Spectroscopy of atomic zinc inside a heat pipe: Towards resonance ionisation

Hattingh, Brandon Dane (2019-04)

Thesis (MSc)--Stellenbosch University, 2019

Thesis

ENGLISH ABSTRACT : Zinc (Zn) reserves in South Africa are not being used to full potential. Separating Zn into its different stable isotopes drastically increases its value, as well as usefulness in certain areas such as research, industry and medicine, for example, 68Zn is a precursor of 68Ga used for Positron-Emission Tomography/Computed Tomography (PET/CT) scans to detect tumours. This project aims to develop a complementary experimental setup and computational model in aid of a larger project that aims to develop an efficient and feasible laser-based process to separate Zn isotopes. Time-delayedreference-beam Atomic Absorption Spectroscopy (AAS) inside a heat pipe is used to optimise experimental parameters. Computationally, Einstein rate equations are used to describe the interaction of atoms with the incident laser beam. The avialable pulsed dye laser provides pulses of 8 ns duration and peak intensity of 7.5 × 108 W/m2 . Using the Einstein rate equations, it is found that, for the dipole allowed 4s2 1S0 → 4s4p 1P1 o transition, 5 ps is needed for the population of the energy levels to reach a steady state with a constant laser intensity of 7.5 × 108 W/m2 . The peak intensity needed to saturate this transition when using a pulse with Gaussian temporal envelope and FWHM of 8 ns is 2 × 107 W/m2 . For the dipole forbidden 4s2 1S0 → 4s4p 3P1 o transition, 30 ns is needed to reach a steady state with a constant laser intensity of 7.5 × 108 W/m2 . When using a Gaussian pulse with a FWHM of 8 ns, a peak intensity of more than 7.5 × 108 W/m2 is needed to saturate this transition. For the 4s2 1S0 → 4s4p 1P1 o transition, the absorption cross-section is found to be σ = 3.3 × 10−16 m2 , using a general equation derived from the rate equations. Moreover, it is found that a temperature of 668 K for the Zn atoms is needed for 50% absorption in the heat pipe, when using an intensity of 7.5 × 108 W/m2 . Experimentally, the measured absorption line was found to have a spectral FWHM of 0.4 nm, which is large compared to the simulated spectral FWHM of 0.0025 nm. The broadening is likely due to power broadening and broadening due to collisions, which is not accounted for in the computational model. Improved conditions, that should be implemented when using the setup in the future, were derived from the combined experimental and computational results.

AFRIKAANSE OPSOMMING : Die sink (Zn) reserwes in Suid-Afrika word nie tot volle potensiaal benut nie. Die skeiding van die verskillende stabiele Zn isotope van mekaar sal die waarde, sowel as die bruikbaarheid in velde soos navorsing, industrie en medisyne, drasties verhoog. 68Zn is byvoorbeeld 'n voorloper van 68Ga wat gebruik word vir Positron-Emissie Tomografie/Rekenaartomografie (PET/CT) skanderings om kanker op te spoor. Hierdie projek se doel is om 'n komplementêre eksperimentele opstelling en numeriese model te ontwikkel wat van hulp sal wees 'n in groter projek waarvan die doel is om 'n doeltre ende en haalbare laser-gebaseerde proses te ontwikkel om Zn isotope te skei. Tyd-vertraagde-verwysingsbundel Atoom Absorpsie Spektroskopie (AAS) in 'n hittepyp word gebruik om die eksperimentele parameters te optimeer. In die numeriese model word die Einstein koersvergelykings gebruik om die interaksie van die atome met die inkomende laserbundel te beskryf. Die beskikbare laser lewer pulse van 8 ns duur met 'n piekintensiteit van 7.5 × 108 W/m2. Deur die gebruik van die Einstein koersvergelykings word gevind dat, vir die dipooltoegelate 4s2 1S0 → 4s4p 1P1 o oorgang, 5 ps nodig is vir die bevolking van die energievlakke om 'n ewewigstoestand te bereik met 'n konstante laser intensiteit van 7.5×108 W/m2. Die piekintensiteit wat nodig is om hierdie oorgang te versadig wanneer 'n laserpuls met 'n Gaussiese tydpro el en 'n volle-wydte-by-half-maksimum van 8 ns gebruik word is 2 × 107 W/m2. Vir die dipool-verbode 4s2 1S0 → 4s4p 3P1 o oorgang is 30 ns nodig om 'n ewewigstoestand te bereik met 'n konstante laser intensiteit van 7.5 × 108 W/m2 . Wanneer 'n Gaussiese puls met 'n wydte van 8 ns gebruik word is 'n piekintensiteit van meer as 7.5 × 108 W/m2 nodig om die oorgang te versadig. Vir die dipool-toegelate 4s2 1S0 → 4s4p 1P1 o oorgang is die absorpsiedeursnit bepaal as σ = 3.3 × 10−16 m2 deur gebruik te maak van 'n algemene uitdrukking, wat afgelei is van die koersvergelykings. Verder is daar gevind dat 'n temperatuur van 668 K vir die Zn atome nodig is om 50% absorpsie in die hittepyp te kry, wanneer 'n intensiteit van 7.5 × 108 W/m2 gebruik word. In die eksperimentele metings is daar gevind dat die absorpsielyn 'n spektrale wydte van 0.4 nm het, wat groot is in vergelyking met die berekende spektrale wydte van 0.0025 nm. Die verbreding is waarskynlik die gevolg van versadiging en drukverbreding wat nie in ag geneem word in die numeriese model nie. Van die gekombineerde eksperimentele en numeriese resultate is verbeterde kondisies afgelei, wat gebruik behoort te word wanneer die opstelling in die toekoms aangewend word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/106035
This item appears in the following collections: