Kossel diffraction observed with X-ray color camera during PIXE of nano-scale periodic multilayer
Introduction
The Kossel process refers to the interference of characteristic X-rays generated within and scattered by the atoms present in a periodic micro-structure. Typical diffraction patterns are formed around the Bragg angle corresponding to the wavelength (or energy) of the emission and the period of the lattice. In 1935, Walther Kossel experimentally demonstrated such interferences for a crystalline structure [1]. The phenomenon was then named after him as “Kossel effect”. Its observation requires two essential factors: first, the ionization of a core level in order to produce the characteristic emission; second, a periodic structure to diffract the emitted X-ray. The source for the ionization can vary. It can be X-ray provided either by an X-ray tube [2], [3], [4] or by synchrotron radiation [5], [6], [7], [8], energetic electrons [9], [10], [11], [12] or energetic charged particles (protons or ions) [13], [14], [15], [16], [17], [18], [19] as we report in this paper.
Studies of both crystals and interferential multilayers using various techniques have been widely reported, but it was not until our previous paper [20] that the study of periodic multilayer by Kossel diffraction of X-rays generated by proton beam was first reported. We successfully observed the Kossel curves of Cr Kα and Sc Kα emissions of a Cr/B4C/Sc periodic multilayer irradiated by 2 MeV protons. However due to the very small solid angle of the highly collimated detector the counting statistics were not ideal even for long acquisition times, and we mentioned in our conclusion that it might be improved by using an X-ray color camera, that is a CCD camera in which each pixel can provide an energy-dispersed X-ray emission spectrum. This allows us to avoid the angular scans in our measurements, thus reducing the overall acquisition time while allowing an improvement of statistics.
The work presented in this paper focuses on the Pd/Y based multilayers, which are designed as reflecting mirrors to work in the 7.5–11 nm wavelength range. Derivative systems are also designed with the motivation of improving the optical performance of the reflecting mirror owing to more abrupt interfaces.
Section snippets
Experimental details
Samples are deposited onto sliced and polished Si (100) wafers by using DC magnetron sputtering. The originally designed structure is B4C(2.5 nm)/[Pd(2 nm)/Y(2 nm)]×40/Si. The B4C capping layer is added to protect the samples against oxidation. A series of samples is fabricated with 0–6% of nitrogen gas N2 in the sputtering gas instead of pure argon because we believe that the formation of YN will help improve the thermal stability of the sample and reduce the interdiffusion between Pd and Y
Results and discussion
In Fig. 6 we present the Kossel curves for Pd Lα emission of all measured samples. To facilitate the comparison of the shapes of the curves, for each sample the origin of the detection angle is set to the Bragg angle, which corresponds to the center of its Kossel oscillation. Fig. 6(a) shows the Kossel curves of the Pd/Y multilayers deposited with a proportion of N2 within the argon sputtering gas ranging from 0 to 6%. For the Pd/Y multilayer grown in pure argon we observe no Kossel feature at
Conclusion
We have shown the feasibility of using proton induced X-ray emission combined with Kossel diffraction to characterize the structure of nanometric multilayers. We have measured the Kossel diffraction of proton induced Pd Lɑ emission in Pd/Y based multilayers. Compared to our previous experiment [20], we have significantly improved the data acquisition with the X-ray color camera. We also demonstrate here that the shape of the Kossel curve is very sensitive to the detailed structure of the
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Kossel interferences of proton-induced X-ray emission lines to study thin film waveguides
2019, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and AtomsCitation Excerpt :PIXE analysis possesses a good sensitivity to measure the radiations emitted by electron state changes and distinguishes the elements of the sample in a wide range of atomic numbers, without destruction of the sample [11,22]. In Ref. [12,20,21], the generated X-ray emission is recorded in Kossel geometry to obtain the so-called Kossel curve, which is the intensity of characteristic X-ray emission as a function of the detection angle. Kossel curves are convenient to study the interfacial environment in multilayers made of a periodic alternation of nanometer-thick films.
Note: Observation of the angular distribution of an x-ray characteristic emission through a periodic multilayer
2018, Review of Scientific Instruments