Accelerator modes

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)...

Fig. 11. MBssbauer spectrum in the constant-acceleration mode. Zero velocity is with respect to 57Co in copper source. Figure according to Boudart el til. (215).

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. 

Despite such intensive studies on MPV reduction chemistry, the corresponding alkylation, i.e., MPV alkylation had never been realized, mainly because of the inertness of alkyl transfer [C] compared with the facile hydride transfer [B] in the MPV reduction, until Maruoka presented the first example of MPV alkynylations for various aldehydes [34]. This truly represents a non-organometallic way of effecting carbonyl alkylation of aldehydes. The success of the approach relies heavily on the discovery of a ligand-accelerated mode for the MPV alkynylations, which has a beneficial effect on the rate of alkynyl transfer. 

Fe Mossbauer spectra were taken in an in situ cell at 300 K. A Co/Cr source was used in constant acceleration mode. The isomer shifts are relative to metallic a-Fe. The spectra of the as prepared as well as the pretreated (evacuation/623 K/N2 or eva-cuation/623 K/N2/H2, duration of each step 1 hour) samples were registered. 

S7Fe Mossbauer spectra were recorded in constant acceleration mode. Lorentzian line shape was presumed for decomposition of separate lines, and no positional parameters were constrained in the fits. The accuracy of positional data is ca. +- 0.03 mm/s. The isomer shift values are related to metallic a-iron. 

In this chapter we considered only a small Hamiltonian system whose Poincare map is the standard map defined on the unit square. It is interesting to consider Hamiltonian systems in a large phase space in which diffusion appears. Specifically, we are interested how the accelerator mode, which causes the anomalous diffusion in the standard map, affects the breaking of the adiabatic invariant. We will continue this study in a forthcoming article [21]. 

For specific values of K, one has the coexistence of many accelerator modes (i.e., ballistic trajectories), and one observes anomalous diffusion in the... 

In addition some of the samples were studied by Fe-57 Mossbauer spectroscopy. The spectrometer was an Apple Ile-controlled Ranger Scientific MS-900 operated in constant acceleration mode, using Co-57 in a matrix of rhodium as source. Spectral data were accumulated in 1024 channels. 

Mossbauer spectra have been recorded in a conventional transmission geometry in the constant acceleration mode. The temperature of the sample could be varied from 4.2 K up to room tenq>era-ture by means of a liquid helium cryostat, the source (57co in Rh) being kept at room temperature. The Mossbauer experiments were carried out in the "Laboratoire d Interactions Hyperfines at the CEN-Grenoble. 

The curvature/strain modes method hold the promise of detecting damage from the processing of experimental data without a structural model. The premise of the method is that damage in thin-wall structures induces local discontinuities that affect strongly the curvature of the flexural modes of the structure [71]. The curvamre of the modes (i.e. the curvature modes) can be determined through space-wise double differentiation of the measured displacement/velocity/acceleration modes. They can be also measured directly with surface mounted strain gauges. 

The parameters of the deterioration process (A)),>o can change at time To At To the mean deterioration rate abruptly increases from a nominal value to an accelerated time dependent rate at time To- The first mode corresponds to a nominal mode denoted by M and the accelerated mode is denoted by M2. 

Conversion electron Mossbauer spectra (CEMS) of the samples were measured at 295 K with the aid of the same Haider Mossbauer spectrometer capable of operating in conventional constant acceleration mode using a backscatter-type gas flow detector. 

We will present the Mossbauer data to show its usefulness in monitoring various stages of the process. Transmission Mossbauer spectroscopy was carried out on powder samples at room temperature with a Co (Rh) source in constant acceleration mode using a thin iron foil for calibration. 

Environmental test rooms Environmental test rooms which permit various combinations of temperature, fogging, humidity levels, and shower effects to simulate rain can reproduce in an accelerated mode many of the factors present in an actual exposure. For the most part, these tests use small cut or machined specimens, but assemblies of simulated components and complete systems can also be tested as shown in Fig. 11.35. 

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