1. 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

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

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

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

6. Fast neutrons are determined from data
Fast neutron sample
< 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)
Scintillator balloon
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
13C & total
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 %
Cross section from JENDL

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

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

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

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

16. Estimate the number of (a, n) events
in the final data set
measure 210Po and 210Bi rates
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
numerical integral
Geant4 based MC
from a and g quench data
measured efficiency

17. 210Po decay rate Number 210Po decay
measure 210Po and 210Bi rates
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
numerical integral
Geant4 based MC
from a and g quench data
measured efficiency

18. 210Po decay rate 210Pb  210Bi  210Po  206Pb a b 13C (a, n) 16O
T1/2 = 22.3y
5.013d
138.4d
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 210Po a 210Bi b run by run
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, and Po agree within error
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. 210Po non-equilibrium Fit with 210Po life time
KamLAND filling (May-Sep, 2001)
Master thesis by K. Ichimura, RCNS Tohoku
(being written in Japanese)

24. 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)

25. a propagation and n yield
measure 210Po and 210Bi rates
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
numerical integral
Geant4 based MC
from a and g quench data
measured efficiency

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. n propagation, detector effects
measure 210Po and 210Bi rates
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
numerical integral
Geant4 based MC
from a and g quench data
measured efficiency

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

43. 6-MeV b.g. (free): best-fit v.s. input
Scaled no oscillation
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

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

45. 6-MeV b.g. vs Reactor component
Oscillation

46. 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.