DE4143530C2 - Device for neutron activation analysis - Google Patents

Abstract

The invention relates to a device for neutron activation analysis, in which the smooth plane surface of a sample or the thin film on a sample is excited by the incident neutron radiation and the emitted gamma radiation is spectrally detected, the device essentially consisting of a radiation source, a monochromator, a sample diaphragm and a measurement acquisition device, for example a semiconductor detector. In order to achieve the object, namely to provide a device, with the aid of which series analyses with regard to selected elements can be carried out under optimum analysis conditions, provision is made for the neutron beam to be directed at grazing incidence onto the smooth surface of the sample or a thin film on the sample, so that this surface or film is excited under total reflection conditions.

Description

The invention relates to a device for neutron activation analysis according to the preamble of claim 1.

To carry out surface-selective and surface-sensitive Investigations on solid surfaces require probes from shallow penetration or penetration depth. Shallow depths of penetration have, for example, low-energy electrons and ions because of their intense interaction with matter. This intense Interaction with matter also leads to these probes interact with the atmosphere in which the sample surface is located located. Therefore, these probes and therefore the samples can can only be used analytically in ultra-high vacuum chambers. Probes with relatively little interaction with matter, such as. B. X-ray photons and neutrons are subject to these restrictions Not. X-ray photons and neutrons have large ones Penetration and penetration depths. The great depth of penetration of such However, probes can be used in the area of surfaces and Selectively reduce interfaces to a few nm if the Probes fall onto the sample surface in such a way that they cause total external reflection Experienced.  

External total reflection occurs when radiation is sufficient flat on smooth surfaces of solid or liquid, more organic or inorganic, amorphous, polycrystalline or crystalline Samples falls and for this radiation the refractive index of the matter in question is smaller than that of the surrounding one Vacuum or the surrounding atmosphere. For X-ray photons and there is no significant difference in the refractive index for neutrons between air and vacuum. In the following, therefore, without Restriction of vacuum by air or another gas atmosphere be replaced.

The refractive index n of X-ray photons (ν) and neutrons (n) in vacuum:

n _ν = 1 (1)

n _n = 1 (2)

and in matter:

n _ν = 1-δ _ν -iβ _ν (3)

n _n = 1-δ _n -iβ _n (4)

With:

In the above formulas, δ is the dispersive decrement of the refractive index, β is the absorptive decrement of the refractive index and b _{ν , n is} the coherent scattering length for X-ray photons (ν) and neutrons (n). The absorptive decrement β can be neglected in the calculation of the total reflection critical angle, since δ <β for X-ray photons (ν) as well as for neutrons (n).

The following applies to the coherent scattering length of X-ray photons:

b _ν = r _e Z (7)

r _e denotes the classic electron radius, which is 2.818 × 10 ^-15 m. Z is the atomic number of the atoms of matter.

Equation (7) applies to λν «λ _K-edge and approximately for all wavelengths λ apart from the absorption _edges . b _ν is monotonically dependent on Z. There are resonances depending on λν.

b _n denotes the coherent scattering length of the neutron-nuclear interaction. b _n is highly isotope-dependent. In addition, there are also resonances depending on λ _n .

In ferromagnets, b _{n has} to be replaced by (b _n + p _n ). The scattering length p _n describes the magnetic interaction. Because of the adjustment of the magnetic moment of the neutron with respect to the magnetic moment of the shell electrons of matter, positive and negative values result.

The atomic density of matter N is calculated as follows:

Herein, N _A denotes the Avogadro constant (N _A = 6.0225 × 10 ²³ mol ^-1 ). ρ is the density of matter. A _r is the relative atomic mass of the atoms of matter.

The following applies to the wavelength of the X-ray photons λ _ν :

E denotes the photon energy.

The de Broglie wavelength of the neutrons λ _n results from:

Herein, E _{n is} the neutron energy.

In the majority of cases, the refractive index for matter is n _ν <1 and n _n <1. External total reflection thus occurs when the angle of incidence R of the X-ray photons or neutrons is smaller than the critical angle R _c :

Typical values of R _c are 0.5 °.

For the angle of incidence O <R <R _c , the penetration depth L _ν , n for X-rays and neutron beams decreases to a few nm. The minimum value L _min is:

The low penetration depth results in surface selectivity. This also allows analysis methods with X-rays or neutron beams perform extremely surface-selective, although this Methods usually not the analysis of the sample surface, but serve the inside of the sample. In total reflection x-ray fluorescence analysis (TRFA, TXRF) and the total reflection neutron activation analysis  (TNAA) is grazing with the incident X-ray photons or neutrons selectively the surface excited. The atoms excited on the surface can emitting x-rays (fluorescent radiation) or return from gamma radiation to a stable state. Out the energy of the radiation (X-ray or gamma radiation) the sample atoms identify themselves chemically. this makes possible a multi-element analysis. The intensity of the radiation is a Measure of the number of sample atoms. TRFA and TNAA thus represent surface-selective analysis methods for determining the element composition of surfaces and thin layers.

Already from the publication Y.Yoneda and T. Horiuchi, Rev.Sci.Instr. 42 (1971) 1069; P. Wobranschek and H. Aiginger, Anal.Chem. 47 (1975) 852 and DE-OS 26 32 001, US Pat. No. 4,358,854, devices for total reflection X-ray fluorescence analysis (TRFA) are known. X-ray sources (X-ray tubes) provide polychromatic X-rays. The X-ray absorption in the sample shows a strong dependence on the X-ray energy. The absorption coefficient and thus the excitation of a certain fluorescence radiation drops sharply above the corresponding absorption edge to higher energies. Nevertheless, in order to ensure selective excitation of the surface atoms (total reflection), the angle of incidence of the excitation radiation must be selected so that total reflection is ensured for the excitation spectrum. Because of the dependence on λ in equation 5, the shortest wavelength still occurring in the excitation spectrum determines the critical angle R _c , even if these short-wave components of the spectrum only make a small contribution to the measurement signal. If one deviated from this principle, the measurement signal of the analyte on the surface would also be superimposed on the inside of the sample with the measurement signal of a similar element. This would falsify the result, since the absolute number of atoms, even of an element with a low concentration inside the sample, is generally much higher than that of an analyte on the surface. Known measuring arrangements had the disadvantage of either having to choose an extremely small angle of incidence or of having to accept excitation of the sample volume in addition to the surface excitation. In further developments, attempts were therefore made to cut off the excitation spectrum below a lower, practically chosen cut-off wavelength. Such cutting off of short-wave radiation was achieved by total reflection on polished quartz glass plates (DE-PS 27 36 960, US-PS 4,426,717 and DE-PS 29 11 596).

Ultimately, for the detection limit of the analysis method the intensity of the background and measurement signal is decisive. There, in particular in the excitation of x-ray photons, the stimulating Radiation is only partially absorbed and fluorescent radiation delivers, but is largely scattered and thus contributes to the subsurface must improve the detection limit the adjacent spectrum can be chosen so that in the fluorescence spectrum of the analytes sought no scattered radiation from the excitation spectrum occurs. On the other hand, the excitation spectrum if possible fall on the absorption edge of the analytes in order to to receive an intense measurement signal. Recently, attempts have been made to to achieve this goal through monochromatic excitation, single crystals or multilayers are used as monochromators (A. Bohg and M. Briska, DE-OS 2727505 and US-PS 4,169,228; M. Brunel, Acta Cryst. A42 (1986) 304; R. S. Becker, J.A. Glovchenko and J.R. Patel, Phys. Rev. Lett. 50 (1983) 153; A. Iida and Y. Gohshi, Jpn. J. Appl. Phys. 23 (1984) 1543; A. Iida, K. Sakurai, A. Yoshinaga and Y. Gohshi, Nucl. Instr. Meth. Phys. Res. A246 (1986) 736; C. T. Yap, R. E. Ayala and P. Wobrauschek, X-Ray Spectrometry, 17 (1988) 171; P. Wobrauschek and  P. Kregsamer, Spectrochimica Acta 44B (1989) 453). According to literature (K. Taniguchi, S. Sumita, A. Saski, K. Nishihagi and N. Fujino, Abstracts 39th Annual Denver Conference on Applications of X-Ray Analysis, Steamboat Springs CO, July 30th August 3, 1990, p. 39; M. Schuster, Abstracts 39th Annual Denver Conference on Applications of X-Ray Analysis, Steamboat Springs CO, 30.07.-03.08.1990, p. 40) the detection limits can be determined compared to the spectrometers with an excitation spectrum that only is cut off to the short-wave side, up to a factor 10 improve.

From the Interference Coatings for Neutrons publication Applied Optics, Vol. 23, No. 20, October 15, 1984, pp. 3525-3527 it is already known that neutron beams with multilayer Monochromators can be polarized.

The object of the present invention is now a device of the type specified at the outset, which in particular for series analysis of selected elements, e.g. B. such which are particularly critical for a manufacturing process, below Based on optimal analysis conditions.

According to the invention, this object is achieved by a device for Neutron activation analysis according to the preamble of the claim 1 solved, in addition the features of the characterizing part has claim 1. Accordingly, the neutron beam grazing the smooth flat surface of the sample or a Thin layer directed on the sample so that this under total reflection conditions is excited.

Preferred embodiments of this solution are in the on the Claim 1 related subclaims specified.  

For example, the monochromator can be constructed from layer structures with normal and lateral periodicity (superlattice) on a crystal substrate. Examples of the embodiments of layer structures with normal and lateral periodicity are shown in FIG. 3. AlAs / GaAs and GaAs / GaAs _1-x Sb _x layer structures are known from the literature (cf. DA Neumann, H. Zabel and H. Morkoc, J. Appl. Phys. 64 (1988) 3024; T. Fukui, H. Saito and Y. Tokura, Jpn. J. Appl. Phys. 27 (1988) L1320; T. Fukui and H. Saito, Appl. Phys. Lett. 50 (1987) 824; T. Fukui and H. Saito, J. Vac. Sci. Technol. B6 (1988) 1373; JM Gaines, PM Petroff, H. Koemer, RJ Simes, RS Geels and JH English, J. Vac. Sci. Technol. 6 (1988) 1378), which are used for Achieving certain electronic properties with MOVPE and MBE were made.

According to a further embodiment, the monochromator consists of an intercalate. These are layer structures consisting of a graphite lattice into which atoms, preferably alkali metals (e.g. Li, Na, K, Rb, Cs) or whole molecules (e.g. FeCl₃, AsF₆) between the carbon atom planes (see MS Dresselhaus and G. Dresselhaus, Advances in Physics 30 (1981) 139; H. Zabel, Structure and Dynamics of Graphite Intercalation Compounds in Proc. Int. Conf. on Neutron Scattering, August 19-23, 1985 , Santa Fe; H. Zabel, WA Kamitakahara and RM Nicklow, Phys. Rev. B26 (1982) 5919). This can be done in different stages (see FIG. 4). In step 1, a graphite layer alternates with an intercalate layer. In step 2, an intercalate layer follows each after 2 graphite layers, etc. (see FIG. 3).

The types of monochromators described above can be used for neutrons if the neutron scattering lengths are taken into account in the choice of material and the periodicity lengths are adapted to the de Broglie wavelength of the neutron radiation. Especially for neutrons, a preferred embodiment results in the monochromator consisting of layer structures with ferromagnetic layers. With this solution for neutron activation analysis (TNAA), the magnetic interaction of neutrons in ferromagnetic materials also offers the possibility of influencing the refractive index by an external magnetic field perpendicular to the scattering vector (note Eq. 4 and 6 and b _n → b _n + p _n . With a clever choice of atomic densities N₁ and N₂ as well as the scattering lengths b₁, b₂, p₁ and p₂ the reflectivity R = 0 for spin-up and R = 0 for spin-down neutrons can be achieved An example of this is an Fe / Ge layer structure (cf. Ch.F. Majkrzak, Appl. Opt. 23 (1984) 3524).

Other advantages of the present invention will be based on the embodiments shown in the figures explained in more detail. Show it:

Fig. 1 is a schematic diagram of a first embodiment of the device according to the invention,

Figure 2a). The schematic overall structure of the monochromator according to a preferred embodiment of the present Erfinndung,

Fig. 2b): the schematic structure of the layer structure of the monochromator gem. the embodiment shown in Fig. 2a),

Fig. 3 shows a schematic overall construction of the monochromator according to another embodiment of the present invention,

FIG. 4 shows a schematic construction of the monochromator according to a third embodiment of the present invention,

Fig. 5 shows a schematic structure of the layer structure of the monochromator according to a fourth embodiment of the present invention,

In Fig. 1 a schematic representation of the apparatus is shown for the irradiation of samples at grazing incidence with monochromatic neutron radiation for neutron activation analysis. The radiation source denoted by 1 is a neutron source, which can be, for example, a reactor or an isotropic source. 4 is to be examined sample, for example a to be examined silicon wafer referred to, above which the sample is the measuring sensor 6, for example, a semiconductor detector for gamma spectrometry arranged, to which a stray radiation diaphragm 5 is connected upstream. An aperture 9 , a monochromator 8 and an aperture 7 are connected downstream of the radiation source 1 in the beam path of the neutron beam 10 . The angle Φ is the diffraction angle at the monochromator unit, while the angle R marked in FIG. 1 denotes the angle of incidence of the radiation on the sample.

From Fig. 2a) of the overall schematic structure of the monochromator is shown according to a first solution of the present invention, which consists of a layer structure on a crystal substrate. In this figure, 1 ' denotes one layer and 2' the other layer of the pair of layers. The entire layer structure is denoted by 3 ' and the crystal substrate by 4' . d _M shows the periodicity length of the layer structure and d _S denotes the periodicity length of the crystal substrate. With d _M1.2 the thickness of the layer 1 ' or 2' is designated. N shows the number of pairs of layers. The depth coordinate is designated with z.

In Fig. 2b) is the schematic structure of the layer structure 3 'of the monochromator acc. shown in FIG. 2a) is closer. An incoherent multilayer is indicated with 5 ' . 6 ′ shows an alloy with a modulated composition. Finally, 7 ' relates to a superlattice (epitaxial layer sequences). These three layer structures shown next to one another thus represent alternative embodiments for the layer structure of the monochromator according to the invention.

Fig. 3 shows the schematic overall structure of a monochromator according to a second solution of the present invention. Here the layer structure with lateral periodicity is shown on a crystal substrate. In the exemplary embodiment shown here, the layer structure consists of sequentially grown AlAs and GaAs. d _{M || '⟂} denotes the lateral or normal periodicity _{length of} the layer structure. The periodicity length of the crystal substrate is again indicated by d _S. The direction [100] denotes the orientation of the crystal substrate. A misorientation of the surface of the crystal substrate is necessary for the growth of lateral periodic structures.

Fig. 4 shows the schematic structure of the monochromator according to a further solution of the present invention. Intercalates based on graphite of levels 1, 2 and 3 are shown here side by side. The periodicity length of the intercalate is denoted by d. This quantity corresponds to the periodicity length d _M in the description of the monochromator type according to the first solution of the present invention. d _c indicates the periodicity length of the undisturbed graphite layers. In the case of higher-level intercalates, this size corresponds to the periodicity length d _S in the description of the monochromator type according to the first solution of the invention.

In Fig. 5, the schematic structure of the layer structure of the monochromator according to a fourth solution of the present invention is given. This is an electrically switchable monochromator for the monochromatization of spin-polarized neutrons. With 1 '' , the non-magnetic layer 1 is marked. 2 '' denotes the ferromagnetic layer 2. Layer 3, which is labeled 3 '' , is non-magnetic and is identical to layer 1. With 4 '' is again a non-magnetic layer 4. D _1,2,3,4 indicate the thicknesses of layers 1 to 4. In addition to the layer structure, the refractive index for spin-polarized neutrons for a switched-off magnetic field H and a switched-on magnetic field H is compared. The refractive index for the switched-off magnetic field H is designated with n _{n, H = 0 and} the refractive index for the switched-on magnetic field h with n _{n, H ≠} ₀. d _{H = 0} gives the periodicity length when the magnetic field H is switched off and d _{H ≠} shows the periodicity length when the magnetic field H is switched on. z is the depth coordinate of the layer structure. Here, the embodiment of a neutron monochromator is shown schematically, which can be controlled by an external magnetic field. With this monochromator, each period is made up of four individual layers. Layers 1 and 3 are made of identical non-magnetic material with the refractive index n _n1 = n _n3 . Layer 4 is also non-magnetic with the refractive index n _n4 ≠ n _n1 , n _n3 . Between layers 1 and 3 there is the ferromagnetic layer 2, whose refractive index is n _{n2, H = 0} = n _n1 , n _n3 without a magnetic field and with a saturating magnetic field for spin-polarized neutrons n _n2, H _≠ ₀ = n _n4 . By switching on the external magnetic field H ≠ 0 and switching off H = 0, the periodicity length of the layer structure for spin-polarized neutrons can be switched between d _{H ≠} ₀ and d _{H = 0} . In the example shown, d _{H = 0} = 2 × d _{H ≠} ₀. If the diffraction angle is determined by diaphragms, the exciting neutron spectrum can be changed in this way by electrical switching.

Claims (8)

1.Device for neutron activation analysis, in which a sample is excited by the incident neutron radiation and the emitted gamma radiation is recorded spectrally, consisting of a radiation source, a monochromator with layer structures with normal periodicity on a substrate, an anti-scatter aperture and a measuring sensor, characterized in that the neutron beam grazes the smooth, flat surface of the sample or a thin layer on the sample so that it is excited under total reflection conditions. 2. Device for neutron activation analysis according to claim 1, characterized in that the monochromator ( 8 ) consists of a layer structure with a high jump in the refractive index for neutrons on a crystal substrate (coherent multilayer). 3. Device for neutron activation analysis according to claim 2, characterized in that the layer structure of Ni / Mn or Ni / Ti. 4. Apparatus for neutron activation analysis according to claim 1, characterized in that the substrate is a crystal substrate and that the layer structures are also lateral Show periodicity. 5. Device for neutron activation analysis according to claim 1, characterized in that the monochromator consists of an intercalate, d. H. a layer structure consisting of a graphite grid, one between each of one or more carbon planes Layer of other atoms or molecules is built, is built up. 6. Device for neutron activation analysis according to claim 1, characterized in that the monochromator from layer structures with ferromagnetic layers for neutrons. 7. Device for neutron activation analysis according to claim 6, characterized in that the monochromator ( 8 ) consists of a layer structure with a ferromagnetic and a non-magnetic layer, for example of FeGe in an external saturating magnetic field for generating spin-polarized monochromatic neutrons. 8. Apparatus for neutron activation analysis according to A. 6 or 7, characterized in that the monochromator ( 8 ) has a layer structure with a period of four individual layers, with the refractive index for spin-polarized neutrons being changeable by an external magnetic field in at least one layer.