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#