1. Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010 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, Kraków, Poland c) Max Planck Institute for Nuclear Physics, Heidelberg, Germany

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

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

4. Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010 Technique applied to tests of surfaces cleaning  Removal of long-lived 222 Rn 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 210 Pb, 210 Bi and 210 Po  Activities of 210 Pb, 210 Bi and 210 Po were measured before and after cleaning using appropriate detectors

5. Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010 226 Ra decay chain 222 Rn 226 Ra 218 Po 214 Pb 214 Bi 214 Po 210 Pb     210 Bi 210 Po 206 Pb   T 1/2 = 1622 y E = 4.8 MeV Br = 94 % T 1/2 = 3.8 d E = 5.5 MeV T 1/2 = 3.1 m E = 6.0 MeV T 1/2 = 26.8 m E m = 0.7 MeV Br = 48 %  T 1/2 = 19.8 m E m = 1.5 MeV Br = 40 % T 1/2 = 164  s E = 7.7 MeV T 1/2 = 22.3 y E m = 0.06 MeV Br = 81 % T 1/2 = 5.0 d E m = 1.2 MeV T 1/2 = 138.4 d E = 5.3 MeV Stable

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

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

8. Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010 Copper  Electrolytic copper used to fabricate sample discs (50 mm in diameter, 1 mm thick)  Etching procedure: - 5 min in 1% H 2 SO 4 + 3% H 2 O 2 - 5 min in 1% citric acid - rinsing with distilled water  Electropolishing procedure: - electrolyte: 85 % H 3 PO 4 + 5 % 1-butanol (C 4 H 10 O) - 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

9. Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010 Steel  Stainless steel from the GERDA cryostat used to fabricate sample discs (50 mm in diameter, 1 mm thick)  Etching procedure: - etching in 20 % HNO 3 + 1.7 % HF - passivation in 15 % HNO 3 - rinsing with distilled water  Weighing the discs before and after cleaning to measure the thickness of a removed surface layer  Both discs surfaces investigated

10. Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010 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

11. Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010 Selected results for Copper Isotope Original activity [cpm] Activity after cleaning [cpm] Reduction factor R Amount of removed Cu Remarks 210 Pb 1.49  0.04 < 0.022> 68 3.91 mg/cm 2 4.4 µm Only side a was investigated 210 Bi 31.17  0.710.77  0.02 40.5 Only side a was investigated 210 Po 2.55  0.012.06  0.01 1.2 Only side a was investigated Etching: Disc side Original 210 Po activity [cpm] 210 Po activity after pol. [cpm] 210 Po reduction factor R Amount of removed Cu Remarks a 2.18  0.020.011  0.001 198 20 mg/cm 2 22.3 µm Facing the cathode 3 times, each time for 30 min b 2.45  0.030.014  0.001 175 Facing the cathode 3 times, each time for 30 min Electropolishing (only results for 210 Po are shown):

12. Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010 Selected results for Steel Isotope Original activity [cpm] After 1 st cleaning [cpm] Reduction factor R Amount of removed Cu Remarks 210 Pb 6.87  0.08 1.48  0.09 0.15  0.01 0.030  0.004 46 49 3.1 mg/cm 2 4.0 m Etching time = 50 min 210 Bi 147  3 18.6  0.4 4.0  0.1 0.60  0.03 37 31 210 Po 16.5  0.5 1.83  0.04 0.88  0.07 0.41  0.02 19 45 Disc No. 1 Isotope Original activity [cpm] After cleaning [cpm] Reduction factor R Amount of removed Cu Remarks 210 Pb 6.34  0.07 2.11  0.03 0.0318  0.0025 0.0159  0.0020 199 132 4.3 mg/cm 2 5.5 m Etching time = 120 min Solution stirred during etching 210 Bi 138  2 36.7  0.4 0.79  0.06 0.21  0.02 174 210 Po 24.7  0.2 5.2  0.1 0.55  0.02 0.30  0.01 45 17 Disc No. 2

13. Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010 Selected results for Ge of optical quality Isotope Disc side Initial activity [cpm] Activity after cleaning [cpm] Reduction factor R Average reduction factor R av Remarks 210 Pb a2.08< 0.02> 104 Amount of removed Ge not measured. After etching side b not measured for 210 Pb. b3.43-- 210 Bi a42.7< 0.18> 237 > 427 b67.9< 0.11> 617 210 Po a42.40.041060 2300 b71.70.023585 Disc No. 1 Isotope Disc Side Initial activity [cpm] Activity after cleaning [cpm] Reduction factor R Average reduction factor R av Remarks 210 Pb A2.09-- > 106Amount of removed Ge not measured. After etching side a not measured for 210 Pb. 210 Bi not measured because it has decayed. b2.12< 0.02> 106 210 Bi a40.7-- - b46.1-- 210 Po a50.00.06820 880 b47.00.05940 Disc No. 2

14. Topical Workshop in Low Radioactivity Techniques, Sudbury, Canada, August 28-29, 2010 Comparison between Cu/Steel/Ge Isotope Average reduction factors for etching CopperSteelGe (Optical) 210 Pb~ 50~ 100 210 Bi~ 50~ 100~ 400 210 Po~ 1~ 20~ 1000

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