RU2258202C2 - NON-DESTRUCTIVE METHOD OF INSPECTION OF LATERAL CHARACTERISTICS OF CELLULAR STRUCTURE HAVING α-RADIOACTIVE LAYER MATERIAL LAYER - Google Patents

Abstract

FIELD: inspection of dynamics of changes in cellular structures.

SUBSTANCE: method concludes angular collimation of α-radiation by means of Soller collimator, registration of energy spectrum of collimated flux of particles, determination of lateral structures from the shape of registered spectrum on the base of its mathematical model.

EFFECT: improved precision; improved speed of measurement.

3 dwg

Description

The invention relates to measuring equipment, in particular to measuring the technological transverse parameters of a layered micron structure (structure thickness of the order of several microns), containing alternating layers of passive (non-radioactive) and active (alpha-radioactive) material (local thicknesses, depth distribution of alpha-radioactive material ) The invention can be used to study the dynamics of changes in these structures when exposed to various internal and external factors, for example, in the interests of nuclear-pumped lasers.

The following methods are known for determining the technological parameters of thin layers: Auger electron spectroscopy (EOS), X-ray diffractometry, and metallography.

- X-ray diffractometry [3] refers to non-destructive methods for controlling a layered structure, however, it is mainly used to study structures with layer thicknesses exceeding 10 microns.

Methods for measuring layered micron structures are the EOS and metallographic methods, but they have several disadvantages.

- The EOS method [1, 2] refers to destructive methods. To obtain data on the composition of the layers (the concentration profile of elements by depth) in the EOS method, thin (hundredths of microns) layers of material are sequentially removed from the surface of the sample by sputtering using a beam of high-energy ions. As the material is removed, an elemental analysis of the new layer that has come to the surface is performed. The duration of an individual measurement by the EOS method can be up to several days.

- In a metallographic study of the structure [2], the initial sample is also destroyed. In this way, it is difficult to obtain reliable information about the transition zones between the individual layers of the structure. In addition, due to the need for separate chemical etching of the materials of the layers and the substrate, the method is characterized by extremely low efficiency (a separate measurement can last about a month).

Thus, a common drawback for the EOS method and metallography is the destruction of the test sample. In addition, both methods are quite laborious and do not differ in efficiency.

The authors are not aware of a non-destructive operational method for monitoring thin-layer micron structures, including layers of alpha-radioactive material.

There are a number of tasks (in particular, the development of efficient and radiation-resistant energy-generating elements used in nuclear-pumped lasers), the solution of which requires operational monitoring of changes in the characteristics of thin-layer structures (containing one or more alpha-radioactive layers). In this case, it is important to maintain the structure of the object during its study:

- for the purpose of its multiple use;

- to control the facility at any stage of use;

- to study the dynamics of changes in structure parameters from influencing factors.

The technical result consists of:

1. In ensuring high-quality non-destructive testing of a thin-layer structure with one or more alpha-radioactive layers due to high spatial resolution (hundredths of a micron), which allows one to study fairly fine effects that slightly change the initial structure, for example, spraying a surface with its own fission fragments.

2. In the efficiency of measurements (the duration of an individual measurement may take only a few minutes), allowing research on a large batch of samples.

To achieve this technical result, when determining the transverse characteristics of a layered structure (layer thickness, thickness of transition zones of layers, distribution of the alpha-radioactive component along the depth of the structure), it is necessary to ensure:

- angular collimation of the flux of alpha particles (natural alpha radiation of a radioactive material) from the surface of the structure;

- obtaining the energy spectrum of natural collimated alpha radiation emerging from the surface of the sample by recording it on a spectrometer with high energy resolution;

- analysis of the shape of the received (registered) energy spectrum of natural collimated alpha radiation emerging from the surface of the sample, in order to determine the technological parameters of the structure containing the alpha radioactive layer;

- the use of a collimator of the Soller type as a collimating device, which allows for a given degree of collimation to drastically reduce the measurement duration compared to other types of collimators, due to the significant transparency of this collimator.

The use of natural radiation of radioactive materials as measured by detecting alpha particles emerging from the surface of the structure is quite obvious from the point of view of obtaining reliable information about the state of the structure containing the radioactive layers.

Indeed, radiation, before reaching the surface, travels a certain distance in the material and changes its energy spectrum. And it is alpha particles, in view of their extremely small mileage in materials (several microns), that are most indicative when studying the properties of micron alpha-radioactive materials. For uranium, for example, linear alpha-particle energy loss (LET) is about 0.5 MeV / μm [3]. Obviously, the energy spectrum of alpha particles after passing through the micron layer carries fairly complete information about the transverse structure of this layer.

Based on the mathematical model constructed by the authors, the energy spectrum of alpha particles recorded upon exiting the layer (with some simplifications) has the form

where n _α is the concentration of alpha particles generated in the active layer (at a depth corresponding to the distance traveled "L") per unit time (it is obvious that this concentration is proportional to the concentration of the nuclei of a radioactive element, such as uranium, at a given layer depth)

S is the area of the active layer;

θ _min (L) and θ _max (L) - the minimum and maximum angles of departure at which the alpha particle passes the path "L" (ie, takes off with energy "E (L)");

dE (L) / dL is a function that reflects the linear energy loss of an alpha particle for a particular material.

As can be seen, the concentration “n _α ”, determined from relation (1), is averaged over a certain thickness of the layer “ΔX '” (X is the coordinate axis directed from the surface of the structure to its depth), which, as shown below, determines the spatial resolution the proposed measurement method. Naturally, the information obtained during the measurement, the better, the less this thickness. Technically, a decrease in the ΔX 'value is achieved by the collimation of the detected radiation (ie, by decreasing the maximum emission angle "θ _max (L)" of the detected alpha particles relative to the normal to the surface of the structure under study). For a given energy “E” with which an alpha particle flies from the surface, the minimum angle “θ _min (L (E))” is uniquely determined by the thicknesses of the active layer “d ₁ ” and the protective film “d ₂ ” (for a structure, for example, of three layers - the active layer 1, the protective film 2, the substrate 3) (figure 1). The maximum angle "θ _max (L (E))" can be reduced by using a collimator that limits the aperture of registration of alpha particles to the angle "θ '".

The use of a collimator of the Soller type (which, in this case, is a plate with a thickness of "h" with several hundreds of through holes with a diameter of "d") (Fig. 2), known in optics as a limiter of the aperture of optical radiation, allows us to implement the mathematical model proposed by the authors ( 1) studies of the structure of alpha-radioactive layers due to the "small-angle" energy selection of alpha particles while maintaining a high intensity of their registration (due to the presence of numerous through channels collimator).

(т.е. зависимость количества альфа-частиц, регистрируемых в единичном энергетическом интервале от их энергии) в данном случае имеет несколько характерных точек (фиг.3):According to model (1), the recorded energy spectrum of alpha particles

(i.e., the dependence of the number of alpha particles recorded in a unit energy interval on their energy) in this case has several characteristic points (Fig. 3):

- E ₀ is the initial energy of an alpha particle with a range of R (for example, for U ²³⁴ E ₀ = 4.77 MeV, for U ²³⁵ E ₀ = 4.4 MeV [4]);

- E ₁ - the maximum energy of the detected alpha particles flying out from the depth x = d ₂ (the inner boundary of the protective film);

- E ₂ - the minimum energy of the detected alpha particles, emitted from a depth of x = d ₂ .

- E ₃ - the maximum energy of the recorded alpha particles flying out from the depth X = d ₁ + d ₂ (inner surface of the uranium layer);

- E ₄ - the minimum energy of the recorded alpha particles flying out from the depth X = d ₁ + d _2.

Thus, the spectrum has several distinct areas (see figure 3):

- “Leading front” - interval of energies E ₁ -E ₂ ;

- "Top" - the energy interval E ₂ -E ₃ ;

- "Back front" energy interval E ₃ -E ₄ .

The thickness “d ₂ ” of the passive layer is uniquely determined through the difference in the values of these energy points (assuming that dE / dL = const≈E ₀ / R)

The difference in values of E ₁ -E ₃ determines the thickness of the active layer

Point "E ₂ " has a purely "aperture" origin and appears under the condition

The shape of the spectrum in the area "E ₂ -E ₃ " gives an idea of the distribution of the alpha-radioactive component along the depth of the active layer. Formula (1) gives a one-to-one correspondence between the value of “dN _α / dE” at some point of the spectrum “E '” belonging to the portion of the spectrum “E ₂ -E ₃ ” and the average concentration of radioactive material in the region from X' to X '+ ΔX 'of the active layer, and it is easy to show that the thickness of this region is

From the foregoing, the importance of the angular collimation of the flux of alpha particles leaving the sample surface when registering their spectrum becomes apparent. Without the use of the Soller collimator, in principle, it is not possible to determine the average concentration of radioactive material by the thickness of the layer (there is no "E ₂ -E ₃ " section). On the other hand, the use of the Soller collimator makes it possible to determine the concentration distribution over the layer thickness with spatial resolution the better, the smaller the collimator aperture “θ '” (see formula (5)). Along with the energy resolution of the measuring path, the use of the Soller collimator determines the accuracy of thickness measurements of individual layers of the structure. At the same time, due to the transparency of the collimator, the efficiency of measurements is ensured. The method allows measurements in two main modes, determined by the choice of the collimation angle "θ '":

- Measurements with high spatial resolution (hundredths of a micron). Similar measurements are carried out when studying subtle effects (for example, spraying the surface of a layer with fission fragments). A single measurement in this mode requires a significant investment of time (several hours).

- Measurements with spatial resolution ~ tenths of a micron. This type of measurement is most often encountered in the study of structures (determining the thickness of the layers of the active and passive layers, the length of the transition zones at the boundary of the layers, etc.). Such measurements differ in efficiency (the duration of an individual measurement takes only a few minutes) and can be performed on a large batch of samples.

The list of figures and graphic images includes:

Figure 1 is a schematic illustration of a section of a layered structure;

Figure 2 - schematic representation of the collimator Soller;

Figure 3 is a characteristic view of the recorded energy spectrum of alpha particles.

As an example, consider the shape of the recorded alpha spectrum for a structure consisting of three layers (see FIG. 1): 1 - a uranium layer with a thickness of “d ₁ ”; 2 - a protective layer of aluminum with a thickness of "d ₂ " and 3 - a substrate.

In accordance with the model, the analysis of the spectral dependence (Fig. 3) allows us to determine the thickness of the active layer - d ₁ = 2 μm (see formula (3)) and the thickness of the protective film - d ₂ = 0.5 μm (see formula (2) )

Figure 2 shows a variant of the device of the collimator Soller, providing an aperture angle "θ '".

To test the proposed method, the thicknesses and concentration distributions of alpha-emitting isotopes were measured over the thickness of standard samples (~ 1 cm ^{2 area} ) with known technological characteristics containing various alpha-active layers: uranium-235, plutonium-238, plutonium-239, americium- 241, radium-226.

The film structure was measured on an alpha spectrometer consisting of a DKDPs semiconductor detector with a holder of an amplitude spectrum analyzer. Alpha particles emitted from the sample with an angular divergence specified by the Soller collimator were recorded. The height of the collimator used in this experiment was h = 2.7 mm, the diameter of the holes d = 4.5 mm, the number of through holes N = 25. These collimator characteristics provide an angle θ '= 59 °. The total energy resolution of the spectrometer was 50 keV. In this case, the spatial resolution is about 0.1 μm. The thickness of the active film according to the measurement results was equal to d ₁ = 2 μm, the thickness of the protective film d ₂ = 0.5 μm. The measured distribution of uranium concentration is uniform over the entire thickness of the layer (the portion of the spectrum "E ₂ -E ₃ " is horizontal) and is about 9 g / cm ³ . The measurement duration (with a general error in measuring the distribution of concentration and thicknesses ~ 20%) was about 10 minutes.

The obtained experimental results give good agreement with the passport data of the tested samples.

List of references

1. Vloch G.V., Sinyansky A.A., Filippov G.E., Kazakov L.L., Kosulin N.S., Cherevatyuk V.N. Film energy-generating elements for nuclear-pumped lasers. - Proceedings of the conference "Physics of Nuclear-Excited Plasma and Problems of Nuclear-Pumped Lasers". - Arzamas-16, 1994, vol. 1, p. 47.

2. Kazakov L.L., Kosulin N.S., Cherevatyuk V.N. - Proceedings of the conference "Physics of Nuclear-Excited Plasma and Problems of Nuclear-Pumped Lasers". - Obninsk, 1993, v. 2, p. 41.

3. Physical quantities: Reference. / A.P. Babichev, N.A. Babushkina, A.M. Bratkovsky, etc. / Ed. I.S. Grigorieva, E.Z. Meilikhova. - M .: Energoatomizdat, 1991 .-- 1232 p.

4. German O.F., Hoffmann Yu.V. Handbook of Nuclear Physics. - Kiev, 1975.

Claims (1)

A method for determining the transverse characteristics of a layered structure containing a layer of alpha radioactive material, including angular collimation of the natural alpha radiation of radioactive material from the surface of the structure by means of a collimator, recording the energy spectrum of the collimated particle stream, determining the transverse characteristics of the structure in the form of the recorded spectrum, characterized in that spectrum of alpha particles emitted from a structure with an angular divergence specified by the Co collimator lehr as a plate with through holes, analyzing the shape of the registered energy spectrum of the alpha particles dNα / dE, the described model dependence

where n _α is the concentration of alpha particles generated in the active layer at a depth corresponding to the distance traveled L; S is the area of the active layer; θ _min (L) and θ _max (L) are the minimum and maximum departure angles at which the alpha particle, passing the path L, takes off with energy E (L); dE (L) / dL is a function that reflects the linear energy loss of an alpha particle for a particular material, determine the characteristic parts of the spectrum and the corresponding characteristic points with certain energy values, the transverse characteristics of the structure are determined taking into account the obtained energy values.

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