JPS61265593A - Method for measuring radioactivity - Google Patents

Abstract

PURPOSE:To measure radioactivity with high accuracy, by receiving a measuring specimen discharging alpha-rays in gas or liquid containing a light element having high probability generating (alpha, n) reaction and converting alpha-rays to neutron by (alpha, n) reaction when alpha-rays were discharged from a radiation source. CONSTITUTION:A specimen 11 is received in a container 12 and hermetically sealed by a lid 12A. In this state, measurement (first measurement) for counting a neutron is performed by using a <3>He neutron detector 19. The output of a preamplifier 23 is amplified by a main amplifier and measured by a counter through the peak analyser of a post-stage. After measurement, air in the container 12 is exhausted by an exhaust pipe 15. Next, gas containing a light element having high probability generating (alpha, n) reaction is injected in the container 12 by a suction pipe 14. In this state, BF3 gas is flowed even into the container 12 of the specimen 11 and the measurement (second measurement) for counting the neutron generated by (alpha, n) reaction between an alpha-ray radiation nuclide and each of elements B, F is performed. alpha-radioactive rays can be evaluated on the basis of the count value obtained by subtracting the first measured value from the second measured value.

Description

DETAILED DESCRIPTION OF THE INVENTION "Industrial Application Field" The present invention relates to a radioactivity measurement method that can accurately measure α radioactivity regardless of the shape of a measurement sample.

"Prior Art" Generally, when measuring the alpha radioactivity of an object, a detector such as a gas flow type proportional counter, a scintillation detector, or a semiconductor detector is used.

FIG. 2 shows a conventional α using a semiconductor detector as an example.
This shows the radioactivity measurement method.

The measurement container 1 is connected to a vacuum pump (not shown) through a vacuum connector 2, and the inside of the measurement container 1 containing the radiation source 3 is maintained at a substantially vacuum state. A detector 4 is arranged within the measurement container 1 facing the radiation source 3 . The α rays emitted from the radiation source 3 travel in vacuum, and the semiconductor detector 4
What is incident on the object is detected. The detection signal is analyzed by a measurement circuit system (not shown), and necessary data such as the α-ray intensity in the measurement sample 3 is obtained.

By the way, alpha rays have a very weak ability to penetrate materials, and even if there is only one piece of paper between the radiation source and the detector, the alpha rays will be blocked by this, making measurement impossible. This can be clearly seen from the fact that, for example, when alpha rays with an energy of 5 MeV are incident on a material with a density of several g/cc, they can only penetrate through a thickness of several micrometers to several tens of micrometers. α
Therefore, the absorption of the wire is significant even in the air. -For example,
If the energy of the alpha ray is similarly 5 MeV, its range in air is only 3.5 cm.

This is the reason why the inside of the measurement container 1 is kept in a vacuum.

Because alpha radiation has such a very weak penetrating power, the radiation source to be measured by the detector cannot have a complex shape or be located far from the detector. The radiation source 3 housed in the measurement container in FIG. 2 is, for example, a disc-shaped or flat-plate object 3A as shown in FIG. It was necessary that the α-ray emitting nuclide 6 be attached to the surface). That is, the alpha rays emitted from the radiation source had to be incident on a detector such as the semiconductor detector 4 without being disturbed on the way.

"Problems to be Solved by the Invention" FIG. 4 shows an example of a radiation source that is not suitable for measuring alpha radioactivity. This object 3B is a cylindrical metal container with a small opening, and an α-ray emitting nuclide 6 is attached to its inner surface. With such a shape, almost no alpha rays can escape from the container, making it impossible to measure them as is. Therefore, conventionally, a radiation source having such a complicated shape has been cut and measured so that the α-ray emitting nuclide 6 directly faces the detector. For this reason, as the shape of the radiation source becomes more complex, the work to cut it becomes more difficult, and the number of cut pieces increases, leading to a corresponding increase in the number of measurements.

In view of these circumstances, an object of the present invention is to provide a radioactivity measurement method that can accurately measure α radioactivity even in objects with complex shapes without destroying the object that serves as a radiation source. shall be.

"Means for Solving the Problems" In the present invention, a measurement sample that emits α rays is housed in a gas or liquid containing light elements that have a high probability of causing an (α, n) reaction, and α rays are emitted from the radiation source. When the line is emitted, we define this as (α, n
) converted into neutrons in a reaction. By measuring the number of neutrons generated in this way, the alpha radioactivity of the source can be indirectly measured. However, depending on the radiation source (α, n)
It is possible that neutrons are emitted separately from the reaction.

Therefore, in order to accurately measure α radioactivity, (α, n
) It is effective to measure neutrons without placing the measurement sample in a gas or liquid that contains light elements that have a high probability of causing a reaction, and then calculate the difference in the measurement results and correct for α radioactivity.

Typical examples of light elements with a high probability of causing a (α, n) reaction include aluminum, boron, oxygen, and fluorine neon. These are placed in a container in a gas or liquid state, and the (α, n) reaction is carried out by the radiation source in this container.

"Example" The present invention will be described in detail with reference to Examples below.

Outline of Measuring Apparatus FIG. 1 shows an example of a measuring apparatus to which the radioactivity measuring method of the present invention is applied. A container 12 containing a measurement sample (radiation source) 11 is arranged in the center of this apparatus, and a measurement section 13 is arranged around the container 12 to surround it. An intake pipe 14 and an exhaust pipe 15 are attached to the lid 12A of the container 12, so that gas is taken into and discharged from the container 12. Lid 1
The lower surface of the container 2A is brought into close contact with the edge of the container body 12B by a sealing member 16, so that air cannot freely flow in from the outside and gas within the container 12 cannot freely flow out.

A polyethylene cylinder 17 with a wall thickness of about 15 cm is arranged in the measuring section 13. Polyethylene is used as a neutron moderator. The cylinder 17 has 10 to 20 through holes 18 equally spaced at equal distances from its central axis, and one 3 He neutron detector 19 is inserted into each of these through holes 18. ing. 3 He each
The upper end of the neutron detector 19 is attached to a connector box 21. The output signals of these detectors 19 are combined and connected as a signal cable 22 to a signal input of a preamplifier 23. A signal cable 24 is connected to the signal output side of the preamplifier 23 for transmitting a signal to a main amplifier (not shown).

The inside of the container 12 is a cylindrical space with a diameter of about 20 cm and a height of about 25 cm. In this example, a stainless steel cylindrical container with a bottom diameter of about 1Qcm is used as the measurement sample 1.
It was accommodated as 1. An α-ray emitting nuclide is attached to the inner wall of this cylindrical container.

As already explained, it has been difficult to measure the measurement sample 11 having such a shape in the past.

Measuring method Measurements of alpha radiation are carried out in the following order:

(1) When the measurement sample 11 is placed in the container 12, the lid 12A
Seal this with. At this time, the inside of the container 12 is filled with surrounding air.

(2) In this state, neutron counts are measured using the 'He neutron detector 8. The output of the preamplifier 23 is amplified by the main amplifier, passes through a pulse height analyzer in the subsequent stage, and is counted by a counter. A timer is used to set the measurement time. The measurement performed in this manner will be referred to as the first measurement.

(3) After the first measurement is completed, the air inside the container 12 is removed using the exhaust pipe 15.

(4) Next, using the intake pipe 14, a gas containing a light element with a high probability of causing the (α, n) reaction is injected into the container 12.

By the way, when an α-ray emitting nuclide such as U (uranium) or Pu (plutonium) is attached to the measurement sample 11, α-rays with energies shown in Table 1 below are emitted.

The energy of α-rays varies depending on the type of nuclide, but it can be seen that α-rays of approximately 5 MeV are emitted.

Table 1: Probability of emitting a neutron when such an α ray with an energy of approximately 5 MeV hits a nuclide, i.e. (α, n)
Nuclides with high neutron emission rates are shown in Table 2 below.

(Margin below) Table 2 In this example, BF3 (boron trifluoride) gas was injected into the container 12 as a compound containing B (boron) with a high (α, n) neutron emission rate. The BF3 gas in the container 12 is circulated through an exhaust pipe 15 and an intake pipe 14 to a gas supply source (not shown). In this state, BF3 gas also flows into the container of the measurement sample 11, and an (α, n) reaction occurs between the α-ray emitting nuclide and each element of BSF (fluorine), and neutrons are emitted.

(5) In this state, neutron counting is performed using the 'He neutron detector 8. The measurement time etc. are the same as the first measurement. The measurement performed in this manner will be referred to as the second measurement.

(6) From the neutron counts obtained in the second measurement, the first
Subtract that of the measurement of. The count value after this subtraction is proportional to the intensity of the alpha rays. Therefore, alpha radioactivity can be evaluated using this method.

In the apparatus of this example, the neutron detection efficiency of all 3 He neutron detectors 8 placed around the measurement sample 11 is 4%. If 1 mci of α-ray emitting nuclide is attached to the inner wall of the measurement sample 11, approximately 5 Me V of α-rays will be emitted. Half of it is (α, n)
Assuming that a reaction occurs, the number of neutrons counted per second is determined by the following formula.

=4. Also, here the value 3.7X10-' is the α-ray emission rate per second, and the other value 5.48X10-6 is (α, n)
is the neutron emission rate.

From this equation, it can be seen that approximately 4 neutrons can be counted per second. This is a sufficiently significant quantity to count. If the measurement time is set to 100 seconds, approximately 4
00 neutron counts will be given.

"Possibility of modification of the invention" Above, we have explained an example in which the (α, n) reaction occurs in a gas, but a similar reaction can be caused in a liquid and indirectly α
It is also possible to measure radiation.

"Effects of the Invention" As explained above, according to the present invention, alpha radiation can be measured without destroying the measurement sample, so the measurement target is not limited and rapid measurement is possible.

[Brief explanation of the drawing]

Fig. 1 is an end view of a measuring device used in an embodiment of the present invention, Fig. 2 is a configuration diagram of a device used in a conventional measuring method, and Fig. 3 is a conventional measuring device capable of non-destructive measurement. A perspective view showing an example of a measurement sample. FIG. 4 is a perspective view showing an example of a measurement sample that has been difficult to measure in the past. ε α-ray emitting nuclide, 11 Measurement sample. Applicant: Japan Atomic Energy Corporation, Agent
Patent attorney Umeo Yamauchi Figure 1

Claims (1)

[Claims] 1. A measurement sample that emits α rays is housed in a gas or liquid containing a light element that has a high probability of causing a (α, n) reaction;
(α, n) A radioactivity measurement method for measuring the α radioactivity of the measurement sample by measuring the number of neutrons produced as a result of the reaction. 2. (α, n) Measure the number of neutrons emitted from the measurement sample in a state where the measurement sample is not contained in a gas or liquid containing a light element that has a high probability of causing a reaction, and calculate the number of neutrons emitted from the measurement sample. 2. The radioactivity measuring method according to claim 1, wherein the measurement result is subtracted from the measurement result of the measurement sample in a state where the measurement sample is accommodated, to obtain the measurement result after correction of α radioactivity. 3. Add beryllium, boron,
2. The method for measuring radioactivity according to claim 1, wherein at least one of the elements oxygen, fluorine, and neon is present in a gas or liquid state.