Fission threshold

It is natural to attribute the step near 6 Mev to fission of the target nucleus. The neutron energy at the step, therefore, should be equal to the fission threshold of the target nucleus the increase in the fission cross section at this point is due to the production of states in the target nucleus with energies above the fission threshold. Above this threshold (Qf), by analogy with (8.9), the cross section is ... 

Szilard, also working at Columbia, became interested around this time in what is now called the fast effect. The fast effect, is the increase in the multiplication constant obtained by the emission of neutrons by which is induced to fission by the fission neutrons before they are slowed down. Szilard measured both the cross section of such fission neutrons to induce fast fission and also their inelastic cross section, i.e., the probability for their being slowed down below the fast fission threshold by an inelastic collision with uranium. He concluded on the basis of these measurements that one may obtain an increase of as much as 6-8% in the multiplication constant by using large and metallic lumps of uranium. Szilard was also somewhat discouraged by the low multiplication constant which Fermi s experiment gave but was far from giving up hope. 

An important factor in the criticality of the actinides is that those containing even number of neutrons characteristically have a fission threshold and experience little or no subthresbold fission. Consequently, the inelastic scattering process, which degrades the neutron spectrum, is particularly Important in determining the criticality of these actinides. Also, the presence of small amounts of moderator has the same effect on the neutron spectrum and can easily reduce k of these nuclides to less than unity. This is Illustrated in Fig. 1, where calculated k values are shown as a function of metal atom concentration in water. For the even nuclides, Pu, Pu, em, and Am, k rapidly becomes less than one upon the addition of water to the theoretical metal system and criticality is no longer possible for these nuclides. 

The agreement vlth experimental values is good except for, ubere the differeiwb lies in the definition of The derivation of dr from the measurement of f is base m the older definition of vhioh is the nuefber of neutrons vhioh slow down the Vr fission threshold for each neutron produced by thermal fission (See Reference 12). An average for the RPR startup element is 0.067. 

An estimate of the fission threshold can be obtained from the energy required to distort the nucleus into an extreme shape which results in complete separation into fragments. It has been shown that this calculation can be based on the liquid-drop model of the nucleus. The two principal contributions to the distortion energy of the nucleus are the surface-tension effect from the nuclear forces between the constituent... 

The types of dosimetry reaction can be divided into fission and non-fission, threshold and non-threshold. The choice of the set of reactions to be used for a particular plication depends on the intensity of the flux, the duration of the irradiation, the type of reactor spectrum (thermal reactor, fast reactor, core, shielding, vessel) and the neutron energy range of importance in the effect being monitored. 

Table 11.2. Values of Cf, (7, v, and Fission Threshold Energy for the Main Fertile Isotopes in a Fast Neutron Spectrum...

The new quantities are (fast nonleakage probability), (capture to fission ratio above fast fission threshold), e (fast-fission factor), and 6, which is defined by the equation... 

Uranium-238 will fission appreciably only for neutrons with energies above one MeV, but, as most of the neutrons are born in fission with energies above one MeV, some of these fast neutrons will initiate fissions. The fissions result in an increased number of neutrons slowing down in the core, and their contribution to the reactor multiplication is considered as the fast fission factor e, where e is defined as the number of neutrons slowing down past the U fission threshold per fast neutron from a fission. 

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