Transcript 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,

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. 13C(a, n)16O (a from 210Po)
Effects on Dm2 measurement
Neutron: serious BG for inverse b decay
The oldest and the strongest
technique for ne detection
ne
pn
pd
g
e+
Prompt
Delayed
Dt ~ 200 ms
(in the KamLAND
< 1.5 m
scintillator) DR ~
Ed = 2.2 MeV
Delayed coincidence ~102~103 BG suppression
Three tags: Dt, DR, and Ed
seems independent, but all are neutron feature
Neutron: serious BG for inverse b decay
The oldest and the strongest
technique for ne detection
n
pd
?
Prompt
g
Delayed
Dt ~ 200 ms
(in the KamLAND
< 1.5 m
scintillator) DR ~
Ed = 2.2 MeV
If only prompt is faked
 perfect delayed coincidence event
e.g. fast neutron: p from np elastic scattering fakes prompt
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
Fast neutrons: m v.s. n
Simple n/m flux ratio:
CHOOZ
> KamLAND
> Pala Verde
Very thick shield
of KamLAND
(see Inoue’s talk)
Kamioka
CHOOZ full paper
(arXiv:hep-ex/0301017)
Sudbury
D ~50-cm water
(active (Che))
D 2.5-m mineral oil
(active (Che))
D 1.0-m scintillatior
(active to recoil
proton)
Fast neutrons are determined from data
Fast neutron sample
Scintillator balloon
Fiducial
volume
Selection: same delayed coincidence
criteria as neutrino events,
but with Outer Detector hit
< 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)
(a, n)
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)
a sources: 232Th series
3.2  105 decay/livetime
(212Bi212Po)
KamLAND single spectrum
5.3 MeV a from 210Po ( 210Pb, T1/2=22y)
S. Enomoto, in the KamLAND collab. meeting
Target: 13C is dominant
(a, n) cross section  abundance in KamLAND
13C
Cross section from JENDL
& total
Abundances
in KL scintillator
nuclei Abundance
in number
13C
0.37 %
14N
0.012 %
15N
4.6105 %
17O
2.1106 %
18O
1.1105 %
13C(a,
n)16O events · · · prompt,
delayed
What fakes prompt signal:
16O ground state
fake
fast n  proton recoil
fast n  12C excitation
“genuine” n capture
(2.2-MeV g)
16O
excited (e+e-)
16O excited (g)
13O
16
C
210Pb
206
Po
a
p
n
d
~200ms
g
e+
e-
Prompt
Delayed
13C(a,
n)16O events · · · prompt,
delayed
What fakes prompt signal:
16O ground state
fake
fast n  proton recoil
fast n  12C excitation
“genuine” n capture
(2.2-MeV g)
16O
excited (e+e-)
16O excited (g)
13O
16
C
210Pb
206
Po
a
p
n
d
12C
g
e+
e-
~200ms
Delayed
Prompt
13C(a,
n)16O events · · · prompt,
delayed
What fakes prompt signal:
16O ground state
fake
fast n  proton recoil
fast n  12C excitation
+eg
n capture
e“genuine”
16O
excited (e+e-)
16O excited (g)
13O
16
C
210Pb
206
Po
a
p
n
(2.2-MeV g)
d
~200ms
g
e+
e-
e+ e
Prompt
-
Delayed
13C(a,
n)16O events · · · prompt,
delayed
What fakes prompt signal:
16O ground state
fake
fast n  proton recoil
fast n  12C excitation
“genuine” n capture
(2.2-MeV g)
16O
excited (e+e-)
16O excited (g)
13O
16
C
210Pb
206
Po
a
p
n
d
~200ms
g
e+
e-
Prompt
Delayed
Estimate the number of (a, n) events
in the final data set
Number
210Po
decay
measure 210Po and 210Bi rates
numerical integral
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
Geant4 based MC
• n propagation (np, n12C scattering, diffusion of thermal n)
• Scintillation quenching for low energy p
from a and g quench data
measured efficiency
• Detector resolution and off-line selection (vertex, energy)
Delayed coincidence rate and prompt energy spectrum obtained
210Po
Number
210Po
decay
decay rate
measure 210Po and 210Bi rates
numerical integral
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
Geant4 based MC
• n propagation (np, n12C scattering, diffusion of thermal n)
• Scintillation quenching for low energy p
from a and g quench data
measured efficiency
• Detector resolution and off-line selection (vertex, energy)
Delayed coincidence rate and prompt energy spectrum obtained
210Po
T1/2 = 22.3y
210Pb
decay rate
5.013d

210Bi
138.4d

b
210Po

a
206Pb
5.3 MeV
Kinetic energy = 1.2 MeV
13C
BG in KamLAND-II (solar)
see Kishimoto’s talk
stable
(a, n) 16O
210Po, 210Bi
decay rate
KamLAND single spectrum
Ph.D thesis by I. Shimizu, RCNS Tohoku
(being written)
210Po, 210Bi
210Po
decay rate
a
Run 3607 (2-hr low-th run)
b
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
210Bi
run by run
Theoretical
Results
Bi, R < 550 cm
2004/happy new yr
y/m/d
2002/Jul./2
Po, R < 550 cm
2004/May/2
Bi, and Po
agree within
error
Stable, and
almost in
equilibrium
~ 33 Hz
210Po
non-equilibrium
Po all volume
Master thesis by K. Ichimura, RCNS Tohoku
(being written in Japanese)
210Po
non-equilibrium
Fit with 210Po life time
KamLAND filling (May-Sep, 2001)
Master thesis by K. Ichimura, RCNS Tohoku
(being written in Japanese)
210Po
non-equilibrium
Fit with free life time
T1/2 = 129 day (fit)
(210Po = 138 day)
KamLAND filling (May-Sep, 2001)
Master thesis by K. Ichimura, RCNS Tohoku
(being written in Japanese)
a propagation and n yield
Number
210Po
decay
measure 210Po and 210Bi rates
numerical integral
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
Geant4 based MC
• n propagation (np, n12C scattering, diffusion of thermal n)
• Scintillation quenching for low energy p
from a and g quench data
measured efficiency
• Detector resolution and off-line selection (vertex, energy)
Delayed coincidence rate and prompt energy spectrum obtained
a propagation and n yield
range ~ 0.04 mm
5.3 MeV
 s (dE/dx)1dE
• 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
S. Enomoto & K. Inoue
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.
Neutron yield and energy spectra
3 to 7 MeV
neutrons from
ground state
events
For excitedstate events,
neutron energy
is negligible
(prompt energy
is from g or
e+e)
n propagation, detector effects
Number
210Po
decay
measure 210Po and 210Bi rates
numerical integral
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
Geant4 based MC
• n propagation (np, n12C scattering, diffusion of thermal n)
• Scintillation quenching for low energy p
from a and g quench data
measured efficiency
• Detector resolution and off-line selection (vertex, energy)
Delayed coincidence rate and prompt energy spectrum obtained
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
Birks constant: quenching effect
Determined from 10 data points
Real Energy [MeV]
a quench
g, e quench
neutrons
Prompt energy spectrum (w/o resolution)
with quenching (“visible energy”)
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
With a-n
Without a-n
With a-n
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
Discussion
Birks constant: quenching effect
Determined from 10 data points
Real Energy [MeV]
a quench
g, e quench
neutrons
Monte Carlo for GoF
Scaled no oscillation
Oscillation
6-MeV b.g. (free):
best-fit v.s. input
Good correlation
between
best-fit and input
Neutrino decay
Neutrino decoherence
6-MeV b.g. can
essentially be
extracted
(excluded) from the
reactor spectra
Scaled no oscillation
Neutrino decay
Oscillation
Neutrino decoherence
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
6-MeV b.g. vs Reactor component
Oscillation
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.