Alexander Vorobyev

Neutron-Induced Fission of 233U, 238U, 232Th, 239Pu, 237Np, natPb and 209Bi Relative to 235U in the Energy Range 1-200 MeV

Fission cross section ratios of 233U, 238U, 232Th, 239Pu, 237Np, natural Pb and 209Bi to 235U have been measured in a wide energy range of incident neutrons from 1 MeV to 200 MeV using a time-of-flight technique at the neutron spectrometer GNEIS based on the 1-GeV proton synchrocyclotron of PNPI. For actinide targets, the threshold cross section method and evaluated data below 14 MeV were used for normalization of the shape measurement data, while the evaluated and recommended fission cross sections of 235U were used to convert the ratio data to absolute fission cross sections. For Pb and Bi targets, an absolute normalization of the measured cross section ratios has been done using the thickness of the targets and detection efficiencies.

Neutron‐Induced Fission Cross Sections of 240Pu, 243Am, and natW in the Energy Range 1–200 MeV

A long‐range research program devoted to measurements of neutron‐induced fission cross‐sections of actinides and stable isotopes is under way at the GNEIS facility. By now the new series of experiments for measurements of fission cross‐section ratios relative to 235U has been completed for 240Pu, 243Am, and natW in a wide energy range of incident neutrons from 1 MeV to 200 MeV in the frame of the ISTC Project ♯1971. The measurements were performed using the multiplate ionization chamber and time‐of‐flight techniques. The results obtained in this measurement are presented in comparison with the other data.

ISNP/GNEIS Facility in Gatchina for Neutron Testing With Atmospheric-Like Spectrum

A description of the testing facility ISNP with spectrum resembling that of terrestrial neutron radiation developed at the PNPI (Gatchina) is given. A broad spectrum (1-1000 MeV) spallation neutron source of the facility with a neutron flux of 4 · 105 n/(cm2·s) is used for accelerated soft error testing. High-quality collimation of the neutron beam in conjunction with the TOF-technique enables to carry out precise and reliable monitoring of the neutron beam. The results of recent tests carried out at the ISNP by the Branch of JSC “URSC” - “ISDE” (Moscow) are presented.

Measurements of Prompt Neutron Multiplicity Distributions in Correlation with Mass-Energy Distribution of Fission Fragments in Spontaneous Fission of 252Cf, 244Cm and 248Cm

The number of prompt neutrons emitted in the fission event have been measured separately for each complementary fragment in coincidence with fragment mass and kinetic energies in spontaneous fission of 252Cf, 244Cm and 248Cm. Two high efficient Gd-loaded liquid scintillator tanks were used for the neutron registration. Approximately 3 106 fission events coincident with prompt neutron emission have been accumulated for each isotope. The mean neutron multiplicity, the dispersion and the covariance of the multiplicity distributions have been obtained as a function of fission fragment mass and kinetic energy. The neutron multiplicity data have been corrected for neutron registration efficiency, background and pile-up. Dependencies of the moments of the multiplicity distributions on the fragment mass and total kinetic energy for different mass bins, as well as mass and total kinetic energy distributions of the fission fragments are presented, discussed and compared for the different isotopes investigated. The results showed a different behavior of the moments of the multiplicity distribution depending on the fragment mass asymmetry that reflects changes in the dynamical effects for different fission modes.

Neutron Total Cross Sections of 204Pb 206Pb, 207Pb and 208Pb and the Neutron Electric Polarizability

The neutron total cross sections have been measured for lead isotopes 208Pb, 206Pb and 207Pb in the range from 1 eV to 20 keV and for 204Pb in the range from 1 eV to 100 eV using the time-of-flight facility GNEIS in Gatchina. An accuracy of the measured neutron total cross section (Δσ/σ) is about 10-3 for 208Pb, 10-2 for 206Pb and 207Pb, 5.10-2 for 204Pb. An estimated value of the neutron electric polarizability from analysis of total cross section of 208Pb is αn = (2.4 ± 1.1) 10-3fm3.

Methodology of a Study of Correlations Between Neutron Multiplicity, Mass and Kinetic Energy of Fission Fragments

The description of an experimental set-up for the study of the excitation energy distribution of fission fragments by means of coincident measurement of fission neutrons is presented. For every fission event the kinetic energy and mass of both fragments are registered simultaneously with the number of prompt neutrons emitted by each of the complementary fission fragments. With this set-up, the fission fragments are detected by a twin ionization chamber and the neutrons - by two large Gd-loaded liquid scintillator tanks.