CN104714245B - The half tomographic gamma scan method that middle cool waste bucket measures - Google Patents

Abstract

A semi-tomographic gamma scanning method for measuring low- and medium-level radioactive waste barrels, comprising: a rotary table, a detector platform, a detector with a collimator, a transmission source platform, a transmission source and its shielding components, and an analysis module. The bucket is divided into several sections along the height direction. By rotating the waste bucket, the surface distribution of radioactive point sources in each section is changed into the line distribution of ring sources along the radial direction. Measure at different eccentric positions in the layer, establish a system of equations for the relationship between the count rate of the reaction detector and the activity of nuclei in each ring grid, and solve the system of equations to obtain the activity of radionuclide in the waste bucket along the diameter direction and height direction distribution, summed to obtain the total activity of radionuclides in the waste bin. The method has high measurement precision and short measurement time, and has high practical value and application prospect.

Description

Semi-tomographic gamma scanning method for the measurement of low and medium radioactive waste barrels

technical field

The present invention relates to a gamma scanning method and activity reconstruction technology for nuclide and activity measurement of medium and low radioactive barreled waste in nuclear power plants, and a corresponding device based on the technology. Specifically, a radiation detector is used to measure uniform velocity Rotating gamma rays emitted in the waste bin, and then reconstructing the scanning technology of the distribution of radionuclides and their activities along the radial and axial directions of the waste bin.

Background technique

With the vigorous development of nuclear power in the country, a large amount of medium and low radioactive waste generated in the operation of nuclear power needs to be disposed of urgently. According to the national standards GB11928-1989, GB9132-1988 and other regulations, these wastes must be tested for surface pollution and dose rate, element identification and activity measurement in the barrel before intermediate temporary storage, transportation and final disposal. Since the measurement object is a large-volume barreled waste containing radioactivity, it is difficult to accurately measure the nuclei and activity in the barrel. At present, the ideal is non-destructive testing technology, gamma scanning technology is one of the most widely used nuclear power plant drum waste detection methods, including segmented gamma scanning technology (Segmented Gamma Sacnning, SGS) and tomographic gamma scanning technology ( Tomographic Gamma Sacnning, TGS). The principle is to measure the amplitude and count of gamma rays by a radiation detector, and then analyze the energy spectrum of gamma rays measured by a multi-channel analyzer, so as to determine the type of nuclide and the characteristic gamma ray count rate emitted by the corresponding nuclide . Since the nuclides are distributed in the barrel, and the emitted rays are also attenuated by the absorption and attenuation of the substances in the barrel, it is necessary to establish the relationship between the measurement count rate and the activity of each nuclide in the barrel. The SGS developed in the 1970s is to divide the waste barrel into several layers along the height direction, assuming that the materials and nuclides in the fault are evenly distributed, so as to establish the relationship between the measurement count rate and the activity, and stipulate the detection The scanning method of the method, that is, the waste barrel rotates at a constant speed (the purpose is to reduce the unevenness of the distribution of radionuclides in the barrel), and the radiation detector is stepping up and down to measure each fault, and the radioactivity can be obtained by this method. Distribution along the axis of the waste bin. Since SGS believes that the radionuclides are evenly distributed in the waste bin, the reconstructed activity has a very large error compared with the real value. Based on the SGS technology, TGS proposed in the 1990s introduced tomographic scanning technology into the measurement of each section layer, which can obtain the three-dimensional distribution of the filling material and radionuclide activity in the barrel through reconstruction and calculation, greatly improving the measurement accuracy. This method requires tomographic scanning, that is, the detector measures the waste bin from various directions and positions, such as the stepping rotation of the waste bin, the eccentric translation of the detector on the horizontal plane and the elevation along the height direction. Therefore, although TGS measures High precision, but the measurement process is complicated and the time is dozens of times that of SGS, which limits its wide application.

In view of the characteristics of short measurement time and poor accuracy of SGS and high measurement accuracy and long time of TGS, it will be of great value and application prospect to seek detection technology with relatively short measurement time and relatively high measurement accuracy. For this reason, an improved SGS technology (Improved Segmented Gamma Sacnning, ISGS) based on dual detectors was proposed. Point source, because the waste bucket rotates at a constant speed during measurement, the point source is a linear ring source. The radius of the ring source is determined by two detectors. The positioning of the source enables accurate calculation of the detection efficiency, so the detection accuracy is greatly improved compared with SGS. . This method has high detection accuracy for the presence of point sources, but the accuracy will decrease in the case of multiple point sources.

Contents of the invention

In view of the deficiencies in the above-mentioned prior art, the present invention proposes a semi-tomography (Semi-Tomography Gamma Scanning, STGS) gamma scanning scanning method for the measurement of medium and low radioactive waste barrels. The method proposed by the present invention is between SGS and TGS. The semi-tomographic gamma scanning between the waste buckets can obtain the distribution of radionuclides (annular existence) in the radius of the segment and the height of the waste bucket during the rotation measurement of the waste bucket. The measurement accuracy is higher than that of SGS, and it is greatly simplified compared with TGS. The measurement process is simplified and the measurement time is shortened.

In order to achieve the above object, the technical scheme adopted in the present invention is as follows:

A semi-tomographic gamma scanning method for measuring medium and low radioactive waste barrels, the scanning device used in the method includes a rotary table, a detector platform, a detector with a collimator, a transmission source platform, a transmission source and its shielding parts , the analysis module, the radioactive waste barrel rotates at a constant speed during scanning and measurement, and the radionuclide in the barrel is equivalent to a circular line source from a point source after rotation relative to the detector outside the barrel, and the waste barrel is divided into several sections along the axial direction, The filling material in each section layer is evenly distributed, and then each section layer is divided into several ring grids, and the detector is measured at several eccentric positions in each section layer, and the radionuclide in each ring grid is obtained through reconstruction calculation. The activity is distributed along the radial and axial directions of the waste barrel, realizing rapid and accurate measurement of the radioactivity of the waste barrel.

Specifically include the following steps:

The first step is to adjust the specific positions of the detector and its collimator, the transmission source and its shielding parts, so that the axis of the detector is aligned with the center of the waste bin and passes through the collimation hole of the transmission source;

The second step is to scan the current layer

The waste bucket rotates at a constant speed at a constant speed to collect energy spectrum data; the distance from the center of the waste bucket to the detector axis is in turn a predetermined distance, and a total of several energy spectrum data of this section are collected;

The third step is to start the detector platform and the transmission source platform from the bottom of the waste bin, move the transmission source platform and the detector platform in the vertical direction at the same time, scan layer by layer, and repeat the second step for each layer of scanning;

The fourth step, data processing

First, the transmission measurement of each layer is carried out to obtain the average attenuation coefficient of the material in this layer to the ray, and then each layer is divided into ring-shaped grids of equal area along the radial direction, and the material attenuation correction of each grid to the detector is calculated The detection efficiency of each ring grid is established to reflect the relationship between the activity of each annular grid radionuclide and the count rate of the detector at each detection position, and the equations are solved to obtain the activity of the radionuclide in the waste barrel along the direction of the barrel diameter and The distribution in the height direction is summed to obtain the total activity of radionuclides in the waste barrel.

In the second step, the waste barrel rotates at a constant speed at a constant speed, and the specific measurement time for collecting energy spectrum data at a measurement position depends on the radioactivity level.

The data processing in the fourth step includes transmission reconstruction and emission reconstruction, wherein the transmission reconstruction is to obtain the attenuation coefficient or density of the ray by the filling material of each section of the waste barrel, and the emission reconstruction is to obtain the radioactive radiation in the waste barrel. Computational analysis of nuclide activity distribution.

The specific method of the transmission reconstruction is:

It is considered that the material is uniformly distributed in each layer, and P _i is set to be equal to the transmittance measured by the detector when the eccentric position of the i segment layer is zero: P _i =C _i /C _max , where: i=1, 2... , I, C _i represent the full-energy peak count rate of gamma photons measured _by the detector at the position of the i segment layer when there is a waste bucket; The relationship between gamma photon total energy peak count rate, transmittance and attenuation coefficient is: μ _i D=-ln(P _i ), where μ _i is the attenuation coefficient of rays to rays by substances in the i-th section layer, and D is the diameter of the waste bucket.

The specific method of the launch reconstruction is:

Since the rays emitted by the radioactive nuclides in the barrel have to interact with the material in the barrel to attenuate before reaching the detector, attenuation correction must be performed. When the detector is facing the jth eccentric position of the i-th section layer, The count rate of the radiation emitted by the radionuclide in the nth ring grid and measured by the detector is: C _mn,ij = α·ε _mn,ij ·χ _mn,ij ·A _mn , where α is the considered ray Energy branch ratio, ε _mn,ij is the detection efficiency of the detector when the radionuclide pair in the nth ring grid of the _mth layer is at the jth eccentric position of the ith segment layer; The activity of radionuclides in n ring grids; the attenuation correction coefficient χ _{mn, ij} = exp(-Σul), μ and l are the attenuation coefficients experienced by each layer from the nth ring grid to the detector in the mth layer and the average track length, considering that the detector can measure the rays emitted by all the grids in the entire waste bin, the count rate measured by the detector at the jth eccentric position of the i-th section layer is: Combine the detection of all positions to form a linear equation system whose number of unknowns is I×N and number of equations is I×J:

The segment number I of the waste bin is 9, the eccentric position number J ranges from 2 to 8, and the ring grid number N of each segment layer is not greater than J.

The number N of annular grids of each segment layer is the same as the number J of eccentric positions.

The number of measured positions, that is, the number of equations, is not less than the number of unknowns, that is, the number of divided grids, that is, P≥Q.

The method for solving the linear equation system is an iterative method based on probability and statistics.

Compared with traditional gamma scanning technology, the present invention has the following advantages:

(1) Compared with SGS, STGS can obtain the distribution of nuclides in the segment layer, so the measurement accuracy is greatly improved, and the measurement time is only increased several times (specifically related to the number of grids). In actual measurement, both STGS and SGS require the waste bucket to rotate at a constant speed, and the detector to step up and down along the height of the waste bucket (for example, 9 times), but STGS also needs to perform several eccentric translations in the plane of each section (such as 4 or 8 times) to achieve tomographic scanning.

(2) Compared with TGS, STGS simplifies the inner surface (two-dimensional) distribution of the segment layer to the radial direction (one-dimensional) distribution by rotating the waste bin, which simplifies the scanning process and shortens the measurement time by an order of magnitude. With the application of technology, the measurement accuracy has not been significantly reduced. In the actual measurement, TGS requires the waste bucket to perform step rotation (such as 24 times), detector step eccentric translation (such as 4 times) and step up and down (such as 9 times), and STGS requires the waste bucket to rotate at a uniform speed, so Reduced the process of doing circular step measurements on waste bins.

(3) Compared with ISGS, STGS reconstructs according to the distribution characteristics of ring sources, and does not use the ISGS point source assumption to set assumptions for measurement technology, so the measurement technology is more reasonable and more suitable for multi-point sources and even nuclide uniform distribution. Measurement. In actual measurement, both STGS and ISGS require the waste bin to rotate at a constant speed and the detector to be raised and lowered step by step, but ISGS only needs to move eccentrically once in the layer plane, while STGS needs to move eccentrically multiple times (such as 4 or 8 times), and the measurement time longer.

The current national standard only requires the measurement of the type and total activity of the nuclei in the waste bin, and does not require obtaining its specific distribution. Gamma scanning technology is used to measure the nuclide and activity of radioactive waste barrels. The accuracy is related to the radioactivity level, the uniformity of the nuclide and filling material, and the detector. For barreled waste with medium and low radioactivity, nuclides with higher radioactivity levels exist in the form of hot spots, and the distribution of radionuclides is poor; at the same time, when the filling material is compressed and packed into barrels, the difference in density distribution is relatively insignificant. Therefore, SGS's assumption about the uniform distribution of radionuclides in each section will cause a large detection error, and at the same time, the three-dimensional distribution of nuclides in the waste bin reconstructed by TGS exceeds the requirements of relevant standards, so STGS is well balanced The advantages and disadvantages of SGS and TGS on measurement accuracy and measurement time are understood, and its practicability is greatly enhanced.

Description of drawings

Fig. 1 is the structural representation of an embodiment of the measuring device that the inventive method adopts;

Fig. 2 is the front view of Fig. 1 embodiment device;

Fig. 3 is a schematic diagram of the rotating table of Fig. 1 embodiment;

Fig. 4 is a schematic top view of the rotary table moving assembly of the embodiment in Fig. 1;

Fig. 5 is a block flow diagram of the inventive method;

Fig. 6 is a schematic diagram of ring grid division in a certain section of the waste bin;

FIG. 7 is a schematic horizontal cross-sectional view of FIG. 6 .

Detailed ways

The embodiments of the present invention are described in detail below. This embodiment is implemented on the premise of the technical solution of the present invention, and detailed implementation methods and specific operating procedures are provided, but the protection scope of the present invention is not limited to the following implementation example.

Example

As shown in Figure 1, this embodiment includes: a waste bucket rotary table 1, a rotary table moving assembly 2, a detector lifting platform assembly 3, a transmission source lifting platform assembly 4, a base 5, and high-purity germanium with a collimator 6 A detector 7, a transmission source 8 with a shielding block and a data processing module 9, wherein: the rotating table moving assembly 2, the detector lifting platform assembly 3 and the transmission source lifting platform assembly 4 are jointly arranged on the base 5, and the waste bucket rotating table 1. The high-purity germanium detector 7 and the transmission source 8 are respectively arranged on the rotating table moving component 2, the detector lifting platform component 3 and the transmission source lifting platform component 4, and the detector lifting platform component 3 and the transmission source lifting platform component 4 The high-purity germanium detectors 7 generate electrical signals and are connected to the data processing module 9. This mechanical system can realize the precise lifting of the transmission source 8 and the detector 7, as well as the rotation and rotation of the waste bucket. panning.

As shown in Figure 3, the waste bin turntable 1 includes: a first servo motor 11 with a first reducer 10, a turntable support 12, a turntable bottom plate 13, a turntable shaft sleeve 14, and a turntable main shaft 15 , rotary table rotating plate 16, rotary table coupling 17, thrust ball bearing 18, angular thrust ball bearing 19, wherein: rotary table bottom plate 13 is connected with rotary table bracket 12, first servo motor 11 is connected with first reducer 10 connected, the first reducer 10 is connected to the main shaft 15 of the main shaft 15 of the main shaft of the rotary table through the coupling 17 of the rotating table, the main shaft of the rotating table 15 is connected to the rotating plate 16 of the rotating table, and thrust balls are respectively arranged between the main shaft of the rotating table 15 and the sleeve 14 of the rotating table Bearings 18 and angular thrust ball bearings 19 are used to support the turntable rotating plate 16 and prevent overturning; when the first servo motor 11 with the first reducer 10 rotates, it passes through the turntable coupling 17 and the turntable main shaft 15 Drive the rotary plate 16 of the rotary table to rotate, and drive the waste bucket to be tested placed on the rotary plate 16 of the rotary table to rotate.

As shown in FIG. 4 , the rotating table moving assembly 2 includes: a second servo motor 20 , a first ball screw 21 , a first guide rail 22 , a first base 23 and a coupling 24 , wherein: the first guide rail 22 Set on the first base 23, the rotating shaft of the second servo motor 20 is connected with the first ball screw 21 through the coupling 24, and the nut on the first ball screw 21 is connected with the turntable bottom plate 13; when the second servo motor When 20 rotates, it drives the first ball screw 21 to rotate and drives the waste bucket rotary table 1 through the screw nut to realize the horizontal movement of the measured object.

As shown in Figure 2, the detector lifting platform assembly 3 includes: a detector bracket 25, a detector platform base 26, a first plate 27 with ribs, a second plate 28, a second ball screw 29, The second guide rail 30, the third ball screw 31, the third guide rail 32, the hand wheel 33 and the third servo motor 35 with the second reducer 34, wherein: the detector bracket 25 is fixedly arranged on the base of the detector platform 26, the second guide rail 30 is set on the detector bracket 25, the third servo motor 35 drives the second ball screw 29 to rotate through the second reducer 34 and drives the first plate 27 to rise and fall, and the third guide rail 32 is set on the first On the flat plate 27, the second flat plate 28 is connected with the third ball screw 31 and parallel to the first flat plate 27, the hand wheel 33 is connected with the third ball screw 31 and drives the second flat plate 28 to translate, and the high-purity germanium detector 7 is placed on the second plate 28.

As shown in FIG. 2 , the transmission source lifting platform assembly 4 includes: a transmission source support 36, a transmission source platform base 37, a third plate 38 with ribs, a fourth ball screw 39, a fourth guide rail 40 and A fourth servo motor 42 with a third reducer 41, wherein: the transmission source support 36 is fixedly arranged on the transmission source platform base 37, the fourth guide rail 40 is arranged on the transmission source support 36, and the fourth servo motor 42 passes through the third The reducer 41 drives the fourth ball screw 39 to rotate and drives the third plate 38 to rise and fall, and the transmission source 8 is placed on the third plate 38 .

The flow chart of the inventive method shown in Fig. 5 is specifically as follows:

The inventive method realizes STGS scanning by following concrete steps:

The first step, setting the initial height positions of the second flat plate 28 and the third flat plate 38, so that the detector 7 on the second flat plate 28 is aligned with the bottom of the waste bucket; adjust the high-purity germanium detector 7 and its collimator 6 At the specific position of the transmission source 8 on the second flat plate 28 and the third flat plate 38 , the axis of the detector 7 is aligned with the collimation hole of the transmission source 8 .

The second step is to scan the current segment layer

The waste bin rotates at a constant speed, and the speed is about 10 rpm. The specific measurement time of a measurement position is determined by the radioactivity level, generally 5-10 minutes, and the energy spectrum data of a detection position is collected; the center of the waste bin reaches the detector 7 The distances of the axes are the specified 8 distances (for example, 0, 3.5, 7.0, 10.5, 14.0, 17.5, 21.0, 24.5cm), and a total of 8 energy spectrum data of this section are collected.

The third step, move the second flat plate 28 and the third flat plate 38 simultaneously in the vertical direction, scan each layer of the waste bin from bottom to top, move 9 times, each vertical distance is 10cm, and repeat the second step for each layer of scanning .

The fourth step is data processing.

It is divided into two parts: transmission reconstruction and emission reconstruction. The transmission reconstruction is to obtain the attenuation coefficient or density of the ray by the filling material of each section of the waste barrel, and the emission reconstruction is to obtain the radionuclide activity in the waste barrel. Computational analysis of degree distribution.

(1) Transmission reconstruction

This method is the same as SGS, considering that the substance is evenly distributed in each layer. Let P _i be equal to the transmittance measured by the detector 7 when the eccentric position of the ith (i=1, 2, ..., I) section layer is zero: P _i =C _i /C _max , wherein: C _i represents The total energy peak count rate of gamma photons measured _by the detector 7 at the position of the i-th section layer when there is a waste bucket; Peak count rate. The relationship between the transmittance and the attenuation coefficient is: μ _i D = -ln(P _i ), where μ _i is the attenuation coefficient of the material in the i-th section layer to the ray, and D is the diameter of the waste bucket.

(2) launch reconstruction

Since the radiation emitted by the radionuclide in the bucket must interact with the material in the bucket to attenuate before reaching the detector 7, attenuation correction must be performed.

Figures 6 and 7 are schematic diagrams of the ring grid division method for each segment layer, assuming that the pixels in each grid are evenly distributed. The grid division adopts the method of equal area division, and the calculation method of the nth ring radius is: Where: n=1, 2..., N, N is the number of divided grids, and R is the radius of the waste bin.

When the detector is facing the jth eccentric position of the i-th section layer, the count rate of the radiation emitted by the radionuclide in the n-th annular grid of the m-th layer and measured by the detector 7 is: Where α is the branching ratio of the considered ray energy, ε _mn,ij is the detection efficiency of the detector 7 when the radionuclide pair in the n-th ring grid of the m-th layer is at the j-th eccentric position of the i-th section layer , the detection efficiency is mainly related to the geometric position and the intrinsic efficiency of the detector 7; A _mn is the activity of the radionuclide in the nth ring grid of the mth layer; the attenuation correction coefficient χ _mn,ij = exp(-Σul) , μ and l are the attenuation coefficients and average track lengths of each layer experienced by the nth annular grid on the mth layer to the detector 7. Considering that the detector can measure the rays emitted by all the grids, the count rate measured by the detector 7 at the jth eccentric position of the i-th segment layer is: Combined with the detection of all positions, a linear equation system whose number of unknowns is I×N and number of equations is I×J can be formed:

Among them, P=I×J, Q=I×N, e=α·ε·χ, usually the number of measured positions (that is, the number of equations) is not less than the number of unknowns (that is, the number of divided grids), P≥Q . The method for solving the linear equation system is usually an iterative method based on probability and statistics, such as the maximum likelihood function iteration method (Maximum Likelihood—Expectation Maximization Algorithm) and the like.

By solving the equations, the distribution of the activity of radionuclides in the waste barrel along the direction of barrel diameter and the direction of height can be obtained. Sum to get the total activity of the entire waste bin

Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art may make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention.

Claims (9)

1. the half tomographic gamma scan method that a kind of middle cool waste bucket measures, scanning means includes used by this method Turntable, detector platform, the detector with collimator, transmission source platform, transmission source and its shield member, analysis module, It is characterized in that, the radwaste bucket is at the uniform velocity rotated in scanning survey, radionuclide opposite bucket after rotation in bucket External detector is equivalent to annular line source by point source, and pail for used dressings is divided to several sections of layers in an axial direction, and each section of layer inner stuffing matter is uniform Distribution, then several cyclic annular grids will be divided in each section of layer, it is measured by several eccentric positions of the detector in each section of layer, Reconstructed calculating obtains radionuclide specific activity in each cyclic annular grid and is realized to pail for used dressings along the radial and axial distribution of pail for used dressings Radioactivity fast and accurately measures；

Specifically include following steps：

The specific location of the first step, adjustment detector and its collimator, transmission source and its shield member makes detector axis be aligned Pail for used dressings center simultaneously passes through transmission source collimating aperture；

Second step, the scanning for carrying out current layer

Pail for used dressings is at the uniform velocity rotated with certain rotating speed, acquires gamma-spectrometric data；Make pail for used dressings center to detector axis distance successively For several scheduled distances, several gamma-spectrometric datas of this section of layer are acquired in total；

Third step, by detector platform and transmission source platform since the pail for used dressings bottom, in vertical direction mobile transmission simultaneously Source platform and detector platform, are successively scanned, and every layer of scanning all repeats second step；

4th step, data processing

The transmission measurement for carrying out each section of layer first obtains mean attenuation coefficient of this section of layer substance to ray, secondly by each section of layer The cyclic annular grid that homalographic is divided along radial direction, calculates detection efficient through substance correction for attenuation of each grid to detector, The side of detector count rate correlation when establishing each cyclic annular grid radionuclide specific activity of reflection in each detecting location Journey group solves equation group acquisition radionuclide specific activity and is obtained along the distribution in bucket diameter direction and short transverse, summation in pail for used dressings Obtain the total activity of radionuclide in pail for used dressings.

2. the half tomographic gamma scan method that middle cool waste bucket according to claim 1 measures, which is characterized in that In the second step, pail for used dressings is at the uniform velocity rotated with certain rotating speed, the specific measurement for measuring its gamma-spectrometric data of station acquisition to one Depending on time is with radioactive level.

3. the half tomographic gamma scan method that middle cool waste bucket according to claim 1 measures, which is characterized in that The data processing of 4th step includes that transmission reconstruction and transmitting are rebuild, and wherein transmission reconstruction is to obtain each section of layer of pail for used dressings The calculating analysis that filler carries out attenuation coefficient or the density of ray, transmitting, which is rebuild, is radiated to obtain in pail for used dressings Property nucleic activity distribution and carry out calculating analysis.

4. the half tomographic gamma scan method that middle cool waste bucket according to claim 3 measures, which is characterized in that The specific method of the transmission reconstruction is：

Think that substance is uniformly distributed in each layer, if P_iIt is measured equal to detector when i-th section of layer position eccentric position is zero Transmissivity：P_i=C_i/C_max, wherein：I=1,2 ..., I, C_iDetector is surveyed i-th section of layer position in the presence of indicating pail for used dressings The gamma photons full energy peak counting rate obtained；C_maxDetector measures when the gamma-rays that expression transmission source is sent out is not decayed by sample The relationship of γ photon full energy peak counting rates, transmissivity and attenuation coefficient is：μ_iD=-ln (P_i), wherein μ_iFor object in i-th section of layer The attenuation coefficient of confrontation ray, D are pail for used dressings diameter.

5. the half tomographic gamma scan method that middle cool waste bucket according to claim 3 measures, which is characterized in that It is described transmitting rebuild specific method be：

Decaying will be generated with matter interaction in bucket before reaching detector due to radionuclide is sent out in bucket ray, because This must carry out correction for attenuation, when detector face i-th section of layer, j-th of eccentric position, in n-th of cyclic annular grid of m layers Radionuclide sends out ray：C_mn,ij=α ε_mn,ij·χ_mn,ij·A_mn, wherein α is is examined The branching ratio of the ray energy of worry, ε_mn,ijIt is radionuclide in the cyclic annular grid of n-th of m layers to being in i-th section of layer j-th The detection efficient of detector when eccentric position；A_mnFor radionuclide specific activity in n-th of cyclic annular grid of m layers；Correction for attenuation system Number χ_mn,ij=exp (- ∑ ul), μ and l are each layer attenuation coefficient and average diameter that n-th of cyclic annular grid of m layers is undergone to detector Mark length considers that detector can measure the ray that all grids are sent out in entire pail for used dressings, partially in j-th of i-th section of layer The counting rate that detector when heart position measures is：In conjunction with the detection of all positions, group It is I × N at unknown number number, equation number is the system of linear equations of I × J：

Wherein, P=I × J, Q=I × N, e=α ε χ, I, J are integer, and M indicates that the section number of plies of pail for used dressings division, N indicate every Cyclic annular grid number, q in a section of layer indicate that the label of cyclic annular grid, Aq indicate the activity of radionuclide, J in q-th of grid The eccentric position number for indicating detector, solves the system of linear equations, obtains the activity of radionuclide in pail for used dressings along bucket diameter The distribution in direction and short transverse, summation obtain the total activity of entire pail for used dressings

6. the half tomographic gamma scan method that middle cool waste bucket according to claim 5 measures, which is characterized in that Pail for used dressings segments I is 9, and eccentric position number J value ranges are 2 to 8, and the cyclic annular grid number N of each section of layer is not more than J.

7. the half tomographic gamma scan method that middle cool waste bucket according to claim 6 measures, which is characterized in that The cyclic annular grid number N of each section of layer is identical as eccentric position number J.

8. the half tomographic gamma scan method that middle cool waste bucket according to claim 5 measures, which is characterized in that Positional number, that is, equation number of measurement, no less than unknown number number are the grid number divided, i.e. P >=Q.

9. the half tomographic gamma scan method that middle cool waste bucket according to claim 5 measures, which is characterized in that The method for solving the system of linear equations is the alternative manner based on probability statistics.