JPS63201586A - Ion chamber type discriminating device for radiation energy quantity - Google Patents

Abstract

PURPOSE:To measure a very minute electric charge quantity by connecting a capacitor provided with an electric charge charging and discharging switch to the input terminal of an amplifier to evaluate a floating capacity in an amplifier feedback capacitor. CONSTITUTION:A self-holding relay switch S1 connected in parallel with an amplified 2 connected to an ion chamber collector electrode 1a is periodically opened and closed. While the switch S1 is opened, an electric charge charging relay switch S2 is closed to charge an electric charge charging capacitor Ci from a power source Ei, and simultaneously, a parallel resultant capacity (fedback capacitor CS+floating capacity CSX) is charged, and the charging voltage of an output terminal 3 is measured to evaluate the floating capacity CSX. When the switch S1 is closed, a switch S3 is closed to discharge the capacity. Since opening and closing of switches S2 and S3 are controlled to evaluate the floating capacity in this manner, measurement is performed with a high precision and a high sensitivity even if the capacity value of the feedback capacitor is reduced.

Description

[Detailed description of the invention] 【overview】

An amount of charge equal to the amount of ionized charge generated in the ionization chamber by radiation incidence is induced in a charge amplifier directly connected to the chamber, such as a feedback capacitor, and a self-holding relay switch connected in parallel to the capacitor is periodically opened and closed. A device that discriminates the energy amount of each incident radiation by actually measuring the integral transition of the charging voltage of the capacitor at each predetermined period, and which switches a charge injection switch and a discharge switch connected in parallel to the capacitor. By controlling opening/closing at predetermined timing using the opening/closing control circuit, static or dynamic stray capacitance that inevitably exists in parallel with the feedback capacitor can be absolutely eliminated, regardless of the charge measurement period or non-measurement period. By evaluating this, we have developed an ionization chamber-type radiation energy system that can always guarantee high-accuracy, high-sensitivity measurements even when the capacitance of the feedback capacitor is set to a very small value equal to or less than the stray capacitance. A quantity discriminator is disclosed in detail below.

[Industrial application field]

The present invention uses an ionization chamber as a detector that performs ionization due to incident radiation and captures the ionized charge, and measures the amount of ionized charge corresponding to the energy amount of the incident radiation, thereby determining the energy amount of each incident radiation with high accuracy. The present invention relates to an ionization chamber-type radiation energy discriminator that is suitable for measuring minute amounts of electricity with high sensitivity and precision.

[Conventional technology]

Conventionally, as shown in FIG. 3, this type of device has an ionization chamber 1, an inverting input terminal 2a connected to a collector electrode 1a of the ionization chamber 1, and a charge collector having a feedback capacitor Cs (electrostatic capacitance Cs). A device consisting of an amplifier 2 and a relay switch S connected in parallel to a feedback capacitor C8 is known. In this ionization box type radiation energy amount discriminator, the collector electrode 1a captures the ionized charges generated in the electron 11 due to the incidence of radiation, and the amount of charge equivalent to the amount of ionized charges QX is induced in the feedback capacitor Os. By extracting the charging voltage Vs++Qx10s from the output terminal 3 of the charge amplifier 2, the energy amount of each incident radiation (α ray, γ ray, etc.) can be discriminated.

[Problems to be Solved] However, the conventional ionization chamber-type radiation energy amount discriminator described above has the following problems. Namely, the amount of ionized charge Q! When measuring with high sensitivity, if the capacitance value of the feedback capacitor C$ is reduced,
As the charging voltage Vs becomes larger, highly sensitive measurement of minute charges is expected, but in fact, in the minute charge measurement region (approximately 10 coulombs or less), a feedback capacitor Cs inevitably exists. Parallel stray capacitance Csx (shown by a broken line) cannot be ignored, and the smaller the capacitance of feedback capacitor Cs, the more pronounced it becomes.
．． (It is said to be on the order of 5 pF),
The true charging voltage Vs is given as Qx/(Cs+Csx), which is the actual stray capacitance Cs! The more you try to perform micro-charge measurements, the more the accuracy of energy discrimination will deteriorate, and especially in the micro-charge measurement area, high-sensitivity measurement and high-precision measurement are antithetical. becomes.

[Purpose of the invention]

The present invention solves the above-mentioned problems, and its purpose is to provide an ionization chamber-type radiation energy discriminator that can measure with high sensitivity and precision even for minute charges, that is, incident radiation with a low energy amount. It's about doing. Solution to Problem C] In order to solve problem E, the configuration of the ionization chamber type radiation energy amount discriminator according to the present invention has the following configuration requirements. ■There must be an ionization chamber. ■There must be a charge amplifier whose input terminal is directly connected to this collector electrode. Here, the term "charge amplifier" usually includes a device having a feedback capacitor, but may not include a feedback capacitor. ■There is a self-holding relay switch connected in parallel to the charge amplifier. ■There is a charge injection capacitor with one end connected to the input terminal. ■There must be a charge injection switch connected to the other end of the charge injection capacitor. ■There must be a discharge switch connected to the other end of the charge injection capacitor. ■Switch opening/closing control circuit that periodically controls the opening and closing of the relay switch, closes the charge injection switch during the opening period, and closes the discharge switch during the opening/closing period of the relay switch. That there is.

[Effect]

Due to ionization in the ionization chamber due to the incidence of radiation, ionization charges corresponding to the energy of the incident radiation are collected on the electrodes of the ionization chamber, and an amount of charge equivalent to the amount of charge is collected in the parallel composite capacitor (
The combined capacitance consisting of the feedback capacitor and the stray capacitance in parallel or only the stray capacitance) is inductively charged, and the charge voltage proportionally amplified to the amount of charge appears at the output terminal of the charge amplifier as it is. The charging voltage increases linearly, or stepwise in the case of pulsed γ-rays or α-rays, but when the self-holding relay switch is closed under the control of the switch opening/closing control circuit, the charge in the parallel combined capacitor increases quickly. The output terminal of the charge amplifier recovers to zero voltage. A series of such integral measurements are carried out.1 For example, when there is no incident radiation (such as when no radiation is present or when the connection between the input terminal of the charge amplifier and the collector electrode of the ionization chamber is broken), , when the charge injection switch is closed during the opening period of the relay switch, the charge injection capacitor is charged instantaneously, and the amount of charge (
Since a charge equivalent to the product of the capacitance of the charge injection capacitor and its applied voltage is induced in the parallel combined capacitor,
An incremental voltage due to charge injection will appear at the output terminal of the charge amplifier. Based on this incremental voltage measurement,
The value of the stray capacitance can be evaluated. Further, when the relay switch is closed, the charges in the parallel combined capacitance are discharged, but within this closing period, the discharge switch is closed and the charge injection capacitor is also rendered uncharged.

【Example】

Next, one embodiment of the present invention will be described based on the drawings. (Configuration of the embodiment) Fig. 1 is a circuit diagram showing an embodiment of the ionization chamber type radiation energy discriminator according to the present invention. It is a gridless ionization chamber that captures the ionization, and is composed of a collector electrode 1a and a high voltage electrode 1c connected to a high voltage power supply tb. 2 is a charge amplifier composed of an operational amplifier, etc., and its inverting input terminal 2a is connected to the ionization chamber l. Its non-inverting input terminal 2b is grounded, and a feedback capacitor Os (static Note that as shown by the broken line, there is inevitably a stray capacitance Csx (electrostatic capacitance Csx) in parallel with this feedback capacitor Cs. This is a self-holding relay switch that is periodically driven to open and close by the control circuit 4, and discharges the charge of the feedback capacitor Cs.Ll opens and closes the self-holding relay switch S1 by excitation by an applied current pulse. It is a magnetic coil to be driven. Ci is a charge injection capacitor (capacitance Ci), one end of which is connected to the inverting input terminal 2a of the charge amplifier 2. S2 is a charge injection relay switch, Connected to the other end of the charge injection capacitor Ci.L2
is a magnetic coil that opens and closes the charge injection relay switch Ci by being excited by an applied current pulse. Ei is a charge injection power source (constant voltage V). These capacitor Ci, relay switch S2, and power source Ei constitute a charge injection circuit that induces and injects a predetermined amount of charge into the feedback capacitor Os. S3 is a discharge relay switch connected to the other end of the charge injection capacitor Ci. L3 is a magnetic coil that opens and closes the discharge relay switch by being excited by an applied current pulse. 4 is a switch opening/closing control circuit that controls opening/closing of the self-holding relay switch St, the charge injection relay switch S2, and the discharging relay switch S3 at predetermined timings. That is, the self-holding relay switch S1 is periodically (periodically) controlled to open and close according to the relationship between the opening period ts and the closing period tr. Furthermore, in this embodiment, after the charge injection relay switch S2 is set to the closed state within the opening period ts of the self-holding relay switch S1, it is returned to the open state before the self-holding relay switch S1 is closed. . Furthermore, after the discharge relay switch S3 is set to the closed state within the closing period tr of the self-holding relay switch S1, in this embodiment, the charge injection relay switch S2 is set to the closed state.
The opening state is restored before the closing point. Note that in the above embodiment, the chargeable ON switch S2 and the discharge relay switch S3 are also self-holding types, but the present invention is not limited thereto, and other switches may be used. <Operations and Effects of the Embodiment> Taking the energy amount discrimination of α-rays as an example, the functions and effects of the above embodiment will be explained.
As shown in the figure, the positive polarity pulse (the first
After it is set to the closed state with a pulse), the
It is quickly returned to the open state with a negative polarity pulse (second pulse), and is opened and closed at a cycle of a closing period tr and an opening time Ls (>>tr). During this opening period ts, due to the ionization phenomenon caused by the α rays entering the ionization chamber 1, ionized charges are collected on the collector electrode 1a, and the ionized charges for each α ray incident are accumulated, and the second As in the illustrated case of period n+1,
The charging voltage Vs changes stepwise. Note that in the case of pulsed γ-rays, the charging voltage Vs changes stepwise, but in the case of γ-rays, the charging voltage V8 changes linearly. Now, in order to evaluate the stray capacitance Csx, when the charge injection relay switch S2 is closed within the opening period ts of the self-holding relay switch S1, the charge injection Qi to capacitor Ci
=CiV is charged almost instantaneously, and a charge Qi is induced to be charged to the parallel composite capacitor (Cs+Csx) consisting of the feedback capacitor Cs and the stray capacitance Csx. As a result, the output terminal 3 has a period n shown in the second diagram.
As in the case of , a charging voltage Vo appears. By actually measuring this charging voltage Vo, it is possible to know the stray capacitance Cs value = (CiV/Vo) -0m. Therefore, when the measurement data is calibrated to take this stray capacitance into account,
The true amount of ionized charge Qx due to α rays is given by C1VVs/Vo. Next, when the discharging relay switch S3 is closed during the closing period tr of the self-holding relay switch St, that is, the charge discharging period of the feedback capacitor Cs and the stray capacitance Csx, the charge of the charge injection capacitor C1 is instantaneously This is to prevent the charge of the charge injection capacitor Ci from inducing a charge in the feedback capacitor Cs during the 0th period when the charge injection capacitor Ci is discharged.If the stray capacitance Csx is to be evaluated also in the 0th period, this discharging relay switch S3 is used. is opened at least before the charge injection relay switch S2 is closed. Note that since the charge injection is instantaneous, in the above embodiment the charge injection relay switch S2 is opened before the self-holding relay switch Sl is closed, but it is closed after the relay switch S3 is closed. You can make it happen.
This is because when the relay switch S3 remains in the closed state, the charge injection capacitor Ci is grounded, so even if there is ionized charge, no induced charge is generated in the charge injection capacitor C1. In this way, the stray capacitance Csx can be evaluated by controlling the opening and closing of the charge injection relay switch S2 and the discharge relay switch S3. Highly accurate and sensitive measurements can be performed with high reliability of approximately 10" to 10-
"'Coulomb minute charge measurement is now possible. In the case of E, the stray capacitance Csz was evaluated in advance before the energy discrimination measurement, but the stray fog * CSX can also be measured by charge injection during the measurement period. It is also possible to evaluate α
Due to the ionization caused by the wire, the charging voltage changes stepwise, and at the time when the self-holding relay switch Sl is closed, the voltage V
However, by closing the charge injection relay switch S2, the charge injection voltage Vo is superimposed, and when the release 2 switch SL is closed, the voltage V2=V1+
Reach Vo. Since the charging voltage Vt for each cycle due only to radiation ionization fluctuates around the statistical average value (vl), the charge injection voltage vO is obtained using V2-<IJI> and the stray capacitance Csx is evaluated. Although the above example is for discrimination of the energy amount of α-rays, it goes without saying that it can also be applied to the case of pulsed γ-rays, and the charging voltage Vs changes linearly. It goes without saying that it can also be applied to γ-rays. [Effects of the Invention] As explained above, the radiation energy amount discriminator according to the present invention connects a charge injection circuit to a charge amplifier, and A discharge switch is provided, charge is injected into the stray capacitance within the opening period of the self-holding relay switch, and after obtaining the charging voltage or its incremental voltage, the charge injection circuit is installed within the closing time of the self-holding relay switch. It has a first-order effect because it discharges the charge of Since it can be constantly evaluated, errors caused by stray capacitance can be calibrated against measurement data (charging voltage associated with ionization phenomena), making it possible to obtain highly accurate radiation energy discrimination. ■Charge When the capacitance value of the feedback capacitor of the amplifier is reduced, the charging voltage increases, and the stray capacitance component becomes non-negligible compared to the capacitance of the feedback capacitor. can also be evaluated, so even if the capacitance of the feedback capacitor is set to a small value and high sensitivity is set, high precision measurement is guaranteed. ■Especially for weak (low-frequency emission) radiation sources and radiation with low energy values. In this case, the amount of charge induced is minute (minimum charge measurement range), so it is effective when performing high-sensitivity measurement by setting the capacitance value of the feedback capacitor of the charge amplifier as small as possible. It is possible to measure the electric charge with high precision. ■Furthermore, since stray capacitance can be appropriately and absolutely evaluated, the capacitance of the feedback capacitor of the charge amplifier can be reduced to zero, that is, the feedback capacitor can be left unloaded. Since the feedback capacitance component can be reduced to only a small stray capacitance, it is possible to measure a very small amount of charge.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of an ionization chamber type radiation energy amount discrimination device according to the present invention. FIG. 2 is a timing chart showing current pulses and feedback capacitor charging voltages for each relay switch in the same embodiment. FIG. 3 is a circuit diagram showing an example of a conventional ionization chamber type radiation energy amount discrimination device. 1.Fist/Ionization chamber, 1a...Collector electrode, 1b+10.High voltage power supply °, 1c...High voltage electrode, 2...Charge amplifier, 2
a... Inverting input terminal, 2b... Non-inverting input terminal, 3
...-output terminal, Os...feedback capacitor Q31
Number * * Stray capacitance, 5L --- Self-holding relay switch, Ci... Capacitor for charge injection, S2... Relay switch for charge injection, Ei/Φ Power supply for charge injection, LL
, L2, L3-・φ magnetic coil, 4... switch opening/closing control circuit, ts... self-holding relay switch S
Opening time of l, tr...Self-holding relay switch Sl
Closing period.

Claims (1)

[Claims] An ionization chamber, a charge amplifier whose input terminal is directly connected to the collector electrode, a self-holding relay switch connected in parallel to the charge amplifier, a charge injection capacitor whose one end is connected to the input terminal, and the like. While periodically controlling the opening and closing of the charge injection switch and the discharge switch connected to the end of the relay switch, the charge injection switch is closed within the opening period, and the closing period of the relay switch is controlled. An ionization chamber type energy amount discriminator comprising a switch opening/closing control circuit for closing the discharge switch.