Minimizing backscattering

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...

Because instrument volume and experiment time must both be minimized for a planetary Mossbauer spectrometer, it is desirable in backscatter geometry to illuminate as much of the sample as possible with source radiation. However, this... 

Cosine smearing. Because instrument volume and experiment time must both be minimized for a planetary Mossbauer spectrometer, it is desirable in backscatter geometry to illuminate as much of the sample as possible with source radiation. However, this requirement at some point compromises the quality of the Mossbauer spectrum because of an effect known as cosine smearing [327, 348, 349] (see also Sects. 3.1.8 and 3.3). The effect on the Mossbauer spectrum is to increase the linewidth of Mossbauer peaks (which lowers the resolution) and shift their centers outward (affects the values of Mossbauer parameters). Therefore, the diameter of the source y-ray beam incident on the sample, which is determined by a... 

Backscattered electrons, however, do give some elemental information about the sample because they are more energetic than secondary electrons and escape from farther within the sample [45,46], On the molecular level, the electron beam can interact with the nucleus of an atom and be scattered with minimal loss of energy. These incident electrons may be scattered more than once and then ejected from the sample as backscattered electrons. The back-scattered electrons originate from a greater depth within the sample and are... 

Bubble-size control was also critical. The intensity of scattering by nonresonant gas bubbles is proportional to the sixth power of the radius of the bubble. Hence, the larger the bubble, the better the scattering intensity. However, the acceptable upper size limit for in vivo administration is determined by the need for bubbles to cross capillary beds. Bubbles larger than 6-8 pm should be avoided as they are trapped in the lung capillaries. The current accepted sizes are in 1-7 pm, preferably around 3 pm, with as narrow a size distribution as possible. Bubble shell material needs to be biodegradable. Soft shells are generally preferable, as they minimally impede US backscatter. 

Calibration of the intensities of the radiation flelds is traceable to the NIST. The ionization chambers and electrometers used by the service laboratories to quantify the intensity of the radiation fields must be calibrated by the NIST or an accredited secondary standards laboratory. The intensity of the field is assessed in terms of air kerma or exposure (free-in-air), with the field collimated to minimize unwanted scatter. Conversion coefficients relate the air kerma or exposure (free-in-air) to the dose equivalent at a specified depth in a material of specified geometry and composition when the material is placed in the radiation field. The conversion coefficients vary as a function of photon energy, angle of incidence, and size and shape of backscatter mediiun. 

Thick or bulky specimens are commonly used in scanning electron microscopy. However, as shown in Fig. 4.4.2, there are two disadvantages in using such thick specimens (1) a low spatial resolution, and (2) interferences from the effects of backscattering, absorption and fluorescence (Russ 1974). The poor spatial resolution can be remedied by using thin specimens (Fig. 4.4.2). The use of cross-sections of 0.5thickness, for example, would permit analysis of the compound middle lamella in cell walls as a separate entity (Saka and Thomas 1982). The second problem can also be minimized or eliminated by using thin specimens (Russ 1974, Saka and Thomas 1982). 

Rutherford Backscattering Spectroscopy (RBS) is an established technique for analysis of inorganic materials. Recently, several applications of RBS on polymer films have been reported however, the effect of ion beams on these surfaces has not been well documented. RBS has been used to determine fluorine distribution in polymers. Since ion beam irradiation of polymers can induce chemical changes, instrumental parameters need to be optimized to minimize damage. 

Glancing angle (a=80 ) Rutherford backscattering spectrometry (RBS) with a 2.0 MeV He" beam was used to obtain the depth-resolved elemental composition of the films. In order to obtain sufficient counting statistics, and at the same time minimize beam-induced damage, each spectrum was accumulated from 20 different spots on the sample. The He ion beam dose was kept below 1 C/cm per spot. Spectral simulation was performed using the computer code RUMP (14). 

Another method for minimizing the proximity effect involves changing the electron beam acceleration potential from the 10 to 20 KV range that is typically used in direct write applications. L. D. Jackel and co-workers have proposed that increasing the acceleration potential to 100 KV will minimize the need for proximity corrections as the backscattered electrons will be averaged over a larger area (2). While this approach may reduce the effect of backscattered electrons, it will also significantly reduce the resist sensitivity since a smaller number of electrons will be deposited in the resist at 100 KV. 

During the April 20th WCR flight experiments, a high-pass filter was placed in front of the AOL fluorosensor. As mentioned earlier, the filter >X as used to remove the on-frequency 532-nm backscatter return from the YAG lasers. The effect of the filter on the spectral response signal from the 532-nm laser was minimal except in the 585-nm spectral region where some minor roll-off distortion occurs in the phycoerythrin fluorescence band. The water Raman line (650 nm) and chlorophyll a band (685 nm) are equally suppressed by a small constant amount. 

Another technique (Fig. 21-13) utilizes an optical system which minimizes the optical path into and out of the sample, including the use of backscatter optics, a moving cell assembly, or setups with the maximum incident beam intensity located at the interface of the suspension to the optical window (Trainer, Freud, and Weiss, Pittsburg Conference, Analytical and Applied Spectroscopy, Symp. Particle Size Analysis, March 1990 upcoming ISO 22412, Particle Size Analysis— Dynamic Light Scattering). 

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