Experimental Investigation of Geologically Produced Antineutrinos with KamLAND: Experimental Investigation of Geologically Produced Antineutrinos with KamLAND Stanford University Department of Physics Kazumi Ishii


Outline: Outline Geologically Produced Antineutrinos (Geoneutrinos) KamLAND Background Events Results


Structure of the Earth: Structure of the Earth Seismic data splits Earth into 5 basic regions: core, mantle, oceanic crust, continental crust, and sediment. All these regions are solid except the outer core. Image by: Colin Rose and Dorling Kindersley


Convection in the Earth: Convection in the Earth The mantle convects even though it is solid. It is responsible for the plate tectonics and earthquakes. Oceanic crust is being renewed at mid-ocean ridges and recycled at trenches. Image: http://www.dstu.univ-montp2.fr/PERSO/bokelmann/convection.gif


Total Heat Flow from the Earth: Total Heat Flow from the Earth Conductive heat flow measured from bore-hole temperature gradient and conductivity Deepest bore-hole (12km) is only ~1/500 of the Earth’s radius. Total heat flow 44.21.0TW (87mW/m2), or 311TW (61mW/m2) according to more recent evaluation of same data despite the small quoted errors. Image: Pollack et. al Bore-hole Measurements


Radiogenic Heat: Radiogenic Heat 238U, 232Th and K generate 8TW, 8TW, and 3TW of radiogenic heat in the Earth Beta decays produce electron antineutrinos


Urey Ratio and Mantle Convection Models: Urey Ratio and Mantle Convection Models Urey ratio indicates what fraction of heat dissipated comes from radiogenic heat. Urey ratio can be defined as Some mantle convection models predict Urey ratio > ~0.7.


Discrepancy?: Discrepancy? The measured total heat flow, 44 or 31TW, and the estimated radiogenic heat produced in the mantle, 13TW, gives Urey Ratio ~0.3 or ~0.5. Problem with Mantle convection model? Total heat flow measured? Estimated amount of radiogenic heat production rate? Geoneutrino can serve as a cross-check of the radiogenic heat production.


Geoneutrino Signal: Geoneutrino Signal KamLAND is only sensitive to antineutrinos above 1800keV Geoneutrinos from K decay cannot be detected with KamLAND.


U and Th in the EarthChondritic Meteorites: U and Th in the Earth Chondritic Meteorites U and Th concentrations in the Earth are based on measurement of chondritic meteorites. Chondritic meteorites consist of elements similar to those in the solar photosphere. Th/U ratio is 3.9 Th/U ratio is known better than the absolute concentrations.


U and Th Distributionsin the Earth: U and Th Distributions in the Earth U and Th are thought to be absent from the core and present in the mantle and crust. The core is mainly Fe-Ni alloy. U and Th are lithophile (rock-loving), and not siderophile (metal-loving) elements. U and Th concentrations are the highest in the continental crust and continental sediment. Mantle crystallized outward from the core-mantle boundary. U and Th prefer to enter a melt phase.


Reference Earth ModelConcentrations of U and Th: Reference Earth Model Concentrations of U and Th Total amounts of U and Th in the Earth are estimated from the condritic meteorites. Concentrations in the sediments and crusts are based on the samples on the surface, seismic data, and tectonic model. Concentrations in the mantle are estimated by subtracting the amounts in the sediments and the crusts.


Geological Uncertainty: Geological Uncertainty Variations in local U and Th concentrations contribute ~3% error in the total flux. U and Th concentration variations due to various crustal types contribute ~7% error in the total flux. We assigned 10% for the observable geological uncertainty. This does not include uncertainties in the total amounts or distributions of U and Th. U concentrations


Neutrino Oscillations: Neutrino Oscillations The weak interaction neutrino eigenstates may be expressed as superpositions of definite mass eigenstates The electron neutrino survival probability can be estimated as a two flavor oscillations:


KamLAND Neutrino Oscillation Measurement: KamLAND Neutrino Oscillation Measurement KamLAND saw an antineutrino disappearance and a spectral distortion. KamLAND result combined with solar experiments precisely measured the oscillation parameters.


The Expected Geoneutrino Flux: A survival probability due to neutrino oscillations, for geoneutrino energy range. The Expected Geoneutrino Flux The decay rate per unit mass The number of antineutrinos per decay chain per unit energy The mass concentration as a function of position in the Earth The density as a function of position in the Earth Given an Earth model and neutrino oscillation parameters, the antineutrino flux per unit energy at KamLAND is given by


Reference Earth Model Flux: Reference Earth Model Flux Expected geoneutrino flux at KamLAND 238U geoneutrinos: 2.34106 cm-2s-1 232Th geoneutrinos: 1.98 106 cm-2s-1


Expected Geoneutrino Detection Rate: Expected Geoneutrino Detection Rate By multiplying the expected geoneutrino flux and cross-sections, detection rates for geoneutrinos from U and Th at KamLAND are 238U geoneutrinos: 3.010-31 per target proton year 232Th geoneutrinos: 0.8510-31 per target proton year


Geoneutrino Map of the Earth: Geoneutrino Map of the Earth KamLAND Simulated origins of geoneutrinos detectable with KamLAND using the reference Earth model


Geoneutrino References: Geoneutrino References


Have Geoneutrinos Been Measured before?: Have Geoneutrinos Been Measured before? Fred Reines’ neutrino detector (circa 1953) By Gamow in 1953


Were Fred Reines Background Events from Geoneutrinos?: Were Fred Reines Background Events from Geoneutrinos? ~30TW


Outline: Outline Geoneutrinos KamLAND Background Events Results


KamLAND Detector: KamLAND Detector Electronics Hut Steel Sphere, 8.5m radius Water Cherenkov outer detector 225 20” PMT’s 1 kton liquid-scintillator Inner detector 1325 17” PMT’s 554 20” PMT’s 34% coverage 1km Overburden Buffer oil Transparent balloon, 6.5m radius


Inside the Detector: Inside the Detector


Determining Event Vertices : Determining Event Vertices Vertex determined using the photon arrival times at PMTs. Calibrated using sources deployed down the center of the detector.


Determining Event Energies: Determining Event Energies The “visible” energy is calculated from the amount of photo-electrons correcting for spatial detector response. The “real” energy is calculated from the visible energy correcting for Cherenkov photons and scintillation light quenching.


Tracking Muons: Tracking Muons Monte Carlo (line) and Data (+)


Detecting Antineutrinos with KamLAND: Detecting Antineutrinos with KamLAND KamLAND (Kamioka Liquid scintillator AntiNeutrino Detector) d p e+ 0.5 MeV  2.2 MeV g n p 0.5 MeV  ne e- Inverse beta decay ne + p → e+ + n E ~ Te + 1.8MeV The positron loses its energy then annihilates with an electron. The neutron first thermalizes then captures a proton with a mean capture time of ~200ms. Prompt Delayed


Selecting Geoneutrino Events: Selecting Geoneutrino Events Δr 1.2m e+ 0.5 MeV  2.2 MeV g 0.5 MeV  Prompt Delayed *These cuts are different from the reactor antineutrino event selection cuts because of the excess background events for lower geoneutrino energies.


Outline: Outline Geoneutrinos KamLAND Background Events Results


Reactor Background Introduction: Reactor Background Introduction KamLAND was designed to measure reactor antineutrinos. Reactor antineutrinos are the most significant background. KamLAND


Reactor Background Measurement: Reactor Background Measurement Reactor antineutrino signals are identical to geoneutrinos except for the prompt energy spectrum. To calculate the reactor antineutrino interaction rate per target proton per year, we need to know the neutrino oscillation parameters, the detection cross-section (~0.2%) and each reactor’s Location Reactor thermal power (~2.1%) Fuel composition (~1.0%) Antineutrino spectrum (~2.5%)


Long-lived Reactor Background: Long-lived Reactor Background Fission fragments with half-lives greater than a few hours (97Zr, 132I, 93Y, 106Ru, 144Ce, 90Sr) may not have reached equilibrium. The reactor antineutrino spectrum is based on the measured β spectrum after ~1day exposure of 235U, 239Pu, and 241Pu to a thermal n flux. Long-lived isotopes occur in the core and spent fuel. Spent fuel is assumed to be at the reactor location. Kopeikin et al. Physics of Atomic Nuclei 64 (2001) 849 235U fission products 239Pu fission products Fractional Increase in energy spectra Antineutrino Energy[MeV]


13C(α,n)16O Background: 13C(α,n)16O Background Alpha source, 210Po→206Pb+α. Natural abundance of 13C is 1.1% 13C(α,n)16O. n loses energy creating a prompt event, and is later captured creating a delayed event. 13C(a,n)16O* n(12C,12C*)n np scattering


Cosmic Muon Induced Background: Cosmic Muon Induced Background Muons produce unstable isotopes and neutrons as they go through the detector. 9Li and 8He -decay producing n, mimicking inverse -decay signals. Any events after muons are vetoed. 2ms after all muons 2s within 3m cylinder of the muon track 2s whole detector for muons with high light yield


Random Coincidence Background: Random Coincidence Background There is a probability that two uncorrelated events pass the coincidence cuts. The random coincidence background event rates are calculated by different delayed event time window (10ms to 20s instead).


Background Event Summary : Background Event Summary The following is a summary of the expected numbers of background coincidence events.


Pulse Shape Discrimination: Pulse Shape Discrimination Antineutrino prompt event is caused by e+ whereas 13C(α,n)16O prompt event is caused by n. These different prompt events produce different scintillation light time distributions allowing a statistical discrimination. Neutrons Gammas From AmBe source


Pulse Shape Discrimination Part 2: Pulse Shape Discrimination Part 2 This study assumes similarities in time distributions of positrons and gammas. This method yields consistent 13C(α,n)16O background event rate. Neutrons Gammas From AmBe source


Outline: Outline Geoneutrinos KamLAND Background Events Results


Data-set: Data-set From March, 2002 to October, 2004. 749.1±0.5 day of total live-time. (3.46 ± 0.17) × 1031 target protons. (7.09 ± 0.35) × 1031 target proton years. 0.687±0.007 of the total efficiency for geoneutrino detection. 14.8 ± 0.7 238U geoneutrinos and 3.9 ± 0.2 232Th geoneutrinos are expected.


Geoneutrino Candidate Energy Distribution: Geoneutrino Candidate Energy Distribution Expected total Expected reactor Expected total background Expected U (,n) Random Expected Th Candidate Data


Rate Analysis: Rate Analysis 152 candidate events 127±13 expected background events. geoneutrinos. / (target proton-year) detected geoneutrino rate.


Likelihood Analysis: Likelihood Analysis Uses un-binned likelihood analysis. Uses the expected prompt event energy distribution. Uses the neutrino oscillation parameters determined from results of KamLAND reactor antineutrino and solar neutrino experiments.


Log Likelihood Equation: Log Likelihood Equation For given NU and NTh, log L is maximized by varying the other parameters.


How Many Geoneutrinos Did We See?: How Many Geoneutrinos Did We See? Expected result from reference Earth model Expected ratio from chondritic meteorites Best fit 3 U geoneutrinos 18 Th geoneutrinos


How Many Geoneutrinos Did We See, Part 2?: How Many Geoneutrinos Did We See, Part 2? Expected result from reference Earth model Central Value 28 2 = 2(logLmax - logL)


Reality Check…: Reality Check…


Conclusions: Conclusions This is the first experimental investigation of geoneutrinos. This is the first chemical analysis of the mantle of the Earth. We observed 4.5 to 54.2 geoneutrinos with 90% C.L. Scaling concentrations in all regions of our reference Earth model, the 99% upper limit on geoneutrino rate corresponds to radiogenic power from U and Th decays of less than 60TW. The measurement is consistent with the current geological models.


Future of Geoneutrino Measurement with KamLAND : Future of Geoneutrino Measurement with KamLAND The reactor background is irreducible for KamLAND. We are working on purifying the liquid scintillator, which will reduce the (,n) background events. More accurate (,n) cross section can lower the error on the (,n) background rate. S. Harissopulos et al. submitted to Phys. Rev. C calculated new (,n) cross sections with more accuracy. G. Fiorentini et al. arXiv:hep-ph/0508048 recalculated the number of geoneutrinos using the above cross sections and our data. They claim that we detected geoneutrinos, ~2.5 above 0.


Future Geoneutrino Experiment Considerations: Future Geoneutrino Experiment Considerations Location and geoneutrino data purity: No nearby nuclear reactors On oceanic crust to probe mantle On continental crust to probe continental crust Needs to be shielded from cosmic muons Low radioactive background People are talking about Hawaii (oceanic crust with no reactors) Canada, South Dakota, Australia, the Netherlands, and South Africa (continental crust with no reactors) Geoneutrino Meeting in Hawaii, December 2005


Acknowledgement: Acknowledgement Prof. E. Ohtani (Tohoku University) and Prof. N. Sleep (Stanford University) Japanese Ministry of Education, Culture, Sports, Science, and Technology United States Department of Energy Electric associations in Japan: Hokkaido, Tohoku, Hokuriku, Chubu, Kansai, Chugoku, Shikoku, and Kyushu Electric Companies, Japan Atomic Power Co. and Japan Nuclear cycle Development Institute Kamioka Mining and Smelting Company


KamLAND Collaborators: KamLAND Collaborators


Geoneutrino Results in Nature: Geoneutrino Results in Nature http://www.nature.com/nature/journal/v436/n7050/full/nature03980.html Nature 436, 499-503 (28 July 2005) | doi: 10.1038/nature03980