Conversion electron Mössbauer spectroscopy (CEMS) is a

Mössbauer spectroscopy Mössbauer spectroscopy is a spectroscopic technique based on the Mössbauer effect. This effect, discovered by Rudolf Mössbauer (sometimes written "Moessbauer", German: "Mößbauer") in 1958, consists of the nearly recoil-free emission and abs ...

technique based on

conversion electron Internal conversion is a non-radioactive, atomic decay process where an excited nucleus interacts electromagnetically with one of the orbital electrons of an atom. This causes the electron to be emitted (ejected) from the atom. Thus, in internal ...

. The CEM spectrum can be obtained either by collecting essentially all the electrons leaving the surface (integral technique), or by selecting the ones in a given energy range by means of a beta ray spectrometer (differential or depth selective CEMS). This method allows the use of simple and inexpensive detecting equipment, mainly flow-type proportional detectors in which large counting rates can be obtained. This last characteristic makes possible the study of samples with the natural abundance of the Mössbauer

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...

. The information furnished by the integral measurements can be increased by using various angles of incidence or by depositing thin layers of inert material on the sample.

Theory

In the energy range used in CEMS, the incident radiation can interact with the absorber through two kinds of processes: (a) conventional interactions –

photoelectric The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid sta ...

and Compton effects, and (b) nuclear resonant absorption –

Mössbauer effect The Mössbauer effect, or recoilless nuclear resonance fluorescence, is a physical phenomenon discovered by Rudolf Mössbauer in 1958. It involves the resonant and recoil-free emission and absorption of gamma radiation by atomic nuclei bound in a ...

. Due to conventional interactions the beam is attenuated and electrons are emitted from the sample. The nuclear de-excitation following the resonant absorption takes place by emission of either a

gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...

or an internal conversion (IC) electron. In the latter case, the atom is left in an ‘excited’ state with a hole in an inner shell; the energy excess is given away with emission of

Auger electrons The Auger effect or Auger−Meitner effect is a physical phenomenon in which the filling of an inner-shell vacancy of an atom is accompanied by the emission of an electron from the same atom. When a core electron is removed, leaving a vacancy, a ...

and/or X-rays. Thus, the electrons emitted from the sample as a consequence of the Mössbauer absorptions are: (a) primary (IC or Auger) electrons originated in the de-excitations of the nuclei excited by the incident beam, and (b) secondary electrons originated by conventional interactions of photons (or resonant absorption of gamma rays) emitted after resonant absorptions.

References

Nuclear Instruments and Methods in Physics Research Bl (1984) 70–84
Mössbauer spectroscopy Instrumental analysis {{spectroscopy-stub