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Search for low-spin states above the 5-α break-up threshold in 20Ne

Swartz, Jacobus Andreas (2014-04)

Thesis (PhD)--Stellenbosch University, 2014.


ENGLISH ABSTRACT: The study of α clustering is a well-established topic of research in nuclear physics. Recent experimental evidence has revealed the first 2+ excitation of the Hoyle state in 12C, which is known to have a strong α cluster structure. The idea of multi-particle α cluster structures in light nuclear matter has received much attention from theoretical investigations of late. This research has profound implications in the fields of both nuclear structure and nuclear astrophysics. The 20Ne nucleus is a good example for α clustering, since many of its states are known to have α clustering structures. Few low spin states are known at high excitation energies of this nucleus. It is predicted that this nucleus contains a 0+ 5-α cluster state, a so-called ‘Hoyle analogue state’, above its 5-α break-up threshold at Ex = 19.17 MeV. This thesis presents a study of the 20Ne nucleus with the 22Ne(p,t)20Ne reaction at laboratory angles θlab =(0◦, 7◦, 16◦, 27◦). The iThemba LABS K600 magnetic spectrometer was employed with a beam of energy Elab = 60 MeV, incident upon a 22Ne gas target held intact by Aramid foils. The aim was to search for low spin states in 20Ne at excitation energies above Ex = 15 MeV, and, possibly, to find an indication of the 5-α cluster state. Three narrow states were discovered at energies of Ex = 20.59 MeV, Ex = 21.16 MeV and Ex = 21.80 MeV. Calculations performed with the isobaric multiplet mass equation indicate that these states may be T = 2 isobaric analogue states of three known states in 20O. However, shell-model calculations indicate that these states may also have T = 0 or T = 1 isospin values. There is also evidence of a new state at Ex = 17.67 MeV and, possibly, of a collection of new states which could not be resolved at Ex = 18.84 MeV. A tentative candidate for the desired 5-α cluster state was observed, but this will require another measurement with cleaner background to be confirmed.

AFRIKAANSE OPSOMMING: Alfa bondelvorming is ’n gevestigde navorsingsonderwerp in kernfisika. Daar is onlangse bewyse vir die ontdekking van die 2+ opwekking van die Hoyle toestand in 12C, wat ’n erkende alfa bondelstruktuur het. Die idee van multi-alfa bondelstrukture in ligte kerne het onlangs baie aandag geniet in teoretiese ondersoeke. Hierdie navorsing het besondere implikasies vir kernstruktuur, sowel as vir kernastrofisika. Die 20Ne kern bied ‘n ideale voorbeeld vir alfa bondelvorming aangesien dit bekend is dat baie van die kern se toestande alfa bondelstrukture het. Min lae-spin toestande is by ho¨e opwekenergie¨e bekend in hierdie kern. Daar word voorspel dat ’n 0+ 5-alfa bondeltoestand, ’n sogenaamde ‘Hoyle analoogtoestand’, bo die drumpel vir 5-alfa verval by Ex = 19.17 MeV bestaan. Hierdie tesis beskryf ‘n studie van die 20Ne kern met die 22Ne(p,t)20Ne reaksie by laboratorium hoeke van θlab =(0◦, 7◦, 16◦, 27◦). Die K600 magnetiese spektrometer van iThemba LABS is gebruik met ‘n proton bundel, by ‘n energie van Elab = 60 MeV, wat op ‘n 22Ne gas teiken omhul met aramid foelies gerig is. Die doel was om lae-spin toestande in 20Ne by opwekenergie¨e bo Ex = 15 MeV op te spoor, en om moontlik ook ‘n aanduiding van die 5-alfa bondeltoestand te vind. Drie smal toestande is by energie¨e van Ex = 20.59 MeV, Ex = 21.16 MeV en Ex = 21.80 MeV opgespoor. Berekeninge wat met die isobariese multiplet massa vergelyking uitgevoer is, dui daarop dat hierdie toestande T = 2 isobariese analoogtoestande van drie bekende toestande in 20O kan wees, hoewel skilmodel berekeninge ook T = 0 en T = 1 kandidate aandui. Daar is ook bewyse van ’n nuwe toestand by Ex = 17.67 MeV, en moontlik van ’n versameling nuwe toestande rondom Ex = 18.84 MeV wat nie uitmekaar geken kon word nie. ‘n Tentatiewe kandidaat vir die gesogte 5-alfa bondel toestand is waargeneem, maar nog ‘n meting met ’n skoner agtergrond word vir bevestiging benodig.

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