A. A. Alexandrov

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.

A time-of-flight spectrometer for unslowed fission fragments

Abstract The one-armed time-of-flight spectrometer mounted in a vertical experimental port of the research reactor at the Moscow Engineering Physics Institute (MEPhI) was designed for measuring mass-energy distributions of unslowed fission fragments. The study of rare fission modes in the yield regions up to 10 −6 is carried out having a target location near the reactor core and an electrostatic particle guide system installed, which increases the efficiency ratio of the spectrometer by a factor of 50. The special “heavy ion generator” operation mode of the spectrometer can be used for studies of fast (∼ MeV/amu) heavy ion interaction with matter. The fission fragment energy is measured by an ionization chamber or a semiconductor detector. The negligible angular divergence of the fragment beam enables fission fragment detection in a channeling registration mode to improve the energy resolution of semiconductor detectors. The fragment velocity is determined by the time of flight between two time pick-off microchannel plate detectors. A new procedure using unseparated fission fragments was developed to obtain the correct value for the time resolution of the system.

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.

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.

Study of the surface-barrier semiconductor detector response to the fission fragments in the channeling mode

Abstract This study of the silicon semiconductor surface-barrier detector response to 239Pu(nth, f) fission fragments separated according to mass, charge and energy shows that, in the axial channeling mode, the energy resolution is two times better than in the ordinary mode. The achieved resolution is about 1.0 MeV (1.5%) for fission fragments of mass 137 amu and energy 67.3 MeV. The estimated energy value for channeled ions derived by the extrapolation of the pulse height to the infinite electric field strength in the detector and an alpha-particle calibration is in good agreement with the true energy. This fact may be the basis of a new calibration procedure.

Measuring the nuclear charge of fission fragments using a large ionization chamber—part of the double-arm time-of-flight spectrometer

Identification of decay products is a relevant task in studying rare collinear decays of low excited heavy nuclei. A technique for measuring the nuclear charge of decay products detected by a wide-aperture ionization chamber—a part of the double-arm time-of-flight spectrometer—is described. Two versions of nuclear charge calibration using data of the reaction 235U(nth, f) have been developed to determine the charge of the decay products. Testing with simulation data shows that the use of charge parameterization based on the Bohr-Willer empirical equation in the calibration procedure makes it possible to determine the nuclear charge of the fission fragments over a wide energy range. The charges of light ions from He to C, predicted on the basis of this approach, appear to be overestimated by two charge units.

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.

Status and prospects of investigations into the collinear cluster decay of heavy nuclei

Basic experimental results confirming the existence a new cluster-decay type called collinear cluster tripartition (CCT) are presented. Decays of this type manifest themselves, in particular, as a two-dimensional region of a locally enhanced yield of fragments (bump) that corresponds to specific missing-mass values in the mass-mass distribution of fission fragments. One of the decay modes that contribute to the bump can be treated as a cluster-decay type that is new in relation to the well-known heavy-ion or lead radioactivity. The conclusions drawn from an analysis of correlation mass distributions are confirmed by the results obtained from neutron-gated data, measurements of the nuclear charge for CCT events, and the direct detection of new-decay products.

Dynamics of superheavy system in 86Kr + 208Pb reaction

The dynamics of the excited superheavy system with Z = 118 in the reaction 86Kr + Pb at EKr = 600 MeV has been investigated. The mass and kinetic energy of binary fragments were measured by the time-of-flight method. Double differential distributions of neutrons and α particles were measured in coincidence with fragments. Neutron and α-particle probes were used for determination of the fragmentation time scale. Evidence of the neck fragmentation was obtained from analysis of double differential α spectra. Properties of the α-particle neck fragmentation component are close to those known from the ternary fission of actinide nuclei, but the multiplicity is much larger. The total kinetic energy distribution of fragments tagged by neutrons or α particles shifts towards lower energies. Fragment yields in the symmetric region increase substantially when fragments are tagged by α particles.

Preliminary Results on Observation of New Shape Isomers

In our previous publications devoted to the collinear cluster tri-partition (CCT) of the low excited nuclei [1, 2] we have discussed the role of scattering medium in the registration of the CCT products.