JP2002365317A - Electrometer circuit for ionization chamber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent reduction in measurement accuracy which results from discharge path of accumulated charges in an electrometer circuit for ionization chamber, for integrating an input current to output voltage. SOLUTION: As the path for discharging an integrator 24 charged with the current from an ionization chamber detector 20, a first discharge path formed of a contact type electromagnetic relay 26 and a second discharge route formed of the serial connection of an electromagnetic relay 28 and a transistor switch 30 are provided in parallel to each other. The first discharge path has the characteristic of causing no leakage current, and the second discharge path can ignore the discharge period forming a dead time by the high-speed electronic operation of a semiconductor. At a low dose rate, where the influence of leakage current can be increased with a long charging time, the electromagnetic relay 28 is set to OFF, and charging and discharging is switched by the operation of the electromagnetic relay 26. At a high dose rate, where the use of a switch having a short dead time is suitable with a long charging period, the electromagnetic relay 26 is set to OFF, and the electromagnetic relay 28 is set to ON for switching the charging and discharging by the operation of the transistor switch 30.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ionization chamber electrometer circuit for converting a current signal from an ionization chamber into a voltage signal, and to an improvement in measurement accuracy over a wide range from a low dose to a high dose.

[0002]

2. Description of the Related Art One of radiation measuring instruments is an ionization chamber. The ionization chamber measures the amount of incident radiation based on the amount of gaseous ions generated by the ionizing action of the radiation. The ions generated in the gas enclosed in the ionization chamber are collected by a voltage-applied electrode, and the amount of ions, and thus the amount of incident radiation, is measured from the resulting current.

[0003] Since the ionization chamber does not utilize current amplification due to secondary ionization, the resulting ion current is very small. An electrometer is used to measure this minute current.

FIG. 4 is a circuit diagram showing a conventional electrometer circuit for an ionization chamber. The current input from the ionization chamber detector 2 to the electrometer circuit 3 is accumulated in the integrator 4, and the integrator 4 outputs a voltage corresponding to the integrated value of the current. In the integrator 4 shown in the figure, a capacitor 8 is arranged in a feedback circuit of an operational amplifier 6. The input current to the integrator 4 charges the capacitor 8, and an output voltage corresponding to the voltage between the terminals of the capacitor 8 is obtained.

[0005] As the charging proceeds, the capacitor 8 saturates, and the linearity between the input current and the output voltage is lost. Therefore, the switch 8 is operated such that the capacitor 8 is discharged and reset as appropriate, and the integration is performed within a range where the linearity of the output of the integrator 4 is maintained. That is, when the switch 10 is turned on, the capacitor 8 is once discharged, and when the switch 10 is turned off, charging of the capacitor 8 is started again.

A switch 10 for controlling the charge and discharge of the integrator
, A mechanical relay or a transistor switch is used.

[0007]

During the period τ _ON in which the switch is turned on to discharge the capacitor, the input current from the ionization chamber is not integrated, and this period is a dead time for radiation measurement. Since a contact-type switch whose contacts are opened and closed mechanically has a relatively low operating speed, if it is used as a switch for charge / discharge control, the dead time becomes long.
FIG. 5 is a schematic graph showing a time change of an output voltage of a conventional electrometer circuit using a contact switch as a charge / discharge control switch. FIG. 5A shows a case where the incident dose rate to the ionization chamber detector is low, and FIG. 5B shows a case where the incident dose rate is high. In the case of a low dose rate, the reset period τ _L which is the interval between the timings at which the capacitors are reset is long, and the ratio of the dead time τ _ON to the capacitor charging period is relatively small. Therefore, in this case, the dead time due to the contact switch does not matter so much. On the other hand, in the case of a high dose rate, the reset period τ _H becomes short. Since the dead time τ _ON is irrelevant to the level of the dose rate, in the case of a high dose rate, there is a problem that the ratio of the dead time to the measurement time becomes so large that it cannot be ignored.

On the other hand, a transistor switch has a high operating speed, and its dead time can be ignored compared to a contact switch. FIG. 6 is a schematic graph showing a time change of an output voltage of a conventional electrometer circuit using a transistor switch as a charge / discharge control switch. FIG. 6A shows the case where the incident dose rate to the ionization chamber detector is low, and FIG. 6B shows the case where the incident dose rate is high. As shown in the figure, in the configuration using the transistor switch, the problem of dead time is avoided. But,
In a transistor switch, leakage current in an off state poses a problem. That is, even if the switch is turned off, the current flowing through the switch does not become completely zero, and the discharge proceeds while the integrator integrates the input current, which causes a measurement error of radiation. The effect of discharge due to leakage current can be ignored when the input current from the ionization chamber detector is large and the dose is high, but when the input current is small and the dose is low, the leakage current relative to the input current can be ignored. There is a problem that the ratio increases and the measurement error increases. Also,
The leakage current of a transistor switch fluctuates with temperature, which makes it more difficult to handle measurements at low dose rates.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and prevents a decrease in measurement accuracy due to a dead time or a leak current caused by a charge / discharge control switch, thereby enabling highly accurate measurement. An object of the present invention is to provide an electrometer circuit for an ionization chamber.

[0010]

According to the present invention, there is provided an electrometer circuit for an ionization chamber, wherein a charge / discharge control means is provided on a discharge path of an integrator, and includes a plurality of types of discharge path switches having different operation characteristics. Discharge path switching means for selecting one of the discharge path switches in accordance with the level of the input current from the ionization chamber and serving as a charge / discharge control switch used for controlling the charge / discharge operation.

According to the present invention, the discharge path does not necessarily have to be a single path, but may include a plurality of paths. The discharge path is provided with a plurality of types of discharge path switches having different operation characteristics. The input current to the integrator depends on the dose rate of the radiation incident on the ionization chamber. In the present invention,
The charge / discharge control switch is changed according to the level of the input current. As the charge / discharge control switch for each input current level, a switch having operation characteristics suitable for the corresponding input current level is selected from the discharge path switches. At a predetermined input current level, whether the integrator is charged or discharged is switched by turning on / off the charge / discharge control switch corresponding to the input current level. The discharge path switching means includes a path switch for specifying a discharge path to be used for discharge when there are a plurality of discharge paths, in order to enable control of the charge / discharge operation by the charge / discharge control switch. be able to. The discharge path switching means selects some of the discharge path switches as charge / discharge control switches according to the input current level, and sets other discharge path switches and The on / off state of the path change switch is controlled. The determination of the level of the input current may be performed by a separately provided input current level determination circuit, or may be determined by a measurer according to the measurement target. The discharge path switching means may be configured to automatically execute a discharge path switching operation in response to a result of the determination by the input current level determination circuit, or to be configured so that the discharge path is switched by a measurer's operation. Can also. The level of the input current is, for example,
It can be defined as multiple current ranges. Also,
A plurality of charge / discharge control switches can be determined for one input current level. The operation characteristics of the discharge path switch include, for example, switching operation speed, leakage current,
There are temperature characteristics and the like.

In another ionization chamber electrometer circuit according to the present invention, the first discharge path switch is a contact type switch for interrupting the discharge path by opening and closing a contact, and the second discharge path switch. Is a semiconductor switch that is arranged in parallel with the first discharge path switch and interrupts the discharge path by electronic operation, wherein the discharge path switching means is connected in series to the second discharge path switch. Path change switch.

According to the present invention, at least two types of discharge path switches, a contact type switch and a semiconductor switch, are provided, each of which can be selected as a charge / discharge control switch. While the contact type switch has a relatively low operation speed, it has an operation characteristic that there is no leakage current when off, while a semiconductor switch has an operation characteristic that it can generate a leakage current when off but has a high operation speed. In the present invention, these two types of discharge path switches are arranged in parallel, and a first discharge path in which a first discharge path switch is disposed, and a second discharge path in which a second discharge path switch is disposed. Exist in parallel. The first discharge path switch, which is a contact switch, is used as a charge / discharge control switch, and the second discharge path needs to be cut off so that the charge / discharge of the integrator is controlled by this switch. This second
In order to cut off the discharge path, a contact-type path change switch is provided in series with the second discharge path switch. By turning off the path changeover switch, the leakage current of the second discharge path switch in the second discharge path is cut off. On the other hand, when the second discharge path switch is a charge / discharge control switch, the contact switch serving as the first discharge path switch is turned off.

In a preferred aspect of the present invention, the discharge path switching means turns off the path switch when the input current is low and selects the first discharge path switch as the charge / discharge control switch. An electrometer circuit for an ionization chamber, characterized in that at a high input current, the path changeover switch is turned on, and the second discharge path switch is selected as the charge / discharge control switch.

In another ionization chamber electrometer circuit according to the present invention, the first discharge path switch is a contact type switch for interrupting the discharge path by opening and closing a contact, and the second discharge path switch. Is a semiconductor switch that is arranged in series with the first discharge path switch, and that switches on and off the discharge path by electronic operation.

According to the present invention, at least two types of contact path switches and semiconductor switches are provided as discharge path switches, and each of them can be selected as a charge / discharge control switch. In the present invention, these two types of discharge path switches are arranged in series on one discharge path.
The first discharge path switch, which is a contact type switch, is used as a charge / discharge control switch, and the charge / discharge of the integrator is controlled by this switch. The discharge path is maintained and the first discharge path switch interrupts the discharge path. Conversely,
The second discharge path switch, which is a semiconductor switch, is a charge / discharge control switch, and the charge / discharge of the integrator is controlled by this switch. It is maintained, and the discharge path is interrupted by the second discharge path switch.

In a preferred aspect of the present invention, the discharge path switching means turns on the second discharge path switch when the input current is low, and uses the first discharge path switch as the charge / discharge control switch. And selecting the second discharge path switch as the charge / discharge control switch at the time of selection and at a high input current, and selecting the second discharge path switch as the charge / discharge control switch. .

A preferred aspect of each of the above-described present inventions is an electrometer circuit for an ionization chamber, wherein the integrator is configured using an operational amplifier.

[0019]

Next, embodiments of the present invention will be described with reference to the drawings.

[First Embodiment] FIG. 1 is a circuit diagram showing an ionization chamber electrometer circuit according to a first embodiment of the present invention. FIG. 1 shows an ionization chamber detector 20 and an electrometer circuit 22 connected thereto.

The ionization chamber detector 20 outputs a current according to the dose rate of the incident radiation. Electrometer circuit 2
2 integrates a minute input current from the ionization chamber detector 20 and outputs a voltage signal corresponding to the integrated value. The output voltage signal is used in a subsequent signal processing circuit (not shown) for obtaining a radiation evaluation amount such as a dose rate. The electrometer circuit 22, as described below,
The operation mode is switched so that suitable measurement is performed depending on whether the radiation incident on the ionization chamber detector 20 has a low dose rate or a high dose rate.

The electrometer circuit 22 includes the integrator 2
4, electromagnetic relays 26 and 28, transistor switch 3
0, including the control unit 32.

The basic configuration of the integrator 24 is as follows.
In which a capacitor 42 is arranged in the feedback circuit. The input current from the ionization chamber detector 20 is stored in the capacitor 42.
The integrator 24 outputs to the output terminal 34 a voltage corresponding to the amount of charge accumulated in the capacitor 42, that is, an integrated value of the input current. The configuration of the integrator 24 using the operational amplifier 40 is convenient for measuring a small current from the ionization chamber detector 20 in that an amplified voltage signal is obtained as an output.

The electromagnetic relay 26 is connected between the input terminal and the output terminal of the integrator 24 and forms a first discharge path for discharging the electric charge stored in the capacitor 42.

The transistor switch 30 and the electromagnetic relay 2
8 are connected in series with each other, and these
Is connected in parallel with the input terminal and the output terminal of the integrator 24. The path formed by connecting the transistor switch 30 and the electromagnetic relay 28 in series constitutes a second discharge path for discharging the electric charge stored in the capacitor 42.

The control section 32 supplies an electric signal to the electromagnetic relays 26 and 28 and the transistor switch 30 to control the on / off operation thereof. Specifically, the control unit 3
2, a mode switching control signal 50 from the outside is input, and in accordance with the mode switching control signal 50, a low dose rate operation mode suitable for low dose rate measurement and a high dose rate suitable for high dose rate measurement are provided. The operation mode of the electrometer circuit 22 is switched to one of the rate operation modes.

Here, the mode switching control signal 50 is generated, for example, by a measurer switching a mode selection switch to select a desired operation mode. Further, a configuration is also possible in which a determination circuit for determining which operation mode is appropriate based on an output signal from the output terminal 34 is provided, and switching to an appropriate operation mode is automatically performed.

FIG. 2 is a schematic graph showing the time change of the output voltage of the present electrometer circuit. FIG. 2 (a)
Shows a low dose rate operation mode, and FIG. 4B shows a high dose rate operation mode.

First, the low dose rate operation mode of the electrometer circuit 22 will be described with reference to FIGS. 1 and 2A. When the low dose rate operation mode is instructed, the control unit 32 turns off the electromagnetic relay 28. Although a leak current may flow even when the transistor switch 30 is turned off, the second discharge path including the transistor switch 30 is reliably shut off by turning off the electromagnetic relay 28. In this state, the control unit 32 controls the charging and discharging of the capacitor 42 by turning on / off the electromagnetic relay 26 to periodically interrupt the first discharge path.

When the electromagnetic relay 26 is turned on,
The electric charge accumulated in the capacitor 42 passes through the first discharge path.
And output from the electrometer circuit 22.
Pressure V _OUTBecomes 0 (t = t_E1). This discharge is basically
Instantaneous, and that the electromagnetic relay 26 is
The interval is a dead time for radiation measurement,
ON period τ of relay 26_ONIs controlled to be as short as possible.
It is. However, the electromagnetic relay 26 is a contact switch.
And the operating speed is relatively slow, the time t_E1Once on
Next time t_S1Before turning off at
Constant ON period τ_ONIs required. Electromagnetic relay 26 is off
In the state, the input current from the ionization chamber detector 20
The charging of the capacitor 42 is started (t = t_S1). sand
That is, the off period is the charging period of the capacitor 42. This
Charging period τ_OFFIs, for example, the integrated value of the input current and the output
Pressure V_OUTIs set within a range where the linearity with is maintained. Filling
Charging period τ_OFF, The electromagnetic relay 26 turns on again.
And the capacitor 42 is discharged, and the output voltage
V_OUTIs reset to 0 (t = t_E2). Control unit 32
Indicates that the operation mode is set to low dose rate operation mode
During this time, the on / off operation of the electromagnetic relay 26 is repeated.
return. During this time, the electromagnetic relay 28 was kept off.
Remains.

At a low dose rate such as when measuring the background, the input current to the integrator 24 is low, and the charging period is relatively long. Therefore, even if the charge leak from the capacitor 42 is slight per unit time, the amount of charge lost during the charging period increases, and the measurement error of radiation increases. In this regard, the transistor switch 30 that can flow the leak current even in the off state is not suitable for the charge / discharge control of the capacitor 42 in the low dose rate operation mode. On the other hand, the electromagnetic relay 26 has a very small leak current and is suitable for the low dose rate operation mode. As described above, although the electromagnetic relay 26 generates a dead time, in the low dose rate operation mode, the charging period is generally set to be much longer than the dead time, so that the influence of the dead time is small. Therefore, even if there is a dead time, the electromagnetic relay 26 can be used in the low dose rate operation mode.

Next, the high dose rate operation mode of the electrometer circuit 22 will be described with reference to FIGS. 1 and 2B. When the high dose rate operation mode is instructed, the control unit 32 turns off the electromagnetic relay 26 and turns on the electromagnetic relay 28. By turning off the electromagnetic relay 26, the first discharge path is cut off. In this state, the control unit 32 turns on / off the transistor switch 30 to intermittently interrupt the second discharge path, and controls charging and discharging of the capacitor 42.

When the transistor switch 30 is turned on, the electric charge accumulated in the capacitor 42 until that time is discharged through the second discharge path, and the electric charge is stored in the electrometer circuit 22.
Output voltage V _OUT from is reset to 0 (t =
t _R1 ). Since the operation speed of the transistor switch 30 is orders of magnitude faster than that of the contact switch, the ON period of the transistor switch 30 can be made negligibly shorter than the _ON period τ _ON of the electromagnetic relay 26. That is, the transistor switch 30 can be turned off again and the next charging period τ ′ _OFF can be started with almost no dead time. The charging period τ ′ _OFF is set, for example, in a range in which the linearity between the integrated value of the input current and the output voltage V _OUT is maintained, similarly to the charging period τ _OFF in the low dose rate operation mode. When the charging period τ ′ _OFF has elapsed, the transistor switch 30 is turned on again, the capacitor 42 is discharged, and the output voltage V _OUT is reset (t =
t _R2 ). The control unit 32 repeats the on / off operation of the transistor switch 30 while the operation mode is set to the high dose rate operation mode. During this time, the electromagnetic relay 26 is kept off, and the electromagnetic relay 28 is kept on.

At a high dose rate, ie, at a high input current,
The charging period is shorter than at low dose rates. Therefore, if the dead time during discharging of the capacitor 42 in the measurement at the high dose rate is the same as the dead time in the low dose rate operation mode, the ratio of the dead time in the measurement period becomes large, and the measurement error of radiation becomes large. Become. The electromagnetic relay 26 having a low operation speed is not suitable for charge / discharge control of the capacitor 42 in the high dose rate operation mode in this regard. On the other hand, the transistor switch 30 can be operated at a very high speed and hardly causes a dead time, and thus is suitable for the high dose rate operation mode. Note that, as described above, the transistor switch 30
In the high dose rate operation mode, the influence of this leakage current is small since the capacitor 42 discharges due to a minute leakage current even during the charging period.
Therefore, even if there is a leak current, the transistor switch 30 can be used in the high dose rate operation mode.

[Second Embodiment] FIG. 3 is a circuit diagram showing an ionization chamber electrometer circuit according to a second embodiment of the present invention. In FIG. 3, the same components as those in the first embodiment are denoted by the same reference numerals, and the description will be simplified. FIG. 3 shows the ionization chamber detector 20 and the electrometer circuit 100 connected thereto.

The electrometer circuit 100 includes the first
As in the case of the electrometer circuit 22 of the embodiment, the operation mode can be switched so that suitable measurement is performed depending on whether the radiation incident on the ionization chamber detector 20 has a low dose rate or a high dose rate. Has features.

The discharge path of the electrometer circuit 100 is different from that of the first embodiment in that the discharge path is composed of an electromagnetic relay 26 and a transistor switch 30 connected in series. Accordingly, the operation of the control unit 102 for controlling these switches is also different from that of the control unit 32 of the first embodiment.

The control unit 102 receives a mode switching control signal 50 from the outside, controls the on / off operation of the electromagnetic relay 26 and the transistor switch 30 in accordance with the mode switching control signal 50, and controls the electrometer circuit 100.
Is switched between the low dose rate operation mode and the high dose rate operation mode.

The change over time of the output voltage in each operation mode of the electrometer circuit is basically the same as that in FIG. When the low dose rate operation mode is instructed, the control unit 102 turns on the transistor switch 30. In this state, the control unit 102 turns on / off the electromagnetic relay 26,
As a result, the discharge path is periodically interrupted and the capacitor 4
2 is controlled.

When the high dose rate operation mode is instructed, the control unit 102 turns on the electromagnetic relay 26. In this state, the control unit 102 turns on / off the transistor switch 30, whereby the discharge path is periodically interrupted,
The charging and discharging of the capacitor 42 is controlled.

[0041]

According to the electrometer circuit for an ionization chamber of the present invention, a plurality of discharge paths capable of controlling charging and discharging of the integrator are provided by using switches having different operation characteristics. Then, a discharge path is selected according to the level of the input current from the ionization chamber. This allows highly accurate measurements over a wide range of dose rates.

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram showing an ionization chamber electrometer circuit according to a first embodiment of the present invention.

FIG. 2 is a schematic graph showing a time change of an output voltage of the present electrometer circuit.

FIG. 3 is a circuit configuration diagram showing an ionization chamber electrometer circuit according to a second embodiment of the present invention.

FIG. 4 is a circuit configuration diagram showing a conventional electrometer circuit for an ionization chamber.

FIG. 5 is a schematic graph showing a time change of an output voltage of a conventional electrometer circuit using a contact switch as a charge / discharge control switch.

FIG. 6 is a schematic graph showing a time change of an output voltage of a conventional electrometer circuit using a transistor switch as a charge / discharge control switch.

[Explanation of symbols]

Reference Signs List 20 ionization chamber detector, 22, 100 electrometer circuit, 24 integrator, 26, 28 electromagnetic relay, 30
Transistor switch, 32, 102 control unit, 40
Operational amplifier, 42 capacitors, 50 mode switching control signal.

Claims (6)

[Claims] 1. An integrator for accumulating an input current from an ionization chamber and outputting a voltage corresponding to an integrated value of the input current, and a charge / discharge control switch provided on a discharge path of the integrator to operate the integrator. A charge / discharge control means for controlling a charge / discharge operation of an integrator, wherein the charge / discharge control means is provided in the discharge path, and a plurality of types of discharge path switches having different operation characteristics are provided. And a discharge path switching means for selecting any one of the discharge path switches according to the level of the input current to be the charge / discharge control switch, and an electrometer circuit for an ionization chamber. 2. The electrometer for an ionization chamber according to claim 1, wherein the first discharge path switch is a contact-type switch that interrupts the discharge path by opening and closing a contact, and the second discharge path switch is A semiconductor switch that is disposed in parallel with the first discharge path switch and that interrupts the discharge path by an electronic operation, wherein the discharge path switching means is a contact type switch connected in series to the second discharge path switch. An electrometer circuit for an ionization chamber, comprising a path changeover switch. 3. The electrometer for an ionization chamber according to claim 2, wherein the discharge path switching means turns off the path change switch when the input current is low, and charges and discharges the first discharge path switch. A control switch, wherein at a high input current, the path changeover switch is turned on, and the second discharge path switch is selected as the charge / discharge control switch. An electrometer circuit for an ionization chamber. 4. The electrometer for an ionization chamber according to claim 1, wherein the first discharge path switch is a contact-type switch that interrupts the discharge path by opening and closing a contact, and the second discharge path switch is An electrometer circuit for an ionization chamber, wherein the semiconductor switch is arranged in series with the first discharge path switch and interrupts the discharge path by electronic operation. 5. The electrometer for an ionization chamber according to claim 4, wherein said discharge path switching means turns on said second discharge path switch when the input current is low, and switches said first discharge path switch. Selecting the charging / discharging control switch, turning on the first discharging path switch when the input current is high, and selecting the second discharging path switch as the charging / discharging control switch. Electrometer circuit for boxes. 6. The electrometer circuit for an ionization chamber according to claim 1, wherein the integrator is configured using an operational amplifier.