Photochemical Sciences Ph.D. Dissertations

Title

New Developments in Positron Annihilation Spectroscopy Techniques: From Experimental Setups to Advanced Processing Software

Date of Award

2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Farida Selim (Advisor)

Second Advisor

Alexander Tarnovsky (Committee Member)

Third Advisor

Mikhail Zamkov (Committee Member)

Fourth Advisor

Ellen Gorsevski (Other)

Abstract

Fully-digital Coincidence Doppler broadening (CDB) spectrometer with a software coincidence unit and energy resolution function of 1.2 keV at 511 keV is assembled at Bowling Green State University. A computer program - TLIST Processor - that fits and subtracts background on the two-dimensional energy spectrum and exports CDB spectrum is developed. Another program (SW Calculator) that performs deconvolution of the CDB spectrum into different contributions (e⁺ annihilation on conduction band, valence and core electrons, para-Ps annihilation in pores) is developed. Program evaluates S and W parameters of the CDB spectra and allows calculation of the Fermi energy for simple metals.

Experimental spectra of pure and well annealed Al, Fe, Cu and Zn samples are measured and processed with a help of the developed software. The obtained values of the Fermi energies and binding energies of core electrons are in a good agreement with the reference values for the investigated metals. This result validates the correct operation of the CDB setup.

Tuning of electronic circuits and optimization of geometry of the detection unit of the conventional analog lifetime (LT) spectrometer were performed annually. Three e⁺ sources were manufactured from aqueous solution of ²²NaCl. A convenient technology that allows sealing the activity in between of thin Kapton foils is suggested. New computer software RooPositron that allows processing the lifetime spectra is developed. Program accounts on a type of the studied materials (metals, powders, liquids) and positron reactions therein. The advantage of the program is ability to integrate custom fitting models with meaningful physical parameters that describe e⁺ behaviour in the studied medium.

LT spectra acquired at different recording time intervals and CDB spectra of Al₂O₃ nanopowders (0.05, 0.3,1 and 3 μm) were measured and interpreted by means of the RooPositron software. It was shown that consistent interpretation of the data is possible when thermalization process of the "hot" Ps atom in the intercrystalline space in nanopowders is taken into account. Kinetics of the Ps thermalization is essentially non-exponential, but the developed software is able to proceed with this.

More detailed model of the Ps formation in liquids is suggested. It takes into account formation of the intermediate/transient Ps state, the quasifree positronium, which further transforms into the Ps bubble state. It provides reasonable explanation of the deviation of I₁/I₃ ratio from the value of 1/3 when conventional 3-exponential decomposition of the LT spectra is applied. It also explains the "juvenile broadening" effect observable in ACAR experiments. Interpretation of the LT spectra in water, i-octane, c-hexane and i-propanol with a help of the developed RooPositron program allowed to obtain the reaction rate constants of the Ps oxidation and Ps ortho-para conversion in these liquids with various concentrations of dissolved O₂.

All the program code developed in current research is publicly available.

Link to Full Text
COinS