Hydrogen Contamination of Niobium Surfaces

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The presence of hydrogen is blamed for dramatic reductions in cavity Q's. Hydrogen concentration is difficult to measure, so there is a great deal of Fear, Uncertainty, and Doubt (FUD) associated with the problem. This paper presents measurements of hydrogen concentration depth profiles, commenting on the pitfalls of the methods used and exploring how material handling can change the amount of hydrogen in pieces of niobium. Hydrogen analysis was performed by a forward scattering experiment with Helium used as the primary beam. This technique is variously known as FRES (Forward Recoil Elastic Scattering), FRS, HFS (Hydrogen Forward Scattering), and HRA ... continued below

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Nguyen-Tuong, Viet & Doolittle, Lawrence October 1993.

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The presence of hydrogen is blamed for dramatic reductions in cavity Q's. Hydrogen concentration is difficult to measure, so there is a great deal of Fear, Uncertainty, and Doubt (FUD) associated with the problem. This paper presents measurements of hydrogen concentration depth profiles, commenting on the pitfalls of the methods used and exploring how material handling can change the amount of hydrogen in pieces of niobium. Hydrogen analysis was performed by a forward scattering experiment with Helium used as the primary beam. This technique is variously known as FRES (Forward Recoil Elastic Scattering), FRS, HFS (Hydrogen Forward Scattering), and HRA (Hydrogen Recoil Analysis). Some measurements were also made using SIMS (Secondary Ion Mass Spectrometry). Both HFS and SIMS are capable of measuring a depth profile of Hydrogen. The primary difficulty in interpreting the results from these techniques is the presence of a surface peak which is due (at least in part) to contamination with either water or hydrocarbons. With HFS, the depth resolution is about 30 nm, and the maximum depth profiled is about 300 nm. (This 10-1 ratio is unusually low for ion beam techniques, and is a consequence of the compromises that must be made in the geometry of the experiment, surface roughness, and energy straggling in the absorber foil that must be used to filter out the forward scattered helium.) All the observed HFS spectra include a surface peak which includes both surface contamination and any real hydrogen uptake by the niobium surface. Some contamination occurs during the analysis. The vacuum in the analysis chamber is typically a few times 10{sup -6} torr, and some of the contamination is in the form of hydrocarbons from the pumping system. Hydrocarbons normally form a very thin (less than a monolayer) film which is in equilibrium between arrival rate and the evaporation rate. In the presence of the incoming ion beam, however, these hydrocarbons crack on the surface into non-volatile components. Equilibrium is lost, and the surface builds up a layer of carbon-based gunk.

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404 Kilobytes pages

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  • 6th RF Superconductivity Workshop, Newport News, VA (US), 10/1993

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  • Report No.: CEBAF-PR-93-065
  • Report No.: DOE/ER/40150-2015
  • Grant Number: AC05-84ER40150
  • Office of Scientific & Technical Information Report Number: 791941
  • Archival Resource Key: ark:/67531/metadc735189

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  • October 1993

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  • Oct. 19, 2015, 7:39 p.m.

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  • Feb. 5, 2016, 8:10 p.m.

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Nguyen-Tuong, Viet & Doolittle, Lawrence. Hydrogen Contamination of Niobium Surfaces, article, October 1993; Newport News, Virginia. (digital.library.unt.edu/ark:/67531/metadc735189/: accessed August 18, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.