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Experimental Investigation of Geologically Produced Antineutrinos with KamLAND

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

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

  2. Outline • Geologically Produced Antineutrinos (Geoneutrinos) • KamLAND • Background Events • Results SLAC seminar

  3. 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

  4. 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

  5. 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 44.2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

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

  7. 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

  8. 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

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

  10. 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.

  11. 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

  12. 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.

  13. 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.

  14. 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

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

  16. 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.

  17. 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

  18. 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

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

  20. 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

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

  22. Were Fred Reines Background Events from Geoneutrinos? ~30TW

  23. Outline • Geoneutrinos • KamLAND • Background Events • Results SLAC seminar

  24. 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

  25. Inside the Detector SLAC seminar

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

  27. 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

  28. Tracking Muons Monte Carlo (line) and Data (+)

  29. 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

  30. 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

  31. Outline • Geoneutrinos • KamLAND • Background Events • Results SLAC seminar

  32. 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

  33. 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%)

  34. 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

  35. 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

  36. 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

  37. 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

  38. Background Event Summary • The following is a summary of the expected numbers of background coincidence events. SLAC seminar

  39. 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

  40. 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

  41. Outline • Geoneutrinos • KamLAND • Background Events • Results SLAC seminar

  42. 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

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

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

  45. 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

  46. : 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.

  47. 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

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

  49. 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

  50. 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.

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