1. Measurement of surface radioactivity by Alpha/Beta detection
Pia Loaiza
LRT2017, Seoul May 2017

2. Surface screening: motivations in DM search
Bolometers in dark matter search, Edelweiss, CDMS :
210Pb on detector surface or directly in contact with the detectors produce events leaking in the nuclear recoil band
To achieve required sensitivities, detectors must have ACTIVE REJECTION
And screening to minimize surface
radioactivity
Surface events

3. Alpha background in 2b0n search
g/b-region
a -region
Qbb 130Te = 2530 keV
Degraded Alphas main background in CUORICINO/CUORE-0
234U
208Tl
210Po
190Pt
226Ra
230Th
238U
222Rn
CUORICINO CUORE-0
factor ~7 reduction in the alpha continuum region thanks to surface contamination reduction
218Po
from CUORICINO/
CUORE-0 cryostat
from surface
contaminations

4. Background in SuperNEMO 2b0n search
Thin 2b0n source in tracker surrounded by calorimeter, 2b0n isotope: 82Se
A 2b0n event is recognized as 2 electrons from the source foil with ∑E = 3 MeV and t1 t2
One of the most dangerous background : 208Tl and 214Bi from source foils and Radon
a’s used to asses contaminations:
~ 200 mm
Number of 212Po a’s going out of the source foil
208Tl b
208Tl b g
Eg = 2.7 MeV
208Pb
Qb=4.9 MeV
In this case, if g is not giving a hit
in the calorimeter it mimics a 2b0n event

5. Alpha/Beta screening
This talk will be restricted to large surface detectors
Not inclusive of the many efforts and detectors existing
My sincere apologies if your favourite detector is not mentioned
Alpha screening:
- XIA’s alpha counter UltraLo-1800
Surface alpha detector using m-TPC
Alphas and Betas
- BetaCage
Alpha/Beta coincidence:
- BiPo3

6. XIA’s Alpha counter – UltraLo-1800
Talk « Improving the limits of detection of low background alpha emission
measurements » by Brendan McNally 
Images courtesy of e-
Talk: ‘Precise measurement of Pb210
and Po210 contamination in bulk copper’
by Kazuyoshi Kobayashi
Poster: ‘An Ar-gas ionization chamber for
alpha particle detection at the Yangyang
underground laboratory’ by Chang Hyon Ha
Ar+ a a
Drift chamber filled with Ar
Electrodes at +1kV

7. XIA’s Alpha counter – UltraLo-1800
Alphas :
≥ 2 MeV
Discrimination of alphas from walls and ceiling
Alphas from sample : no guard signal amplitude
Total sample surface: 1800 or 707 cm2
Background
sea-level, unshielded ~ 10-3 a/cm2/hr
underground, shielded ~ 10-4 a/cm2/hr
Sensitivity < 0.5 mBq/m2 for 210Po
Images courtesy of 7 7

8. Surface alpha detector using m-TPC
Talk: « Development of a low alpha emitting m-PIC for NEWAGE direction-sensitive
dark matter search » by Takashi Hashimoto
Position sensitive
Located at Kamioka mine
Background (outside sample region):
0.119 ± events/cm2/h
Total sample surface: (10 x 10) cm2

9. The BetaCage
Poster: « The BetaCage: An Ultra-sensitive screener for surface contamination »
by Eric Miller
Drift chamber with Multi-wire proportional
counter
Sample in the gas (90 % Ne, 10% methane)
A ‘trigger’ MWPC creates the trigger
region
Drift region contains all 200 keV e- and alphas
A ‘bulk’ MWPC collects the charges
Total sample surface: 0.45 m2 = 4500 cm2

10. The BiPo-3 detector Measurement of 212Bi(208Tl) and 214Bi
Initially developed to measure SuperNEMO 2b0n source foils
Detection principle: b-a delayed coincidence detection

11. Detector principle
The foil of interest is installed between two thin low radioactive organic scintillators
Source Foil
Scintillator
Light Guide
PMT

12. Assembling of BiPo sub-OMs in the structure
The set-up
Optical sub-module
Assembling of BiPo sub-OMs in the structure
Optical sub-module:
Polyestyrene scintillators
coupled to 5’’ PMTs through a PMMA guide
20 pairs of optical sub-modules
assembled in one Module
BiPo3= 2 modules
Total detector surface=
3.6 m2 = cm2
Installed in Canfranc
Underground Laboratory

13. Time window for event selection
212BiPo
214BiPo
Eprompt= 576 keV
Eprompt= keV
Dt= 40 ms
Edelay= keV
Edelay= 967 keV
Dt=390 ns
The delay time between the prompt and the delayed signal is 20 ns < Dt < 1500 ns (212Po T1/2 = 300 ns)
The delay time between the prompt and the delayed signal is 10 ms < Dt < 1000 ms (214Po T1/2 = 164 ms)

14. Backgrounds Scintillator surface Scintillator bulk Random coincidences
contamination
Radon in the sensitive
volume
Random coincidences
Compton e- from external g’s
Scintillator bulk
contamination
rejected

15. 212BiPo background measurement
Three dedicated background measurements (200.4 days of measurement)
One measurement: 75 days x 3.42 m2
12 212BiPo candidates
All events from surface background:
a’s deposit all their energy inside the scintillator → peak at about 1000 keV
source
a energy
scint
For events with Ea < 700 keV (bulk activity)
background free measurement
Surface background:
A(208Tl) = 0.9 ± 0.2 mBq/m2
(= events/cm2 hr)

16. 214BiPo background measurement
Two dedicated background measurements (111.9 days of measurement)
One measurement: 75 days x 3.42 m2
30 214BiPo candidates
Events from surface background:
a’s depositing all its energy inside the scintillator → peak at about 800 keV
Random coincidences at low
energies→ dominant background
Surface background:
A(214Bi) = 1.0 ± 0.4 mBq/m2
a energy
(= events/cm2 hr)
Random coincidences average:
= 0.12 cts/day/m2
(= counts/cm2 hr) 16

17. Sample measurements SuperNEMO foils 82Se powder mixture:
A(208Tl) mBq/kg 90% CL
A(214Bi) mBq/kg 90% CL
A(214Bi) mBq/m2 90% CL
Mylar
< 49
<195
< 5
Irradiated Mylar
100 ± 53
< 690
3.7 ±
PVA
< 12
< 505
31 ± 8
6.8
3.5
212BiPo
SuperNEMO foils
Se mixture
Mylar
scint. surface contamination
82Se powder mixture:
A(208Tl) = (21 ± 11) mBq/kg, 90% CL
a energy

18. Sensitivity Sensitivity to Se powder mixture:
A(208Tl) < 2 mBq/kg , 90% CL
in 6 months
A(214Bi) < 140 mBq/kg , 90% CL
in 6 months
Using total BiPo3 area = 3.6 m2
Mylar ideally radiopure
Surface background and random coincidences from dedicated bkg measurements

19. Summary
A number of dark matter and neutrino experiments are sensitive to surface contaminants
Surface screening is needed to control and reduce surface contaminants
Alpha detection used to asses surface and bulk radiopurity levels
Current background in alpha detectors ~ 10-4 a/cm2/hr. Sensitivities < 0.5 mBq/m2 for 210Po
Beta/Alpha coincidence used to asses 212Bi/214Bi radioactivity levels.
Sensitivity reached A(208Tl) < 10 mBq/kg