Independent fission yields

Figure 8.16. Independent fission yields for the fission of by thermal neutrons (charge distribution).

Later, many fast radiochemical separations and ingenious automatic chemical separation methods were developed (Herrmann and Denschlag 1969,1982 Rengan and Meyer 1993) and were applied to the determination of independent fission yields (Denschlag 1986, 1997). Summarily, one can say that the methods developed allow isolating any element formed from the complex mixture of fission products within the time span of a few seconds (exception rare earth elements that require a time span of minutes). 

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. 

O Table 56.7 lists independent fission product yields (England and Rider 1994) per 100 fissions for the most important fissionable nuclides. For thermal, fast neutron (fission spectrum), and high-energy neutron (14 MeV) induced, for fast neutron induced,... 

As an illustration of the way to disentangle the events that had taken place 2 billion years ago, the measurement of the neodymium isotopes (142-146,148, and 150) wiU be presented. It is important to know that neodymium remained in the rocks without migrating. In addition, it is advantageous that the content of natural neodymium in the minerals is small. This content could be determined since Nd is practically not formed in nuclear fission. As it results from the systematics of fractional fission yields (see Chap. 4 in Vol. 1), the (independent, direct) yield of Nd is very small. An alternative formation by the P decay of more neutron-rich nuclides of the same mass (i.e., isobars) is not possible because the direct precursor Ce is stable. Consequently, the content of the mineral in Nd reflects the amount of natural neodymium not produced by fission. Using the known isotopic composition of natural neodymium, the contributions of natural origin of all neodymium isotopes could be subtracted. This is illustrated in Table 57.1 (Columns 1-3). 

Wunderlich, F. Measurement of Independent Iodine Fission Yields In the Thermal Fission of U. Radiochim. Acta 7. 105 (1967). (In German). 21 38324... 

The general observation in the published work has been that for species such as hexamminechromium, thiocyanatopentammine, or the hexa-aquo ion where O18 exchange was looked at, irradiation produced a substitution reaction and nothing else. Moreover the reaction mode was independent of wave length, and the quantum yields did not change much. From a morphological point of view, there are essentially three types of explanations. First all excited states independently lead to the same chemical sequence, and we suppose that the primary act is simply a heterolytic bond fission. 

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 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 ... 

A plot of the total quantum yield versus the concentration of pentachloro-benzene provides a very nice linear plot (r = 0.995) (Fig. 2), suggesting that (kT + k,d) > k2[l]. Thus, by extrapolation of [1] to zero, one can calculate the quantum yield independent of triplet excimer (siaglet + triplet)- Subtraction of the quantum yields for direct fission from singlet and triplet states ( si gIe, + triplet) from ToM provides the expression for the dependence of the remainder ( ) upon concentration (Eq. 7). A plot of the reciprocal (l/cx) versus the reciprocal of the concentration of the substrate is linear (r = 0.950), which is consistent with the mechanism and is illustrated in Fig. 3 [2]. 

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 cross section of low-cross-section fission products, Op, has a single constant value independent of the fissile nuclide from which the fission products were produced and independent of the flux time to which the fission products were exposed. This assumption is an oversimplification, because the yield of individual fission products is different from each fissile nuclide, and individual fission products with hi er cross sections tend to be converted to those of lower cross section as irradiation progresses. Walker [Wl] has given tables from which may be determined the effective cross sections of fission products from U, U, Pu, and Pu as a function of the flux and flux time to which the fission products have been exposed. 

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