Experimental study of cluster effects in binary and ternary decays of low excited actinides
Malaza, Vusi David (2018-12)
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.
AFRIKAANSE OPSOMMING : Die eksperimentele studie van rare verval modus van drieledige splyting wat uitgevoer is deur die FOBOS groep by die Joint Institute for Nuclear Research (JINR) in Dubna, Rusland, wys ŉ nuwe drieledige verval mode uit vir lae opgewekte swaar kerne. Hierdie nuwe drieledige verval mode word verwys na as ŉ ko-liniêere bondel Tri-partisie (CCT). Die onderskeie kenmerke van die CCT sluit in die massa middelpunte van een van die verval fragmente wat in ŉ teenoorgestelde rigting beweeg, relatief tot die massa middelpunt van die twee ander verval fragmente. ŉ Ander eksperimentele kenmerk wat die CCT uitwys is dat een van die verval fragmente ‘n voorkeurgetal (Magic number) konfigurasie het. Die eerste eksperimentele waarneming van die CCT het te voorskyn gekom in ŉ studie van spontane verval van 252Cf en is uitgevoer deur gebruik te maak van die sogenaamde gemodifiseerde-FOBOS spektrometer wat installeer is by die JINR. Hierdie verskynsel was waargeneem onder die verlore massa metode, waar twee fragmente waargeneem was, en die derde deeltjie die verlore deeltjie is. Hierdie verlore massa was bepaal deur die verskil tussen die massas van die waargenome fragmente en massa van die oorspronklike kern te bereken. Die CCT word deur ŉ piek in die massa-massa distribusie van die splytings fragmente van die verval van 252Cf gemanifesteer. Verdere bevestiging van die CCT onder dieselfde verlore massa metode was verkry vanaf die reaksie 235%(nth,") waar ŉ eksperiment gedoen is met ŉ spektrometer wat verwys word na as mini-FOBOS. Hierdie eksperiment maak gebruik van ŉ neutronbundel wat verskaf word deur die IBR-2 Reaktor by die Frank Laboratorium vir Neutron Fisika by die JINR. In beide eksperimente was die CCT uitgelig as ŉ piek in die massa-massa distribusie wat gekoppel is aan die voorkeur (“magic”) nikkel bondel. Die piek word na verwys as die Ni-piek. Dit het vinnig duidelik geword vanaf vroeër eksperimente dat ŉ direkte waarneming van al die vervalprodukte van die CCT, ŉ meer geloofwaardige eksperimentele benadering sal wees. In ŉ poging om al die verval produkte waar te neem, was ŉ dubbele arm vlugtyd (TOF) spektrometer, gebaseer op ŉ mosaïek detektor sisteem, wat gefokus is op die waarneming van al die CCT produkte, ontwerp en word dit in hierdie werk voorgelê. Hierdie spektrometer word verwys na as die COMETA (Correlation Mosaic Energy – Time Array). Die COMETA registreer die energie en tydseine van swaar ione deur gebruik te maak van ŉ mosaïek PIN diode detektore in elke arm. Hierdie nuwe spektrometer benodig ŉ nuwe parameteriserings prosedure om die massa van die geregistreerde fragmente van die CCT te bereken. Die parameteriserings prosedure vorm ook deel van hierdie tesis. Hierdie prosedure neem in ag die welbekende eksperimentele uitdagings wanneer die swaar ione geregistreer word deur gebruik te maak van semigeleier-gebaseerde detektore soos die PIN diodes. Hierdie uitdagings is die sogenaamde pulshoogte afwyking wat ŉ negatiewe effekt het wanneer die energie van die swaar ione geregistreer word en ook die plasmavertraging wanneer die tydsein vir swaar ione geregistreer word. Om hierdie prosedure te toets is ŉ spesiale eksperimentele opstelling genoem die Ligte Ion Spektrometer vir Suid Afrika (LIS-SA) aanmekaar gesit. In hierdie werk is ŉ eksperiment uitgevoer met die LIS-SA opstelling, wat die prosedure toets vir die massa her-konstruksie van die splytsingsfragmente van die CCT. Die resultate van die COMETA spektrometer wat bevestig dat die CCT teenwoordig is word ook voorgelê. In die massa-massa distribusie van die splytings fragmente vanaf die COMETA eksperiment, word die CCT geopenbaar as ŉ reghoekige struktuur wat begrens word deur die bekende vervormde voorkeurbondel kerne soos 123Mo, 63Sr asook voorkeurbondel kerne soos 128Sn. Hierdie strukture kom te voorskyn in dieselfde gebied waar die Ni-piek waargeneem was. Verdere analiese van hierdie strukture wys uit dat hulle gekoppel is aan die Ni-piek. Nie net gee hierdie werk ŉ meer gerieflike benadering tot die studie van veeldeeltjie verval van lae opgewekte kerne (die CCT) nie, maar die bestaan van die CCT verskynsel was suksesvol bevestig deur die direkte waarneming van al die verval produkte van die CCT. Die CCT was ook hier bevestig as ŉ verval proses wat plaasvind in ŉ tweestap opbreek van die oorspronklike drie deeltjie ketting kern konfigurasie met ŉ verlengde sentrale bondel.
Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105013
Relativistic Brueckner-Hartree-Fock theory in nuclear matter without the average momentum approximation ﻿