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

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, 15 2005 Neutrino Geophysics, Honolulu

KamLAND Detector: Design and Features December, 15 2005 Neutrino Geophysics, Honolulu

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, 15 2005 Neutrino Geophysics, Honolulu

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, 15 2005 Neutrino Geophysics, Honolulu

Anti-neutrino Spectrum Number of observed events (1/MeV) Reactor spectrum Observed spectrum Interaction cross- section (~10-43cm2) E (MeV) December, 15 2005 Neutrino Geophysics, Honolulu

Neutrino Geophysics, Honolulu 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 < 2.7 x 10-16 g/g December, 15 2005 Neutrino Geophysics, Honolulu

Motivation for the Experimental Geo-reactor Search with KamLAND

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, 15 2005 Neutrino Geophysics, Honolulu

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, 15 2005 Neutrino Geophysics, Honolulu

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, 15 2005 Neutrino Geophysics, Honolulu

Geo-reactor Analysis Anti-neutrino Event Selection and Backgrounds December, 15 2005 Neutrino Geophysics, Honolulu

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 = 1.806 MeV Prompt Event γ γ e+ γ νe p n 2.2MeV Eprompt = E - 0.8 MeV 200 μs Delayed Event December, 15 2005 Neutrino Geophysics, Honolulu

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 (89.8 + 1.5)% - 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, 15 2005 Neutrino Geophysics, Honolulu

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, 15 2005 Neutrino Geophysics, Honolulu

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, 15 2005 Neutrino Geophysics, Honolulu

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% December, 15 2005 Neutrino Geophysics, Honolulu

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 (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) December, 15 2005 Neutrino Geophysics, Honolulu

Anti-neutrino Flux at KamLAND December, 15 2005 Neutrino Geophysics, Honolulu

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 > 3.4 MeV E > 2.4 MeV December, 15 2005 Neutrino Geophysics, Honolulu

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 0.0102 events/TW·day 0.0137 events/TW·day December, 15 2005 Neutrino Geophysics, Honolulu

Neutrino Geophysics, Honolulu Analysis December, 15 2005 Neutrino Geophysics, Honolulu

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, 15 2005 Neutrino Geophysics, Honolulu

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, 15 2005 Neutrino Geophysics, Honolulu

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, 15 2005 Neutrino Geophysics, Honolulu

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, 15 2005 Neutrino Geophysics, Honolulu

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, 15 2005 Neutrino Geophysics, Honolulu

Neutrino Geophysics, Honolulu Analysis Results Geo-reactor power < 19 TW at 90% C.L. 16 geo-reactor events in the data sample PRELIMINARY December, 15 2005 Neutrino Geophysics, Honolulu

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, 15 2005 Neutrino Geophysics, Honolulu

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, 15 2005 Neutrino Geophysics, Honolulu

Summary and Conclusion December, 15 2005 Neutrino Geophysics, Honolulu

Comparison of the Best Fit Result with Geological Data PRELIMINARY 31-44 TW 19-31 TW 0-12 TW December, 15 2005 Neutrino Geophysics, Honolulu

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, 15 2005 Neutrino Geophysics, Honolulu