Auger and conversion electron spectroscopy of medical isotopes and the construction of a new atomic radiation database

Abstract

A localized source of electrons can be provided by radioisotopes that emit Auger electrons through atomic relaxation as part of their electron capture and/or internal conversion decays. Most of these Auger electrons could deposit their energies over short distances (nanometre to micrometre) in human body, making them an attractive tool to be used for highly-targeted radiation therapy. In this thesis, a semi-empirical energy correction method is developed to correct for the overestimated X-ray and Auger transition energies in the Monte-Carlo atomic relaxation model BrIccEmis. The improved model is tested against selected Auger electron experimental data, including our own measurements.

The Auger and conversion electrons of the widely-used medical isotopes, 125I and 99Mo, are measured together at high-resolutions (3 eV to 9 eV) using the cylindrical mirror analyzer and the hemispherical electrostatic spectrometer. The relatively well-known conversion electrons are used to calibrate the Auger electrons to obtain accurate Auger yields.

The full electron spectra (0 eV to 35 keV) from the electron capture of 125I are measured and analyzed. A new set of nuclear parameters, namely the penetration parameter and mixing ratio, of the 35.5-keV M1 transition from the decay of 125I is obtained. The measurements are compared with BrIccEmis calculations and overall a reasonable agreement is found.

A method to prepare thin 99Mo sources on alumina substrates is developed, and the resulting sources are found to have thicknesses of about 20 nm. The electron spectra (200 eV to 2.3 keV) from the beta-decay of 99Mo are measured and analyzed. However, due to the relatively thick molybdenum sources, a quantitative result could not be obtained from the measured Auger electron spectra. One possible way to improve the source quality, which is to use an evaporated aluminum on alumina substrate for the deposition, is discussed.

Finally, an atomic radiation database called BrIccEmisDB is constructed based the calculations on the BrIccEmis code. The database could provide quick and detailed X-ray and Auger spectra from nuclear decays, particle-impact ionizations, or any other type of ionization events. The BrIccEmisDB database is shown to be more accurate than the widely used radiation programs MIRD-RADTABS and Geant4, especially at low energies (less than 1 keV), through comparisons with experiments. Show more