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

Abstract

The performance of superconducting radio frequency (SRF) cavities is strongly influenced by various impurities within the penetration depth (∼50 nm) of niobium (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 techniques. However, 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 analyzes various impurities within the penetration depth of Nb samples, treated by SRF cavity processing techniques such as colloidal silica polishing (simulating centrifugal barrel polishing), buffer chemical polishing (BCP), high pressure rinsing (HPR) and degassing under a high vacuum (HV) condition at 600 °C for 10 h. Static, dynamic and slow sputtering modes of the time of flight secondary ion mass spectrometry (TOFSIMS) technique were employed to study the effect of the above treatments on interstitial impurities, hydrocarbons, oxides, acidic residues, reaction products and metallic contaminations. The study confirms that the impurity distribution in Nb is not only sensitive to the surface treatments, 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. The HV degassing treatment was capable of reducing hydrogen contamination, but oxygen, carbon and metallic impurities were introduced into bulk Nb due to poor isolation from furnace contamination. On the other hand, BCP treated samples exhibited minimum hydrocarbon and metallic contamination along with the thinnest oxide layer at ∼2.8 nm, but led to extensive contamination of the oxide layer with residuals and reaction products of acids used in the BCP solution. HPR treatment, on the other hand, was effective in reducing the acidic impurities on the top surface. Variability of the distribution of impurities on samples as a function of SRF treatments was also discussed.