JPS5824744B2 - Energy monitor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a radiation energy monitor designed to simultaneously monitor the dose of radiation from an ion chamber and also monitor the energy of the radiation.

As a conventional example, an electron accelerator for medical use will be explained below.

Rattan 11. Sumo 9 shows an example of the 4th medical doctor Q and the electric power station.
・So, 21 is a threat of 5, and an intermittent resistance! , X-rays or electron beams.

1 is an electron beam that has been crushed by Kanbenriyoyu 1 theory, and 1 ter γ
・Getsuto or input key 1 yaftaling huoi east

3 is an electron gun, 4 is a control system for the incident beam to the acceleration tube, 5 is the acceleration tube, and 6 is a bending electromagnet system that guides the accelerated electron beam 2 to the target or scatter link foil. This system performs arbitrary deflection such as 90° deflection using one electromagnet, 110° deflection or 270° deflection using one complicated electromagnet, but in the case of low-energy radiation, this deflection electromagnet system is not available and acceleration is required. The central axis of the tube 5 and the central axis of the beam of generated radiation may be coaxial.

7 is a source of radiation; in the case of X-rays, 1 is a target and flatness filter; in the case of electron beams, it is a scattering foil.

8 is a microwave source for acceleration in the acceleration tube 51; 9
and 10 are transmission type ion chambers for measuring X-rays or electron beams, 9 is referred to as a primary chamber, and 10 is referred to as a secondary chamber].

Reference numeral 11 denotes an amplification circuit for the ionizing current produced by the secondary and secondary chambers 9 and 101, and a display system for the dose rate and cumulative dose.

Both the primary and secondary chambers 9 and 10 are shown in Figure 2a.
, b.

That is, in FIG. 2, 13 is a high-voltage electrode for causing the ion collector electrode 1 ionized by the radiation 1 to migrate;
Reference numeral 14 denotes a shield plate for preventing leakage current due to the insulator surface 1, and the potential 1 is grounded.

Reference numeral 15 is a collecting electrode for ionizing current, and reference numeral 16 is a grounding electrode. Each of the electrodes 13 to 16 is held by an insulator.

Then, the radiation 1 is irradiated in 121 directions.

The radiation dose is monitored by the current flowing through the collector electrodes 15 of the primary and secondary chambers 9, 10.

In this conventional electron accelerator, dose monitoring was performed in the primary and secondary chambers 9 and 101, but there was no method for energy monitoring from this point.

That is, conventionally, the energy is estimated from parameters such as accelerating voltage, microwave power, and deflection electromagnet current, and minute fluctuations are adjusted from the microwave wave number and the incident electron beam of the accelerating tube 5, and the energy is usually measured. This was done when adjusting the equipment.

However, beam energy or radiation energy is the most important parameter in radiation therapy, and there has been a need for a system that can constantly monitor this.

This invention is based on the above-mentioned point 1. When measuring the dose rate of radiation generated by a radiation generating device such as an electronic linac, the measurement medium has a finite thickness and mass, so a build-up effect occurs. Therefore, different ionizing currents flow in two independent ion chambers placed on the central axis of the radiation flux, and according to the build-up effect, each ionizing current has an energy of 1. The slopes of the characteristic curves are different.

Therefore, we measure the energy by taking the ratio of the former)
This is what led to this invention.

The details of this invention will be explained below. Figure 3a shows the depth rate curve of the underwater depth of X-rays.

Each curve A, B, C is 6MeV, 8MeV and 10MeV
It represents a curve where the energy of V is increased by 1.

These figures show that there is a build-up effect when radiation enters water at one point, and even in a layer shallower than the point where the build-up effect is maximum, the deep rate curve has a different value depending on the energy. It represents what will happen.

Typically, transmission type ion chambers are 0.1-0.2 mu
Thickness of A [Although it is composed of one plate, the specific gravity of A-, l is 2.
69, the depth on the water side of A7 with a thickness of 0.2 is 0.
．． This is equivalent to 538mm.

Two primary and secondary chambers 9,1 as shown in FIG.
0 as a dose monitor, the radiation 1 passes through two A [plates] until it reaches the ionization space I in the primary chamber 9; Passing through 7 A [plates including the partition (If one plate is 0.2 mm thick, the depth of the water side in the primary chamber 9 is 1.076 mm, and the secondary chamber 10 This corresponds to measuring the dose at a depth of 3.766 mm.

In Figure 3, Figure a, and Figure 4, a1, dl represents the isolateral depth of 1.076 nm, and d2 represents 3.766 ynm.

Here, when the depth of d and d2 is 1, the energy (in contrast, the points on the depth rate curve are plotted as shown in Figure 3 for X-rays, Figure 4 for electron beams) .

In Fig. 3b1, the curve has the relationship between d and l in Fig. 3a, and the curve has the relationship between d21 and d21.

Also, in the curve shown in Figure 4, ' is dl in Figure 4 a, and curve L' is
is the relationship in d2.

In the first aspect of the invention, attention is paid to the fact that dl and d21 have different slopes with respect to the deep R amount f:)' energy.

Moreover, each of the curves in Figure 3 a and Figure 4 a is expressed not as an absolute value of the measured ionizing current, but as a percentage of the value when the build-up effect is maximum], so dl or d2 It must be noted that energy monitor I is never used for dose measurements in this field.

Therefore, in the present invention, this problem is overcome in the following manner.

That is, the dose at dll is Dl, the dose at d2 is D2, and the point at which the build-up effect is maximum is Dmax.

Now, if we consider DR as the ratio of doses D1 and D2, ] This is the point on the depth rate curve.

In dose measurement I, the percentage depth rate curve is uniquely determined with respect to radiation of a certain energy, so energy measurement is possible by monitoring the value on this percentage depth base oil line. It becomes.

Obtaining the dose signals at dl and d21 and calculating the ratio of the two means measuring points on the 1% deep base oil line (equal to the dose D), as is clear from equation (1).
1. If the energy remains constant even if D2 changes, the ratio DR
is always a constant value, and as is clear from FIGS. 3 and 4, when the energy changes, the value of the ratio DR in equation (1) changes.

Therefore, the value of the ratio DR can be regarded as an amount representing the energy of radiation, making it possible to monitor the energy.

This situation is shown in FIGS. 5a and 5bl.

The curve W in FIG. 5a shows the case of X-rays, and the curve W of FIG. 5S shows the case of electron beams.

In this figure, the ratio DR becomes a decreasing function in a region where the energy is low, and the ratio DR becomes an increasing function in a region where the energy is higher than a certain value.

The energy of radiation obtained by accelerating an electron beam is usually 3
There is a lot of energy above M e V, and the ratio DR in Figure 5
This corresponds to a region where DR is an increasing function, and in this region, the relationship between the ratio DR and energy is uniquely determined, so energy monitoring according to the present invention 1 can be performed.

When calculating the ratio DR in equation (1) from the electric circuit,
By moving either the dose D1 or D2 curve in parallel along the vertical axis of the graph in Figure 3 or Figure 4, the magnitude of the arbitrary ratio DR can be determined, and the energy of the ratio DFL can be determined. It is possible to find the relationship where the slope is the steepest.

FIG. 6 is a block diagram showing an embodiment of the present invention based on the above principle.

In FIG. 6, 20 is a voltage conversion and amplification circuit for the ionization current in the primary chamber 9, and 21 is a secondary chamber 10.
An amplifier circuit 22 has the same function as the amplifier circuit 20, and 22 is a level converter for parallelly shifting the output level of the amplifier circuit 20, on the output side of the amplifier circuit 21. This connection may also be made.

Reference numeral 23 is a division circuit for calculating the ratio of the output of the level converter 22 from the amplifier circuit 21, and an amplifier circuit if necessary, and in the case of X-rays or electron beams, 1 can adjust the output level as appropriate. do.

24 is a meter or similar display system for displaying energy.

According to this configuration, the ratio of the ionizing currents in the primary chamber 9 and the secondary chamber 10 can be obtained, so that the radiation energy can be measured from that value.

Recently, the use of two transmission type ion chambers in medical linear detectors has been increasing, but medical linear detectors such as the one in this invention, as well as medical Betatrons, are also used in non-destructive testing equipment. Of these, radiation generating devices with the function of accelerating electrons] can be widely used.

As explained in detail above, this invention uses two transmission type ion chambers, one for dose measurement, installed in a radiation generator, and calculates the energy of the radiation by taking the ratio of the measured ionization currents. Since the radiation energy can be easily monitored, it is useful for detecting abnormalities and the radiation generator can be used with peace of mind.

In addition, in the past, it was not possible to measure the radiation energy at any time while generating radiation, so it was not possible to know important parameters during operation, and it was not possible to understand the characteristics of the machine, but this invention 1. For example, it is possible to constantly monitor the energy of radiation.

Therefore, innovative effects such as easy establishment of a control method for stabilizing energy can be obtained.

[Brief explanation of drawings]

Figure 1 is a schematic configuration diagram showing an example of a conventional medical electron accelerator, Figures 2a and b are sectional views and plan views showing details of the transmission type ion chamber in Figure 1, and Figure 3a is a Figure 3 is a graph showing the depth rate characteristic of X-rays at underwater depth; Figure 4a is a diagram showing the relationship between the percentage of X-ray energy and the depth dose; Figure 4a;
b is a diagram similar to Figures 3a and b in the case of electron beams, Figure 5a
6 is a diagram showing the ratio of the outputs of two transmission type ion chambers to the X-ray energy l, and FIG. 6 is a block diagram showing an embodiment of the present invention. In the figure, 9 is the primary chamber, 10 is the secondary chamber, 2
0 is a voltage conversion and amplification circuit, 21 is an amplification circuit, 22 is a level converter, 23 is a division circuit, and 24 is a display system. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Claims] [1] A radiation generating device that accelerates an electron beam to generate radiation and is equipped with two transmission-type ion chambers with a double safety system] where the center of the radiation flux of the radiation generating device is A radiation energy monitor characterized by comprising a circuit for obtaining radiation energy by calculating the ratio of the ionization currents of the two independent ion chambers installed on the axis.