1. Experimental Status of Geo-reactor Search with KamLAND Detector
Jelena Maričić
University of Hawaii at Manoa

2. Neutrino Geophysics, Honolulu
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
December,
Neutrino Geophysics, Honolulu

3. KamLAND Detector: Design and Features
December,
Neutrino Geophysics, Honolulu

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 νe sources.
- Nuclear plant
December,
Neutrino Geophysics, Honolulu

5. Reactors as Neutrino Sources and KamLAND
Nuclear reactor is an excellent source of electron anti-neutrinos from β decay.
Average 3 GWth plant has a flux of 6•1020 anti-neutrinos/s!
• KamLAND - disappearance experiment νe νe
Look for a deficit of νe at a distance L νe νe
nuclear
reactor νe νe
KamLAND
νx ? νe νe νe
νe detector L
December,
Neutrino Geophysics, Honolulu

6. Anti-neutrino Spectrum
Number of observed events (1/MeV)
Reactor
spectrum
Observed spectrum
Interaction cross-
section (~10-43cm2)
E (MeV)
December,
Neutrino Geophysics, Honolulu

7. Neutrino Geophysics, Honolulu
Detector Scheme
1kton of LS surrounded by buffer oil and acrylic Rn barrier.
” PMTs
554 20” PMTs
34% photocatode coverage
225 20” PMTs - veto
water Cherenkov detector
• p.e./MeV observed at the center.
*KamLAND oil has the best radiopurity ever achieved
in the world:
U ( ) x g/g
Th ( ) x g/g
K < 2.7 x g/g
December,
Neutrino Geophysics, Honolulu

8. Motivation for the Experimental Geo-reactor Search with KamLAND

9. Neutrino Geophysics, Honolulu
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 > 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.
December,
Neutrino Geophysics, Honolulu

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!
December,
Neutrino Geophysics, Honolulu

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?
December,
Neutrino Geophysics, Honolulu

12. Geo-reactor Analysis Anti-neutrino Event Selection and Backgrounds
December,
Neutrino Geophysics, Honolulu

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 γ rays
Delayed event: 2.2 MeV
γ ray about 200 μs later.
ne + p+ ® e+ + n
Ethreshold = MeV
Prompt
Event γ γ e+ γ νe p n
2.2MeV
Eprompt = E MeV
200 μs
Delayed Event
December,
Neutrino Geophysics, Honolulu

14. Neutrino Geophysics, Honolulu
Event Selection Cuts
- target volume cut (R < 5.5 m)
4.61 x 1031 target protons,
- inverse β decay cut
- timing correlation cut (0.5μs < ΔT < 1000μs)
- vertex correlation cut (ΔR < 2.0 m)
- delayed energy cut (1.8MeV < Edelay < 2.6MeV)
***Efficiency of inverse β decay cut ( )%
- prompt energy analysis threshold (2.6 MeV < Eprompt< 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.
December,
Neutrino Geophysics, Honolulu

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.
December,
Neutrino Geophysics, Honolulu

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 (Eprompt>2.6MeV) or
362 events (Eprompt>1.6MeV)
December,
Neutrino Geophysics, Honolulu

17. Estimated Systematic Uncertainties
The largest
contribution
Target volume %
Energy threshold 2.3%
Efficiency of cuts 1.6%
Livetime %
Reactor power %
Fuel composition 1.0%
Anti-neutrino spectra 2.5%
Anti neutrino cross-section 0.2%
Total %
December,
Neutrino Geophysics, Honolulu

18. Neutrino Geophysics, Honolulu
Analysis Backgrounds
E > 3.4 MeV E > 2.4 MeV
- Geo-neutrinos coming from
the radioactive decay chains
of 238U and 232Th negligible (14 + 5)
Accidental backgrounds ( ) ( )
9Li/8He Background ( ) ( )
13C(,n)16O background ( ) ( )
Total ( ) ( )
December,
Neutrino Geophysics, Honolulu

19. Anti-neutrino Flux at KamLAND
December,
Neutrino Geophysics, Honolulu

20. Anti-neutrino Flux from Man-made Reactors
- 79% is within range km
ave. dist. 180 km
- Expected number of events in days of livetime:
(syst)
(syst.)
in the unoscillated case.
E > 3.4 MeV
E > 2.4 MeV
December,
Neutrino Geophysics, Honolulu

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 > 2.4 MeV
events/TW·day
events/TW·day
December,
Neutrino Geophysics, Honolulu

22. Neutrino Geophysics, Honolulu
Analysis
December,
Neutrino Geophysics, Honolulu

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 νe flux on the top of varying νe 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.
December,
Neutrino Geophysics, Honolulu

24. Neutrino Geophysics, Honolulu
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 > 3.4 MeV.
Cross-check analysis with lower energy threshold
E > 2.4 MeV
December,
Neutrino Geophysics, Honolulu

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
December,
Neutrino Geophysics, Honolulu

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.
December,
Neutrino Geophysics, Honolulu

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(α,n)16O background (constrained)
Δ m2 (constrained)
sin2 2θ (constrained)
} BG
}OP
*Geoneutrino background from terrestrial
uranium is also treated as a fit parameter in
the lower energy cross-check analysis.
December,
Neutrino Geophysics, Honolulu

28. Neutrino Geophysics, Honolulu
Analysis Results
Geo-reactor power < 19 TW at 90% C.L.
16 geo-reactor events in the data sample
PRELIMINARY
December,
Neutrino Geophysics, Honolulu

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
December,
Neutrino Geophysics, Honolulu

30. The Δχ2 Test as a Function of Geo-reactor Power
The best fit with SNO old (2003)
choice of mixing parameters
PRELIMINARY
Very wide minimum
December,
Neutrino Geophysics, Honolulu

31. Summary and Conclusion
December,
Neutrino Geophysics, Honolulu

32. Comparison of the Best Fit Result with Geological Data
PRELIMINARY
31-44 TW
19-31 TW
0-12 TW
December,
Neutrino Geophysics, Honolulu

33. Neutrino Geophysics, Honolulu
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 (>99.99%) measurement.
Hawaii presents an excellent choice for a definite
geo-reactor measurement (Hanohano).
PRELIMINARY
December,
Neutrino Geophysics, Honolulu