Mass Distribution of Fission Fragments

The fission reaction, again taking the example of 235U, can be written in the somewhat general form7

This expression represents the fission of 236U into two fragments (with atomic mass numbers Ai and A2) and N neutrons. From conservation of charge and of number of nucleons, Zi + Z2 = 92 and Ai + A2 + N = 236. N is a

7 Here, we ignore relatively rare additional particles, most notably alpha particles.

small number, typically 2 or 3. If A1 = A2, or nearly so, the fission process is called symmetric fission. Otherwise, it is asymmetric fission. The relative yields of fission fragment nuclei of different atomic mass number are plotted in Figure 6.2.8

As seen in Figure 6.2, thermal fission of 235U leads overwhelmingly to asymmetric fission.9 The fission yield is dominated by cases where one fragment has a mass number A between about 89 and 101, and the other has a mass number between about 133 and 144. The fission yields, namely the fraction of the events that produce nuclei of a given A, lie between about 5% and 7% in this region, with a single anomalously high value at A = 134. For symmetric fission (A of about 116), the fission yield at a given value of A is only about 0.01%. The integral of the area under the curve of Figure 6.2 is 200%, as there are two fragments per fission.

Fission Fragment

Fig. 6.2. Yield of fission fragments as a function of atomic mass number A for thermal fission of 235U (in percent per fission).

ATOMIC MASS NUMBER

Fig. 6.2. Yield of fission fragments as a function of atomic mass number A for thermal fission of 235U (in percent per fission).

8 Data for Figure 6.2 are the yields after delayed neutron emission, as listed in Ref. [6, Table I].

9 This fission process for neutrons incident on 235U is commonly referred to as "fission of 235U," although, of course, the fissioning nucleus is 236U. Usually, this does not lead to ambiguity, because the meaning is clear from the context.

Although in most cases the massive products from the breakup of the fissioning nucleus are the two fission fragments and neutrons, there is also a small possibility of emission of other nuclei, especially alpha particles. Alpha-particle emission, with energies up to 20 MeV, occurs in 0.2% of fission events [7, p. 590]. While of some interest in its own light, this contribution is too small to be of any importance in considering nuclear reactors.

The fission fragment yields for other fissile targets, in particular 233U and 239Pu, are similar to those in Figure 6.2, although the minima are not quite as deep and there are small displacements in A due to the differences in nuclear mass number. Fast fission yields also have the same general character, for both fissile and fertile targets [8]. Again, asymmetric fission dominates, with peak yields primarily in the range between 5% and 7%, although there are some exceptions and differences in detail.

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