JP5829745B1 - Radiation measurement equipment - Google Patents

Abstract

A radiation measuring device capable of measuring an extremely low radiation dose rate with high accuracy by compensating for a dark current generated in a radiation detector and reducing an influence of noise generated by a circuit. A radiation measuring apparatus 1A according to the present invention includes a radiation detector 10 having a first electrode 101 and a second electrode 102, and charges generated by the radiation detector 10 in accordance with an incident radiation dose rate. A first power supply 21 that supplies a first voltage HT for reaching the first electrode to the second electrode 102, and negative ions and positive ions generated in the radiation detector 10 according to the incident radiation dose rate Are recombined to supply a second voltage dV to the second electrode 102 for minimizing the amount of charge flowing into the first electrode 101, the first power supply 2 or A switch 30 for selecting the second power supply 22 and connecting it to the second electrode 102 is provided. [Selection] Figure 1

Description

  The present invention relates to a radiation measuring apparatus that compensates for a dark current of a radiation detector and measures radiation with a low radiation dose rate (or radiation dose) with high accuracy.

  When investigating environmental pollution by radioactive materials, it is necessary to measure whether the radiation dose rate (hereinafter sometimes referred to as the dose rate) is within or exceeds the range that naturally exists in nature. . Furthermore, by accurately measuring changes in dose rate over time at the same measurement point, or by accurately measuring dose rates at many measurement points, it is possible to accurately and appropriately grasp changes and distribution of environmental pollution caused by radioactive substances. Therefore, it is necessary to judge decontamination or take measures to prevent contamination.

  In dose rate measurement, which is the basis for such judgments, low dose rates are increased to appropriately determine the impact on human health, etc., and to properly grasp changes over time and regional distribution of contamination. It is necessary to measure with accuracy. If the measurement accuracy of the low level dose rate is low, it becomes difficult to appropriately determine the presence or absence of contamination, determination of decontamination, evaluation of decontamination, and the like.

  However, since the radiation measuring apparatus amplifies a very weak output current of the radiation detector by a high gain amplifier circuit, it is affected by a dark current generated by the radiation detector or the like. This dark current is a very small current generated between each electrode of the radiation detector and the insulating member (strictly speaking, there are elements that generate a dark current in the peripheral circuit of the radiation detector, for example, a high gain amplifier circuit). You can.) In low-level dose rate measurement, the weak output current and dark current corresponding to the extremely low level dose rate cannot be distinguished, and an error occurs in the dose rate measurement. Occurs.)

  Therefore, for example, when measuring the dose rate in farmland where the criteria for radioactive contamination are severe, it is necessary to use a radiation measurement device that can measure the low dose rate with high accuracy by eliminating the influence of the dark current of the radiation detector. There is.

  As such a radiation measuring apparatus, a dark current compensation type ionization chamber current measuring device is known (Patent Document 1). This dark current compensation type ionization chamber current measuring device uses an ionization chamber as a radiation detector, and this ionization chamber has a collecting electrode for detecting negative ions (or electrons) generated by ionization due to radiation and dark current compensation. Terminal (electrode) for use (the box of the ionization chamber acts as a high voltage electrode). Here, the collector electrode outputs a current corresponding to the dose rate, and the dark current compensation terminal outputs a current independent of the dose rate, that is, a dark current. Therefore, this dark current compensation type ionization chamber current measuring device can compensate the dark current included in the output current of the collector electrode by the electronic circuit based on the output current of the dark current compensation terminal, and the dose rate is low. Even so, highly accurate measurement can be performed.

JP 2003-156563 A

  However, in the dark current compensation type ionization chamber current measuring instrument disclosed in Patent Document 1, the collector electrode and the dark current compensation terminal are separate components in the ionization chamber, and they are different in shape and structure, for example. The positional relationship with the high-voltage electrode, the capacitance, etc. are different.

  Therefore, it cannot be denied that there is a slight difference (error in dark current compensation) between the dark current generated in the collector electrode and the output current of the dark current compensation terminal. Further, the collector electrode and the dark current compensating terminal are different from each other in shape, capacitance and the like. Therefore, when the amplifier on the collector electrode side and the amplifier on the dark current compensation terminal side are compared, the pole generated due to the relationship between the amplifiers and their peripheral circuits (for example, the stray capacitance of the collector electrode and the dark current compensation terminal). Low level noise is different. Such error and noise differences in dark current compensation can be a source of error in the measurement of very low level radioactive contamination.

  These error factors can be even more problematic when measuring extremely low dose rates. This is because, when measuring a very low level of dose rate, it is necessary to increase the gain of the amplifier. However, if the gain of the amplifier is increased, errors and extremes in dark current compensation between the collector electrode and the dark current compensation terminal are required. This is because the low level noise difference increases as the gain increases.

  Therefore, the present invention improves the accuracy of compensation of dark current generated in the radiation detector and reduces the influence of noise that may occur due to the relationship between the electrode and the electronic circuit as much as possible, thereby achieving an extremely low radiation dose rate with high accuracy. The realization of a radiation measurement device that can be measured by the method was an issue.

  In order to solve the above problems, a radiation measuring apparatus according to the present invention includes a first electrode that outputs a current according to an incident radiation dose rate, and a second electrode to which a voltage for generating an electric field is applied. An electric field is generated between the radiation detector and the first electrode and the second electrode, and the electric charge generated in the radiation detector according to the incident radiation dose rate reaches the first electrode. And a first power source for supplying the first voltage to the second electrode (usually minus by setting the potential of the second electrode lower than the potential of the first electrode. Ions reach the first electrode). Therefore, the radiation measuring apparatus can obtain a current corresponding to the incident radiation dose rate from the first electrode of the radiation detector.

  Further, the radiation measuring apparatus generates an electric field between the first electrode and the second electrode, and negative ions generated between the first electrode and the second electrode according to the incident radiation dose rate. A second voltage that minimizes the amount of charge flowing into the first electrode by recombining (negatively charged particles) and positive ions (positively charged particles) is supplied to the second electrode. A power source, and a switch for selecting and connecting the first power source and the second power source to the second electrode.

  Therefore, the radiation measuring apparatus recombines negative ions and positive ions generated in the radiation detector according to the incident radiation dose rate by applying a second voltage to the second electrode by a switch. Thus, the current output from the first electrode can be minimized.

  That is, the current (dark current) output from the first electrode can be reduced to zero by recombining the negative ions and the positive ions so that the amount of charge reaching the first electrode is zero. Item 1).

  According to a second aspect of the present invention, the radiation measuring apparatus further includes a load impedance into which the current output from the first electrode flows, and an amplifier for obtaining an output corresponding to a voltage generated in the load impedance. The voltage or current corresponding to the radiation dose rate can be output.

  This amplifier is, for example, a high-gain DC amplifier (hereinafter sometimes referred to as an amplifier), and has an inverting input terminal, a non-inverting input terminal, and an output terminal. The load impedance is connected between the inverting input terminal and the output terminal of the amplifier, and the non-inverting input terminal of the amplifier is connected to a point that becomes a reference potential (for example, ground point), and the amplifier constitutes a negative feedback amplifier circuit.

  Therefore, the higher the input impedance of the amplifier, the more the current output from the first electrode flows into the load impedance, and the higher the gain of the amplifier, the smaller the potential difference between the inverting input terminal and the non-inverting input terminal. Become. That is, the potential of the first electrode is maintained substantially the same as the potential of the non-inverting input terminal of the amplifier (for example, the ground potential), and the current output from the first electrode flows into the load impedance, and the output voltage of the amplifier Is equal to the voltage generated in the load impedance by the current output from the first electrode.

  According to a third aspect of the present invention, the radiation measurement apparatus includes an offset adjuster for adjusting an offset of the amplifier, thereby adjusting a direct current offset voltage (or current) of the direct current amplifier so that the radiation measurement is performed. The zero point display adjustment (DC offset adjustment) of the display provided in the apparatus can be performed.

  As described in claim 4, the radiation measuring apparatus can use an ionization chamber as a radiation detector. In this case, the first electrode is a collector electrode of the ionization chamber, and the second electrode is a high voltage electrode of the ionization chamber.

  The radiation measuring apparatus according to the present invention having the above-described configuration adjusts the second voltage applied to the second electrode to make the influence of the dark current zero (compensating for the influence of the dark current). Since the dark current included in the output current of the first electrode can be compensated, and this dark current compensation is performed in the same state as the dose rate measurement, the dark current compensation adjustment and the dose rate measurement There is no difference in noise generated by an amplifier circuit or the like, and high-accuracy measurement can be performed even at an extremely low radiation dose rate.

It is a figure which shows schematic structure of one Example in the radiation measuring device concerning this invention. It is a figure which shows schematic structure of the display circuit part with which the radiation measuring apparatus shown in FIG. 1 is provided. It is a figure which shows the example of the characteristic (electric field vs. collector electrode current characteristic) of the output current of a collector electrode with respect to the electric field between the collector electrode of a radiation detector and a high voltage electrode in the radiation measuring apparatus shown in FIG. It is a figure which shows the characteristic of the low electric field area | region in the characteristic view shown in FIG.

  The radiation measuring apparatus according to the present invention will be described below with reference to the drawings.

  FIG. 1 shows a schematic configuration of an embodiment of a radiation measuring apparatus according to the present invention.

<Schematic configuration of radiation measuring apparatus>
The radiation measuring apparatus 1A includes a radiation detector 10, a high voltage power source (first power source) 21, an adjustment power source (second power source) 22, a switch 30, an amplifier 40, a load impedance R41, and a display circuit unit 50. .

<Schematic configuration of radiation detector>
The radiation detector 10 is a so-called ionization chamber, and is a collector electrode (first electrode) 101 that outputs an electric current according to an incident radiation dose rate, and a high-voltage electrode (second electrode) that forms an internal space for enclosing a predetermined gas. Electrode 102), a first insulating member 103a and a second insulating member 103b for electrically insulating the collector electrode 101 and the high-voltage electrode 102.

  The high voltage power supply 21 outputs a high voltage (first voltage) HT. The output voltage of the high voltage HT is, for example, −25 volts. The adjustment power supply 22 outputs an adjustment voltage (second voltage) dV that is lower in absolute value than the high voltage HT. The voltage regulator 221 included in the adjustment power supply 22 is for adjusting the adjustment voltage dV to an arbitrary positive voltage or negative voltage. The switch 30 of the radiation measuring apparatus 1 </ b> A is for selecting either the high-voltage power supply 21 or the adjustment power supply 22 and connecting it to the high-voltage electrode 102 of the ray detector 10.

  The collector electrode 101 of the radiation detector 10 is connected to the amplifier 40 and biased to zero potential. The high voltage electrode 102 of the radiation detector 10 is connected to the high voltage power source 21 and applied with a high voltage HT when measuring the radiation dose rate. Then, a high electric field is generated between the collector electrode 101 and the high-voltage electrode 102, and the gas molecules enclosed in the radiation detector 10 are ionized according to the incident radiation dose rate. Negative ions generated by ionization are captured by the collector electrode 101, and positive ions are captured by the high voltage electrode 102. Therefore, by forming a closed circuit between the collector electrode 101 and the high-voltage electrode 102, the radiation detector 10 can output the collector current Is from the collector electrode 101 in accordance with negative ions generated by ionization.

<About amplifiers>
The amplifier 40 converts the collector electrode current Is into a voltage together with a load impedance R41 (typically a resistor). The amplifier 40 is a high gain DC amplifier, for example, and has a non-inverting input terminal 401, an inverting input terminal 402, and an output terminal 403.

  The non-inverting input terminal 401 of the amplifier 40 is grounded, a load impedance R41 is connected between the inverting input terminal 402 and the output terminal 403 of the amplifier 40, and the amplifier 40 constitutes a negative feedback amplifier circuit (load). The impedance R41 becomes the feedback impedance of the negative feedback amplifier circuit.) An inverting input terminal 402 of the amplifier 40 is connected to the collector electrode 101 of the radiation detector 10.

  Therefore, if the amplifier 40 is a high-gain DC amplifier, the potential difference between the inverting input terminal 402 and the non-inverting input terminal 401 is small. If the input impedance of the amplifier 40 is high, the collector output from the collector electrode 101 is reduced. All electrode current Is flows into load impedance R41.

  Thus, the potential of the collector electrode 101 is maintained at substantially the same potential (substantially ground potential) as the potential of the non-inverting input terminal 401 of the amplifier 40, and the collector electrode current Is flows into the load impedance R41. Outputs voltage according to the rate.

  A collector electrode current Is flowing from the collector electrode 101 of the radiation detector 10 into the load impedance R41 passes through the amplifier 40, the power source (not shown) of the amplifier 40, and the high-voltage power source 21 or the adjustment power source 22. Thus, a closed circuit that reaches 10 high-voltage electrodes 102 flows.

  Here, if the load impedance R41 is composed only of a resistor, the voltage output from the amplifier 40 becomes a voltage corresponding to the radiation dose rate, and if the load impedance R41 is composed only of a capacitor, the voltage output from the amplifier 40 is the radiation dose. It becomes the voltage according to.

  In general, a DC input offset voltage exists between the inverting input terminal 402 and the non-inverting input terminal 401 of the amplifier 40, and this DC input offset voltage generates a DC output offset voltage at the output of the amplifier 40. Therefore, the radiation measuring apparatus 1A is configured so that the DC output offset voltage of the amplifier 40 can be adjusted to zero using the offset adjuster R42.

<About the display circuit section>
As shown in FIG. 2, the display circuit unit 50 includes an input terminal 501 for inputting the output voltage of the amplifier 40, an amplifier 502, a display 503, and the like, and amplifies the output voltage of the amplifier 40 to the display 503. indicate. Of course, the value displayed on the display 503 is the value of the radiation dose rate.

  The display circuit unit 50 has a function of adjusting the voltage gain or the like of the amplifier constituted by the amplifier 502, so that the measurement range of the radiation dose rate can be changed. For example, the measurement sensitivity can be increased by setting the voltage gain of the amplifier constituted by the amplifier 502 to be high, and the dose rate at an extremely low level can be measured. On the other hand, the measurement sensitivity can be set by setting the voltage gain to be low. Can be used to measure high levels of dose rate.

  It is desirable that the display circuit unit 50 can display a slightly negative value in order to smoothly adjust the dark current and the DC offset described later.

  Of course, the display circuit unit is not limited to the above-described configuration. For example, the display circuit unit has an analog / digital converter and digitally displays the radiation dose rate by converting the output voltage of the amplifier 40 from analog to digital. Also good.

<Measurement of radiation dose rate>
When the radiation dose rate is measured, the switch 30 is operated to connect the high voltage power source 21 to the high voltage electrode 102 of the radiation detector 10 and apply the high voltage HT (−25 (V)). At this time, an electric field generated between the collector electrode 101 and the high voltage electrode 102 of the radiation detector 10 is defined as an electric field Ev (in this specification, when a negative voltage is applied to the high voltage electrode 102, the electric field Ev is a positive electric field. To do.) Of course, the electric field Ev changes according to the voltage of the high voltage HT.

  As the electric field Ev increases, the negative ions and positive ions ionized according to the radiation dose rate incident on the radiation detector 10 are accelerated and are less likely to recombine, and the collector current Is increases. When the electric field Ev reaches a certain electric field strength, the collector electrode current Is is saturated even if the electric field Ev is further increased.

  FIG. 3 shows the characteristics of the electric field Ev versus the collector electrode current Is when the incident radiation dose rate is a constant value. The collector electrode current Is becomes substantially zero when the electric field Ev is substantially zero, and increases as the electric field Ev increases. Saturates over time.

<About dark current>
FIG. 4 shows the electric field Ev versus collector current Is characteristic when the electric field Ev is low. Even if the voltage applied to the high-voltage electrode 102 is set to zero (V) and the electric field Ev is set to zero, the negative ions ionized by radiation reach the collector electrode 101 without being recombined with positive ions, though very little. There is also. Therefore, even if the electric field Ev is zero, the collector electrode 101 outputs a very small collector electrode current Is as shown in FIG.

  It can be said that the collector electrode current Is at this time is a combination of a current generated by radiation and a dark current. On the other hand, the dark current can be said to be the collector electrode current Is when no radiation is incident on the radiation detector 10.

<Dark current compensation>
When compensating for the dark current in the radiation measuring apparatus 1A, the power supply connected to the high-voltage electrode 102 of the radiation detector 10 is set as the adjustment power supply 22 by operating the switch 30. Here, the adjustment power supply 22 outputs the adjustment voltage dV having a smaller absolute value than the high voltage HT as described above.

  In the radiation measurement apparatus 1A, adjustment of dark current compensation is performed as follows using radiation from a relatively high level radiation source (for example, a reference radiation source).

(1) First, the high-voltage power supply 21 is selected by the switch 30, and the radiation measuring apparatus 1A is operated to confirm that the radiation from the radiation source can be detected.
(2) Next, the adjustment power source 22 is selected by the switch 30 and the adjustment voltage dV is set, for example, in the vicinity of zero (V). Then, the dose rate detection sensitivity of the radiation detector 10 is lowered, but if the radiation source is at a relatively high level, the collector current Is is not zero, and the display 503 of the display circuit unit 50 has a numerical value of a certain level or higher. Is displayed. At this time, it is desirable that the radiation measuring apparatus 1A is set in a highly sensitive state.
(3) Next, the adjustment voltage dV output from the adjustment power supply 22 is changed. By adjusting the adjustment voltage dV, the electric field generated between the collector electrode 101 and the high-voltage electrode 102 is set to −dEv to dEv. When the adjustment voltage dV is changed (for example, when the adjustment voltage dV is slightly positive), positive ions are repelled by the high-voltage electrode 102, while negative ions are repelled by the collector electrode 101, and negative ions and positive ions are Can promote recombination. By further finely adjusting the adjustment voltage dV, almost all of the ionized negative ions and positive ions can be recombined.
Specifically, the adjustment voltage dV is adjusted so that the display on the display 503 becomes the minimum value. However, since the amplifier 40 has a DC offset, the display 503 generally displays a slightly positive value or a slightly negative value even when the collector electrode current Is output of the radiation detector 10 becomes zero. indicate.
(4) Therefore, after adjusting the adjustment voltage dV so that the display on the display 503 becomes the minimum value, the offset adjuster R42 is adjusted so that the display on the display 503 becomes zero.
(5) After that, the adjustment voltage dV is readjusted again to minimize the display on the display 503. Further, the offset adjuster R42 is adjusted so that the display on the display 503 becomes zero. By repeating the adjustment voltage dV and the DC offset adjustment several times in this way, dark current compensation and DC offset adjustment in the radiation measuring apparatus 1A can be performed.

  In this way, the radiation measuring apparatus 1A can directly detect and compensate for the dark current included in the collector electrode current Is without using a dark current compensation electrode (terminal) separate from the collector electrode, and can also reduce the DC offset to zero. Can be adjusted.

  In this way, the radiation measuring apparatus 1A can measure the dose rate at a very low level with high sensitivity, eliminating the influence of dark current, and setting the voltage gain of the amplifier circuit composed of the amplifier 502 low. High dose rate can be measured (radiation dose rate can be measured with high accuracy in a wide measurement range).

<Influence of noise generated in amplifier circuits>
Noise generated in the amplifier circuit is noise inherent to the first-stage amplifier (amplifier 40 in the radiation measurement apparatus 1A) constituting the amplifier circuit, and stray capacitance on the input side of the first-stage amplifier (collector electrode 101 and high-voltage electrode in the radiation measurement apparatus 1A). And the like, and the load impedance (load impedance R41 in the radiation measuring apparatus 1A) into which the signal current (collecting electrode current Is in the radiation measuring apparatus 1A) flows.

  In the radiation measuring apparatus 1A, when the dark current compensation adjustment is performed, the operation of the switch 30 is performed. What is changed by this operation is a power source connected to the high voltage electrode 102 of the radiation detector 10, and the collector electrode 101 ( The stray capacitance between the high voltage electrode 102 and the high voltage electrode 102 is not changed. Of course, the noise inherent to the amplifier 40 and the load impedance R41 are not changed by the operation of the switch 30.

  As described above, the radiation measuring apparatus 1A does not include an element that changes noise between the measurement of the radiation dose rate and the adjustment of the dark current compensation. Therefore, the dark current compensation and the amplifier 40 are not affected by the noise generated in the amplifier circuit. Thus, the radiation dose rate can be measured with high sensitivity and high accuracy.

  The radiation measuring apparatus according to the present invention has been described above, but the present invention can be implemented with appropriate modifications without changing the gist thereof.

  For example, the output of the radiation detector is determined by the charge captured by the first electrode, the output charge per unit time from the first electrode is a current, and this current flows into the resistor and becomes a voltage and Therefore, even if it is a case where a radiation measuring device makes the output of a radiation detector flow in into a capacitor, it is a radiation measuring device concerning the present invention.

  The radiation detector is not limited to an ionization chamber. The radiation detector includes a first electrode (collector electrode) capable of outputting an electric current according to an incident radiation dose rate, and a second electrode for generating an electric field between the first electrode and the first electrode. And changing the electric field between the first electrode and the second electrode to change the incident radiation dose rate versus the output current characteristic of the first electrode, so that radiation is incident on the radiation detector to some extent. Even if it is, it is sufficient if the output current of the first electrode can be only a dark current.

  Since the radiation measuring apparatus according to the present invention can be manufactured industrially, etc., can be used industrially and commercially, and can be used for commercial transactions, the present invention is economical. It is an invention that has value and can be used industrially.

1A Radiation measurement device 10 Radiation detector (ionization chamber)
101 First electrode (collector electrode)
102 Second electrode (high voltage electrode)
21 First power supply (high voltage power supply)
22 Second power supply (adjustment power supply)
30 Switch 40 Amplifier R41 Load impedance R42 Offset adjuster HT First voltage (high voltage)
dV Second voltage (adjustment voltage)

Claims (4)

A radiation detector comprising a first electrode that outputs a current in accordance with an incident radiation dose rate, and a second electrode to which a voltage for generating an electric field is applied;
An electric field is generated between the first electrode and the second electrode, and the electric charge generated in the radiation detector according to the incident radiation dose rate reaches the first electrode. A first power supply for supplying a first voltage to the second electrode;
A voltage having an absolute value lower than that of the first voltage is supplied to the second electrode, and an absolute value of the electric field generated between the first electrode and the second electrode by the first power source low electric field was raw time difference between said first electrode and said second electrode, and a negative charge particles generated inside the radiation detector in response to incident radiation dose rate and positively charged particles re A second power source with a voltage regulator that, by coupling, minimizes the amount of charge flowing into the first electrode;
A radiation measuring apparatus comprising: a switch for selecting the first power source and the second power source and connecting the second power source to the second electrode. The radiation measurement apparatus according to claim 1,
The radiation measurement apparatus further comprising: a load impedance into which the current output from the first electrode flows; and an amplifier for obtaining an output corresponding to a voltage generated in the load impedance.   3. The radiation measuring apparatus according to claim 2, further comprising an offset adjuster for adjusting an offset of the amplifier. The radiation detector is an ionization chamber;
The first electrode is a collector electrode of the ionization chamber;
The second electrode is a high-voltage electrode of the ionization chamber to which a high voltage is applied to generate an electric field that causes the charge generated inside the radiation detector to reach the collecting electrode according to the incident radiation dose rate. The radiation measuring apparatus according to claim 1, wherein:

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