Spectroscopy of 22Mg relevant to explosive nucleosynthesis in classical novae and X-ray bursts

Brummer, Johann Wiggert (2020-04)

Thesis (PhD)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: This thesis discusses the spectroscopy of 22Mg. This was done by performing the 24Mg(p,t) 22Mg reaction at iThemba LABS using the K600 magnetic spectrometer in coincidence with the CAKE silicon-detector array. Using this experimental setup, resonances in 22Mg were studied and proton decays from this nucleus to the ground state and excited states in 21Na were detected with the CAKE. The 22Mg nucleus was studied for two different reasons. Firstly, it is the compound nucleus in the 18Ne(α, p) 21Na HCNO breakout reaction in X-ray bursts (XRBs). Secondly, the 24Mg(p,t) 22Mg dataset also revealed that it is possible to use the excitation energy spectrum below the α-particle separation threshold, Sα, at 8.142 MeV to study the 21Na(p, γ) 22Mg reaction. The 18Ne(α, p) 21Na reaction is one of a number of crucially-important reactions that greatly influence t he e nergy g eneration i n X RBs. T herefore, it also affects the shape of the lightcurve considerably which is the only realistic observable of these events and only means to detect and study stellar XRBs. Direct measurements of this reaction are challenging and, to date, many studies have performed a variety of different types of measurements to determine the thermonuclear reaction rate. In particular, one such study performed the time-reversed reaction. The data from that study were used and reevaluated in this thesis. The resultant excitation energy spectrum from the 24Mg(p,t) 22Mg experiment was analysed in two sections. Above Sα the spectrum was analysed in 100-keV bins up to an excitation energy of 13 MeV. The proton branching ratios Bpi (i ∈ [0, 4]) were calculated for each bin. A moving-average function was determined for the proton branching ratio to the ground state, Bp0 , to calculate the cross section as a function of energy, σ(E), using cross section data from the previous study of this reaction. This was used to recalculate the thermonuclear reaction rate of 18Ne(α, p) 21Na. From this study it is concluded that the reaction rate varied by no more than a factor of 2.3 higher as compared to the average reaction rate from the previous study that performed the time-reversed reaction. The recalculated rate is in good agreement, within uncertainties, with another study that made an estimate of this rate by means of evaluating all pre-existing data. In that study, the reaction rate was determined by evaluation of the uncertainties from previous 18Ne(α, p) 21Na reaction-rate measurements and determining the regions where those uncertainties overlap. Results from this thesis were used with a stellar simulation code, MESA, to simulate the lightcurve of a particular XRB event. Clear changes to the lightcurve, due to the recalculated reaction rate, were apparent. The 21Na(p, γ) 22Mg thermonuclear reaction rate is linked to the production of 22Na by means of β emission from the 22Mg nucleus. The production of 22Na plays an important role in the ability of astronomers to detect and study the astrophysical phenomenon of classical novae. For the 21Na(p, γ) 22Mg reaction the compound nucleus is also 22Mg. The region of the excitation energy spectrum below Sα was resolved and as a result isolated, narrow resonances were studied. The resonance energies and widths were determined by fitting the spectrum using Gaussian and Voigt functions. The proton branching ratios were calculated and possible Jπ assignments were discussed. However, owing to the physical and electronic thresholds of the silicondetector array, the protons from the decay of resonances in the astrophysicallyrelevant Gamow energy region were too low in energy to detect successfully. Therefore, it was not possible to use their branching ratios to calculate the thermonuclear reaction rate of 21Na(p, γ) 22Mg. To achieve this, subsequent experiments that aim to calculate this rate may need to use silicon detectors that can operate at lower thresholds. Alternatively, active-target detectors can be used as they have very low energy thresholds coupled with high detection efficiencies.

AFRIKAANSE OPSOMMING: Hierdie tesis bespreek die spektroskopie van 22Mg. Dit is moontlik gemaak deur ’n eksperiment waarin die 24Mg(p,t) 22Mg reaksie uitgevoer is by iThemba LABS met die K600 magnetiese spektrometer in voorwaartse hoeke en die CAKE silikondetektor opstelling gelyktydig in terugwaartse hoeke. Met hierdie eksperimentele opstelling was resonans toestande in 22Mg bestudeer. Proton verval van hierdie kern na die grond toestand en verskeie opgewekte toestande in 21Na was gevolglik opgespoor met die CAKE. Die 22Mg kern is bestudeer vir twee afsonderlike redes. Eerstens, dit is die saamgestelde kern in die 18Ne(α, p) 21Na HCNO wegbreek reaksie in X-straal ontploffings in sterre. Tweedens, data vanuit die 24Mg(p,t) 22Mg eksperiment het aan die lig gebring dat dit moontlik is om die opwekkings-energie spektrum tot en met Sα te gebruik om die 21Na(p, γ) 22Mg reaksie te bestudeer. Die 18Ne(α, p) 21Na reaksie is een van ’n aantal hoogs-belangrike reaksies wat die energie produksie in hierdie astrofisiese o ntploffings g rootliks beïnvloed. Om hierdie rede beïnvloed die reaksie ook die vorm van die X-straal ontploffing ligkurwe aansienlik. Dit is die enigste prakties-waarneembare maatstaaf van hierdie gebeurtenisse en die enigste manier om hierdie soort ontploffings op te spoor en te bestudeer. Direkte metings van hierdie reaksie is uitdagend and tot op hede het menigde vorige studies hierdie reaksie op verskillende wyse ondersoek om sodoende die reaksietempo te probeer meet. Een so ’n spesifieke s tudie h et die tyd-omgekeerde reaksie uitgevoer. Die data van daardie studie is in hierdie tesis gebruik en herevalueer. Die gevolglike opwekkings-energie spektrum van die 24Mg(p,t) 22Mg reaksie is in twee afdelings geanaliseer. Bo die α-deeltjie verval drumpel, Sα, by 8.142 MeV is die spektrum in 100-keV energie gedeeltes geanaliseer tot by ’n opwekkings-energie van 13 MeV. Die proton vertakkings verhoudinge Bpi (i ∈ [0, 4]) was vir elke 100-keV energie gedeelte bereken. ’n Bewegendegemiddelde funksie is bepaal vir die proton vertakkings verhoudinge na die grond toestand, Bp0 , om die reaksiekansvlak as ’n funksie van energie, σ(E), te bereken m.b.v. data vanuit ’n vorige studie van hierdie reaksie. Die kansvlak was gebruik om die nuwe reaksietempo van die 18Ne(α, p) 21Na reaksie te bereken. Die reaksietempo wat gevolglik in hierdie tesis bereken is is hoogstens 2.3 maal groter as die reaksietempo wat bereken is deur die vorige studie wat die tyd-omgekeerde reaksie uitgevoer het. Die herberekende reaksietempo vergelyk baie goed, binne die onsekerheidsvlakke, met ’n vorige studie wat ’n skatting van hierdie reaksietempo uitgewerk het deur alle vorige data van hierdie reaksie te herevalueer. Die skatting is bepaal deur die mees waarskynlike oorvleueling van die onsekerhede van datapunte vir die 18Ne(α, p) 21Na reaksietempo te neem. Resultate vanuit hierdie tesis is gebruik in ’n astrofisiese simulasie program, MESA, om die ligkurwe van ’n spesifieke X-straal ontploffing gebeurtenis te simuleer. Duidelike verskille in die ligkurwe is opgemerk weens die verskille in die nuwe reaksietempo. Die 21Na(p, γ) 22Mg reaksietempo is gekoppel aan die produksie van 22Na d.m.v. β verval van die 22Mg kern. Die produksie van 22Na speel ’n baie belangrike rol in die vermoë van sterrekundiges om die astrofisiese verskynsel van klassieke novae op te spoor en te bestudeer. Die saamgestelde kern van die 21Na(p, γ) 22Mg reaksie is ook 22Mg. Die gedeelte van die opwekkings-energie spektrum tot en met Sα was opgelos vanweë die goeie eksperimentele resolusie. Dit was dus moontlik om die geïsoleerde, vernoude resonans toestande te bestudeer. Die resonans energieë en wydtes was bereken deur ’n passing te maak in die spektrum met Gaussian en Voigt funksies. Hierdeur was dit moontlik om die proton vertakkings verhoudinge te bereken en om moontlike Jπ toekennigs te bespreek. Dit was egter nie moontlik om resonans toestande in die astrofisies-relevante Gamow-energie gebied suksesvol te verken nie omdat die toestande te laag in energie is. Dit is vanweë die fisiese en elektroniese drumpels in die silikon detektors. Dus kon hul proton vertakverhoudings nie gebruik word om die 21Na(p, γ) 22Mg reaksietempo te bereken nie. Om hierdie probleem te oorkom in moontlike daaropvolgende eksperimente kan silikon detektors gebruik word wat by laer fisiese en elektroniese drumpels kan funksioneer. As ’n alternatief kan aktiewe-teiken detektors gebruik word omdat hierdie soort toerusting by baie lae-energie drumpels kan meet met ’n baie hoë doeltreffenheid.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107992
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