JP5499384B2 - Radioactivity measurement method and apparatus - Google Patents

Description

  The present invention relates to a radioactivity measurement method and apparatus.

  An imaging plate formed by applying a photostimulable phosphor powder to a film accumulates radiation information (radiation dose) on the phosphor when irradiated with radiation (forms a radiation dose latent image). The phosphor that has accumulated radiation information emits light according to the amount of accumulated radiation when irradiated with stimulation light. By photoelectrically converting this light emission into an electrical signal (pixel data), the imaging plate is irradiated. The emitted radiation dose can be detected as an electrical signal (radiation dose image data).

  A radioactivity measurement method and apparatus for measuring the radiation dose from a transuranium element such as plutonium using such an imaging plate has been proposed.

  In a conventional radioactivity measurement method and apparatus, radiation dose image data (PSL (Photo-Stimulated Luminescence) image data) obtained by using an imaging plate includes a dose (background) derived from natural radiation existing in nature. (Radiation dose) is also included, there is no method for determining the existence region of the radioactive substance to be measured. For this reason, it is difficult to calculate the radioactivity amount with a fixed range.

JP 2007-24629 A JP 2005-304952 A JP-A-10-132940

Journal of Japanese Society for Radiation Safety Management Vol.5 No.2 (27-32) "Analysis of PSL intensity distribution recorded on imaging plate"

  An object of the present invention is to provide a radioactivity for measurement purposes from the radiation dose image data (PSL image data) obtained by the imaging plate in consideration of the influence of the radiation dose (background radiation dose) derived from natural radiation existing in nature. An object of the present invention is to realize a radioactivity measurement method and apparatus capable of determining a presence region of a substance and calculating a radioactivity amount of a determined region with a predetermined accuracy.

  When a sample to which a radioactive substance that emits α rays (for example, plutonium) is attached is brought into contact with the imaging plate and irradiated with radiation, a latent image corresponding to the amount of irradiated radiation is recorded on the imaging plate. Then, when the latent image on the imaging plate on which the latent image is recorded is read, a radiation dose image is obtained by the density of the PSL value proportional to the background and the radiation dose of the radioactive substance as shown in FIG.

  In FIG. 1, a and b are pixel collection areas (spots) recorded by radiation emitted from a radioactive material, and c is a pixel collection area (background) recorded by natural radiation.

  FIG. 2 is a three-dimensional image in which the radiation dose image data is represented by an amount (PSL value) proportional to the radiation dose in the height direction. When a pixel is selected from the three-dimensional image (a) using an arbitrary PSL value as a threshold, it can be extracted by being divided into spots of innumerable pixels (b).

  The background is a small spot due to natural radiation incident on the imaging plate almost uniformly, and is different from a spot concentrated on a part of the pixel area due to radiation emitted continuously from the radioactive material. Therefore, the presence or absence of a radioactive substance different from the background can be identified by comparing each spot area (number of pixels) for each individual spot. Moreover, the radioactivity amount of each spot can be evaluated by comparing the radiation dose of a known radioactive substance with the PSL value of the spot.

  In the present invention, a value at which a predetermined number of pixels of background radiation dose image data is equal to or smaller than the threshold value is set as a threshold value, and a region (spot) in which pixels of radiation dose image data equal to or greater than the threshold value are continuous over a predetermined number. Is determined as the location of the radioactive substance to be measured.

  Specifically, the threshold value is a value including 99% (˜99.9%) of pixels of background radiation dose image data below that value, and the spot determined as the presence position of the radioactive substance for measurement is , A region where 40 pixels or more are continuous.

  Then, the radioactivity of the radioactive substance to be measured is calculated by the following equation (Equation 1) obtained from the least square method using a representative sample of one spot, and the relative error is calculated by the following equation (Equation 2). calculate.

According to the present invention, from the radiation dose image data (PSL image data) obtained by the imaging plate, the effect of the radiation dose (background radiation dose) derived from natural radiation existing in the natural world is taken into consideration, and the radioactivity for measurement purposes. It is possible to realize a radioactivity measurement method and apparatus that can determine the presence region of a substance and calculate the amount of radioactivity with a predetermined accuracy. In particular, by contacting a sample with a radioactive substance in contact with an imaging plate and irradiating with radiation, a latent image corresponding to the amount of irradiated radiation is recorded on the imaging plate, and the latent image recorded on the imaging plate is read. 99% of background radiation dose image data read from an area other than the sample contact area on the imaging plate in a radioactivity measurement method and apparatus for creating grayscale radiation dose image data proportional to the background and the radiation dose of radioactive material Is set as a threshold value, and in the radiation dose image data read from the sample contact region in the imaging plate, a region where the pixels of the radiation dose image data equal to or larger than the threshold value are continuous over 40 pixels or more. The location of radioactive material for measurement purposes By you to determine easily it is possible to perform efficiently identify the spot by radioactive measurement purposes.

It is the radiation dose image by the density of the PSL value proportional to the background recorded on the imaging plate and the radiation dose of a radioactive substance. 2 is a three-dimensional image in which the height direction of the data of the radiation dose image shown in FIG. 1 is expressed by an amount (PSL value) proportional to the radiation dose. It is a flowchart of the radioactivity amount measuring method by this invention. It is a functional block diagram of the radioactivity measuring apparatus which implements the radioactivity measurement method of this invention. The background and the distribution of the PSL value of the radioactive substance (plutonium) in the radiation dose image data are shown. The result of performing a pixel extraction experiment from radiation dose image data with PSL values such that about 90.0%, 99.0%, and 99.9% of the background pixels are equal to or less than that value is shown. The distribution of spot areas (number of pixels) of pixels extracted from the background is shown. The relationship between the PSL value of the spot by the irradiation of plutonium, the amount of radioactivity, and the irradiation time is shown. A pixel is extracted with a value at which the PSL value of 99.0% of the background pixels is equal to or less than that value as a threshold, and is identified quantitatively as a radiation irradiation spot by a radioactive substance by a method of identifying a spot of 40 pixels or more. An example is shown. The comparative example of the radioactivity amount evaluated from the PSL value of 10 plutonium samples of 1 spot and the radioactivity amount evaluated by the radiation detector is shown. The comparative example of the radioactivity amount evaluated from the PSL value of 10 plutonium samples of a plurality of spots and the radioactivity amount evaluated by the radiation detector is shown.

  In the present invention, a value at which 99% to 99.9% of pixels of background radiation dose image data (pixel data) are equal to or lower than the threshold value is set as a threshold value, and radiation dose data equal to or higher than the threshold value is continuous for 40 pixels or more. The area (spot) is determined as the location of the radioactive substance to be measured, and the amount of radioactivity of the radioactive substance to be measured is calculated by the formula (Equation 1) obtained from the least square method using a representative one-spot sample. The relative error is calculated by the equation (2) based on the error obtained from the least square method.

  FIG. 3 is a flowchart of the radioactivity measurement method of the present invention.

Step S1
Collect dust (including radioactive materials such as plutonium) floating on the surface of equipment and dust, etc., floating in the air of the measurement environment on filter paper for dust collection.

Step S2
The dust collecting filter paper or the like is brought into contact with the imaging plate, and the imaging plate is irradiated with radiation (α rays) of a radioactive substance (plutonium or the like) contained in the collected dust. Record the latent image.

Step S3
The dust latent image recorded on the imaging plate is read, and dust PSL image data proportional to the radiation dose is acquired.

Step S4
Background PSL image data is acquired from a predetermined area other than the dust latent image recording area on the imaging plate.

Step S5
A value at which 99% (˜99.9%) of the pixels constituting the background PSL image is equal to or less than that value is set as the PSL image evaluation threshold value.

Step S6
Pixel data equal to or higher than the evaluation threshold is extracted from the dust image data.

Step S7
Among spots where extracted pixels are continuous, a spot having an area of a predetermined number (40 pixels) or more is identified as a radiation irradiation spot by a radioactive substance.

Step S8
The dust PSL value of the irradiation spot is obtained by subtracting the background from the PSL value of the identified irradiation spot.

Step S9
A relational expression between the PSL value of the radiation irradiation spot and the radioactive amount of the radioactive substance is obtained in advance by an experiment using a representative sample.

Step S10
Calculation of the radioactive amount of the radioactive substance measured from the dust PSL value of the radiation irradiation spot and its calculation error are obtained.

  FIG. 4 is a functional block diagram of a radioactivity measuring device that implements the radioactivity measurement method described above.

  Reference numeral 1 denotes a dust collecting filter paper or the like used as a sample, which is installed in a measurement environment dust collecting device and used for collecting dust floating on the air in the measurement environment or dust attached to the surface of equipment. 2 is the particle | grains of the radioactive substance contained in the collected dust.

  Reference numeral 3 denotes an imaging plate, which records a dust latent image 2a by contacting dust collecting filter paper 1 which is a sample that collects dust and irradiating dust radiation (α-rays) containing the collected radioactive substance 2. Use to do. The imaging plate 3 is brought into contact with the dust collecting filter paper 1 to record a dust latent image 2a, and is positioned outside the dust latent image recording region 3a to be a background latent image region 3b. Is provided.

  The background latent image area 3b is an area for evaluating the background on the imaging plate 3, and the sample is not brought into contact therewith.

  Reference numeral 4 denotes a reader, which includes stimulation light irradiation means and a photoelectric conversion element array, photoelectrically scans the surface of the imaging plate 3 and photoelectrically converts a dust latent image and a background latent image into an electric signal, thereby forming a latent image (radiation). PSL image data proportional to the dose is output.

  Reference numeral 5 denotes an image data processing apparatus, which acquires PSL image data output from the reader 4, sets it as a PSL image evaluation threshold, extracts pixel data equal to or higher than the evaluation threshold in dust image data, and extracts the extracted pixels. A region of a predetermined number (40 pixels) or more of consecutive spots is identified as a radiation irradiation spot by a radioactive substance, and the background is subtracted from the PSL value of the identified radiation irradiation spot to obtain a dust PSL value of the radiation irradiation spot. The calculation of the radioactive amount of the radioactive substance measured from the dust PSL value of the radiation irradiation spot and the calculation error are performed.

  The threshold of the PSL value for extracting the irradiation spot is set based on the distribution of the background PSL value in order to minimize the influence of the background.

FIG. 5 shows the distribution of the background PSL value, and also shows the case where a radioactive substance (plutonium) is included for reference. The PSL value of the radioactive substance has a distribution higher than the background, and identification by the PSL value is effective. The background distribution varies depending on the irradiation time and measurement location. As shown in FIG. 5, considering the distribution of background PSL values, PSL such that about 90.0%, 99.0%, and 99.9% of the background pixels are less than or equal to that value, respectively. A pixel extraction experiment was performed using the value as a threshold value.
Further, an average PSL value (PSL / pixel) of the background is obtained from this distribution. The average background PSL value is used in calculating the net PSL value of the irradiated spot.

  FIG. 6 shows the result of this pixel extraction experiment. Each white pixel extracted with reference to the threshold value forms a spot. (A) is pixel extraction that thresholds a value at which about 90.0% of the background pixels are less than or equal to that value. Many spots are distributed over the entire surface, and separation is not sufficient. The identification processing time is also long, and the efficiency is poor.

  On the other hand, (b) and (c) are pixel extractions in which about 99.0% and 99.9% of the background pixels are threshold values, and the number of background spots is reduced. Identification is easy and efficient. In consideration of the identification processing time and the measurement up to a low radiation dose, the optimum threshold value to be used is such that about 99.0% of the background pixels are equal to or less than that value.

  In order to identify the irradiation spot by a radioactive substance such as plutonium by the area of the spot (number of pixels), the background was evaluated by leaving a plurality of imaging plates in the room with different standing times. For each imaging plate, pixels were extracted with a threshold value at which about 99.0% or more of the pixels of the background image were below that value, and the spot area distribution was evaluated.

  FIG. 7 shows a distribution of spot areas (number of pixels) of pixels extracted from the background. Regardless of the standing time, the background spots decreased as the area increased (the number of pixels increased), and spots of 40 pixels or more were not observed. From this, a spot of 40 pixels or more can be regarded as a radiation irradiation spot by a radiation substance other than natural radiation.

  The relationship between the PSL value I of the irradiation spot by plutonium and the amount of radioactivity A was evaluated by the least square method using one-spot samples with different irradiation doses. The PSL value I of the spot was obtained by subtracting the background value from the total value of the PSL values of the pixels extracted by the threshold value and the spot area. The background value was obtained by multiplying the average PSL value of the background obtained in FIG. 5 by the spot area.

  The relational expression between the PSL value and the amount of radioactivity considered the effect of fading. Using a few representative samples of one spot, a coefficient is obtained by applying the least square method to this relational expression. The relative error σ was obtained from the error of the coefficient obtained from the least square method. The amount of radioactivity A of the spot and the relative error σ according to the relational expression can be calculated by the formulas (1) and (2) based on the PSL value I and the irradiation time t.

  FIG. 8 shows the relationship between the PSL value of a spot, the amount of radioactivity, and the irradiation time obtained by irradiation of plutonium obtained by applying the least square method to several representative one-spot samples.

  FIG. 9 shows that in a PSL image of an imaging plate in which a latent image is recorded by irradiating radiation with a sample to which radioactive substance (plutonium) particles are attached, the PSL value of a 99.0% background pixel is less than that value. An example is shown in which pixels are extracted using values as threshold values and quantitatively identified as radiation irradiation spots by radioactive substances by a method of identifying spots of 40 pixels or more. The overlap of multiple spots can also be identified as a collection of large spots, and as a sample in which radioactive material is sparsely distributed, not only dust collection filter paper but also a distribution of radioactive material particles collected is uneven It can also be applied to the measurement of smear filter paper.

  FIG. 10 shows a comparative example of the amount of radioactivity evaluated from the PSL values of 10 plutonium samples in one spot and the amount of radioactivity evaluated by the radiation detector. About the sample (No.7-No.10) which was not used for calculation of (Formula 1) type | formula and (Formula 2) type | formula, the radioactivity amount evaluated from the PSL value was measured with the radiation detector within the range of the error. It is in good agreement with the amount of radioactivity.

  FIG. 11 shows a comparative example of the amount of radioactivity evaluated from the PSL values of 10 plutonium samples of a plurality of spots and the amount of radioactivity evaluated by the radiation detector. The measurement samples are three smear filter papers, three samples collected with adhesive paper, and four dust collection filter papers in the air. Even when applied to a sample of multiple spots, the amount of radioactivity evaluated from the PSL value is in good agreement with the amount of radioactivity measured by the radiation detector within the range of error.

  By this evaluation method, it became possible to quantitatively identify the radioactive substance spot from the PSL image of the imaging plate, and to evaluate the radioactivity amount of each spot within the error range from the PSL value. Thereby, distribution of radioactivity, such as contamination, can be evaluated.

  DESCRIPTION OF SYMBOLS 1 ... Dust collection filter paper, 2 ... Radioactive particle, 2a ... Dust latent image, 3 ... Imaging plate, 3a ... Dust latent image recording area, 3b ... Background area, 4 ... Reader, 5 ... Image data processing apparatus .

Claims (3)

A sample with a radioactive substance attached is brought into contact with the imaging plate and irradiated with radiation, whereby a latent image corresponding to the radiation dose is recorded on the imaging plate, and a background image is read by reading the latent image recorded on the imaging plate. In the radioactivity measurement method to create the radiation image data of the light and shade proportional to the radiation dose of the radioactive material and
The radiation dose image data read from the sample contact area in the imaging plate is set as a threshold value at which 99% of the background radiation dose image data read from the area other than the sample contact area in the imaging plate is less than that value. A method for measuring a radioactivity amount, comprising: determining a region in which pixels of radiation dose image data equal to or greater than the threshold value are continuous over 40 pixels or more as the presence position of a radioactive substance to be measured. In claim 1 , the amount of radioactivity of the radioactive substance to be measured is calculated by the equation (Equation 1) obtained by applying the least square method to a plurality of representative one-spot samples, and the relative error is expressed as 2) A method for measuring the amount of radioactivity, which is calculated by the equation.

An imaging plate that records a latent image corresponding to the amount of irradiation radiation by bringing a sample with a radioactive substance into contact with it and irradiating the radiation, and a background and a radioactive substance by reading the latent image recorded on the imaging plate In a radioactivity measuring device comprising a reader that creates grayscale radiation dose image data proportional to the radiation dose and an image data processing device that processes the radiation dose image data,
  The image data processing apparatus sets, as a threshold value, a value at which 99% of the background radiation dose image data read from an area other than the sample contact area on the imaging plate is equal to or less than that value, and the sample contact area on the imaging plate An apparatus for measuring a radioactivity amount, comprising: determining, in a radiation dose image data read from the above, a region where pixels of radiation dose image data equal to or greater than the threshold value are continuous over 40 pixels or more as an existing position of a radioactive substance to be measured.

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