Undergraduate Research at Jefferson Lab

Studying the Effectiveness of Varying pH Rinsing Solutions on Deliberately Contaminated Niobium Surfaces

Student: Matthew Mendonca

School: College of William & Mary

Mentored By: Michael Kelley

At Jefferson Lab, contaminant particles adsorbed on the niobium superconducting radiofrequency (SRF) cavities act as unwanted electron emitters that can significantly limit accelerator performance. Particle adhesion to the cavity surfaces is influenced by particle size and surface charge. Particles immersed in water acquire a composition-dependent surface charge, causing electrostatic forces between them and cavity surfaces to become repulsive or attractive depending on the sign of both charges. Our goal was to learn if varying pH, thus changing surface charge, affects the adhesion of particles to niobium. We prepared suspensions of aluminum oxide (Al₂O₃), niobium oxide (Nb₂O₅), and chromium oxide (Cr₂O₃) and deposited them on 10 x 10 mm polished niobium substrates. We used optical microscopy to image different sample regions, comparing the number of particles before and after treatment with rinsing solutions of varying pH. It was found that rinsing, in general, dispersed a majority of the particle aggregates and rinsing with alkaline solution removed the largest percentage of Al₂O₃ particles, but there seemed to be no direct correlation between using different pH solutions on the retention of Nb₂O₅ and Cr₂O₃. Assuming all findings are accurate, it appears that surface charge cannot be considered the major cause of the retention of metal oxide particles to niobium.