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Experimental Investigation of Geologically Produced Antineutrinos with KamLAND. Stanford University Department of Physics Kazumi Ishii. Outline. Geologically Produced Antineutrinos (Geoneutrinos) KamLAND Background Events Results. Structure of the Earth.
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Experimental Investigation of Geologically Produced Antineutrinos with KamLAND Stanford University Department of Physics Kazumi Ishii
Outline • Geologically Produced Antineutrinos (Geoneutrinos) • KamLAND • Background Events • Results SLAC seminar
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 SLAC seminar
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 SLAC seminar
Total Heat Flow from the Earth Bore-hole Measurements • 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 44.21.0TW (87mW/m2), or 311TW (61mW/m2) according to more recent evaluation of same data despite the small quoted errors. Image: Pollack et. al
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 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. SLAC seminar
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. SLAC seminar
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 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 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. SLAC seminar
Reference Earth ModelConcentrations 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 • 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 U and Th concentration variations due to various crustal types contribute ~7% error in the total flux. Variations in local U and Th concentrations contribute ~3% error in the total flux.
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: SLAC seminar
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 • Given an Earth model and neutrino oscillation parameters, the antineutrino flux per unit energy at KamLAND is given by • 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 • A survival probability due to neutrino oscillations, • for geoneutrino energy range.
Reference Earth Model Flux • Expected geoneutrino flux at KamLAND • 238U geoneutrinos: 2.34106 cm-2s-1 • 232Th geoneutrinos: 1.98 106 cm-2s-1
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: 3.010-31 per target proton year • 232Th geoneutrinos:0.8510-31 per target proton year
Geoneutrino Map of the Earth Simulated origins of geoneutrinos detectable with KamLAND using the reference Earth model KamLAND
Geoneutrino References • G. Marx, Menyhard N, Mitteilungen der Sternwarte, Budapest No. 48 (1960) • M.A. Markov, Neutrino, Ed. "Nauka", Moscow, 1964 • G. Eders, Nucl. Phys., 78 (1966) 657 • G. Marx, Czech. J. of Physics B, 19 (1969) 1471 • G. Marx and I. Lux, Acta Phys. Acad. Hung., 28 (1970) 63 • C. Avilez et al., Phys. Rev. D23 (1981) 1116 • L. Krauss et al., Nature 310 (1984) 191 • J.S. Kargel and J.S. Lewis, Icarus 105 (1993) 1 • R.S. Raghavan et al., Phys. Rev. Lett. 80 (1998) 635 • C.G. Rothschild, M.C. Chen, F.P. Calaprice, Geophys. Rev. Lett. 25 (1998) 1083 • F. Montovani et al., Phys. Rev. D69 (2004) 013001 SLAC seminar
Have Geoneutrinos Been Measured before? Fred Reines’ neutrino detector (circa 1953) By Gamow in 1953
Outline • Geoneutrinos • KamLAND • Background Events • Results SLAC seminar
1km Overburden KamLAND Detector Electronics Hut Steel Sphere, 8.5m radius Inner detector 1325 17” PMT’s 554 20” PMT’s 34% coverage 1 kton liquid-scintillator Transparent balloon, 6.5m radius Buffer oil Water Cherenkov outer detector 225 20” PMT’s SLAC seminar
Inside the Detector SLAC seminar
Determining Event Vertices • Vertex determined using the photon arrival times at PMTs. • Calibrated using sources deployed down the center of the detector. SLAC seminar
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. SLAC seminar
Tracking Muons Monte Carlo (line) and Data (+)
Detecting Antineutrinos with KamLAND Delayed Prompt • KamLAND (Kamioka Liquid scintillator AntiNeutrino Detector) 2.2 MeVg 0.5 MeV e- e+ 0.5 MeV n p • Inverse beta decay ne + p → e+ + n E ~ Te+ 1.8MeV p d ne • 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. SLAC seminar
Δr < 1m 0.5μs < ΔT < 500μs 1.7MeV < E,p< 3.4MeV 1.8MeV < Ed< 2.6MeV Veto after muons Rp, Rd < 5m ρd>1.2m Selecting Geoneutrino Events Delayed Prompt 2.2 MeVg 0.5 MeV e+ 0.5 MeV *These cuts are different from the reactor antineutrino event selection cuts because of the excess background events for lower geoneutrino energies. SLAC seminar
Outline • Geoneutrinos • KamLAND • Background Events • Results SLAC seminar
Geoneutrinos Reactor Background with oscillation Reactor Background Introduction • KamLAND was designed to measure reactor antineutrinos. • Reactor antineutrinos are the most significant background. KamLAND SLAC seminar
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 Fractional Increase in energy spectra • 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. 235U fission products 239Pu fission products Antineutrino Energy[MeV] Kopeikin et al. Physics of Atomic Nuclei 64 (2001) 849
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. npscattering 13C(a,n)16O* n(12C,12C*)n SLAC seminar
Muon Veto Fiducial Volume 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 SLAC seminar
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). SLAC seminar
Background Event Summary • The following is a summary of the expected numbers of background coincidence events. SLAC seminar
Pulse Shape Discrimination From AmBe source • 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 SLAC seminar
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. From AmBe source Neutrons Gammas SLAC seminar
Outline • Geoneutrinos • KamLAND • Background Events • Results SLAC seminar
Data-set • From March, 2002 to October, 2004. • 749.1±0.5day 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 ± 0.2232Th geoneutrinos are expected. SLAC seminar
Geoneutrino Candidate Energy Distribution Expected total Candidate Data Expected total background Expected reactor (,n) Expected U Random Expected Th
Rate Analysis • 152 candidate events • 127±13 expected background events. • geoneutrinos. • / (target proton-year) detected geoneutrino rate. SLAC seminar
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. SLAC seminar
: Number of candidate events observed • : Number of candidate events expected • : (,n) background energy scaling factor • : (,n) background rate scaling factor Log Likelihood Equation For given NU and NTh, log L is maximized by varying the other parameters.
How Many Geoneutrinos Did We See? Expected ratio from chondritic meteorites Best fit 3 U geoneutrinos 18 Th geoneutrinos Expected result from reference Earth model SLAC seminar
How Many Geoneutrinos Did We See, Part 2? 2 = 2(logLmax - logL) Expected result from reference Earth model Central Value 28 SLAC seminar
Reality Check… • Could all “geoneutrinos” come from an undiscovered uranium deposit? • Not likely • The antineutrino flux from a 100kton uranium deposit (the world’s largest) located 1km away from KamLAND would be only 3% of expected geoneutrino flux. SLAC seminar
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.