Portable becquerel sievert survey meter

Abstract

To provide a portable becquerel sievert survey meter which is portable and can measure radioactivity at a site.SOLUTION: A portable becquerel sievert survey meter includes a dose meter, a range meter and a display part, and has a function of determining a dose rate D of a measurement object and a function of measuring a distance L with the measurement object, and further has a function of determining measured radioactivity by a following expression A on the basis of a measured representative distance LM, a function of determining a longest distance LMax on the basis of the distance L measured by the distance meter, and determining upper limit radioactivity by a following expression B on the basis of the longest distance LMax, and a function of displaying the measured radioactivity and the upper limit radioactivity on a display part. (Expression A) Measured radioactivity=(DLM2) / Γ; and (Expression B) Upper limit radioactivity=(DLMax2) / Γ.SELECTED DRAWING: Figure 7

Description

[Technical field]

[0001] The present invention relates generally to a technology related to a becquerel monitor, and more particularly to a portable becquerel-sievert survey meter that can be carried around and used to measure the radioactivity of an entire object on-site. The portable becquerel-sievert survey meter of the present invention allows the safety of an object to be evaluated in real time on-site. [Background technology]

[0002] Following the nuclear accident at the Fukushima Daiichi Nuclear Power Plant in March 2011, radiation levels in the surrounding environment (μSv, μSv / h) and radioactivity levels in contaminated soil (Bq, Bq / kg) have been measured at work sites such as reactor decommissioning sites and radioactive contaminated material storage sites.

[0003] A dosimeter (hereinafter referred to as a "survey meter") is used to measure radiation dose (Patent Document 1). Survey meters are generally small and lightweight, easy to carry, and not particularly difficult to use, so that workers at the site can use them relatively freely.

[0004] On the other hand, a becquerel monitor is used for measuring radioactivity (Patent Document 2). A becquerel monitor is a device specialized for measuring radioactivity, and is a large, heavy object that includes a shield. For this reason, most becquerel monitors are installed in dedicated facilities or rooms (measurement rooms), and measurements are performed by specific personnel.

[0005] Workers use survey meters on-site to measure radiation levels in real time, and if necessary, they also collect samples of radioactive materials, i.e., objects that contain or are likely to contain radioactive nuclides, and take the collected contaminated soil, etc. to a radiation measurement facility to have their radiation levels measured.

[0006] Survey meters and becquerel monitors also differ in the way they are calibrated to maintain and verify their performance. In the case of survey meters, calibration is performed using a radiation field created by an irradiation source with a known exposure dose rate. In contrast, a becquerel monitor uses a standard radiation source. This standard radiation source is made of a material with the same or similar physicochemical shape as the measurement target collected in a measurement container, and its radioactivity content is priced by a reliable organization. In addition, irradiation radiation sources and standard radiation sources differ greatly in terms of their working lives (period of use). The working life of an irradiation radiation source is, for example, 15 years, while that of a standard radiation source is, for example, 3 years.

[0007] As can be seen from the above, in the workplace, radiation dose measurement and radioactivity measurement are understood to be separate fields. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2019-109120 A [Patent Document 2] JP 2020-193811 A Summary of the Invention [Problem to be solved by the invention]

[0009] Now, 10 years after the Fukushima Daiichi Nuclear Power Plant accident, gamma-ray fields emitted mainly from Cs-137 are formed at work sites such as intermediate storage facilities related to the accident (gamma-ray fields). Portable survey meters suitable for measuring the radiation dose of such gamma-ray fields are used. Examples include survey meters using silicon semiconductors, survey meters using gas detectors, and survey meters using solid scintillators (hereafter referred to as "scintillation survey meters"). Scintillation survey meters are particularly commonly used. Survey meters can measure radiation doses in real time on-site.

[0010] At work sites, there is a demand to evaluate whether the radioactivity of the entire object being measured is within a safe range to ensure the safety of workers. In other words, there is a demand to perform a safe radioactivity evaluation in real time on-site with the same convenience as a survey meter, in order to ensure the safety of workers.

[0011] An object of the present invention is to provide a portable becquerel-sievert survey meter that is portable and capable of evaluating radioactivity on-site from a safety perspective.

[0012] A further object of the present invention is to provide a portable becquerel-sievert survey meter that is capable of quickly and safely evaluating radiation on site with the same ease as a portable survey meter. [Means for solving the problem]

[0013] In order to solve the above technical problems, the present invention provides A portable becquerel-sievert survey meter that is equipped with a dosimeter, a distance meter, and a display unit and can be carried by a worker, A dose calculation unit for calculating a dose rate D of a measurement object by the dosimeter; A distance measurement processing unit that measures a distance L to the measurement object; A memory storing a 1 cm dose equivalent rate constant Γ corresponding to the dose rate; The representative distance L measured toward the object to be measured M An actual radioactivity calculation unit that calculates the actual radioactivity based on the following formula A; Based on the distance L measured by the range finder, the longest distance L Max a maximum distance determination unit for determining The longest distance L Max and an upper limit radioactivity calculation unit for calculating an upper limit radioactivity by the following formula B based on the A portable becquerel-sievert survey meter is provided, characterized in that the actual measured radioactivity and the upper limit radioactivity can be displayed on the display unit. (Formula A) Measured radioactivity = (DL M2 ) / Γ (Formula B) Upper limit radioactivity = (DL Max 2 ) / Γ

[0014] The effects and other objects of the present invention will become apparent from the detailed description of the following embodiments. In the description of the embodiments, the portable becquerel sievert survey meter may be abbreviated as "becquerel sievert survey meter", "BS survey meter", or "portable becquerel meter". [Brief description of the drawings]

[0015] [Figure 1] FIG. 13 is a diagram for explaining the results of a radiation measurement simulation performed using a virtual circular contamination model. [Diagram 2] FIG. 1 is a diagram for explaining the physical meaning of parameters included in Equation 4 and Equation 5. [Diagram 3] FIG. 1 is a diagram for explaining the theory for determining the theoretical radioactivity emitted by a real radiation source. [Figure 4] 1 is a diagram for explaining the meaning of the terms shortest distance LMin and longest distance LMax in surface contamination. FIG. [Diagram 5] 1 is a diagram for explaining the meaning of the terms shortest distance LMin and longest distance LMax, taking a cylindrical three-dimensional contaminant as an example. FIG. [Figure 6] FIG. 1 is a diagram for explaining the hardware configuration of a portable Becquerel Sievert survey meter according to an embodiment of the present invention. [Figure 7] FIG. 2 is a functional block diagram of a portable Becquerel Sievert survey meter according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLES

[0016] The following describes the concept underlying the present invention, taking as a typical example the case of using a general-purpose survey meter to determine the radiation dose in a gamma ray field (hereinafter referred to as a "Cs-137 gamma ray field") created by a measurement object that contains or may contain Cs-137 (hereinafter referred to as a "Cs-137 radiation source"). In this example, the survey meter determines the "1 cm dose equivalent rate (hereinafter referred to as the "dose rate")" as the radiation dose in the Cs-137 gamma ray field using the following formula 1.

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[0017] As a modified example, 1 cm dose equivalent (hereinafter referred to as "dose") may be measured instead of dose rate. In this case, the original data obtained by the measurement is the integrated count rather than the count rate. The count rate N required to calculate Equation 1 can be obtained by mechanically dividing the integrated count value by the integration time. Therefore, dose rate and dose are essentially equivalent.

[0018] Work sites such as intermediate storage facilities related to the Fukushima Daiichi nuclear accident are subject to gamma ray fields emitted mainly from Cs-137, and since the nuclide is fixed, the dose rate (μSv / h) of the Cs-137 gamma ray field measured with a survey meter can be calculated using the following formula 2.

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[0019] If we rewrite the above equation 2 into a formula to calculate radioactivity A, we get the following equation 3.

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[0020] In the above formula 3, a survey meter with a distance measurement function (hereinafter referred to as a "portable becquerel sievert survey meter" or its abbreviation "becquerel If the distance L is measured using a BS survey meter, sometimes called a "Bequerel Meter," "BS Survey Meter," or "Portable Becquerel Meter," the radiation A can be calculated on-site based on the above formula 3. Here, the dose rate D obtained with a BS survey meter relates to a point source. Therefore, the radiation calculated based on formula 3 is the radiation at a single point contained in the object being measured.

[0021] To ensure the safety of workers on-site, what they want to know is not the radioactivity of a point, but the radioactivity of an object to be measured, which has an area and volume. In other words, workers want to know in real time whether it is within the safe range (hereinafter referred to as "safety side estimated radioactivity"). Therefore, the radioactivity calculated based on the above formula 3 is not necessarily the correct radioactivity. To confirm this, a radioactivity measurement simulation was performed assuming a circular contamination model spreading on a plane. The results are shown in Figure 1.

[0022] In Figure 1, the horizontal axis shows circular contamination in 10cm increments, with a maximum radius of 400cm. The circles indicate the correct amount of radioactivity contained in the contamination. The triangles indicate the results of measurements calculated using a portable Becquerel Sievert Survey Meter (BS Survey Meter). Comparing the circles and triangles, it was found that the difference between the two was within 10% up to a contamination radius of about 50cm. This makes it possible to assess radioactivity as a guideline for safety.

[0023] However, as the radius increases, the underestimation increases. In fact, the results of simulations of radiation measurements for circular contamination with radii of 100 cm, 200 cm, 300 cm, and 400 cm, expressed in terms of relative radiation, were 0.80, 0.57, 0.44, and 0.35. These are inappropriate as a safe estimate of radiation levels.

[0024] However, in cases where the shape and contents are almost fixed, such as with contaminated soil flexi-con bags, an appropriate correction factor (safety correction factor) can be determined in advance based on measurements and surveys to make an appropriate, safe evaluation, and then by multiplying the measured value, it is possible to obtain an approximate radioactivity that is not an underestimate. For example, in the case of circular contamination with a maximum radius of 400 cm in Figure 1, multiplying by "5" can obtain radioactivity that is corrected to the safe side and is not an underestimate. However, this method can only be applied in special cases.

[0025] To solve this problem of underestimation, we investigated practical measurable quantities (practical measurable quantities) that never underestimate radioactivity from the perspective of ensuring worker safety. Practical measurable quantities based on a safe interpretation that does not underestimate radioactivity can be obtained by calculation based on the following formulas 4 and 5.

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[0026] The physical meaning of the parameters appearing in the above formulas 4 and 5 will be explained based on the actual radiation source shown in Figure 2. With reference to Figure 2, As in formulas 4 and 5 is the safety assessment radioactivity. M is the 1 cm dose equivalent rate (μSv / h) measured by a dosimeter that is part of the BS survey meter. M is the distance (representative distance) from the effective center to the measurement point used in the calculation based on the above formula 3. This representative distance L M means the actual measured distance to the measurement point when an operator points the BS survey meter at an actual radiation source.

[0027] L in the above formulas 4 and 5 Min , L Max are the shortest and longest distances from the effective center to the actual radiation source (Figure 2). Usually, the operator points the survey meter at the point that is considered to be the closest. With this in mind, the representative distance LM is the shortest distance L Min It corresponds to the representative distance L M = shortest distance L Min When it is considered that the radiation doses A1, A2, ..., An of the point radiation sources are generated, the dose rate D of the gamma ray field is D1, D2, Di, Dn. The dose rate D of the entire real radiation source can be determined from the sum of the dose rates D. The shortest distance L Min Regarding the method of calculating the distance, the representative distance L measured by the operator using a distance meter M The representative distance L changes every time the measurement point to which the BS survey meter is pointed is changed. M The displayed value is changed in real time, and the smallest measured value, that is, the representative distance L M The shortest distance L Min It may be determined as:

[0028] Considering the theoretical radioactivity emitted by a real radiation source with reference to FIG. 3, it can be said that the radioactivity of the real radiation source is the sum of the radioactivities of the multiple point radiation sources Pi contained in the real radiation source. In other words, the real radiation source can be considered to be composed of N point radiation sources Pi (i=1 to N). The distance Li shown in FIG. 3 is the distance from any point radiation source Pi to the effective center. Di indicates the dose rate formed by the point radiation source Pi at the effective center. Ai is the radioactivity of the point radiation source Pi. A T is the theoretical activity of the actual radiation source. Based on the above formula 3, the radiation activity Ai of the point radiation source Pi can be calculated based on the following formula 6.

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[0029] Theoretical radioactivity of real radiation source A T Since is the sum of the radioactivity Ai of the point source Pi, the theoretical radioactivity A is calculated based on the following equation 7. T can be obtained.

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[0030] The distance L from the measurement point on-site to the effective center is the same as that of all point radiation sources P N The representative distance L to the effective center of M In other words, assuming that the actual distance is the actual radiation source A obtained by measurement M can be expressed by the following Equation 8.

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[0031] All point sources P included in the real source N In each point source P N The distance to the effective center is the shortest distance L Min Assuming that, the radioactivity of the real source is a minimum of A Min (the "lower activity limit"). Conversely, for all point sources P N The distance to the effective center is the maximum distance L Max Assuming that the radiation of the actual source is the maximum value A Max ("Upper limit radioactivity"). This relationship is expressed by the following inequality 9.

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[0032] The above inequality 9 can be expressed as the following inequality 10.

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[0033] In order to obtain the upper and lower limits of radioactivity using a BS survey meter, the maximum distance L Max and the shortest distance L Min must be determined by actual measurement or other methods, and then calculated by a calculation circuit. Below, we will outline how to determine this, assuming that a circular surface contamination and a cylindrical three-dimensional contamination are the radiation source as typical examples.

[0034] (1) For circular surface contamination: The characteristic of circular surface contamination is that it has an area but no volume. Therefore, the radiation source has no depth. This allows the longest and shortest positions on the radiation source to be visually observed. Therefore, the longest distance L can be determined by direct measurement using a distance meter (e.g., a laser distance meter) equipped on the BS survey meter. Max and the shortest distance L Min can be obtained.

[0035] Referring to FIG. 4, FIG. 4 illustrates a case where a BS survey meter is placed at a position shifted from the center of a circular surface contamination. The distance actually measured by the distance meter of the BS survey meter is the shortest distance L Min The distance from the measurement point to the farthest point is the longest distance L Max If the BS survey meter is installed in alignment with the center of the circular surface contamination, the maximum distance L Max Since the time period is shorter, the upper limit of radioactivity becomes smaller.

[0036] Even if the surface contamination is an elliptical surface or an inclined surface, as long as the surface contamination is a surface contamination with no depth, the longest / shortest positions can be determined by visual inspection. Therefore, the longest distance L Max and the shortest distance L Min By measuring the distance L and inputting it into the circuit, the upper and lower limits of radioactivity can be calculated. Max and the shortest distance L Min Regarding the measurement of the representative distance L, which changes every time the measurement point to which the BS survey meter is pointed is changed, M The displayed value is changed in real time and the smallest representative distance is displayed. M The shortest distance L Min or the largest representative distance M The longest distance L Max It may be determined as follows.

[0037] In case of steric contamination: The decontamination waste is packed into bags called "flexi-con bags" and piled up in temporary storage areas. The flexi-con bags containing the decontamination waste are roughly cylindrical in shape. If we call this a "cylindrical three-dimensional contaminated object," the characteristic of a cylindrical three-dimensional contaminated object is that it has volume as well as surface area. Therefore, the radiation source has depth. Even with such a radiation source, if the longest and shortest positions are visible, the longest distance L can be determined by direct measurement using the distance meter of a Becquerel Sievert survey meter. Max and the shortest distance L Min Therefore, the measured maximum distance L Max and the shortest distance L Min The upper and lower limits of radioactivity can be calculated based on the above formula. However, in many cases, the maximum distance L Max The location of the radiation source is behind the source, which cannot be seen from the Becquerel Sievert survey meter. Therefore, it is not possible to see it directly, so the distance to the longest point L Max This problem can be solved by using a distance meter or measuring tool other than the Becquerel-Sievert survey meter, or by manually inputting estimated values ​​based on surveys and experience into the subject circuit. For example, the shortest distance L Min Display the shortest distance L Min By inputting the estimated value, for example the diameter of the flexible container bag, the estimated maximum distance L Max Of course, the shortest distance L Min Maximum distance L from Max When it is possible to estimate the maximum distance L, the operator can manually input it. Max may be determined.

[0038] Referring to FIG. 5, the three-dimensional contaminant shown has a cylindrical shape, typically a FIBC bag. The shortest distance L Min The location of the BS survey meter can be measured directly because it is visible. However, the maximum distance L MaxThe position of the source is not visible because it is on the back side of the cylinder. Therefore, it cannot be measured directly with the distance meter of the BS survey meter. In such a case, if you assume the farthest position from the shape of the radiation source, estimate the distance to that position, and manually input that value into the presentation circuit, it is possible to calculate and output the upper and lower limit radioactivity. To increase the convenience, the shortest distance L measured with the distance meter of the BS survey meter is Min By displaying the maximum distance L, the operator can select the maximum distance L based on experience and the diameter of the FIBC bag. Max This is solved by manually inputting the estimated value of into the subject circuit.

[0039] FIG. 6 is a schematic overall configuration diagram of the BS survey meter of the embodiment. Referring to FIG. 6, the BS survey meter 20 of the embodiment is a dosimeter, and is configured by a scintillation survey meter 4 that is widely used as a dosimeter, and a measurement target ( 137 The device has a hardware structure to which a range finder 6 for measuring the distance to a target Cs point source is added, and also has a display unit 8. As is well known, the range finder 6 generally uses a laser beam for distance measurement, but a range finder using ultrasonic waves, infrared rays, etc. may also be used.

[0040] As is already known, the scintillation survey meter adopted as the dosimeter 4 photoelectrically converts and amplifies the fluorescence emitted when the scintillator detects radiation, and determines the dose rate or dose based on the count rate or count of the radiation incident from the object to be measured. The distance L to the object to be measured can be measured by the distance meter 6 included in the BS survey meter 20.

[0041] 7 is a functional block diagram of the BS survey meter 20. The BS survey meter 20 has a dose measurement unit 22 related to the dose measured by the scintillation survey meter 4, a distance measurement unit 24 related to the distance to the radiation source measured by the distance meter 6, and a radioactivity measurement unit 26 related to the radioactivity of the radiation source.

[0042] In the dose measurement unit 22, the dose detected by the radiation detector 4, that is, the survey meter, is calculated as a dose rate by the dose calculation circuit 220 through the above-mentioned calculation, and dose rate data is generated.

[0043] In the distance measurement unit 24, the distance to the radiation source measured by the distance detector 6, that is, typically a laser distance meter, is found by a distance calculation circuit 240, and distance data is generated.

[0044] The radioactivity measurement unit 26 includes a data memory 260 and a manual input unit 262. The data memory 260 stores the above-mentioned formulas and the 1 cm dose equivalent rate constant Γ and threshold values ​​for safety judgment, which are necessary for calculations based on these formulas. The manual input unit 262 stores the maximum distance L Max The manual input unit 262 is used to select and input the necessary information such as the outline shape of the modeled contaminated surface or the model of the contaminated solid, for example, a cylindrical shape. In the radioactivity calculation and evaluation unit 264 of the radioactivity measurement unit 26, the upper limit radioactivity, the measured radioactivity, and the lower limit radioactivity of the radiation source are typically calculated based on the above-mentioned formulas, and a safe side radioactivity evaluation is calculated by comparing with the threshold value for safety judgment, and if it is not safe, an alarm signal is generated. The operator can select the maximum distance L using the manual input unit 262. Max When you set the maximum distance L Max is the longest distance L Max and this determined maximum distance L Max In this case, the radiation measuring unit 26 constitutes the "maximum distance determining unit."

[0045] The display unit 8 (FIG. 6) of the BS survey meter 20 can display the following items. (1) Radiation Dose (2) Distance measured by rangefinder 6 (3) Upper limit radioactivity (4) Lower limit radioactivity (5) Upper and lower limits of radioactivity (6) Measured radioactivity (7) Upper limit, lower limit and measured radioactivity (8) Upper limit of radioactivity and actual measured radioactivity (9) Margin of safety (10) Margin and measured radioactivity (11) Notification of measurement results, such as safe radioactivity assessment (12) Alarm (including sound)

[0046] The display unit 8 (FIG. 6) of the BS survey meter 20 may be switchable in display mode, and it is preferable that the operator can select one of the display modes alternatively. Specifically, the first mode is a radiation dose display mode that displays the radiation dose measured by the dosimeter 4, and the second mode is a measured distance or the longest distance L measured by the distance meter 6. Max , the shortest distance L Min and a third mode is a radiation display mode that displays the above items (3) to (12) related to radiation. For the third display mode, the operator may be allowed to select the display items.

[0047] Of course, the radiation dose of item (1) and any of items (3) to (12) may be displayed on the display unit 8 (FIG. 6) at the same time. Also, the distance of item (2) and any of items (3) to (12) may be displayed on the display unit 8 at the same time. Also, the radiation dose of item (1), the distance of item (2), and any of items (3) to (12) may be displayed on the display unit 8 at the same time. Here, the "margin" of item (9) above means a threshold value that can be determined as safe, which is registered in advance, and the difference between this threshold value and the measured radioactivity or the upper limit radioactivity. For example, when the measured radioactivity or the upper limit radioactivity is significantly lower than the threshold value, the margin can be displayed by displaying a double circle. Also, when the margin is large, the measurement result may be notified by voice, "It is safe."

[0048] The BS survey meter 20 can display information about radioactivity in real time on-site, just like the widely used scintillation survey meter 4, and workers can get a real-time indication of safety on-site by looking at the display unit 8 (Fig. 6). The amount of radioactivity and distance may be displayed simultaneously, or the three values ​​of the amount of radioactivity, dose rate or dose, and distance may be displayed simultaneously.

[0049] Considering the situation at the site, the information that workers want to know in real time is whether the measured radiation is on the safe side or not, and also how safe it is. With this in mind, in most cases, workers will select one of the measurement result notifications on the display unit 8 (Fig. 6), such as the above item (7) upper limit radiation, lower limit radiation, and measured radiation, item (8) upper limit radiation and measured radiation, item (9) margin, (10) margin and measured radiation, or (11) radiation evaluation on the safe side.

[0050] When a numerical value is displayed on the display unit 8 (FIG. 6), the worker can recognize the measured value by moving his / her eyes from the object to be measured to the display unit 8. In a situation where it is impossible to take one's eyes off the object to be measured at the work site, for example, when the measurement work is proceeding continuously while looking at the object to be measured, the display on the display unit 8 cannot be visually confirmed. In this situation, the BS survey meter 20 of the embodiment may generate a number of clicking sounds (counts / sec) according to the level of the measured value or the degree of safety, or may add to the measurement result notification by reading out the measured value, measurement result, and evaluation by voice. The same applies to the alarm generation in item (9) above.

[0051] Also, instead of or in addition to the above-mentioned sound notification, the notification may be by flashing a number of lights or by varying the strength of vibration according to the level of the measured value. This allows the worker to perceive safety-related information through sound, light, and vibration without moving his / her eyes to the display unit 8 (FIG. 6).

[0052] In the above embodiment, the maximum distance L from the point source constituting the real radiation source to the measuring device is Max , and the shortest distance L Min It was explained that a safe estimate of radioactivity can be obtained based on a formula that takes this into account.

[0053] In a concrete example, in addition to the distance, there are also effects of self-shielding, e.g. due to volume. As a result, the dose rate generated by each point source i that constitutes the real source is 1 / F Pi Here, "F Pi " is called the shielding effect coefficient, and the shielding effect coefficient F Pi differs for each point source i. Shielding effect coefficient F Pi The maximum value of F Max ", the safe side assessment radioactivity can be calculated using the following equations 11 and 12.

[0054] Representative distance L M and the shortest distance L Min If they match, then Equation 11 below applies.

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[0055] Representative distance L M and the shortest distance L Min If they do not match: Equation 12 below is applied.

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[0056] The above Equations 11 and 12 represent the safety assessment radiation activity that ensures that the radiation activity of the actual radiation source is not greater than this when there are influences other than distance. [Explanation of symbols]

[0057] 20 Portable Becquerel Sievert Survey Meter (BS Survey Meter) 4. Scintillation survey meter (dosimeter) 6 Distance meter 8 Display unit

Claims

1. A portable becquerel-sievert survey meter that is equipped with a dosimeter, a distance meter, and a display unit and can be carried by a worker, a dose calculation unit for calculating a dose rate D of a measurement object by the dosimeter; A distance measurement processing unit that measures a distance L to the measurement object; A memory storing a 1 cm dose equivalent rate constant Γ corresponding to the dose rate; A representative distance L measured toward the object to be measured Ｍ An actual radioactivity calculation unit that calculates the actual radioactivity based on the following formula A; Based on the distance L measured by the range finder, the longest distance L Ｍax a maximum distance determination unit for determining The longest distance L Ｍax and an upper limit radioactivity calculation unit for calculating an upper limit radioactivity by the following formula B based on the A portable becquerel-sievert survey meter characterized in that the actual measured radioactivity and the upper limit radioactivity can be displayed on the display unit. (Formula A) Measured radioactivity = (DL Ｍ 2 ) / Γ (Formula B) Upper limit radioactivity = (DL Ｍax 2 ) / Γ

2. The portable becquerel-sievert survey meter of claim 1, An evaluation unit that compares the upper limit radioactivity calculated by the radioactivity calculation unit with a safety judgment threshold value registered in the memory to evaluate the safety of the measurement object, A portable becquerel-sievert survey meter in which the safety evaluated by the evaluation unit is displayed on the display unit.

3. The portable becquerel-sievert survey meter according to claim 2, Based on the distance L measured by the range finder, the shortest distance L Ｍin A shortest distance setting unit that determines and a lower limit radioactivity calculation unit for calculating a lower limit radioactivity based on the following formula C: A portable becquerel-sievert survey meter capable of displaying the lower limit radioactivity calculated by the lower limit radioactivity calculation unit on the display unit. (Formula C) Lower limit radioactivity = (DL Ｍin 2 ) / Γ

4. In claim 3, The display unit has first to third display modes that can be alternatively selected as display modes, In the first display mode, the radiation dose measured by the dosimeter 4 is displayed. In the second display mode, the actual distance measured by the rangefinder 6 and the longest distance L Ｍax , the shortest distance L Ｍin is displayed, A portable Becquerel-Sievert survey meter whose third display mode displays information about radioactivity.

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