Yu. V. Pyatkov

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.

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.

EXPERIMENTAL EVIDENCES OF CLUSTERING IN LOW EXCITED HEAVY NUCLEAR SYSTEMS

Results of the experiments aimed at revealing of manifestations of collinear cluster tri-partition (CCT) of the 252Cf and 236U* nuclei are reported. Presumably, the ternary decay of nuclear molecules based on heavy magic clusters such as isotopes of Ni, Ge, Sn, Te gives rise to the effect observed. Experimental conditions allow one to distinguish two CCT partners flying almost in the same direction, while analysis of their charges ensures the discrimination of the background due to scattering. A total yield of the CCT originated events found at the order of 10-3 per binary fission.

FISSION POTENTIAL ENERGY SURFACES IN TEN-DIMENSIONAL DEFORMATION SPACE

Various fission processes are described in terms of high-dimensional potential energy surface in the frame of the Strutinsky shell correction method for actinide region. The complete deformation space is necessary to study the potential energy minima responsible for the cluster radioactivity, cold fission and cold multi-fragmentation valleys. The nuclear shape families for the different fission configurations are obtained without any specific change of the parameters. The coordinate system based on the Cassini ovaloids makes it possible to increase the number of independent deformation parameters without divergence. The higher orders of the deformation are shown to play an important role in the description of the potential energy surface structure.

Collinear cluster tripartition channel in the reaction 235U(nth, f)

Investigation of the 235U(nth, f) reaction using the miniFOBOS double-arm time-of-flight spectrometer of fission fragments confirmed manifestations of the earlier unknown many-body, at least ternary, decay involving almost collinear decay-product escape, which were first observed in the spontaneous fission of 252Cf(sf). The use of variables sensitive to the nuclear charge of fission fragments allowed the reliability of identification of decay events to be increased and new decay modes to be revealed.

Applications of 3He neutron detectors

Neutron detectors with 3He-filled proportional counters are described. The use of these detectors in measuring the probability of neutron emission (in particular, multiparticle neutron emission) after the β decay of neutron-rich nuclei and in studying rare events of spontaneous fission of superheavy nuclei is considered.

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.

COLLINEAR CLUSTER TRIPARTITION IN FISSION OF ACTINIDE NUCLEI

A new ternary decay channel connected with magic constituents from Cf and U has been observed in recent experiments. It was called Collinear Cluster Tripartition (CCT) because the decay products fly apart almost collinearly. It is a decay of the fissioning system into the three fragments of comparable masses from the strongly elongated configuration. A total yield of the CCT events was found at the order of 10-3 per binary fission. The first theoretical description of the high lying energy minima on the potential energy landscape are presented for U and Cf. They have been obtained by the Strutinsky shell correction calculations in the multidimensional deformation space.