A flat plate ionization chamber for high energy electron beam dose measurement

Abstract

The present invention discloses a flat plate ionization chamber for high-energy electron beam dose measurement, comprising an outer electrode and an inner electrode; the outer electrode is a cap-mounted structure made of air equivalent material, and the outer electrode and the lower outer electrode back cover form a closed flat air cavity, and an insulating plate is provided at the bottom of the air cavity; the inner electrode is fixed on the insulating plate, and a protective ring is surrounded on the periphery. The air equivalent material used in the present invention as the outer electrode makes the measurement more accurate; it solves the problem of measuring electron beam dose in the prior art, and can truly reflect the gradient change of the radiation beam; it adopts a fixed cap edge structure, which is simple to assemble, has good contact, and has few air gaps, thereby improving the consistency of the cavity ionization chamber, improving the qualified rate of the product, and realizing the firmness and durability of the product.

Description

Technical Field

[0001] The invention relates to the technical field of electron beam dose measurement, in particular to a flat plate ionization chamber used for high energy electron beam dose measurement. Background Art

[0002] In 1958, Shonka, FR, Rose, JE, Failla, G, etc. published articles on conductive plastic equivalent tissue, air, and polystyrene, pioneering the research on air equivalent plastics and applying air equivalent materials to the production of ionization chambers. The use of air equivalent materials to make ionization chamber walls has approximately the same absorption coefficient as air for primary radiation and approximately the same atomic stopping power for secondary electrons. The physical properties are strong, durable, and stable, making it an excellent material for the production of ionization chambers.

[0003] Foreign countries started to develop air equivalent materials very early, and there are ready-made products, such as C-552 air equivalent materials. Foreign countries have used air equivalent materials to produce standard ionization chambers, such as FC23-C Farmer, PR-06CFarmer, PR-06G Farmer, etc. In China, Sichuan Zhongce Radiation Technology Co., Ltd. is engaged in the research and development of this material. The CT long-rod ionization chamber produced has been used for CT dose measurement, with an energy response of less than 3%. A patent has been applied for [Pen-type ionization chamber for CT index measurement, patent number: ZL200520036437.7].

[0004] In view of the research on air-equivalent materials and the production of ionization chambers in China, the China Testing Technology Research Institute has carried out research on the production process of ionization chambers and developed a finger-shaped ionization chamber with a volume of 0.6 cm3 for absolute dose measurement in radiotherapy. This ionization chamber is strong, durable and stable, but cannot be used for high-energy electron beam dose measurement.

[0005] Medical linear accelerator is an accelerator that uses microwave electromagnetic field to accelerate electrons and has a linear motion track. It is mainly used for radiotherapy of tumor patients and has been widely used. However, for the dose detection of high-energy electron beam, finger-shaped ionization chamber is usually used. Since high-energy electron beam is mainly used to treat shallow tumors on the surface of the skin, its dose attenuation gradient varies greatly. Therefore, it is not suitable to use finger-shaped ionization chamber to measure the absorbed dose of electron beam with lower energy ≤10MeV.

[0006] In summary, the existing technology has two main shortcomings: 1. Finger-shaped ionization chambers are usually used for measurement. At present, finger-shaped ionization chambers are usually used to measure water absorbed dose in China. However, when the electron beam energy is low, the finger-shaped ionization chamber cannot truly reflect the characteristics of the gradient change of the radiation beam.

[0007] 2. Use organic glass coated with graphite as the chamber wall material. There are flat plate ionization chambers abroad for measuring water absorption dose with low electron beam energy, but the chamber wall uses organic glass coated with graphite conductive layer, whose effective atomic number is slightly smaller than that of air, so the ionization charge generated in the cavity is slightly less than that of the free air ionization chamber. Summary of the invention

[0008] In view of the above problems, the object of the present invention is to provide a flat ionization chamber for high-energy electron beam dose measurement, which has a compact structure, small intracavity scattering disturbance effect, small disturbance influence, and easy determination of effective measurement points.

[0009] The technical scheme of the present invention is as follows: a flat-plate ionization chamber for high-energy electron beam dose measurement, comprising an outer electrode and an inner electrode; the outer electrode is a cap-mounted structure made of air-equivalent material, which forms a closed flat air cavity with the outer electrode back cover below, and an insulating plate is provided at the bottom of the air cavity; the inner electrode is fixed on the insulating plate and is surrounded by a protective ring.

[0010] Furthermore, a first insulating support ring and a second insulating support ring are supported between the insulating plate and the outer electrode; the second insulating support ring is located in the outer circle, and an inner concave circle is provided on the inner side of its bottom; the first insulating support ring is located in the inner circle, and an outer concave circle is provided on the outer side of its bottom and an inner concave circle is provided on the inner side; the protective ring is embedded between the inner concave circle of the second insulating support ring and the outer concave circle of the first insulating support ring; the inner electrode is embedded in the inner concave circle of the first insulating support ring and is in the same plane as the protective ring.

[0011] Furthermore, a concave platform is provided on the outer edge of the bottom of the outer electrode, and a convex platform is provided on the outer edge of the rear cover of the outer electrode to be mounted in cooperation with the concave platform.

[0012] Furthermore, it also includes a front end of a support, one end of which is connected to the outer electrode through a thread, and the other end of the front end of the support is fixedly connected to the rear end of the support.

[0013] Furthermore, the insulating plate is provided with small grooves for routing the inner electrode and the guard ring and for filling the ionization cavity at the back.

[0014] Furthermore, the inner electrode is a graphite-coated organic glass with a diameter of 10 mm, and a gold-plated pressing sheet is provided on its edge; the thickness of the incident window is 1 mm.

[0015] Furthermore, the width of the protection ring is not less than 1.5 times the height of the air cavity.

[0016] The beneficial effects of the present invention are as follows: the present invention adopts air equivalent material as the outer electrode, and the measurement is more accurate; the difficult problem of measuring electron beam dose in the prior art is solved, and the gradient change of the radiation beam can be truly reflected; the outer electrode and the outer electrode back cover adopt a fixed cap edge structure, which is simple to assemble, has good contact, and has few air gaps, thereby improving the consistency of the cavity ionization chamber, improving the qualified rate of the product, and realizing the firmness and durability of the product. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The figure is a top view of a flat-plate ionization chamber for high-energy electron beam dose measurement according to the present invention.

[0018] Figure 2 for Figure 1 Cross-section view at the middle BB.

[0019] In the figure: 1-external electrode; 2-external electrode back cover; 3-protective ring; 4-inner electrode; 5-front end of the support; 6-rear end of the support; 7-insulating plate; 8-first insulating support ring; 9-second insulating support ring. DETAILED DESCRIPTION

[0020] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0021] like Figure 1 As shown, a flat-plate ionization chamber for high-energy electron beam dose measurement includes an outer electrode 1 and an inner electrode 4; the outer electrode 1 is a cap-mounted structure made of air equivalent material, and it forms a closed flat air cavity with the outer electrode back cover 2 below, and an insulating plate 7 is provided at the bottom of the air cavity; the inner electrode 4 is fixed on the insulating plate 7, and is surrounded by a protective ring 3.

[0022] This embodiment uses air-equivalent plastic as the chamber wall material and epoxy resin as the adhesive, which has the advantages of small inter-electrode spacing, small disturbance influence, and easy determination of effective measurement points. It has obvious advantages and higher measurement accuracy for the calibration of electron beam absorbed dose and the measurement of areas with large dose gradient changes along the beam incident direction.

[0023] like Figure 2As shown, in this embodiment, a first insulating support ring 8 and a second insulating support ring 9 are also supported between the insulating plate 7 and the outer electrode 1; the second insulating support ring 9 is located in the outer circle, and an inner concave circle is provided at the inner bottom; the first insulating support ring 8 is located in the inner circle, and an outer concave circle is provided at the outer side of the bottom, and an inner concave circle is provided at the inner side; the protection ring 3 is embedded between the inner concave circle of the second insulating support ring 9 and the outer concave circle of the first insulating support ring 8; the inner electrode 4 is embedded in the inner concave circle of the first insulating support ring 8, and is in the same plane as the protection ring 3. The first insulating support ring 8 and the second insulating support ring 9 are made of highly insulating polytetrafluoroethylene, and the processed concave circles are respectively placed to place the protection ring and the inner electrode to ensure that the protection ring and the inner electrode are in the same plane; high insulation isolation is achieved for the protection ring 3 and the inner electrode 4, and the processed surface is smooth to reduce leakage loss.

[0024] The ionization chamber of this embodiment uses air equivalent material as the outer electrode of the ionization chamber to maintain the firmness and durability of the ionization chamber; there is a flat air cavity with a height of 2 mm, and the inner electrode is surrounded by a protective ring with a width not less than 1.5 times the height of the air cavity. In order to reduce the influence of radial beam inhomogeneity, the diameter of the inner electrode is relatively small, 10 mm, and the thickness of the incident window is 1 mm.

[0025] The outer electrode of this embodiment is made of air equivalent material, and cooperates with the back cover to form an independent electric field space to reduce external electric field interference. Specifically, a concave platform is provided at the outer edge of the bottom of the outer electrode 1, and a convex platform is provided at the outer edge of the outer electrode back cover 2 to be installed in cooperation with the concave platform. The cap edge structure is easy to process, has accurate position control during installation, is simple to assemble, has good contact, and has few air gaps, which improves the consistency of the cavity ionization chamber, improves the qualified rate of the product, and realizes the firmness and durability of the product; the external seams of the ionization chamber are all bonded with epoxy resin to ensure good waterproof performance.

[0026] The inner electrode is made of 10mm diameter organic glass coated graphite, which is easy to process and has small deformation. Its sensitive height is determined by the first insulating support ring 8 and the second insulating support ring 9. A gold-plated pressing sheet is used at the edge of the inner electrode for welding the lead wire to collect the ionized charge on the inner electrode. The graphite coated on the inner electrode has good conductivity and reduces the loss of collected charge.

[0027] The guard ring is made of pure copper plated with gold for easy welding, with a width of 3mm, ensuring that the electric field at the edge of the sensitive volume remains uniform and reduces leakage loss; small grooves are opened on the insulating plate for routing the inner electrode and the guard ring and filling the ionization cavity on the back to prevent the increase of additional ionization volume. Adding guard electrodes reduces leakage loss and ensures that the electric field at the edge of the sensitive volume remains uniform.

[0028] It also includes a support front end 5, one end of which is connected to the outer electrode 1 through a thread, and the other end of the support front end 5 is fixedly connected to the support rear end 6. The support front end 5 and the support rear end 6 are made of aluminum alloy, connected to the high-voltage pole of the cable, and connected to the outer electrode through a thread, which can realize the non-attenuation conduction of the voltage and protect the cable from being damaged by external force, and can be fixed by a clamp.

Claims

1. A flat plate ionization chamber for high energy electron beam dose measurement, characterized in that: It comprises an outer electrode (1) and an inner electrode (4); the outer electrode (1) is a cap-mounted structure made of an air equivalent material, and together with the outer electrode back cover (2) below, forms a closed flat air cavity, and an insulating plate (7) is provided at the bottom of the air cavity; the inner electrode (4) is fixed on the insulating plate (7), and is surrounded by a protective ring (3); A first insulating support ring (8) and a second insulating support ring (9) are also supported between the insulating plate (7) and the outer electrode (1); the second insulating support ring (9) is located in the outer ring, and an inner concave ring is provided on the inner side of its bottom; the first insulating support ring (8) is located in the inner ring, and an outer concave ring is provided on the outer side of its bottom, and an inner concave ring is provided on the inner side; the protective ring (3) is embedded between the inner concave ring of the second insulating support ring (9) and the outer concave ring of the first insulating support ring (8); the inner electrode (4) is embedded in the inner concave ring of the first insulating support ring (8), and is in the same plane as the protective ring (3).

2. A flat-plate ionization chamber for high-energy electron beam dose measurement according to claim 1, characterized in that: The outer edge of the bottom of the outer electrode (1) is provided with a concave platform, and the outer edge of the outer electrode rear cover (2) is provided with a convex platform that is mounted in cooperation with the concave platform.

3. A flat-plate ionization chamber for high-energy electron beam dose measurement according to claim 1, characterized in that: It also comprises a support front end (5) whose one end is connected to the outer electrode (1) via a thread, and the other end of the support front end (5) is fixedly connected to the support rear end (6).

4. A flat-plate ionization chamber for high-energy electron beam dose measurement according to claim 1, characterized in that: The insulating plate (7) is provided with small grooves for routing the inner electrode (4) and the guard ring (3) and for filling the ionization cavity at the back.

5. A planar ionization chamber for high energy electron beam dose measurement according to claim 1, characterized in that: The width of the protective ring (3) is not less than 1.5 times the height of the air cavity.

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