Laser Ionisation Studies - Applications in Nuclear Waste Monitoring

[摘要] This thesis is concerned with the development of the new, laser-based analytical technique of Resonance Ionisation Spectroscopy (RIS) for monitoring of components of nuclear waste materials, in the environment, which are difficult to assay by radiometry. Chapter one begins with a review of the literature on RIS since its inception in 1975, and concludes with a brief discussion of the theory of the technique: multiphoton ionisation is described and saturation conditions derived; the basic resonance ionisation schemes, plus variants which do not involve photons in the ionisation step, are given. Chapter two deals with the nuclear power industry. Basic reactor design and waste management procedures are detailed, with particular emphasis on the ways in which radioactive species are generated and how they may subsequently, by accident or authorised discharge, be released to the environment. The unsuitability of counting techniques for monitoring of low energy, beta-active nuclides (particularly if present in low concentration) is noted, as is the suitability of RIS for this task. From a literature-based assessment, 135Cs55 was deemed to be the low energy beta-emitter of greatest environmental significance and, consequently, a study of multiphoton transitions in caesium was undertaken. Chapter three contains an outline of the apparatus and equipment employed in the experimental work. The laser system and caesium detection/ detection chambers are described in detail. The study of background ionisation in proportional counter systems similar to that used for caesium comprises Chapter four. The significance of this work is that non-resonant background ionisation arising in impurities in the counter gas may swamp resonant signals, limiting the sensitivity of RIS. From the wavelength dependence of the background ionisation, phenol and toluene were identified as contaminants. Chapter five contains the results of experimental work on the detection of caesium vapour emanating from a solid sample of the metal. Both two photon (one photon resonant) ionisation and three photon (two photons resonant via a virtual level) ionisation were used. The dependence of the observed transitions on laser fluence was studied. The efficiency of ionisation at different wavelengths and using different ionisation schemes is compared and, for analytical work, detection of caesium by resonant excitation to Rydberg levels followed by collisional ionisation is recommended. A value of the single photon ionisation cross section for caesium of 1.16 x 10e-19 cm2 is obtained as are values of the two photon resonant ionisation cross sections for the 6s1/2 - 7p3/2 and the 6s 1/2 - 7P1/2 transitions. In Chapter six, future work on RIS at Glasgow, which will principally consist of the construction and use of resonance ionisation mass spectrometers, is described. General conclusions are drawn concerning the achievable detection limit of such devices and on their future applicability, in particular to nuclear waste monitoring. The Appendix describes a non-analytical applications of laser induced ionisation in calibration of multi-wire drift chambers for high energy physics experiments. Results are presented of a project aimed at selection of a suitable ionisable additive (seeding agent) for the time projection chamber at CERN.

[发布机构] University:University of Glasgow

[关键词] Nuclear chemistry, Nuclear engineering