Constant-velocity Mossbauer

Figure 5.11 A constant velocity Mossbauer experiment reveals the kinetics of the denitridation of an iron nitride in different gases at 525 K. The negative part of the time scale gives the transmission of the most intense peak of the nitride at time zero the gas atmosphere is changed to the desired gas. Denitridation occurs relatively fast in H2, but is retarded by CO, whereas the nitride is stable in an inert gas such as helium (from Hummel etal. [33]).

Even more interesting is a new spectrometer that has been built with a novel moving system that synchronizes the movement of the radioactive source with that of the detector (206). This system allows the accumulation of Mossbauer spectra either in constant acceleration or constant velocity modes, therefore giving better signal-to-noise ratios and also reducing the time necessary for accumulation of a spectrum. Narrower line widths were also obtained with this equipment design. 

One of the more difficult experimental aspects of Mossbauer spectroscopy is the accurate determination of the absolute velocity of the drive. The calibration is comparatively easy for constant-velocity instruments, but most spectrometers now use constant-acceleration drives. The least expensive method, and therefore that commonly used, is to utilise the spectrum of a compound which has been calibrated as a reference. Unfortunately, suitable international standards and criteria for calibration have yet to be decided. As a result, major discrepancies sometimes appear in the results from different laboratories. The problem is accentuated by having figures quoted with respect to several different standards, necessitating conversion of data before comparison can be made. However, calibration of data from an arbitrary standard spectrum will at least give self-consistency within each laboratory. 

The Mossbauer measurement requires the generation of a precise, controllable relative motion between the source and the absorber. A large variety of drive systems has been developed. The majority of drives work on electromechanical, mechanical, hydrauHc, and piezoelectric principle. The spectrometers can be classified into constant-velodty spectrometers and velocity-sweep spectrometers. The mechanical drives, hke a lead screw or a cam, move with constant velocity. They have advantages for the thermal scan method and because their absolute velocity calibration is straightforward. The velocity-sweep spectrometers are usually of electromechanical nature (like loudspeaker-type transducers) and normally used in conjunction with a multichannel analyzer. The most commonly used M(t) functions are rectangular (constant velocity), triangular (constant acceleration), trapezoidal, and sinusoidal. A typical Mossbauer spectrometer is shown schematically in O Fig. 25.24. 

In the case of the Mossbauer thermal scan method, the spectrum point belonging to zero (or a given constant) velocity is only measured at different temperatures (in the case of zero velocity without Doppler moving). By this simple technique, phase transition temperatures can be detected. 

Figure 8.47 Principle of Mossbauer spectroscopy. An electromagnetic drive system (EDS) moves the energy source (Sj towards and awayficm the absorber (A) with a constant velocity 8. Transmitted radiation is measured by the proportional counter (Z). lead collimators (C) dejme beam geometry.

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)...

Fig. 3.10 Variation of the spectrometer aperture as a function of the source motion for Mossbauer spectrometers operated in constant acceleration mode with triangular velocity profile, and the resulting nonlinear baseline distortion of the unfolded raw spectra. For simplicity a point-source is adopted, in contrast to most real cases (Rib mm active spot for Co in Rh)...

Figure 12, in which the calibration constant, k, in mm./sec. channel, is shown as a function of the analyzer address. In this manner the linearity of the velocity scale can be readily determined over the range of Doppler velocities of interest in most iron and tin Mossbauer experiments.]... 

Further identification of the particles is made with 57Fe Mossbauer spectroscopy. Mossbauer spectra were recorded with a conventional constant acceleration spectrometer with 57Co in Rh matrix as a y-ray source. Velocity calibration was made using a 5-pm a-Fe foil at 293 K. Figure 1.6.10 shows the Mossbauer spectra of the sample recorded at 293 K and 4.2 K. Spectra were fitted with theoretical... 

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. 

Experimentally, the most convenient way to achieve such movement is to fix the source on a constant-acceleration oscillator allowing imposition of velocities in positive and negative direction depending on whether the source moves toward or away from the absorber E — Eq (1 p/c) (Fig. 1). For convenience, Mossbauer spectrum is taken to correspond to the plot of the intensity of the transmitted or diffused X-rays as a function of the Doppler velocity v instead of the corresponding energy. 

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... 

Constant acceleration spectra were obtained with an Austin Science Associates, Inc. S-600 Mossbauer spectrometer equipped with an electromagnetic Doppler velocity motor. The source was 50 m Ci of Co diffused into a palladium matrix, and it was obtained from New England Nuclear, Inc. The pulses from the proportional counter detector (Reuter Stokes) were amplified, shaped and gated using Austin Science Associates electronics. These shaped pulses were then sent to a Tracor Northern NS-900 multichannel analyzer. The MCA was interfaced directly to a PDP-11 minicomputer, greatly facilitating data storage and analysis. 

The Mossbauer measurements makes it possible to study a variety of interesting effects that may be brought about by the introduction of impurity in the lattice as also the modifications brought about by imperfection in the crystal lattice, since the nuclei must be bound in a crystal for this study of resonant emission (or absorption) of y-rays. Three dynamical quantities of interest, which are possible in the Mossbauer studies, are (i) Zero-phonon absorption cross-section giving the Lamb-Mossbauer factor, (ii) One-phonon absorption cross section yielding the time information as well as the information of the localized modes and through this the information on the force constant between impurity and the host atom, (iii) the second order Doppler effect yielding information about the mean square velocity of the Mossbauer probe. 

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