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In cases for many (~50) nuclei from $^{51}$V to $^{209}$Bi the experimental data on photoneutron partial reactions ($\gamma$,1n), ($\gamma$,2n), ($\gamma$,3n) cross sections directly obtained using beams of quasimonoenergetic annihilation photons [1] do not satisfy objective physical criteria of data reliability [2–5]. The reasons are systematic uncertainties of experimental photoneutron multiplicity sorting method basing on partial reactions separation via measurement of neutron energies. Therefore, the experimental-theoretical method for partial reaction cross-section evaluation basing on physical criteria was used for analysis of reliability of data obtained using quite different method on the beams of bremsstrahlung [6]. Partial reaction cross sections are separated and determined in such kind experiment using statistical theory corrections to the neutron yield cross section $\sigma$($\gamma$,xn) = $\sigma$($\gamma$,1n)+2$\sigma$($\gamma$,2n)+3$\sigma$($\gamma$,3n)+… measured at first. Experimental cross sections of the reactions ($\gamma$,1n) and ($\gamma$,2n) are definitely unreliable in the cases of $^{51}$V, $^{52}$Cr, $^{59}$Co, but enough reliable in the case of $^{90}$Zr. The reason is that the role of two-nucleon reaction ($\gamma$,1n1p) was not taken into account, though this reaction competes with also two-nucleon reaction ($\gamma$,2n). It was shown via the results of calculation in the frame of the Combined photonuclear reaction model [5] that energy positions and amplitudes of cross sections of ($\gamma$,1n1p) and ($\gamma$,2n) reactions are very close to each other in the cases of $^{51}$V, $^{52}$Cr, $^{59}$Co, but in the case of $^{90}$Zr the value of ($\gamma$,1n1p) reaction cross section is very small and could be negligible. This conclusion is analogous to that of the preliminary investigation of the cases of $^{127}$I, $^{165}$Ho, $^{181}$Ta [7]. It means that in the cases of relatively light nuclei $^{51}$V, $^{52}$Cr, $^{59}$Co, as well as $^{58,60}$Ni [8] the reaction ($\gamma$,1n1p) plays important role in nucleus photodisintegration but its contribution is not correctly described by statistical theory corrections.
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Section | Experimental and theoretical studies of nuclear reactions |
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