JPS6218873B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the structure of a GM counter used for radiation measurement.

GM counters are most widely used as radiation detectors, and a typical example is an organic gas extinguishing type end window GM counter using a 25mmφ mica thin window for measuring low-energy β-beta rays. can be given. FIG. 1 is a schematic cross-sectional view of an end window GM counter, in which 1 is an entrance window, 2 is a central anode wire, 3 is a cylindrical cathode, 4 is a glass tank, 5 is a counting discharge effective part,
6 is a case, and 7 is a base. In order to obtain good characteristics, the above-mentioned GM counter usually uses the diameter of the entrance window 1 as the length of the discharge effective section 5, that is, the center anode line 2.
Alternatively, the length is set to 1/2 or less of the length of the cylindrical cathode 3. In addition, for a GM counter used in a survey meter, the length of the central anode wire 2 or cylindrical cathode 3 is approximately 100 mm for the diameter of the entrance window 1 of 20 mmφ.
The structure of the general GM counter tube described above is extended in the axial direction. Therefore, to help explain the GM counter having the structure of the present invention and its application effects, the relationship thereof will be explained below by way of example using this GM counter for a survey meter.

First, let's consider measuring the proximity dose of a point gamma-ray source using a survey meter. That is, when a GM counter is placed close to a γ-ray source, a difference in distance from the γ-ray source occurs at the detection portion of the counter. Since this greatly affects the dose rate at each site, it is unclear at which position the measured value corresponds to the dose rate. To further illustrate this numerically, if a point source is placed 300 mm axially from the entrance window 1 of the GM counter for a survey (the measurement method for survey meters is usually to place the point source at the axial entrance window of the GM counter). It is arranged on a line extending outward from 1),
In the part facing the entrance window 1 of the counting discharge effective part 5,
The distance will be 400mm, and the distance difference will be 3:4. On the other hand, the dose rate is 1mR/h at 100mm from a point source.
1/9; 1/16, with a slight difference in the part of the counter, it is approximately
This results in a 1/2 dose rate difference. Separately from the above, considering the solid angle of the counter with respect to the radiation source, the solid angle becomes smaller as you go deeper than entrance window 1, and the measured dose rate is the average of the multiplicative composites of these. From the relationship displayed as a value, it is not easy to determine at what position inside the counter the true dose rate is.

Furthermore, although the glass tank 4 is provided to extend the counting life, the boundary with the counting discharge effective section 5 varies somewhat depending on the energy and quality of the incident radiation, so it may affect the measurement effective volume. This also results in a decrease in the accuracy of the measured values.

The above is an explanation of the state in which the point source and the GM counter are arranged on their axes, but when the GM counter is rotated by 90 degrees at the entrance window 1, the measured dose rate is higher than before rotation. That is, since the measured value varies depending on the orientation of the counter, it is necessary to keep the measurement conditions constant in order to maintain measurement accuracy.

Furthermore, in actual radiation measurements, radiation sources are distributed or non-directional radiation fields including scattered radiation are measured, so it is extremely difficult to measure the true dose rate, and it is difficult to measure the true dose rate. This will be a rough measurement. In this case, in order to measure the dose rate in a certain direction, it is possible to attach a collimator in the target direction and shield the outer surface of the GM counter other than the collimator, but this would result in an extremely large increase in weight and volume. It does not meet the requirements for a survey meter, such as ease of portability.

Therefore, by adopting the structure of the present invention in a GM counter, it is possible to solve the above-mentioned problems and to create a detector that can be used not only for survey meters but also for a wide range of applications with easy and simple operation and accuracy. Therefore, its configuration and application effects will be explained below. First, regarding the structure, FIG. 2 is a schematic sectional view of the GM counter of the present invention, where 8 is a counter, 9 is a shield, 10 is an entrance window, 11 is a case, and 12 is a base. First, regarding the counting section 8, the purpose is to make it unnecessary to obtain the true dose rate for point-like gamma rays. That is, by making the thickness of the GM counter as thin as possible relative to the diameter of the entrance window 10, the counter detection portion becomes short, and the difference in the solid angle of the counter 8 with respect to the point radiation source becomes smaller, reducing measurement errors. These provide the effect of eliminating the need for calculation, determination, etc. of the position at which the true dose rate value is obtained. Next, the thickness of the detection part is set to 1/3 or less of the effective diameter of the entrance window 10, and the entrance window 1
The thickness should be reduced to a level that does not impair the efficiency of radiation incident on the zero plane. For example, the thickness of the detection part is 8 mm with respect to the diameter of the entrance window 10 of 25 mm, and the ratio is set to 1/3 or less. That is, in the counting section 8, the thickness of the effective sensitivity section is kept constant, so that the entire window surface has equal sensitivity with respect to the entrance window surface. The window surface shape may be circular or oval.

FIG. 3 is a schematic cross-sectional view of a parallel disk-shaped halogen GM counter to show an example of the configuration of the counter.
3 is a cathode entrance window, 14 is an anode, 15 is a side wall insulator, and 16 is a counting discharge filled with halogen annihilation gas. The cathode entrance window 13 has a Nesa film on its inner surface,
This serves as a cathode, and a counting discharge section 16 is formed by the anode 14 and the side wall insulating section 15 facing each other.

The NESA film is a conductive thin film obtained by vapor-depositing or sputtering tin chloride or the like on a mica thin window surface and then heating it to a high temperature in an oxygen atmosphere. The resistance is constant and does not change, and it is chemically stable against halogen gas. On the other hand, as for the anode 14, a part of the envelope of the counting discharge section is used in the embodiment, but
Almost all surfaces of the counting discharge section except for the cathode entrance window 13 may be used as an anode.

The counting discharge part 16 forms a short cylindrical hollow part, and has a window diameter and thickness of 25 mmφ and 8 mm as described above, and has a diameter of 25 mm and a thickness of 8 mm.
The thickness of the GM counter tube has been reduced to a certain level, and the wall structure is such that it has a cylindrical hollow section. This wall also serves as a vacuum-resistant container for the GM counter tube.

Next, the shielding body 9 for providing directivity will be described. In general, as a common means of giving directionality,
This is done by a combination of a collimator and a shield, but
In the present invention, a collimator is not used, but a separately manufactured shield 9 is provided to block radiation from entering from the back side of the entrance window 10. On the other hand, the effective cross-sectional area of the counting discharge section 16 of the counter for radiation coming from the side of the counter is reduced by reducing the thickness of the counter, making the detector effective only for radiation entering the entrance window 10. It is something. To further explain these shielding parts, the shielding body is a shielding body of a constant thickness with a lead equivalent of 30 mm or more, such as lead,
It is made of a material with a large mass number, such as gold, silver, copper, iron, or stainless steel, and is integrated with the counter tube. In addition, these are formed to have the same external structural dimensions as the general GM counter shown in Figure 1, and are incorporated into the case 6 and base 7 in Figure 1, and are used not only in survey meters but also in general GM counters.
This makes it compatible with counter tubes and promotes the use of its advantages.

Finally, when the GM counter of this example is used inside a lead-shielded box, the background counting rate is reduced, and both inside and outside the lead-shielded box of 50 mm, compared to a general GM counter, respectively. Approximately 1/2 or less (in numerical terms, the background count rate values of a general GM counter tube and the one of the present invention within a lead shield of 50 mm are 13 ± 2 cpm and 7 ± 1 cpm, and are the same outside the shield) 23±3cpm and 13.5±cpm). These effects are due to the extremely small volume of the discharge counting effective section 8 and the installation of the shield 9 to provide directivity.
This contributes to improving the maximum detection sensitivity in general measurements other than survey meters, especially in the measurement of trace amounts of radiation.

[Brief explanation of the drawing]

FIG. 1 shows a conventional GM counter, FIG. 2 shows an embodiment of the present invention, and FIG. 3 shows a detailed diagram of the counting section. 8... Counter, 9... Shield, 10... Entrance window, 13... Cathode entrance window, 14... Anode, 15...
...Side wall insulation.

Claims (1)

[Claims] 1. The thickness of the counting discharge part forming a rectangular cylindrical hollow part is 1/3 or less of the diameter of the radiation entrance window, and a Nesa film is attached to the inner surface of the radiation entrance window to serve as a cathode, forming the counting discharge part envelope. An end window GM counter characterized in that all or part of the other parts are anodes, and a shield with a lead equivalent of 3 cm or more is provided on the back side facing the entrance window.