Back-Scattering Spectrometer

REELS will continue to be an important surface analytical tool having special features, such as very high surface sensitivity over lateral distances of the order of a few pm and a lateral resolution that is uniquely immune from back scattered electron effects that degrade the lateral resolution of SAM, SEM and EDS. Its universal availability on all types of electron-excited Auger spectrometers is appealing. However in its high-intensity VEELS-form spectral overlap problems prevent widespread application of REELS. 

In Raman measurements [57], the 514-nm line of an Ar+ laser, the 325-nm line of a He-Cd laser, and the 244-nm line of an intracavity frequency-doubled Ar+ laser were employed. The incident laser beam was directed onto the sample surface under the back-scattering geometry, and the samples were kept at room temperature. In the 514-nm excitation, the scattered light was collected and dispersed in a SPEX 1403 double monochromator and detected with a photomultiplier. The laser output power was 300 mW. In the 325- and 244-nm excitations, the scattered light was collected with fused silica optics and was analyzed with a UV-enhanced CCD camera, using a Renishaw micro-Raman system 1000 spectrometer modified for use at 325 and 244 nm, respectively. A laser output of 10 mW was used, which resulted in an incident power at the sample of approximately 1.5 mW. The spectral resolution was approximately 2 cm k That no photoalteration of the samples occurred during the UV laser irradiation was ensured by confirming that the visible Raman spectra were unaltered after the UV Raman measurements. 

Thomas W. Hegels E. Slijkhuis S. Spurr R. and Chance K. (1998b). Detection of trace species in the troposphere using back-scatter spectra, obtained by the GOME spectrometer. Geophys. Res. Lett., 25, 1317-1320. 

Viking landers carried out Rutherford back-scattering and X-ray fluorescence (XRF) analyses of soils at two sites (Clark et al., 1982). Rocks and soils were analyzed by an alpha-proton X-ray spectrometer (APXS) on the Mars Pathfinder rover (Rieder et al., 1997) (Figure 1). Early APXS analyses (Rieder et al., 1997 McSween... 

The unpolarized Raman spectra were excited by 50 W cm (15 mW incident power) of the 514.5 nm Ar ion laser line on approximately 100 pm by 300 pm spots (Fig. 1). A monochromator was used to eliminate laser plasma lines from the spectra. Light was collected in a 45° back scattering geometry, and dispersed by a 3/4-m Spex double spectrometer with 7 cm spwctral... 

Brooker (1997) measured the Raman spectra using a Laser Raman Microprobe Renishaw and a conventional spectrometer Coderg PHO. A super-notch filter served as a monochromator in front of the entrance slit of a single grating, which in turn disperses the Raman beam onto a 400 x 600 CCD detector. The Laser Raman Microprobe was equipped with a 632.8 nm helium-neon laser of 10 mW power and a 514.5nm argon ion laser of 50 mW power with the appropriate super-notch filters. The laser beam was focused into the sample by a lens with an Olympus microscope and the back-scattered Raman light was collected by the same lens. Samples of molten salts were sealed in capillary tubes under dry nitrogen or vacuum. 

Ice particle measurements in the expansion experiment with 40% OC soot aerosol markedly differ from the 16% OC sample. Note that the optical particle spectrometer hardly detects any ice particles. Additionally, extinction signatures of ice are barely visible in the infrared spectra and diere is only a weak intensity increase of the back-scattered laser light in course of the expansion. The number concentration of ice crystals is less than 10 cm, thus < 1% of the seed aerosol particles act as deposition ice nuclei. In contrast to the 16% OC experiment, no precise critical ice saturation ratio can be specified for the 40% OC soot sample. RHi continues to increase to 190% because very little water vapour is lost on the small surface area of the scarce ice crystals. In summary, die comparison of the two expansion experiments provides first evidence that a higher fraction of organic carbon notably suppresses the ice nucleation potential of flame soot particles. 

The chemical composition of minerals was determined in 29 polished carbon-coated thin sections using a Cameca Camebax BX50 microprobe equipped with three spectrometers and a back-scattered electron detector (BSE). Operating conditions were 20 kV acceleration voltage, 8 nA (for carbonates and clay minerals) to 12 nA (for feldspars) measured beam current, and a 1-10 pm beam diameter (depending on the extent of homogeneous areas). Standards and count times were wollastonite (Ca, 10 s), orthoclase (K, 5 s), albite (Na, Si, 5 and 10 s, respectively), corundum (Al, 20 s), MgO (Mg, 10 s), MnTiOj (Mn, 10 s) and hematite (Fe, 10 s). Precision of analysis was better than 0.1 mol%. 

The chemical composition of minerals was determined in a total of 17 polished, carbon-coated thin sections. A Cameca Camebax BX50 microprobe equipped with three spectrometers and a back-scattered electron detector (BSE) was used for quan-... 

Figure 3 demonstrates the electron spectrometer part of a depth-resolved conversion electron MOssbauer spectrometer specially designed for such measurements in our laboratory (10, 11). The electron spectrometer is of the cylindrical mirror type back-scattered K conversion electrons from resonantly excited Fe nuclei are resolved by the electrostatic field between the inner and outer cylinders (cylindrical mirror analyzer) and then detected by a ceramic semiconductor detector (ceratron). The electron energy spectra taken with this spectrometer indicate that peaks of 7.3-keV K conversion electrons, 6.3-keV KLM Auger electrons, 5.6-keV KLL Auger electrons, etc., can be resolved well, with energy resolution better than 4%. 

Details of the Rutherford Back Scattering technique as applied to polyethylene have been described elsewhere (3). The IRS measurements were carried out using a Perkin Elmer 621 infrared spectrometer. A KRS-5 reflection element with dimensions 50 mm X 2 mm was used with a 45° angle of incidence. Further details can be found elsewhere (I). Most of the atomic absorption measurements were performed by the Fairfield Testing Laboratory, Fairfield, NJ. The x-ray fluorescence measurements were carried out on a GE-XRD6 spectrometer. 

In place of the diffractometer discussed in Section 2.5.3, a spectrometer is used, which allows measurement of the energy spectrum of scattered neutrons at different scattering angles. There are four main types of spectrometers in use today, the tripleaxis spectrometer, the time-of-flight spectrometer, the back-scattering spectrometer, and the spin-echo spectrometer, each of which is briefly described in the following section. 

A schematic view of a back-scattering spectrometer is shown in Figure 8.13. The incident neutrons brought in a straight guide tube are back-scattered from a monochromator crystal mounted on a velocity drive. An example of the monochromator crystal... 

Figure 8.13 Schematic view of a back-scattering spectrometer. The neutrons incident from the neutron guide are back-scattered by the monochromator mounted on a Doppler drive, deflected by a graphite crystal to the sample, scattered to the analyzers and then back-scattered again to the detectors located close to the sample. The chopper interrupts the beam and makes it possible to discriminate the neutrons scattered directly into the detectors.

Fig. 9.1 Schematic representation of a Brillouin spectrometer for measurements in three different scattering geometries back-scattering (180°), 90R scattering and 90A scattering (beam path defined by shutters and mirrors). The laser power can be modu-...

---