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Experimental measurements of angular and energy distributions of fission neutrons $n(\theta,E)$ per fission event were carried out in [1-4] assuming that $n(\theta,E)=n_{ev}(\theta,E)+n_{sc}(\theta,E)$, where $E$ is asymptotic kinetic energy of a neutron, $\theta$ is angle between the directions of the outgoing neutrons and the light fission fragment, $n_{ev}(\theta,E)$ is number of prompt neutrons evaporated from light and heavy fission fragment, $n_{sc}(\theta,E)$ is number of scission neutrons. These measurements corresponded to the cases of spontaneous fission of the $^{252}$Cf nucleus and thermal neutron-induced fission of $^{233}$U, $^{235}$U and $^{239}$Pu target nuclei. Also, in these papers a theoretical calculation of the prompt neutron distributions $n_{ev}(\theta,E)$ was carried out. The value $B(\theta,E)=\frac{n(\theta,E)}{n_{ev}(\theta,E)}=1+\frac{n_{sc}(\theta,E)}{n_{ev}(\theta,E)}$ was further constructed, and it can be used to calculate $n_{sc}(\theta,E)$ by the formula $n_{sc}(\theta,E)=\frac{(B(\theta,E)-1)n(\theta,E)}{B(\theta,E)}$.
Unfortunately, the approaches used for calculations $n_{sc}(\theta,E)$ require a certain adjustment, since at definite angles $\theta$ they lead to $B(\theta,E)<1$, which corresponds to $n_{sc}(\theta,E)<0$,which is impossible due to the positive definition of $n_{sc}(\theta,E)$.
In the present paper, for all studied nuclei, the appearance of a peak for $n_{sc}(\theta,E)$ in the vicinity of the angle $\theta=90^{\circ}$ for $50^{\circ}\leq \theta \leq 125^{\circ}$ is demonstrated, which is a direct indication of the emission of scission neutrons from the neck of a compound fissile nucleus. At the same time the energy spectrum $n_{sc}(\theta,E)$ in the specified range of angles corresponds to the energies of neutrons lying in the range of $0\leq E \leq 1$ MeV. Since these neutrons are emitted from neutron states of a compound fissile nucleus corresponding to binding energies about (- 6) MeV, so the appearance of these neutrons in continuous spectrum states with positive energies $0\leq E \leq 1$ MeV
and emission angles in the vicinity of the angle $\theta = 90^{\circ}$ can be explained [5] by considering the emission of these neutrons from the neck of a compound fissile nucleus analogously to the light particle emission in ternary nuclear fission.
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A. Vorobyev et al., JETP, 152, 730 (2017).
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A. Vorobyev et al., JETP, 154, 774 (2018).
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A. Vorobyev et al., Bull. RAS Ser. Phys. 82, 1373 (2018).
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A. Vorobyev et al., EPJ Web of Conferences 239, 05008 (2020).
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S.G. Kadmensky, L.V. Titova, D.E. Lyubashevsky, Phys. Atom. Nucl. 83, 298 (2020).
