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ENDF/B-VIII.0 239Pu PFNS [8, 60] are used as examples to illustrate structures commonly observed around second- and third-chance fission thresholds: from thermal Einc to the on-set of second chance fission, curves similar in shape to the thermal PFNS are expected. In a window of Einc = 1–2 MeV around second-chance fission (red dashed line), one sees a characteristic structure of the PFNS appear in the 100 s of keV stemming from a second-chance fission contribution to the PFNS that is only non-zero up to these energies due to energy conservation. Around the third-chance fission threshold (blue solid line), this structure is like a broader shoulder in Eout above 100 keV. One also sees a sharp structure at high Eout (several MeV) coming from the pre-equilibrium neutron emission from the compound nucleus before fission happens.
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