HEPD-Activities/Nuclear Physics

Study of Nuclear Structure with Quasielastic Proton Scattering at 1 GeV.

The shell structure of nuclei was studied at PNPI by (p, 2p) and (p, np) reactions using the 1 GeV proton beam of the PNPI syncrocyclotron . Both reactions were studied in identical kinematical conditions. The scattered proton was detected with a magnetic spectrometer, while the knocked out nucleon (p or n) was detected with a time-of-flight spectrometer. A number of nuclei (more than 20, from ⁶Li to ²⁰⁸Pb) were studied, and detailed information on the proton- and neutron-shell energies was obtained. Prior these measurements, the neutron shell energies were in most cases unknown. An important result of these studies was the observation of the 1s_1/2 and 1p_1/2 proton and neutron shells in heavy nuclei including ⁹⁰Zr and ²⁰⁸Pb. The shell splitting was observed in many of the studied nuclei. This shell splitting was explained in terms of the spatial deformation of the self-consistent nuclear field. It has been shown that the data on the proton and neutron shell energies allow one to study the deformation parameters of the nuclear density distributions both for protons and neutrons. Also, the measured spectra show high sensitivity to the differences between the proton and neutron root-mean square radii. The obtained data can serve as a solid basis for testing the existing nuclear models and for developing new theoretical approaches to microscopic descriptions of the nuclear structure.
(For more detail review see article "Study of nuclear structure with quasielastic proton scattering at 1 GeV" in PNPI report of the High Energy Physics Division "Main Scientific Activities 1971-1996").

Ternary Fission.

A series of experiments on ternary nuclear fission were performed at the Gatchina WWR-M neutron reactor. Two alternative approaches in description of the nuclear fission process were discussed in the 60s. Within the framework of the statistical fission model, descending from the saddle point was considered to be adiabatically slow, so that formation of the fragments occurs just before the scission point. On the contrary, in the dynamical model of fission the formation takes place at the barrier, and further separation of the fragments goes so rapidly that no significant redistribution of the fragment masses occurs. The ternary fission has occurred to be useful to tell between these two fission models. The inclusive spectra of light nuclei (^2,3H, ^4,6,8,He, ^7,8,9Li, ^9,10,11Be, ^11,12,13,14B, ^14,15,16C, ²⁰O) formed in the ternary fission were measured with a magnetic time-of-flight mass-spectrometer. The experiments were carried out with the ²³³U, ²³⁵U, ²³⁹Pu, and ^242mAm targets exposed to thermal neutron fluxes. The next stage was the correlation experiment with a special setup which enabled simultaneous detection of the heavy fragments and the light nuclei ³H, ⁴He, ⁶He, ¹⁰Be. Detailed studies of the ²³⁵U and ²³⁹Pu thermal neutron induced fission, as well as spontaneous fission of ²⁵²Cf, were performed. As a result of these studies, the experimental data were obtained which created the basis for a detailed kinematics analysis aimed at determination of the initial conditions of the fragment separation. Such an analysis resulted in a rather large value (» 30 MeV) of the fragments initial energy. This value excludes the statistical model of nuclear fission.
(For more detail review see article "Ternary fission" in PNPI report of the High Energy Physics Division "Main Scientific Activities 1971-1996").

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