Laser induced breakdown detection for the assessment of colloid mediated radionuclide migration

Colloids play an important role in the transport of pollutants in the environment. Harmful substances can undergo transport over large distances if bound to colloids in aqueous surrounding. One important example is the migration of Pu(IV) at unexpectedly high rates over several miles at a Nevada nuclear detonation test site. For long term safety assessments of radioactive waste repositories, it is hence crucial to know about the amount, size distribution and chemical composition of colloids in the ground water. Standard methods (e.g. light scattering) can be applied for high concentrations and large sizes of particles. Colloids smaller than 50 nm, however, are detected with very low efficiency. Laser induced breakdown detection (LIBD) can fill this gap. A new instrumentation is presented, which as compared to previous instruments, opens tip a much wider operational dynamic range, now covering three orders of magnitude in size (5-1000 nm) and seven orders of magnitude in particle concentration (1 ppt - several ppm). The technique is based on plasma formation on colloidal particles inside the focus of a pulsed laser. The plasma plume is detected by three-dimensional optical observations and by means of its shock wave with a piezo-detector. For mathematical modelling, detailed knowledge on the photon fluence distribution in the focal region is indispensable. For the first time a true Gaussian TEM00 mode has been achieved in the focus of a LIBD apparatus and great care has been taken to guarantee long-term stability of the optical parameters. Automated control of the laser pulse energy and beam shape is introduced to allow routine reproducible measurement. The apparatus combines acoustic detection with three-dimensional optical monitoring of the focal region with two CCD cameras placed perpendicular to each other in order to gain additional size information. The breakdown events are systematically characterized with respect to the number density and size of aquatic colloids as a function of the laser pulse energy. Whereas the threshold energy (irradiance) only depends on the colloid size, the breakdown probability at higher pulse energies is a direct function of the number density of colloids. A correlation of the two facts allows the speciation of the colloidal size distribution. (C) 2002 Elsevier Science B.V. All rights reserved.