Conversion Electron Mössbauer Spectroscopy utilises the emission of conversion electrons from the decay of the state in the absorber to record the spectrum. This is useful for samples with thick substrates which would block transmission of gamma-rays or for studies of the surfaces of samples rather than the bulk. As the ratio of conversion electrons to gamma-rays emitted by the Mössbauer event in Fe is 8.21 the counting efficiency of CEMS is much greater than the transmission method.
A schematic diagram of the CEMS spectrometer as used in this thesis is shown in Figure 4.3. As the emitted electrons are of a low energy they would be attenuated by a detector window hence the sample is inserted directly into the detector.
The source irradiates the sample through the window. An anode wire is maintained at a high voltage ( ) directly in front of the sample. Electrons emitted from the sample are accelerated towards the anode, ionising atoms in the counting gas, producing an avalanche effect which amplifies the signal from the original emitted electron. This electronic pulse is detected and recorded as for Transmisson Mössbauer Spectroscopy.
The counting gas, a helium and methane mix, is constantly replenished by a small flow from a cannister. This flushes out any contaminants from the atmosphere, such as oxygen, which would reduce the counting efficiency. The methane acts as a quenching gas, preventing helium ions reaching the cathode whilst maintaining a high gain. The counting gas mixture of helium/methane has negligible reaction with the incident gamma-rays - making it transparent to the incident radiation - but when electrons are emitted from the sample the normal proportional counter effect described above takes place. The perspex window attenuates the x-rays from the source which would otherwise produce a large background signal from non-resonantly emitted electrons, but only attenuates the gamma-ray by a small amount.
Measurements with this system are restricted to room temperature due to the bulk of the detector itself and the condensation of the methane gas at low temperatures. A magnetic field of up to can be applied in the plane of the sample. This is supplied by placing the CEMS detector between the soft iron poles of an electromagnet. Fields greater than are prohibited due to the curved trajectory of the emitted electrons in a magnetic field producing a reduced counting efficiency.
Samples are mounted upon the aluminium faceplate of the sample mount using a thin layer of vacuum grease. The samples are then covered with a sample shroud constructed from aluminium foil to present a smooth, continuous face to the anode wire. This surface is cleaner than the sample mount which can become coated with contaminants stuck to the vacuum grease and prevents electric fields building at sharp corners or edges on the sample which may produce electron discharges towards the anode and hence increase the background noise.
Dr John Bland, 15/03/2003