Fission charge distribution

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.)...

For the mass 140 chain, Zp = 54.55 (Wahl, 1988). Note that this tabulated value of Zp/A (=54.55/140) is very close to that of the fissioning system, 92/236, that is, the N/Z ratio of the fragments is approximately that of the fissioning system. This idea is called the UCD (unchanged charge distribution) prescription. Therefore... 

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. 

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

In addition to measuring the variation of mass yield, the variation of fission yield in isobaric mass chains as a function of the proton number has bear studied. In Figure 14.10, "individual yield data are presented for the A = 93 chain. In gmeral, the charge distribution yields follow a Gaussian curve with the maximum displaced several units below the value of Z for stable nuclides with the same A. For A = 93 the yield is largest for Z = 37 and 38 most probable charge, Zp) compared to the stable value of Z = 41. 

Abstract This chapter first gives a survey on the history of the discovery of nuclear fission. It briefly presents the liquid-drop and shell models and their application to the fission process. The most important quantities accessible to experimental determination such as mass yields, nuclear charge distribution, prompt neutron emission, kinetic energy distribution, ternary fragment yields, angular distributions, and properties of fission isomers are presented as well as the instrumentation and techniques used for their measurement. The contribution concentrates on the fundamental aspects of nuclear fission. The practical aspects of nuclear fission are discussed in O Chap. 57 of Vol. 6. 

As a consequence, the Zp model allows one to describe the nuclear charge distribution of a specific fission reaction by four parameters ... 

Physical Implications of the Nuclear Charge Distribution in Fission... 

It is interesting to note that the spectra of ternary particles extend to different atomic numbers, which coincide nearly with the size of the neck that results from the postulate extracted from the mass yield and nuclear charge distributions. To correlate the data, it has been postulated that the two spheres of the dumbbell configuration shown in O Fig. 4.11 for the fission of are practically the same for all asymmetrically fissioning nuclei from Z= 90 to about 99 and, consequently, that the variation in the neutron/proton numbers of the different compound nuclei must be connected with the size of the neck. O Fig. 4.29 is the direct experimental proof for this assumption in the fission of uranium, the neck size is (92 — 82 =) 10 protons in the fission of californium, the neck size is (98 — 82 =) 16 protons. The situation is similar for neutrons and for the total mass. This is, however, less convincing due to prompt neutron emission. 

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... 

In considering the physical forces acting in fission, use may be made of the Bohr liquid drop model of the nucleus. Here it is assumed that in its uonual energy state, a nucleus is spherical and lias a homogeneously distributed electrical charge. Under the influence of the activation eneigy furnished by the incident nentron, however, oscillations are set up which tend to deform the nucleus. In the ellipsoid form, the distribution of the protons is such that they are concentrated in the areas of the two foci. The electrostatic forces of repulsion between the protons at the opposite ends of the ellipse may then further deform the nucleus into a dumbbell shape. Rrom this condition, there can be no recovery, and fission results. 

---