Experimental Status of Georeactor Search with KamLAND Detector

1
Experimental Status of Geo-reactor Searchwith
KamLAND Detector

• Jelena Maricic
• University of Hawaii at Manoa

2
Outline

• KamLAND detector design and features
• Motivation for experimental geo-reactor search in
  KamLAND
• Geo-reactor analysis anti-neutrino
• event selection and backgrounds
• Anti-neutrino flux at KamLAND
• Analysis
• Summary and conclusion

3
KamLAND Detector Design and Features
4
KamLAND Purpose and Location

• - KamLAND - anti-neutrino detector built
  to study
• anti-neutrino oscillations.
• - Japan - natural choice for
  location of
• anti-neutrino detector
• - large number of nuclear
  plants.
• - Nuclear plants - the largest
  man-made ?e sources.

- Nuclear plant
5
Reactors as Neutrino Sources and KamLAND

• Nuclear reactor is an excellent source of
  electron anti-neutrinos from ß decay.
• Average 3 GWth plant has a flux of 61020
  anti-neutrinos/s!
• KamLAND - disappearance experiment

?e
?e
Look for a deficit of ?e at a distance L
?e
?e
nuclear reactor
?e
?e
KamLAND
?x ?
?e
?e
?e detector
?e
L
6
Anti-neutrino Spectrum
Number of observed events (1/MeV)
Observed spectrum
Reactor spectrum
Interaction cross- section (10-43cm2)
E? (MeV)
7
Detector Scheme

• 1kton of LS surrounded by buffer oil and acrylic
  Rn barrier.
• 1325 17 PMTs
• 554 20 PMTs
• 34 photocatode coverage
• 225 20 PMTs - veto
• water Cherenkov detector
• 300 p.e./MeV observed at the center.
• KamLAND oil has the best radiopurity ever
  achieved
• in the world

U (3.5 0.5) x 10-18 g/g Th (5.2
0.8) x 10-17 g/g K lt 2.7 x 10-16 g/g
8
Motivation for the Experimental Geo-reactor
Searchwith KamLAND
9
Introduction

• Natural nuclear fission reactor with power up to
  10 TW in the center of the Earth was proposed by
  M. Herndon as the energy source of geo-magnetic
  field.
• 4.5 billion years ago, 235U/238U ratio was high
  enough for the nuclear fission reaction to occur.
• If such a reactor exists, its
• anti-neutrino flux would be
• visible by KamLAND.

235U/238U gt 5
Fast breeder nuclear reactor was simulated using
the SCALE code package (by D. Hollenback and M.
Herndon) and shown feasibility and
sustainability for 4.5 billion years.
10
Motivation for Geo-reactor Search
Large error!
90 C.L.

• Rate from the putative geo-reactor very small!
• Incoming daily flux varies due
• to nuclear reactors varying work regime.

Small positive offset of 0.03e/day with VERY
LARGE ERROR may be present, for 0 ev/day
expected!
11
Is the Event Excess for Real and if So, What is
the Source ?

• The possible surplus of detected events implies
  that there may be another source of
  anti-neutrinos that have not been accounted for.
• Proposed 3-10 TW georeactor if exists would
  produce anti-neutrino signal of 4-14 of the
  KamLAND signal.
• The goal of this analysis is to
• set the upper limit on the power of the
  putative geo-reactor.

Is it there and if so, how large is it?
12
Geo-reactor Analysis Anti-neutrino Event
Selection andBackgrounds
13
Detection Reaction in KamLAND

• Inverse beta decay reaction combined with delayed
  neutron capture reaction.
• Distinctive signature
• in time and space
• Prompt event e - e- annihilation 2 ? rays
• Delayed event 2.2 MeV
• ? ray about 200 µs later.

ne p e n
Ethreshold 1.806 MeV
Prompt Event
?
?
e
?
?e
p
n
2.2MeV
Eprompt E? - 0.8 MeV
Delayed Event
200 µs
14
Event Selection Cuts

• - target volume cut (R lt 5.5 m)
• 4.61 x 1031 target protons,
• - inverse ß decay cut
• - timing correlation cut (0.5µs lt ?T lt 1000µs)
• - vertex correlation cut (?R lt 2.0 m)
• - delayed energy cut (1.8MeV lt Edelay lt
  2.6MeV)
• Efficiency of inverse ß decay cut (89.8
  1.5)
• - prompt energy analysis threshold (2.6 MeV lt
  Epromptlt 8.5 MeV)
• - cosmic ray muon spallation event cut
  (spallation - shattering of a
  nucleus by a highly energetic cosmic-ray
  particle)
• As a cross-check, analysis with lower energy
  threshold of 1.6 MeV prompt energy
  has been performed as well. Data sample increase
  40. However, lower energy threshold requires
  additional background subtraction.

15
Cosmic Ray Muon Spallation Cuts

• Cosmic muon rate in KamLAND is 0.34 Hz.
• 2 ms veto is applied after each tagged muon
• 2 sec veto is applied after showering muon
• 2 sec veto along LS muon track with 3 m radius
• Spallation cuts introduce
• around 9.7 additional dead time.

16
Anti-neutrino Candidates

• From March 9th 2002 to January 11th 2004 total
  livetime is
• 515.1 days
• After applying selection cuts, the number of
  selected anti-neutrino candidates is
• 258 events (Epromptgt2.6MeV)
• or
• 362 events (Epromptgt1.6MeV)

17
Estimated Systematic Uncertainties
The largest contribution

• Target volume 4.7
• Energy threshold 2.3
• Efficiency of cuts 1.6
• Livetime 0.06
• Reactor power 2.1
• Fuel composition 1.0
• Anti-neutrino spectra 2.5
• Anti neutrino cross-section 0.2
• Total 6.5

18
Analysis Backgrounds
E? gt 3.4 MeV E? gt 2.4 MeV

• - Geo-neutrinos coming from
• the radioactive decay chains
• of 238U and 232Th
  negligible (14 5)
  
• Accidental backgrounds (2.69
  0.05) (10.73 0.04)
• 9Li/8He Background (4.8
  0.9) (6.2 1.0)
• 13C(?,n)16O background (10.3
  7.1) (13.5 10.8)
• Total
  (17.73 8.05) (44.43 16.84)

19
Anti-neutrino Flux at KamLAND
20
Anti-neutrino Flux from Man-made Reactors

• - 79 is within range 138-214km
• ave. dist. 180 km
• - Expected number of events in 515.1 days of
  livetime
• 365 23.7 (syst)
• 493.2 32.0 (syst.)
• in the unoscillated case.

E? gt 3.4 MeV
E? gt 2.4 MeV
21
Anti-neutrino Spectrum from Geo-reactor

• Reactor spectrum for the deep Earth reactor is
  assumed to be a typical commercial reactor
  spectrum.
• It is assumed that its output is very stable (on
  the data taking scale)

E? gt 2.4 MeV
0.0102 events/TWday
0.0137 events/TWday
22
Analysis
23
Detecting a Geo-reactor

• Geo-reactor signal - 0 to 14 (10 TW) of the
  signal at KamLAND.
• KamLAND can detect signature spectrum from
• geo-reactor, as a constant ?e flux on the top of
  varying ?e flux from terrestrial reactors.
• - Upper limit on the geo-reactor thermal power
  set using statistical approach Maximum
  Likelihood Method.
• Maximum likelihood estimation (MLE) is a
  popular statistical method used to make
  inferences about parameters of the underlying
  probability distribution of a given data set.

24
Analysis Outline

• The analysis is based on 776 ton-year exposure of
  KamLAND to neutrinos.
• Geo-reactor power is treated as a completely free
  parameter
• Analysis consists of 2 parts
• Rate Spectrum shape analysis using global solar
  solution for oscillation parameters (independent
  of KamLAND) for E? gt 3.4 MeV.
• Cross-check analysis with lower energy threshold
• E? gt 2.4 MeV

25
Two Different Choices for Global Solar
Oscillation Parameters

• Two different sets of oscillation parameters
  used.
• Effects on the geo-reactor power output results
  tested.

?m2 6.45 10-5 eV2
SNO old 2003
sin2 2? 0.82
?m2 6.5 10-5 eV2
SNO new 2005
sin2 2? 0.86
26
Time dependent survival probability

• Survival probability changes daily due to the
  distance flux variation (reactors being turned
  off etc.).
• Also energy spectrum is time dependent.

Difference in shape due to the difference
in oscillation parameters.
27
Choice of Maximum Likelihood Function

• Analysis takes into account both daily rate and
  spectrum shape information with flux time
  variation included.
• Variable parameters in the fit are
• Geo-reactor rate (free)
• Detection efficiency (constrained)
• 9Li muon spallation background (constrained)
• 13C(a,n)16O background (constrained)
• ? m2 (constrained)
• sin2 2? (constrained)

BG
OP
Geoneutrino background from terrestrial uranium
is also treated as a fit parameter in the lower
energy cross-check analysis.
28
Analysis Results
Geo-reactor power lt 19 TW at 90 C.L.
16 geo-reactor events in the data sample
PRELIMINARY
29
Energy Spectrum for the Best Fit Result
Observed spectrum is time integrated, while the
best fit is obtained from the time varying
maximum likelihood function best fit.
PRELIMINARY
30
The ??2 Test as a Function of Geo-reactor Power
The best fit with SNO old (2003) choice of
mixing parameters
PRELIMINARY
Very wide minimum
31
Summary and Conclusion
32
Comparison of the Best Fit Result with Geological
Data
PRELIMINARY
31-44 TW
19-31 TW
0-12 TW
33
Conclusion

• Upper limit on the power of the geo-reactor have
  been set for the first time.
• The best fit is
• Upper limit on geo-reactor power is 19 TW at 90
  C.L.
• Final result greatly influenced by the input
  oscillation parameters.
• KamLAND size detector far away from nuclear
  reactors
• needed for high confidence (gt99.99)
  measurement.
• Hawaii presents an excellent choice for a
  definite
• geo-reactor measurement (Hanohano).

PRELIMINARY
34
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35
The Existence of Geo-reactor
can explain the following unresolved
question - provide the energy source for
driving the Earths magnetic field (0.02-10 TW of
power running for more than 3 billion
years!!!). - easily explains reversals of the
geo-magnetic field (171 reversals recorded in
the last 70 million years). - provide
explanation for the up to 40 times higher
measured ratios (comparing to average atmospheric
ratio) of 3He/4He observed in volcanic plumes in
Hawaii, Iceland some other places.
36
Earth Models

• Traditional Model (BSE) content of the inner
  core based on carbonaceous, chondrites. As a
  result, U and Th are in the form of oxides, act
  as lithophiles and can exist in the crust and
  mantle only.
• Nuclear Earth Model (by M. Herndon) content
  of the inner core based on rare enstatite
  chondrites. U and Th are alloyed with Fe or S,
  act as siderophiles and due to high density can
  exist in the inner core and particularly the
  Earths center.

37
Geo-reactor Sustainability
235U/238U gt 5
- 4.5 billion years ago, 235U/238U ratio was
high enough for the nuclear fission reaction to
occur. - Fast breeder nuclear reactor was
simulated using the SCALE code package (by M.
Herndon) and shown feasibility and sustainability
for 4.5 billion years.
Fission products must be removed
38
Geo-reactor and 3He/4He anomaly

• SCALE code package was used (by M. Herndon) to
  estimate tritium production, since tritium decays
  into 3He with 12 years lifetime.
• Estimated ratios resemble observations from vents
  in Hawaii, Iceland and MORBs.

39
Has Natural Nuclear Reactor Ever Been Observed?

• YES! Natural nuclear reactor has already been
  seen in nature.
• And not just one, but 17 of them were found.
• Although, these reactors came to be, by a
  different process, they operated 2 billion years
  ago, for about a million years in total, as fast
  breeder reactors.
• They were discovered in Oklo uranium mine in
  Gabon, Africa in 1972.
• 2 billion years ago, natural nuclear
  reaction could occur, due to the larger
  percentage of 235U (3)

40
Rate Term

• Rate term is described by Poisson distribution

Ndays is a number of days in the chosen data
set µi Eff Lti (P0 R0 P1i Ri ) (Lti
/TotalLt)(Nli NC Nacc) P0 is geo-reactor
survival probability R0 is geo-reactor expected
daily rate P1 is terrestrial reactors survival
probability dependent on mixing parameters and
distribution of reactor flux Ri is terrestrial
reactors expected daily rate Nli, NC and Nacc are
total Li, carbon and accidental background in the
data sample
41
Shape Term
In the binned analysis, events are divided into
0.1 MeV energy bins. Each bin is described by
Poisson distribution.

• In the unbinned analysis, at each events energy
  contributions are added according to their
  spectral shape.
• The geo-reactor spectrum looks like unoscillated
  spectrum.

42
Constraints Term

• Gaussian distribution is used to constrain
  efficiency and mixing parameters.

• Gaussian distribution is used to constrain
  backgrounds.

Binned
Unbinned
43
The ??2 Test as a Function of Geo-reactor Power
and ?m2
?m2 region favored by solar data
?m2 region favored by KamLAND data
R0TW
log10 ?m2 eV2
44
The ??2 Test as a Function of Geo-reactor Power
and sin22?
Wide valley for sin22?!
R0TW
sin2 2?
45
Unbinned RateShape Analysis

• Contributions of Rate, Shape and Constraint
  Likelihood Terms for the Constrained RateShape
  Unbinned Analysis

Shape term the most constraining!
SK I
SNO
SK II
Rate
Rate
Rate
Shape
Shape
Shape
Constr
Constr
Constr
46
Unbinned RateShape Analysis
The best fit around 6 TW 90 C.L. around 19 TW
R0 6.9 TW
R0 4.9 TW
R0 5.9 TW
R90 20.7 TW
R90 18.2 TW
R90 18.2 TW
The best fit moves toward larger ?m2 favored by
KamLAND data