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Experimental Investigation of Geologically Produced Antineutrinos with KamLAND

Experimental Investigation of Geologically Produced Antineutrinos with KamLAND. Nature 436 , 499-503 (28 July 2005). Nikolai Tolich. KamLAND Collaborators. Acknowledgement. Prof. E. Ohtani (Tohoku University) and Prof. N. Sleep (Stanford University)

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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 Nature 436, 499-503 (28 July 2005) Nikolai Tolich

  2. KamLAND Collaborators INPA seminar

  3. 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 INPA seminar

  4. Outline • Geologically Produced Antineutrinos (Geoneutrinos) • KamLAND • Background Events • Results INPA seminar

  5. 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 INPA seminar

  6. 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 INPA seminar

  7. 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

  8. Radiogenic Heat • U, Th, and K concentrations in the Earth are based on measurement of chondritic meteorites • Chondritic meteorites consist of elements similar to those in the solar photosphere • U, Th, and K concentrations in Bulk Silicate Earth (BSE) are 20ppb, 80ppb, and 240ppm, respectively • This results in U, Th, and K heat production of 8TW, 8TW, and 3TW, respectively. • Th/U ratio of 3.9 is known better than the absolute concentrations INPA seminar

  9. Discrepancy? • The measured total heat flow is 44 or 31TW. • The estimated radiogenic heat produced is 19TW. • Models of mantle convection suggest that the radiogenic heat production rate should be a large fraction of the measured heat flow. • Problem with • Mantle convection model? • Total heat flow measured? • Estimated amount of radiogenic heat production rate? • Geoneutrinos can serve as a cross-check of the radiogenic heat production. INPA seminar

  10. Geoneutrinos • Beta decays produce electron antineutrinos

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

  12. 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 The Expected Geoneutrino Flux • Given an Earth model and neutrino oscillation parameters, the geoneutrino flux per unit energy at KamLAND is given by INPA seminar

  13. 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 highest in the continental crust. • Mantle crystallized outward from the core-mantle boundary. • U and Th prefer to enter a melt phase. INPA seminar

  14. 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.

  15. 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. INPA seminar

  16. Neutrino Oscillations • The electron neutrino survival probability can be estimated as a two flavor oscillations: • For the geoneutrino energy range, due to the distributed geoneutrino generation points the second sine term averages to 0.5 INPA seminar

  17. KamLAND Neutrino Oscillation Measurement • KamLAND saw antineutrino disappearance and spectral distortion. • KamLAND results combined with solar experiments precisely measured the oscillation parameters.

  18. Reference Earth Model Flux • The expected 238U and 232Th geoneutrino fluxes at KamLAND are 2.34106 cm-2s-1 and 1.98 106 cm-2s-1, respectively • Multiplying by the cross-section the expected 238U and 232Th geoneutrino rates are 3.010-31 per target proton year and 0.8510-31 per target proton year, respectively

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

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

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

  22. Outline • Geoneutrinos • KamLAND • Background Events • Results INPA seminar

  23. 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 INPA seminar

  24. Inside the Detector INPA seminar

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

  26. 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. INPA seminar

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

  28. Delayed Prompt 2.2 MeVg 2.2 MeVg 0.5 MeV 0.5 MeV e- e- e+ e+ e+ 0.5 MeV 0.5 MeV n n p p d • Inverse beta decay ne + p  e+ + n ne Detecting anti-neutrinos at KamLAND • KamLAND (Kamioka Liquid scintillator Anti-Neutrino Detector) • The positron loses its energy then annihilates with an electron. • The neutron first thermalizes then is captured by a proton with a mean capture time of ~200ms. INPA seminar

  29. Dr < 1m 0.5ms < DT < 500ms 1.7MeV < E,p< 3.4MeV 1.8MeV < Ed< 2.6MeV Veto after muons Rp, Rd < 5m rd>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. INPA seminar

  30. Outline • Geoneutrinos • KamLAND • Background Events • Results INPA seminar

  31. Geoneutrinos Reactor Background with oscillation Reactor Background • KamLAND was designed to measure reactor antineutrinos. • Reactor antineutrinos are the most significant background. • Reactor antineutrino signals are identical to geoneutrinos except for the prompt energy spectrum. KamLAND INPA seminar

  32. 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 b 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 INPA seminar Antineutrino Energy[MeV] Kopeikin et al. Physics of Atomic Nuclei 64 (2001) 849

  33. 13C(a,n)16O Background • Alpha source, 210Po206Pb+a. • Natural abundance of 13C is 1.1% • 13C(a,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 INPA seminar

  34. 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 INPA seminar

  35. 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). INPA seminar

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

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

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

  39. Outline • Geoneutrinos • KamLAND • Background Events • Results INPA seminar

  40. 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. INPA seminar

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

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

  43. 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. INPA seminar

  44. 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 INPA seminar

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

  46. 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. INPA seminar

  47. 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. INPA seminar

  48. 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. INPA seminar

  49. 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, Netherlands, and South Africa (continental crust with no reactors) • Geoneutrino Meeting in Hawaii, December 2005 INPA seminar

  50. Future U prospecting Conclusion Nature Journal says that “Future observations at KamLAND, and at the Borexino detector under the Gran Sasso mountain in central Italy, which begins operation in 2006, will generate more data and provide greater sensitivity in testing the nature and sources of geoneutrinos.” "Before the revolution really comes to fruition, I think it'll take some time," Gratta told Live Science, "I would imagine one or two decades, before we have more of those detectors and maybe larger ones built in the appropriate place for geophysics.'' Clearly this is still some time off commercial application in the mining sector, and these scientists probably aren’t too concerned with how this could improve exploration success like 3-D seismic has done for oil and gas. They are looking at things like the heat of the earth’s core. But when the BHP’s of this world get their hands on this, they could well put it to good use, as long as it doesn’t have the unintended consequence of scaring up too many new deposits and depressing commodity prices. Either way, it will make for an interesting future. Science buffs can access the tremendously complicated results of the KamLAND study here.

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