11C background suppression in the CNOpep solar neutrino energy region with Borexino

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11C background suppression in the CNO-pep solar
neutrino energy region with Borexino
XCIV Congresso Nazionale della Società Italiana
di Fisica 22-27 Settembre 2008, Genova

• Davide Franco for the Borexino collaboration
• Milano University INFN

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pep and CNO neutrinos

• pep neutrinos
• Directly related with the pp fusion reaction in
  the Sun
• Improves our knowledge of the solar neutrino
  luminosity
• Test of the MSW-LMA
• CNO neutrinos
• First experimental observation of the CNO cycle
  presence in the Sun
• Helpful in the age estimation of the Globular
  Clusters
• Non-standard interactions, mass-varying
  neutrinos, CPT

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Detection signature in Borexino

• Neutrino elastic scattering off electrons
• Energy range of observation
• 0.8 1.4 MeV
• Expected flux in BOREXino-like scintillator
  (BS07LMA)
• pep-n 9x10-3 d-1 ton-1
• CNO-n 6x10-3 d-1 ton-1

3 years statistics in 100 tons of scintillator
Overall signal rate 1.5 c/d in 100 tons
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Detector layout and main features
Stainless Steel Sphere 2212 PMTs 1350 m3
Scintillator 270 t PCPPO in a 150 mm thick
nylon vessel
Nylon vessels Inner 4.25 m Outer 5.50 m
Water Tank g and n shield m water C detector 208
PMTs in water 2100 m3
Carbon steel plates
20 legs
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The signal and the background
Energy range 0.8 1.4 MeV
11C contamination
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11C production and the three-fold coincidence

• Triple coincidence among
• cosmic muon
• rate at LNGS (3700 m.w.e.) 1.16 hr-1 m-2
• average energy 320 GeV
• gamma from neutron capture
• energy 2.2 MeV
• capture time 250 ms
• positron from 11C decay
• deposited energy between 1.022 and 1.982 MeV
  (but quenched!)
• mean life 30 min!!!

m
11C
n
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Vetoing the detector
Depends on detector efficiencies in tagging
neutrons and in tagging and tracking muons
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11C expected rate from NA54 Cern experiment
S/B 1/6
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Detecting (and rejecting) cosmic muons
m pulse

• m are identified by ID and OD
• OD eff 99
• ID based on pulse shape analysis
• Rejection factor
• gt 103 (conservative)

scintillation pulse
Rm (1.210.05) h-1m-2
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m-track (in progress)
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Intrinsic inefficiency
SNOLab
Cross sections for 11C production from 12C as a
function of energy
SNOLab
Cumulative range of m-induced secondaries
Neutrons are produced in association with 95.5
of the muon-induced 11C
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Muon induced neutrons

• Since January 2008 we were unable to identify
  high-multiplicity events
• Electronics has been properly modified a very
  long gate (1.6 ms) is opened after each muon
• New clustering algorithm has been developed
• A parallel FADC multichannel system is under
  development to estimate the efficiency
• From preliminary results gt 90 neutrons detected
• Still problems in very high multiplicity events

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Test of the coincidence technique with the
Counting Test Facility

• 4 tons of scintillator
• 1 m radius vessel housing the scintillator
• 2 m radius shroud
• 3.6 p.e./PMT for 1 MeV electron
• Muon veto
• 100 PMT (OC 21)
• Buffer of water
• Energy saturation 6 MeV

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Data selection in CTF

• Muon selection
• cut on the number of photoelectrons detected by
  the muon-veto

• Neutron selection
• For each detected m, the following event in the
  time window Tn 20, 2000 µs is selected as a
  candidate event for a neutron capture ?
• E lt 2.6 MeV

• 11C selection
• After each m-g coincidence, 11C candidates are
  selected in a subsequent time window Tw 300
  min, 10 times the 11C mean life.
• Optimal energy range 1.15 lt E lt 2.25 MeV
• Distance between 11C event and gamma lt 35 cm

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Measured 11C production rate in CTF
Phys.Rev.C74045805,2006.
R(11C) 13.0 2.6(stat) 1.4(syst)10-2 d-1
ton-1
Goal reached?
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Three Fold Coincidence in Borexino
sample of 11C with strong cuts
S 500 12 pe/MeV
Q/E 80

• The total rate is not known yet
• Unknown efficiency in tagging neutrons
• Too low statistics

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larger rate in Borexino
from spectral analysis

• Inconsistency between NA54/CTF and Borexino?
• different size of the detector?
• other background sources?

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Conclusion

• Electronics and DAQ have been properly modified
  and strongly improved
• New FADC system for increasing the neutron
  detection efficiency is almost ready
• The CTF measurement has demonstrated that the
  three-fold coincidence technique is powerful in
  localizing in space and time 11C decays
• 11C can be removed by blinding detector volumes
  around it
• Still problems inconsistency between the rate
  measured by CTF and NA54 and Borexino
• Waiting for more statistics
• pep and CNO neutrino spectroscopy is coming soon!!