Independent yields

The kinetic data fit a mechanism of successive reactions sequent to only one primary ion equally well, provided that the first step can yield 1.37 methyl radical/100 e.v. and is pressure dependent and that the succeeding pressure independent step yields methyl radicals with a lesser efficiency and leads to a pressure independent yield of 0.58 methyl radicals/100 e.v. If the first step is either Reaction 9a or Reaction 17b, one can once more use the rate constant ratios given earlier to estimate the yields of the possible primary precursor ions. Hence, either G-(C2H2+) = 1.9 ions/100 e.v., or G(C2H4+) = 1.52 ions/100 e.v. The... 

Kinetic examination of the methane yield shows behavior quite similar to that of methyl radical a pressure dependent yield of 0.406 molecule/100 e.v., a pressure independent yield of 0.126 molecule/100 e.v., and a rate constant ratio of kq/kf = 1.5 X 106 mole-1 cc. for the competing steps. 

Values a and b for the fission product isotopes and the partition factors ai and a2 are listed in Table V au for a given isotope, is the fraction which was retained by the local fallout glass particles, and < > is the fraction released to the cloud. Thus, from Table V, i137 is 0.153 which indicates that 15.3% of the 137Cs is retained by the local glass particles. It is interesting to note that the independent yield of cesium in the 137 mass chain is approximately 17%—the balance of the chain is formed as tellurium, iodine, and xenon. 

The behavior of the 131 and 132 chains can also be explained on the basis of escape. Tellurium has a larger fractional independent yield in the 132 chain than in the 131 chain (12) and also has a larger fractional release than iodine at elevated temperatures (11). 

The population of fission product elements as a function of time is changing rapidly. These may be estimated from a knowledge of the half-lives of the fission product chain members, the mass chain yield, and the independent yield distribution along the mass chains. Although there are some uncertainties in these procedures largely because of lack of data on short-lived species, and a less than perfect understanding of the charge distribution function, reasonable estimates of radioactive atom... 

The yield of any given nuclide in fission is called its independent yield. It can be shown that the independent yield of isobars in fission has a Gaussian form ... 

In discussions of fission, one frequently hears the terms cumulative yield and independent yield. The independent yield of a nuclide is just what it appears, the yield of that nucleus as a primary fission product. Because the fission products are all (3 emitters, they decay toward the bottom of the valley of (3 stability, populating several different members of an isobaric series, as, for example, with A = 140 fragments ... 

Example Problem In the above example, what is the independent yield of 140Ba for the thermal neutron-induced fission of 235U and what is its cumulative yield ... 

Allan and Scholes (2) suggested an independent yield of H atoms in neutral solutions. As will be discussed later, e aq can be converted to H atoms by reaction with H30 + and other scavengers. 

The mass distribution curves in Figs. 8.13 to 8.15 give the total yields of the decay chains of mass numbers A. The independent yields of members of the decay chains, i.e. the yields due to direct formation by the fission process, are more diflicult to determine, because the nuclides must be rapidly separated from their precursors. Only a few so-called shielded nuclides (shielded from production via decay by a stable isobar one unit lower in Z) are unambiguously formed directly as primary... 

P Z) is the relative independent yield and C is a constant with a mean value of 0.80 + 0.14. This charge distribution is plotted in Fig. 8.16 for the fission of by thermal neutrons and holds for all mass numbers. For even numbers of Z the yields are systematically higher than those for odd numbers of Z. Zp, the most probable value of Z, is about 3 to 4 units lower than the atomic number of the most stable nuclide in the sequence of isobars. Nuclides with Zp are obtained with about 50% of the total isobaric yield, nuclides with Z = Zp + 1 with about 25% each and nuclides with Z = Zp + 2 with about 2% each. 

The xenon poisoning effect is well known in the field of nuclear engineering as the effect that prevented early reactors from rapid startup after shutdown. In addition to Xe being created in high relative yields as an independent yield fission product in the fission of U and Pu, it is also created in high amounts by a chain-yield fission product from decay of Te and I via ... 

The resulting pKB value is 8.09 + 0.145. The data was then refit to the power departure version of the equation to yield the Clark plot shown in panel C. The calculated F for comparison of the simple model (slope = unity) to the more complex model (slope fit independently) yielded a value for F that is not greater than that required for 95% confidence of difference. Therefore, the slope can be considered not significantly different from unity. Finally, the data was again refit to the quadratic departure version of the equation to yield the Clark plot shown in panel D to test for nonlinearity. The resulting F indicates that the plot is not significantly nonlinear. 

For the fission of each heavy nucleus, it is important that we have as much information as possible about the fission products, for both practical and theoretical reasons. In general, we wish to identify all the products, to determine their genetic relations one to the other, to determine their nuclear properties, half-lives, neutron cross sections, etc., and finally, to determine the fission yields of the various nuclides (independent yields) and of the various mass chains (cumulative yields or total of the independent yields along a chain). The yield is defined as the percentage probability per fission that a given nuclide or mass chain will be formed. 

If Te13B has a higher independent yield than Sb134, then the 134 chain will gain more than it loses by this mechanism and the Xe134 yield will be high. 

For the most part, however, independent yield data are confined to about a dozen nuclides that are near stability and are shielded by stable or relatively long-lived isobars. 

The accurate independent yields of I128 and I130 obtained for U233, U235, and Pu239 fission provide a severe test for any hypothesis of charge distribu-... 

Fig. 11. Comparison of the independent yield data of Im and Ilao (62) with the charge-distribution curve of (A) Glendenin (37) and (B) Pappas (88).

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