Removal+of+long%lived+ Rn+daughters+by++ electropolishing ... ·...
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Longlived alpha and beta emi2ers in the Rn222 decay chain on detector surfaces may be the limi;ng background in many experiments a2emp;ng to detect dark ma2er or neutrinoless double beta decay. Removal of tens of microns of material via electropolishing has been shown to be effec;ve at removing radon daughters implanted into material surfaces (Zuzel and Wó́jcik 2012). Some applica;ons, however, require the removal of uniform and significantly smaller thicknesses. Here, we demonstrate that electropolishing < 1 micron from stainless steel plates efficiently reduces surface contamina;on. Examina;on of electropolished wires with a scanning electron microscope confirms that the thickness removed is reproducible and reasonably uniform. Together, these tests demonstrate the effec;veness of removal of radon daughters for a proposed lowradia;on, mul;wire propor;onal chamber (the BetaCage), without compromising the screener's energy resolu;on. More generally, electropolishing thin layers of stainless steel may be an effec;ve means of removing radon daughters without compromising precisionmachined parts. Removal of longlived 222 Rn daughters by electropolishing thin layers of stainless steel Syracuse University: R.W. Schnee, M.A. Bowles, R. Bunker, K. McCabe, J. White Uniformity of Electropolished Wires Detec;on Systems University of Minesota: P. Cushman, M. Pepin University of South Dakota: V.E. Guiseppe Before Electropolish A[er Electropolish Electropolishing reduced diameter of SS fine wire by 2 microns per minute. Nonuniformity may be dominated by handling deforming wire to oval cross sec;on. Even for 2 microns removed, uniformity is sufficient to provide <10% gain varia;on, mee;ng BetaCage specifica;on for gain uniformity. 24 25 26 0 2 4 6 Wire Diameter (microns) Frequency 22 23 24 0 2 4 6 Wire Diameter (microns) σ=440 nm σ=600 nm UMinn GOPHER ntype HPGe 25% rela;ve detec;on efficiency samples directly on 1.6 mm aluminum window 36% efficiency for 210 Pb 46 keV xray emi2ed down ORTEC alpha counter for 210 Po 1.6 ± 0.2 background events / day in 210 Po energy region 14% efficiency for these samples (geometrical), confirmed by rate of calibrated source sample 5000 5200 5400 10 ï2 10 ï1 10 0 10 1 Energy (keV) Counts/(keV day) Before electropolish A[er 670 nm removed Sample Prepara;on Electropolishing Setup 4 unpolished (mill finish) 316 stainless steel samples, 2” x 2” x 0.1875” thick ~8.6 μm surface roughness (AFM) One 1.5” x 1.5” #2B finish in progress for dependence on surface roughness ~0.4 μm surface roughness Exposed to 5.42 x 10 6 Bq m 3 day 1 at University of South Dakota Electropolished in small bath (see figure to right) Thickness removed determined from precision scale (guarded against dri[s by massing standard immediately before and a[er samples) 4 nm/second on average Sample #1 counted in HPGe and alpha counter a[er electropolishing Sample #2 (control) counted in HPGe and alpha counter wo/electropolishing Other samples counted in alpha counter before and a[er every ~50 nm removed by electropolishing Results Removal of 1.1 microns from Sample #1 reduced 210 Pb rate from 13.5 ± 1.3 Bq/m 2 to <1.1 Bq/m 2 reduc;on factor of >12 210 Po rate from 11.5 ± 1.5 Bq/m 2 to 0.22 ± 0.05 Bq/m 2 reduc;on factor of ~50 (ini;al ac;vity from alpha coun;ng and calcula;ons) Removal of 160 nm from samples reduced alpha rate by factor ~20 Roughly exponen;al profile Rate of reduc;on with thickness removed shows some varia;on Reduc;on of 210 Po by > 100 (to level consistent with counter background) demonstrated by removing 660 nm Reduc;on stalled at level of 5 alphas/day un;l improved technique Rate of “unexposed” steel sample ~2x higher than background rate 0 200 400 600 800 1000 1200 10 0 10 1 10 2 10 3 Mean Thickness Removed (nm) Alphas/day Background level Suspect limited by 210 Po in rinse water Sample 1 #2b finish Uncertainty smaller than symbol if no error bar shown Preliminary (No correc6ons yet for dri8 of scale or energy range, growin of 210 Po) 40 42 44 46 48 50 52 Energy (keV) 700 600 500 400 300 200 100 0 Counts 40 42 44 46 48 50 52 Energy (keV) 600 500 400 300 200 100 0 Counts Sample #1 a[er EP: <1.1 Bq/m 2 Sample #2 (control): 14.6 ± 1.3 Bq/m 2 DC Power Supply + – – Plas2c Rods Stainless9Steel Vessel Level of Acid Solu2on (40% H 3 PO 4 40% H 2 SO 4 ) Copper Electrode (Cathode) Stainless9Steel Sample (Anode) A V
Transcript of Removal+of+long%lived+ Rn+daughters+by++ electropolishing ... ·...
Long-‐lived alpha and beta emi2ers in the Rn-‐222 decay chain on detector surfaces may be the limi;ng background in many experiments a2emp;ng to detect dark ma2er or neutrinoless double beta decay. Removal of tens of microns of material via electropolishing has been shown to be effec;ve at removing radon daughters implanted into material surfaces (Zuzel and Wó́jcik 2012). Some applica;ons, however, require the removal of uniform and significantly smaller thicknesses. Here, we demonstrate that electropolishing < 1 micron from stainless steel plates efficiently reduces surface contamina;on. Examina;on of electropolished wires with a scanning electron microscope confirms that the thickness removed is reproducible and reasonably uniform. Together, these tests demonstrate the effec;veness of removal of radon daughters for a proposed low-‐radia;on, mul;-‐wire propor;onal chamber (the BetaCage), without compromising the screener's energy resolu;on. More generally, electropolishing thin layers of stainless steel may be an effec;ve means of removing radon daughters without compromising precision-‐machined parts.
Removal of long-‐lived 222Rn daughters by electropolishing thin layers of stainless steel
Syracuse University: R.W. Schnee, M.A. Bowles, R. Bunker, K. McCabe, J. White
Uniformity of Electropolished Wires
Detec;on Systems
University of Minesota: P. Cushman, M. Pepin University of South Dakota: V.E. Guiseppe
Before Electropolish A[er Electropolish
Electropolishing reduced diameter of SS fine wire by 2 microns per minute. Nonuniformity may be dominated by handling deforming wire to oval cross-‐
sec;on. Even for 2 microns removed, uniformity is sufficient to provide <10% gain
varia;on, mee;ng BetaCage specifica;on for gain uniformity.
24 25 260
2
4
6
Wire Diameter (microns)
Freq
uenc
y
22 23 240
2
4
6
Wire Diameter (microns)
σ=440 nm σ=600 nm
UMinn GOPHER n-‐type HPGe 25% rela;ve detec;on efficiency samples directly on 1.6 mm aluminum
window 36% efficiency for 210Pb 46 keV
x-‐ray emi2ed down
ORTEC alpha counter for 210Po 1.6 ± 0.2 background events / day
in 210Po energy region 14% efficiency for these samples
(geometrical), confirmed by rate of calibrated source
sample
5000 5200 540010ï2
10ï1
100
101
Energy (keV)
Coun
ts/(k
eV d
ay)
Before electropolish
A[er 670 nm removed
Sample Prepara;on Electropolishing Set-‐up 4 unpolished (mill finish) 316 stainless steel samples, 2” x 2” x 0.1875” thick
~8.6 μm surface roughness (AFM) One 1.5” x 1.5” #2B finish in progress for dependence on surface roughness
~0.4 μm surface roughness Exposed to 5.42 x 106 Bq m-‐3 day-‐1 at University of South Dakota
Electropolished in small bath (see figure to right) Thickness removed determined from precision scale (guarded against
dri[s by massing standard immediately before and a[er samples) 4 nm/second on average
Sample #1 counted in HPGe and alpha counter a[er electropolishing Sample #2 (control) counted in HPGe and alpha counter wo/electropolishing Other samples counted in alpha counter before and a[er every ~50 nm
removed by electropolishing
Results
Removal of 1.1 microns from Sample #1 reduced 210Pb rate from 13.5 ± 1.3 Bq/m2 to <1.1 Bq/m2
reduc;on factor of >12 210Po rate from 11.5 ± 1.5 Bq/m2 to 0.22 ± 0.05 Bq/m2
reduc;on factor of ~50 (ini;al ac;vity from alpha coun;ng and calcula;ons)
Removal of 160 nm from samples reduced alpha rate by factor ~20 Roughly exponen;al profile Rate of reduc;on with thickness removed shows some varia;on
Reduc;on of 210Po by > 100 (to level consistent with counter background) demonstrated by removing 660 nm Reduc;on stalled at level of 5 alphas/day un;l improved technique
Rate of “unexposed” steel sample ~2x higher than background rate
0 200 400 600 800 1000 1200100
101
102
103
Mean Thickness Removed (nm)
Alp
has/
day
Background level
Suspect limited by 210Po in rinse water Sample 1
#2b finish
Uncertainty smaller than symbol if no error bar shown
Preliminary (No correc6ons yet for dri8 of scale or energy range, grow-‐in of 210Po)
40 42 44 46 48 50 52 Energy (keV)
700 600 500 400 300 200 100 0
Coun
ts
40 42 44 46 48 50 52 Energy (keV)
600
500
400
300
200
100
0
Coun
ts
Sample #1 a[er EP: <1.1 Bq/m2
Sample #2 (control): 14.6 ± 1.3 Bq/m2
DC#Power#Supply#
+# –#–#
Plas2c#Rods#
Stainless9Steel#Vessel#
Level#of#Acid#Solu2on#(40%#H3PO4####40%#H2SO4)#
Copper#Electrode#(Cathode)#
Stainless9Steel#Sample#(Anode)#
A#
V#
