JP2015143663A - radioactive gas monitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radioactive gas monitor which has a compact and relatively simple configuration and is capable of measuring radioactivity concentration of a detection target gas for each nuclide.SOLUTION: A radioactive gas monitor 1 includes; a vented inner ionization chamber 104 having a pair of electrodes comprising a core wire 106 and an outer electrode 107 with a flow path for allowing passage of a detection target gas G located in between; an outer ionization chamber 105 located surrounding the inner ionization chamber 104 and having a pair of electrodes comprising an inner electrode 110 and outer electrodes 111a-111n with a hollow space containing ionized gas in between; and an energy absorbing material 109 which is provided between the inner ionization chamber 104 and outer ionization chamber 105 to absorb energy of radiation generated by the detection target gas G. The amount of energy absorbed by the energy absorbing material 109 changes along the flow path in the inner ionization chamber 104.

Description

  The present invention relates to a radioactive gas monitor that discriminates and measures the radioactivity concentration of a detection target gas for each nuclide.

  Conventionally, a radioactive gas monitor installed in a nuclear facility, a radioisotope handling facility, or the like has measured the radioactivity concentration of a detection target gas by using a plastic scintillator, an ionization chamber, or the like. However, in general, the number of β-rays can be measured with plastic scintillators and ionization chambers, but because it is difficult to measure the energy of β-rays, the nuclide could not be identified. For this reason, it has been difficult to measure the radioactivity concentration of a plurality of individual nuclides because it has only been converted into the radioactivity concentration of a specific nuclide from only the measured β ray count data.

Japanese Patent No. 2930513

  If β-ray energy can be measured, the radioactivity concentration can be measured for each nuclide contained in the gas to be measured. For example, it is easy to identify the cause of the malfunction such as where the leak occurred in the facility. There are advantages such as.

  In the method proposed in Patent Document 1, an inner ionization chamber and an outer ionization chamber having different sensitive volumes are provided, and β rays are discriminated using the fact that the range of β rays depends on energy. In other words, by introducing the detection target gas sequentially into the inner ionization chamber with a small sensitive volume and the outer ionization chamber with a large sensitive volume, and calculating the ratio of the ionization current of both ionization chambers, the β-rays are divided into two energy regions. The radioactivity concentration can be measured by roughly classifying the nuclide into two types.

  However, when there are three or more detection target nuclides, it is necessary to provide a large number of ionization chambers having different sensitive volumes in order to discriminate those nuclides. Therefore, a separate detector cell must be prepared for each energy region to be discriminated, and the entire detection system becomes complicated and large. On the other hand, in the configuration as in Patent Document 1, in reality, the nuclide can only be roughly divided into two, and the concentration of each radioactive substance cannot be measured.

  An object of the present invention is to provide a radioactive gas monitor capable of measuring the radioactivity concentration of a detection target gas for each nuclide with a small and relatively simple configuration.

In order to achieve the above object, the radioactive gas monitor according to the present invention comprises:
A ventilated inner ionization chamber having a pair of electrodes in which a flow path for allowing the gas to be detected to pass is interposed;
An outer ionization chamber having a pair of electrodes provided outside the inner ionization chamber and interposing a cavity containing ionized gas;
An energy loss material provided between the inner ionization chamber and the outer ionization chamber, for losing the energy of radiation generated by the detection target gas;
The energy loss amount of the energy loss material varies along the flow path.

  According to the present invention, when the detection target gas passes through the flow path of the inner ionization chamber, β rays are emitted from the radionuclide contained in the gas. Such beta rays are first detected by the inner ion chamber, then gradually lose energy as they pass through the energy loss material, and are detected again by the outer ion chamber. At this time, since the amount of energy loss of the energy loss material changes along the flow path, the number of β rays that can reach the outer ionization chamber varies depending on the location that reaches the outer ionization chamber. As a result, the magnitude of the β-ray energy of the measurement nuclide can be associated with the detection position in the outer ionization chamber, and the β-ray energy can be spatially distinguished.

It is a perspective view which shows the structure of the radioactive gas monitor by Embodiment 1 of this invention. It is a longitudinal cross-sectional view along the central axis of a radioactive gas monitor. It is explanatory drawing which shows the other example of the electrical structure of a radioactive gas monitor. It is a longitudinal cross-sectional view which shows the structure of the radioactive gas monitor by Embodiment 2 of this invention. It is a longitudinal cross-sectional view which shows the structure of the radioactive gas monitor by Embodiment 3 of this invention.

Embodiment 1 FIG.
FIG. 1 is a perspective view showing a configuration of a radioactive gas monitor according to Embodiment 1 of the present invention. FIG. 2 is a longitudinal sectional view along the central axis of the radioactive gas monitor. The detection target gas G includes a radionuclide that emits β rays, such as Xe-133, Kr-85, Ar-41, and the like.

  The radioactive gas monitor 1 includes a ventilated inner ionization chamber 104, an outer ionization chamber 105 provided outside the inner ionization chamber 104, and an energy loss material 109 provided between the inner ionization chamber 104 and the outer ionization chamber 105. Etc.

  The inner ionization chamber 104 includes a housing having side surfaces parallel to the central axis and both end surfaces, a conductive core wire 106 installed on the central axis, and an outer electrode 107 installed at a distance from the core wire 106. . The outer electrode 107 may be installed along the inner wall of the side surface of the housing, or can also be used as the side surface of the housing. A gas inlet 102 is provided on one end face of the housing, and a gas outlet 103 is provided on the other end face of the housing. The housing cavity functions as a flow path through which the detection target gas passes. The detection target gas G flows from the gas inlet 102 toward the gas outlet 103. The core wire 106 and the outer electrode 107 function as a pair of electrodes having a flow path between them.

  The outer ionization chamber 105 includes a housing for forming a cavity, an inner electrode 110 configured as a single electrode, and outer electrodes 111a, ..., 111n configured as a plurality of electrodes. The cavity is filled with an ionizing gas (for example, a chemically inert gas such as air, nitrogen, or xenon). The inner electrode 110 and the outer electrodes 111a,..., 111n function as a pair of electrodes with a cavity interposed therebetween.

  The energy loss material 109 has a function of losing the energy of β rays generated by the detection target gas G, and the amount of energy loss is also increased when the thickness and material of the energy loss material 109 are spatially changed. It is distributed so as to change along the flow path of the ionization chamber 104. As an example, the energy loss material 109 has a truncated cone shape or a truncated pyramid shape whose outer diameter continuously changes along the flow path of the inner ionization chamber 104, as shown in FIG. May be included. In this case, the lower the position, the larger the amount of β-ray energy loss, while the higher the position, the smaller the amount of β-ray energy loss. The energy loss material 109 may have a function as a support member for the inner ionization chamber 104 and the outer ionization chamber 105.

  Next, the electrical configuration will be described. A high DC voltage is applied between the core wire 106 of the inner ionization chamber 104 and the outer electrode 107 and between the inner electrode 110 of the outer ionization chamber 105 and each of the outer electrodes 111a,. A high voltage power supply 108 is connected. A plurality of direct current microammeters (A in the figure) for measuring the currents flowing through the outer electrodes 111a,..., 111n of the outer ionization chamber 105 are provided. Similarly, a direct current microammeter (A in the figure) for measuring the current flowing through the outer electrode 107 of the inner ionization chamber 104 is provided. The core wire 106 and the outer electrodes 111a, ..., 111n are grounded. The output signal of each direct current microammeter is taken into the arithmetic unit 115 including a microprocessor or the like.

  Next, the operation will be described. When the detection target gas G passes through the flow path of the inner ionization chamber 104 from the gas inlet 102, when the radionuclide emits β rays and collides with gas molecules, it is ionized into electrons and ions. When ionization is performed in the inner ionization chamber 104, electrons and ions flow in opposite directions along the electric field between the core wire 106 and the outer electrode 107, and a current is generated. On the other hand, when ionization is performed in the outer ionization chamber 105, electrons and ions flow in opposite directions along the electric field between the inner electrode 110 and each outer electrode 111a, ..., 111n, and a current is generated. At this time, since current flows through the outer electrodes 111a, ..., 111n closest to the ionized position, the ionized position can be spatially discriminated by identifying the outer electrode through which the current flows. The value of the generated current is measured by a direct current microammeter.

  The range of β rays emitted from the detection target gas G is longer as the energy of β rays is higher, and conversely is shorter as the energy is lower. In FIG. 2, as an example, there are β particles “β1”, “β2”, and “β3”, which are divided into three regions, and are called high energy β particles, medium energy β particles, and low energy β particles. I will do it. The energy referred to here is the β-ray energy emitted from the radionuclide for convenience, and the β-ray energy varies for example from about several keV at low to about several MeV at high. is there.

  In the inner ionization chamber 104, any β rays of “β1”, “β2”, and “β3” can ionize gas molecules. Further, β rays of “β1” and “β2” having relatively high energy pass through the outer electrode 107, the energy loss material 109, and the inner electrode 110 and reach the cavity of the outer ionization chamber 105. As described above, the range of β rays is longer as the energy of β rays is higher, and conversely is shorter as the energy is lower. The same applies to the tapering energy loss material 109. Therefore, the thickness of the energy loss material 109 that can pass, that is, the distance in the radial direction, differs depending on the energy of β rays. For example, since “β1” having high energy is thick enough to pass, it can reach region 1 indicated by reference numeral 113 in FIG. On the other hand, “β2” having lower energy than “β1” can reach the region 2 indicated by reference numeral 114, but cannot reach the region 1 indicated by reference numeral 113. Therefore, in region 1, the gas is ionized only by “β1”, and in region 2, the gas is ionized by “β1” and “β2”.

  Since the radial dimension of the cavity of the outer ionization chamber 105, that is, the distance between the electrodes, is almost constant at any position, the energy imparted to the cavity by the β rays that have passed through the energy loss material 109 is substantially constant. Therefore, the amount of ionization in the region 1 and the region 2 does not substantially depend on the initial energy of β rays, and is almost proportional to the number of β rays that have reached the cavity. Since a plurality of outer electrodes 111a,..., 111n divided in the axial direction are provided on the outer inner surface of the outer ionization chamber 105, the amount of ionization in the corresponding region 1, region 2, etc. can be measured.

  As described above, in the inner ionization chamber 104, gas molecules are ionized by any β rays of “β1”, “β2”, and “β3”, and in the region 2 indicated by reference numeral 114 of the outer ionization chamber 105, “β1”, “ Gas molecules are ionized by β rays of “β2”, and gas molecules are ionized by β rays of “β1” in the region 1 indicated by reference numeral 113 of the outer ionization chamber 105. Then, an ionization current value corresponding to each case is measured by each DC microammeter. The output value of each DC microammeter is transmitted to the arithmetic unit 115, and the individual energy values of “β1”, “β2”, and “β3” are obtained by calculation. As a result, β-rays can be measured separately for each energy region.

  In the above example, for the sake of easy understanding, the β-ray energy has been described as the three categories of “β1”, “β2”, and “β3”. However, depending on the number of the outer electrodes 111a,. Since the cavity region can be arbitrarily divided, it is possible in principle to discriminate β-ray energy continuously.

  Therefore, the radioactive gas monitor 1 according to the present embodiment can discriminate the β-ray energy of the detection target gas G into an arbitrary region, so that the β-rays of the detection target gas G are finely and continuously discriminated for each energy. Individual nuclides can be identified. Further, since the radioactive gas monitor 1 can be constituted by two detector cells, the inner ionization chamber 104 and the outer ionization chamber 105, the apparatus can be reduced in size and cost.

  FIG. 3 is an explanatory diagram showing another example of the electrical configuration of the radioactive gas monitor. In order to measure the energy of β rays, the ionization current is measured using a direct current microammeter in the configuration of FIG. 2, but the pulse count using a charge-type amplifier is adopted in the configuration of FIG.

  3, a high DC voltage is applied between the core wire 106 and the outer electrode 107 of the inner ionization chamber 104 and between the inner electrode 110 of the outer ionization chamber 105 and each of the outer electrodes 111a,. As described above, the high-voltage power supply 108 is connected via a load resistor. A charge amplifier 213 set to an appropriate time constant is connected to the core wire 106 and the outer electrodes 111a,. The charge-type amplifier 213 has a function of converting the input charge into a voltage pulse signal, and the counter circuit 214 counts the output to obtain the number of pulses per unit time. The number of pulses per unit time is taken into the arithmetic unit 115 including a microprocessor and converted into β-ray energy. As a result, it becomes possible to discriminate β-ray energy, and the radioactivity concentration of the nuclide can be measured.

  As described above, in the configuration of FIG. 3, by performing pulse measurement, it is possible to count each β-ray particle, so that even a very low radioactive concentration can be measured with high sensitivity.

  In the present embodiment, the inner ionization chamber 104 has a cylindrical shape and the outer ionization chamber 105 has a truncated cone shape. However, the shape is not limited to this shape, and may be, for example, a flat plate shape. Further, the energy loss material 109 is assumed to have a uniform material and a changed thickness. However, even if the composition, density, etc. are changed, there is an effect of changing the energy of β-rays. Even if the material is changed, the same effect can be obtained.

Embodiment 2. FIG.
FIG. 4 is a longitudinal sectional view showing the configuration of the radioactive gas monitor according to the second embodiment of the present invention. The present embodiment is different from the first embodiment in that the inner ionization chamber 104 and the energy loss material 109 have the same configuration, but the outer electrode of the outer ionization chamber 105 is configured as a single electrode. To do. Note that a duplicate description of the same configuration as that of the first embodiment is omitted.

  The outer ionization chamber 105 includes a housing for forming a cavity, an inner electrode 110 configured as a single electrode made of a conductive material, and an outer electrode 311 configured as a single electrode made of a resistive material. And performs an operation like a PSD (position detection element) that can detect the position of the center of gravity of the charge distribution. The conductive material is a general electrode material, such as gold, silver, copper, or aluminum, while the resistive material is a material having a higher electrical resistivity than the conductive material, such as graphite or tungsten. .

  Next, the electrical configuration will be described. A high voltage power source 108 is connected between the core wire 106 and the outer electrode 107 of the inner ionization chamber 104 via a load resistor so that a high DC voltage is applied. On the other hand, lead wires are connected to the upper end portion and the lower end portion of the outer electrode 311, respectively. A high voltage power supply 108a is connected between the inner electrode 110 of the outer ionization chamber 105 and each lead wire of the outer electrode 311 via a load resistor so that a high DC voltage is applied.

  Charge-type amplifiers 213a to 213d set to appropriate time constants are connected to the lead wires of the outer electrode 311, the inner electrode 110, and the outer electrode 107, respectively. The charge-type amplifiers 213a to 213d have a function of converting input charges into voltage pulse signals, and pulse height values of the pulses are respectively measured by pulse wave height measurement circuits 314a to 314d. The measured pulse peak value is taken into the arithmetic unit 115 including a microprocessor or the like.

  Next, the operation will be described. Β rays of “β1” and “β2” having relatively high energy pass through the outer electrode 107, the energy loss material 109 and the inner electrode 110 and reach the cavity of the outer ionization chamber 105 to ionize gas molecules. . At this time, an ionization current is generated and flows into an upper lead wire and a lower lead wire through an outer electrode 311 formed of a resistive material. This is because the distance to the end of the outer electrode 311 (where the lead wire is connected) changes according to the ionization position, so the current value is proportionally distributed according to the electrical resistance value to the end depending on the ionization position. It is to be done.

  Specifically, in FIG. 4, the length of the outer electrode 311 along the generatrix direction is L, and the distance from the position where “β1” is incident and the gas is ionized to the upper end of the outer electrode 311 is x. If the amount of charge generated by ionization is Q, the output pulse peak value of the charge amplifier 213a is proportional to [1- (x / L)] Q, and the output pulse peak value of the charge amplifier 213b is (x / L) Proportional to Q. The charge amount Q is proportional to the pulse peak value of the charge amplifier 213c.

  Therefore, the output pulse peak values of the charge amplifiers 213a, 213b, and 213c are measured by the pulse peak measurement circuits 314a, 314b, and 314c, respectively, and the arithmetic unit 115 calculates the distance x from these measured values. Then, by counting the number of pulses corresponding to the distance x, the number of counts for each β-ray energy can be obtained, so that β-ray energy discrimination can be continuously performed.

  Similarly, in the inner ionization chamber 104, the output pulse peak value of the charge amplifier 213d is measured by the pulse peak measurement circuit 314d, and the arithmetic unit 115 calculates the initial energy of β rays from the measured value.

  In the present embodiment, the generated charges are divided by analog, so that in principle, the ionization position can be obtained continuously. That is, since the position can be specified with extremely high resolution, the β-ray energy corresponding thereto can be discriminated with high resolution, and as a result, measurement capable of discriminating various nuclides becomes possible.

Embodiment 3 FIG.
FIG. 5 is a longitudinal sectional view showing a configuration of a radioactive gas monitor according to Embodiment 3 of the present invention. In this embodiment, the configuration of the inner ionization chamber 104 is the same as that in the first embodiment, but the energy loss material 409 has an outer diameter that changes in a step shape along the flow path of the inner ionization chamber 104. It differs in that it has a staircase pyramid shape. Accordingly, the outer ionization chamber 105 also has a stepped pyramid shape, a housing for forming a cavity, an inner electrode 110 configured as a single electrode, outer electrodes 111a configured as a plurality of electrodes,. , 111n. Note that a duplicate description of the same configuration as that of the first embodiment is omitted.

  The energy loss material 409 has a laminated body shape composed of a plurality of cylinders or prisms having different outer diameters for each stage, and the height in the axial direction of each stage is set to be substantially the same. On the other hand, the length of each stage in the radial direction is different, and is preferably set to a value corresponding to the range of various β rays to be measured. By setting a plurality of lengths corresponding to the range of the nuclide to be measured, the nuclides can be accurately distinguished. As an example, the length in the radial direction of each stage of the energy loss material 409 can be simply set as follows.

  Now, the measurement target nuclides are assumed to be Xe-133 (β-ray average energy: 115 keV), Kr-85 (β-ray average energy: 229 keV), and Ar-41 (β-ray average energy: 400 keV). For example, when the material of the energy loss material 409 is polyethylene, the length in the radial direction is set to be approximately equal to the β-ray average energy of each nuclide in the polyethylene.

The empirical equation of the range in aluminum for single energy electrons is published in the following equation (5.6) in page 33 of the document "Isotope Handbook (3rd revised edition)" (Maruzen, published in 1984): ing.
R = 0.512E ^{1.265-0.0954lnE} (2.5>E> 0.01 MeV) (1)

However, the unit of the range R is [g · cm ⁻² ], and the unit of the electron energy E is [MeV]. Although this formula is for aluminum, it is known that it does not change much depending on the substance, so the range (weight per unit area) in polyethylene is calculated from this formula, and the thickness is calculated by density conversion. And

First, since the average energy of Xe-133 is 115 keV, the range R per unit density can be obtained by substituting into the above equation.
R (Xe-133) = 0.177 [g · cm ⁻² ] (2)

By dividing this value by the density of polyethylene 0.9 [g · cm ⁻³ ], the actual range Re expressed in cm is obtained.
Re (Xe-133) = 0.1.9 [cm] = 0.19 [mm] (3)

The other nuclides are calculated as follows.
Re (Kr−85) = 0.58 [mm] (4)
Re (Ar-41) = 1.33 [mm] (5)

  Therefore, by setting the length of each step in the radial direction to 0.19 mm, 0.58 mm, and 1.33 mm, it becomes possible to discriminate energy corresponding to the above three types of nuclides and minimize the influence of other nuclides. To the limit. As a result, the radioactivity concentration for each nuclide can be determined accurately.

  In this embodiment, the design method is exemplified for three types of nuclides of Xe-133, Kr-85, and Ar-41. However, for each of four or more types of nuclides including other nuclides, each step is performed in the same procedure. The length in the radial direction can be calculated.

1, 2, 3, 4 Radioactive gas monitor, 102 Gas inlet, 103 Gas outlet,
104 inner ionization chamber, 105 outer ionization chamber, 106 core wire,
107 outer electrode, 108 high voltage power supply, 109,409 energy loss material,
110 inner electrode, 111a to 111n, 311 outer electrode,
113 region 1, 114 region 2, 115 arithmetic unit,
213, 213a to d charge type amplifier, 214 counter circuit,
314a-d Pulse height measurement circuit, G detection target gas.

Claims (7)

A ventilated inner ionization chamber having a pair of electrodes in which a flow path for allowing the gas to be detected to pass is interposed;
An outer ionization chamber having a pair of electrodes provided outside the inner ionization chamber and interposing a cavity containing ionized gas;
An energy loss material provided between the inner ionization chamber and the outer ionization chamber, for losing the energy of radiation generated by the detection target gas;
The radioactive gas monitor characterized in that the amount of energy loss of the energy loss material varies along the flow path.   2. The radioactive gas monitor according to claim 1, wherein in the outer ionization chamber, the inner electrode of the pair of electrodes is configured as a single electrode, and the outer electrode of the pair of electrodes is configured as a plurality of electrodes. A charge-type amplifier connected to the outer electrode;
The radioactive gas monitor according to claim 2, further comprising a counter circuit that counts pulse signals output from the charge amplifier.   In the outer ion chamber, the inner electrode of the pair of electrodes is configured as a single electrode made of a conductive material, and the outer electrode of the pair of electrodes is configured as a single electrode of a resistive material. The radioactive gas monitor according to claim 1. Two charge-type amplifiers connected to both ends of the outer electrode;
5. The radioactive gas monitor according to claim 4, further comprising a pulse height measuring circuit for measuring a peak value of a pulse signal output from each charge amplifier.   The radioactive gas monitor according to claim 1, wherein the energy loss material has a truncated cone shape or a truncated pyramid shape whose outer diameter continuously changes along the flow path.   The radioactive gas monitor according to claim 1, wherein the energy loss material has a step pyramid shape whose outer diameter changes stepwise along the flow path.

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