JPS5821066Y2 - Flat electrode type ionization chamber - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a flat electrode type ionization chamber that increases ionizing current using a flat electrode and measures the radiation exposure dose.

Generally, when the electrode of a flat electrode type ionization chamber is composed of a thin film,
When ionizing current is collected at a current extraction terminal provided at one point on the electrode, a loss occurs on the electrode surface due to the electrical resistance of the electrode, and the measured dose is proportional to the area of incidence of radiation at a constant intensity. The area proportionality and the dose rate proportionality proportional to radiation intensity at a constant incident area are inferior to those using metal plate electrodes that are good electrical conductors.

For example, in radiation therapy, ionization chambers that are placed between the radiation source and the irradiated body to monitor the irradiation dose are made of transparent film to confirm the irradiation field and to avoid absorption of radiation. A transmission type ionization chamber is used, which has a thin transparent flat plate electrode on which a conductive material such as a metal oxide is vapor-deposited. ) is high and the ionizing current increases at one point (-), the impedance on the electrode to that point of increase becomes uneven, and as a result, the ionizing current is extracted (the impedance loss is lost while it reaches one point). However, since the loss also varies depending on the position on the electrode, the proportionality of the electrode area and the dependence on the dose rate are poor as described above, and improvements have been required.

This invention aims to realize a flat plate electrode type ionization chamber that meets this demand, and one embodiment of this invention will be described below.

Figure 1 is a cross-sectional view explaining the main parts of a flat plate electrode type ionization chamber according to one embodiment of this invention. Reference numerals 1 and 2 are transparent flat plate electrodes in which metal oxides are vapor-deposited on Mylar film, and radiation from the direction of arrow S is shown. is incident, passes through these flat electrodes 1 and 2, and irradiates the object to be irradiated.

Note that a high voltage is applied between these plate electrodes 1 and 2, and the electrode on the high potential side collects ionized ions between both electrodes 1 and 2.

3 is a spacer made of electrically insulating material that holds the above-mentioned electrodes 1 and 2 parallel to each other; 4 is an electrode fixing frame made of a metal material that is a good electrical conductor, and is tightly attached to the outer edges of both electrodes 1 and 2 through spacer 3; Fix it.

Reference numeral 5 denotes a terminal disposed in a part of the fixed frame, to which a lead wire 6 is connected.

On the other hand, in the conventional flat electrode type ionization chamber, as shown in Fig. 2, the ionizing current extraction piece 7 is arranged at one corner of each of the flat plate electrodes 1 and 2, and the impedance from any point on the electrode surface is uneven. The ionizing current suffered loss due to impedance while reaching the ionizing current extraction piece 7, and its area proportionality and dose dependence were poor.

In this device, the electrodes 1 and 2 are electrically connected along their entire circumference in close contact with a frame 4 that is a good electrical conductor.
The impedance of the ionizing current extraction path from each point on the electrode surface is equalized, and area dependence and dose rate dependence are improved.

In addition, a conductive coating film 8 such as silver paint is provided around the four circumferences of the conductive thin film of the electrode within a range that does not interfere with the incident radiation flux, and instead of adding a straight lead wire to the conductive metal frame, a conventional ionizing current extraction piece is used. You can get exactly the same effect by using

In the example, a low-resistance conductive part was formed around the periphery of the five electrodes 1 and 2, but it is also possible to form a low-resistance conductive part around the periphery of one of the electrodes, which serves as a collecting electrode for trapping ion current. good.

However, if a low-resistance conductive portion is formed around both electrodes as in the embodiment, the electric field between both electrodes will be uniform, and area proportionality or dose dependence will be further improved.

As detailed above, the effects of this invention are that it is possible to realize a flat electrode type ionization chamber with improved area proportionality and dose rate dependence, and it also uses a conductive thin film with lower X-ray absorption as the electrode than conventional ones. , it is expected that the irradiation field of the irradiated object can be confirmed easily, and a flat electrode type ionization chamber with less radiation absorption can be realized.

[Brief explanation of the drawing]

Fig. 1 is a sectional view illustrating the main parts of a flat plate electrode type ionization chamber according to one embodiment of this invention, Fig. 2 is a plan view illustrating an example of the electrode of a conventional plate electrode type ionization chamber, and Fig. 3 is an explanatory view of an example of the electrode of a conventional plate electrode type ionization chamber. FIG. 7 is an explanatory plan view of an electrode used in another example. 1.2... Flat plate electrode, 3... Spacer, 4... Electrode fixed frame, 5... Terminal, 6...
... Lead wire, 8 ... Conductive coating film.

Claims (1)

[Scope of utility model registration request] In an ionization chamber in which two flat plate electrodes each having a conductive material such as a metal oxide deposited on a substrate such as a transparent film are opposed in parallel to each other and one is used as a collector electrode, a flat plate that acts as at least one of the electrodes is used as a collector electrode. A flat plate electrode type ionization chamber characterized by forming a low resistance conductive part on the entire outer peripheral surface of the electrode.