage power supply for supplying a predetermined voltage to the common electrode;
A first charge integration circuit for integrating a signal output from the signal line;
A second charge integration circuit for integrating a signal output from the dummy signal line;
A dose distribution measuring apparatus comprising: a signal processing circuit that subtracts the output of the second charge integration circuit from the output of the first charge integration circuit. The signal processing circuit previously stores sensitivity calibration constants corresponding to fluctuations in signals output from the signal lines and fluctuations in signals output from the dummy signal lines when receiving radiation of the same intensity. And
15. The dose distribution measuring apparatus according to claim 14, wherein a subtraction process is performed after the sensitivity calibration process is performed on the outputs of the first and second charge integration circuits.