Study of impurity distribution in mechanically polished, chemically treated and high vacuum degassed pure Niobium samples using TOFSIMS technique

TL;DR

This study investigates how different surface treatments and their sequences affect impurity distribution in niobium samples used for SRF cavities, using TOFSIMS to analyze impurities within the surface layer.

Contribution

It demonstrates the impact of treatment sequence on impurity levels in niobium and validates TOFSIMS as an effective tool for analyzing SRF surface treatments.

Findings

BCP reduces hydrocarbons and metallic contamination

HPR effectively reduces acidic impurities

Impurity distribution depends on treatment sequence

Abstract

The performance of Superconducting radio frequency cavities (SRF) is strongly influenced by various impurities within the penetration depth (~50nm) of Nb, which in turn depends on the applied surface treatments. The effect of these surface treatments on the impurities of Nb has been explored using various surface analytical treatments. But, the results are still inadequate in many aspects and the effect of sequential SRF treatments on the impurity distribution has not been explored. The present study analyses various impurities within the penetration depth of Nb samples, treated by SRF cavity processing techniques like colloidal silica polishing (simulating centrifugal barrel polishing), buffer chemical polishing (BCP), high pressure rinsing (HPR) and degassing under high vacuum (HV) condition at 600{\deg}C for 10hrs. Static, dynamic and slow sputtering modes of Time of flight secondary ion mass spectrometry (TOFSIMS) technique was employed to study the effect of the above treatments on interstitial impurities, hydrocarbons, oxides, acidic residuals, reaction products and metallic contaminations. The study confirms that the impurity distribution in Nb is not only sensitive to the surface treatment, but also to their sequence. Varying the treatment sequence prior to HV degassing treatments affected the final impurity levels in HV degassed bulk Nb samples. BCP treated samples, exhibited minimum hydrocarbon and metallic contamination but, led to extensive contamination of the oxide layer with residuals and reaction products of acids used in BCP solution. HPR treatment, on the other hand was effective in reducing the acidic impurities on the top surface. The study also establishes the application of TOFSIMS technique to analyze and evolve SRF treatments.