Detection of Stable Isotopes Using Resonance Ionisation Spectroscopy

Drysdale, Shauna Lynn Taylor (1989) Detection of Stable Isotopes Using Resonance Ionisation Spectroscopy. PhD thesis, University of Glasgow.

Abstract

The laser ionisation studies group was formed in 1983 with the initial task of using the resonance ionisation technique to detect impurities in counter gases used in multiwire drift chambers at CERN. After completion of this high energy physics application, the technique was then directed towards the development of a combination of resonance ionisation with mass spectrometry (RIMS) for the purpose of ultra sensitive trace analysis. To this end, two resonance ionisation time of flight mass spectrometers have been designed and constructed, one designed primarily for ultra trace element analysis and the other for depth profiling in semiconductors. In order to maximise the sensitivity of the RIMS technique, the physics behind the resonance ionisation process must be clearly understood. Therefore, the ionisation spectroscopy of the alkali metals caesium and rubidium was investigated using a proportional counter and also a quadrupole mass spectrometer to distinguish between the two isotopes of rubidium. Chapter 1 gives a brief history of resonance ionisation spectroscopy, and explains the basic ideas behind the technique. Theoretical rate equations relating to the two photon ionisation case are given, and the appropriate saturation conditions are derived. The experimental apparatus is described in detail in Chapter 2. All types of lasers used are described, and the operation of the three detectors used (proportional counters, quadrupole mass spectrometers and time of flight mass spectrometers) is explained. Chapters 3 and 4 document the results obtained from the proportional counter and the quadrupole mass spectrometer. The spectroscopy of caesium metal using a proportional counter is described in Chapter 3. Two and three photon ionisation spectra are shown, and investigations made into the requirements for saturation of these transitions is discussed. Rubidium ionisation spectra using both the proportional counter and the quadrupole mass spectrometer are presented in Chapter 4. The author was largely responsible for molecular mass spectra obtained using the quadrupole mass spectrometer. Chapter 5 explains the choice of time of flight mass spectrometers, and describes the construction and operation of the two laser ionisation mass spectrometers at Glasgow. The technique of laser desorption is also explained. In the later sections of the chapter, methods for optimising the resolution of the laser mass spectrometers are discussed. The initial results taken from the trace analysis instrument at Glasgow are presented in Chapter 6. This chapter can be divided into three distinct sections. Firstly, spectra of the background ionisation within the spectrometer are shown and discussed, and experiments to discover the source of the ionisation are described. As carbon was found to be a major contaminant in the spectra obtained, further spectra of pure graphite were recorded. This progressed to a study of carbon clustering effects, documented in the final section of this chapter. In the second section, laser desorbed spectra of standard reference material (NBS coal samples) were obtained in order to investigate the qualitative and quantitative accuracy of this non resonant technique. Results of resonant ionisation detection of rubidium at sub parts per million levels are then given, and are compared with the non resonant detection technique to establish the sensitivity and suitability of RIMS in trace element detection. The author was largely responsible for the experimental results and solely responsible for the data analysis in this chapter. The concluding chapter suggests improvements which could enhance the detection sensitivity, and describes future experiments on both spectrometers at Glasgow. Research work relating to high energy physics is described in Appendix 1. The first section deals with the identification of two sources of background ionisation, phenol and toluene. Section 2 describes research carried out on a gas purification system for the ALEPH Time Projection Chamber at CERN, and establishes its effectiveness at removing oxygen and water contaminants from the counter gas. The author was responsible for the study of the gas purification system. Appendix 2 describes the second laser resonant ionisation mass spectrometer constructed at Glasgow for the main purpose of depth profiling in semiconductors.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Nuclear physics and radiation
• Date of Award: 1989
• Depositing User: Enlighten Team
• Unique ID: glathesis:1989-77966
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 30 Jan 2020 15:46
• Last Modified: 30 Jan 2020 15:46
• URI: https://theses.gla.ac.uk/id/eprint/77966