JP5570541B2 - Method for measuring γ-rays radiated from live animals to the body surface - Google Patents

Description

  The present invention relates to a method for measuring γ-rays radiated from the live body of a living body to the body surface, which makes it possible to estimate the amount of radioactive cesium present in the muscle of livestock such as cattle before slaughtering.

  As a result of the accident at TEPCO's Fukushima Daiichi Nuclear Power Station following the Great East Japan Earthquake, it was found that radioactive cesium was contained in the muscles of domestic animals that ingested rice straw and pasture containing radioactive fallout.

  Currently, in areas around Fukushima Prefecture, we conduct radioactivity tests after slaughter using a germanium spectrometer on cattle shipped as meat (fattened fertilized / retired dairy cows). However, there were cases where consumers could not sell because they felt uneasy even if they were well below the national regulations.

  In order to ensure the safety of consumers and protect the production base of beef cattle in the region, develop a method that predicts the radioactivity in the muscles to a certain extent before birth and prevents shipment of contaminated cattle above a certain level. There is a need.

  A commercially available device that can measure radioactive material in muscle in the state of a living body before slaughtering is known, but it is not portable because it is a large and heavy device. Due to the high price, it is very difficult to install it in various places where radioactivity inspection is desired. There is a demand for an intramuscular radioactivity measurement system that is inexpensive and has excellent portability.

Manual for Measuring Radioactivity of Foods in Emergency March 2002 Monitoring and Safety Division, Food Health Department, Ministry of Health, Labor and Welfare

  An object of the present invention is to provide an inexpensive and highly portable measurement system that makes it possible to estimate the amount of radioactive cesium in muscle from live animals before slaughtering.

  As a result of earnest research, the inventors of the present invention combined a commercially available small NaI scintillation survey meter with a lead shielding ring suitable for the probe diameter of the survey meter, and inserted the probe into the shielding ring in the tip of the probe. By measuring while keeping the part and the shielding ring in close contact with the surface of the livestock body, it is possible to measure the gamma rays radiated from the muscles of livestock to the body surface by greatly shielding the background environmental radiation, It has been found that the amount of radioactive cesium in the muscles of livestock can be estimated with good accuracy by using the measurement system, and the present invention has been completed.

That is, the present invention measures the γ dose from the body surface by bringing the lower surface of the shielding member and the probe tip portion into close contact with the body surface of the livestock animal in a state where the periphery of the probe tip portion of the survey meter is surrounded by the shielding member. A method for measuring γ-rays radiated from the livestock body to the body surface .

  According to the present invention, a novel method has been provided that makes it possible to measure gamma rays on the body surface of livestock before slaughtering and to estimate the amount of radioactive cesium in muscle. The system of the present invention is excellent in portability, and can be prepared at a much lower cost than known devices that perform measurements on living animals. Since a commercially available scintillator with a small probe diameter can be used, the shielding member can be reduced in size and weight, and the measurement operation can be easily performed by a human hand without a special support tool. Since the shielding member is small, it is easy to make it close to the cow body, and it is possible to effectively prevent the entry of γ rays from the gap. Moreover, since the shielding member and the scintillator are separated from each other in the system of the present invention, it is easy to cope even when the livestock being measured suddenly moves, and the measurement work can be continued. Furthermore, by adjusting the thickness of the shielding member, it is possible to flexibly construct a measurement system that matches the background γ-ray radiation amount of the measurement site.

It is a figure which shows one example of the shielding member used by this invention. It is a figure which shows the waist angle part of a cow.

  In the present invention, a survey meter that measures γ-rays and a shielding member that shields the probe tip of the survey meter from radiation from outside the measurement target are used.

  As the survey meter, a commercially available NaI scintillation survey meter can be preferably used. The probe diameter of the NaI survey meter varies depending on the model, and there are probe diameters of 1 inch and 2 inches. Any of these may be used, for example, depending on the γ dose of background environmental radiation. Can be used. In order to enhance the shielding effect by the shielding member, a small model having a probe diameter of 1 inch can be preferably used.

  The shielding member preferably has a ring shape as shown in FIG. At the time of γ-ray measurement, measurement is performed with a probe inserted into the through hole at the center of the ring. Therefore, the inner diameter of the ring is set to a size corresponding to the probe diameter of the survey meter. For example, when a survey meter having a probe diameter of 1 inch is used, the inner diameter of the ring may be about 4.5 cm to 5.5 cm, preferably about 5 cm. The ring thickness of the shielding member can be appropriately selected according to the background γ dose at the place where the measurement is performed. For example, in an area where background radiation by radioactive cesium is relatively high such that 662 ± 10% keVγ rays exceed 100 to 130 cpm, a shielding member having a ring thickness of about 2.5 cm to 3.5 cm can be preferably used. The height of the shielding member is not particularly limited as long as it is high enough to shield the NaI crystal at the tip of the probe from γ rays from outside the measurement target. For example, the height can be about 5 cm to 10 cm.

  The material of the shielding member is not particularly limited as long as it is a material that can shield background radiation that is not a measurement target, and for example, lead can be preferably used. It may be a lead lump formed in a ring shape, or may be manufactured by winding a lead plate having a thickness of several mm into a ring shape as described in the following examples.

  The livestock to be measured is preferably a large livestock, and examples include, but are not limited to, cows, horses, and pigs. It is preferable that the measurement site is the posterior part of the hip angle from the viewpoint of grasping the muscles of the thigh within the range. FIG. 2 is a diagram for explaining the waist angle portion of a cow. It is possible to measure from the upper part of the longest muscle of the thigh to the lower part of the thigh, once for each of the left and right behind the hip angle with the spine as the center. The measurement time can be appropriately selected and is usually about 1 minute to 10 minutes, for example, about 3 minutes to 7 minutes.

  In the method of the present invention, it is preferable to measure the γ dose of the negative control water phantom in the vicinity of the measurement place where the livestock is measured. As the water phantom, for example, several commercially available polytanks filled with water (distilled water, reverse osmosis water, etc.) and having a capacity of about 20 liters can be used with a height of about 50 to 100 cm. The gamma dose of the water phantom is measured using a shielding member as in the case of livestock. Specifically, a shielding member is placed on the top of a tank filled with water, and the tip of the probe is inserted into the inner hole of the shielding member, and the same time measurement as the livestock measurement is performed. The water phantom is measured at least once, for example, twice. It is not necessary to measure the water phantom for each livestock, and if a plurality of individuals are measured at the same place on the same day, one set may be performed. When at least one of the measurement date and the measurement location is changed, one set of water phantom measurement may be performed each time it changes.

  The amount of radioactive cesium in livestock muscle can be estimated from the measured value (count number) of livestock and the measured value (count number) of the water phantom.

  Hereinafter, a specific example in the case of performing gamma dose measurement of livestock and estimating the amount of radioactive cesium in muscle by the measurement method of the present invention will be described.

  The measurement of livestock is performed once for each of the left and right (see FIG. 2) behind the hip angle with the spine at the center. Measurement is performed vertically above the left and right sides of the lumbar vertebra at the back of the hip angle, vertically downward or perpendicular to the body surface. When using a NaI scintillation survey meter with a 1 inch probe diameter, a lead ring (about 6.0 kg as lead) with an inner diameter of 5 cm, an outer diameter of 11 cm, and a height of 7 cm can be used as a shielding member. A lead ring is brought into close contact with the measurement site, a probe of a survey meter is inserted into the inner hole of the ring, and the tip of the probe is brought into close contact with the body surface. In this state, measurement is performed for 5 minutes. The obtained measurement values are referred to as “left measurement value” and “right measurement value”, respectively.

  On the other hand, one set of water phantom measurement is performed near the livestock measurement site. A 20-liter poly tank filled with distilled water is placed near the measurement location, a lead ring is placed on the top surface of the tank, a probe of a survey meter is inserted into the inner hole of the ring, and the probe tip is in contact with the poly tank surface. In this state, the measurement for 5 minutes is performed twice. The obtained measurement values are referred to as “water measurement value 1” and “water measurement value 2”, respectively.

The measured radiocaesium equivalent count value (Cscpm (cpm)) of livestock is calculated by the following formula.
Cscpm = {(Left measurement value + Right measurement value) − (Water measurement value 1 + Water measurement value 2)} ÷ 10 (Formula 1)

Assuming that the counting efficiency is X (Bq / kg / cpm), the amount of radioactive cesium Cs (Bq / kg) in the muscles of livestock can be estimated by the following equation.
Cs (Bq / kg) = Cscpm (cpm) x X (Bq / kg / cpm) (Formula 2)

  The counting efficiency X is appropriately determined according to the probe hole diameter of the survey meter to be used, the size of the shielding member, and the like. The specific method for determining X is as described in the examples below. Specifically, for multiple specimen livestock, Cscpm (cpm) is obtained as described above, and the amount of radioactive cesium in the muscle after slaughter is measured with a germanium semiconductor detector or germanium scintillator (Ge measurement value). (Bq / kg)) is obtained, and a value obtained by dividing the Ge measurement value by Cscpm is obtained for each individual sample livestock, and the average value of the values can be adopted as X.

  Also, as in the following examples, when there is a sample livestock that showed an abnormal value, using the average value and standard deviation calculated based on the data excluding the abnormal value excluded by statistical processing, the risk factor The counting efficiency X ′ that is 1% or less is obtained by the formula “average value (Bq / Kg / cpm) + 3 × standard deviation (Bq / Kg / cpm)”, and the obtained counting efficiency X ′ is set as X in the above formula 2. Can be used to determine the upper limit of Cs. In livestock where the amount of radioactive cesium in muscle should be estimated, if the upper limit exceeds the regulation value, the amount of radioactive cesium in the muscle measured in the sample after slaughtering exceeds the regulation value Since there is a risk, it is desirable to carry out a more precise examination or continue breeding until radioactive cesium in the body is naturally excreted.

  For example, a 1-inch diameter NaI scintillation survey meter is used to shield a lead ring (approx. 6.0 kg as lead) with an inner diameter of 5 cm, an outer diameter of 11 cm, and a height of 7 cm manufactured by processing several millimeters of lead plate. When used as a survey meter and lead ring, the counting efficiency obtained from the data of 28 cows excluding cattle that showed abnormal values was 6.5 ± 1.7 Bq / kg / cpm. X in Formula 2 may be 6.5. At this time, X ′ = average value 6.5 (Bq / Kg / cpm) + 3 × standard deviation 1.7 (Bq / Kg / cpm) = 11.6 (Bq / Kg / cpm), so 11.6 is used as X in Equation 2. Cs upper limit value can be obtained.

  Note that the value of X described above is a provisional value, and a more preferable value of X can be obtained by collecting more livestock data using the same system. Once X is determined, as long as the measurement is performed using the same system in an environment where the background γ-ray radiation is about the same, the estimated value of the radioactive cesium amount and Preferably, the upper limit value can be obtained. It is also possible to create a quick reference table in which an estimated value (and preferably an upper limit value) of the amount of radioactive cesium in the muscle can be searched from the negative control measurement value and the livestock measurement value.

  Hereinafter, the present invention will be described more specifically based on examples. However, the present invention is not limited to the following examples.

  At farms in Fukushima Prefecture, gamma rays on the surface of cattle were measured using the following system. The experiment was conducted on 29 beef cattle. Background radiation from radioactive cesium in Fukushima Prefecture during the period of measurement was 179 to 1,000 cpm (662 ± 10% keVγ ray) at the farm in Otamamura, and 147 to 229 cpm (662 ± 10 at the beef cattle mooring facility in Fukushima Prefecture). % keVγ ray).

Measuring device: Fuji Electric Survey Meter NHC7 (NaI scintillation survey meter, 1 inch probe diameter)
Water phantom: Assembled four commercially available 20-liter plastic tanks filled with distilled water Shielding member: Lead ring with an inner diameter of 5 cm, an outer diameter of 11 cm, and a height of 7 cm (about 6.0 kg as lead) (Figure 1)

  The shielding lead ring was prepared by dividing a 50 cm square × 2 mm thick lead plate into 7 equal parts of about 7 cm width and winding them tightly with vinyl tape so that the inner diameter was 5 cm.

<Shielding efficiency of shielding lead ring>
A lead ring was placed at the center of the upper part of the water tank phantom, and a survey meter probe was inserted into the hole of the ring, and the measurement was made vertically from the center of the upper part of the tank. The measurement for 5 minutes was performed twice. Moreover, the same measurement was performed without using a lead ring.

  As a result, the count number of the water phantom without shielding by the lead ring was 112.0cpm, whereas the count number of the water phantom with shielding by the lead ring was 22.6cpm (shielding efficiency 0.80). By using a lead ring, the lowest measurable value could be lowered.

<Measurement of gamma dose measurement and counting efficiency of bovine body>
The water phantom plastic tank was placed near the place where cattle were measured. A lead ring was placed on the center of the upper part of the polytank, and a survey meter probe was inserted into the hole of the ring, and the measurement was made vertically from the center of the upper part of the polytank (5 minutes × 2 times). The measurement values are “water measurement value 1” and “water measurement value 2”, respectively.

  The measurement of the bovine individual was performed from the upper part of the longest muscle on both the left and right sides of the back of the hip angle (arrows in FIG. 2) vertically downward (once each left and right). The lead ring was brought into close contact with the measurement site, the probe was inserted into the hole of the ring, and the probe tip was brought into close contact with the body surface for measurement. The left and right measured values are referred to as “left measured value” and “right measured value”, respectively.

  The cow individual was slaughtered after the measurement to obtain a sample of the meat portion, and the amount of radioactive cesium in the muscle was measured with a germanium scintillator. The measured value with a germanium scintillator was converted to Bq / kg by a conventional method, and used as the measured value of Ge.

The value measured by the above system was calculated by the following method, and “NaI measurement conversion value (cpm)” was obtained for each cow individual.
NaI measurement conversion value (cpm) = (left measurement value + right measurement value) / 10-(water measurement value 1 + water measurement value 2) / 10

For each cow individual, the counting efficiency was determined by the following formula.
Counting efficiency (Bq / kg / cpm) = [Ge conversion value (Bq / kg)] ÷ [NaI measurement conversion value (cpm)]

  The calculation results are shown in Table 1. In addition, in the measurement value of the Ge scintillator for meat, for the data in which Cs137 is finite and Cs134 is ND, a value 0.9 times Cs137 was added to the total Cs data as Cs134 data.

Using the measurement count thus obtained, the amount of radioactive cesium in meat (Cs134, Cs137) can be estimated by the following formula.
Amount of radioactive cesium in muscle (Bq / kg) = [NaI converted value (cpm)] x 6.5

In addition, since the average value calculated based on the data excluding the abnormal No. 6 value is 6.5 and the standard deviation is 1.7, the counting efficiency when the risk rate is 1% or less is 6.5Bq / Kg / cpm + 3 x standard deviation 1.7 Bq / Kg / cpm = 11.6 Bq / Kg / cpm
It became. This counting efficiency 11.6 can be used as the counting efficiency for obtaining the upper limit value.

Claims (7)

  Including measuring the γ-ray dose from the body surface by bringing the lower surface of the shielding member and the probe tip portion into close contact with the body surface of the livestock body in a state in which the periphery of the probe tip portion of the survey meter is surrounded by the shielding member A method for measuring gamma rays emitted from the body to the body surface.   The method according to claim 1, wherein the shielding member is a ring-shaped member having a through-hole having a size into which the probe can be inserted.   The method according to claim 2, wherein the shielding member has a ring thickness of 2.5 cm to 3.5 cm.   The method according to claim 3, wherein a shielding member having a ring inner diameter of 4.5 cm to 5.5 cm is used for a one inch diameter probe.   The method according to claim 1, wherein the shielding member is made of lead.   The method according to any one of claims 1 to 5, wherein the livestock is a cow.   The method according to any one of claims 1 to 6, wherein the measurement site is a body surface behind a hip angle of a livestock animal.

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