Fission Product Charge Distributions

If one were to plot the yield of fission fragments as a function of their atomic numbers (as in Fig. 11.16), the result would look very much like the fission mass 

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  

The yield of each member of the isobaric series integrates, by virtue of the intervening (3 decay, the yields of its precursors. Such yields are referred to as cumulative yields. For example, the cumulative yield of the mass 140 chain in the thermal neutron-induced fission of 235U is 6.25%. 

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  

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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... 

Figure 11.18 Yields of products from the thermal neutron-induced fission of 235U. (From A. C. Wahl. Nuclear Charge Distribution in Fission, in New Directions in Physics, N. Metropolis, D. M. Kerr, and G. C. Rota, Eds. Copyright 1987 by Academic Press, Inc. Reprinted by permission of Elsevier.)...

Oil Contamination of Helium Gas. For more than 20 years, helium gas has been used in a variety of nuclear experiments to collect, carry, and concentrate fission-recoil fragments and other nuclear reaction products. Reaction products, often isotropically distributed, come to rest in helium at atmospheric concentration by coUisional energy exchange. The helium is then allowed to flow through a capillary and then through a pinhole into a much higher vacuum. The helium thus collects, carries, and concentrates products that are much heavier than itself, electrically charged or neutral, onto a detector... 

Up to this point, we have focused on describing the factors that control the probability of fission to occur. Now we will focus our attention on the distributions of the products in mass, energy, charge, and so forth. In doing so, we will mostly be discussing scission point or postfission phenomena. Our treatment of these phenomena is, of necessity, somewhat superficial, and the reader is referred to the excellent monograph of Vandenbosch and Huizenga (1973) for a more authoritative account. 

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