Surface cleaning techniques

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

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

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

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

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 3.1 m E 6.0 MeV
T1/2 164 ?s E 7.7 MeV
T1/2 138.4 d E 5.3 MeV
6
Loading the samples
Exposure time 6 8 months
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

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

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

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

11
Selected results for Copper
Etching
Isotope Original activity cpm Activity after cleaning cpm Reduction factor R Amount of removed Cu Remarks
210Pb 1.49 ? 0.04 lt 0.022 gt 68 3.91 mg/cm2 4.4 µm Only side a was investigated
210Bi 31.17 ? 0.71 0.77 ? 0.02 40.5 3.91 mg/cm2 4.4 µm Only side a was investigated
210Po 2.55 ? 0.01 2.06 ? 0.01 1.2 3.91 mg/cm2 4.4 µm Only side a was investigated
Electropolishing (only results for 210Po are
shown)
Disc side Original 210Po activity cpm 210Po activity after pol. cpm 210Po reduction factor R Amount of removed Cu Remarks
a 2.18 ? 0.02 0.011 ? 0.001 198 20 mg/cm2 22.3 µm Facing the cathode 3 times, each time for 30 min
b 2.45 ? 0.03 0.014 ? 0.001 175 20 mg/cm2 22.3 µm Facing the cathode 3 times, each time for 30 min
12
Selected results for Steel
Disc No. 1
Isotope Original activity cpm After 1st cleaning cpm Reduction factor R Amount of removed Cu Remarks
210Pb 6.87 ? 0.08 1.48 ? 0.09 0.15 ? 0.01 0.030 ? 0.004 46 49 3.1 mg/cm2 4.0 ?m Etching time 50 min
210Bi 147 ? 3 18.6 ? 0.4 4.0 ? 0.1 0.60 ? 0.03 37 31 3.1 mg/cm2 4.0 ?m Etching time 50 min
210Po 16.5 ? 0.5 1.83 ? 0.04 0.88 ? 0.07 0.41 ? 0.02 19 45 3.1 mg/cm2 4.0 ?m Etching time 50 min
Disc No. 2
Isotope Original activity cpm After cleaning cpm Reduction factor R Amount of removed Cu Remarks
210Pb 6.34 ? 0.07 2.11 ? 0.03 0.0318 ? 0.0025 0.0159 ? 0.0020 199 132 4.3 mg/cm2 5.5 ?m Etching time 120 min Solution stirred during etching
210Bi 138 ? 2 36.7 ? 0.4 0.79 ? 0.06 0.21 ? 0.02 174 174 4.3 mg/cm2 5.5 ?m Etching time 120 min Solution stirred during etching
210Po 24.7 ? 0.2 5.2 ? 0.1 0.55 ? 0.02 0.30 ? 0.01 45 17 4.3 mg/cm2 5.5 ?m Etching time 120 min Solution stirred during etching
13
Selected results for Ge of optical quality
Disc No. 1
Isotope Disc side Initial activity cpm Activity after cleaning cpm Reduction factor R Average reduction factor Rav Remarks
210Pb a 2.08 lt 0.02 gt 104 gt 104 Amount of removed Ge not measured. After etching side b not measured for 210Pb.
210Pb b 3.43 - - gt 104 Amount of removed Ge not measured. After etching side b not measured for 210Pb.
210Bi a 42.7 lt 0.18 gt 237 gt 427 Amount of removed Ge not measured. After etching side b not measured for 210Pb.
210Bi b 67.9 lt 0.11 gt 617 gt 427 Amount of removed Ge not measured. After etching side b not measured for 210Pb.
210Po a 42.4 0.04 1060 2300 Amount of removed Ge not measured. After etching side b not measured for 210Pb.
210Po b 71.7 0.02 3585 2300 Amount of removed Ge not measured. After etching side b not measured for 210Pb.
Disc No. 2
Isotope Disc Side Initial activity cpm Activity after cleaning cpm Reduction factor R Average reduction factor Rav Remarks
210Pb A 2.09 - - gt 106 Amount of removed Ge not measured. After etching side a not measured for 210Pb. 210Bi not measured because it has decayed.
210Pb b 2.12 lt 0.02 gt 106 gt 106 Amount of removed Ge not measured. After etching side a not measured for 210Pb. 210Bi not measured because it has decayed.
210Bi a 40.7 - - - Amount of removed Ge not measured. After etching side a not measured for 210Pb. 210Bi not measured because it has decayed.
210Bi b 46.1 - - - Amount of removed Ge not measured. After etching side a not measured for 210Pb. 210Bi not measured because it has decayed.
210Po a 50.0 0.06 820 880 Amount of removed Ge not measured. After etching side a not measured for 210Pb. 210Bi not measured because it has decayed.
210Po b 47.0 0.05 940 880 Amount of removed Ge not measured. After etching side a not measured for 210Pb. 210Bi not measured because it has decayed.
14
Comparison between Cu/Steel/Ge
Isotope Average reduction factors for etching Average reduction factors for etching Average reduction factors for etching
Isotope Copper Steel Ge (Optical)
210Pb 50 100 100
210Bi 50 100 400
210Po 1 20 1000
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 a-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
  (gt 98 )
• Electropolishing of Copper removes 210Pb and
  210Bi more effective than etching (99.5 of
  210Bi and gt 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