Basic Mössbauer Spectrometer

Figure 4.2 shows a schematic diagram of a simple Mössbauer spectrometer. The source velocity is controlled by a transducer which is oscillated with constant acceleration. A waveform generator sends a reference waveform (triangular in the spectrometers at Liverpool) to the drive amplifier, via a Digital to Analogue Converter. This signal is sent to the vibrator where it is converted to a mechanical oscillation of the drive shaft and source. A small coil within the vibrator provides a feedback signal to correct any deviations from the reference waveform.

Figure 4.2: Mössbauer spectrometer schematic.

The detector is a proportional counter containing a $ 90\%$ argon and $ 10\%$ methane gas mixture. It uses an applied bias voltage of $ -2.0$ to $ -2.5\ensuremath{\unskip\,\mathrm{keV}}$ and has a $ 65\%$ detection efficiency for $ 14.41\ensuremath{\unskip\,\mathrm{keV}}$ gamma-rays. The pulse magnitude from the detector is directly proportional to the gamma-ray energy and is sorted by a single channel analyser after amplification. This allows the selection of the Mössbauer gamma-ray from any other radiation emitted from the source.

The detector counts and source velocity are synchronised by a microprocessor system. The counts accumulate in 576 channels for one complete cycle, which contain two complete spectra: one for positive acceleration and one for negative acceleration of the source. As the acceleration is constant the time interval is equal for all velocity intervals, hence each channel records for the same amount of time. During analysis the full spectrum is folded around a center point to produce a single spectrum. This increases the number of counts (and hence gives better statistics) and flattens the background profile produced by the difference in intensity of the source radiation as the source moves relative to the absorber and detector.

Dr John Bland, 15/03/2003