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The SuperNemo BiPo detector
Jean-stephane Ricol CENBG - CNRS VIeme
rencontres du Vietnam Hanoi August 2006
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Motivation
Current bb0n experiment sensitivity on neutrino
effective mass 0.2-1 eV SuperNemo aimed
sensitivity lt 50 meV T1/2 (82Se 150Nd) gt 1026
yrs - BG lt 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 lt 2 µBq/kg and 214Bi lt
10 µBq/kg
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BiPo detection
Use the Bi-Po coincidence in the decay chain
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BiPo detection
Use the Bi-Po coincidence in the decay chains
a delay T1/2 300 ns Drift time µsec / cm
212Po a cant be detected in the wire chamber ?
need a dedicated detector
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BiPo detection
Use the Bi-Po coincidence in the decay chains
a
a delay T1/2 300 ns Edeposited 1 MeV
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Efficiency
Total efficiency 6
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Two possible designs studied in RD

• Alpha scintillator with electron tracking
  detector
• e- tagging

• Multilayer scintillator plates without tracking

• 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
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Parallel RD 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
Foil to be measured
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Parallel RD Ultra thin scintillator
Thickness of UTS All a detected if. gt 90
µm Optimal for e- 200-500 µm Crossing
efficiency 65-50 DE 100-200 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 ?
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Ultra Low Background Detector
5 kg of 82Se source foil ( 12 m2, 40 mg/cm2)
50 (e-, delay a) 212Bi decays / month
2 mBq/kg of 208Tl
3-12 decays / month
e 6-25
Background lt 1 event/month is required !
Ultra high radiopurity required for the surface
of the scintillator
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Main origin of background
Surface contamination of 208Tl on the entrance
surface of the lower scintillator
Bulk contamination
Surface contamination
Bkg event NOT rejected
Bkg event rejected
e- ltdeposited energygt 50 keV in 100 µm of
scintillator
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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
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Prototype BiPo-1

• 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
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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
Radon-tight tank (pure iron)
Free radon air
Lead shield (13 tons)
Water shield
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Prototype BiPo-1 Phase II
Bg measurement of multi layers design
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Conclusion

• BiPo detector must reach a sensitivity of few
  µBq/Kg
• Different designs are under study, they will be
  tested during 2007-2008 with first prototypes
• The final BiPo detector is planned to be built
  and installed in the Canfranc laboratory in 2009