I. D. Alkhazov

Neutron and fragment yields in proton-induced fission of 238U at intermediate energies

The primary fission fragment mass and kinetic energy distributions, and neutron multiplicities as function of fragment mass have been measured in the proton-induced fission of 238 U at energies Ep ¼ 20, 35, 50 and 60 MeV using time-of-flight technique. Pre-scission and post-scission neutron multiplicities have been extracted from double differential distributions. The fragment mass dependence of the post-scission neutron multiplicities reveals the gross nuclear shell structure effect even at the higher proton energies we measured. The yields ofneutron-rich fission products in the fission of 238 U by 25 MeV protons were measured using an ion guide-based isotope separator technique. The results indicate enhancement for superasymmetric mass division at intermediate excitation energy of the fissioning nucleus. The experimental results have been analysed in the framework of a time-dependent statistical model with inclusion ofnuclear f effects in the fission process. # 2001 Elsevier Science B.V. All rights reserved.

Neutron multi-detector system: mutual influence of its modules

Abstract A position-sensitive neutron detector (PSND) has been designed and tested as a module of a multi-detector array. “Cross-talk” effects, contributing a major distortion factor for all closely packed neutron detector systems, have been measured. It was established, using neutrons from the spontaneous fission of 252 Cf (setting neutron energy threshold at 0.7 MeV), that the distortions of neutron energy and of neutron angular distribution are insignificant (within the level of statistical accuracy). The influence of cross-talk on the coincidence counting rate was measured to range from 48% to 16% for distances between detectors axes of 12 to 24 cm.

Fragment mass distribution in superasymmetric region in proton-induced fission of U and Th

SummaryFission fragment mass distributions down to super-asymmetric mass region and both pre- and post-scission neutron multiplicity for238U(p,fission) reaction atEp = 20, 35, 50, 60 MeV and for232Th(p, fission) reaction atEp = 50, 60 MeV were measured using HENDES set-up. The results indicate enhancement for super-asymmetric mass division at intermediate excitation energies.

HENDES-high efficiency neutron detection system for correlation measurements with HI beams

HENDES is a new device for correlation measurements of fission fragments, neutrons and light charged particles produced in HI induced reactions. Double differential neutron spectra are measured with the help of Position Sensitive Neutron Detectors, fission fragment spectra are recorded with large Position Sensitive Avalanche Counters, and light charge particle spectra are measured with arrays of dE-E telescopes. Collective dynamical parameters of nuclear matter are extracted from the data. New results were obtained from 40Ar+180Hf reaction at EAr=190–250 MeV. Work is in progress to study the influence of neutron excess on the fusion-fission dynamics in different combinations of Ni beams on Sn targets.

Inelastic deuteron breakup followed by fission at intermediate energy

The results of experimental and theoretical studies of the double-differential proton and neutron spectra measured in coincidence with fission fragments in the deuteron-induced reaction on a 238U target at Ed=65 MeV are presented. These spectra measured in the forward direction are analyzed in the plane-wave Born approximation by using the modified model of stripping into a continuum. The pre-neutron emission fission fragment mass distributions were measured for the (d, f), (d, pf), and (d, nf) reaction channels. The enhancement of highly asymmetric mass division in the (d, pf) channel for the low-energy part of the breakup proton spectrum was observed. The (d, pf) channel can be used to imitate the neutron-induced fission at intermediate energy. The fission characteristics were analyzed in the model taking into account nuclear friction and relevant fission modes.

Pre- and post-scission neutron emission in the reaction180Hf(40Ar, fission) atElab=216 MeV

Neutron and fission fragment spectra following the180Hf(40Ar,f) reaction atElab=216MeV were measured. The following neutron total, pre- and post-scission multiplicities were extracted:Mntot=7.2±1.3,Mnpre=2.7±0.9,Mnpost=4.5±0.9. The average temperature parameters of the neutron spectra areTpre=(1.63±0.16) MeV andTpost=(1.14±0.26)MeV. The mean total kinetic energy of the fission fragments is TKE=(166±10) MeV and the measured width is σTKE=17.3MeV. The width of the fragment mass distribution is σA=18.3u. The same reaction was analyzed using a modified statistical code which includes nuclear dissipation effects and particle and γ-ray emission in the equilibrium compound nucleus state and saddle-to-scission transition. The extracted value of nuclear friction coefficient indicates over-damped large-scale fission motion. The obtained total fission time τfiss=9·10−20s is in agreement with other existing data.