Backscattering measurement geometry

Because of the complexity of sample preparation, backscatter measurement geometry (see Fig. 3.19) is the choice for an in situ planetary Mossbauer instrument [36, 47 9]. No sample preparation is required, because the instmment is simply presented to the sample for analysis. On MER, the MIMOS II SH is mounted on a robotic arm that places it in physical contact with the analysis target (e.g., rock or soil) [36, 37]. 

Fig. 3.19 Schematic illustration of the measurement geometry for Mossbauer spectrometers. In transmission geometry, the absorber (sample) is between the nuclear source of 14.4 keV y-rays (normally Co/Rh) and the detector. The peaks are negative features and the absorber should be thin with respect to absorption of the y-rays to minimize nonlinear effects. In emission (backscatter) Mossbauer spectroscopy, the radiation source and detector are on the same side of the sample. The peaks are positive features, corresponding to recoilless emission of 14.4 keV y-rays and conversion X-rays and electrons. For both measurement geometries Mossbauer spectra are counts per channel as a function of the Doppler velocity (normally in units of mm s relative to the mid-point of the spectrum of a-Fe in the case of Fe Mossbauer spectroscopy). MIMOS II operates in backscattering geometry circle), but the internal reference channel works in transmission mode...

Sirotyuk (ref. 25) found that the complete removal of solid particles from a sample of water increased the tensile strength by at most 30 percent, indicating that most of the gas nuclei present in high purity water are not associated with solid particles. Bernd (ref. 15,16) observed that gas phases stabilized in crevices are not usually truly stable, but instead tend to dissolve slowly. This instability is due to imperfections in the geometry of the liquid/gas interface, which is almost never exactly flat (ref. 114). Medwin (ref. 31,32) attributed the excess ultrasonic attenuation and backscatter measured in his ocean experiments to free microbubbles rather than to particulate bodies this distinction was based on the fact that marine microbubbles in resonance, but prior to ultrasonic cavitation (ref. 4), have acoustical scattering and absorption cross sections that are several orders of magnitude greater than those of particulate bodies (see Section 1.1.2). 

Advances in two other areas have been introduced in the past five years. One is the use of the backscattering experimental geometry [35,88]. From a theoretical standpoint, backscattering ICP ROA offers approximately eight times higher ROA intensities compared to the standard depolarized right angle measurement. 

Figure 6 Raman measurement geometries. The solid line represents laser light and the dashed line represents Raman scattered light from the sample. Laser light and Raman light overlap between the beam splitter and the sample in 180° backscattering.

Raman spectroscopy is a very convenient technique for the identification of crystalline or molecular phases, for obtaining structural information on noncrystalline solids, for identifying molecular species in aqueous solutions, and for characterizing solid—liquid interfaces. Backscattering geometries, especially with microfocus instruments, allow films, coatings, and surfaces to be easily measured. Ambient atmospheres can be used and no special sample preparation is needed. 

Fig. 7.73 Mossbauer spectra of Pt (99 keV) in ferromagnetic alloys measured in backscattering geometry with the scatterer kept at 29K. The count rate at infinite velocity is normalized to 1 (from [330])...

Samples were characterized by X-ray diffraction, magnetic susceptibility and chemical analysis with some results summarized in Table 1. The electrical resistivity measurements were made down to 80 K using a four-probe method. Raman scattering experiments used the excitation line A = 514.5 nm of an Ar+ laser in a quasi-backscattering geometry. The laser power of 5 mW was focused to a 0.1 mm diameter spot on the (010) surface. The averaged laser power density amounts to 6 105 W/m2 which is much less compared to earlier Raman studies in manganites [12-15],... 

It can be seen from these equations that it is clearly preferable to use the 180 ° configuration for the measurement of ROA, since the CID in backscattering is four times more intense. Together with a two-fold increase in Raman intensity the improved signal-to-noise ratio allows for an eight-fold reduction of the measurement time. The first measurements with this geometry have been published by Hecht et al. (1989). 

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