JPS5923610B2 - proportional counter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a proportional counter, and particularly to an improvement in a proportional counter that accurately measures the concentration of trace amounts of radioactive substances contained in a sample gas while taking background into account.

゛For measurement of trace amounts of radioactive substances contained in sample gas,
For example, gas phase proportional counters have been used in the past.

In a conventional proportional counter, in order to improve the S/N ratio during measurement, a center counter for sample measurement is surrounded by a guard counter for shielding cosmic rays, and the outside thereof is further surrounded by a heavy shield.

FIGS. 1 and 2 are explanatory diagrams of a conventional proportional counter having such a structure.

In the proportional counter shown in FIG. 1, a center counter 10 and a guard counter 12 surrounding the center counter 10 are formed separately.
A central center counter 10 is surrounded by a plurality of guard counters 12.

In addition, the proportional counter shown in FIG.
It has a double cylindrical structure in which the guard counter 12 and the cover are integrated, and the anode wire 10a for the center counter is stretched in the center, and around it are the cathode tube 10b for the center counter, the anode wire 12a for the guard counter, and the anode wire 12a for the guard counter. Cathode tube 1
2b are sequentially provided concentrically.

Reference numeral 14 denotes a heavy shield that surrounds the guard counter 12 and reduces disturbances during measurement.

Here, the measurement of trace amounts of tritium in the air using each of the proportional counter tubes is performed by passing sampled air through the center counter 10 and utilizing the ionization phenomenon caused by the β rays emitted by the tritium. It will be done.

However, in addition to tritium, the air generally contains other natural radioactive isotopes, such as Rn, Th, and their daughter nuclides, and like tritium, the alpha rays emitted by these natural radioactive isotopes also This causes ionization within the air, contributing to an increase in the S/N ratio.

In order to prevent deterioration of the S/N ratio due to such an increase in background, alpha rays emitted from the natural radioactive isotopes are made to enter the guard counter 12, the dose is detected, and both counters 10 , 12 may be processed by the backward counting method.

However, in the proportional counter shown in FIG. 1, the radiation emitted from the natural radioactive isotope within the center counter 10-, such as α-rays, does not enter the guard counter 12Vc, and therefore the α-ray dose mentioned above is cannot be detected, and an increase in the S/N ratio is unavoidable.

Further, in the proportional counter shown in FIG. 2, the cathode cylinder 10b for the center counter is formed using a synthetic resin thin film coated with double-sided gold or aluminum vapor deposition, or a metal pipe.
The cathode tube 10b prevents the beta rays of tritium or the like emitted within the center counter 10 from entering the guard counter 12.

However, when the cathode tube IQb having such a structure is used, the radiation emitted from the natural radioactive isotope in the center counter 10 is also absorbed by the cathode tube 10b, and hardly reaches the guard counter 12. As a result, the guard counter 12 cannot measure the radiation emitted from the natural radioisotope, and an increase in background due to this is unavoidable.

In addition, since each of the above proportional counter tubes seals a sample gas containing radioactive substances into the center counter 10, radioactive substances adhere to the inner wall of the cathode cylinder 10b, which may cause an increase in the background when measuring the sample gas later. Cause.

As a means for removing contamination caused by such radioactive substances, a method is generally used in which the center counter 10 is heated and evacuated to an ultra-vacuum state.

However, if the center counter cathode tube 10b is formed of the synthetic resin thin film described above, it cannot be heated to a high temperature and it is difficult to remove the radioactive substance contamination.

For this reason, contamination increases each time it is used, and it cannot be used stably for a long period of time.

The present invention was made in view of the above-mentioned conventional problems, and its purpose is to provide a proportional counter that can stably and accurately measure the concentration of any radioactive substance contained in a sample gas for a long period of time. There is a particular thing.

In order to achieve the above object, the present invention provides a center counter comprising a center counter anode wire stretched in the center and a mesh-like center counter cathode tube arranged on the outside;
A mesh-like guard counter cathode tube is arranged coaxially on the outside of the center counter cathode tube, and a plurality of guard counter anode wires parallel to the center counter electrode wire are stretched in a ring shape on the outside of the mesh-like guard counter cathode tube. and a guard counter consisting of a guard counter, and a gap between the center counter cathode tube and the guard counter cathode tube is a β-ray absorption layer.

Next, preferred embodiments of the present invention will be described based on the drawings.

FIG. 3 is a side sectional view showing an embodiment of a proportional counter according to the present invention. In this embodiment, a proportional counter for measuring the concentration of trace amounts of tritium contained in air will be explained as an example.

The proportional counter of the present invention includes a sealed case body 20, and is housed and fixed in the case body 20, and allows a mixed gas of the above-mentioned air sampling air and methane gas to pass through the inside of the case body 20. At this time, the proportional counter tube is caused by the β-rays emitted by the above-mentioned tritium. A center counter 10 that measures the concentration of tritium in the air using the ionization generated by the air, and a center counter 10 that is formed integrally with the center counter 10 to measure the concentration of tritium in the air, and a center counter 10 that measures the concentration of tritium in the air using ionization. and a guard counter 12 that measures ionization caused by radiation of radioactive isotopes.

The sealed case 20 consists of a cylindrical outer tube 22 and a lid 28.30 whose openings at both ends are fixed to the outer tube 22 with closing screws 24.26, and between the outer tube 22 and the lids 28, 30. Airtightness is maintained by ring-shaped packings 32 and 34.

A mixed gas of sampling air and methane gas flows into the airtight case 20 from an inflow duct 36 disposed on one lid 28, and a mixed gas of sampling air and methane gas flows into the airtight case 20 from an exhaust duct 38 disposed on the other lid 30. The gas is exhausted.

・40.42 is a base for mounting a pair of counters provided at opposing positions in the outer cylinder 22, one base 40 is attached to the inner surface of the outer cylinder 22 on the inflow duct side, and the other base 4
2 is attached to the inner surface of the lid 3o.

Further, the center counter 10 is formed by extending a center counter anode wire 10a at the center and arranging a mesh-like center counter pole part '10b around the center counter anode wire 10a.

Here, since a high voltage is applied to the anode wire 10a, one end of the anode wire 10a is connected to a Teflon insulator 44 at the center of the base 40.
The other end is attached to a connector 50 provided at the center of the lid 30 through an insertion hole 48 formed at the center of the other base 42.

Note that Teflon 46 is provided on the inner surface of the insertion hole 48 of the base 42 in order to maintain insulation from the anode wire 10a.

The center counter cathode tube 10b is made of a stainless steel photoetching plate formed into a cylindrical shape, and the cathode tube 10b has the anode wire 10a located at the center of the cylinder, and both ends of the cathode tube 10b are connected to the respective bases 40, 42. Fixed.

Mixed gas passage holes 40a are provided in both bases 40 and 42 so that the mixed gas flowing from the inflow duct 36 into the center counter 10 thus formed passes.

42a is formed.

Further, as shown in FIG. 4, the guard counter 12 has a coaxial cylindrical mesh-like guard counter cathode tube 12b disposed outside the center counter cathode tube 10b, and further includes a center counter anode wire 10a. A plurality of guard counter anode wires 12a parallel to the cathode tube 1
It is formed in a coaxial cylindrical shape extending around the outer periphery of 2b.

The guard counter cathode tube 12b, like the center counter cathode tube 10b described above, is made of a cylindrical stainless steel photo-etched plate, and both ends thereof are attached to each base 40, 42.

Further, since a high voltage is applied to the plurality of guard counter anode wires 12a arranged in a cylindrical shape, a pair of Teflon support members 52, 54 are provided in a ring shape on the inner wall at both ends of the outer cylinder 22. Both ends are stretched.

Note that the support member 52 provided on the inflow duct 36 side allows the mixed gas flowing from the inflow duct 36 to pass through the guard counter 12.
The gap between the base 40 and the inner wall of the outer cylinder 22 is closed to prevent it from flowing to the side.

56 is an anode wire 12a provided on the lid 28 for guard counter.
It is a connector that is connected to the

In this example, the total capacity of the proportional counter is 264
1, and the effective volume of the center counter 10 was set to 0.51.

In addition, each of the cathode tubes 10b and 12b has an aperture ratio of 91 by photoetching, and the cathode tubes 10b and 12b have an aperture ratio of 91.
The gap length 1 between the gaps 12b was set to about 7 mm in order to function as a β-ray absorption layer to be described later.

The proportional counter formed in this manner is used inside a heavy shield (not shown) in order to reduce measurement errors during use.

The present invention has the above configuration, and its operation will be explained next.

First, in order to measure the concentration of trace amounts of tritium in the air, it is necessary to effectively ionize the inside of the center counter 10.

For this purpose, a mixed gas of methane gas and sampling air mixed at an appropriate ratio is caused to flow into the proportional counter tube from the inflow duct 36.

This mixed gas flows into and through the center counter 10 through a passage hole 40a formed in the base 40, and further passes through a passage hole 42a of the other base 42 and is discharged to the exhaust duct 38.

Due to the flow of the mixed gas in the center counter 10, the emitted β rays of tritium contained in the mixed gas cause ionization, and by measuring this ionization state, the tritium concentration in the air can be measured. I can do it.

Here, as mentioned above, there are natural radioactive isotopes other than tritium in the air, and alpha rays emitted from these elements and cosmic rays incident from space also cause ionization within the center counter 10. Wake up.

Therefore, accurate measurements cannot be made unless the background amount is subtracted from the measured value of the center counter 10.

In the present invention, since this background component can be accurately measured by the guard counter 12, accurate tritium concentration can be determined using the reverse counting method in which the measured value of the guard counter 12 is subtracted from the measured value of the center counter 10. Measurements can be taken.

That is, in the present invention, the maximum range 11 of β-rays emitted by tritium contained in the air is approximately 6 molar range in air, and the maximum range 11 of β-rays emitted by tritium contained in the air is approximately 6 molar ranges, and the maximum range 11 of β-rays emitted by tritium contained in the air is about 6 miles. Taking advantage of the fact that the distance 1□ is about 5 cmH degree, by setting the distance 1 between the center counter cathode tube 10b and the guard counter cathode tube 12b to 7 mm, which is the maximum range of β rays of 6 mm or more, The cathode cylinder 10b emits β rays emitted by tritium contained in the sampling air.
All of the β rays are attenuated and absorbed within the gap A surrounded by 12bK, thereby preventing the β rays from entering the guard counter 12.

Furthermore, by providing pores with an aperture ratio of 91 on the surface of each cathode tube 10b and 12b, alpha rays emitted by natural radioactive isotopes contained in the sampling air pass through the pores of the center counter cathode tube 10b. The light then passes through the pores of the guard counter cathode tube 12b and enters the guard counter 12.

Therefore, in the guard counter 12, in addition to cosmic rays,
Background components such as α rays emitted by natural radioactive isotopes contained in the sampling air in the center counter 10 can be accurately measured.

In addition, the mixed gas containing radioactive materials is transferred to the center counter 10.
The cathode cylinder 1 of the center counter 10
The inner wall of the 0b becomes gradually contaminated with radioactive substances such as tritium, which becomes a factor in increasing the background when measuring the concentration of tritium to be performed later.

However, unlike conventional center counters, the center counter 10 of the present invention uses a photo-etched plate with an aperture ratio of 91 for its cathode tube 10b, so compared to the conventional cathode plate, the effective surface area is approximately It is easily understood that the radioactive contamination is also reduced to about 10 times the conventional level.

Therefore, there is little background increase due to radioactive contamination, and tritium concentration can be measured stably for a long period of time.

Furthermore, the cathode tube 10b is made of stainless steel and can be easily designed to withstand vacuum, so that even if the radioactive contamination occurs, the center counter 10 can be heated and evacuated to a high vacuum to prevent the radioactive contamination. can be easily removed.

In this example, a mixed gas of sampling air and methane gas was used at a ratio of 1:9.

FIG. 5 shows a plateau curve for β rays emitted by tritium contained in such a mixed gas, where a indicates the plateau curve for the guard counter 12;
b is a plateau curve related to the center counter 10.

The following table shows the obtained characteristics.

As is clear from the table, the proportional counter of the present invention can measure the tritium concentration contained in the air with a high detection efficiency of 80 factors.

Based on this, the detection limit of tritium in air aμCi
/ce is calculated from the following formula: NBG background 4.8 cpmE: Triticum detection efficiency 0.
80 (2): Center counter effective volume 500c, c Therefore, a=7.8X 10'μci/Cr:.

In this example, a proportional counter used for measuring tritium concentration in air was explained, but the present invention
The present invention is not limited to this, and it is also possible to measure the concentration of any trace amount of radioactive substance contained in other sample gases.

However, in this case, it is necessary to make the distance between the cathode tube for the center force sensor and the detent section for the guard sensor larger than the maximum range of the β rays emitted by the radioactive substance whose concentration is to be measured.

As described above, according to the present invention, the cathode tube for the center force counter and the cathode tube for the guard counter are formed in a mesh shape, are arranged in a coaxial cylindrical shape, and the intervals between the cathode tubes are adjusted to absorb β-rays. By forming a layer, it becomes possible to accurately measure the concentration of any radioactive substance contained in the detection gas.

In addition, by forming the surface of each cathode cylinder in a mesh shape and increasing the aperture ratio, the area that is contaminated with radioactivity can be reduced, and as a result, accurate measurements can be stably performed over a long period of time. .

Furthermore, since the radioactively contaminated cathode tube can be made heat resistant, contamination can be easily removed by heating.

[Brief explanation of the drawing]

Figures 1 and 2 are structural explanatory diagrams of a conventional proportional counter;
FIG. 3 is a side sectional view of one embodiment of the proportional counter according to the present invention, FIG. 4 is an explanatory diagram showing the main parts thereof, and FIG. 5 is a plateau for β rays emitted by tritium contained in a mixed gas. It is a curve diagram. Corresponding members in each figure are given the same reference numerals, 10 is a center counter, 10a is an anode wire for the center counter, 10
b is an anode wire for the center counter, 12 is a guard counter, 12a is an anode wire for the guard counter, and 12b is a cathode tube for the guard counter.

Claims (1)

[Scope of Claims] 1. A center counter comprising a center counter anode wire stretched in the center and a mesh-shaped center counter cathode tube arranged outside the anode wire, and a mesh-shaped center counter cathode tube outside the center counter cathode tube. a guard counter comprising cathode tubes for guard counter arranged coaxially and a plurality of anode wires for guard counter parallel to the anode wire for center counter stretched in a ring shape on the outside thereof; A proportional counter characterized in that the gap between the cathode tube for the guard counter and the cathode tube for the guard counter is made into a β-ray absorption layer. 2. In the proportional counter according to claim 1, the distance between the cathode tube for the center counter and the cathode tube for the guard counter is set to be greater than or equal to the maximum range of β rays emitted by the radioactive substance measured in the center counter. A proportional counter characterized by setting.