Mossbauer drives

Mossbauer spectra are usually recorded in transmission geometry, whereby the sample, representing the absorber, contains the stable Mossbauer isotope, i.e., it is not radioactive. A scheme of a typical spectrometer setup is depicted in Fig. 3.1. The radioactive Mossbauer source is attached to the electro-mechanical velocity transducer, or Mossbauer drive, which is moved in a controlled manner for the modulation of the emitted y-radiation by the Doppler effect. The Mossbauer drive is powered by the electronic drive control unit according to a reference voltage (Fr), provided by the digital function generator. Most Mossbauer spectrometers are operated in constant-acceleration mode, in which the drive velocity is linearly swept up and down, either in a saw-tooth or in a triangular mode. In either case. 

Fig. 3.2 Triangular velocity reference signal top) and drive error signal bottom) of a Mossbauer drive operating in constant acceleration mode. The error signal is taken from the monitor output F of the drive control unit (see Fig. 3.1). Usually it is internally amplified by a factor of 100. Here, the deviations, including hum, are at the 2%o level of the reference. The peaks at the turning points of the triangle are due to ringing of the mechanical component, induced by the sudden change in acceleration (there should be no resonance line at the extremes of the velocity range)...

A memory sweep is initialized when the MCA receives a start pulse from the function generator at the trigger input (7). The start pulse is synchronized with the sweep of the reference voltage (Vr) for the Mossbauer drive. It opens the first MCA channel when the source velocity passes through the minimum (cf. Fig. 3.3). After this start trigger, a train of 512 channel advance pulses follows with exact delay times of about 100 ps each. On receiving such a channel advance pulse, the... 

Since the actual motion of the Mossbauer drive, as for any frequency transmission system, can show phase shifts relative to the reference signal, the ideal folding point (FP) of the raw data in terms of channel numbers may be displaced from the center at channel number (N — l)/2 (= 255.5 in the example seen earlier). The folding routine must take this into account. Phase shift and FP depend on the settings of the feedback loop in the drive control unit. Therefore, any change of the spectrometer velocity tuning requires the recording of a new calibration spectrum. 

Interestingly, the correct polarity of the Mossbauer drive can be checked by using the isomer shift of oc-iron with respect to the materials in Table 3.1. After folding of the raw data, the center of the calibration spectrum without further correction must be at —0.12 mm s relative to the Co/Rh source material. 

Fig. 3.15 Left External view of the MIMOS II sensor head (SH) with pyramid structure and contact ring assembly In front of the Instrument detector system. The diameter of the one Euro coin is 23 mm the outer diameter of the contact-ring is 30 mm, the inner diameter is 16 mm defining the field of view of the Instrument. Right. Mimos II SH (without contact plate assembly) with dust cover taken off to show the SH Interior. At the front, the end of the cylindrical collimator (with 4.5 mm diameter bore hole) Is surrounded by the four SI-PIN detectors that detect the radiation re-emltted by the sample. The metal case of the upper detector is opened to show its associated electronics. The electronics for all four detectors Is the same. The Mossbauer drive is inside (in the center) of this arrangement (see also Fig. 3.16), and the reference channel is located on the back side In the metal box shown In the photograph...

The Mossbauer transmission spectra were recorded in the constant acceleration mode with an Elscint Mossbauer drive unit and a model MFG 3A Elscint function generator, an MVT-3 linear velocity transducer and an MD-3 transducer driving unit, y-ray detection was done with a Reuter-Stokes Kv-CH4 proportional counter driven by an Ortec 401A/456 high voltage power supply. Voltage pulses were introduced into an Ortec 142 PC preamplifier and an Ortec 571 spectroscopy amplifier. Data were collected on a Tracor-Northern NS-701A multichannel analyzer. The data were later analyzed on an IBM 360/370 computer. 

Figure 1 Mossbauer spectrometer with velocity calibrator Mossbauer drive and detector. The sample is placed between drive and detector. For measurements at cryogenic temperatures, the sample is moimted in a cryostat equipped with Mylar or beryllium windows. (Reproduced with permission Wissel Gmbh)...

Most Mossbauer spectrometers use triangular velocity profiles. Saw-tooth motion induces excessive ringing of the drive, caused by extreme acceleration during fast fly-back of the drive rod. Sinusoidal operation at the eigen frequency of the vibrating system is also found occasionally and... 

Fe which have full width 2r at 0.2 mm s . Other isotopes are less demanding, e.g., Au, for which the lines are ten times wider. Most spectrometers are equipped with electromechanical Mossbauer velocity transducers of the loudspeaker type. This technique is suitable for velocity variations ranging from less than 1 mm s full scale up to several cm s and covers the whole reach of hyperfine splitting for most of the common isotopes. Kalvius, Kankeleit, Cranshaw, and others [1-5] have been pioneers in the field, who laid foundations for the development of high-precision drives with feedback amplifiers for proper linear velocity scales with high stability and low hum. Other techniques for Doppler modulation have been developed for isotopes with extremely narrow hyperfine lines, e.g., Zn. For such isotopes, piezoelectric transducers are mostly used [6, 7], more details of which are found in Sect. 7.2.1. 

The polarity of both the drive coil and pick-up coil of the Mossbauer motor can be changed together without changing the performance. 

The absolute velocity imparted to the drive shaft can be determined either directly or indirectly (30, 32, 87, 88). In the latter technique, the spectrum of a compound with well-established Mossbauer parameters is collected, and to the positions in the spectrum where resonances appear, specific absolute velocities can be assigned. The velocities at other positions in the spectrum are then inferred by interpolation between these known velocities. This indirect calibration is then used in the interpretation of other spectra obtained with the same drive unit. Unfortunately, compounds with well-established Mossbauer parameters may not be available for the Mossbauer isotope of interest. For 57Fe, however, this is not a problem, and metallic iron foils and sodium nitroprusside are often used for calibration purposes. Thus, the 57Fe resonance may be used to calibrate the drive unit, and this unit can then be used to study other Mossbauer isotopes if the drive unit is operated under identical conditions. 

X-Ray Diffraction (XRD) patterns were recorded on a Philips HTK-KC diffractometer with a CuKa X-ray source, linked to Philips 386 computer. Thermo-Gravimetry (TG), was recorded on a Setaram TG-DTA 92 thermobalance. Diffuse Reflectance Spectra (DRS) were recorded on a Cary-5 spectrofotometer with a BaS04 integration-sphere in the UV-VIS-NIR region. Mossbauer spectra are recorded on a vertical constant acceleration drive in transmission geometry with a 28mCi Co(Rh) source. Isomer shift data are expressed relative to metallic Fe at 293 K which has an isomer shift of8 = -0.0888 mm/s relative to natural a-Fe. 

A Mossbauer experiment is performed in practice (Fig. 2 a) by mounting the radioactive source S on a velocity drive L. By applying well-known velocities v to the source one can modulate the energy of the emitted... 

Fe Mossbauer spectra were obtained using a digital constant acceleration spectrometer having a symmetrical triangular velocity drive waveform. A 10 m Ci 5 Fe in Pd source was used and all experiments were carried out at room temperature. The... 

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