WO2020054511A1 - Ionization chamber and method for producing same - Google Patents

Abstract

This ionization chamber is provided with: a cylindrical external electrode (101); a rod-like inner electrode (102) which is internally contained in the external electrode (101) so as to extend in an axial direction L from one end (101a) side of the external electrode to the other end (101b) side; a cylindrical guard electrode (103) which is provided between the external electrode (101) and the inner electrode (102) on the one end (101a) side so as to surround the side wall of the inner electrode (102); an insulation part (104) which is provided between the inner electrode (102) and the guard electrode (103) on the one end (101a) side; and an electron shielding part (105) which is formed of an electrically insulating material and is provided in a region A that spreads beyond the front end (103a) of the guard electrode (103) toward the other end (101b) side.

Description

Ionization chamber and its manufacturing method

The present invention relates to an ionization chamber and a method for manufacturing the same.
Priority is claimed on Japanese Patent Application No. 2018-171919, filed Sep. 13, 2018, the content of which is incorporated herein by reference.

Radiation-based cancer treatments make use of the fact that cancer cells have lower resistance to radiation than normal cells. Since the difference in resistance is generally not so large, in order to perform radiation therapy without damaging normal cells, the dose (dose) of radiation must be determined accurately. The dose used for cancer treatment is the absorbed water dose [Gy = J / kg], which is defined as the amount of energy radiation gives to a unit mass of water. The National Institute of Advanced Industrial Science and Technology (AIST) has established a primary standard for water absorption dose, and has calibrated a pneumatic ionization chamber (for example, see Non-Patent Document 1) brought in from a user (mainly a hospital). The constant N [Gy / C] is given. The user uses the charge output Q [C] of the ionization chamber when irradiating the ionization chamber to determine the water absorption dose as NQ [Gy]. This makes it possible to accurately determine the dose at a specific point (reference point) in the water.

(4) Since the patient to be treated has a finite size and the position of the organ changes with respiration, etc., the radiation dose differs for each position and time of the patient (dose distribution). For this reason, it is clear that determining the dose at the reference point is not enough. In radiation therapy, dose calculation using the Monte Carlo method or the like predicts the dose distribution in the patient's body and determines the relationship with the dose at the reference point, so that treatment can be performed regardless of the size of the patient. ing. On the other hand, with respect to the change in the position of the organ of the patient during treatment, measures such as fixing the patient as much as possible are taken, but this is not a drastic measure.

In recent years, radiation therapy devices that have the potential to improve the accuracy of dose distribution determination have been developed. This radiotherapy apparatus includes a magnetic resonance image (MRI) imaging apparatus, and is configured to be able to cope with a change in the position of an organ of a patient by irradiating radiation while imaging an MRI in real time.

Standard Imaging Exradin Chamber Product Catalog URL: https://www.standardimaging.com/uploads/files/Exradin_BR_1180-27.pdf

(4) In order to acquire an MRI image, a strong DC magnetic field of the order of Tesla is constantly applied to the above radiation therapy apparatus. Electrons entering the ionization chamber receive Lorentz force due to the strong magnetic field, and their trajectories are greatly bent. For this reason, it is known that the output of the ionization chamber changes depending on the angle between the direction of the magnetic field and the axial direction of the ionization chamber on a plane parallel to the direction. This phenomenon is a problem in determining the radiation dose from the output (sensitivity) of the ionization chamber in a magnetic field.

The present invention has been made in view of the above circumstances, and provides an ionization chamber capable of reducing a change in sensitivity of the ionization chamber due to an axial direction of the ionization chamber with respect to a magnetic field direction even in a magnetic field, and a method for manufacturing the same. With the goal.

た め In order to solve the above problems, the present invention employs the following solutions.

(1) An ionization chamber according to one aspect of the present invention includes a cylindrical external electrode, a rod-shaped internal electrode included in the external electrode, and extending in the axial direction from one end to the other end of the external electrode. A cylindrical guard electrode provided between the external electrode and the internal electrode on one end side and surrounding a side wall of the internal electrode, and between the internal electrode and the guard electrode on the one end side An insulating portion provided, and an electron shielding portion made of an electrically insulating material and provided in a region A extending to the other end side from the tip of the guard electrode.

According to the above aspect, the electron shielding portion made of an electrically insulating material is provided in the region A extending from the tip of the guard electrode to the other end. By shielding electrons coming and going between the region A and the region extending to the other end of the region A, when the axial direction of the ionization chamber changes with respect to the magnetic field applied to the ionization chamber, the electrons coming and going are changed. Reduce the effects of As a result, it is possible to suppress a variation in the number of ions to which the electrons adhere to reach the internal electrodes. Therefore, in the ionization chamber of the above aspect, even in a magnetic field, the sensitivity change of the ionization chamber due to the axial direction of the ionization chamber in a plane parallel to the magnetic field direction with respect to the magnetic field direction, that is, the anisotropy of the directional characteristics of the ionization chamber sensitivity Can be reduced.

(2) In the ionization chamber according to (1), assuming that the region A is filled with air, a motion trajectory of ions generated by a calculation according to the Statistical {Diffusion} Simulation (SDS) method is obtained. It is preferable that this is a region where ions reaching the guard electrode are generated.

(3) In the ionization chamber according to (2), it is preferable that the electron shielding portion is provided in a portion of the region A that is in contact with a region B that extends to the other end side from the region A.

(4) In the ionization chamber according to any one of (1) to (3), it is preferable that the electron shielding portion is provided on the entire area A.

(5) In the ionization chamber according to any one of (1) to (4), it is preferable that the electrically insulating material has a higher density than air.

(6) In the ionization chamber according to (5), it is preferable that the electric insulating material is made of at least one of a resin material and an inorganic material.

(7) In the ionization chamber according to any one of (1) to (6), it is preferable that the electron shielding portion is formed so as to cover at least a tip of the guard electrode.

(8) The method for manufacturing an ionization chamber according to one aspect of the present invention includes a rod-shaped external electrode and a rod-shaped electrode included in the external electrode and extending axially from one end to the other end of the external electrode. An internal electrode, on one end side, provided between the external electrode and the internal electrode, a cylindrical guard electrode surrounding a side wall of the internal electrode, and on one end side, the internal electrode and the guard electrode. A method of manufacturing an ionization chamber having an insulating portion provided between the ionization chamber and the ionization chamber, wherein it is assumed that the inside of the ionization chamber is filled with air, and a calculation is performed by the SDS method to obtain a motion trajectory of the generated ions. Estimating a region where ions reaching the guard electrode are generated, and forming an electron shielding portion made of an electrically insulating material in the estimated region.

According to the above aspect, assuming that the inside of the ionization chamber is filled with air, the movement trajectory of the generated ions is calculated by performing the calculation by the SDS method, and an electric field is generated in a region where the ions reaching the guard electrode are generated. An electron shield made of an insulating material is provided. It was applied to the ionization chamber by shielding electrons coming and going between a region sandwiched between the guard electrode and the external electrode by the electron shielding portion, and a region extending to the other end side from the tip of the guard electrode. When the axial direction of the ionization chamber changes with respect to the magnetic field, the effect of the coming and going electrons can be reduced, and the variation in the number of ions with attached electrons reaching the internal electrode can be suppressed. Therefore, in the manufacturing method of the above aspect, even in a magnetic field, the sensitivity change of the ionization chamber due to the axial direction of the ionization chamber in a plane parallel to the magnetic field direction with respect to the magnetic field direction, that is, the anisotropy of the directional characteristics of the ionization chamber sensitivity It is possible to manufacture an ionization chamber with a reduced amount.

(A)-(c) It is principal part sectional drawing of the ionization chamber which concerns on one Embodiment of this invention. It is a figure which shows the result of having simulated about the potential distribution (a) formed when an ionization chamber is driven, and the movement (b) of an ion. (A), (b) It is principal part sectional drawing of the ionization chamber which becomes a modification. FIG. 4 is a diagram illustrating a result of a simulation of the configuration of the ionization chamber and the distribution of sensitive and insensitive regions according to the first embodiment. It is a figure showing the result of having simulated about the composition of the ionization chamber of Example 2, and distribution of a sensitive area and an insensitive area. FIG. 14 is a diagram illustrating a simulation result of the configuration of the ionization chamber and the distribution of the sensitive region and the insensitive region according to the third embodiment. It is a figure showing the result of having simulated about the composition of the ionization chamber of Example 4, and the distribution of the sensitive area and the insensitive area. It is a figure showing the result of having simulated about the composition of the ionization chamber of Example 5, and distribution of the sensitive area and the insensitive area. (A) It is the graph which compared the measured result and the simulated result about the output in the sensitive area of the ionization chamber of comparative example 1. (B) A graph showing a simulation result of an output in a dead area of the ionization chamber of Comparative Example 1. (C) A graph obtained by synthesizing simulation results with respect to the output in the sensitive region and the output in the non-sensitive region of the ionization chamber of Comparative Example 1. It is the graph which compared the output of the ionization chamber of Example 6 of this invention, and Comparative Example 2. (A) to (d) are graphs comparing the outputs of Examples 7 to 10 and Comparative Examples 3 to 6 for the ionization chambers of Examples 2 to 5.

Hereinafter, an ionization chamber according to an embodiment to which the present invention is applied and a method for manufacturing the same will be described in detail with reference to the drawings. In addition, in the drawings used in the following description, in order to make the characteristics easy to understand, the characteristic portions may be enlarged for convenience, and the dimensional ratios and the like of the respective components are not necessarily the same as the actual ones. Absent. In addition, the materials, dimensions, and the like illustrated in the following description are merely examples, and the present invention is not limited thereto, and can be implemented with appropriate changes without departing from the scope of the invention.

FIGS. 1A and 1B are cross-sectional views schematically showing a main part of an ionization chamber 100 according to an embodiment of the present invention. The ionization chamber 100 includes an external electrode 101 extending in the axial direction, an internal electrode (center electrode) 102 extending in the same axial direction as the external electrode, a guard electrode 103, an insulating section 104, and an electron shielding section 105. And FIG. 1A is a cross-sectional view along the axial direction of the external electrode 101 and the internal electrode 102, and FIG. 1B is a cross-sectional view along a direction perpendicular to the axial direction. In FIG. 1A, a magnetic field B is applied from the back of the paper to the front. The solid arrow indicates the direction of incidence of electrons on the ionization chamber, and the broken arrow indicates the direction in which electrons subjected to Lorentz force travel.

The external electrode 101 is a cylindrical electrode made of a conductive member, for example, graphite, metal, or the like, and is open at one end 101a side and closed at the other end 101b side, and forms an outer frame of the ionization chamber 100. The space surrounded by the external electrode 101 is filled with air at room temperature and atmospheric pressure.

The internal electrode 102 is made of a conductive member, for example, metal or the like, is included in the external electrode 101, and extends from one end 101a side of the external electrode to the other end 101b side in a rod-shaped electrode extending along the axial direction L of the external electrode. It is. The position of the central axis of the internal electrode 102 overlaps the position of the central axis of the external electrode.

The guard electrode 103 is made of a metal member, is provided between the external electrode 101 and the internal electrode 102 on one end 101a side, and is a cylindrical electrode surrounding the side wall of the internal electrode 102. The position of the central axis of the guard electrode 103 also overlaps with the position of the central axis of the external electrode.

The insulating portion 104 is made of an electrically insulating material, and is provided between the external electrode 101 and the guard electrode 103 and between the internal electrode 102 and the guard electrode 103 on one end 101a side. The guard electrode 103 has a function of preventing the generated current from flowing into the internal electrode 102 when the radiation directly enters the insulating portion 104.

When operating the ionization chamber 100, the external electrode 101 is electrically grounded, and a high voltage (several hundred volts) is applied between the external electrode 101 and the internal electrode 102 and between the external electrode 101 and the guard electrode 103, respectively. ) Is connected to the power supply (not shown). As a result, an electric field is generated from the external electrode 101 toward each of the internal electrode 102 and the guard electrode 103. Note that the high voltage and the electrode to be grounded may be reversed.

電子 In this state, the electrons incident on the ionization chamber 100 react with air molecules to generate many ion pairs. Of the generated ion pairs, positive ions are attracted to the external electrode 101 and negative ions are attracted to the internal electrode 102.

数 The number of ion pairs generated in the air is proportional to the energy that electrons entering the ionization chamber give to the air. Therefore, by connecting a counting device between the internal electrode 102 and the power supply and measuring the number of anions reaching the internal electrode 102 from the output result, the number of incident electrons and, consequently, the radiation dose can be estimated. .

As described above, when a voltage is applied between the external electrode 101 and the internal electrode 102 and between the external electrode 101 and the guard electrode 103, the space in the ionization chamber 100 is moved from the external electrode 101 to the internal electrode 102 due to the potential distribution. a region R ₁ of the electric field towards occurs, is divided into a region R ₂ an electric field is generated extending from the external electrode to the guard electrode. In this specification, a region _R 1, _{R 2,} respectively sensible regions _{R 1,} is defined as a dead region _{R 2.} In the ionization chamber 100 will measure the radiation dose ions from only generated in sensible region R _1.

FIG. 1C is an enlarged view of a peripheral region R of the tip 103a of the guard electrode 103 in FIG. 1A. Dead region _{R 2} is constituted by a first region _{R 21} sandwiched between the guard electrode 103 and the external electrode 101, a second region (region _{A) R 22} spread to the other end 101b than the tip 103a of the guard electrode Have been. In the space ionization chamber 100 (a space surrounded by the outer electrode 101), the first region _{R 21,} third region excluding the second region _{R 22} (region B) _{R 1} is a sensible region _{R 1.}

Electron shield portion 105 is provided on at least the second region _{R 22.} In Figure 1, the electron shield portion 105, as is illustrated for the case provided in the whole of the second region R _22, described below as a modified example, electron shield 105, at least, a sensitive area R it may be provided at a boundary portion between the ₁ and the dead region R _2. The electron shield 105 is preferably formed so as to cover the tip 103a of the guard electrode. The electron shield 105 is made of an electrically insulating material, and is preferably made of a material having a higher density than air. The electric insulating material is preferably a resin material, glass, and an inorganic material, for example, an insulating metal oxide such as Al ₂ O ₃ , MgO, and SiO ₂ .

The distribution of the sensitive region R ₁ and the insensitive region R ₂ (the first region R ₂₁ and the second region R ₂₂ ) in the space inside the ionization chamber 100 (hereinafter sometimes referred to as an ionization cavity) is actually Assuming that the inside of the ionization chamber 100 is filled with air, the ionization chamber 100 can be obtained by performing a calculation by the statistical diffusion simulation (SDS) method. Specifically, the shape of the ionization chamber 100 is extracted as three-dimensional CAD data, and this data is read by a computer using simulation software (SCIENTIFIC INSTRUMENT SERVICES, SIMION (registered trademark)) to perform calculations. In consideration of the fact that the ionization chamber is cylindrically symmetric (axially symmetric), when the external electrode 101 is grounded and a high voltage of several hundred volts is applied to the internal electrode 102 and the guard electrode 103, the potential in the ionization chamber 100 Calculate the distribution. The SDS method is a method of calculating the movement of ions in a high-density medium to which an electromagnetic field is applied, in consideration of ion diffusion and ion drift.

FIGS. 2A and 2B are diagrams showing simulation results of potential distribution and ion movement formed when the ionization chamber is driven. FIG. 2A is a diagram illustrating a result of calculation of a potential distribution in an A1SLMR ionization chamber 100A manufactured by Standard @ Imaging. A voltage of 0 V is applied to the external electrode 101A and a voltage of +300 V is applied to the internal electrode 102A. Equipotential lines P are drawn in the space inside the ionization chamber 100A. Since the distance from the external electrode 101A differs between the internal electrode 102A and the guard electrode 103A, the equipotential line P is largely bent at the ionization chamber stem 100S.

Ion initial kinetic energy of the occurring ionizing cavity, considering the ideal gas equation can be evaluated and _{^{MV 2/2 = 3k B T}} / 2. Here, M average molecular mass of air (approximately 28.95u), V is the average velocity of the ion, k _B is the Boltzmann constant, T is meant the absolute temperature. Assuming that the temperature T is room temperature, the initial kinetic energy of the ions can be estimated to be about 0.04 eV.

Referring to FIG. 2B, it is a diagram showing a result of calculating the motion of ions generated in the ionization chamber by the SDS method. Here, ions having an average molecular mass are generated at equal intervals in the axial direction L near the external electrode 101A. The ions are moving almost perpendicular to the equipotential surface because the ions frequently lose kinetic energy due to collisions. From this result, the trajectory of ions, a boundary of a sensitive area R ₁ boundary is divided into a guard electrode 103 and the internal electrode 102 and the dead region R _2, it is possible to estimate the boundary C.

It should be noted that even when a strong magnetic field is applied, the Lorentz force qvB is a force proportional to the velocity, and the velocity of the ions is lost due to collisions. ing.

Returning to Figure 1, the second region R ₂₂ electron shield portion 105 is provided, since the electrons are difficult to move, the electrons in the second region R ₂₂ is a sensitive area (third area) hardly pops out R _1. Also, electrons in the first region _{R 21} also electron shield portion 105 of the second region _{R 22} becomes a wall, not easily jump out the sensible region _{R 1.} That is, by providing the electron-blocking portion 105, toward the dead region R ₂ in the sensible region R _1, electrons can be prevented from moving. On the other hand, the electron blocking unit 105 may repel electrons trying to enter from the sensitive region R _1, as a result, the electron blocking unit 105 from the sensitive region R ₁ toward the dead region R _2, electrons You can also prevent them from moving.

FIGS. 3A and 3B are cross-sectional views of main parts of a modification of the ionization chamber 100 shown in FIG. In the ionization chamber 100, an electron blocking portion 105 is provided on the entire second region _{R 22} constituting the dead region _{R 2.} In contrast, in the ionization chamber shown in FIG. 3 (a), an electron blocking portion 105 of the second region _{R 22,} a sensitive region _{R 1} (third extending on the other end 101b side from the second region _{R 22} It is provided only in the region) and contact portions (near the boundary between a sensitive region R _1). Other configurations of the ionization chamber are the same as the configuration of the ionization chamber 100, and portions corresponding to the ionization chamber 100 are denoted by the same reference numerals regardless of the difference in shape. Electrons in was ionization chamber provided small electron shield portion 105 hardly moves, but not so much as ionization chamber 100 has a function of shielding the back and forth of electrons between the sensible region R ₁ and the dead region R ₂ .

The ionization chamber shown in FIG. 3 (b), an electron blocking portion 105, the second region R ₂ is formed which includes the tip of the guard electrode 103 is also formed in the first region R ₁ in addition to. That is, the first region R ₁ and the second region R ₂ is formed of an electrically insulating material. Other configurations of the ionization chamber are the same as the configuration of the ionization chamber 100, and portions corresponding to the ionization chamber 100 are denoted by the same reference numerals regardless of the difference in shape. Since the electron shielding portion 105 that makes it difficult for electrons to move is provided in the first region R ₁ and the second region R ₂ , compared with the ionization chamber 100, the portion between the sensitive region R ₁ and the insensitive region R ₂ is smaller. It has a high function of blocking the flow of electrons. Note that the electron shielding unit 105 may be formed so as to cover at least a part of the insulating unit 104.

The above-described ionization chamber may be manufactured by any procedure. For example, it can be manufactured by the following procedure. First, in the ionization chamber, the position of the boundary of the sensitive region R ₁ and the dead region R ₂ is estimated, so that at least the boundary portion is blocked, determining the shape and size of the electron shield portion. The position of the boundary can be obtained by performing calculation by the SDS method, assuming that the inside of the ionization chamber is filled with air. Specifically, the sensible region R ₁ of the electric field directed from the outer electrode 101 to the inner electrode 102 is produced, determined by calculating the distribution of the dead region R ₂ of the electric field directed from the outer electrode 101 to the guard electrode 103 is generated, Estimate the location of those boundaries.

(4) Based on the estimated position of the boundary, a mold for preparing the electron shielding portion 105 having a predetermined shape (such as a ring shape) and a size is prepared. A material for the electron shield 105 is placed on the mold and solidified. Subsequently, the solidified electron shielding portion 105 is removed from the mold, and this is fitted to a predetermined position on the inner wall surface of the external electrode 101 made of a cylindrical metal member or the outer wall surface of the inner electrode 102 made of a rod-shaped metal member. Let it.

Subsequently, the internal electrode 102 and the guard electrode 103 are inserted from the one end 101a side of the external electrode into a space surrounded by the external electrode 101 so that the electron shielding portion 105 is fitted between the external electrode 101 and the internal electrode. . At this time, the central axes of the external electrode 101, the internal electrode 102, and the guard electrode 103 are aligned. Note that one or both of the electron shielding portion 105 and the inner wall surface of the external electrode 101 and the electron shielding portion 105 and the outer wall surface of the internal electrode 102 may be bonded using an adhesive or the like. Lastly, by filling the material of the insulating portion 104 between the internal electrode 102 and the guard electrode 103, the ionization chamber as described above can be obtained.

As described above, in the ionization chamber 100 of the present embodiment, the dead region R ₂ consisting of a first region R ₂₁ and the second region R _22, between the sensible region R ₁ in its outside, electron shield part 105 is provided with, the electrons back and forth between the sensible region R ₁ and the dead region R _2, is configured to be disturbed. Therefore, with the change in orientation of the ionization chamber 100 to the magnetic field, it is possible to avoid the electronic problems several changes to alternate between the sensible region R ₁ and the dead region R _2, as a result, the internal electrodes Variations in the number of ions to which the electrons reaching the layer 102 are attached can be suppressed. Therefore, in the ionization chamber 100 according to the present embodiment, even in an environment where a magnetic field is present, a change (directional characteristic) of the measurement result due to the rotation with respect to the magnetic field can be suppressed to be small, and a stable output can be obtained. .

Hereinafter, the effects of the present invention will be made clearer by examples. It should be noted that the present invention is not limited to the following embodiments, and can be implemented with appropriate changes within the scope of the present invention.

と し て As Examples 1 to 5, calculations were performed on ionization chambers of various shapes and sizes by the SDS method, and the potential distribution in the internal space was obtained to divide the internal space into sensitive areas and insensitive areas.

Figure 4-8 is a diagram showing an example a sensitive region _{R 1} of the configuration and the internal space of the ionization chamber 130-134 1-5 and dead region _{R 2.} The ionization chambers 130 to 134 have the same configuration as the ionization chamber 100 shown in FIG. Also in the ion chamber 130-134, dead region _{R 2,} the second region R that protrudes from the first region _{R 2} 1 sandwiched between the external electrode 101 and the guard electrode 103, from there to the other end 101b side of the external electrode is composed of a _22, the region of the other end 101b side is a sensible region _{R 1} than the second region _{R 22.} However, each of the ionization chambers 130 to 134 has a portion different in shape and size from the ionization chamber 100.

Next, as Comparative Example 1, the ionization chamber 130 of Example 1 shown in FIG. 4 which was replaced with air except for the electron shielding portion 105 was used, and the direction of the magnetic field of 0.35T was changed in the axial direction of the ionization chamber. Was rotated, a simulation (Monte Carlo code (EGS5)) of an output current obtained for each rotation angle was performed in the sensitive region. Further, using the ionization chamber of Comparative Example 1, actual measurement of the output current obtained for each rotation angle in the sensitive region was also performed.

FIG. 9A is a graph comparing simulation results obtained by the ionization chamber of Comparative Example 1 with actual measurement results. The horizontal axis of the graph indicates the rotation angle [°] of the ionization chamber with respect to the magnetic field. The vertical axis of the graph indicates the relative output current obtained by normalizing the output current (output of the ionization chamber) detected by the counter connected to the internal electrode with the output current at a rotation angle of 0 °.

The simulation and the measured data show that the output obtained varies by about 5% due to the rotation of the ionization chamber. In addition, since the simulation almost coincides with the actually measured data, it can be seen that the simulation method applied here can accurately reproduce the actually measured data.

シ ミ ュ レ ー シ ョ ン A simulation of the output current obtained for each rotation angle was performed in the dead area of the ionization chamber of Comparative Example 1. FIG. 9B is a graph showing the result, and the horizontal axis and the vertical axis of the graph are the same as those in FIG. 9A. From the comparison with the graph of FIG. 9A, it can be seen that the directional characteristics of the insensitive region show the directional characteristics opposite to those of the insensitive region.

FIG. 9C is a graph showing the combined output of the sensitive area and the insensitive area obtained for each rotation angle by simulation in the ionization chamber of Comparative Example 1, and the horizontal axis and the vertical axis of the graph are shown in FIG. This is the same as (a) and (b). From this graph, it is understood that the output fluctuation due to the rotation of the ionization chamber is almost suppressed in the entire ionization chamber.

Next, when the ionization chamber 130 of the first embodiment in FIG. 4 is used as the sixth embodiment and the axial direction of the ionization chamber 130 arranged in a plane perpendicular to the magnetic field is rotated, the ionization chamber 130 is obtained for each rotation angle. A simulation of the output current obtained was performed. Total dead region (first region R _{2 1} and the second region R ₂₂₎ as an electron blocking unit 105, as a material thereof was used a polyacetal resin (POM) which is an insulating material. Further, as Comparative Example 2, the ionization chamber 130 except for the electron shielding portion 105 was used, rotated under the same conditions as in Example 1, and the output current obtained for each rotation angle was measured.

FIG. 10 is a graph showing a simulation result of Example 6 and a measurement result of Comparative Example 2. The horizontal axis and the vertical axis of the graph are the same as in FIG.

The variation of the output of Example 6 for each rotation angle is significantly reduced as compared with the variation of the output of Comparative Example 2, and the variation is within 0.5% of the average value of the simulation results. It is suppressed. From this result, it can be seen that in the ionization chamber of Example 6, the directional characteristics of the ionization chamber output with respect to the magnetic field were almost eliminated.

As Embodiments 7 to 10, the ionization chambers 131 to 134 of Embodiments 2 to 5 in FIGS. 5 to 8 are used, and the axial directions of the ionization chambers 131 to 134 are in a plane parallel to the direction of the magnetic field. The simulation of the output current obtained for each rotation angle when was rotated.

As Comparative Examples 3 to 6, the ionization chambers 131 to 134 were replaced with air except for the electron shield 105, and rotated under the same conditions as in Example 6, and the output current obtained for each rotation angle was measured. .

FIG. 11A is a graph comparing the results of Example 7 and Comparative Example 3. FIG. 11B is a graph comparing the results of Example 8 and Comparative Example 4. FIG. 11C is a graph comparing the results of Example 9 and Comparative Example 5. FIG. 11D is a graph comparing the results of Example 10 and Comparative Example 6. The horizontal and vertical axes of the graph are the same as in FIGS.

で は In the graph of FIG. 11B, the directional characteristics of Example 7 are slightly smaller than Comparative Example 3. This is because, when the volume of the space inside the ionization chamber is large, the directional characteristics of the electrons bent by the Lorentz force are greater than the contribution of the electrons coming and going between the sensitive region and the insensitive region (traffic effect). This is because the contribution (bending effect) of the orbital length is larger.

On the other hand, in the graphs of FIGS. 11A, 11C, and 11D, the directional characteristics of Examples 7, 9, and 10 are significantly smaller than the directional characteristics of Comparative Examples 4 to 6, respectively. I have. This is because the volume of the space inside the ionization chamber is small, and the contribution of the back-and-forth effect is larger than the contribution of the bending effect on the directional characteristics, and the effect of the electron shielding portion is remarkable. is there.

100, 100A, 130 to 134: ionization chamber 100S: stem 101, 101A: external electrode 101a: one end of external electrode 101b: other end of external electrode 102, 102A: internal electrode 103, 103A ... guard electrode 103a ... tip of guard electrode 104 ... insulating part 105 ... electron shielding part C ... boundary L ... axial direction P ... equipotential line R ... - the guard electrode tip near region R ₁ · · · sensible region (third region, region B)
R ₂ · · · dead zones _{R 21} · · · first region _{R 22} · · · second region (region A)

Claims (8)

A cylindrical external electrode,
A rod-shaped internal electrode included in the external electrode and extending in the axial direction from one end to the other end of the external electrode;
On the one end side, a cylindrical guard electrode provided between the external electrode and the internal electrode and surrounding a side wall of the internal electrode,
On the one end side, an insulating portion provided between the internal electrode and the guard electrode,
An ion shielding unit provided in an area A extending from the tip end of the guard electrode to the other end side and made of an electrically insulating material. The area A is an area in which, assuming that the area is filled with air, a motion trajectory of ions generated by a calculation based on the Statistical {Diffusion} Simulation (SDS) method is obtained, and ions reaching the guard electrode are generated. The ionization chamber according to claim 1. 3. The ionization chamber according to claim 2, wherein the electron shielding portion is provided in a portion of the region A that is in contact with a region B that extends to the other end side from the region A. 4. (4) The ionization chamber according to any one of (1) to (3), wherein the electron shielding portion is provided on the entire area A. (5) The ionization chamber according to any one of (1) to (4), wherein the electrically insulating material has a higher density than air. The ionization chamber according to claim 5, wherein the electrically insulating material is made of at least one of a resin material and an inorganic material. The ionization chamber according to any one of claims 1 to 6, wherein the electron shielding part is formed so as to cover at least a tip of the guard electrode. A cylindrical external electrode, a rod-shaped internal electrode included in the external electrode and extending in the axial direction from one end to the other end of the external electrode, and at one end, the external electrode and the internal electrode; Between the internal electrode and the guard electrode on the one end side, and a method for manufacturing an ionization chamber having an insulating section provided between the internal electrode and the guard electrode on the one end side. So,
Assuming that the inside of the ionization chamber is filled with air, a calculation by a statistical diffusion simulation (SDS) method is performed to obtain a motion trajectory of generated ions, and a region where the ions reaching the guard electrode are generated is estimated. Steps and
Forming an electron shielding portion made of an electrically insulating material in the estimated area.

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