Resonant ionization spectroscopy for laser isotope separation of zinc isotopes

De Bruyn, Andre (2021-12)

Thesis (PhD)--Stellenbosch University, 2021.


ENGLISH ABSTRACT: The vast zinc (Zn) reserves in South Africa are currently being underutilized, as the Zn ore refinement and isotope enrichment occurs abroad. The enrichment process most commonly used thus relies only on mass separation techniques and is therefore limited when separating isotopes of similar masses like 68Zn and 67Zn. Pure 68Zn and 67Zn are important stable nuclides in the medical industry as these isotopes are used for production of gallium-based radiopharmaceuticals used in positron emission tomography (PET) and single-photon emission computerised tomography (SPECT) scans. Additionally, depleted 64Zn is used in the cooling water of nuclear reactors for the prevention of stress corrosion cracking as well as limiting the formation of cobalt radioisotopes. This study aimed to investigate, model and optimize resonant ionization spectroscopy (RIS) schemes for the Zn isotopes 68Zn and 67Zn that are suitable for laser-based enrichment of these isotopes from natural Zn. This study reports on the development and implementation of the first experimental RIS system combined with a time-of-flight mass spectrometer at Stellenbosch University. This system was used to investigate a resonance ionization scheme that has potential for laser-based enrichment of Zn isotopes from a natural Zn sample. The ionization scheme was chosen by considering transition wavelengths, the hyper fine splitting of 67Zn, and transition strengths of intermediate levels. The effect of physical conditions and laser parameters was experimentally investigated. The experimental results were complemented by a rate equation model of the RIS scheme developed in this study. The experimental measurements included laser induced fluorescence (LIF) from both the first and second intermediate levels in the RIS scheme. The experimental LIF results contributed to optimization of the experiment and refining the rate equation model by confirming the laser bandwidth and helping to estimate an unknown transition probability. The experimental RIS measurements confirmed the model results for the existing experimental conditions, including the effects of laser tuning, pulse timing and laser pulse energy. The limiting factor to the isotope selectivity of the RIS experiment was the bandwidth of the dye laser used for the first excitation step. The simulated results of the rate equation model that was developed in this study was tested against experimental results and was found to be in good agreement. The model was also applied to conditions which could be found in industrial scenarios. The results indicated that when using a commercially available narrow bandwidth Ti:sapphire laser ( 300 MHz) it is possible to achieve 85% enrichment of 67Zn if the Zn vapour is suffciently cooled ( 100 K).

AFRIKAANSE OPSOMMING: Die groot sink (Zn) reserwes in Suid-Afrika word tans onderbenut, omdat die Zn erts se rafinering en isotoopverryking in die buiteland gedoen word. Die verrykingsproses wat die meeste gebruik word berus slegs op massaskeidingstegnieke en het daarom beperkings as soortgelyke massas soos 68Zn en 67Zn geskei word. Suiwer 68Zn en 67Zn is belangrike stabiele isotope in die mediese industrie omdat hierdie isotope gebruik word vir produksie van galliumgebaseerde radiogeneeskundige middels wat gebruik word in positron emissie tomografie (PET) en enkel-foton emissie rekenaar-tomografie (SPECT) skanderings. Addisioneel word gesuiwerde 64Zn gebruik in die verkoelingswater van kernreaktors om spanningskorosie en kraakvorming te verhoed, en om die vorming van die cobalt-60 radioaktiewe isotoop te beperk. Hierdie studie het ten doel om resonante ionisasie-spektroskopie (RIS) skemas wat geskik is vir laser-gebaseerde verryking van die Zn isotope 68Zn en 67Zn te ondersoek, te modelleer en te optimeer. Die studie doen verslag oor die ontwikkeling en implementering van die eerste eksperimentele RIS systeem by Stellenbosch Universiteit. Hierdie sisteem is gebruik om 'n resonante ionisasie skema te ondersoek wat potensiaal het vir die laser-gebaseerde verryking van Zn isotope vanaf natuurlike Zn. Die ionisasie skema is gekies met oorweging van oorgangsgol engtes, die hiperfyn splitsing van 67Zn, en oorgangsterktes van intermediêre energievlakke. Die effek van fisiese toestande en laser parameters is eksperimenteel ondersoek. Die eksperimentele resultate is aangevul deur 'n tempovergelykingmodel van die RIS skema wat as deel van die studie ontwikkel is. Die eksperimentele metings het laser-geïnduseerde uoressensie (LIF) vanaf beide die eerste en tweede intermediêre vlakke in die RIS skema ingesluit. Die eksperimentle LIF resultate het daartoe bygedra om die eksperiment te optimeer en die tempovergelykingmodel te verfyn, deurdat die laser bandwydte bevestig is en dit gehelp het om 'n onbekende oorgangswaarskynlikheid af te skat. Die eksperimentele RIS metings het die modelresultate bevestig vir die bestaande eksperimentele kondisies, insluitende die effekte van laser afstemming, puls aankomtye en laser puls energie. Die beperkende faktor in die isotoopselektiwiteit van die RIS eksperiment was die bandwydte van die kleurstoflaser wat vir die eerste opwekkingstap gebruik is. Die gesimuleerde resultate van die tempovergelykingmodel wat ontwikkel is in hierdie studie is gekontroleer deur vergelyking met eksperimentele resultate en dit was in ooreenstemming. Die model is ook toegepas op toestande wat in industriële senarios gevind kan word. Die resultate dui aan dat dit moontlik is om 'n verryking van 67Zn to 85% te kry wanneer daar gebruik gemaak word van 'n kommersiële smalband Ti:sapphire laser ( 300 MHz) en as die Zn-damp voldoende afgekoel is ( 100 K).

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