JP2003130958A - Ionization chamber detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ionization chamber detector of which the self-staining dose rate due to an outside electrode made of aluminum and a cover made of aluminum does not exceed a predetermined value of 10 nGy/h or less without forming a shielding film. SOLUTION: When the predetermined value of the self-staining dose rate is set to SnGy/h, material aluminum used in the outside electrode and the cover is restricted to a material wherein the count number of β-rays emitted from material aluminum is 80 S count/10 h or less or a material wherein the concentrations of<238> U,<232> Th and<39> K in material aluminum are a 0.25 S unit or less in the sum total of three nuclides when 1 ppm of<238> U is set to a 1.05 unit, 1 ppm of<232> Th is set to a 0.30 unit and 1 ppm of<39> K is set to a 3.0×10<-5> unit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ionization chamber detector (hereinafter referred to as an ionization chamber) for detecting γ rays and X rays.

[0002]

2. Description of the Related Art An ionization chamber is often formed in a spherical shape in order to improve its directional characteristics, and detects γ-rays and X-rays (hereinafter represented by γ-rays). Although a spherical ionization chamber will be described below, the ionization chamber is not limited to a spherical ionization chamber. FIG. 1 is a partial sectional front view showing the appearance and electrode configuration of such a spherical ionization chamber. The spherical central electrode 12 at the center of the ionization chamber 1 has its lower portion held by an electrode holding member (not shown), functions as a collecting electrode, and is connected to an amplifier (not shown) for detecting a signal current. Has been done. Outside the central electrode 12, a spherical outer electrode 11 having the same center as the center of the central electrode 12 is electrically insulated from the central electrode 12, and is held by the outer peripheral portion of the electrode holding member not shown. Then, it is connected to a high-voltage power source (not shown) to form a high-voltage electrode. A spherical cover 14 having the same center as the center of the central electrode 12 is arranged further outside the outer electrode 11.

A sealed container is formed by the outer electrode 11 and a holding member therebelow, and a pressurized sealed gas 13 is sealed in the sealed container. When γ-rays enter the enclosed gas 13, the enclosed gas 13 is ionized by the interaction between the γ-rays and the enclosed gas 13, and the ionized enclosed gas is applied between the outer electrode 11 and the central electrode 12. The generated electric field separates into ions and electrons, and as a result, a current signal is taken out from the central electrode 12, which is the collector electrode. From this current value, the dose rate of incident γ rays can be obtained.

In the prior art, the central electrode 12, the outer electrode 11 and the cover 14 are made of stainless steel and the enclosed gas 13
Argon gas is used for. As an example of the dimensions and the like in this case, the thickness of the outer electrode 11 and the thickness of the cover 14 are both 2 mm, and the pressure of the enclosed argon gas is 0.91 MPa. However, in the case of the above configuration, γ-rays in the low energy region of 200 keV or less cannot be measured with high accuracy, so in the ionization chamber disclosed in JP 2001-166059 A, the outer electrode 11 and The material of the cover 14 is changed from stainless steel to aluminum (thickness 3 mm), and the enclosed gas 13 is changed from argon gas to nitrogen gas in which a small amount of argon gas is mixed, so that the energy characteristics in the low energy region are improved. Hereinafter, the ionization chamber in which the material of the outer electrode 11 and the cover 14 is aluminum is referred to as an aluminum ionization chamber.

FIG. 2 is a partial sectional view showing the electrode structure of such an aluminum ionization chamber. The stainless steel outer electrode 11 (thickness 2 mm) and the stainless steel cover 14 (thickness 2 mm) are replaced with aluminum outer electrode 11a (thickness 3 mm) and aluminum cover 14a (thickness 3 mm), respectively. A desired energy characteristic is obtained by changing the gas-only enclosed gas 13 to the enclosed gas 13a in which nitrogen gas is mixed with about 3% of argon gas. But the outer electrode
Radiation emitted from natural radionuclides contained in aluminum such as 11a reaches the space between the central electrode and the outer electrode 11a, ionizes the enclosed gas 13a, and raises the background value. And the aluminum outer electrode 11a
A shielding layer 111 consisting of a copper plating layer of 5 μm and a nickel plating layer of 5 μm was provided on the inner surface of the to completely shield the α rays emitted from the aluminum, and the background value due to the material being changed to aluminum. The increment is from the background value increment of 19.9 nGy / h when the shielding layer 111 is not provided.
It is reduced to 6.2 nGy / h.

The dose rate due to the radiation emitted from the radionuclide contained in the constituent material of the ionization chamber is called the self-contamination dose rate, which should be distinguished from the background value due to the radiation existing in the environment. Is. The above background value increase of 6.2 nGy / h is due to the self-contamination dose rate detected through the shield layer 111 in the radiation from the aluminum outer electrode 11a and the aluminum cover 14a, and the shield layer.
It corresponds to a value obtained by subtracting the self-contamination dose rate by the outer electrode 11 made of stainless steel and the cover 14 made of stainless steel from the sum of the self-contamination dose rate by 111. In addition, the stainless steel outer electrode 11 and the stainless steel cover
The self-contamination dose rate from 14 is said to be low.

In the above description, the central electrode 12
Is used as the collecting electrode and the outer electrode 11 is used as the high-voltage electrode, but the polarities of both electrodes are exchanged, and the central electrode 12 is used as the high-voltage electrode.
The outer electrode 11 can also be used as a collecting electrode.

[0008]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
According to Japanese Patent Publication No. 59, the shield layer 111 is made of an aluminum outer electrode 11a.
It is possible to reduce the increment of the background value to a little less than 1/3 to about 6 nGy / h by forming it on the inner surface of. However, it is possible to form a shielding layer 111 having a uniform thickness and a low defect occurrence rate such as peeling, swelling, or unplating on the inner surface of the aluminum outer electrode 11a, although it is possible at the current technical level, but difficult processing is possible. Or need management. The most common plating technique for forming the shielding layer 111 will be explained. For example, in order to form a uniform plating layer on the inner surface of the aluminum outer electrode 11a having a spherical shape, the cleanliness and roughness of the surface to be plated are required. However, it is necessary to control the composition of the plating solution and to circulate the plating solution to uniformly expose the surface to be plated to the plating solution. Circulation of the plating solution is technically difficult, and variations in plating thickness and plating defects are liable to occur, which not only raises costs but also fails to secure desired products. Further, as explained in the section "Means for solving the problem", since the range of β rays is long, the shielding effect in the shielding layer is small, and the self-contamination dose rate by β rays should be significantly reduced. Because I can't
When a large number of β-decaying nuclides are included (including uranium series and thorium series), it may be impossible to reliably increase the background value below a predetermined value.

The object of the present invention is to form a shield layer having the above-mentioned problems, and the self-contamination dose rate from aluminum used for the outer electrode and the cover is 10 nGy / h, which is the minimum measured value of the ionization chamber. To provide an ionization chamber made of aluminum smaller than the following predetermined values.

[0010]

The inventor of the present invention has made it possible to obtain high-purity aluminum that is usually available (hereinafter referred to as "normal high-purity aluminum") and aluminum that has been further purified (hereinafter referred to as "high-purity aluminum"). Treated aluminum ”) and the content of ³⁹ K and ^{238 of} natural radionuclides contained in each aluminum.
The concentration of U and ²³² Th was measured by ICP-MS mass spectrometry, and the number of β rays radiated from each aluminum was measured to determine the concentration of natural radionuclides. Whether there is an excellent correspondence between the self-contamination dose rate due to α-rays and the count of β-rays from aluminum, and whether the material aluminum for the outer electrode and cover is limited by the concentration of radionuclides, etc. The inventors have found that the above-mentioned problems can be achieved by limiting the count rate of radiated β rays, and have reached the present invention.

Table 1 shows the above measurement results. The nuclides for concentration measurement are ²³⁸ U, ²³² Th, and ³⁹ K.
And the discreet level for β-ray measurement is 20 keV.

[0012]

[Table 1] The reason for limiting the target nuclides for concentration measurement to ²³⁸ U, ²³² Th, and ³⁹ K is as follows.

Among the natural radionuclides expected to be contained in aluminum, the nuclides expected to be involved in the self-pollution dose rate are alkali metals ⁴⁰ K and ^87, which are estimated from the abundance and half-life of the crust. Rb, a uranium decay series with ²³⁸ U as a parent, an actinium decay series with ²³⁵ U as a parent, and a thorium decay series with ²³² Th as a parent. Among them, alkali metal is 0.1p during the refining process of aluminum.
Since it is said that the concentration is lower than pm, it was confirmed that the concentration of ³⁹ K was significantly reduced by measuring the concentration of ³⁹ K and comparing it with the abundance of the crust. For reference, the crustal abundance of K is 53,500 (atoms / 10 ⁶ Si atoms),
As shown in Table 1, the concentration of ³⁹ K in aluminum, which accounts for about 93% of K, was measured to be less than 2.0 ppm, and it is certain that the concentration will be significantly reduced in the alkali metal refining process. Moreover, the abundance of Rb in the crust is 107 (atoms / 10 ⁶ Si atoms), and the concentration in aluminum can be assumed to be sufficiently low to be ignored.

Based on the results shown in Table 1, the self-contamination of the aluminum ionization chamber was carried out according to the measurement results of the concentrations of ²³⁸ U, ²³² Th and ⁴⁰ K of natural radionuclides or the measured β ray counts. Explain that the dose rate can be controlled. First, the uranium series having ²³⁸ U as a parent nuclide, the actinium series having ²³⁵ U as a parent nuclide, the thorium series having ²³² Th as a parent nuclide, and ⁴⁰ K will be described.

[0015] ²³⁸ U is the half-life of 4.47 × 10 ⁹ years, ²³⁸
In the uranium series with U as the parent nuclide, 8 times of α decay and 6
There are two β-decays, and the total energy of α-rays emitted by α-decay is about 43 MeV, and the total energy of β-rays emitted by β-decay is about 5.3 MeV. ²³⁵ U has a half-life of 0.70 × 10 ⁹ years, and there are 7 α-decays and 4 β-decays in the actinium series with ²³⁵ U as the parent nuclide, and the total α-rays emitted by α-decay. Energy is about 42 MeV, β
The total energy of beta rays emitted by decay is about 3.1 MeV.

²³² Th has a half-life of 14.1 × 10 ⁹ years and is ²³² T
There are 6 α-decays and 4 β-decays in the thorium series with h as the parent nuclide, and the total energy of α-rays emitted by α-decay is about 36 MeV, and β-rays emitted by β-decay. Has a total energy of about 3.6 MeV. ⁴⁰ K has a half-life of 1.28 × 10 ⁹ years and emits two levels of β rays with an average of 1.23 MeV and undergoes β decay.

²³⁸ U and ²³⁵ U are isotopes, and their abundance ratio is 99.275: 0.720, and ³⁹ K and ⁴⁰ K are isotopes, and their abundance ratio is 93.26: 0.0117. First of all, we will consider how radiation types contribute to self-contamination dose rates. When the radiation from the above-mentioned natural radionuclides is α-ray, its energy is in the range of 4 MeV to 9 MeV. Even with the maximum value of 8.8 MeV, the range in the enclosed gas is as short as 10 mm or less, and the α rays reaching the enclosed gas inside the ionization chamber consume all of its energy to ionize the enclosed gas. Therefore, all α rays reaching the enclosed gas contribute to the self-contamination dose rate. However, since the range of α-rays in the constituent materials is short, the entire constituent materials do not contribute to the self-contamination dose rate, but rather the thickness of 40 μm or less (for aluminum) from the surface in contact with the enclosed gas. Only materials in the region contribute to the self-contamination dose rate, and the natural radionuclides contained in this region are of concern. The portion outside this region does not contribute to the self-contamination dose rate, because the material inside is a shielding layer for α rays.

On the other hand, when the radiation is β rays, its energy is distributed from the 10 keV level to about 3 MeV, and the range greatly varies depending on the energy, so it is possible to easily estimate how it contributes to the self-contamination dose rate. It's difficult. The outline is as follows. Β-rays with energy of about 100 keV have a range similar to that of α-rays. Therefore, judging from the fact that the energy level is one tenth of that of α rays, it can be judged that β rays having an energy of 100 keV or less do not contribute much to the self-contamination dose rate.

However, β rays with energy of about 1 MeV are
The range in the enclosed gas is about 400 mm, which is slightly larger than the inner diameter of the outer electrode, and the range in aluminum is also large, which is about 1/2 of the thickness of the outer electrode (for example, 3 mm). Therefore, unlike the case of α-ray, the inside of the outer electrode is about 1.5 mm.
Natural radionuclides contained in
Since the energy is large and a part of the energy becomes the signal current of the ionization chamber, it can be judged that β rays in the vicinity of 1 MeV emitted from the outer electrode are strongly involved in the self-pollution dose rate.

For β rays having a larger energy, the range in the enclosed gas is larger than the inner diameter of the outer electrode, and the proportion of energy consumed in the enclosed gas is reduced, but the range in aluminum is the range of the outer electrode. Equal to or greater than the thickness,
Beta rays from the cover also contribute to the self-contamination dose rate. Therefore, it can be judged that β-rays exceeding 1 MeV emitted from the outer electrode and the cover also contribute to the self-pollution dose rate to some extent.

Since γ-rays have a low probability of interacting with the enclosed gas, it is considered that they hardly contribute to the self-pollution dose rate. According to the above-mentioned Japanese Patent Laid-Open No. 2001-166059, the background value is increased by 19.9 nGy / by forming a shielding layer composed of a copper layer of 5 μm and a nickel layer of 5 μm on the inner surface of the outer electrode of the aluminum ionization chamber. From h to 6.2 nGy / h
Has been reduced to. In other words, the α rays from the aluminum outer electrode that can be completely shielded by the shielding layer occupy 2/3 or more of the self-contamination dose rate due to the aluminum outer electrode and the aluminum cover, and shield the remaining 1/3 or less. It can be said that the α-rays and β-rays from the layer and the β-rays from the aluminum outer electrode and the aluminum cover, which cannot be shielded much by the shielding layer, occupy. Considering that the shielding layer is a high density and thin plating layer, it can be expected that there are few α rays and β rays from the shielding layer, so most of the remaining 1/3 or less is the aluminum outer electrode and the aluminum cover. Can be thought of as β rays.

Next, regarding α rays from the aluminum outer electrode, which accounts for slightly more than 2/3 of the self-pollution dose rate, the correspondence between the concentration analysis result and the self-pollution dose rate is quantitatively examined.
In the case of α rays, as described above, it can be assumed that all the energy of the α rays that has entered the enclosed gas from the outer electrode contributes to the ionization of the enclosed gas. The method of contribution will be examined in the case of being proportional to the total energy of α rays for each decay sequence and in the case of being proportional to the sum of squares of the energy of each decay sequence. The idea that it is proportional to the sum of squares of energy takes into account that the range increases as the energy increases, and the thickness of the part related to the self-contamination dose rate increases.

The self-contamination dose rate of α rays for each decay series is
Proportional to the total energy of α rays due to concentration and α disintegration of the parent nuclide, and inversely proportional to its half-life, of ²³⁸ U 1
Assuming that the self-contamination dose rate due to α-rays from the uranium series corresponding to ppm is 1 unit, the self-contamination dose rate due to α-rays from the actinium series is [ ²³⁸ if the concentration of ²³⁸ U is expressed in ppm. U concentration] x (0.720 x 4.47 x 10 ⁹ x 42 / 99.275
X 0.70 x 10 ⁹ x 43) ≒ [ ²³⁸ U concentration] x 0.045 (unit), and the self-contamination dose rate due to α rays from thorium series corresponding to 1 ppm of ²³² Th is (4.47 x 10 ⁹ x 36 / 14.1 × 10 ⁹ × 43) ≈ 0.265 (unit).

Replacing the above total energy with the sum of squares of the decay energies, the self-contamination dose rate due to α rays from the actinium series ≈ [
The concentration of ²³⁸ U] × 0.050 (unit), and the self-contamination dose rate due to α rays from thorium series equivalent to 1 ppm of ²³² Th ≈ 0.300 (unit).

Since ⁴⁰ K is only β decay, the self-contamination dose rate due to α rays from ⁴⁰ K is zero. Therefore, the self-contamination dose rate due to α rays corresponding to 1 ppm of ²³⁸ U in the analysis result is 1.045 units or 1.050 units for the uranium series and the actinium series. The concentration of ²³² Th, which corresponds to one unit, is 1 / 0.265 ≈ 3.77
ppm or 1 / 0.300 ≈ 3.33 ppm.

Based on the measurement results shown in Table 1, the number of units of ordinary high-purity aluminum and highly-purified aluminum was calculated, and in the case of ordinary high-purity aluminum, the total energy was 1.045 × 2.9 + 0. .265 × 0.2 ≈ 3.084 (unit) The energy sum of squares gives 1.050 × 2.9 + 0.300 × 0.2 ≈ 3.105 (unit).

In the case of highly-purified aluminum, the total energy is 1.045 × 0.6 + 0.265 × (0.1-0) ≈0.654-0.627.
(Unit) According to the sum of squares, 1.050 x 0.6 + 0.300 x (0.1 to 0) ≈ 0.660 to 0.630
(Unit)

The ratio of the unit numbers of both is 4.72 to 4.92 in the case of total energy, and 4.70 to 4.93 in the case of sum of squared energies. The ratio of the β-ray count numbers shown in Table 1 = 991/209 = 4.74, and they are in good agreement. Similarly, for β rays which are considered to occupy a little less than 1/3 of the self-contamination dose rate, the correspondence between the concentration analysis result and the self-pollution dose rate is quantitatively examined.

In the case of β rays, as described above, not all the energy of the β rays that has entered the enclosed gas from the outer electrode contributes to the ionization of the enclosed gas. Although it is not possible to simply determine how to contribute to the detection signal, it is assumed here that it is proportional to the total energy of β rays for each decay sequence. The β-ray self-contamination dose rate for each decay series is proportional to the concentration of its parent nuclide and the total energy of β-rays associated with β decay, and is inversely proportional to its half-life,
^{If the} self-contamination dose rate due to β rays from the uranium series corresponding to 1 ppm of ²³⁸ U is 1 unit, the self-contamination dose rate due to β rays from the actinium series is the concentration of ²³⁸ U in ppm.
When expressed in units, [ ²³⁸ U concentration] x (0.720 x 4.47 x 10 ⁹ x 3.1 / 99.2
75 x 0.70 x 10 ⁹ x 5.3) ≒ [ ²³⁸ U concentration] x 0.0
27 (unit), and the self-contamination dose rate due to β rays from the thorium series corresponding to 1 ppm of ²³² Th is (4.47 × 10 ⁹ × 3.6 / 14.1 × 10 ⁹ × 5.3) ≈ 0.215 (unit).

The self-contamination dose rate due to β rays corresponding to 1 ppm of ⁴⁰ K is (4.47 × 10 ⁹ × 1.23 / 1.28 × 10 ⁹ × 5.3) ≈0.810 (unit), and when converted to 1 ppm of ³⁹ K, 0.810 × 0.0117 / 93.26) ≈ 0.000102 (unit).

Therefore, 1 of ²³⁸ U in the analysis result
The self-contamination dose rate due to β-rays equivalent to ppm is 1.027 units for uranium series and actinium series. The concentration of ²³² Th, which corresponds to 1 unit, is 4.65 ppm.
Therefore, the concentration of ³⁹ K corresponding to 1 unit is 10,000 ppm. In the case of β-rays, as in the case of α-rays, the number of units of ordinary high-purity aluminum and highly-purified aluminum is calculated based on the measurement results shown in Table 1. In case of 1.027 × 2.9 + 0.215 × 0.2 + 0.00 ≈ 3.021 (unit) In case of high-purity treated aluminum, 1.027 × 0.6 + 0.215 × (0.1 to 0) + 0.00 ≈ 0.638 ~
It is 0.616 (unit), and the ratio of the number of units of both is 4.74 to 4.90. Compared with the ratio of the number of β-ray counts = 991/209 = 4.74 shown in Table 1, as in the case of α-rays, Match.

In any case, the concentration of ²³² Th, which has a long half-life and is weakly involved in the self-contamination dose rate, was less than 7% of the concentration of ²³⁸ U, so the ratio of the number of units was ^238. It is a value close to the ratio of U concentration of 2.9 / 0.6 = 4.83,
Can be assumed. As described above, both the unit number ratio in the case of α rays and the unit number ratio in the case of β rays are in good agreement with the count number ratio of β rays. As explained above regarding the way in which the type of radiation contributes to the self-contamination dose rate, while it is clear how α rays are involved in the self-contamination dose rate,
How β-rays are related to the self-contamination dose rate is complicated, and the number of units calculated for each of the α-rays and β-rays above,
Of the self-contamination dose rates due to the aluminum outer electrode and the aluminum cover, as described in the section "Means for solving the problem", the α ray component occupies a little over 2/3 and the β ray component accounts for less than 1/3. ²³⁸ U and ²³² Th, ³⁹
The number of units corresponding to 1 ppm of K is determined as follows.

The unit number corresponding to ²³⁸ U and ²³² Th is the largest value and the value based on the sum of squares of energy considered to be the most reasonable, and is 1 ppm of ²³⁸ U.
Is 1.05 unit, 1ppm of ²³² Th is 0.30 unit, and ³⁹ K
The unit number corresponding to is ⁴⁰ K is only β decay, so 1 ppm of ³⁹ K is 3.0 × 10 as a contribution of 30% of the above calculated value.
^-5 units.

As described above, the unit number ratio and the β ray count
In other words, the good agreement with the number ratio means that
In²³⁸U or²³²Th,³⁹If you measure the concentration of K, the unit
The number of β rays emitted from aluminum
A β-ray coun
If you measure the number of²³⁸U or²³²Th, ³⁹
It is possible to estimate the total number of units by K, etc.
Is to say. Therefore, in aluminum²³⁸U or²³²
Th,³⁹Measuring the concentration of K or radiating from aluminum
By measuring the counted number of β rays,
Aluminum of the ionization chamber made of aluminum when using other aluminum
Estimate the self-contamination dose rate value by the side electrode and aluminum cover
The maximum allowable self-contamination dose rate
If decided,²³⁸U or²³²Th,³⁹From the concentration of K
An upper limit is set for the calculated number of units or β ray counts.
Defined,²³⁸Concentration of U etc. or measurement result of β ray count
Depending on the result, whether to use the aluminum material can be decided.
it can.

The present invention is based on the above idea.
According to the invention of claim 1, the central electrode in the central portion, the outer electrode surrounding the outer side of the central electrode, and the cover covering the outer side of the outer electrode are electrically insulated from each other at their lower portions to form a holding member. An ionization chamber in which a sealed container is formed by holding the outer electrode and a holding member, and a pressurized gas is enclosed in the sealed container. The outer electrode and the cover are made of aluminum. Also, in order to reduce the self-contamination dose rate by the cover to a value less than a predetermined value SnGy / h of 10 nGy / h or less, the aluminum material is a material in which the number of β rays emitted from 20 keV or more is 80S count / 10h or less. And

As described in the section of "Embodiment of the Invention", the aluminum material for making the outer electrode and the cover is made to have a β ray count of 20 keV or more and a counting number of 80 S count / 10 h or less. When limited to a certain aluminum material, the self-contamination dose rate from the aluminum used for the outer electrode and the cover is SnGy / h or less, and if S = 10, the minimum measurement value of the ionization chamber is 10nGy / h or less.

According to a second aspect of the present invention, the central electrode in the central portion, the outer electrode surrounding the outer side of the central electrode, and the cover covering the outer side of the outer electrode are electrically insulated from each other at their lower portions. An ionization chamber that is held by a holding member, and a closed container is formed by the outer electrode and the holding member, and a pressurized gas is sealed in the closed container.The outer electrode and the cover are made of aluminum. , in order to reduce the self-contamination dose rate by the outer electrodes and the cover than 10nGy / h following the predetermined value SnGy / h, the aluminum material, by the concentration of ²³⁸ U, ²³² Th and ³⁹ K contained therein, ²³⁸ 1 ppm of U is 1.05 units, 1 ppm of ²³² Th is 0.30 units, ³⁹ K
With 1 ppm of 3.0 x 10 ^-5 units, the total concentration of the three nuclides should be 0.25 S units or less.

As described in the section of “Embodiment of the Invention”, the natural radionuclides ²³⁸ U and ²³² Th and the natural radionuclides ⁴⁰ contained in the high-purity aluminum as the material of the outer electrode and the cover are used. By limiting the content concentration with ³⁹ K, which is an isotope of K, as described above, the self-contamination dose rate from aluminum used for the outer electrode and the cover is SnGy /
If h is less than or equal to S, and if S = 10, the minimum measurement value of the ionization chamber is less than or equal to 10 nGy / h.

[0039]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of an ionization chamber according to the present invention will be described with reference to examples. Since the configuration of this embodiment is exactly the same as that of FIG. 1, description of common parts will be omitted and only different parts will be described. In this embodiment, the outer electrode 11 which is a high-voltage electrode and the cover 14 are made of high-purity aluminum, and no shielding layer is provided.

The high-purity aluminum used is the same as the one whose measurement results are shown in Table 1, namely "normal high-purity aluminum" and "high-purification-treated aluminum". Outer electrode
The thickness of both 11 and cover 14 is 3 mm. An aluminum ionization chamber was assembled using the outer electrode 11 and the cover 14 made of the above two kinds of aluminum materials, and the background value of each aluminum ionization chamber in the iron chamber was measured.

Table 2 shows the measurement results of the background values of the respective aluminum ionization chambers, and the β-ray measurement values of Table 1.
²³⁸ U, described in the section “Means for solving problems”
"Unit number" calculated with the self-contamination dose rate by α-rays of the uranium series corresponding to 1 ppm as 1 unit.

[0042]

[Table 2] The self-contamination dose rate (hereinafter simply referred to as the self-contamination dose rate) by the outer electrode 11 and the cover 14 of the aluminum ionization chamber is
As is clear from the explanation in the section "Means for solving the problem", it is considered that it is proportional to the "β-ray count number" or "unit number". If the "background value" is calculated as the sum of the "original background value" and the "proportional amount of β-ray counts", that is, the self-contamination dose rate, which is proportional to the "count number", "Background value" is
22.4nGy / h, the self-contamination dose rate for "normal high-purity aluminum" is 12.2nGy / h, and the self-contamination dose rate for "high-purity treated aluminum" is 2.6nGy / h.

Similarly, when the simultaneous equations are solved assuming that the self-contamination dose rate is proportional to the “unit number”, the “original background value” is 22.4 to 22.6 nGy / h, which is equal to that of “normal high-purity aluminum”. In case of self-contamination dose rate is 12.2 ~ 12.0n
Gy / h, and the self-contamination dose rate in the case of "highly purified aluminum" is 2.6 to 2.4 nGy / h. The value calculated from the "β-ray count number" and the value calculated from the "unit number" are in good agreement, and the self-contamination dose rate for "high-purity treated aluminum" is "normal high-purity aluminum". It is significantly lower than the self-contamination dose rate.

As is clear from the above results, when the aluminum material is used, the "β-ray count number" or " ²³⁸
If the “concentration of U etc.” is measured, the self-contamination dose rate of the aluminum ionization chamber to be manufactured can be estimated from the result, so by setting the upper limit of this self-contamination dose rate, Number "or" concentration of ²³⁸ U etc. "
It is possible to set an upper limit value on the “number of units” calculated from the measurement result, and it is possible to determine whether or not to adopt the aluminum material.

In order to set the upper limit of the self-contamination dose rate by the outer electrode 11 and the cover 14 to 10 nGy / h or less, which is the minimum measurement value of the ionization chamber, the "β ray count number" is 812 counts / 10 h or less. , ²³⁸ U contained in aluminum, ²³²
The concentration of Th and ³⁹ K is 1.05 unit of 1 ppm of ²³⁸ U, ²³²
With 1 ppm of Th as 0.30 unit and 1 ppm of ³⁹ K as 3.0 × 10 ^-5 units, the total concentration of the three nuclides will be 2.54 to 2.59 units or less. Therefore, in order to keep the self-pollution dose rate below 10 nGy / h with some allowance,
It is desirable that the "β-ray count number" be 800 counts / 10 hours or less, or that the "unit number" be 2.5 units or less.

As a matter of course, when the upper limit value is further reduced, the “β-ray count number” or the “unit number equivalent to the concentration of three nuclides” may be reduced in proportion to it, and the self-contamination dose In order to set the rate to SnGy / h or less, the "β ray count number" may be set to 80 S count / 10 h or less, or the "unit number" may be set to 0.25 S unit or less.

Although the present invention is limited to the ionization chamber made of aluminum, even when the outer electrode and the cover are made of a material other than aluminum, the applicability of the material is determined by the same idea. It will be clear that this can be done.

[0048]

According to the invention of claim 1, in the ionization chamber made of aluminum, in order to keep the self-contamination dose rate (hereinafter referred to simply as the self-contamination dose rate) by the outer electrode and the cover below SnGy / h, Since the aluminum material that makes up the electrodes and cover is the aluminum material that has a count of β rays emitted from it of 80S count / 10h or less,
As described in the section of "Embodiment of the Invention", the self-contamination dose rate of the aluminum ionization chamber can be controlled to a desired value or less. For example, if S = 10, the self-contamination dose rate of the aluminum ionization chamber can be made smaller than the minimum measurement value of 10 nGy / h of the ionization chamber.

According to the second aspect of the present invention, in the aluminum ionization chamber, in order to make the self-contamination dose rate SnGy / h or less, the aluminum material for forming the outer electrode and the cover is included in the aluminum material ²³⁸ U. , ²³² Th and ³⁹ K concentration,
^Since 1 ppm of ²³⁸ U is 1.05 units, 1 ppm of ²³² Th is 0.30 units, and 1 ppm of ³⁹ K is 3.0 × 10 ^-5 units, the total concentration of three nuclides is 0.25 S units or less. As described in the section "Embodiment", the self-contamination dose rate of the aluminum ionization chamber can be controlled to be equal to or lower than a desired value. For example, if S = 10, the self-contamination dose rate of the aluminum ionization chamber is 10 nGy which is the minimum measurement value of the ionization chamber.
/ H can be made smaller.

[Brief description of drawings]

FIG. 1 is a partial sectional front view showing the configuration of a spherical ionization chamber.

FIG. 2 is a partial sectional view showing an electrode structure of an aluminum ionization chamber according to a conventional technique.

[Explanation of symbols]

1 Ionization chamber detector 11 Outer electrode 12 Center electrode 13 Filled gas 14 Cover 1a Aluminum ionization chamber detector 11a Aluminum outer electrode 111 Shielding layer 13a Filled gas 14a Aluminum cover

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 2G088 EE09 FF02 FF04 GG01 JJ01                       JJ09 JJ31 JJ37 LL02 LL06                 5C038 DD03 DD04

Claims (2)

[Claims] 1. A central electrode in the center, an outer electrode surrounding the outer side of the central electrode, and a cover for covering the outer side of the outer electrode,
Ionization chamber detection in which a sealed container is formed by the outer electrode and the holding member, which are electrically insulated from each other in their lower part and are held by a holding member, and in which the pressurized gas is enclosed. The outer electrode and the cover are made of aluminum, and the self-contamination dose rate by the outer electrode and the cover is 10 nGy / h.
In order to make the aluminum material smaller than the following predetermined value SnGy / h, the aluminum material is a material whose counted number of β rays emitted from 20 keV or more is 80S count / 10h or less. vessel. 2. A central electrode at the center, an outer electrode surrounding the outer side of the central electrode, and a cover for covering the outer side of the outer electrode,
Ionization chamber detection in which a sealed container is formed by the outer electrode and the holding member, which are electrically insulated from each other in their lower part and are held by a holding member, and in which the pressurized gas is enclosed. The outer electrode and the cover are made of aluminum, and the self-contamination dose rate by the outer electrode and the cover is 10 nGy / h.
In order to make it smaller than the following predetermined value SnGy / h, the above-mentioned aluminum material is included in this, ²³⁸ U, ²³² Th and ³⁹
Depending on the concentration of K, 1 ppm of ²³⁸ U is 1.05 units, ²³² T
An ionization chamber detector characterized in that 1 ppm of h is 0.30 unit and 1 ppm of ³⁹ K is 3.0 × 10 ^-5 unit, and the total concentration of three nuclides is 0.25 S unit or less.