Profiling of Hydrogen in Zirconium Surfaces by Laser Ablation with Resonance Ionization
- PDF / 225,234 Bytes
- 4 Pages / 420.48 x 639 pts Page_size
- 85 Downloads / 215 Views
PROFILING OF HYDROGEN IN ZIRCONIUM SURFACES BY LASER ABLATION WITH RESONANCE IONIZATION
G.A. BICKEL, G.A. MCRAE AND L.W. GREEN
AECL Research, Chalk River, Ontario, Canada KOJ IJO ABSTRACT
Elemental distributions in the bulk and metal oxide surface layers of zirconium alloys play key roles in the fracture toughness of the alloys. In particular, localized hydrogen build-up leads to hydride formation and delayed hydride cracking. Parts per million levels of H in Zr have been detected using the 1.06 /.m or 355 nm output of a Nd:YAG laser for ablation followed by 2+1 resonance ionization detection of H and D. Analysis of the ablation plume has shown that it consists predominately of atomic species in thermal equilibrium between 2000 and 3600 0 C. Ablation of thin foils has shown that the ablation rate is on the order of mono-layers per shot and increases exponentially with increasing fluence. Laser ablation depth profiling results of H distributions in an anodically grown oxide film compare qualitatively with nuclear-reaction-analysis profiling of the same sample. INTRODUCTION
We are investigating the technique of laser ablation for profiling the hydrogen distributions in zirconium fuel channel components used in CANDU reactors. Because hydride formation in Zr components may lead to embrittlement and fracture, it is important to understand the mechanisms of hydrogen ingress in order to increase the component lifetimes. Laser ablation microprobe techniques may prove valuable for determining the mechanisms of corrosion and hydrogen ingress. The laser can be focused to sub-micron spot sizes for high lateral spatial resolution, and control of the ablation energy can result in material removal rates on the order of monolayers per shot. Resonance ionization spectroscopy (RIS), which has the potential for high sensitivity, enables efficient multiphoton ionization of the ablated species of interest [1]. Because the RIS technique uses atomic resonances that are unique to each element, most species can be detected with high selectivity. This paper presents methods of ablation and detection, a description of plume characteristics and comments on the capability for depth profiling. EXPERIMENTAL RESULTS
The ablation was carried out in vacuum with a Q-switched Nd:YAG laser operating at either 1.06 1m or 355 nm. The focused spot size varied for different experiments but, generally the beam was Gaussian and had a radius at l/e 2 of = 150 1pm. The fluence at beam center was kept between 1 and 3 J/cm2 . The ablated neutrals were resonantly ionized via the ls 2S, - 2s2S,, two photon transition at 243.06 and 242.99 nm for H and D respectively. These wavelengths were obtained by frequency doubling the output of a XeCI excimer pumped dye laser. The dye used was Coumarin 480 and the doubling crystal was BBO. Timing between the ablation and photoionization pulses was controlled and could be continuously adjusted with a digital delay generator. The photo-ions were then detected in a time-of-flight mass spectrometer. Mat. Res. Soc. Symp. Pr
Data Loading...
