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Surface cleaning techniques

Surface cleaning techniques. B. Majorowits a , M. Wójcik b , G. Zuzel b,c a) Max Planck Institute for Physics, Munich, Germany b) Institute of Physics, Jagielonian University, Krak ó w, Poland c) Max Planck Institute for Nuclear Physics, Heidelberg, Germany. Outlook. Motivation

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Surface cleaning techniques

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  1. Surface cleaning techniques B. Majorowitsa, M. Wójcikb, G. Zuzelb,c a) Max Planck Institute for Physics, Munich, Germany b) Institute of Physics, Jagielonian University, Kraków, Poland c) Max Planck Institute for Nuclear Physics, Heidelberg, Germany Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010

  2. Outlook • Motivation • Technique applied to tests of surface cleaning - Loading samples with the Rn daughters - Measurement of 210Pb, 210Bi and 210Po • Copper, Steel and Germanium surface treatment - Etching and electropolishing of Copper - Etching of Steel - Etching of Germanium (optical quality) • Obtained results • Conclusions Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010

  3. Motivation • Equilibrium in the 226Ra decay chain is usually broken at the 210Pb level • 210Pb may stay as main residual surface contamination after cleaning (will appear after some years as e.g. 210Po) • Radio-chemistry of 210Po not well understood, most probably quite different than chemistry of Pb and Bi • Long-lived 222Rn/210Pb daughters deposited on surfaces (or implanted into a sub-surface layer) may significantly contribute to the background of many experiments Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010

  4. Technique applied to tests of surfaces cleaning • Removal of long-lived 222Rn daughters form different surfaces, like Copper, Steel and Germanium was investigated • Samples in a form of discs with 50 mm diameter were used • To increase the sensitivity sample surfaces were artificially loaded with 210Pb, 210Bi and 210Po • Activities of 210Pb, 210Bi and 210Po were measured before and after cleaning using appropriate detectors Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010

  5. 226Ra decay chain 210Pb 206Pb 226Ra 214Pb T1/2 = 22.3 y Em = 0.06 MeV Br = 81 % T1/2 = 1622 y E = 4.8 MeV Br = 94 % T1/2 = 26.8 m Em = 0.7 MeV Br = 48 %    Stable 222Rn 214Bi 210Bi T1/2 = 19.8 m Em = 1.5 MeV Br = 40 %    T1/2 = 3.8 d E = 5.5 MeV T1/2 = 5.0 d Em = 1.2 MeV 214Po 218Po 210Po T1/2 = 164 s E = 7.7 MeV  T1/2 = 3.1 m E = 6.0 MeV   T1/2 = 138.4 d E = 5.3 MeV Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010

  6. Loading the samples Exposure time 6 – 8 months Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010

  7. Measuring 210Pb/210Bi/210Po • Screening of 210Po with an alpha spectrometer 50 mm Si-detector, bcg ~ 2 /d (1-10 MeV) sensitivity ~ 20 mBq/m2 (100 mBq/kg, 210Po) • Screening of 210Bi with a beta spectrometer 250 mm Si(Li)-detectors, bcg ~ 0.18/0.40 cpm sensitivity ~ 10 Bq/kg (210Bi) • Screening of 210Pb (46.6 keV line) with a gamma spectrometer 16 % - HPGe detector with an active and a passive shield Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010

  8. Copper • Electrolytic copper used to fabricate sample discs (50 mm in diameter, 1 mm thick) • Etching procedure: - 5 min in 1% H2SO4 + 3% H2O2 - 5 min in 1% citric acid - rinsing with distilled water • Electropolishing procedure: - electrolyte: 85 % H3PO4 + 5 % 1-butanol (C4H10O) - drying in a nitrogen stream • Weighing the discs before and after cleaning to measure the thickness of a removed surface layer • Both discs surfaces investigated Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010

  9. Steel • Stainless steel from the GERDA cryostat used to fabricate sample discs (50 mm in diameter, 1 mm thick) • Etching procedure: - etching in 20 % HNO3 + 1.7 % HF - passivation in 15 % HNO3 - rinsing with distilled water • Weighing the discs before and after cleaning to measure the thickness of a removed surface layer • Both discs surfaces investigated Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010

  10. Germanium of optical quality • Optical quality Germanium used for a “test run” before using HPGe • Samples cut out from bigger Ge pieces, no special surface treatment after cutting • 2 discs 50 mm in diameter and 3 mm thick were prepared • Discs etched by Canberra according to their standard procedure applied to HPGe crystals • Amount of removed material not measured Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010

  11. Selected results for Copper Etching: Electropolishing (only results for 210Po are shown): Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010

  12. Selected results for Steel Disc No. 1 Disc No. 2 Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010

  13. Selected results for Ge of optical quality Disc No. 1 Disc No. 2 Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010

  14. Comparison between Cu/Steel/Ge Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010

  15. Conclusions • Etching/electropolishing removes some m of treated material (depending on the treatement time) • 210Po deposited on- or just below the surface (relatively narrow α-peaks observed) • Etching does not remove 210Po from Copper, it is re-deposited (209Po added to the solution was found after etching on the surface) • Long electropolishing of Copper reduces 210Po activity by a factor of ~200 – much more effective than etching • Etching of Copper removes most of 210Pb and 210Bi (> 98 %) • Electropolishing of Copper removes 210Pb and 210Bi more effective than etching (99.5 % of 210Bi and > 99.9 % of 210Pb removed) • Etching of Steel works fine for all isotopes and it is more efficient than etching of Copper • In a multi-stage etching process of Steel removal of all isotopes successively drops (passivation makes the process less effective) • Removal efficiency of all long-lived 222Rn daughters from Ge is very high • Etching of Germanium seems to be more efficient than etching of Copper and Steel (especially for 210Po) • Etching tests of HPGe discs ongoing Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010

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