CEMS and Mössbauer Sprectroscopy
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This document discusses conversion electron Mössbauer spectroscopy (CEMS), a surface-sensitive technique used to analyze the atomic structure of materials. CEMS works by detecting internal conversion electrons emitted during the radioactive decay of certain isotopes like 57Fe. It provides a few key advantages over traditional Mössbauer spectroscopy such as the ability to probe surface layers only a few nanometers thick. The document outlines the basic principles of CEMS, provides examples of its applications in analyzing steel corrosion, metal alloys, and thin films, and discusses both its advantages in high sensitivity surface analysis and limitations related to electron scattering in materials.
CEMS and Mössbauer Sprectroscopy
- 1. Conversion Electron Mössbauer Spectroscopy (CEMS) : Basis and applications. Sameh Hamzamy Ingrid Gnemegna MESC9
- 3. 1957 → Rudolf Mössbauer achieved 1rst experimental observation of resonant adsorption of γ-radiation and discovered recoil free in solid emission. Introduction The Mössbauer effect → basis of Mössbauer spectroscopy and CEMS. Need radioactive source which decays via excited state called Mössbauer isotope (Fe,Sn).
- 4. Mössbauer effect Nuclear decay scheme
- 5. Mössbauer effect Solid matrix = no recoil Ephoton = Enucleartransition Means to probe hyperfine interactions of atoms nucleus and their surrounding! Ephoton = Enucleartransition-Erecoil
- 7. As radiation leave the sample, only a layer at the surface contributes to the backscattered spectrum. The thickness of this layer depends on the range of the radiation. Resulting method: CEMS Mössbaueur spectroscopy CEMS and Mössbauer Sprectroscopy
- 9. A nucleus promoted to an excited state by gamma ray adsorption can de-excite by two categories of mechanism : Radiative: emission of a gamma ray with a probability of N(γ) Non-Radiative: internal conversion and the ejection of an atomic electron with a probability of N(e) Internal conversion coefficient α: α=8.21 for 57Fe , internal conversion is 8.21 times more probable than γ-ray (photon) emission. Conversion coefficient
- 10. Emitted electron energy Ec = Eγ – Eb Eγ :energy of the transition Eb:binding energy of the electron In 57Fe the internal conversion can occur from K, L and M shells, in order of probability. Emitted electron energy
- 11. Equipement for CEMS → Source irradiate sample →Electrons emitted from the sample accelerated towards the anode →ionising atoms in the counting gas → produce avalanche effect amplifying signal → electronic pulse detected and recorded
- 12. Applications Steel Corrosion Analysis Metal Alloy Characterization Ultra Thin Films Analysis
- 13. Steel Corrosion Analysis CEM spectra of the corrosion layers form on iron by immersing it in 3% sodium chloride solution of initial pH 6.5 at 318 K for: (a)12h (pH changed to 7.0) (b)24h (pH changed to 7.3), (c) 48 h (pH changed to 7.7) (d) 96 h (pH changed to 8.0) Journal of Radioanalytical and Nuclear Chemistry, Articles, Vol. 202,Nos 1-2 (1996) 103-199.
- 14. Characterization of tin coated Al alloy Depth selective CEMS spectra of Sn coated Al alloy plate untreated in a molybdate solution Journal of Radioanalytical and Nuclear Chemistry, Vol. 257, No. 1 (2003) 97–103
- 15. Depth selective CEMS spectra of Sn coated Al alloy plate treated in a molybdate solution Schematic cross section of Sn coated Al alloy treated with molybdate ions
- 16. Ultra thin films of Fe on Mo substrate CEMS spectrum of the 3 A film evaporated at 600 K on polycrystalline Mo substrate. Spectrum was measured at 300 K. T. Tyliszczak, J.A. Sawicki and W. Wilk
- 17. Surface analysis Spray Deposition(SD)Technique. Artificial oxide films Surface and interface reaction Ion implanted layers Other applications
- 18. CEMS is a highly-selective and non-destructive method. Conversion electrons are very easy to detect efficiently. Information on the configuration of magnetic spin in the surface layer can be obtained. CEMS has become an important tool of materials surface analysis. Advantages
- 19. Loss of electron energy in their passage through matter cannot be calculated. Compton scattering and the photoelectric effect produce "non-resonant electrons". Electrons do not move through matter in straight lines. Only windowless detectors may be used. Disadvantages
- 20. Conclusion CEMS seems a valuable technique for either thin films or the surface of bulk samples. Applicable to a wide range of low-area materials containing iron (or tin) while Mössbauer spectroscopy requires high-area solids. The high sensitivity and attractive probing depth from the first atomic layer on the surface to 1μm below the surface.
- 22. References https://wiki.umn.edu/MXP/MossbauerLabIntroduction Mössbauer Spectroscopy in Materials Science,Marcel Miglierini,Dimitris http://korek.uci.agh.edu.pl/cemsnew.html Mössbaueur1, Structural Investigation and Characterization of Materials, Carlos familyname http://phobos.chemie.uni-mainz.de/moessbauer.php
Editor's Notes
- #14: For electrons of energy in a low kiloelectronvolt range, which is typical of conversion electrons, the escape length is of the order of 1000 A. In case of X-ray detection, the escape length of X-rays in the kiloelectronvolt range is of the order of a few microns, and all the atoms within that distance from the surface will contribute to the conversion Mossbauer spectrum. Fig: CEMS is an useful technique for analyzing final and stable oxides produced on surface layers of iron and steels
- #15: The surface of aluminum alloy is treated by Sn substitution plating in order to reduce the friction resistance of moving parts of automobiles In this paper, the surface of tin coated aluminum alloy was characterized by CEMS and the results were described. 119Sn CEMS was used to clarify the Sn-plating structures and chemical states before and after the post-molybdate treatment.
- #16: - In the case of the tin coating containing Mo ions, the Sn(II) doublet peaks increase little even if the coatings were heat-treated for 16 hours. - This result indicates that the tin coatings with Mo ions have good resistance against oxidation.
- #17: A film of iron with an average thickness of about 3 A was deposited on polycrystalline Molybdenium foil kept at 600K . - The spectrum measured at 300 K indicates the magnetic field H = 33.2 T of metallic iron, we can attribute these paramagnetic components to iron atoms at the interface but other explanations such as surface contamination or island structure cannot be excluded either and more experiments are needed before firm conclusions can be reached.
- #19: - The line widths of resonant peaks do not depend so much on the thickness of thicker samples and the saturation of peak intensity is not observed. - The approximate energy distribution function for electrons related to depth and detection angle with energy can be calculated. The intensity ratio of magnetic split peaks obtained can yield effective information on the configuration of magnetic spin in the surface layer. CEMS has become an important tool of materials surface analysis because of its depth selectivity and increased sensitivity.
- #20: Magnetic split peaks of complex CEM spectra do not always follow the constant intensity ratio. Some fitting must be done on the some assumption. Windowless detector because of its high sensitivity for low energy electrons
- #21: - The conversion electron Mossbauer spectroscopy, either using the simple gas-detector or detecting the conversion electrons in a vacuum with an electron energy analyzer, which allows measurements down to 4.2 K, seems a valuable technique for doing Mossbauer measurements either on thin films (less than 1000 a) or on the surface of bulk samples (corrosion studies or analysis of implanted layers) - The high sensitivity of CEMS, which is able to detect less than a monolayer of 57Fe and the very good fit which exists between the depth of implantation and the depth explored by conversion electron Mossbauer spectroscopy, makes this technique a very valuable tool to analyze implanted surfaces.