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11 C background suppression in the CNO-pep solar neutrino energy region with Borexino

11 C 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. pep and CNO neutrinos. pep neutrinos:

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11 C background suppression in the CNO-pep solar neutrino energy region with Borexino

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  1. 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

  2. pep and CNO neutrinos • pep neutrinos: • Directly related with thepp 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… Davide Franco – Università di Milano & INFN

  3. Events/3years/100 tons/0.05 MeV Detection signature in Borexino • Neutrino elastic scattering off electrons • Energy range of observation: • 0.8 – 1.4 MeV • Expected flux in BOREXino-like scintillator (BS07++LMA): • 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 Davide Franco – Università di Milano & INFN

  4. Detector layout and main features Stainless Steel Sphere: 2212 PMTs 1350 m3 Scintillator: 270 t PC+PPO 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 Č detector 208 PMTs in water 2100 m3 Carbon steel plates 20 legs Davide Franco – Università di Milano & INFN

  5. The signal and the background Energy range: 0.8 – 1.4 MeV 11C contamination Davide Franco – Università di Milano & INFN

  6. m (+ secondaries) + 12C → m (+ secondaries) + 11C + n n + p → d + g 11C → 11B + e+ + ne 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 Davide Franco – Università di Milano & INFN

  7. m PC+PPO 11C n g Vetoing the detector Depends on detector efficiencies in tagging neutrons and in tagging and tracking muons Davide Franco – Università di Milano & INFN

  8. 11C expected rate from NA54 Cern experiment S/B ~1/6 Davide Franco – Università di Milano & INFN

  9. m are identified by ID and OD OD eff: ~ 99% ID based on pulse shape analysis Rejection factor > 103 (conservative) m crossing the buffer only m crossing the scintillator Detecting (and rejecting) cosmic muons m pulse scintillation pulse Rm = (1.21±0.05) h-1m-2 Davide Franco – Università di Milano & INFN

  10. m-track (in progress) Davide Franco – Università di Milano & INFN

  11. NA54 Kamioka LNGS C. Galbiati et al., Phys. Rev. C 71, 055805 (2005) 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 Davide Franco – Università di Milano & INFN

  12. 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 > 90% neutrons detected • Still problems in very high multiplicity events Davide Franco – Università di Milano & INFN

  13. 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 Davide Franco – Università di Milano & INFN

  14. 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 γ • E < 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 < E < 2.25 MeV • Distance between 11C event and gamma < 35 cm Davide Franco – Università di Milano & INFN

  15. Measured Rate = 0.89 ± 0.20 Expected Rate(NA54) Measured 11C production rate in CTF Phys.Rev.C74:045805,2006. R(11C) = [13.0 ± 2.6(stat) ± 1.4(syst)]×10-2 d-1 ton-1 Goal reached? Davide Franco – Università di Milano & INFN

  16. 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 Davide Franco – Università di Milano & INFN

  17. Measured Rate = 1.7 ± 0.1 Expected Rate(NA54) …larger rate in Borexino from spectral analysis • Inconsistency between NA54/CTF and Borexino? • different size of the detector? • other background sources? Davide Franco – Università di Milano & INFN

  18. 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 techniqueis 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!! Davide Franco – Università di Milano & INFN

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