Browsing by Author "Malaza, Vusi David"
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- ItemExperimental study of cluster effects in binary and ternary decays of low excited actinides(Stellenbosch : Stellenbosch University, 2018-12) Malaza, Vusi David; Mkaza, N. M.; Wyngaardt, Shaun M.; Pyatkov, Yu V.; Kamanin, D. V.; Stellenbosch University. Faculty of Science. Dept. of Physics.ENGLISH ABSTRACT : The experimental study of rare decay modes of ternary fission performed by the FOBOS group at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, revealed a new ternary decay mode of low excited heavy nuclei. This new ternary decay mode is referred to as Collinear Cluster Tri-partition (CCT). The distinct features of the CCT include the centre of mass of one of the decay fragments and the centre of mass of the two other decay fragments moving in opposite directions relative to each other. Another experimental signature for identifying the CCT is that one of the three decay fragments have a magic nucleon number configuration. The first experimental observation of the CCT was revealed in a study of spontaneous decay of 252Cf performed using a so-called modified FOBOS spectrometer facility installed at the JINR. This phenomenon was observed using the missing-mass method, where two fragments were detected, the third one being missing. The missing mass was accounted for by the difference in the masses of the detected fragments and the mass of the initial nucleus. The CCT manifested itself through a bump in the mass-mass distribution of fission fragments from the decay of 252Cf. Further confirmation of the CCT using the same missingmass method was obtained from the reaction 235%(nth,") where an experiment was performed using a spectrometer referred to as miniFOBOS. This experiment made use of a neutron beam delivered by the IBR-2 Reactor from the Frank Laboratory of Neutron Physics at the JINR. In both experiments the CCT was revealed as a bump in the massmass distribution which was linked to the magic nickel cluster. The bump was referred to as the Ni-bump. It quickly became clear from the early experiments that a direct detection of all the decay products of the CCT will be a more convincing experimental approach. In an effort to detect all decay products, a new double arm time-of-flight (TOF) spectrometer, based on a mosaic detection system, aimed at detecting all the CCT products was designed and is presented in this work. This spectrometer is referred to as COMETA (Correlation Mosaic Energy – Time Array). The COMETA spectrometer registers the energy and time signals of heavy ions using mosaics of PIN diode detectors in each arm. This new spectrometer required a new parametrization procedure to calculate the masses of the registered fragments of the CCT. The parametrization procedure also forms part of this work. This procedure takes into consideration the well-known experimental challenges when registering heavy ions using semiconductor-based detectors such as PIN diodes. The first challenge is the so-called pulse-height defect, which is manifested when registering the energy of heavy ions. The other challenge is the so-called plasma delay, which occurs when registering the time signal for the heavy ions. To test this procedure a special experimental setup called Light Ion Spectrometer for South Africa (LIS-SA) was put together. In this work, an experiment performed with the LIS-SA setup, that tested this procedure in the mass reconstruction of the fission fragments of the CCT, is also presented. The results from the COMETA spectrometer that confirmed the existence of the CCT are also presented. In the mass-mass distribution of fission fragments from the COMETA, the CCT reveals itself as rectangular structures bounded by known deformed magic clusters such as 123Mo, 63Sr and also magic clusters such as 128Sn. These structures appeared in the same vicinity where the Ni-bump was observed earlier. Further analysis of these structures revealed that they are linked to the Ni-bump. This work did not only provide a more convincing approach in the study of multi-body decay of low excited nuclei (the CCT), the existence of the CCT phenomenon was successfully confirmed through the direct detection of all the decay products of the CCT. The CCT has been confirmed as a decay process that takes place as a two-stage break-up of the initial three body chain-like nuclear configuration with an elongated central cluster.
- ItemMulti-detector registration system for the study of multi-body decay of heavy body nuclei(Stellenbosch : Stellenbosch University, 2012-12) Malaza, Vusi David; Jacobs, N. M.; Pyatkov, Yu V.; Kamanin, D. V.; Stellenbosch University. Faculty of Military Sciences. School of Science and Technology. Dept. of Physics.Nuclear ssion is commonly known as a process where a heavy nucleus such as Uranium or Thorium decays into two fragments of roughly equal mass. On occasion however, instead of decay into two parts a process known as binary ssion, the nucleus can decay into three fragments. In this decay channel known as ternary ssion, the nucleus splits into three fragments with the third particle being too light compared to the main ssion fragments. There are also instances where heavy nuclei split into three fragments of comparable masses, the so called \true ternary ssion" as was predicted by the theoretical calculations of Strutinsky [Str63]. While theoretical predictions hold promises for this decay mode, experimental attempts had little success in proving the existence of true ternary ssion in low energy ssion. The challenges and di culties faced by experimentalist in con rming the existence of true ternary ssion also proved that this ssion mode is a very rare phenomenon. This thesis is devoted to the investigation of ternary ssion know as collinear cluster tripartition (CCT) in spontaneous ssion of 252Cf, and the design and development of two time-of- ight spectrometers aimed at identifying all collinear multi-body decay partners directly. Prior to this study the only technique that was used at identifying decay partners in CCT was the \missing mass" approach. In this approach only two partners are identi ed directly with the third partner being identi ed by subtracting the two observed partners from the initial mass of the nucleus. The experimental results from the two spectrometer setups showed that it was possible to identify all three partners of the CCT channel. The results also con rmed the existence of the so called \Sn lost" CCT mode which was already observed in earlier experiments.