Photoneutron energy spectra from 181Ta, 208Pb, and 209Bi, and the problem of partial photoneutron reaction cross section reliability

Varlamov, V.V.; Orlin, V.N.; Peskov, N.N.

Авторы: Varlamov V.V., Orlin V.N., Peskov N.N.
Сборник: Международная конференция «Ядро-2017» (67-е Совещание по ядерной спектроскопии и структуре атомного ядра), 12-15 сентября 2017 г., Алматы, Республика Казахстан. Тезисы
Тезисы
Год издания: 2017
Место издания: РГП «Институт ядерной физики» Алматы, Республика Казахстан
Первая страница: 31
Последняя страница: 31
Аннотация: Most of the cross sections of total and partial photonuclear reactions were obtained using quasimonoenergetic annihilation photon beams at the National Lawrence Livermore Laboratory (USA) and France Centre d’Etudes Nucleaires de Saclay. Both laboratories used similar methods to separate reactions with different multiplicities based on the assumption that the neutron spectra of (g, 1n) and (g, 2n) reactions are fundamentally different. Quite different methods, however, were used to measure kinetic energy of neutrons: at Livermore the so called “ring-ratio” method was used and at Saclay a specifically calibrated large Gd–loaded liquid scintillator was used. As a result there are well–known complex systematic discrepancies in partial photoneutron reaction cross sections obtained at Livermore and Saclay: as a rule the (g, 1n) reaction cross sections are larger at Saclay, and the (g, 2n) cross sections are larger at Livermore. Therefore Saclay–Livermore data difference is in some cases up to about 60–100%. These disagreements were a subject of special studies for several years [3–6]. Since the total number of neutrons detected at both laboratories agrees, it has been suggested that the difference of the partial reaction cross sections originated from the noticeable systematic uncertainties of the procedures used to separate counts into 1n and 2n events. Using the objective physical criteria of partial photoneutron reaction cross sections reliability it was shown that for many nuclei those data obtained using quasimonoenergetic annihilation photon beams and photoneutron multiplicity sorting method are nor reliable. In various energy ranges the ratios F1 = SIGMA(g, 1n)/SIGMA(g, xn) = SIGMA(g, 1n)/SIGMA[(g, 1n) + 2(g, 2n) + 3(g, 3n) + …] have physically forbidden negative values and at the same time corresponding ratios F2 = SIGMA(g, 2n)/SIGMA(g, xn) have physically unreliable values larger 0.50. New data free of the uncertainties mentioned above were evaluated for many nuclei (for example, 91,94Zr, 115In, 112–124Sn, 133Cs, 159Tb, 181Ta, 186-192Os, 197Au, 208Pb, 209Bi) using the experimental-theoretical method. The only experimental reaction cross section SIGMAexp(g, xn) free from problems of experimental determination of neutron multiplicity was used and shared into the partial parts using the equations Ftheori = SIGMAtheor(g, 1n)/SIGMAtheor(g, xn) of the combined model of photonuclear reactions (CMPNR). The way of new cross sections evaluation was the following – SIGMAeval(g, in) = Ftheori • SIGMAexp(g, xn). Using the CMPNR the energy spectra of various chances outgoing neutrons from the reactions (g, 1n), (g, 2n) and (g, 3n) were calculated for nuclei 181Ta, 208Pb, and 209Bi. Calculations were carried out for nucleus excitation energies 15, 20, 25, 30, 40, and 50 MeV. For all three nuclei it was obtained that if nucleus excitation energy increase the part of high-energy neutrons in the inclusive neutron spectra increase also. At the same time the shapes and averaged energies of various neutron spectra are near and the main maxima are place in small neutron energies ~ 0.7–1.0 MeV. The reason could be that there are many neutron transitions to the excited states of final nucleus. Therefore the dependence of neutron multiplicity with its energy is very complex.
Добавил в систему: Варламов Владимир Васильевич

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Photoneutron energy spectra from 181Ta, 208Pb, and 209Bi, and the problem of partial photoneutron reaction cross section reliabilityтезисы доклада

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