JP2023109040A - portable becquerel meter - Google Patents

Abstract

To provide a portable Becquerel meter which can be carried around and used for on-site radioactivity measurement.SOLUTION: A portable Becquerel meter is provided, comprising a first computation unit configured to derive a dose rate according to a count rate of radiation detected by a radiation detector, a ranging processing unit for measuring a distance L to a measurement target, data memory storing a 1 cm dose equivalent rate constant Γ corresponding to the dose rate, and a second computation unit configured to derive radioactivity A using the following equation: A=(DL2)/Γ, where A represents radioactivity (Mbq) of the measurement target, D represents a dose rate (μSv/h) measured by a survey meter, L represents a distance (m) measured by a range finder, and Γ represents the 1 cm dose equivalent rate constant (μSv m2/MBq/h).SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention generally relates to a technology related to a becquerel monitor, and more particularly to a portable becquerel meter capable of carrying and measuring radioactivity on site.

After the nuclear accident at the Fukushima Daiichi Nuclear Power Station in March 2011, radiation dose measurements (μSv, μSv/h) and Radioactivity measurements (Bq, Bq/kg) of contaminated soil, etc. are being carried out.

A dosimeter (hereinafter referred to as a “survey meter”) is used for radiation dose measurement (Patent Document 1). Survey meters are generally small and lightweight, easy to carry, and not too difficult to handle.

On the other hand, a becquerel monitor is used for radioactivity measurement (Patent Document 2). A Becquerel monitor is a device specialized for radioactivity measurement and is a large and heavy object including a shield. For this reason, many Becquerel monitors are installed in dedicated facilities or rooms (measurement rooms) and measurements are performed by specific personnel.

Workers use survey meters on site to measure radiation levels in real time, while collecting radioactive materials, that is, measurement objects that contain or are likely to contain radionuclides (contaminated soil, etc.) as necessary. , the sampled object to be measured is brought to a radioactivity measurement facility and a request for radioactivity measurement is performed.

Survey meters and becquerel monitors also differ in calibration for maintaining/confirming performance. Survey meters are calibrated using radiation fields produced by radiation sources with known radiation dose rates. In contrast, the Becquerel monitor uses a standard radiation source. This standard radiation source is made of a material that has the same or similar physicochemical shape as the measurement object collected in the measurement container or the like, and the contained radioactivity is priced by a reliable institution.

In addition, irradiation sources and standard radiation sources differ greatly in terms of working life (usable period). The working life of the radiation source is eg 15 years, the standard radiation source is eg 3 years.

As can be seen from the above, radiation dosimetry and radioactivity measurement are understood to be separate fields in the workplace.

JP 2019-109120 A JP 2020-193811 A

Ten years have passed since the Fukushima Daiichi Nuclear Power Plant accident, and at work sites such as interim storage facilities related to the accident, a field of γ-rays emitted mainly from Cs-137 is formed (γ-ray field). Suitable portable survey meters are used to measure the radiation dose of these gamma fields. For example, a survey meter using a Si semiconductor, a survey meter using a gas detector, and a survey meter using a solid scintillator (hereinafter referred to as "scintillation survey meter") can be exemplified, and the scintillation survey meter is particularly often used. there is A survey meter can measure the radiation dose in real time on site. At work sites, there is a demand to measure radioactivity in real time with the same convenience as this survey meter.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a portable Becquerel meter that is portable and capable of on-site radioactivity measurement.

A further object of the present invention is to provide a portable becquerel meter that is as simple as a portable survey meter and capable of rapid radioactivity measurement on site.

It is a further object of the present invention to provide a portable becquerel meter that does not require a standard source of relatively short working life.

Radiation dose in a γ-ray field (hereinafter referred to as "Cs-137 γ-ray field") created by a measurement object that contains or may contain Cs-137 (hereinafter referred to as "Cs-137 radiation source") using a general-purpose survey meter The concept of the present invention will be described below using the case of obtaining the as a typical example. In this example, the survey meter obtains the “1 cm dose equivalent rate (hereinafter referred to as “dose rate”)” as the radiation dose of the Cs-137 γ-ray field by the following formula 1.

(Formula 1) D= _CND ×N
D: Dose rate of Cs-137 γ-ray field (μSv/h)
N: Count rate of Cs-137 γ-ray field (cps)
C _ND : ND coefficient of Cs-137 (μSv/h/cps)
(ND coefficient is a coefficient for converting count rate to dose rate)

As a modification, 1 cm dose equivalent (hereinafter referred to as "dose") may be measured instead of the dose rate. The original data obtained from the measurement at this time is not the count rate but the cumulative count. By mechanically dividing the value of the integrated count by the integrated time, the count rate N required for the calculation of Equation 1 can be obtained. For this reason, dose rate and dose are substantially equivalent.

The portable becquerel meter according to the present invention obtains the amount of radioactivity according to the following equation 2 in relation to the survey meter.

(Equation 2) A=(DL ² )/Γ
A: Radioactivity of Cs-137 source (MBq)
D: Dose rate of Cs-137 γ-ray field measured by survey meter (μSv/h)
L: Distance (m) measured with a rangefinder
Γ: 1 cm dose equivalent rate constant of Cs-137 (μSv・m ² /MBq/h)

Among the three parameters included in Equation 2 above, the dose rate D can be obtained by the same method as for existing survey meters. Also, the 1 cm dose equivalent rate constant Γ of Cs-137 should be registered in advance as internal data in the "data memory" (see 144 in Fig. 1) of the portable becquerel meter. The 1 cm dose equivalent rate constant Γ can be obtained from well-known literature such as the Isotope Notebook.

Currently, Cs-137 is considered to be the main radionuclide contained in the objects to be measured at Fukushima work sites. Therefore, in Equations 1 and 2, it is assumed that the value of Cs-137 is used for the ND coefficient C _ND and the 1 cm dose equivalent rate constant Γ to obtain the radioactivity of Cs-137 contained in the measurement object. When the nuclide contained in the object is different from Cs-137, the measurement principle can be similarly explained by using the ND coefficient _CND and the 1 cm dose equivalent rate constant Γ of each nuclide.

In addition, the radiation dose measured by survey meters at many sites is the "1 cm dose equivalent rate." For this reason, Γ can be defined as "1 cm dose equivalent rate constant" in Equation 2 above. However, for survey meters that measure radiation doses in other units, it is necessary to use coefficients and constants according to the measured doses. For example, if there is a survey meter that measures the "effective dose rate" as radiation dose, when measuring with that survey meter, the ND coefficient C _ND is the coefficient that converts the count rate into the effective dose rate, and Γ is the effective dose A rate constant should be used. That is, coefficients and constants corresponding to the respective radiation doses to be measured are used as the values of _CND and Γ.

A portable Becquerel meter in accordance with the present invention includes a range finder that measures the distance to an object. Known rangefinders using ultrasonic waves, infrared rays, and laser light, for example, can be exemplified as the rangefinder.

A functional block diagram of the portable becquerel meter of the present invention is shown in FIG. The portable becquerel meter 2 of the present invention firstly emits from a measurement object (limited to FIG. 1, hereinafter referred to as a “radiation source”) that contains or may contain a radionuclide such as Cs-137. It has a radiation dose processing unit 10 that captures and counts the emitted radiation, converts the counting rate into a dose rate and quantifies (digitizes), and secondly, measures the distance to the object to be measured. and a processing unit 12 .

The radiation dose processing unit 10 includes a “radiation detector” 102 that detects radiation from the radiation source and a “signal processing/dose calculation unit” that calculates the dose rate based on the radiation count rate detected by the “radiation detector” 102. circuit" 104, and preferably has a "μSv/h display" 106 that generates display data of the dose rate (μSv/h) obtained by the "signal processing/dose calculation circuit" 104 as the radiation dose measurement result. .

The distance measurement processing unit 12 includes a “distance detector” 122 that detects the time difference and phase difference between light projection and light reception in order to measure the distance to the radiation source, and the time difference and position obtained by the “distance detector” 122 . and a ``signal processing/distance calculation circuit'' 124 that calculates a distance based on the phase difference. Preferably, a ``distance ( m) Display '126.

The portable becquerel meter 2 of the present invention further includes a radiation dose processor 10 and a radioactivity processor 14 that obtains dose rate data and distance data from the ranging processor 12 to determine the radioactivity of the radiation source. The radioactivity processing unit 14 has a "radioactivity/calculation and/or evaluation circuit" 142 that obtains the radioactivity of the radiation source by calculation based on Equation 2 explained in the above-described specific example. In the above-described specific example, the 1 cm dose equivalent rate constant Γ required to determine the radioactivity of the radiation source is stored in the "data memory ” 144.

The portable becquerel meter 2 preferably generates "radioactivity (Bq) display" 146, "measurement result notification" 148, " ALARM TRANSMISSION” 150.

The portable Becquerel meter of the present invention can theoretically measure accurate radioactivity for point radiation sources, but the measured values for plane radiation sources and volume radiation sources are approximate values. However, it is portable and allows for on-site radioactivity measurement, and enables rapid on-site radioactivity measurement with the same convenience as a portable survey meter.

The effects of the present invention and other objects of the present invention will become clear from the detailed description of the embodiments below.

1 is a functional block diagram of the portable becquerel meter of the present invention; FIG. FIG. 4 is a diagram for explaining the hardware configuration of the portable becquerel meter of the embodiment;

Preferred embodiments of the invention are described below with reference to the accompanying drawings. Referring to FIG. 2, the portable becquerel meter 20 of the embodiment measures the distance to the measurement object (radiation source in FIG. 1, ¹³⁷ Cs point radiation source in FIG. It has a hardware structure to which a rangefinder 6 is added, and a display unit 8 is provided. As is known, the rangefinder 6 illustratively uses ultrasonic waves, infrared rays, and laser light for distance measurement.

As is already known, the scintillation survey meter 4 (FIG. 2) photoelectrically converts and amplifies the fluorescence emitted by the scintillator detecting radiation, thereby measuring the object to be measured (the radiation source in FIG. 1, the radiation source in FIG. 2). The dose rate or dose is obtained based on the count rate or counting of the radiation incident from the ¹³⁷ Cs point source in

The portable becquerel meter 2 (corresponding to 20 ⁱⁿ FIGS. 1 and 2) further has a rangefinder 6 (FIG. 2). The distance L to the point source) is measured.

Referring to FIG. 1, the portable becquerel meter 2 has a second computing section (incorporated in 142) that determines the radioactivity A based on Equation 2 above. The second calculation unit takes in the dose rate D, the distance L, and the 1 cm dose equivalent rate constant Γ registered in the "data memory" 144 to obtain the radioactivity A (MBq).

Strictly speaking, the above equation 2 is valid for a point radiation source. On the other hand, an actual measurement object has an area or volume. For this reason, the portable becquerel meter 2 (corresponding to 20 in FIGS. 1 and 2) cannot accurately measure the radioactivity of the object to be measured at the work site in a strict sense. However, the portable becquerel meter 2 can measure approximate radioactivity. At the work site, there is a demand to know the amount of radioactivity in real time, even if it is just an approximate value. This need has increased in recent years as processes involving the transport of contaminants have increased. The portable becquerel meter 2 of the present invention can meet this need.

Errors contained in the portable becquerel meter 2 (corresponding to 20 in FIGS. 1 and 2) can be compensated for by measurement experiments and simulations. For example, a widely polluted wall can be considered to consist of an infinite number of n point radiation sources. Therefore, if the dose rate D of the γ-ray field created by the radioactivity A1, A2, . . . An of each point radiation source is D1, D2, . can be specified.

The portable becquerel meter 2 (corresponding to 20 in FIGS. 1 and 2) has a display section 8 (FIG. 2), and the measurement results are displayed on the display section 8 in real time. Thereby, the user can read the value of radioactivity in real time by looking at the display unit 8 at the site.

The portable becquerel meter 2 (corresponding to 20 in Figs. 1 and 2) has a basic display mode that displays the measured radioactivity numerical value on the display unit 8 (Fig. 2), and a numerical display of the dose rate or dose on the display unit. and a distance display mode for numerically displaying the distance to the object to be measured (radiation source in FIG. 1, ¹³⁷ Cs point radiation source in FIG. 2). Then, it is preferable that the radioactivity amount, the dose rate or the dose, and the distance are alternatively displayed on the display unit 8 according to the user's selection.

As a modification, the amount of radioactivity and dose rate or dose may be displayed at the same time, the amount of radioactivity and distance may be displayed at the same time, or the three values of radioactivity, dose rate or dose, and distance may be displayed. may be displayed at the same time.

The portable becquerel meter 2 (corresponding to 20 in FIGS. 1 and 2) of the present invention can selectively display radioactivity, dose rate or dose, and distance on the display unit 8 (FIG. 2) according to the user's selection. By doing so, it can be used as a becquerel meter, a survey meter, and a rangefinder.

When numerical values are displayed on the display unit 8 (FIG. 2), the user moves the line of sight from the object to be measured to the display unit 8 of the portable becquerel meter 2 (corresponding to 20 in FIGS. 1 and 2) to recognize the measured value. be able to. In a work site, when the object to be measured cannot be taken off from the eye, for example, when the measurement work is continuously progressed while looking at the object to be measured, the display on the display unit 8 cannot be visually confirmed. Assuming this situation, the portable becquerel meter 2 of the present invention has an additional function of generating a number of click sounds (counts/sec) according to the level of the measured value, or reading out the measured value aloud. A "measurement report" 148 (Fig. 1) may be included to provide. The same is true for "alarm transmission" 150 (FIG. 1).

As another option of the "measurement result notification" 148, instead of or in addition to the sound notification, the number of flashes of light or vibration strength notification according to the level of the measurement value is provided. It is good to be As a result, the operator can see the "radioactivity (Bq) display" 146 by moving the line of sight and perceive the approximate measured value through sound, light, and vibration without confirming the measured value. can. The same is true for “alarm transmission” 150 .

2 portable becquerel meter 4 general-purpose scintillation survey meter 6 distance meter 8 display unit

Claims (2)

a radiation detector that detects radiation from an object to be measured;
a first calculation unit for obtaining a dose rate based on the radiation count rate detected by the radiation detector;
a distance measurement processing unit that measures the distance L to the measurement object;
a data memory storing a 1 cm dose equivalent rate constant Γ corresponding to the dose rate;
A portable becquerel meter, comprising: a second calculation unit for obtaining radioactivity A based on the following formula.
A = ( ^DL2 )/Γ
A: Radioactivity of object to be measured (MBq)
D: Dose rate measured by survey meter (μSv/h)
L: Distance (m) measured with a rangefinder
Γ: 1 cm dose equivalent rate constant (μSv・m ² /MBq/h) The portable becquerel meter of claim 1, wherein
It further has a display unit with an expanded radioactivity (Bq) display function,
As the display mode of the display unit,
A basic display mode that displays the measured radioactivity values,
A dose display mode for numerically displaying the dose rate or dose obtained by the first calculation unit;
a distance display mode for numerically displaying the distance to the measurement object,
These basic display mode, dose display mode, and distance display mode can be selected by the user.

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