Low atomic weight

In summary, it is evident that the only moderators of merit are based on elements of low atomic weight. Practically, this limits the choice to elements of atomic number less than sixteen. Gases are of little use as moderators because of their low density, but can be used effectively in chemical compounds such as HjO and DjO. The choice of potential moderators of practical use thus rapidly reduces to the four materials shown in Table 3. Comparing the candidate moderators in Table 3 with our requirements listed above we may note ... 

In the studies described above, the samples were supported in low atomic weight matrices, melted in situ, and measured in transmission mode. Similarly, second... 

The two covalent carbides have low density, low atomic weight, and useful semiconductor properties. They are extremely hard and strong materials which exhibit typical ceramic characteristics. 

Figure 4.1-9 Schematic of the sample preparation methods used to study liquid EXAFS (a) thin liquid film sandwich between low atomic weight plates, and (b) the liquid (circles) dispersed in a low atomic number matrix (polyhedrons). The figure has been redrawn from reference 40 with permission.

A general requirement is that the average molecular weight of exhaust gas should be low. This restricts the choice of solid propellant components to these containing elements of low atomic weight. Low atomic weight elements often release a greater heat of reaction. 

The use of radiotracers is dependent on certain basic assumptions being fulfilled. The first assumption, mentioned above, is that the radioactive isotopes of a given element behave identically as the stable isotopes of the same element. Actually, this assumption is not exactly true. The difference in masses between radiotracer nuclei and stable nuclei can cause a shift in the reaction rate or equilibria (the isotope effect). It is true, however, that in most cases the isotope effect does not significantly affect the utility of the radioisotope method. Since the degree of chemical bond stability due to vibrational motion is directly related to the square root of the masses of the isotopes involved, it is apparent that an isotope effect will be of significance only for elements of low atomic weight (at wt <25). 

In 1917 Harkins found that on the average elements of even atomic number (Z) are about 70 times as abundant in meteorites as those of odd Z he further noted that the first seven elements in the order of their abundance are all even-numbered and make up almost 99% of the material in meteorites. [28] Four years later he elaborated his assumption that the relative abundances of the atomic species of low atomic weight may be used as an index... of their relative stability. He now suggested several more rules, including that atoms with even A (mass number) and an odd number of electrons are extremely rare. [29] These rules were claimed to be valid for isotopes, and not merely elements. In 1931, after more data on the distribution of isotopes had been collected, he reported that even-A nuclei were much more frequent than odd-A nuclei. [30]... 

Active carbon (AC), carbon nanotubes, carbon fibers, and other carbonaceous adsorbents, owing to their low atomic weight, high surface area, pore volume, and suitable mechanical properties, are excellent options as adsorbents for hydrogen storage [81,147,165-184],... 

The most important transmutations by a particles are of the (a,p) and (a,n) types. The (a,p) processes (for example, Na23(a,p)Mg26) are common with targets of low atomic weights (Z > 25) as has been seen, these were the first artificial transmutations to be studied. The (a,n) reactions (and the closely related reactions in which two, three, or more neutrons are ejected by a particles of high energy) are of considerable interest in connection with the synthesis of the transuranium elements and of astatine (element 85). The following are typical and important examples ... 

The atomic heats are therefore 5-83, 5-74 and 6-26 over the three ranges of temperature (1), (2), (3). There is a slight deviation from Dulong and Petit s law 9 at the lower temperatures, in the same sense as that met with in the case of the elements carbon, boron and silicon. But although phosphorus has a relatively low atomic weight, it also has a low melting-point, and the atomic heat as usual assumes the normal value at temperatures near the melting-point. 

Recent EXAFS results have provided the most detailed information on the coordination sphere of the Mn. Studies on PSII particles posed in the S, level disclose that at least two metals occur as a dinuclear species with a Mn Mn separation of approximately 2.7 A (250). Low-atomic-weight atoms, N or O, are found at distances of 1.75 and 1.98 A, and are characteristic of bridging oxide (or presumably hydroxide) ligands and terminal ligands, respectively. 

Therefore, intensity of cathode emission increases with greater cathode current and with higher atomic weights of the filler gas species. Helium, with its low atomic weight and relatively high ionization potential, generally is least suitable (Tl). 

The most common moderators are substances of low atomic weight such as heavy water (deuterium oxide) or graphite. Hydrides (binary compounds corrtaining hydrogen), hydrocarborrs, and beryllium and beryllium oxide have also been used as moderators in certain specialized kinds of reactors. 

The specific heat varies most rapidly with the temperature in the case of non-metals of low atomic weight. Particularly is this true of carbon, above all in the non-conducting crystalline form of diamond. The specific heat of diamond increases from 01128 to 046, that is to say, about four-fold in the interval between 0° and 1000°. 

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