1. VIeme rencontres du Vietnam
The SuperNemo
BiPo detector
Jean-stephane Ricol
CENBG - CNRS
VIeme rencontres du Vietnam
Hanoi August 2006

2. Motivation High level of purification for the source foils
Current bb0n experiment sensitivity on neutrino effective mass ~ eV
SuperNemo aimed sensitivity < ~ 50 meV
T1/2 (82Se 150Nd) > ~ 1026 yrs BG < 1 evt/100kg/yr
High level of purification for the source foils
Goal of the BiPo detector :
Measure the contamination in 208Tl and 214Bi of the bb source foils before the installation in SuperNEMO
5 kg of source (12 m2, 40 mg/cm2) in 1 month with a sensitivity of
208Tl < 2 µBq/kg and 214Bi < 10 µBq/kg

3. BiPo detection Use the Bi-Po coincidence in the decay chain 238U 232Th
(164 ms)
(300 ns)
232Th
212Bi
(60.5 mn)
208Tl
(3.1 mn)
212Po
208Pb
(stable)
36%
238U
214Bi
(19.9 mn)
210Tl
(1.3 mn)
214Po
210Pb
22.3 y
0.021%
Bi-Po Process
Source foil
(40 mg/cm2)
e- Scintillators
+ PMT
T0, Qb(214Bi)=3.2 MeV
Tracking
(wire chamber)
a (delay 164µs) e-

4. BiPo detection Use the Bi-Po coincidence in the decay chains 238U
(164 ms)
(300 ns)
232Th
212Bi
(60.5 mn)
208Tl
(3.1 mn)
212Po
208Pb
(stable)
36%
238U
214Bi
(19.9 mn)
210Tl
(1.3 mn)
214Po
210Pb
22.3 y
0.021%
Bi-Po Process
Source foil
(40 mg/cm2)
e- Scintillators
+ PMT
T0, Qb(214Bi)=3.2 MeV
Tracking
(wire chamber)
a (delay 300 ns) e-
a delay T1/2 ~ 300 ns
Drift time ~ µsec / cm
212Po a cant be detected in the wire chamber  need a dedicated detector

5. BiPo detection Use the Bi-Po coincidence in the decay chains 238U
(164 ms)
(300 ns)
232Th
212Bi
(60.5 mn)
208Tl
(3.1 mn)
212Po
208Pb
(stable)
36%
238U
214Bi
(19.9 mn)
210Tl
(1.3 mn)
214Po
210Pb
22.3 y
0.021%
Bi-Po Process
Source foil
(40 mg/cm2)
e- Scintillators
+ PMT e-
T0, Qb(212Bi)=2.2 MeV
a Scintillator
Tracking
(wire chamber) a
a delay T1/2 ~ 300 ns
Edeposited ~ 1 MeV

6. Efficiency Total efficiency ~ 6% e ~ 0.5 e- goes up
e ~ a goes down a
Thickness of the foil (mg/cm2)
Efficiency
Initial energy of the a: E = MeV
e ~ a escapes from the foil
with a energy > 1 MeV (~150 keV for
energy deposited in the scintillator due
to the quenching)
Total efficiency ~ 6%

7. Two possible designs studied in R&D
Alpha scintillator with electron tracking detector
e- tagging
Multilayer scintillator plates without tracking
0.8m e- a g
Gamma tagging
Foil to be measured
Scintillator plate
Thickness=1cm
(as MOON-1 prototype)
Efficiency x 4
Compact geometry & less channels
Measurement of 214Bi is not possible
(214Po T1/2 = 164 µs  high random coincidence bkg)
 Radon emanation detector developed by Heidelberg

8. Parallel R&D : Ultra thin scintillator
Ultra-thin scintillating detector (plastic or fiber) for a measurement and e- tagging
(e- cross the a calorimeter)
Advantages:
e ~ 25%
Can be used in both designs e- a e- a
Technique can be very usefull for a and e- identification with the multi-layers design
Foil to be measured

9. Parallel R&D : Ultra thin scintillator
Thickness of UTS :
All a detected if. > 90 µm
Optimal for e- ~ µm Crossing efficiency ~ 65-50%
DE ~ keV
Material possibilities :
Plastic : Kharkov produce 2m long x few cm large x 200 µm
Fibers : Bicron produces scint. fiber 250 µm (square or round section)
To be tested : Light yield ? Radiopurity ?

10. Ultra Low Background Detector
5 kg of 82Se source foil (~ 12 m2, 40 mg/cm2)
2 mBq/kg of 208Tl
50 (e-, delay a) 212Bi decays / month
e ~ 6-25 %
3-12 decays / month
Background < 1 event/month is required !
Ultra high radiopurity required for the surface of the scintillator

11. Main origin of background
Surface contamination of 208Tl
on the entrance surface of the lower scintillator
Bulk contamination
Surface contamination
Prompt e-, T0 e- a
delay a, T1/2 ~ 300 ns
Bkg event rejected
Bkg event NOT rejected
e- <deposited energy> ~ 50 keV in 100 µm of scintillator

12. If all comes from mylar wrapping : 2.5 mBq/kg
Analysis of such BG in NEMO-3 data
1642 events observed
in 1 year of data
Factor 10 Too High !!!
If all comes from mylar wrapping : 2.5 mBq/kg T0
electron
(trigger)
40 ns < Tdelay < 130 ns e- a
Fit between 40 and 130 ns :
T 1/2 = (212 +/- 65) ns
~ 300 ns expected
Dt between a and e- (in ns)
Qb ~ 2.2 MeV
electron energy (MeV)
quenching
a energy (MeV)

13. Prototype BiPo-1 Capsule BiPo-1 PM 5”
e =1 cm
Goal of this prototype: Background measurement
Random coincidence from single counting rate of the scint. + PMT
scintillator blocs: 20 x 20 x 1 cm
Surface contamination 212Bi on scintillator entrance surface
Surface treatment :
Very thin layer e = 200 nm of ultrapure aluminium deposit on the scintillator surface
NEMO-3 equipments: radiopure 5” PMTs, radiopure scintillators
First capsule installed in Canfranc laboratory end of september 2006

14. Prototype BiPo-1 Shield Test Facility: external: 2.3 m x 2.3 m x 2 m
internal: 1.45 m x 1.45 m x 1.05 m
Up to 25 capsules can be installed in Phase I
2000
1050
300
2300 x 2300
1450 x 1450
Radon-tight tank
(pure iron)
Free radon air
Lead shield (13 tons)
Water shield

15. Prototype BiPo-1 Phase II
Bg measurement of multi layers design
70 cm

16. Conclusion BiPo detector must reach a sensitivity of few µBq/Kg
Different designs are under study, they will be tested during with first prototypes
The final BiPo detector is planned to be built and installed in the Canfranc laboratory in 2009