Methods and equipment for investigating the distribution of non-sedimentary radionuclides

Abstract

The present invention discloses a method and equipment for investigating the distribution of non-sedimentary radioactive nuclides, including a sand density measuring device and a layer-by-layer peeling measurement model. The sand density measuring device is composed of a rectangular bracket, a slide plate, a detector, a mixed point source, a sleeve, a drill rod and a handle. The layer-by-layer peeling measurement model includes a detector, air, a sand attenuation layer, a virtual surface source and a volume source. The new technology and method for determining the distribution depth by using a double peak ratio is studied, which greatly improves the pertinence and accuracy of the measurement. The present invention designs and manufactures a special density measuring device on site, which can quickly and accurately give the density parameters of the measured object on site, eliminating the sampling and measurement process, greatly reducing financial expenses, and reducing workload and time. The present invention is mainly for 239 Pu and 241 The innovation of Am nuclide is therefore universal for other situations where multiple nuclides coexist in proportion and emit multiple energy rays, and its application range is very wide.

Description

Technical Field

[0001] The present invention relates to a detection technology for the distribution of non-deposited radioactive nuclides in the application field of radiation protection (ray detection) technology, and in particular to a detection and investigation technical method and procedure as well as detection equipment for contaminated areas where the measurement objects are not distributed according to a regular pattern. Background Art

[0002] In the earth's environment, with the development of global nuclear energy, radioactive nuclides are released from nuclear reactors into the atmosphere, diffuse with the atmosphere, and continuously settle and accumulate on the surface of the environment, and their concentration becomes higher and higher. This sedimentary nuclide will also migrate and diffuse below the surface over time. Sedimentary radioactive nuclides generally have a definite distribution pattern, and there are mature investigation techniques and methods. However, there is currently little research on the distribution of non-sedimentary radioactive nuclides, such as waste barrels, bulldozers, landfills, or sedimentary contaminated sites that have been damaged or disturbed. This invention is mainly aimed at this

[0003] For radioactive contaminated areas that are artificially bulldozed or mechanically collected and sealed, the present invention proposes to first conduct on-site sand density measurement, then conduct layer-by-layer measurement and stripping of the covering layer (attenuation layer), then use the α surface contamination meter to determine the upper boundary of the contaminated area, and finally conduct pollution source cleaning and accompanying measurement research (double peak ratio method).

[0004] The main measurement objects are 241 Am and 239 Pu two nuclides, therefore, the present invention focuses on the research of these two nuclides. Summary of the invention

[0005] The object of the present invention is to provide a method and device for investigating the distribution of non-deposited radionuclides to solve the problems raised in the above-mentioned background technology.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A method and equipment for investigating the distribution of non-sedimentary radionuclides, comprising a sand density measuring device and a layer-by-layer peeling measurement model, wherein the sand density measuring device is composed of a rectangular bracket, a slide plate, a detector, a mixed point source, a sleeve, a drill rod and a handle, and the layer-by-layer peeling measurement model includes a detector, air, a sand attenuation layer, a virtual surface source and a volume source, the depth range of the measurement object is within 10.0 cm, and the detector of the on-site gamma spectrometer is 36 cm from the ground; as a further solution of the present invention: the measuring bracket of the sand density measuring device is made of stainless steel, the upper bracket is designed as a slide rail, which can carry a slide plate, and there are scale lines engraved on it, so as to facilitate reading the position of the slide plate; a sleeve is welded at the center of one side of the lower bracket, and the sleeve is oriented obliquely upward and forms a certain angle with the ground; the slide plate is made of stainless steel, and the detector is suspended at the center of the slide plate; the detector is a gamma The drill rod is made of stainless steel, with a sharp drill head. The mixed point source is embedded at a certain distance from the drill head. The drill rod is inserted into the sand through a sleeve. There are scale marks on the drill rod to facilitate reading the insertion depth. There is a handle on the end of the drill rod to facilitate inserting into the sand. The nuclides contained in the mixed point source are 241 Am, 137 Cs, 60 Co, the mixed point source is wrapped by polyethylene material and embedded in the drill pipe; the lower part of Figure 1 is an enlarged image of the drill bit, and its positioning is calibrated by the scale line of the drill pipe, which requires the position to be in a straight line with the detector up and down; the direction of the mixed point source and the red dot on the handle are consistent, both facing upward, so as to ensure that the rays emitted by the mixed point source after being inserted into the sand can be detected by the detector.

[0008] As a further solution of the present invention: the top layer of the measurement model is a sand attenuation layer, and the lower layer is a radioactive body source, and the radioactive body source includes 239 Pu and 241 The measurement of radioactive body source of Am can be converted into the measurement of virtual surface source. The virtual surface source is a virtual surface source of the body source. The virtual surface source is in a fixed position inside the body source, not on the surface of the body source, and has a certain distance from the surface of the body source. The body source part above the virtual surface source is generally assumed to be a sand attenuation layer.

[0009] As a further solution of the present invention: the area of ​​the sand attenuation layer is large and it is difficult to make in terms of technology. The solution is to divide the large sand attenuation layer into small pieces, that is, to splice about 300 rectangular sand samples into a large area of ​​sand attenuation layer. The sample box is made of PP plastic, which is light and has a density of only 0.91g / cm 3 , use chemically pure sand according to the above sand composition to make the sand attenuation layer.

[0010] A method for investigating the distribution of non-sedimentary radionuclides, the steps of the investigation method are as follows:

[0011] Step 1: Measurement of sand density;

[0012] In on-site measurement, we must first know the density parameters of the measured object, the accuracy of which directly affects the accuracy of the final measurement results. For the nuclide distribution depth thickness of about 2.0 cm, this survey technology is established 241 The relationship between the ratio of the double peaks of Am (59.54keV and 26.4keV) and the density of sand is: Y=2.06+2.02x (where Y is the ratio of the double peaks and x is the density of sand), field measurement 241 The bimodal ratio of Am combined with this relationship can give the density of sand. If the nuclide distribution depth is unknown, the sand density can be actively measured using non-sedimentary radionuclide distribution survey equipment (equipment specifically for measuring density) to establish the relationship between the bimodal ratio and the sand density;

[0013] Step 2: Stripping of sand attenuation layer;

[0014] The sand layering experiment was used to determine the functional relationship between the surface source detection efficiency and the attenuation layer height, Ф(h), and the fitting curve. If the volume source activity concentration reaches the α waste standard of 4×10 6 Bq / kg (generally the surface source thickness is 0.10cm and the density is set to 1.43g / cm 3 , then the standard value of surface activity is 4×10 6 / 0.1 / 1.43=2.8×10 7 Bq / cm 2 ), then according to this activity concentration combined with the experimental determination of the surface source efficiency Ф(h) data, calculate the peak count rate fitting curve under the α waste standard under different attenuation layers. If the waste level is one order of magnitude higher than the α waste standard, it can be treated as one order of magnitude higher activity concentration; if the measured peak count rate is higher than the peak count rate under the α waste standard and is at the middle value of the fitting curve, then the attenuation layer corresponding to the count rate in the figure can be peeled off, and the measurement can be continued after peeling off; if the measured peak count rate is close to the peak count rate under the α waste standard and is at a lower value of the fitting curve, the thicker sand attenuation layer needs to be peeled off. At this time, for the sake of security, the thinner attenuation layer is peeled off first, and the measurement and judgment are continued after peeling off; when the measured peak count rate is close to the peak count rate under the α waste standard and is at a higher value of the fitting curve, the thinner sand attenuation layer needs to be peeled off. Whether this thin layer is the assumed sand attenuation layer of the upper body source of the virtual surface source (there is a large uncertainty in the determination of the interface between the actual sand attenuation layer and the upper surface of the contaminated area) requires the selection of an α particle detector for surface measurement and judgment;

[0015] Step 3: Technology for determining the boundary of the upper surface of the contaminated area;

[0016] How to define the interface between the covering layer or attenuation layer and the upper surface of the contaminated area is a key issue. After cleaning most of the covering layer, special instruments and equipment are needed to measure the surface to determine whether the interface has been cleaned. Therefore, it is necessary to clean one layer and measure and judge once. This surface measurement and judgment can be selected using an α particle detector. The scheme of the present invention uses two detection technologies: one is to use an α particle surface contamination instrument to measure, fix the probe of the contamination instrument on a special test stand, and measure and judge 5.0mm away from the body source surface; the other is a long-distance α contamination detection technology;

[0017] Step 4: Determine the thickness of the contaminated sand layer using the bimodal ratio method;

[0018] After the upper sand attenuation layer is cleaned, the contaminated sand layer (also called body source) should be cleaned. Therefore, the thickness of the contaminated sand layer needs to be known. The present invention proposes to use the radionuclide double peak ratio method to determine the thickness of the contaminated sand and establish a functional relationship between the double peak ratio and the thickness of the contaminated sand. For the case where the contaminated sand is thin, it is generally chosen to use 241 The double peak ratio of 59.54keV and 26.4keV Am rays; for thicker contaminated sand, it is generally used 239 Pu's 129keV and 241 Am 59.54keV ( 239 Pu is generally 241 Am is the ratio of the double peaks of the radiation (which exist in proportion). As the thickness of the source increases, the double peak ratio changes monotonically. The thickness of the contaminated area can be determined based on the measured double peak ratio combined with the established function curve, and then the contamination can be removed.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. The present invention studies the technology of non-deposited radionuclide distribution survey, establishes the method steps and procedures for survey and cleaning, and studies the new technology and method of determining the distribution depth by double peak ratio, which greatly improves the pertinence and accuracy of the measurement;

[0021] 2. The present invention designs and manufactures a special density measuring device on site, which can quickly and accurately give the density parameters of the measured object on site, eliminating the sampling and measurement process, greatly reducing expenses, workload and time;

[0022] 3. The present invention is mainly to 239 Pu and 241 The innovation of Am nuclides is universal to other situations where multiple nuclides coexist in proportion and emit multiple energy rays, and its application range is very wide. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Schematic diagram of the sand density measuring device.

[0024] Figure 2 This is a curve showing the change of the bimodal ratio with the density of sand.

[0025] Figure 3 This is a schematic diagram of the survey on the distribution of non-sedimentary radionuclides.

[0026] Figure 4 This is the area source measurement map.

[0027] Figure 5 The Ф(h) function of sand sample is shown in the figure below. The experimental result on the left and the theoretical calculation result on the right (density 1.36 g / cm 3 )picture.

[0028] Figure 6 is the theoretical calculation result of the Ф(h) function of sand sample (density 1.6 g / cm 3 )picture.

[0029] Figure 7 This is the simulation result diagram of the volume source double peak ratio.

[0030] As shown in the figure: a rectangular bracket 1, a slide plate 2, a detector 3, a mixed point source 4, a sleeve 5, a drill rod 6, a handle 7, a detector 8, air 9, a sand attenuation layer 10, a virtual surface source 11 and a volume source 12. DETAILED DESCRIPTION

[0031] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0032] Please refer to Figures 1 to 7. In the embodiment of the present invention, the method and equipment for investigating the distribution of non-sedimentary radionuclides include a sand density measuring device and a layer-by-layer peeling measurement model. The sand density measuring device is composed of a rectangular bracket 1, a slide plate 2, a detector 3, a mixed point source 4, a sleeve 5, a drill rod 6 and a handle 7. The layer-by-layer peeling measurement model includes a detector 8, air 9, a sand attenuation layer 10, a virtual surface source 11 and a volume source 12. The depth range of the measured object is within 10.0 cm, and the on-site γ spectrometer detector is 36 cm from the ground; the measuring bracket 1 of the sand density measuring device is made of stainless steel, and the upper bracket is designed as a slide rail, which can carry the slide plate 2, and there are scale lines engraved on it, so as to facilitate the reading of the position of the slide plate 2; a sleeve 5 is welded at the center of one side of the lower bracket, and the sleeve 5 is oriented obliquely upward and forms a certain angle with the ground; the slide plate 2 is made of stainless steel, and the detector 3 is suspended at the center of the slide plate; the detector 3 is a γ spectrometer, and the type is not required; the drill rod 6 Made of stainless steel, the drill rod head is sharp and embedded in the mixed point source 4 at a certain distance from the drill rod head. The drill rod 6 is inserted into the sand through the sleeve 5. The drill rod is marked with scale lines to facilitate reading the insertion depth. The handle 7 is connected to the end of the drill rod to facilitate inserting into the sand. The nuclides contained in the mixed point source 4 are 241 Am, 137 Cs, 60 Co, the mixed point source is wrapped by polyethylene material and embedded in the drill pipe; the lower part of Figure 1 is an enlarged image of the drill bit, and its positioning is calibrated by the scale line of the drill pipe, requiring the position to be in a straight line with the detector up and down; the orientation of the mixed point source 4 and the red dot at the handle 7 are consistent, both facing upward, so as to ensure that the rays emitted by the mixed point source 4 after being inserted into the sand can be detected by the detector.

[0033] The top layer of the measurement model is a sand attenuation layer 10, and the lower layer is a radioactive body source 12. The radioactive body source 12 includes 239 Pu and 241 The measurement of the radioactive body source of Am can be converted into the measurement of the virtual surface source 11. The virtual surface source 11 is the virtual surface source of the body source 12. The virtual surface source 11 is fixed in the body source 12, not on the surface of the body source 12, and has a certain distance from the surface of the body source 12. The body source part above the virtual surface source is generally assumed to be a sand attenuation layer 10; the area of ​​the sand attenuation layer 10 is large, and it is difficult to make in terms of technology. The solution is to divide the large sand attenuation layer 10 into small pieces, that is, to use about 300 rectangular sand samples to splice into a large area of ​​sand attenuation layer 10. The sample box is made of PP plastic, which is light and has a density of only 0.91g / cm 3 The sand attenuation layer 10 is made of chemically pure sand according to the above sand composition.

[0034] The steps of the non-sedimentary radionuclide distribution survey method are as follows:

[0035] Step 1: On-site measurement of sand density;

[0036] In on-site measurement, the density of the sand on site is a very important parameter. The accuracy of the density directly affects the accuracy of the final measurement result. Therefore, the present invention conducts in-depth research on the density parameter. 241 The ratio of the double peaks (59.54keV and 26.4keV) of Am was studied, and the Monte Carlo program was used for simulation calculation. Monte Carlo program simulation calculation parameters: the detector is an on-site 335A detector (crystal diameter 5.45cm, crystal length 5.71cm, cold finger diameter 1.02cm, cold finger length 4.89cm, magnesium window 1.5mm, magnesium window distance crystal 3.0mm), the detector is 20.0cm from the ground, the thickness of the ground sand layer is set to 2.0cm, the cylindrical radius is 10.0cm, the sand composition is shown in Table 1, and the sand density ranges from 1.1 to 2.1g / cm 3 , assuming that the nuclides are uniformly distributed, the simulation results are shown in Figure 2; From Figure 2, we can see that as the density of sand increases, 241 The ratio of the double peaks (59.54keV and 26.4keV) of Am also increases, and the data is linear, satisfying the linear function relationship. The prerequisite for determining the density of sand by the double peak ratio method is that the ratio must change significantly with the density of sand, that is, the slope of the linear function must be large, only in this way can the density of sand be accurately determined. When measuring, as long as the double peak ratio is known, the corresponding sand density can be found through the curve.

[0037] The actual situation on site is that the nuclide distribution depth is not a constant value but an unknown variable, so the conclusion of Figure 2 cannot be directly applied. Therefore, it is necessary to use a mixed point source fixed position to actively measure the double peak ratio on site, and the measurement device is shown in Figure 1;

[0038] Step 2: Peel off and measure the covering layer (attenuation layer) layer by layer;

[0039] After the radioactive nuclides are artificially bulldozed or mechanically collected and sealed, their measurement model can be simplified as follows: Figure 3 The model shown in the figure. The top layer is the sand attenuation layer, and the lower layer is the radioactive body source. The radioactive body source can be converted into a virtual surface source for measurement. The virtual surface source is fixed in the body source, not on the surface of the body source, but at a certain distance from the surface of the body source.

[0040] The purpose of the stratification experiment is to determine the functional relationship Ф(h) between the peak detection efficiency of the virtual surface source and the height of the sand attenuation layer (in order to simplify the model, the upper body source part of the virtual surface source is first classified as the sand attenuation layer). First, sand samples were collected from the Gobi Desert, and the sand composition was verified by an external collaborator. The mass percentage verification results are shown in Table 1. Secondly, a sand attenuation layer was made. It is difficult to make a large area of ​​sand attenuation layer in terms of technology. Therefore, it was chosen to divide the large attenuation layer into small pieces, that is, to use about 300 rectangular sand samples to splice into a large area of ​​attenuation layer on a regular hexagonal 241Am large surface source with a side length of 0.5m in the laboratory. The sample box is made of PP plastic, which is light and has a density of only 0.91g / cm 3 (smaller than water). Use chemically pure sand to make the attenuation layer according to the above sand composition, with a density of 1.36g / cm 3 Finally, the measurement and data results are as follows Figure 4 , 5 and 6 as shown.

[0041] Figures 5 and 6 above show that the functional relationship between the detection efficiency of the surface source and the height of the attenuation layer is determined by the layered experiment. The deviation between the theory and the experiment is within 18%, and the method is feasible. If the body source activity concentration reaches the α waste standard of 4×10 6 Bq / kg (converted to surface activity value: the source thickness is 0.10cm, the source density is 1.43g / cm 3 , then the standard value of surface activity is 4×106 / 0.1 / 1.43=2.8×10 7 Bq / cm 2 ), then according to this activity concentration combined with the efficiency data of Figure 5 or 6, calculate the peak count rate under the α waste standard of different attenuation layers. If the measured count rate is higher than the peak count rate under the α waste standard and is at an intermediate level, then the attenuation layer corresponding to the count rate in the figure can be peeled off, and the measurement can be continued after peeling. If the waste level is one order of magnitude higher than the α waste standard, it can be processed according to the activity concentration of one order of magnitude higher. If the measured count rate is close to the peak count rate under the α waste standard and is at a lower level, as shown in the figure, it is necessary to peel off the thicker sand attenuation layer. At this time, for the sake of guarantee, the thinner attenuation layer is peeled off first, and then the measurement and judgment are continued after peeling. Since the upper body source of the virtual surface source is classified as the sand attenuation layer, there is a large uncertainty in the determination of the interface between the attenuation layer and the upper surface of the contaminated area. When the measured count rate is close to the peak count rate under the α waste standard and is at a higher level, as shown in the figure, it is necessary to peel off the thinner sand attenuation layer. At this time, for the sake of guarantee, the surface of the instrument measuring α particles is selected to measure and judge whether it reaches the upper surface of the contaminated area.

[0042] Step 3: Determine the upper surface boundary of the contaminated area;

[0043] How to define the interface between the covering layer or attenuation layer and the upper surface of the contaminated area is a key issue. After cleaning most of the covering layer, special instruments and equipment are needed to measure and determine whether the interface has been cleaned. Therefore, it is necessary to clean one layer and measure and determine it once. This surface measurement and judgment can choose an instrument that measures α particles. The scheme of the present invention uses two detection technologies, one is to use an α particle surface contamination instrument for measurement, and the other is a long-distance α contamination detection technology.

[0044] The surface contamination meter can obtain the radioactivity intensity of α particles on the sand surface. The range of α particles in the air is: R0=0.381E 3 / 2 , R0: Range of α particles in 101324.72Pa, 15°C air (cm); E: Energy of α particles (MeV). Fix the probe of the contamination meter on a special test stand, 5.0mm away from the surface of the body source. Range of α particles in other substances Rm

[0045] It can be calculated using the Bragg-Kleemm empirical formula: Where, ρ—medium density (g / cm 3 ), Am is the mass number of the medium nucleus. After stripping off a layer of sand, test and record it, and judge whether the stripping reaches the upper surface boundary of the body source based on the measured data.

[0046] Long Range Alpha Detection (LRAD) is a method of indirectly measuring alpha pollution activity by using an ionization chamber detector to collect ion pairs produced by alpha particles ionizing air molecules and converting them into measurable current signals. Normally, atoms or molecules in the air are not charged. When alpha particles collide with atoms or molecules in the air, energy is gradually lost, causing the air molecules to be ionized or excited. The originally neutral air molecules will become an electron and a positive ion with a positive charge, that is, many positive and negative ion pairs are generated. The ionization effect ends when the energy of the alpha particle is gradually consumed to a level lower than the ionization potential of the gas molecules. The total number of ion pairs that an alpha particle can produce is N=E / W, where E is the incident radiation energy and W is the average ionization work. Theoretical calculations show that a general alpha particle can produce 10 5ion pairs. It takes a long time for the generated ion pairs to recombine in the air. Here, the air drive device drives the air flow through the fan, and the ion pairs enter the current ionization chamber through the air guide tube. The positive and negative ion pairs move under the action of the electric field of the ionization chamber, generating weak current signals. The weak ionization current coming out of the ionization chamber is converted into an effective digital signal by the signal acquisition unit, and finally input into the data processing and display unit, and the results are displayed after processing. LRAD technology overcomes the shortcomings of short range and weak penetration of α particles, and has higher detection sensitivity than direct detection of α particle radiation. LRAD technology can be used for the detection of α radiation on the soil surface, contaminated nuclear waste and natural radioactivity. At present, low power and portability have been achieved, and industrial products have been realized.

[0047] Step 4: Bimodal ratio method;

[0048] When the upper sand attenuation layer is cleaned, the contaminated sand layer (also called body source) should be cleaned. Therefore, the thickness of the contaminated sand layer needs to be known. The present invention proposes to use the radionuclide double peak ratio method to determine the thickness of the contaminated sand.

[0049] The present invention uses Monte Carlo simulation to establish the functional relationship between the double peak ratio and the peeling thickness. For thinner contaminated sand, the energy peak ratio of 26.4keV and 59.54keV rays is generally used. For thicker contaminated sand, 239 Pu's 129keV and 241 Am 59.54keV ( 239 Pu is generally 241 Am (existing in proportion) ray energy peak ratio. MCNP simulation calculation has a polyethylene attenuation layer thickness of 0.45cm and a probe distance of 5.0cm from the source. The sand composition is set to M8 8016-31.37 14028 -18.4 13000 -3.3 26056 -1.33 20040 -4.83 12024 -0.8419039 -1.28 11023 -13.926012 -2.33 17000 -22.4, the sand density is set to 1.43 g / cm3, and the nuclides are assumed to be uniformly distributed. The ray energy and sampling probability are si1 L 0.0264 0.05954 0.129, sp1 D 0.05240.753 0.00195.

[0050] The MCNP simulation calculation uses the 335A HPGe detector model, and the calculation results are shown in Table 2 and Figure 7: From Table 2 and Figure 7, we can draw the following conclusions: As the thickness of the source increases, the double peak ratio changes monotonically.

[0051] The thickness of the contaminated area can be determined based on the measured double peak ratio, and then the contamination can be cleaned and peeled off.

[0052] Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments, or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for investigating the distribution of non-deposited radionuclides, characterized in that: The non-sedimentary radionuclide distribution survey method adopts a non-sedimentary radionuclide distribution survey device comprising a sand density measuring device and a layer-by-layer peeling measurement model. The sand density measuring device comprises a rectangular bracket (1), a slide plate (2), a first detector (3), a mixed point source (4), a sleeve (5), a drill rod (6) and a handle (7). The layer-by-layer peeling measurement model comprises a second detector (8), air (9), a sand attenuation layer (10), a virtual surface source (11) and a body source (12). The depth range of the measurement object is within 10.0 cm, and the detector of the on-site gamma spectrometer is 36 cm away from the ground. The non-sedimentary radionuclide distribution survey method has the following steps: Step 1: Measurement of sand density; In the on-site measurement, we must first know the density parameters of the measured object. The accuracy of the parameters directly affects the accuracy of the final measurement results. For the nuclide distribution depth thickness of 2.0 cm, this survey technology is established. 241 The relationship between the bimodal ratio of Am and the density of sand is: Y=2.06+2.02x, where Y is the bimodal ratio and x is the density of sand. 241 The bimodal ratio of Am combined with this relationship gives the density of sand. If the nuclide distribution depth is unknown, the non-sedimentary radionuclide distribution survey equipment is used to establish the relationship between the bimodal ratio and the density of sand. Step 2: Stripping of sand attenuation layer; The sand layering experiment was used to determine the functional relationship between the surface source detection efficiency and the height of the sand attenuation layer Ф(h) and the fitting curve. If the volume source activity concentration reaches the α waste standard of 4×10 6 Bq / kg, then according to this activity concentration combined with the experimental determination of the surface source efficiency Ф(h) data, calculate the peak count rate fitting curve under the α waste standard under different attenuation layers. If the waste level is one order of magnitude higher than the α waste standard, it is treated as one order of magnitude higher activity concentration; if the measured peak count rate is higher than the peak count rate under the α waste standard and is at the middle value of the fitting curve, then the attenuation layer corresponding to the count rate in the figure can be peeled off, and the measurement can be continued after peeling off; if the measured peak count rate is close to the peak count rate under the α waste standard and is at a lower value of the fitting curve, it is necessary to peel off the thicker sand attenuation layer, first peel off the thinner attenuation layer, and then continue to measure and judge after peeling off; when the measured peak count rate is close to the peak count rate under the α waste standard and is at a higher value of the fitting curve, it is necessary to peel off the thinner sand attenuation layer. Whether this thin layer is the sand attenuation layer assumed by the upper body source of the virtual surface source requires the selection of an α particle detector for surface measurement and judgment; Step 3: Technology for determining the boundary of the upper surface of the contaminated area; How to define the interface between the covering layer or attenuation layer and the upper surface of the contaminated area is a key issue. After cleaning most of the covering layer, special instruments and equipment are needed to measure the surface to determine whether the interface has been cleaned. It is necessary to clean one layer and measure and judge once. This kind of surface measurement and judgment selects α particle detector, uses α particle surface contamination instrument to measure, fixes the probe of the contamination instrument on a special test stand, and measures and judges 5.0mm away from the body source surface, or long-distance α contamination detection technology; Step 4: Determine the thickness of the contaminated sand layer using the bimodal ratio method; After the upper sand attenuation layer is cleaned, the contaminated sand layer is cleaned, and the radionuclide double peak ratio method is used to determine the thickness of the contaminated sand. A functional relationship between the double peak ratio and the thickness of the contaminated sand layer is established. For the case where the contaminated sand is thin, the 241 The double peak ratio of 59.54keV and 26.4keV rays of Am; for thicker contaminated sand, use 239 Pu 129keV and 241 The double peak ratio of Am's 59.54keV rays changes monotonically with the increase of the source thickness. The thickness of the contaminated area can be determined based on the measured double peak ratio and the established function curve, and then the contamination can be cleaned and peeled off.

2. The method for investigating the distribution of non-deposited radionuclides according to claim 1, characterized in that: The rectangular bracket (1) of the sand density measuring device is made of stainless steel, the upper bracket is designed as a slide rail, equipped with a slide plate (2), which is engraved with scale lines to facilitate reading the position of the slide plate (2); a sleeve (5) is welded at the center of one side of the lower bracket, and the sleeve (5) is oriented obliquely upward and forms a certain angle with the ground; the slide plate (2) is made of stainless steel, and the first detector (3) is suspended at the center of the slide plate; the first detector (3) is a gamma spectrometer, and the type is not required; the drill rod (6) is made of stainless steel, and the drill rod head is designed to be sharp, and the mixed point source (4) is embedded at a certain distance from the drill rod head, and the drill rod (6) is inserted into the sand through the sleeve (5); the drill rod is marked with scale lines to facilitate reading the insertion depth; the upper end of the drill rod is connected to a handle (7) to facilitate inserting into the sand with force; the nuclides contained in the mixed point source (4) are 241 Am, 137 Cs, 60 Co, the hybrid point source is wrapped with polyethylene material and embedded in the drill pipe.

3. The method for investigating the distribution of non-deposited radionuclides according to claim 1, characterized in that: The top layer of the measurement model is a sand attenuation layer (10), and the lower layer is a radioactive body source (12). The radioactive body source (12) includes 239 Pu and 241 The measurement of the radioactive body source of Am is converted into the measurement of the virtual surface source (11). The virtual surface source (11) is a virtual surface source of the body source (12). The virtual surface source (11) is at a fixed position inside the body source (12), is not on the surface of the body source (12), and is at a certain distance from the surface of the body source (12). The body source part above the virtual surface source is assumed to be a sand attenuation layer (10).

4. The method for investigating the distribution of non-deposited radionuclides according to claim 1, characterized in that: The large sand attenuation layer (10) is divided into small pieces, that is, 300 rectangular sand samples are spliced ​​into a large-area sand attenuation layer (10). The sample box is made of PP plastic with a density of only 0.91 g / cm 3 The sand attenuation layer (10) is made of chemically pure sand according to the above sand composition.

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