Fission Cross Sections with Fast and Thermal Neutrons

For orientation purposes, the cross sections for fission in 235U, 238U, and 239Pu are shown in Figure 6.1. (The data are extracted from detailed plots given in Ref. [5].) In the thermal region, the cross sections in 235U and 239Pu are hundreds of barns. In general, they then fall with increasing energy, with very rapid fluctuations in the resonance region. In the 1-10 MeV region, the fission cross sections are in the vicinity of 1 or 2 b for these nuclei.

The cross section for 238U fission is negligible at low neutron energies. For neutrons with energies of several electron volts, af is typically in the neighborhood of 10~5-10~4 b [5], and it is still under 0.02 b at En = 1.0 MeV. However, it then rises rapidly to a plateau of about 0.6 b, which extends from 2 to 6 MeV. In this region, af in 238U is about one-half the magnitude of af in 235U. Thus, although 238U is not "fissile" in the sense that the term is used—namely having a significant fission cross section for thermal neutrons— the 238U fission cross sections becomes appreciable for neutron energies above about 1 MeV.

0.01

100 104 NEUTRON ENERGY (eV)

10 6

Fig. 6.1. Fission cross sections as a function of neutron energy En for neutrons on 235U, 238U, and 239Pu. The shaded region is a schematic of the resonance region where the cross section varies rapidly. [The top and bottom boundaries of the shaded region are not limits on the peaks and valleys of the resonance cross sections, nor are there sharp boundaries at low and high energies.]

0.01

100 104 NEUTRON ENERGY (eV)

10 6

Fig. 6.1. Fission cross sections as a function of neutron energy En for neutrons on 235U, 238U, and 239Pu. The shaded region is a schematic of the resonance region where the cross section varies rapidly. [The top and bottom boundaries of the shaded region are not limits on the peaks and valleys of the resonance cross sections, nor are there sharp boundaries at low and high energies.]

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