Download - Neutron backgrounds in KamLAND
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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 KamLANDe.g. 13C(, n)16O (from 210Po)Effects on m2 measurement
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Neutron: serious BG for inverse decay
Delayed coincidence ~102~103 BG suppressionThree tags: t, R, and Ed
seems independent, but all are neutron feature
Prompt Delayed
The oldest and the strongesttechnique for e detection
pne pd
e+
t ~ 200 sR < 1.5 m~
Ed = 2.2 MeV(in the KamLAND scintillator)
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Neutron: serious BG for inverse decay
If only prompt is faked perfect delayed coincidence event
e.g. fast neutron: p from np elastic scattering fakes prompt
Prompt Delayed
The oldest and the strongesttechnique for e detection
n pd
?
t ~ 200 sR < 1.5 m~
Ed = 2.2 MeV(in the KamLAND scintillator)
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Possible neutron sources
Cosmic-ray Fast neutronsLong-lived spallation products emitting neutrons
RadioactivitySpontaneous fission(, n)(, n)
Atmospheric Solar
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Fast neutrons: v.s.
Simple / 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 pr
oton)
Sudbury
Kamioka
CHOOZ full paper(arXiv:hep-ex/0301017)
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< 5 fast n’s in the 5.5-m fiducial (for data set of 2nd reactor result)OD 92% efficient: < 0.4 for OD muonFor rock muon < 0.5 from MC (MC only for relative contribution)Total < 0.89 fast n (258 events in sample)
Fast neutrons are determined from data
Selection: same delayed coincidence criteria as neutrino events, but with Outer Detector hit
Fiducialvolume
Fast neutron sample
Scintillator balloon
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(, n)
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sources: 238U series
KamLAND single spectrum
2.5 106
decay/livetime (234Pa)
1.2 104 decay/livetime (214Bi214Po)
1.3 109 decay/livetime (210Bi, 210Po)
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sources: 232Th series
KamLAND single spectrum3.2 105 decay/livetime (212Bi212Po)
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5.3 MeV from 210Po ( 210Pb, T1/2=22y)
S. Enomoto, in the KamLAND collab. meeting
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Target: 13C is dominant
Cross section from JENDL
(, n) cross section abundance in KamLAND
13C & total nuclei Abundance in number
13C 0.37 %
14N 0.012 %
15N 4.6105 %
17O 2.1106 %
18O 1.1105 %
Abundancesin KL scintillator
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13C(, n)16O events · · · prompt, delayed
16O ground statefast n proton recoilfast n 12C excitation
16O excited (e+e-)16O excited ()
210Po13C
Prompt Delayed
~200s
What fakes prompt signal:
fake
“genuine” n capture (2.2-MeV )
d
pn
e+
e-
206Pb16O
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13C(, n)16O events · · · prompt, delayed
16O ground statefast n proton recoilfast n 12C excitation
16O excited (e+e-)16O excited ()
210Po13C
PromptDelayed
~200s
What fakes prompt signal:
fake
“genuine” n capture (2.2-MeV )
d
pn
e+
e-
12C206Pb
16O
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13C(, n)16O events · · · prompt, delayed
16O ground statefast n proton recoilfast n 12C excitation
16O excited (e+e-)16O excited ()
210Po13C
Delayed
~200s
What fakes prompt signal:
fake
“genuine” n capture (2.2-MeV )
d
pn
e+
e-
Prompte+ e-
e+e-
206Pb16O
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13C(, n)16O events · · · prompt, delayed
16O ground statefast n proton recoilfast n 12C excitation
16O excited (e+e-)16O excited ()
210Po13C
Prompt Delayed
~200s
What fakes prompt signal:
fake
“genuine” n capture (2.2-MeV )
d
pn
e+
e-
206Pb16O
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Estimate the number of (, n) events
Number 210Po decay
propagation and (, n) rate: dE/dx and range of and 13C(, 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
in the final data set
measure 210Po and 210Bi rates
numerical integral
Geant4 based MC
measured efficiency
from and quench data
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210Po decay rate
Number 210Po decay
propagation and (, n) rate: dE/dx and range of and 13C(, 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
measure 210Po and 210Bi rates
numerical integral
Geant4 based MC
measured efficiency
from and quench data
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210Po decay rate
210Pb 210Bi 210Po 206Pb
13C (, n) 16O
T1/2 = 22.3y 5.013d 138.4d
5.3 MeVKinetic energy = 1.2 MeV
stable
BG in KamLAND-II (solar)see Kishimoto’s talk
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210Po, 210Bi decay rate
KamLAND single spectrum
Ph.D thesis by I. Shimizu, RCNS Tohoku(being written)
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Run 3607 (2-hr low-th run)
NsumMax
210Po 210Bi
Theoretical
For fiducail cut: low-th (th=35) runFor all volume: history run
Evis~260 keVgaussian+ax+b
R < 550 cm R < 550 cm
210Po, 210Bi decay rate run by run
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Results
y/m/d
Bi, and Po agree within error
Stable, and almost in equilibrium
~ 33 Hz
2002/Jul./2
2004/May/2
Bi, R < 550 cm
Po, R < 550 cm
2004/happy new yr
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210Po non-equilibrium
Master thesis by K. Ichimura, RCNS Tohoku(being written in Japanese)
Po all volume
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Master thesis by K. Ichimura, RCNS Tohoku(being written in Japanese)
210Po non-equilibrium
Fit with 210Po life time
KamLAND filling (May-Sep, 2001)
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Master thesis by K. Ichimura, RCNS Tohoku(being written in Japanese)
210Po non-equilibrium
KamLAND filling (May-Sep, 2001)
Fit with free life timeT1/2 = 129 day (fit)(210Po = 138 day)
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propagation and n yield
Number 210Po decay
propagation and (, n) rate: dE/dx and range of and 13C(, 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
measure 210Po and 210Bi rates
numerical integral
Geant4 based MC
measured efficiency
from and quench data
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propagation and n yield
• All sources and targets are included in actual calculation
• is actually differential cross section to obtain neutron energy spectrum (see next)
• dE/dx table from GEANT3
5.3 MeVrange ~ 0.04 mm
(dE/dx)1dE
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S. Enomoto & K. Inoue
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16O excited state
JENDL gives only theoretical cross sectionsThe absolute number of events from 16O excited state is treated as a free parameter in final oscillation analysis.
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Neutron yield and energy spectra
3 to 7 MeV neutrons from ground state eventsFor excited-state events, neutron energy is negligible (prompt energy is from or e+e)
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n propagation, detector effects
Number 210Po decay
propagation and (, n) rate: dE/dx and range of and 13C(, 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
measure 210Po and 210Bi rates
numerical integral
Geant4 based MC
measured efficiency
from and quench data
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n propagation, detector effects
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 from 1
2C excitation is clearly seen
Genat4 based MC, cross-check by GENAT3
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Birks constant: quenching effect
Real Energy [MeV]
Determined from 10 data points
quench
e quench neutrons
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Prompt energy spectrum (w/o resolution)with quenching (“visible energy”)
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Prompt energy spectrum (with resolution)expected number of events in the data sample
~10 events above theanalysis thr. of 2.6 MeV
low-energy part is preliminary
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With -n
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Without -n
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With -n
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Summary
13C(, n)16O : main neutron source in KamLAND
Estimation of rate and energy spectra has been done~10 BG events from 13C(, n)16O
(total candidates: 258 events)Effects on oscillation analysis (m2 measurement) is very smallMore study needed for low energy region below 2.6 MeV
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Discussion
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Birks constant: quenching effect
Real Energy [MeV]
Determined from 10 data points
quench
e quench neutrons
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Monte Carlo for GoF
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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
Scaled no oscillation Oscillation
Neutrino decay Neutrino decoherence
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6-MeV b.g. vs Reactor component
Horizontal axes: 6-MeV b.g. (best-fit) - (input of MC)
Vertical axes: m2, neutrino life
time etcShows how “misfit” of 6-MeV b.g. affects analysis of reactor component
Scaled no oscillation Oscillation
Neutrino decay Neutrino decoherence
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6-MeV b.g. vs Reactor component
Oscillation
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6-MeV b.g. vs Reactor component
-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 m2, LMA-0 favored
Just as we experienced.
Oscillation