JPH048743B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for non-destructive measurement of the uniformity of radioactive solidified bodies.

(Background technology and its problems) Radioactive materials generated at nuclear facilities are stored using hardening resin, glass, etc. for safe storage and stabilization during storage.
Although it is solidified using cement, asphalt, etc., measuring the uniformity of this radioactive solidified material is important for the management of the radioactive solidified material.

A common method for inspecting the uniformity of a radioactive solidified material is to cut the radioactive solidified material, observe it, and analyze it, but since this method destroys the radioactive solidified material, the radioactive The solidified material cannot maintain its properties and performance before inspection, and cannot be applied to 100% inspection.

As an alternative to such destructive measurement methods, it is possible to use X-ray tomography, but the radioactive solidified material to be measured emits γ-rays.
Since the rays are detected together with X-rays, the gamma-ray (X-ray) absorption rate at each location of the resulting radioactive solidified body is influenced by the type and amount of radioactive substances contained in the radioactive solidified body, and is highly accurate. Unable to measure.

(Object of the Invention) The present invention has been made based on the above-mentioned circumstances, and is a method for measuring the uniformity of a radioactive solidified material, which can non-destructively and accurately measure the radioactivity of the radioactive solidified material and the uniformity of the solidified material base material. The purpose is to obtain.

(Summary of the Invention) That is, the present invention provides a gamma ray absorption rate measurement system in which a gamma ray source, a collimator, and a gamma ray detector having an energy discrimination function are arranged in a straight line, between the gamma ray source and the collimator. The radioactive solidified body is placed and the γ
The linear absorption measurement system and the radioactive solidified body are moved relative to each other to obtain projection data of the transmitted γ-ray intensity from at least two directions on any cross section of the radioactive solidified body.
The radioactive solidified body is placed in front of the collimator of a gamma ray intensity measurement system including a ray detector, and the gamma ray intensity measurement system and the radioactive solidified body are moved relative to each other, so that at least Projection data of emitted γ-ray intensities from two directions are obtained, these projection data of emitted γ-ray intensities are subtracted from the projection data of the transmitted γ-ray intensity, and the projection data of the subtracted γ-ray intensities are calculated from the projection data of the subtracted γ-ray intensities within the tomographic plane. This is a non-destructive method for measuring the uniformity of a radioactive solidified body, which is characterized by obtaining the gamma ray absorption rate at each position.

(Example of the invention) The details of the present invention will be explained below.

FIG. 1 is a diagram schematically showing an apparatus for carrying out the method for nondestructively measuring the uniformity of a radioactive solidified body according to the present invention.

In the figure, reference numeral 1 indicates an external radiation source containment vessel. For example, ⁶⁰ Co
A γ-ray source 2 consisting of a γ-ray source 2 is housed, and its surroundings are covered with a γ-ray shield. A shutter 3 is provided in a part of the γ-ray shield, and this shutter 3 can be opened and closed as desired by a shutter drive mechanism 4.

A radioactive solidified material drive mechanism 6 is disposed in front of the shutter 3 of the external radiation source containment vessel 1 to fix a radioactive solidified material 5 to be measured and to make it rotate and linearly move within an arbitrary tomographic plane. There is.

Furthermore, a collimator 7 with a gamma-ray passage window in the center is disposed on the extension connecting the external radiation source containment vessel 1 and the radioactive solidified body 5 to narrow down the gamma-ray beam. 7 collimator window 7a
A gamma ray detector 8 capable of discriminating energy, such as a Ge detector or a Na1 (Tl) detector, is disposed behind the detector. Since each radioisotope in the solidified body emits gamma rays with a unique energy, the radioisotope can be identified by using a detector capable of discriminating energy. especially
Ge detectors have high energy resolution and are therefore effective in identifying radioactive isotopes.

Further, in front of the collimator window 7a of the collimator 7, a collimator window 7a is opened by the γ-ray absorber drive mechanism 9.
γ-ray absorber 10 that can be inserted arbitrarily into the front surface of
is installed.

The gamma ray absorber 10 has a thickness that blocks the gamma rays emitted from the radioactive solidified body 5 and allows the gamma rays emitted from the gamma ray source 2 to be attenuated and transmitted.

A multiplex wave analyzer 12 is connected to the output end of the gamma ray detector 8 via an amplifier 11, and the gamma ray detection signal detected by the gamma ray detector 8 is amplified by the amplifier 11, and the wave height is determined by the multiplex wave height analyzer 12. Analysis is performed.

Reference numeral 13 denotes an electronic computer, which performs arithmetic processing for image reconstruction from the data of the above-mentioned analysis results.

The measurement method of the present invention is carried out as follows using the above-mentioned apparatus. First, the radioactive solidified material 5 to be measured is fixed to the radioactive solidified material drive mechanism 6, and the collimator window 7a is set to the width of the required position resolution. .

Next, the shutter 3 is opened to release the γ-rays from the γ-ray source 2.
While irradiating the radioactive solidified body 5 with radiation, the radioactive solidified body drive mechanism 6 is operated to cause the radioactive solidified body 5 to perform, for example, rotational movement and linear movement across the γ-ray irradiation direction, and the radioactive solidified body 5 is exposed to γ-rays. A position is taken on the line connecting the source 2 and the gamma ray detector 8 to obtain projection data from at least two directions for image reconstruction.

The transmitted γ-rays from the γ-ray source are transmitted through the arranged radioactive solidified body 5 and passed through the collimator window 7a of the collimator 7, and are detected by the γ-ray detector 8 and analyzed by the multiple wave height analyzer 12. For example, a gamma ray energy spectrum as shown in FIG. 2 is obtained. From this gamma ray energy spectrum
The γ-ray intensity of 1332 KeV of ⁶⁰ Co is obtained by calculating the peak counting area in the shaded area in the same figure using the electronic computer 13. A plurality of pieces of 1332 KeV gamma ray intensity data obtained by performing such gamma ray intensity measurements in each direction are processed by the electronic computer 13 to reconstruct individual images on the tomographic plane using, for example, the convolution method. The distribution of gamma ray absorption rate at the position is determined.

At this time, the radionuclide in the radioactive solidified body 5 is γ
When different from that of radiation source 2, for example, when the nuclide in radioactive solidified body 5 is ¹³⁷ Cs and the γ-ray source is ⁶⁰ Co, and the γ-ray intensity of ¹³⁷ Cs is higher than a certain level, γ-ray absorption It is necessary to use your body to shield gamma rays from ¹³⁷ Cs.

That is, the gamma ray detector, amplifier, and multiple pulse height analyzer each have an upper limit on their measurable counting rate (the number of gamma rays counted per unit time).

Moreover, in order to carry out measurements efficiently and in a short time, it is desirable that the count not exceed this upper limit, and moreover be close to this upper limit.

Therefore, in this case, by driving the γ-ray absorber drive mechanism 9 and inserting the γ-ray absorber in front of the collimator window 7a of the collimator 7, the γ-rays from the radioactive solidified body 5 can be sufficiently shielded. If the intensity of the gamma rays can be counted close to the upper limit of the count rate of the detection system, the S/N ratio will be improved, the detection accuracy will be improved, and the measurement time can be shortened.

However, if there are non-uniform parts such as voids or separated substances in the radioactive solidified material, the gamma ray absorption rate of that part will be different from other parts. uniformity can be determined.

In the above method, if the radioactive nuclides in the radioactive solidified material contain the same types as those in the external source, the γ-rays emitted from the nuclides in the radioactive solidified material are superimposed on the γ-rays from the external source. This will cause errors.

In this case, similar operations are performed on the gamma rays emitted from the radioactive solidified material to obtain emitted gamma ray intensity data in the same direction within the same cross-sectional plane of the radioactive solidified material, and this emitted gamma ray intensity data is used to calculate the transmitted gamma ray intensity data. True γ-ray absorption data can be obtained by subtracting from the intensity data.

That is, in this case, after (or before) the above operation, close the shutter 3 of the external radiation source containment vessel 1, perform the same operation to obtain emitted γ-ray data,
The arithmetic processing for image reconstruction is performed using the data obtained by subtracting the corresponding transmitted gamma ray data.

Detected when shutter 3 is opened now.
If the 1332KeV gamma ray intensity of ⁶⁰ Co is Co, the 1332KeV gamma ray intensity from an external source is _C1 , and the 1332KeV gamma ray intensity from the radioactive solidified body is _C2 , then Co= _C1 + _C2Also , when the shutter is closed ⁶⁰ Co detected in case
If the γ-ray intensity of 1332KeV is Cc, Cc=C ₂ Therefore, the 1332KeV γ-ray intensity from an external source C ₁
can be determined by C ₁ =Co−Cc.

In this method, when measuring transmitted gamma rays, the gamma ray absorber 10 is inserted in front of the collimator window 7a, and when emitted gamma rays are measured, it is moved back to measure the gamma rays. γ-ray absorber 1 is added to the measurement data of emitted γ-rays measured by
The S/N ratio can be improved by multiplying the γ-ray attenuation rate by 0 and subtracting it from the corresponding transmitted γ-ray data.

In other words, the γ-ray intensity of 1332 KeV when the shutter 3 is opened and the γ-ray absorber is inserted is Co′γ
If the γ-ray attenuation rate by the ray absorber is a, then Co' = aC ₁ + aC ₂ (C ₁ and C ₂ are the same as above) aC ₁ = Co' - aC ₂ Therefore, the γ-rays emitted from the radioactive solidified body By multiplying the data by the gamma ray attenuation rate and subtracting it from the transmitted gamma ray data at this time, the value obtained by multiplying the gamma ray intensity from the external source by a constant can be obtained. From this value, the influence of γ rays from the radioactive solidified body has been removed, and the S/N ratio has been improved.

As described above, the uniformity of the radioactive solidified body can be determined from the distribution of γ-ray absorption rate of the radioactive solidified body.

Each of the above processes and control of each device is performed by an electronic computer.

Although the above embodiments have been described using one collimator and one gamma ray detector, the present invention is not limited to such embodiments.
A plurality of sets of collimators and gamma ray detectors may be arranged centripetally with respect to the gamma ray source. In this case, even if the radioactive solidified material driving device is driven in one direction, for example, in the rotational direction, the same result as scanning the radioactive solidified material from two directions can be obtained by combining the outputs of each detector. This makes it possible to perform image reconstruction processing.

(Effects of the Invention) As explained above, according to the present invention, it is possible to determine the γ-ray absorption rate at each position of an arbitrary tomographic plane of a radioactive solidified body without being affected by the γ-rays emitted from the radioactive solidified body. It is possible to non-destructively evaluate the uniformity of the radioactive solidified material.

[Brief explanation of drawings]

FIG. 1 is a block diagram schematically explaining an embodiment of the present invention, and FIG. 2 is a graph showing the energy spectrum of γ-rays irradiated from an external source and transmitted through a radioactive solidified body. 1... External radiation source containment vessel, 2... γ-ray source, 3...
... Shutter, 4... Shutter drive mechanism, 5... Radioactive solidified body, 6... Radioactive solidified body drive mechanism, 7...
...collimator, 8...gamma ray detector capable of energy discrimination, 9...gamma ray absorber drive mechanism, 10...γ
Line absorber, 12...Multiple wave height analyzer, 13...Electronic computer.

Claims (1)

[Claims] 1 γ-ray source, collimator, and γ-ray detector with energy discrimination function arranged in a straight line
A radioactive solidified body is placed between the γ-ray source and the collimator of the radiation absorption rate measurement system, and the γ-ray absorption rate measurement system and the radioactive solidified body are moved relative to each other, so that at least two Transmission from direction γ
While obtaining radiation intensity projection data, the radioactive solidified body is placed in front of the collimator of an emitted gamma ray intensity measurement system consisting of a collimator and a gamma ray detector having an energy discrimination function, and the emitted gamma ray intensity measurement system is performed. The system and the radioactive solidified body are moved relative to each other to obtain projection data of emitted γ-ray intensities from at least two directions on the arbitrary tomographic plane, and these projection data of emitted γ-ray intensities are used as a projection of the transmitted γ-ray intensity. A method for non-destructively measuring the uniformity of a radioactive solidified body, characterized in that the gamma-ray absorption rate at each position within the tomographic plane is obtained from the projection data of the subtracted gamma-ray intensity by subtracting the gamma-ray intensity from the data.