Non-destructive method of inspection of lateral characteristics of cellular structure having α-radioactive layer material layer

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

 

The invention relates to measurement techniques, in particular the transverse measurements of process parameters micron layered structure (thickness patterns of the order of several microns)containing alternating layers of passive (non-radioactive) and active (alpha-radioactive) material (local thickness distribution along the depth of alpha-radioactive material). The invention can be used to study the dynamics of changes of these structures under the influence of various internal and external factors, for example, in the interests of nuclear-pumped lasers.

Known methods of determination of technological parameters of thin layers: a method of auger electron spectroscopy (AES), x-ray diffraction, metallography.

- X-ray diffraction [3] refers to non-destructive methods of control of the layered structure, however, it is used mainly for studies of structures with layer thicknesses in excess of 10 microns.

Methods allowing to measure micron layered structure, is a method of EOS and metallographic, but they have drawbacks.

- EOS method [1, 2] refers to the destructive methods. To obtain data on the composition of the layers (the concentration profile of elements in depth) in the method of the EOS is consistent removal of fine (hundredths dollars is micron) layers of material from the sample surface by spraying using a beam of high-energy ions. As material removal is performed elemental analysis of a new layer left on the surface. The duration of a single measurement method of the EOS can be up to several days.

- When the metallographic study of the structure [2] there is also the destruction of the original sample. In this way it is difficult to obtain reliable information on the transition zones between the individual layers of the structure. In addition, due to the necessity of application of separate chemical etching of the material layer and the substrate, the method characterized by extremely low efficiency (separate dimension can last about a month).

Thus, the total lack of method for EOS and metallography is the destruction of the sample. In addition, both methods are rather time-consuming and not very efficient.

The authors are not known for rapid non-destructive method of control thin-micron structures comprising layers of alpha-radioactive material.

There are a number of tasks (in particular, the development of effective and radiation-resistant energovydelenii elements used in nuclear-pumped lasers), for which the necessary operational control of the changing characteristics of thin-film structures containing one or more alpha-radioactive layers). While it is important to maintain structure the tours of the object when the study:

- with a view to its reuse;

- for control of the object at any stage of use;

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

The technical result consists of:

1. In providing quality non-destructive testing of thin-layer structure with one or more alpha-radioactive layers due to the high spatial resolution (a few hundredths of a micron), allowing to study subtle effects, slightly changing the original structure, such as spraying the surface by its own fission fragments.

2. In efficiency measurement (duration of a single measurement can only take a few minutes), allowing it to conduct research on a large batch of samples.

To achieve this technical result in the determination of the transverse characteristics of the layered structure (thickness of layers, the thickness of the transition zone layers, the distribution of alpha-radioactive component of the depth structure), you must provide:

the angular collimation of the alpha-particle flux (natural alpha-emitting radioactive material) from the surface of the structure;

- obtaining the energy spectrum of natural collimated alpha radiation emerging from the sample surface, by registering a spectrometer is a high energy resolution;

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

- use as a collimating device of the collimator type Soller, allowing for the degree of collimation drastically reduce the duration of the measurement in comparison with other types of collimators, due to high transparency of this collimator.

The use of natural radiation radioactive materials as measured by the Desk facing surface patterns of alpha particles is quite obvious from the point of view of obtaining information about the condition of the structure containing the radioactive layers.

Indeed, the radiation before it can reach the surface, passes a certain distance in the material and changes its energy spectrum. And that alpha particles, because of their extremely small mileage in materials (several microns), most significantly, studies of the properties of micron alpha-radioactive materials. For uranium, for example, linear energy alpha particle (let) is about 0.5 MeV/µm [3]. It is obvious that the energy spectrum of alpha particles after the passer who placed them micron layer carries sufficient information about the transverse structure of this layer.

On the basis constructed by the authors of the mathematical model of the energy spectrum of alpha particles, which are recorded by the exit layer (with some simplifications), has the form

where nαthe concentration of alpha-particles generated in the active layer (at a depth corresponding to the path travelled "L") per unit of time (obviously, this concentration is proportional to the concentration of nuclei of a radioactive element such as uranium, at a given depth layer)

S - area of the active layer;

θmin(L) and θmax(L) the minimum and maximum angles at which the alpha particle goes "L" (i.e. crashes with energy E(L)");

dE(L)/dL - function, reflecting the linear energy loss of alpha particles for a particular material.

As can be seen, the concentration of nα"determined from the relation (1)averaged over a certain layer thickness "ΔX'" (X - coordinate axis from the surface of the structure in its depth), which, as shown below, and determines the spatial resolution of the proposed method of measurement. Naturally, the information obtained in the measurement, the better than this thickness less. Technically, the reduction of size "ΔX'" is the collimation of the registered radiation (i.e. reduction of the maximum angle of departure "θmx (L)" registered alpha-particles relative to the normal to the surface of the investigated structure). For a given energy E, with which the alpha particle is ejected from the surface, the minimum angle "θmin(L(E))" uniquely determined by the thickness of the active layer "d1" and the protective film d2(patterns, for example, of the three layers is an active layer 1, the protective film 2, the substrate 3) (figure 1). The maximum angle "θmax(L(E))can be reduced by using a collimator that limits the aperture of the registration of alpha particles corner "θ'".

The use of the collimator type Soller (representing, in this case, the plate thickness "h" with a few hundred through holes with a diameter "d") (2), known in optics as a limiter aperture of optical radiation, allows to implement the mathematical model (1) studies of the structure of alpha-radioactive layers due to the "small-angle neutron energy selection of alpha particles while maintaining the intensity of their registration (due to the presence of numerous cross-cutting channels in the collimator).

According to the model (1) of the recorded energy spectrum of alpha particles(i.e. the number of alpha particles registered in the unit energy interval from their energy) on nom case has several characteristic points (figure 3):

- E0- the initial energy of the alpha particles with mileage R (for example, for U234E0=4,77 MeV for U235E0=4,4 MeV [4]);

- E1- maximum energy registered alpha-particles emitted from a depth of x=d2(the inner boundary of the protective film);

- E2- minimum energy registered alpha-particles emitted from a depth of x=d2.

- E3- maximum energy registered alpha-particles emitted from a depth of X=d1+d2(the inner surface of the uranium layer);

- E4- minimum energy registered alpha-particles emitted from a depth of X=d1+d2.

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

- "Front" is the interval of energies E1-E2;

- "Top" - the energy interval E2-E3;

- "Back front" the energy interval E3-E4.

The thickness "d2" passive layer is uniquely determined by the difference of the values of these energy points (assuming that the dE/dL=const≈E0/R)

The difference of values of E1-E3defines the thickness of the active layer

Point "E2" is purely "aperture" origin and appears when the condition

The shape of the spectrum in the area of "E2-E3" gives an idea about the distribution of alpha-radioactive component of the depth of the active layer. Formula (1) gives a one-to-one correspondence between the value of dNα/dE" at some point spectrum "E'"belonging to the spectral range "E2-E3"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 equal to

From the foregoing it becomes apparent the importance of the angular collimation of alpha-particles flux leaving the surface of the sample, when registering their spectrum. Without the use of Soller collimator in principle not possible to determine the average concentration of radioactive material on the layer thickness (no plot "E2-E3"). On the other hand, the use of Soller collimator allows to determine the distribution of concentration over the thickness of the layer with spatial resolution the better, the smaller the aperture of the collimator "θ'" (see formula (5)). Along with the energy resolution of the measuring path using Soller collimator determines the accuracy of the measurements of the thicknesses of the individual layers of the structure. However, due to the transparency of the collimator ensures efficiency measurements. the manual allows measurements in two main modes, determined by the choice of angle collimation "θ'":

Measurements with high spatial resolution (a few hundredths of a micron). Such measurements are carried out in the study subtle effects (e.g., spraying the surface layer of the fission fragments). A single measurement in this mode requires a considerable investment of time (several hours).

Measurements with a spatial resolution ˜ several tenths of a micron. This type of measurement is most common in the study of structures (determination of thickness of layers of active and passive layers, the length of the transition zones at the border of layers etc). Such measurements are speed (the duration of a single measurement takes only a few minutes) and can be carried out on a large batch of samples.

List of figures and graphics includes:

Figure 1 is a schematic depiction of a cross section of the layered structure;

Figure 2 - schematic representation of the Soller collimator;

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

As an example, consider the form of the recorded alpha spectrum for a structure consisting of three layers (see figure 1): 1 - layer of uranium, the thickness "d1"; 2 - the protective layer of aluminium of a thickness "d2and 3 - substrate interface.

In accordance with the model analysis of the spectral dependence (figure 3) allow you to plug the et to determine the thickness of the active layer is d1=2 μm (see formula (3)) and the thickness of the protective film d2=0.5 µm (see formula (2)).

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

To test the proposed method was measured thickness and distribution of the concentration of alpha-emitting isotopes thickness of standard samples (area ˜ 1 cm2with known technological characteristics, containing various alpha-active layers: uranium-235, plutonium-238, plutonium-239, americium-241, radium-226.

The measurement structure of the films was carried out on an alpha spectrometer, consisting of a semiconductor detector type "DDPs" holder of the amplitude of the spectrum analyzer. Registered alpha particles emitted from the sample with an angular divergence defined by the Soller collimator. The height used in this experiment collimator was h=2.7 mm, hole diameter d=4.5 mm, number of holes, N=25. These features provide collimator angle θ'=59°. The total energy resolution of the spectrometer was 50 Kev. While the spatial resolution is about 0.1 μm. The thickness of the active film according to the results of the measurements was equal to d1=2 μm, the thickness of the protective film d2=0.5 µm. The measured distribution of the concentration of uranium rawname the but across the thickness of the layer (a part of the spectrum "E 2-E3" horizontal) and is about 9 g/cm3. Measurement duration (with a total error of measurement of the concentration distribution and thicknesses ˜ 20%) was about 10 minutes.

The experimental results give a good agreement with the published data of the tested samples.

References

1. Vlokh, GV, sinyanskiy A.A. Filippov GA, Cossacks L.L., Kozulin NS, Cherewatuk NR. Film energovydelenii elements for nuclear-pumped lasers. - Proceedings of the conference "Physics of nuclear induced plasmas and problems of nuclear-pumped lasers". - Arzamas-16, 1994, Vol.1, p.47.

2. Cossacks L.L., Kozulin NS, Cherewatuk NR. - Proceedings of the conference "Physics of nuclear induced plasmas and problems of nuclear-pumped lasers". - Obninsk,1993, Vol.2, p.41.

3. Physical quantities: a Handbook. / Appalaches, Nagbabasa, Amiradaki and others/edited Isegoria, Esenaliev. - M.: Energoatomizdat, 1991. - 1232 S.

4. German AF, Hoffman Y. Handbook of nuclear physics. - Kiev, 1975.

The method for determining the transverse characteristics of laminated structure comprising a layer of alpha-radioactive material, including the angular collimation of natural alpha-emitting radioactive material with the surface structure by means of the collimator, the registration of the energy spectrum of the collimated flux h is CI, the definition of the transverse characteristics of the structure on the shape of the measured spectrum, wherein the register range of the alpha particles emitted from the structure with an angular divergence defined by the Soller collimator in the form of a plate with through holes, analyzing the registered form of the energy spectrum of alpha particles dNα/dE described by the model dependence

where nαthe concentration of alpha-particles generated in the active layer at a depth corresponding to the passed path L;

S - area of the active layer;

θmin(L) and θmax(L) the minimum and maximum angles at which the alpha particle passing through the path L, flies with energy E(L);

dE(L)/dL - function, reflecting the linear energy loss of alpha particles for a particular material,

defines the characteristic spectrum and the corresponding characteristic points with certain values of energy, the transverse characteristics of the structure is determined taking into account the obtained energy values.



 

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