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Neutron backgrounds in KamLAND

Neutron backgrounds in KamLAND. Tadao Mitsui Research Center for Neutrino Science, Tohoku University (For the KamLAND collaboration). 12-14 December, 2004 Low Radioactivity Techniques 2004, Sudbury, Canada. Neutron backgrounds in KamLAND e.g. 13 C( a , n ) 16 O ( a from 210 Po)

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Neutron backgrounds in KamLAND

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  1. Neutron backgrounds in KamLAND Tadao Mitsui Research Center for Neutrino Science, Tohoku University(For the KamLAND collaboration) 12-14 December, 2004Low Radioactivity Techniques 2004, Sudbury, Canada • Neutron backgrounds in KamLAND • e.g. 13C(a, n)16O (a from 210Po) • Effects on Dm2 measurement

  2. ne Dt ~ 200 ms DR < 1.5 m Ed = 2.2 MeV (in the KamLAND scintillator) ~ Neutron: serious BG for inverse b decay The oldest and the strongest technique for ne detection p n p d • Delayed coincidence ~102~103 BG suppression • Three tags: Dt,DR,and Ed seems independent, but all are neutron feature e+ g Prompt Delayed

  3. Dt ~ 200 ms DR < 1.5 m Ed = 2.2 MeV (in the KamLAND scintillator) ~ Neutron: serious BG for inverse b decay The oldest and the strongest technique for ne detection n p d • If only prompt is faked  perfect delayed coincidence event • e.g. fast neutron: p from np elastic scattering fakes prompt ? g Prompt Delayed

  4. Possible neutron sources • Cosmic-ray m • Fast neutrons • Long-lived spallation products emitting neutrons • Radioactivity • Spontaneous fission • (g, n) • (a, n) • Atmospheric n • Solar n

  5. Fast neutrons: m v.s. n • Simple n/m flux ratio: CHOOZ > KamLAND > Pala Verde • Very thick shield of KamLAND (see Inoue’s talk) • ~50-cm water (active (Che)) • 2.5-m mineral oil (active (Che)) • 1.0-m scintillatior (active to recoil proton) Kamioka CHOOZ full paper (arXiv:hep-ex/0301017) Sudbury

  6. Fast neutrons are determined from data Fast neutron sample Scintillator balloon • < 5 fast n’s in the 5.5-m fiducial (for data set of 2nd reactor result) • OD 92% efficient: < 0.4 for OD muon • For rock muon < 0.5 from MC (MC only for relative contribution) • Total < 0.89 fast n (258 events in n sample) Fiducial volume Selection:same delayed coincidence criteria as neutrino events, but with Outer Detector hit

  7. (a, n)

  8. a sources: 238U series 2.5  106 decay/livetime (234Pa) KamLAND single spectrum 1.2  104 decay/livetime (214Bi214Po) 1.3  109 decay/livetime (210Bi, 210Po)

  9. a sources: 232Th series 3.2  105 decay/livetime (212Bi212Po) KamLAND single spectrum

  10. 5.3 MeV a from 210Po ( 210Pb, T1/2=22y) S. Enomoto, in the KamLAND collab. meeting

  11. Target: 13C is dominant (a, n) cross section  abundance in KamLAND Abundances in KL scintillator • Cross section from JENDL 13C & total

  12. fake “genuine” n capture (2.2-MeV g) ~200ms 13C(a, n)16O events · · · prompt, delayed What fakes prompt signal: • 16O ground state • fast n proton recoil • fast n 12C excitation • 16O excited (e+e-) • 16O excited (g) 13C 16O d 206Pb 210Po Prompt Delayed g a p e+ n e-

  13. fake “genuine” n capture (2.2-MeV g) ~200ms 13C(a, n)16O events · · · prompt, delayed What fakes prompt signal: • 16O ground state • fast n proton recoil • fast n 12C excitation • 16O excited (e+e-) • 16O excited (g) 13C 16O d 206Pb 210Po 12C Delayed g a p e+ Prompt n e-

  14. fake “genuine” n capture (2.2-MeV g) g e+e- ~200ms Prompt e+ e- 13C(a, n)16O events · · · prompt, delayed What fakes prompt signal: • 16O ground state • fast n proton recoil • fast n 12C excitation • 16O excited (e+e-) • 16O excited (g) 13C 16O d 210Po 206Pb Delayed g a p e+ n e-

  15. fake “genuine” n capture (2.2-MeV g) ~200ms 13C(a, n)16O events · · · prompt, delayed What fakes prompt signal: • 16O ground state • fast n proton recoil • fast n 12C excitation • 16O excited (e+e-) • 16O excited (g) 13C 16O d 206Pb 210Po Delayed g Prompt a p e+ n e-

  16. measure 210Po and 210Bi rates numerical integral Geant4 based MC from a and g quench data measured efficiency Estimate the number of (a, n) events in the final data set • Number 210Po decay • a propagation and (a, n) rate: dE/dx and range of a, and 13C(a, n)16O (or 16O*) cross section Neutron energy spectrum obtained • n propagation (np, n12C scattering, diffusion of thermal n) • Scintillation quenching for low energy p • Detector resolution and off-line selection (vertex, energy) Delayed coincidence rate and prompt energy spectrum obtained

  17. measure 210Po and 210Bi rates numerical integral Geant4 based MC from a and g quench data measured efficiency 210Po decay rate • Number 210Po decay • a propagation and (a, n) rate: dE/dx and range of a, and 13C(a, n)16O (or 16O*) cross section Neutron energy spectrum obtained • n propagation (np, n12C scattering, diffusion of thermal n) • Scintillation quenching for low energy p • Detector resolution and off-line selection (vertex, energy) Delayed coincidence rate and prompt energy spectrum obtained

  18. T1/2 = 22.3y 5.013d 138.4d 210Po decay rate stable 210Pb  210Bi  210Po  206Pb a b 5.3 MeV Kinetic energy = 1.2 MeV 13C (a, n) 16O BG in KamLAND-II (solar) see Kishimoto’s talk

  19. 210Po, 210Bi decay rate KamLAND single spectrum Ph.D thesis by I. Shimizu, RCNS Tohoku (being written)

  20. 210Po, 210Bi decay rate run by run 210Po a 210Bi b Run 3607 (2-hr low-th run) R < 550 cm R < 550 cm Evis~260 keV gaussian+ax+b NsumMax For fiducail cut: low-th (th=35) run For all volume: history run Theoretical

  21. Results Bi, R < 550 cm • Bi, and Po agree within error • Stable, and almost in equilibrium • ~ 33 Hz 2004/happy new yr y/m/d 2002/Jul./2 Po, R < 550 cm 2004/May/2

  22. 210Po non-equilibrium Po all volume Master thesis by K. Ichimura, RCNS Tohoku (being written in Japanese)

  23. KamLAND filling (May-Sep, 2001) 210Po non-equilibrium Fit with 210Po life time Master thesis by K. Ichimura, RCNS Tohoku (being written in Japanese)

  24. KamLAND filling (May-Sep, 2001) 210Po non-equilibrium Fit with free life time T1/2 = 129 day (fit) (210Po = 138 day) Master thesis by K. Ichimura, RCNS Tohoku (being written in Japanese)

  25. measure 210Po and 210Bi rates numerical integral Geant4 based MC from a and g quench data measured efficiency a propagation and n yield • Number 210Po decay • a propagation and (a, n) rate: dE/dx and range of a, and 13C(a, n)16O (or 16O*) cross section Neutron energy spectrum obtained • n propagation (np, n12C scattering, diffusion of thermal n) • Scintillation quenching for low energy p • Detector resolution and off-line selection (vertex, energy) Delayed coincidence rate and prompt energy spectrum obtained

  26. a propagation and n yield • All sources and targets are included in actual calculation • s is actually differential cross section to obtain neutron energy spectrum (see next) • dE/dx table from GEANT3 range ~ 0.04 mm 5.3 MeV  s (dE/dx)1dE

  27. S. Enomoto & K. Inoue

  28. 16O excited state • JENDL gives only theoretical cross sections • The absolute number of events from 16O excited state is treated as a free parameter in final oscillation analysis.

  29. Neutron yield and energy spectra • 3 to 7 MeV neutrons from ground state events • For excited-state events, neutron energy is negligible (prompt energy is from g or e+e)

  30. measure 210Po and 210Bi rates numerical integral Geant4 based MC from a and g quench data measured efficiency n propagation, detector effects • Number 210Po decay • a propagation and (a, n) rate: dE/dx and range of a, and 13C(a, n)16O (or 16O*) cross section Neutron energy spectrum obtained • n propagation (np, n12C scattering, diffusion of thermal n) • Scintillation quenching for low energy p • Detector resolution and off-line selection (vertex, energy) Delayed coincidence rate and prompt energy spectrum obtained

  31. n propagation, detector effects Genat4 based MC, cross-check by GENAT3 • Birk’s quenching is included (see next) • Low-energy (< 2.6 MeV) results are very preliminary (more study is needed for quenching) • 4.4-MeV g from 12C excitation is clearly seen

  32. Birks constant: quenching effect Determined from 10 data points Real Energy [MeV] a quench neutrons g, e- quench

  33. Prompt energy spectrum (w/o resolution) with quenching (“visible energy”)

  34. Prompt energy spectrum (with resolution) expected number of events in the data sample low-energy part is preliminary ~10 events above the analysis thr. of 2.6 MeV

  35. With a-n

  36. Without a-n

  37. With a-n

  38. Summary • 13C(a, n)16O : main neutron source in KamLAND • Estimation of rate and energy spectra has been done • ~10 BG events from 13C(a, n)16O (total n candidates: 258 events) • Effects on oscillation analysis (Dm2 measurement) is very small • More study needed for low energy region below 2.6 MeV

  39. Discussion

  40. Birks constant: quenching effect Determined from 10 data points Real Energy [MeV] a quench neutrons g, e- quench

  41. Monte Carlo for GoF

  42. Oscillation Scaled no oscillation 6-MeV b.g. (free):best-fit v.s. input • Good correlation between best-fit and input • 6-MeV b.g. can essentially be extracted (excluded) from the reactor spectra Neutrino decay Neutrino decoherence

  43. Scaled no oscillation Oscillation 6-MeV b.g. vs Reactor component • Horizontal axes: 6-MeV b.g. (best-fit) - (input of MC) • Vertical axes: Dm2, neutrino life time etc • Shows how “misfit” of 6-MeV b.g. affects analysis of reactor component Neutrino decay Neutrino decoherence

  44. 6-MeV b.g. vs Reactor component Oscillation

  45. 6-MeV b.g. vs Reactor component Oscillation • -1: our previous preprint (“truth” is 7, we “fitted” it as 0, then (fit-input)/7=-1 • In this case, LMA-II: disfavored, LMA-I: higher Dm2, LMA-0 favored • Just as we experienced.

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