Time features of delayed neutrons and partial emissive-fission cross sections for the neutron-induced fission of 232 Th
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CLEI Experiment
Time Features of Delayed Neutrons and Partial Emissive-Fission Cross Sections for the Neutron-Induced Fission of 232 Th Nuclei in the Energy Range 3.2–17.9 MeV V. A. Roshchenko* , V. M. Piksaikin**, G. G. Korolev, and A. S. Egorov*** Institute of Physics and Power Engineering, pl. Bondarenko 1, Obninsk, Kaluga oblast, 249033 Russia Received October 9, 2009
Abstract—The energy dependence of the relative abundances of delayed neutrons and the energy dependence of the half-lives of their precursors in the neutron-induced fission of 232 Th nuclei in the energy range 3.2–17.9 MeV were measured for the first time. A systematics of the time features of delayed neutrons is developed. This systematics makes it possible to estimate the half-life of delayed-neutron precursors as a function of the nucleonic composition of fissile nuclei by using a single parameter set for all nuclides. The energy dependence of the partial cross sections for emissive fission in the reaction 232 Th(n, f ) was analyzed on the basis of data obtained for the relative abundances of delayed neutrons and the aforementioned halflives and on the basis of the created systematics of the time features of delayed neutrons. It was shown experimentally for the first time that the decrease in the cross section after the reaction threshold in the fission of 232 Th nuclei (it has a pronounced first-chance plateau) is not an exclusion among the already studied uranium, plutonium, and curium isotopes and complies with theoretical predictions obtained for the respective nuclei with allowance for shell, superfluid, and collective effects in the nuclear-level density and with allowance for preequilibrium neutron emission. DOI: 10.1134/S1063778810060013
1. INTRODUCTION Data on integrated time spectra of delayed neutrons in the neutron-induced fission of heavy nuclei are information of interest for applied problems. Delayed-neutron time features obtained on the basis of such data in low-energy nuclear fission induced by neutrons play a decisive role in calculating the kinetics of nuclear reactors [1]. The role of this information in studying the properties of neutronrich nuclei and the fission process proper is not less important. Delayed neutrons are emitted at the final stage of the nuclear-fission process—in the chain of the beta decays of fission products. Although the time interval separating the instants of nuclearfission and delayed-neutron events is enormous on nuclear scales, the features of delayed neutrons carry information about the properties of nuclei undergoing fission and about their fission products. For example, data on the distribution of the nuclear charge of fission fragments versus the excitation energy of compound nuclei undergoing fission were obtained in [2] on the basis of data on the total yield of delayed neutrons *
E-mail: [email protected] E-mail: [email protected] *** E-mail: [email protected] **
and their time distributions in the fission of 235 U and 239 Pu nuclei. These data are also necessary for developing methods for a nondestructive tests
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