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

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)

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)

Possible neutron sources

Cosmic-ray Fast neutronsLong-lived spallation products emitting neutrons

RadioactivitySpontaneous fission(, n)(, n)

Atmospheric Solar

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)

< 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

(, n)

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)

sources: 232Th series

KamLAND single spectrum3.2 105 decay/livetime (212Bi212Po)

5.3 MeV from 210Po ( 210Pb, T1/2=22y)

S. Enomoto, in the KamLAND collab. meeting

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

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

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

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

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

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

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

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

210Po, 210Bi decay rate

KamLAND single spectrum

Ph.D thesis by I. Shimizu, RCNS Tohoku(being written)

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

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

210Po non-equilibrium

Master thesis by K. Ichimura, RCNS Tohoku(being written in Japanese)

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

KamLAND filling (May-Sep, 2001)

Fit with free life timeT1/2 = 129 day (fit)(210Po = 138 day)

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

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

S. Enomoto & K. Inoue

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.

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)

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

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

Birks constant: quenching effect

Real Energy [MeV]

Determined from 10 data points

quench

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

~10 events above theanalysis thr. of 2.6 MeV

low-energy part is preliminary

With -n

Without -n

With -n

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

Discussion

Birks constant: quenching effect

Real Energy [MeV]

Determined from 10 data points

quench

e quench neutrons

Monte Carlo for GoF

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

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

6-MeV b.g. vs Reactor component

Oscillation

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