JPH068859B2 - Device for measuring β-radionuclide content in food - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to an apparatus for measuring β-radionuclide content in foods, particularly various β-radioactive substances in acceptance inspection of imported foods and investigation of radionuclide content in foods. The present invention relates to the configuration of a device for measuring

[Prior Art] In recent years, the radionuclides contained in food have become a problem due to the occurrence of the nuclear accident at Chernobyl, and there are various radionuclides in nature, and radioactive nuclides mixed in food have been identified. Radiation measuring devices for investigation include gas counter measuring devices, scintillation detectors and semiconductor detectors.

FIG. 2 shows a schematic configuration of a gas counter measuring device. The detector 10 is provided with an entrance window 12 having a thickness of about 1.5 to 3.0 mg / cm ² , and the detector 10 A detection electrode (anode) 14 is arranged in the center of the inside. Also, the detector 1
An application electrode (cathode) 15 is provided on the inner wall of 0, whereby a predetermined high voltage is applied. And the detector 1
Radiation is detected by enclosing or circulating a counting gas such as argon or helium in 0.

A single-channel or multi-channel wave height analyzer 18 is connected to such a detector 10 via a preamplifier 16, and the wave height analyzer 18 selects radiation of a predetermined energy to be measured. The wave height analyzer 18 is provided with a counter 20 and a display 22. The pulse signal output from the wave height analyzer 18 is counted by the counter 20 and this counting device is displayed on the display 22. To be done.

Conventionally, the radioactive nuclide contained in the food is detected by measuring the γ-ray in the food using such a gas counter tube. In general, most radionuclides that emit β-rays simultaneously emit γ-rays, so that the radioactive concentration in food can be measured by measuring the γ-rays.

In addition, the radioactivity concentration in the food is the scintillation detector,
It is also performed using a semiconductor detector.

[Problems to be Solved by the Invention] However, when the radiation in food is measured using the conventional γ-ray measuring device, there are the following problems.

In the γ-ray measurement, the ionizing charge generated by the photoelectric effect is effective for the spectrum measurement, and the Compton effect or the electron pair generation is not used for the measurement.
If only a part of the line is detected and the specific activity is low, a large amount of sample must be measured.

Measurement takes time.

The equipment is expensive.

Has professional skills in operation.

Maintenance costs are required for semiconductor detectors. For example, in a semiconductor detector, liquid nitrogen is used for cooling in order to maintain the detection performance of the semiconductor, and it is expensive to maintain this liquid nitrogen.

In addition, routine measurements for measuring predetermined radionuclides are performed at airports and the like using conventional γ-ray measuring instruments. In this routine measurement, the energy region to be measured is set to a predetermined region by a channel of a wave height analyzer, and only specific radiation is measured in a flow-like manner. The following problems occur in this routine measurement. is there.

The electric circuit for measurement is complicated, and the measured value is also likely to fluctuate.

The background is too high to detect the radiation dose of the target nuclide well.

Since a relatively large amount of sample is required and the energy measurement condition is an electrical measurement condition, for example, the voltage is large, calibration must always be performed using a standard γ-ray source.

As described above, the detection of γ-rays is easy, but there are various problems in the measurement of trace amounts of radionuclides in foods.

Therefore, in the present invention, β in food released simultaneously with γ-rays
Focusing on the radiation, we propose to measure this β-ray to measure the radioactivity concentration in food.

Conventionally, β-ray measuring devices themselves exist, but conventional devices cannot separate and measure low-energy β-rays emitted from foods and other substances, and cosmic rays cause measurement errors. There was a problem that it appeared as.

The present invention has been made as a problem to solve the above conventional problems, the object is a β-radionuclide content in a simple food that can be separately measured β-rays in food, routine measurement is easy It is to provide a measuring device.

[Means for Solving the Problem] In order to achieve the above object, the β-radionuclide content measuring device in food according to the present invention is a first detection for detecting low-energy β-rays emitted from food. And a second detector having an absorption layer that is provided on the rear side of the first detector and that does not transmit the low-energy β-rays, and is provided on the rear side of the second detector. A third detector having an absorption layer that does not transmit β rays other than low energy, and the first detector.
An inverse coincidence counting circuit connected to the detection unit and the second detection unit for counting only pulses due to low energy β rays emitted from the food, and a low concurrency circuit connected to the second detection unit and the third detection unit. And an inverse coincidence counting circuit for detecting β rays other than energy.

[Operation] According to the above configuration, β of low energy is detected in the first detection unit.
Rays are detected, β rays other than low energy are detected by the second detector, and cosmic rays (hard cosmic rays) are detected by the third detector, but low energy is detected by the first detector. Other than β
Since both rays and cosmic rays will be incident at the same time, only the electric pulses that were not detected simultaneously by the output of the first detector and the output of the second detector by the anti-simultaneous counter circuit are extracted, and low energy β rays are generated. To be measured. As a result, for example, ¹³⁴ Cs
(Cesium), ¹³⁷ Cs can be measured.

In addition, since cosmic rays are incident on the second detector at the same time,
Inverse coincidence counting of the output of the second detector and the output of the third detector is performed to measure β rays other than low energy,
Thereby, for example, ⁴⁰ K (potassium) or ⁹⁰ Sr (strontium) ^-90 Y (yttrium; daughter nuclide of ⁹⁰ Sr) can be measured.

[Embodiment] A preferred embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows the configuration of a β-radionuclide content measuring device in food according to an example, and Sample 24 is 500 mg / cm ²
The thickness is set to about 79 cm ² (diameter: 10 cm) so that β rays in the sample can be easily detected. The sample 24 only needs to have a certain thickness, and it is not necessary to increase the sample amount. This is because it is sufficient that the thickness of the sample 24 is equal to or larger than the maximum range of low energy β rays, and the maximum range is short.

Therefore, in the present invention, it is possible to measure radiation with a small amount of sample, and it is possible to measure with an amount of about 1/10 as compared with the conventional γ-ray measurement. Since the β ray of ⁹⁰ Sr- ⁹⁰ Y is about 2.4 MeV, the range in water is 1 cm, and the sample 24 having the above thickness can be detected without any problem.

A first detection unit 26 including a disc-shaped gas flow counter and a plastic scintillator is arranged at the upper surface of the sample 24, and an entrance window 28 is formed in the first detection unit 26. The incident window 28 has an area larger than that of the sample 24, for example, 79 cm ² , and a thickness of 0.9 to ² mg / cm ^2, and the incident window 28 transmits β-rays of low energy or higher and high energy. Let

The second detector 3 is provided on the rear side of the first detector 26.
0 is arranged, and the first absorption layer 32 is formed on the second detection unit 30. This second detection unit 30 has a first
The detection unit 26 has the same configuration as that of the first detection unit 26, and its detection surface is made slightly wider than that of the first detection unit 26, so that β-rays obliquely transmitted through the first detection unit 26 can be detected.

The first absorption layer 32 is formed of aluminum or plastic having a thickness of 200 mg / cm ² , which prevents the transmission of low energy β-rays, while ⁴⁰ K or ⁹⁰ Sr- ^90.
Beta rays such as Y and cosmic rays (hard cosmic rays) are transmitted.

Further, a third detector 3 is provided on the rear side of the second detector 30.
4 is arranged, and the second absorption layer 36 is formed on the third detection section 34. Like the first detection unit 26, the third detection unit also includes a disc-shaped gas flow counter or a plastic scintillator, and has a detection surface slightly wider than that of the second detection unit 30. As a result, β rays that obliquely pass through the second detection unit 30 are detected.

The second absorption layer 36 is formed of aluminum or plastic having a thickness of about 1000 mg / cm ² , which blocks transmission of all β rays while transmitting hard cosmic rays.

The detection units 26, 30, 34 stacked in this manner
And the sample 24 is housed in a heavy shield 38 of lead 10 cm,
External radiation (α rays, X
The line) is cut, and the heavy shield 38 can reduce the background to about 2 to 3 cpm.

The first reverse coincidence counting circuit 40 is connected to the first detection unit 26 and the second detection unit 30,
The second inverse coincidence counting circuit 42 is provided so as to be connected to the third detection unit 34 and the first inverse coincidence counting circuit 40 measures low energy β-rays.
Measure β rays other than low energy by 2.

That is, low-energy β-rays are detected only by the first detection unit 26, so that the first inverse coincidence counting circuit 40 counts the electric pulses only when the first detection unit output is received, Beta rays of energy can be measured. In this case, β rays and cosmic rays other than low energy are detected by both detectors 26 and 30, and the electric pulses thereof are simultaneously supplied to the first reverse coincidence counting circuit 40, so that they are not counted.

In addition, medium and high energy β rays other than low energy are detected by the second detector 30 but not by the third detector 34, so the second inverse coincidence counting circuit 42 detects the second detector. By counting the electric pulses only when there is an output, β rays other than low energy can be measured. In this case, cosmic rays are detected by both detectors 30 and 34, and their outputs are simultaneously supplied to the second inverse coincidence counting circuit 42, so they are not counted.

The first inverse coincidence counting circuit 40 and the second inverse coincidence counting circuit 42 respectively include a processing circuit 44 for performing post-counting processing,
46 is provided.

The embodiment has the above configuration, and its operation will be described below.

Low-energy β radionuclides contained in food, eg ¹³⁴
The energy of Cs and ¹³⁷ Cs are 0.66 MeV and 0.5, respectively.
1 MeV, maximum range 230 mg / cm ² , 160 mg / cm ²
Therefore, the distance in water is about 2 mm and 1.6 mm.
Therefore, when measuring ham or the like as the sample 24, since the specific gravity of ham is 1.2 g / cm ³ , a thickness of about 2 mm is sufficient, and as described above, the width of 79 cm ² is used. 1 is inserted below the detection unit 24.

In this way, in the case of ham, about 19g is enough.
Compared to the case of line measurement, it is possible to measure β-radionuclide content with an extremely small amount.

The low-energy β rays emitted from the ham sample 24 are detected only by the first detection unit 26 and do not reach the other detection units. Therefore, β doses such as ¹³⁴ Cs and ¹³⁷ Cs are accurately measured by the first inverse coincidence counting circuit 40.

Further, a β-ray of medium and high energy ⁴⁰ K and ^{90 Sr-90} Y
Passes through the first detection unit 26, reaches the second detection unit 30, does not reach the third detection unit 34, and the second detection unit 30
Is counted in. Therefore, the β dose other than low energy is measured by the second inverse coincidence counting circuit 42.

In this case, the medium and high energy β-rays are also detected by the first detection unit 26, which is the same as the first detection unit 26 and the second detection unit 26.
Since the output pulses of the detection unit 30 of 1 appear at the same time, they are not counted in the first reverse coincidence counting circuit 40 and are erased.

Further, cosmic rays (hard cosmic rays) are captured by all the detection units of the first detection unit 26, the second detection unit 30, and the third detection unit 34, but are erased by the double-inverse coincidence counting circuits 40 and 42. By not being counted and counted, this makes it possible to reduce the background sufficiently.

Then, the first and second inverse coincidence counting circuits 40 and 42
Is supplied to the processing circuits 44 and 46, and after the predetermined processing is performed, the counting device is displayed on a display (not shown).

Since the limit of the provisional radioactivity concentration in imported food is 370 Bq (becquerel) / Kg, the low energy β-ray measurement device is a β-ray emission rate from the sample (rate that appears on the sample surface).
Even if is set to 1/20, there are about 20 counts per minute (cpm), and sufficient measurement is possible.

As for the radionuclide in the imported food, at present, only ¹³⁴ Cs and ¹³⁷ Cs that emit low energy β rays need to be considered, but ⁴⁰ K existing in nature and ⁹⁰ Sr flying in the air can be used.
-The influence of substances that emit medium- and high-energy β-rays such as ⁻⁹⁰ Y will become a problem in the future, so the significance of not only measuring low-energy ones but also measuring separately from other than low-energy ones will increase. .

[Effects of the Invention] As described above, according to the present invention, the first detection unit that detects low-energy β rays and the second detection unit that has a different absorption layer
, And the third detection unit are arranged, and these are arranged so as to be overlapped for inverse coincidence counting. Therefore, low energy β rays such as ¹³⁴ Cs and ¹³⁷ Cs and ⁴⁰ K, ⁹⁰ Sr- ⁹⁰ Y, etc. Beta rays other than low energy can be separated and measured, and routine measurement for each target nuclide can be easily performed.

Further, the present invention can provide an apparatus for measuring β-radionuclide in food by measuring β-rays, can solve various problems occurring in γ-ray measurement, reduce product cost, shorten measurement time. Therefore, it is possible to reduce the capacity of the measured object.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a β-radionuclide content measuring device for food according to an embodiment, and FIG. 2 is a diagram showing a configuration of a conventional scintillation counter. 12, 28 ... Incident window 18 ... Wave height analyzer 20 ... Counter 22 ... Indicator 26 ... First detector 30 ... Second detector 32 ... First absorption layer 34 ... Third detector 36 ... Second Absorption layer 38 ... Heavy shield 40 ... First coincidence counting circuit 42 ... Second coincidence counting circuit.

Claims (1)

[Claims] 1. A first detection part for detecting low energy β-rays emitted from food and an absorption which is provided over the rear side of the first detection part so as not to transmit the low energy β-rays. A second detection unit having a layer, a third detection unit having an absorption layer that is provided behind the second detection unit and that does not transmit β rays other than low energy, and the first detection unit. And an inverse coincidence counting circuit that is connected to the second detection unit and that counts only the pulses due to the low-energy β rays emitted from the food,
An apparatus for measuring β-radionuclide content in food, comprising an inverse coincidence counting circuit connected to the second detection unit and the third detection unit to detect β rays other than low energy.