In this work we describe the design and construction of a Mössbauer spectrometer based in the use of a Channel Electron Multiplier to detect the electrons emitted as a consequence of the resonant nuclear absorption of γ-Rays by ⁵⁷Fe containing samples. The main resonant signals being detected are the Fe(K), Fe(L) and Fe(M) conversion electrons as well as the Fe(KLM), Fe(KLL) and Fe(LMM) Auger electrons. The design is compact since all the components of the spectrometer are mounted into a single flange, so it can be easily installed in any conventional vacuum chamber dedicated to the study of surfaces. Its performance was tested by acquiring Mössbauer spectra from a pure iron metal sheet and from an iron metal thin film deposited onto a stainless steel plate.

En el presente trabajo se describe el diseño y construcción de un espectrómetro Mössbauer basado en el uso de un multiplicador de electrones tipo canal para detectar los electrones emitidos como consecuencia de la absorción resonante de Rayos γ por muestras que contengan el isótopo ⁵⁷Fe. Las principales señales resonantes que se detectan son las de electrones de conversión Fe(K), Fe(L) y Fe(M), así como las de electrones Auger Fe(KLM), Fe(KLL) y Fe(LMM). El diseño es compacto ya que todos los componentes del espectrómetro están montados en una sola brida, la cual puede ser fácilmente instalada en cualquier cámara de vacío convencional dedicada al estudio de superficies. Se probó su desempeño mediante la adquisición de espectros Mössbauer de una lámina de hierro metálico puro y de una película delgada de hierro metálico depositada sobre una placa de acero inoxidable.

The Mössbauer effect consists in the emission of characteristic γ-Rays photons by nuclei of a given species (source) and its resonant absorption by nuclei of the same species (absorber). Given the small width of the nuclear levels, for this resonant absorption to occur it is necessary that the nuclei do not recoil neither in the emission nor in the absorption processes, conditions that are achieved if they are bound to the structure of a solid state substance. In ⁵⁷Fe Mössbauer Spectroscopy the source typically consists of radioactive ⁵⁷Co nuclei embedded into a Rh or Pd matrix and the absorber is the iron containing solid under study. The ⁵⁷Co nuclei decay by electron capture into excited ⁵⁷Fe* nuclei that have a half life of 270 days. Upon de-excitation, these ⁵⁷Fe* nuclei emit characteristic γ-Rays with well defined energies E₁ = 122.0 keV, E₂ = 136.4 keV and E₃ = 14.4 keV. The γ-Ray photons of energy E₃ are the ones used in ⁵⁷Fe Mössbauer Spectroscopy. They are produced when the excited nuclei suffer transitions from the first excited state (I = 3/2) to the ground state (I = 1/2).

The interaction of the ⁵⁷Fe nuclei with the surrounding atoms in the particular sample under study can cause level shifts and/or splits that impede the resonant absorption of the 14.4 keV γ-Rays emitted by the source. However, the resonance condition can be recovered if the energy of the emitted γ-Rays is modulated by Doppler effect. This can be done by mounting the source on an oscillatory mechanical device or velocity transducer which moves the source to and fro with constant acceleration up to a maximum speed vM in each sense. In this way the energy of the γ-Rays is modulated continuously within the interval E₃(1 ± v_M/c) and the resonant absorption is obtained when the photon energy coincides with an allowed transition between two nuclear levels in the absorber. In each motion cycle the condition for a given resonant absorption is achieved twice; one when the source moves forward and the other when it moves backward.

Conventional Mössbauer Spectroscopy deals with γ-Rays absorption spectra, but one can also obtain Mössbauer emission spectra if one detects the γ-Rays, the X-rays or the electrons emitted from the sample as a consequence of the de-excitation of the 57Fe nuclei which have been excited by the resonant absorption. In the present work we describe the installation of an electron emission Mössbauer spectrometer in an ultra-high vacuum (UHV) chamber used for analysis of surfaces with conventional surface sensitive techniques.

It is useful to compare the performance of the Mössbauer Emission Spectrometer with that of a conventional Mössbauer Absorption Spectrometer. The absorption detection system used in conventional spectrometers consists of a simple transmission arrangement composed of the velocity transducer holding the γ-Ray source, followed by the absorber and then by a proportional counter. Absorption Mössbauer spectra are obtained by plotting the intensity of the transmitted γ-Ray signal as a function of the relative velocity between source and absorber. The spectra show an absorption baseline caused mainly by photoelectric effect in all types of atoms present in the sample region traversed by the γ-Rays, superimposed on which there are Mössbauer absorption resonances due only to the presence of ⁵⁷Fe isotopes. The statistical quality of the spectra depends mainly on the activity of the source, on the acquisition time used and on the ratio of ⁵⁷Fe isotopic mass to the total mass of the absorber. The absorption structure observed in the spectra corresponds to the interaction of the absorbing nuclei with the surrounding atoms, structure that is evaluated by the so called "hyperfine parameters" IS, QS and H. The isomer shift (IS) measures the electrostatic interaction between the absorbent nucleus and the surrounding electrons; it is characterized by a shift of the spectrum relative to that of a reference material, usually α-iron. The quadrupole splitting (QS) arises from the interaction of the nuclear quadrupole moment with the local electric field gradient. When an asymmetry in the electric charge distribution around the 57Fe nuclei produces an electric field gradient, this interaction causes a splitting of the I=3/2 nuclear state into two levels, resulting in two distinct spectral lines. The hyperfine magnetic field (H) measures the splitting caused by the interaction of the nuclear magnetic moment with the local magnetic field at the nucleus site. When this internal magnetic field is present, this interaction lifts all degeneracy between nuclear levels allowing six spectral lines to be observed. Through a fitting procedure of the experimental data one obtains the hyperfine parameters which are characteristic of the particular iron phase under study. For α-iron the Mössbauer spectrum is a sextet with the following values of hyperfine parameters: IS=0 mm/s, QS=0 mm/s and H=330 kOe.

Once the Mössbauer resonant absorption has occurred, the excited ⁵⁷Fe nuclei return rapidly to their ground states by two possible mechanisms: the re-emission of γ-Rays or the radiationless emission of atomic core electrons, a process known as Internal Conversion (Krane, 1988). The probability of de-excitation by γ-Ray re-emission is of only 9%, so most of the ⁵⁷Fe excited nuclei (91%) de-excite themselves by the emission of conversion electrons; 81% of them emit K electrons (7.3 keV kinetic energy), 9% L electrons (13.6 keV) and 1% M electrons (14.3 keV). The emission of conversion electrons leaves the atoms with a hole in a core level, so there is a further de-excitation of these atoms through the emission of either characteristic X-rays or Auger electrons. The main Auger electron signals being produced are the KLM (6.3 keV kinetic energy), the KLL (5.4 keV), the LMM (0.65 keV) and the MVV (0.047 keV). Secondary, low energy, electrons are also produced in the process. We see that there is an important amount of electronic signals that are consequence of the resonant absorption of γ-Rays which then could be used to perform Mössbauer Spectroscopy if these signals are properly detected. The spectra obtained in this fashion are emission spectra and the technique is known as Conversion Electron Mössbauer Spectroscopy (CEMS) [Nomura et al. 1996]. Superimposed to the mentioned Mössbauer  electron signals there are also non resonant electron signals contributing to the spectrum background, produced mainly by photoelectric and Compton effects due to the interaction with the sample of the γ and X-rays emitted by the source. As electrons are particles with electric charge, in general they interact with matter more strongly than γ or X-Rays do, so one expects that the electrons that escape from the sample in Conversion Electron Mössbauer Spectroscopy come from a region close enough to the sample surface that they could be used in the characterization of surfaces, interfaces and thin films.

The electron detector used, shown in Figure 3, was a DeTech 512 Channel-electron-multiplier, operated in the pulse counting mode although it was originally designed to operate in the current mode as part of an Auger Electron Spectrometer. This device has a 19 mm input aperture and requires electrical connections for high voltage bias, input cone bias and signal collector (BURLE, 2009). This electron multiplier is bakeable and normally works under high or ultra-high vacuum conditions, giving a maximum gain of 10⁶. The circuit diagram of Figure 4 shows the connections used for its operation. The graph of Figure 5 shows its detection efficiency as a function of the kinetic energy of the electrons impinging on the surface of its entrance cone; it acts as a high-pass filter whose efficiency drops abruptly for kinetic energies below 500 eV.

The radioactive source used was a 10 mm diameter disc of ⁵⁷Co in a Rh matrix which was placed at the end of an extension rod, connected to the velocity transducer and inserted into the lead-lined tube at approximately 15 mm from the collimator aperture.

Given the geometry of our Mössbauer Emission Spectrometer, it is important to know the area irradiated on the sample position in order to determine the optimum size of the sample to be used. This was determined using an X-Ray dental film to measure the exposed area. Figure 6 shows a picture of the exposed film in which one observes a dark shadow corresponding to the irradiated area; it is elliptical since the film was placed on the sample holder, tilted 45º. The clear disk superposed on the dark shadow corresponds to the 25 mm diameter circular sample used in our measurements, placed on top of the exposed film. One observes that the whole sample area is irradiated by the γ-Rays.

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Figure 6. Image of an X-Ray film exposed to the collimated γ radiation, with a 25 mm diameter sample superimposed on it

The effect of the Plexiglas filter can be appreciated in Figure 7, where we show energy dispersive spectra of the source obtained both without the filter (7a) and with the filter (7b) covering it. Besides the 14.4 keV γ-Ray signal, we observe in Figure 7a the presence of the 6.40 keV Fe Kα , the 7.06 keV Fe Kβ and the 20.21 keV Rh Kα X-Ray signals. The Fe K signal is the result of the de-excitation of the iron atoms produced in the 57Co electron capture process, while the Rh K signal is the result of fluorescence caused by the high energy γ-Rays on the Rh matrix. With the Plexiglas filter, we see in Figure 7b that the Fe K signal disappears without reducing much the 14.4 keV γ-Ray signal. According to these spectra, the use of the filter reduces the intensity of the 14.4 keV γ-Rays to 72% of its original value. However, in this case 86% of the photons that irradiate the sample are the 14.4 keV γ-Ray photons and only 14% are X-Ray photons; while without the filter, 60% of photons that irradiate the sample are the 14.4 keV γ-Ray photons and 40% are the X-Rays photons, which contribute to the spectrum noise by generating photoelectrons. The use of the Plexiglas filter reduces the non-resonant electron signals by eliminating the Fe K X-Rays generated by the radioactive source.

Figure 7. Energy dispersive emission spectra of the ⁵⁷Co(Rh) radioactive source. a) without the Plexiglas filter, b) with the Plexiglas filter

Figure 8 compares an emission conversion electron Mössbauer spectrum from an iron plate with a conventional absorption Mössbauer spectrum from a 6.4 μm thick iron foil. The acquisition time was about 24 hours for both spectra, although the source activity was 30 mCi in the emission experiment and 6 mCi in the absorption one. As expected, both spectra consist of a sextet whose hyperfine parameters agree with those corresponding to α-iron. The signal to noise ratio is clearly smaller in the CEMS spectrum despite the larger source activity that was used. In the case of the absorption spectrum the sample region generating the Mössbauer signals corresponds to the whole thickness, i.e. 6.4 μm. In the case of the emission spectrum the mean-escape-depth of the signals from metallic iron detected by our spectrometer has been calculated to correspond to approximately only 80 nm, taking into account both the conversion electrons and the Auger electrons, as well as the coincidence effects in their detection (Moutinho et al. 2010).

An ICEMS Mössbauer spectrometer for surface studies in iron containing samples was designed and successfully constructed. It is based in the use of a channel electron multiplier for the detection of the integrated conversion and Auger electron signals emitted by the 57Fe isotopes as a consequence of the resonant nuclear absorption of γ-Rays. As this spectrometer is contained within a single flange it can be easily installed in any conventional ultra- high vacuum chamber dedicated to surface physics studies, enhancing in this way its analytical capability.

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