Absorption, cross section for

Commercial-grade zirconium contains from 1 to 3% hafnium. Zirconium has a low absorption cross section for neutrons, and is therefore used for nuclear energy applications, such as for... 

Because the element not only has a good absorption cross section for thermal neutrons (almost 600 times that of zirconium), but also excellent mechanical properties and is extremely corrosion-resistant, hafnium is used for reactor control rods. Such rods are used in nuclear submarines. 

Boron trifluoride is also employed in nuclear technology by uti1i2ing several nuclear characteristics of the boron atom. Of the two isotopes, B and B, only B has a significant absorption cross section for thermal neutrons. It is used in " BF as a neutron-absorbing medium in proportional neutron counters and for controlling nuclear reactors (qv). Some of the complexes of trifluoroborane have been used for the separation of the boron isotopes and the enrichment of B as (84). 

Heavy water (p. 39) is now manufactured on the multikilotonne scale for use both as a coolant and neutron-moderator in nuclear reactors its absorption cross-section for neutrons is much less than for normal water [Pg.623]

Figure 8.2e shows the dependence of the fluorescence intensity on the excitation power of the NIR light for the microcrystals measured with a 20x objective. In this plot, both axes are given in logarithmic scales. The slope of the dependence for the perylene crystal is 2.8, indicating that three-photon absorption is responsible for the florescence. On the other hand, slopes for the perylene and anthracene crystals are 3.9 for anthracene and 4.3 for pyrene, respectively. In these cases, four-photon absorption resulted in the formation of emissive excited states in the crystals. These orders of the multiphoton absorption are consistent with the absorption-band edges for each crystal. The four-photon absorption cross section for the anthracene crystal was estimated to be 4.0 x 10 cm s photons by comparing the four-photon induced fluorescence intensity of the crystal with the two-photon induced fluorescence intensity of the reference system (see ref. [3] for more detailed information). 

Fig. 14. Plot of the quantum efficiency for the formation of C>2(v < 26) as a function of wavelength. The value at 193 nm is taken from Stranges et aJ.44 A 10% branching ratio into the triplet channel is assumed. The absorption cross-section for ozone is plotted as the dashed curve on the right-hand axis.

Neutron diffraction on Cd compounds is not feasible, as one of the Cd nuclides (113Cd) has an extremely high absorption cross-section for neutrons, yielding an average [Pg.1255]

The dopant concentration depends both on the isotopic abundance and on the neutron absorption cross-section. For natural Ge, a unit neutron flux produces a Ga and As concentration of [14,15] ... 

Uses. Low melting solders, low melting alloys and metallurgical additives. Bismuth is a metal with some unusual properties like Ge and Ga its volume increases in solidification. It is the most diamagnetic metal, its alloys show large thermoelectric effect with the exception of Be has the lowest absorption cross-section for thermal neutrons. 

The extraction of deuterium from natural water feed forms an excellent case study of the application of large scale distillation and exchange distillation to isotope separation. The principal historical demand for deuterium has been as heavy water, D20, for use in certain nuclear reactors. Deuterium is an excellent neutron moderator, and more importantly, it has a low absorption cross section for slow neutrons. Therefore a reactor moderated and cooled with D20 can be fueled with natural uranium thus avoiding the problems of uranium isotope enrichment. This was the... 

As a result of the second step process, the population of the emitting state 2 (N3 0, due to the feeding of level 2 by level 3), increases with the population of level 1, with ctu (the absorption cross section for the transition 1 3), and... 

The direct detection of the S <- Sj absorption in organic compounds has so far been achieved by a nanosecond or picosecond laser flash photolysis method. The general features of transient absorption spectra of metalloporphyrins actually suggest the presence of strong absorption bands in visible or ultraviolet region (38-40). However, as the transient absorption of the state often overlaps with that of ground state depletion, it is usually difficult to evaluate the absolute absorption cross sections for the transition by... 

FIGURE 4-19 Absorption cross sections for N205 at room temperature (adapted from Harwood et al., 1998). 

Table 4.24 summarizes absorption cross sections for HCHO averaged over 2.5-nm intervals centered at the wavelengths shown. 

Table 4.26 summarizes the absorption cross sections for the relatively unstructured absorptions by other small aldehydes and ketones (Martinez et al., 1992). For acetaldehyde, there are three possible decomposi-... 

Figure 4.39 shows the absorption spectrum of HOC1 and Table 4.33 summarizes the absorption cross sections, for which most measurements are in relatively good agreement. The major products are OH + Cl, with a quantum yield of 1 above 290 nm (e.g., Vogt and Schindler, 1992 Schindler et al., 1997) ... 

Figures 4.44 and 4.45 show absorption spectra of some simple chlorofluoro-methanes and ethanes, respectively (Hubrich and Stuhl, 1980). Tables 4.37 and 4.38 give the recommended absorption cross sections for some of these compounds (DeMore et al., 1997). None of these compounds absorb in the actinic region above 290 nm, but do around 180-200 nm, wavelengths only found in the stratosphere. As discussed in Chapter 12, it is photolysis at these short wavelengths to generate atomic chlorine that is responsible, along with bromine and perhaps in some cases, iodine atoms, for the chain destruction of stratospheric ozone.

Finally, alkyl iodides, some of which have natural sources, are of interest for both the troposphere and stratosphere. Figure 4.46 shows the absorption spectra of some simple alkyl iodides and Table 4.42 the absorption cross sections for CH3I (Roehl et al., 1997). 

Burkholder and McKeen (1997) report, based on their measurements of the absorption cross sections for... 

Ganske, J. A., H. N. Berko, and B. J. Finlayson-Pitts, Absorption Cross Sections for Gaseous C1N02 and Cl2 at 298 K Potential Organic Oxidant Source in the Marine Troposphere, J. Geophys. Res., 97, 7651-7656 (1992). 

Hynes, A. J., E. A. Kenyon, A. J. Pounds, and P. H. Winer, Temperature Dependent Absorption Cross-Sections for Acetone and n-Butanone—Implications for Atmospheric Lifetimes, Spec-trochim. Acta, 48A, 1235-1242 (1992). 

In this case, a is the differential optical absorption cross section for the absorption band. In practice, of course, there are many different absorbers, t, present at different concentrations and absorbing at different wavelengths over the path length L. 

A major advantage of this approach is that the fundamental spectroscopic parameters for OH, including the absorption cross sections for various transitions, are well known (e.g., see Mount, 1992 Dorn et al., 1995b), so that absolute concentrations of OH can be calculated based solely on the absorption spectra. Another major advantage is that the laser beams are expanded so that generation of OH in the beam itself by photolysis of ozone is not the problem that it has been in LIF measurements (vide infra). 

As is the case for LIF, calibration to obtain absolute concentrations is a challenge. In the instrument shown in Fig. 11.45, a calibration source based on the photolysis of water at 185 nm is installed in the inlet. From the absorption cross section of HzO gas at 185 nm, its concentration, the light intensity, and the sample flow rate, the concentration of OH generated by the photolysis can be calculated. However, not only is there significant uncertainty in the absorption cross section for HzO at 185 nm (e.g., see Lazendorf et al., 1997 Hofzumahaus et al., 1997, 1998 and Tanner et al., 1997), but the measured calibration factor was highly variable from day to day, by as much as a factor of two (Tanner et al., 1997). 

Similarly, the absorption cross sections for the reservoir species BrONQ2 are much larger than those of... 

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