A ghostly tail of the Hoyle state and its breathing-mode excitation

Li, Kevin Ching Wei (2020-04)

Thesis (PhD)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: 12C is almost exclusively produced in stars from a combination of 4He and unstable 8Be which form an excited three-alpha resonance named the Hoyle state, located just above the 3α separation energy at Ex = 7.65407(19) MeV. Whilst this resonance is understood to be the archetypal α-cluster state, 12C is understood to straddle the region between the shell-model and collective pictures as its ground state presents with a large overlap with shell-model states. The significant work by the nuclear physics community to theoretically describe 12C is therefore of both astrophysical and structural significance. An indication that an unlisted source of monopole strength may manifest at Ex ≈ 9 MeV was given by the peak-fitting and Multipole Decomposition Analysis (MDA) of M. Itoh et al. which suggested a double-peaked, broad monopole structure in the excitation-energy region of Ex ≈ 8-13 MeV just above the Hoyle state. Some recent theoretical works which employ formalisms such as AMD, 3α OCM and GCM suggest that this unestablished origin of monopole strength may be understood as the breathing-mode excitation of the 0+ 2 Hoylestate, corresponding to a low-energy fragment of the isoscalar giant monopole resonance (ISGMR). The primary motive of this thesis was to search for any unidentified sources of monopole strength in the Ex ≈ 7-16 MeV excitation-energy region and investigate if they exhibit strong collectivity. A new measurement of the 14C(p,t)12C reaction at θlab = 0◦ was performed with the K600 magnetic spectrometer and the CAKE array to detect coincident charged-particle decay. This data was combined previous datasets of 14C(p,t)12C at 21◦ and 12C(α,α0)12C at θlab = 0◦(×2), 6◦, 8.5◦ and 10◦ to be simultaneously analysed with a new multilevel multichannel R-matrix analysis fitting code. A total of 6 hypotheses were explored and the best quality model corresponded to the monopole structure in the Ex ≈ 7-16 MeV excitation-energy region being modeled by a two-level approximation with coherent interference and an additional 0+ resonance at Ex = 9.270(14) MeV with a width of Γtot = 1581(58) MeV. The extracted parameters for this resonance are in excellent agreement with both the aforementioned measurement by M. Itoh et al. as well as various theoretical predictions. There is particularly good agreement with the OCM calculation by C. Kurokawa and K. Kato¯ which yielded a resonance energy of Ex ≈ 8.95 MeV and a width of Γtot = 1.48 MeV for the 0+ 3 state [1]. The angular distribution of α0 decay in the associated interval of 8.5 MeV < Ex < 9.0 MeV was found to exhibit isotropy which implies that the strength is predominantly monopole. The identification of this new source of monopole strength may play a significant role in the 3α rate in certain astrophysical conditions.

AFRIKAANSE OPSOMMING: 12C word in sterre geproduseer deur die absorpsie van ’n 4He kern deur ’n onstabiele 8Be kern om die opgewekte drie-alfa resonans toestand in 12C, die sogenaamde Hoyle toestand, te vorm. Hierdie toestand bestaan by Ex = 7.65407(19) MeV wat net bokant die opbreek drumpel na drie alfas gelëe is. Alhoewel die Hoyle toestand die klassieke voorbeeld van ’n alpha-toestand is, dek 12C die gebied tussen die skilmodel en kollektiewe-model beskrywing aangesien dat die grond toestand tog ’n groot oorvleuling met skilmodel toestande het. Die beduidende werk wat deur die fisika gemeenskap gedoen is om 12C teoreties te beskryf is derhalwe vir beide astrofisika en kernstruktuur studies van belang. Daar bestaan aanduidings van ’n ongelyste toestand met moontlike monopool eienskappe wat by Ex ≈ 9 MeV voorkom, soos getoon in die werk van Itoh et al. In hul navorsing het Itoh et al. deur middel van piek passings en multipool dekomposisie ’n breë 0+ dubbelpiek struktuur in die energie gebied Ex ≈ 8-13 MeV geidentifiseer, net bokant die Hoyle toestand. In teoretiese berekeninge wat onlangs verskyn het en wat van verskeie formalismes soos AMD, 3α OCM en GCM gebruik gemaak het, is daar aanduidinge dat die oorsprong van hierdie monopool sterkte moontlik verklaar kan word as ’n sg. opgewekte asemhalings variante van die Hoyle toestand, wat in dieselfde energie gebied voorkom as die lae-energie fragment van die Reuse Isoskalare Monopool Resonansie (ISGMR). Die primere motivering van hierdie tesis was om vir ongeidentifiseerde bronne van monopool sterkte in die Ex ≈ 7-16 MeV opwekkingsenergie gebied te soek wat sterk kollektiewe eienskappe het. ’n Nuwe meting by θlab = 0◦ met die 14C(p,t)12C reaksie is met die K600 magnetiese spektrometer gemaak in koinsidensie met die CAKE detektor ten einde gelaaide verval deelties waar te neem. Daarna is hierdie data met voorheen gemete 14C(p,t)12C data by 21◦, 12C(α,α0)12C data by θlab = 0◦ (twee stelle), 6◦, 8.5◦ en 10◦ gekombineer en gelyktydig met ’n nuwe multivlak, multikanaal R-matriks analiese kode gepas. Daar is ses hypotesise getoets vir passings in die Ex ≈ 7-16 MeV gebied. Die beste model is ’n dubbele 0+ toestand sisteem met koherente interferensie tussen die toestande, waar die laagste toestand by Ex = 9.270(14) MeV gevind is met ’n wydte van Γtot = 1581(58) MeV. Dit is in goeie ooreenstemming met die Ex = 8.95 MeV, Γtot = 1.48 MeV, tot nou toe ongelyste 0+ toestand soos voorspel deur C. Kurokawa and K. Kato¯. Die hoekverdeling van die verval wat geassosieer is met die interval in opwekkings energie 8.5 MeV < Ex < 9.0 MeV is isotroop wat impliseer dat die opgewekte toestand oorwegend monopool is. Die bevestiging en positiewe identifikasie van hierdie nuwe bron van monopool sterkte mag ’n beduidende rol speel in die 3α reaksie tempo onder astrofisiese toestande.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/108301
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