surface cleaning techniques

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

Surface cleaning techniques

B. Majorowitsa, M. Wójcikb, G. Zuzelb,c

a) Max Planck Institute for Physics, Munich, Germanyb) Institute of Physics, Jagielonian University, Kraków, Polandc) Max Planck Institute for Nuclear Physics, Heidelberg, Germany


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


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


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


226Ra decay chain

222Rn

226Ra

218Po

214Pb

214Bi

214Po

210Pb

210Bi

210Po

206Pb

T1/2 = 1622 yE = 4.8 MeV Br = 94 %

T1/2 = 3.8 dE = 5.5 MeV

T1/2 = 3.1 mE = 6.0 MeV

T1/2 = 26.8 mEm = 0.7 MeVBr = 48 %

T1/2 = 19.8 mEm = 1.5 MeV Br = 40 %

T1/2 = 164 sE = 7.7 MeV

T1/2 = 22.3 yEm = 0.06 MeV Br = 81 %

T1/2 = 5.0 dEm = 1.2 MeV

T1/2 = 138.4 dE = 5.3 MeV

Stable


Loading the samples

222Rnsource

(1.4 MBq)

Pump

FilterDiscs

Exposure time6 – 8 months


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 250 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


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


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


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


Selected results for Copper

IsotopeOriginal

activity [cpm]

Activity after cleaning

[cpm]

Reduction factor R

Amount of removed Cu

Remarks

210Pb 1.49 0.04 < 0.022 > 68

3.91 mg/cm2

4.4 µm

Only side a was investigated

210Bi 31.17 0.71 0.77 0.02 40.5Only side a was

investigated

210Po 2.55 0.01 2.06 0.01 1.2Only side a was

investigated

Etching:

Disc side

Original 210Po activity [cpm]

210Po activity after pol.

[cpm]

210Po reduction factor R

Amount of removed

CuRemarks

a 2.18 0.02 0.011 0.001 19820 mg/cm2

22.3 µm

Facing the cathode 3 times, each time for 30 min

b 2.45 0.03 0.014 0.001 175Facing the cathode 3 times,

each time for 30 min

Electropolishing (only results for 210Po are shown):


Selected results for Steel

IsotopeOriginal

activity [cpm]After 1st

cleaning [cpm]Reduction factor R


Remarks

210Pb6.87 0.081.48 0.09

0.15 0.010.030 0.004

4649

3.1 mg/cm2

4.0 mEtching time = 50 min210Bi

147 3 18.6 0.4

4.0 0.1 0.60 0.03

3731

210Po16.5 0.5

1.83 0.04 0.88 0.07 0.41 0.02

1945

Disc No. 1

IsotopeOriginal

activity [cpm]After cleaning

[cpm]Reduction factor R


Remarks

210Pb6.34 0.07 2.11 0.03

0.0318 0.0025 0.0159 0.0020

199132

4.3 mg/cm2

5.5 m

Etching time = 120 minSolution stirred during

etching

210Bi138 2

36.7 0.4 0.79 0.06 0.21 0.02

174174

210Po24.7 0.2 5.2 0.1

0.55 0.02 0.30 0.01

4517

Disc No. 2


Selected results for Ge of optical quality

IsotopeDisc side

Initial activity [cpm]


[cpm]

Reduction factor R

Average reduction factor Rav

Remarks

210Pba 2.08 < 0.02 > 104

> 104

Amount of removed Ge not measured. After etching side b not measured for 210Pb.

b 3.43 - -

210Bia 42.7 < 0.18 > 237

> 427b 67.9 < 0.11 > 617

210Poa 42.4 0.04 1060

2300b 71.7 0.02 3585

Disc No. 1

IsotopeDiscSide

Initial activity [cpm]


[cpm]

Reduction factor R

Average reduction factor Rav

Remarks

210PbA 2.09 - -

> 106 Amount of removed Ge not measured. After etching side a not measured for 210Pb. 210Bi not measured because it has decayed.

b 2.12 < 0.02 > 106

210Bia 40.7 - -

-b 46.1 - -

210Poa 50.0 0.06 820

880b 47.0 0.05 940

Disc No. 2


Comparison between Cu/Steel/Ge

IsotopeAverage reduction factors for etching

Copper Steel Ge (Optical)210Pb ~ 50 ~ 100 ~ 100210Bi ~ 50 ~ 100 ~ 400210Po ~ 1 ~ 20 ~ 1000


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

surface cleaning techniques

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