JPS6280579A - Measuring instrument for radioactive concentration - Google Patents

Abstract

PURPOSE:To measure accurate radioactive concentration by correcting the slit width of a variable slit width collimator according to the transmissivity of gamma rays, correcting an error in measurement due to the horizontal ray source state in a sample to be measured, and inputting measurement data from a gamma ray detector and calculating the radioactive concentration by nuclear species. CONSTITUTION:A container 2 to be measured is mounted on a sample mount table 9 and a sample on the table 9 is divided lengthwise into plural columnar segments for radiation evaluation; and transmitted gamma rays from an external gamma ray source 10 are measured by a gamma ray detector 11 at a position opposite to the ray source 10 across the container 2 and an arithmetic device calculates the mean concentration in a segment from the transmissivity by using a specific expression. Then, the slit width of the rectangular variable slit width collimator 12 installed in front of the detector 11 is determined on the basis of the gamma ray transmissivity. Then, the arithmetic device calculates the peak count rate of an object nuclear species from gamma ray spectrum data inputted from respective detectors and calculates the radioactive concentration by nuclear species.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a radioactivity concentration measuring device that measures 82 degrees of radioactive material such as atypical solid radioactive waste filled in a regular container for each nuclide.

[Technical background of the invention] Among atypical solid wastes such as piping and concrete waste generated from nuclear facilities, those that cannot be subjected to volume reduction treatment such as incineration or acid decomposition are directly stored in drums, etc. They are stored in standard containers and disposed of at waste storage facilities, etc. When moving such waste to other facilities or evaluating a facility's permitted storage capacity (radioactive ♀ exchange wharf),
It is necessary to measure the radioactivity contained in waste. In such cases, NaI(
Tβ) Measurement is performed from outside the container using a scintillation detector, germanium semiconductor detector, etc., without taking out the solid waste inside.

However, quantifying the radioactivity of such waste is accompanied by large errors. In general, the radioactivity of the radioactive waste filled in the container is localized, and the fraction of the gamma ray absorber is often uneven, and the difference between the detector and the This is because the distance between and variations in the gamma ray absorber directly affect the measured values. Such errors become more pronounced as the measurement target becomes a nuclide that emits low-energy gamma rays.

In order to eliminate these effects as much as possible,
A conventional method for analyzing low-energy emitting nuclides around 0 KeV is shown in FIGS. 5 to 8. Figure 5 shows a method in which a container to be measured 2 is placed on a rotary table 1, and the detector 3 is moved in the longitudinal direction of the container to be measured to carry out measurement. In this method, a cylindrical container 2 to be measured is placed on a roll 5 rotated by a motor 4 to avoid rotation, and a detector 3 placed below is placed on a roll 5 that is rotated by a motor 4.
The measurement is carried out while moving in the axial direction.

Both of these can be done by rotating the container to be measured.
The aim is to average out the variations in distance due to non-uniform radioactivity distribution and the variations in the fraction of gamma ray absorbers.

FIG. 7 shows an improved version of the above method, in which a parallel slit collimator 6 is placed in front of the detector 3 to limit the longitudinal field of view of the container to be measured 2, and the container to be measured 2 is moved along with the rotating mechanism by a lifting platform 7. Measurement is performed by moving the device. In this method, of the gamma rays emitted from the free substances in the container 2 to be measured, the component that crosses diagonally in the longitudinal direction up to the detector 3 is cut, so compared to the methods shown in FIGS. 5 and 6, The influence of non-uniformity within the container can be reduced.

FIG. 8 shows a slightly improved method of the method shown in FIG. 7, in which a parallel slit collimator 6 placed in front of the detector 3 divides the container 2 to be measured into several segments, and At the same time, the container to be measured 2 is raised in a stepwise manner, the amount of radioactivity is evaluated for each segment, and the amount of radioactivity in the container to be measured 2 is evaluated based on the total. An external comma ray source 8 other than the nuclide to be measured is placed at a position opposite to the nuclide to be measured, and the average density inside the segment is determined based on the gamma ray transmittance, and gamma ray absorption correction is performed for each segment.

[Problems with the blue slip technology] In the conventional method, it is possible to perform comma ray absorption correction with some accuracy regarding the radioactivity distribution or density distribution in the longitudinal direction of the container; If a distribution exists, large differences may occur in the radioactivity evaluation values. The graph in FIG. 9 shows, as an example of a case where radioactivity is locally present, a filling material with a specific gravity of 1.25 is placed in an S-interposed drum container with a volume of 200λ,
This shows the measurement results when one point-shaped radiation source is embedded in the drum and measured using the method shown in Figure 7. Curve A shows the evaluation value when the radiation source is located on the surface of the drum. The graph shows the ratio of the comma ray energy of the nuclide to be measured to the evaluation value when the drum is heated uniformly.Curve B shows the same evaluation when the radiation source is located at the center of the drum. It represents the ratio of values. As is clear from this graph, when the release material is scattered in a pot, the lower the energy of gamma rays, the greater the tendency for the amount of radioactivity to be overestimated or underestimated. This results in a large error. This is because as the gamma ray energy decreases, the proportion of comma rays absorbed by the filling increases.

[Purpose of the Invention] The present invention has been made to address the above-mentioned problems, and even when there is a density or radioactivity distribution in the radial direction of a regular container filled with solid radioactive waste, gamma ray absorption by the substance can be prevented. The object of the present invention is to provide a radioactivity concentration measuring device that can extremely reduce accompanying measurement and evaluation errors.

[Summary of the Invention] In other words, the present invention includes a comma ray detector that measures comma rays from around a measurement sample, and an external detector that measures the transmittance of comma rays by irradiating the measurement sample with comma rays. a radiation source;
A variable slit width collimator is arranged in front of the gamma ray detector, and the slit width of the variable slit width collimator is controlled based on the transmittance to compensate for measurement errors due to the horizontal distribution state of the radiation source within the measurement sample. This is a radioactivity concentration measuring device characterized by comprising: a calculation device which inputs measurement data from the gamma ray detector and calculates radioactivity concentration for each nuclide.

[Embodiment of the Invention] Hereinafter, the present invention will be described in detail with regard to an embodiment shown in the drawings.

Figures 1 and 2 show a detection section of an embodiment of the radioactive concentration measuring device of the present invention, with Figure 1 schematically showing a cross section and Figure 2 schematically showing a longitudinal section. . This detection unit includes a sample mounting table 9 on which the container to be measured 2 is placed, a rotation mechanism (not shown) that rotates the sample mounting table 2 at a constant speed, an elevating mechanism (not shown) that raises and lowers the sample mounting table 2, and a an external gamma ray source 10 that transmits gamma rays of energy different from the nuclide to be measured into the container 2; and a plurality of external gamma ray sources that detect gamma rays emitted from within the container 2 to be measured and gamma rays transmitted through the container 2 from the external gamma ray source. A comma ray detector 11 (for example, a germanium semiconductor detector or a NaI (Tλ) scintillation detector), and these gamma ray detectors 1
A rectangular slit width variable collimator 12 is placed in front of the collimator 1.

This rectangular variable slit width collimator 12 is shown enlarged in FIG. The width of the slit in the height direction is fixed to a predetermined width, but the width of the slit in the horizontal direction can be changed freely by sliding the slit indicated by reference numeral 13 along the guide pin 14 in the figure. can. The slit width in the horizontal direction is controlled by an arithmetic unit (not shown).

First, the container 2 to be measured is placed on the sample mounting table 9 so that its central axis coincides with the rotation axis. The sample on this sample mounting table 9 is divided into a plurality of cylindrical segments in the longitudinal direction of the sample for radioactivity evaluation, and the cutting activity is evaluated for each segment, and the total radioactivity is calculated as the sum. ♀ is required. Such measurement division limits the longitudinal field of view by the width of the slit in the height direction of the collimator 12 placed in front of the gamma ray detector 11, and raises the sample stepwise by a distance corresponding to this field of view. It can be obtained by measuring while rotating at a constant speed each time.

The amount of 110"J conversion for each segment is calculated through the following steps. First, the transmitted gamma rays from the external comma ray source 10 are detected by the gamma ray detector 11 at a position facing the external gamma ray source 10 with the sample sandwiched between them. The average density ρ of the legmen 1 to 1 is calculated from the transmittance by the calculation device based on the following equation.

l0Q (I/Io) ρ=- μmt Here, Io is the gamma ray count rate from the external gamma ray source 10 when there is no sample, and ■ is the gamma ray count rate from the external gamma ray source 10 when the sample is placed. , μm is the mass absorption coefficient corresponding to the gamma ray energy from the external gamma ray source 10, and t is the inner radius of the vessel to be measured.

Next, the width of the slit 1 of the rectangular variable slit width collimator 12 installed in front of the comma ray detector 11 is determined from the gamma ray transmittance, which is based on the following principle. When radionuclides are unevenly distributed in the center of the sample, the measured value at the gamma ray detector 11 decreases due to internal absorption of the substance between the sample and the comma ray detector 11. This rate of decrease increases exponentially with increasing density, leading to underestimation of measurements. On the other hand, when viewing the sample through the gamma ray detector 11, the wider the horizontal slit width, the wider the field of view. Since the measurement sample is rotating at a constant speed, the time it takes for radioactivity to cross the field of view of the collimator is different between the center and the periphery, and becomes shorter toward the periphery. This contrast makes the culm stiffer as the width of the slit becomes narrower.

Therefore, by setting an appropriate width to the sulin 1 of the collimator 12, the absorption of both comma rays when the source is at the center and at the periphery can be achieved in accordance with the radial portion of the radioactivity. It is possible to correct the difference. However, if the packing density is the same and the nuclide to be measured is different, the absorption rate of gamma rays emitted from the nuclide to be measured will change, so the relationship between the transmittance and the s1 knot width will be different. Therefore, the nuclide to be measured is classified in advance into a plurality of groups according to the emitted energy of gamma rays, and a dedicated comma ray detector 11 and a remeter 12 are used for each group to measure the gamma ray transmission as shown in FIG. The relationship between ratio and slit width is derived, and the slit width of each collimator 12 is determined based on this result. For example, when two gamma ray detectors 11 are used, the energy range is classified as 100 to 1500, where the degree of change in gamma ray absorption coefficient is relatively small.
One possible method is to classify it into KeV and 100KeV or less.

The slit width of the collimator 12 in front of the gamma ray detector 11 corresponding to each energy region is determined from the relationship between the gamma ray transmittance and the slit width in each energy region classified into several in this way, and then the comma Line spectrum measurements are taken. The calculation device inputs the gamma ray spectrum data from each gamma detector 11, calculates the peak number 1 number rate of the nuclide to be measured, and then calculates it as a function of the slit width 8 density or obtains it from actual measurement using a radiation source. Calculate the radioactivity concentration by nuclide using the detection efficiency.

In this example, one of the gamma ray detectors for spectrum measurement was also used for gamma ray transmittance measurement, but for gamma ray transmittance measurement, a gamma ray detector separate from the gamma ray detector for spectrum measurement was used. A container may also be used.

[Effects of the Invention] As is clear from the above explanation, the radioactivity concentration measuring device of the present invention can reduce measurement errors by adjusting the density and radioactivity distribution in the longitudinal direction of the container to be measured. In addition, regarding the density and radioactivity distribution in the radial direction of the container, it is possible to correct internal absorption effects over a wide energy range, and it is possible to accurately measure the radioactivity concentration even if there are variations in the radial direction.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing the detection section of an embodiment of the radioactive concentration measuring device of the present invention, FIG. 2 is a longitudinal section view of the detection section shown in FIG. 1, and FIG. 3 is the same as shown in FIG. 1. A perspective view showing a rectangular variable slit width collimator of a radioactive concentration measuring device, FIG. 4 is a graph showing an example of the relationship between gamma ray transmittance and slit width, and FIGS. 5 to 8 are a conventional radioactive concentration measuring device Figure 9 is a side view showing the detection part of the drum can with filler, and Figure 7 shows the case where one point-shaped radiation source is positioned at the center of the drum can, and the side view when it is positioned on the surface of the drum can. This is a graph showing the radioactivity evaluation values determined by the Go device. 2...... Container to be measured 9... Test sample stand 10... External gamma ray source 11...
・・・・・・Gamma ray detector 12・・・・・・・・・Rectangular slit width variable collimator Fig. 1 Fig. 2 Fig. 5 Minoh Fig. 6 Fig. 8

Claims (2)

[Claims] (1) A gamma ray detector that measures gamma rays from around the measurement sample, an external gamma ray source that irradiates the measurement sample with gamma rays and measures its transmittance, and a slit placed in front of the gamma ray detector. a variable width collimator;
The slit width of the variable slit width collimator is controlled from the transmittance to correct measurement errors due to the distribution state of the radiation source in the horizontal direction within the measurement sample, and measurement data from the gamma ray detector is input. A radioactivity concentration measuring device comprising: a calculation device for calculating radioactivity concentration for each nuclide. (2) The radioactive concentration measuring device according to claim 1, wherein the slit width in the height direction of the variable slit width collimator is fixed, and only the slit width in the horizontal direction is variable.