V. G. Tishchenko

Manifestation of clustering in the 252Cf(sf) and 249Cf(nth,f) reactions

Abstract A comparative analysis of the high-statistics mass-energy distributions of the fission fragments formed in the 252 Cf(sf) and 249 Cf(n th ,f) reactions is performed on the basis of the potential energy surface calculations. The available experimental and theoretical results provide evidence for the existence of fission modes due to the clustering of the fissioning nucleus. In 252 Cf one of the modes can be treated as a heavy-cluster decay involving the formation of two fragments close to the magic nuclei of Sn. A sharp drop of the proton odd-even effect is observed at an excitation energy above E ∗ ≈ 40 MeV at the scission point, which is presumably associated with the complete clusterization of the fissioning nucleus.

Manifestation of fine structures in the fission fragment mass-energy distribution of the 233U(nth,f) reaction

Methods of identifying structure peculiarities in two-dimensional distributions of experimental observables are discussed. Structures different from that produced by proton odd-even staggering were revealed for the first time in the mass-energy distribution of fission fragments in the 233 Uðnth; fÞ reaction. The new structures could presumably be linked with fission modes and collective vibrations of the fissioning system. r 2002 Elsevier Science B.V. All rights reserved.

Cluster approach to description of fission modes

A verification for the feasibility of the bicluster mechanism of fission-mode formation and for the clusterization of a neck of a heavy fissioning nucleus predominantly on alpha particles is fulfilled. A correlation was established between the peculiarities (fine structure) in mass-energy distribution of fission fragments being completely different from that produced by the proton odd-even effect and the clusterization of the fissioning nucleus. Calculated results in the cluster approach are compatible with the experimental data.

New indications of collinear tripartition in 252Cf(sf) studied at the modified FOBOS setup

An attempt has been made to search for collinear tripartition in spontaneous fission of 252Cf by using the FOBOS setup coupled with the neutron detector belt. A group of rare events were detected characterized by reduced total kinetic energy and total nuclear charge gated by the large neutron multiplicity measured. This fact is considered to be an experimental indication of collinear tripartition in 252Cf. The theoretical indication of the possible existence of the collinear cluster tripartition valley was obtained for the first time.

Nontrivial manifestation of clustering in fission of heavy nuclei at low and middle excitations

An analysis of the preferable fission path manifesting itself as a fine structure of the total kinetic energy-mass distribution of the fission fragments compelled us to put forward an absolutely new shape of the fissioning system on the descent from the fission barrier. It is a multicomponent nuclear molecule constituted by two magic clusters and a torus-like neck between.

NEUTRON CHANNEL OF THE FOBOS SPECTROMETER FOR THE STUDY OF SPONTANEOUS FISSION

The 4π spectrometer FOBOS has been completed with the multidetector system for registration of neutrons aimed at experiments to search for collinear tripartition of heavy spontaneously fissile nuclei. A simple empirical model developed for the description of the measured neutron multiplicity is tested on the data block comprising 6×107 events.

THE MODIFIED MINI-FOBOS SETUP

The 4π spectrometer FOBOS was proved to be a high-efficiency tool for investigation of the characteristics of multi-body decays of hot heavy nuclei produced in nuclear reactions induced by heavy ions. The time-offlight measurements of charged fragments employing the radio-frequency (RF) signal of the cyclotron U400-M to deduce the START signal require no special START detectors and, hence, the big fraction of the total solid angle is available for particle detection. The lack of the RF signal in experiments aimed at search for the exotic decay modes in spontaneous fission enforces to use an additional START detector. The complicated target station equipped by a START detector shadows a large portion of detector modules making thus no sense to use the whole powerful detection system in such experiments. Therefore, it has been decided to build a simplified facility consisting of several standard FOBOS modules only. The miniFOBOS setup, a prototype of the FOBOS spectrometer, has been chosen for that purpose. It was exploited mainly to solve various method tasks related to the construction of the FOBOS spectrometer. The general idea of the modified miniFOBOS setup (MMF) consists in using a small reaction chamber which can be then unique for each experiment and the basic universal system maintaining the detectors. The special mounting cones are used to joint detectors with the reaction chamber (Fig. 1, 2). Such cones can be used, in particular, to build the high efficiency system joining six modules in the form of 3Dcross.

Searching for Rare Decay Modes in the Reaction 238U+4He (40 MeV)

In the present paper we describe an experiment performed on the reaction 238U + 4He (40 MeV) using a two‐arm TOF‐E (time‐of‐flight vs. energy) spectrometer with micro‐ channel plate detectors and mosaics of PIN diodes. This experiment has aimed at searching for fine structures in the total kinetic energy vs. fragment mass (TKE ‐ M) distributions of binary events, and disclosing rare ternary decay modes others than already known from ternary fission. Details of the procedures of data handling are briefly described as well.

Fine structure in fragment mass-energy distribution from 238U+40Ar (275 MeV)

Existence of a new feature in fragment mass-energy distribution is reported. Careful analysis of the data obtained in the reaction 238U + 40Ar (275 MeV) shows that small but statistically significant ripples visible already in the gross mass spectrum come from extended and regular 2D patterns in the TKE vs. mass matrix. Intensity distributions of these patterns coincide with the location of heavy clusters such as 78Ni, 108Mo, or 132Sn. Presumably, the observed patterns show the dominant trajectories in the elongation vs. mass-asymmetry space of the decaying system. This information, unknown in the past, can shed a new light even on the previously well-studied reactions.