JPS61272679A - Measuring container for radioactive gas - Google Patents

Abstract

PURPOSE:To ignore the influence of braking radiation and to improve measurement precision by lining a container where radio-active gas is run and reserved with a material with a low atomic number to thickness larger than the maximum range of beta rays. CONSTITUTION:The tank 1 for radioactive gas measurement is lined 6 with the material with a low atomic number to thickness (range) with which beta rays are cut off to suppress the generation of braking radiation. When the thickness of this lining 6 is less than the range of beta rays, the beta rays reach stainless steel as the material of the tank 1 to generate braking radiation, so the thicker the lining 6, the better so as to cut off the braking radiation. When the thickness is extremely large, on the other hand, the attenuation of gamma rays to be measured themselves become extreme, causing a decrease in precision. Thus, the lining 6 is provided to ignore the influence of braking radiation and also improve the measurement precision.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to the analysis of radioactive gases from nuclear power plants, etc., and in particular to radioactive gas analysis suitable for accurately quantifying radionuclides with extremely low gamma ray emission rates. Regarding gas measurement tanks (commonly known as gas samplers).

[Background of the invention]

A conventional radioactive gas measurement tank (tank for gamma ray measurement) is shown in Fig. 2. This tank 1 is made entirely of stainless steel, and has a structure in which radioactive gas is allowed to flow in from a sample inlet 2 at the bottom and out from a sample outlet 3 at the upper side. A well structure 4 is created in the center of the tank, and a radiation detector 5 is provided therein. Radiation emitted from the radioactive gas in the tank 1 passes through the stainless steel tank 1 and reaches a radiation detector 5 made of NaI (TQ) or the like.

If the radiation incident on the tank 1 is gamma rays and the nuclide has a high gamma ray emission rate, the radiation dose can be measured accurately. However, when the gamma ray emission rate is small (below 1%), the influence of the X-rays generated by the interaction between the beta rays emitted at the same time as the gamma rays and the tank 1 material becomes relatively large, and the accurate radiation Unable to measure. The data for representative radioactive gases are shown in Table 1. Many gamma-ray-emitting nuclides are accompanied by the emission of beta-rays. Table 1 shows the γ-ray energy γ(E), its emission rate B(γ), and the energy of β-rays emitted at the same time B(E).
, its release rate B(β). As can be seen from Table 1, the gamma ray emission rate of 85Kr is very small at 0.43%.

Table 1, Figure 3 shows each gamma ray energy and radioactivity concentration conversion coefficient (
γμCi/cm"・Cp8). The solid line is the calculated value, and the X mark is the value obtained from the experimental (measured) results. As is clear from this result, "Kr (7-ray energy 514KeV) is , it can be seen that there is a large difference from the calculated value. The cause of this is, as described above, that the stainless steel, which is the material of the tank 1, interacts with the beta rays of e5Kr, and the bremsstrahlung X-rays are emitted. Although the β-rays themselves are blocked by the surface layer of the tank, the X-rays generated by the interaction have a large penetrating power and pass through the tank 1, reaching the radiation detector. The actual measurement results in Figure 3 appear to be radioactive concentration conversion coefficients (γμci/cm'・cps) due to the influence of this bremsstrahlung radiation.
is a small value.

As mentioned in Section 2, in conventional radioactive gas measurement tanks,
It is impossible to measure gamma rays with high precision when measuring radioactive gases with low gamma ray emission rates.

[Purpose of the invention]

An object of the present invention is to provide a tank for measuring radioactive gases that greatly improves the measurement accuracy of radioactive gases having a low gamma ray emission rate.

[Summary of the invention]

The present invention examines the interaction between β rays, which are emitted at the same time as γ rays, and the tank material, and has determined that the main factor in the generation of X-rays caused by the interaction between β rays and tank material is bremsstrahlung radiation of β rays. This is due to the discovery of

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIG. Radioactive gas is caused to flow in from the lower part of the radioactive gas measurement tank 1 and flowed out from the upper side surface. A well structure 4 is created in the center of the tank 1, and a radiation detector 5 is provided therein. The inner surface of the tank 1 is lined with a lining 6 of a material with a low atomic number such as beryllium metal or polytetrafluoroethylene (Teflon). The thickness of the lining shall be the same as or greater than the maximum range of β-rays of Kr.The maximum range of β-rays R
is determined by the following formula.

R (m g/cm2) = 407 ・E””E: Maximum energy of β ray (MeV) The maximum energy of β ray is 687 K e V (0,
687M e V), and the maximum range is 242mg/
cm2. The thickness of the lining can be determined by multiplying the maximum range of this β ray by the density ρ- of the material used for the lining. For beryllium, it is approximately 4.5 mm, and for Teflon, it is approximately 5.5 mm. The emission rate of bremsstrahlung radiation is β
It depends on the atomic number of the material the ray is incident on.

In other words, the lower the atomic number of the material, the less bremsstrahlung radiation is generated. The amount of bremsstrahlung radiation generated differs depending on the material and the energy of the incident β rays, but the amount of bremsstrahlung radiation generated between the β rays of Kr and stainless steel is approximately the same as the number of γ rays emitted.

Based on the above, the radioactive gas measurement tank should be made of low atomic number materials (atomic number (2) of beryllium = 4, Teflon (
Bremsstrahlung radiation can be suppressed by lining Z)-8-8) with a thickness that can block Kr's β-rays. If the range is below the range, β rays will reach the stainless steel (atomic number (Z) of the main component iron = 26), which is the material of tank 1, and generate bremsstrahlung radiation. The thickness of the lining is preferably thicker in order to prevent the generation of bremsstrahlung radiation.However, if it is too thick, the attenuation of the gamma rays itself to be measured will be significant, resulting in a decrease in measurement accuracy. From this point, the thickness of the lining material is β
It can be said that it is good to have the same range as the line.

FIG. 4 shows the γ-ray energy and radioactivity concentration conversion coefficient (γμci) when using the radioactive gas measurement tank of the present invention.
/cm''·cpm).Similar to FIG. 3, the solid line is the calculated value, and the X mark is the actually measured value.Compared to the conventional method shown in FIG. 3, more reliable results can be obtained.

As a modification of the present invention, it is possible to line only the area surrounding the well structure with a material having a low atomic number, or to line a tank or piping other than the well structure with the same material. Furthermore, although the present invention has been described with reference to an embodiment that deals only with Kr, it can be similarly applied to other radioactive substances that have a low gamma ray emission rate and pose a problem in terms of bremsstrahlung radiation.

□ [Effect of the invention] According to the present invention, the influence of bremsstrahlung radiation can be ignored when measuring radioactive gases such as Kr that have a low gamma ray emission rate, and the accuracy is improved by more than 50% compared to conventional measurement tanks. can be achieved.

[Brief explanation of the drawing]

Fig. 1 shows an embodiment of the present invention, Fig. 2 shows a conventional radioactive gas measurement tank, and Fig. 3 shows the gamma ray energy and radioactivity concentration conversion coefficient of a conventional radioactive gas measurement tank. FIG. 4 is a diagram showing the function of γ-ray energy and radioactivity concentration conversion coefficient by the tank of the present invention. DESCRIPTION OF SYMBOLS 1...Tank, 2...Sample inlet, 3...Sample outlet, 4...Well structure, 5...Radiation detector, 6...
- Lining made of low atomic number material.

Claims (1)

[Claims] 1. In a container that measures gamma rays by installing a radiation detector outside the container that ventilates and stores radioactive gas or inside the well structure, a material with a low atomic number is released into the container simultaneously with the gamma rays to be measured. A radioactive gas measuring container characterized by being lined with a thickness greater than the maximum range of β rays.