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Geoneutrinos

Geoneutrinos. Mark Chen Queen’s University OCPA Workshop on Underground Science Hong Kong, China. What are Geoneutrinos?. the antineutrinos produced by natural radioactivity in the Earth. radioactive decay of uranium, thorium and from potassium-40 produces antineutrinos. n e.

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Geoneutrinos

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  1. Geoneutrinos Mark Chen Queen’s University OCPA Workshop on Underground Science Hong Kong, China

  2. What are Geoneutrinos? the antineutrinos produced by natural radioactivity in the Earth radioactive decay of uranium, thorium and from potassium-40 produces antineutrinos ne assay the entire Earth by looking at its “neutrino glow” Image by: Colin Rose, Dorling Kindersley M. Chen OCPA Underground Science

  3. Uranium, Thorium and Potassium • note: 40K also has 10.72% EC branch • QEC=1.505 MeV • 10.67% to 1.461 MeV state (En = 44 keV) • 0.05% to g.s. (En = 1.5 MeV) • thus also emits ne from G. Fiorentini 0.0117% isotopic abundance M. Chen OCPA Underground Science

  4. How to Detect Geoneutrinos • inverse beta decay: • good cross section • threshold 1.8 MeV • liquid scintillator has a lot of protons and can easily detect sub-MeV events • delayed coincidence signal • t= 0.2 ms, neutron capture on H • detect delayed 2.2 MeV g • rejects backgrounds • e+ and n correlated in time and in position in the detector threshold figure from KamLAND Nature paper M. Chen OCPA Underground Science

  5. KamLAND First Detection in 2005 • Expected Geoneutrinos • U-Series： 14.9 • Th-Series：4.0Backgrounds • Reactor： 82.3±7.2 • (α,n) ： 42.4±11.1 • Accidental：2.38±0.01BG total： 127.4±13.3 • Observed： 152 reactor neutrinos geo-n Number of Geoneutrinos: 25 ＋19 －18 M. Chen OCPA Underground Science

  6. Preliminary KamLAND 2008 Geoneutrino Results • factor two more data • 13C(a,n) background error reduced • improved reconstruction (off-axis calibration) • larger fiducial volume • accounting for reactor background time variations from S. Enomoto f(U+Th geo-n)= (4.4 ± 1.6)  106 cm−2 s−1 M. Chen OCPA Underground Science

  7. Geoscience from KamLAND 2008 Preliminary • measured flux consistent with the “Bulk Silicate Earth” model • 99%CL upper limit to the geoneutrino flux, fixing the crust contribution, gives heat < 54 TW from S. Enomoto M. Chen OCPA Underground Science

  8. Switch Gears • first partwas about neutrino detection • what does this tell us about geoscience? • no so much yet…the geoneutrino measurement still has large uncertainties (because of backgrounds) • future improvements from KamLAND (e.g. more statistics, reduced errors) will help • other experiments: Borexino (taking data), SNO+ (initial construction, partially funded), Hanohano (R&D, proposed) • second part will be about the geoscience that we want to learn from geoneutrinos M. Chen OCPA Underground Science

  9. Important Questions in Geosciences • what is the planetary K/U ratio? • can’t address until we can detect 40K geoneutrinos • radiogenic contribution to heat flow? • geoneutrinos can measure this • radiogenic elements in the core? • in particular potassium! • test fundamental models of Earth’s chemical origin • test basic models of the composition of the crust material in subsequent slides from W.F. McDonough M. Chen OCPA Underground Science

  10. Earth’s Total Surface Heat Flow • Conductive heat flow measured from bore-hole temperature gradient and conductivity Data sources Total heat flow Conventional view 463 TW Challenged recently 311 TW

  11. this is what we think gives rise to the measured heat flow

  12. Urey Ratio and Mantle Convection Models • Mantle convection models typically assume: mantleUrey ratio: 0.4 to 1.0, generally ~0.7 • Geochemical models predict: mantleUrey ratio 0.3 to 0.5 radioactive heat production Urey ratio = heat loss

  13. Discrepancy? • Est. total heat flow, 46 or 31TW est. radiogenic heat production 20TW or 31TW give Urey ratio ~0.3 to ~1 • Where are the problems? • Mantle convection models? • Total heat flow estimates? • Estimates of radiogenic heat production rate? • Geoneutrino measurements can constrain the planetary radiogenic heat production.

  14. Chemical Composition of the Earth • chondrites are primitive meteorites • thought to represent the primordial composition of the solar system • why? • relative element abundances in C1 carbonaceous chondrites matches that in the solar photosphere for “refractory elements” • U and Th are refractory elements • K is moderately volatile M. Chen OCPA Underground Science

  15. H O C N Solar photosphere (atoms Si = 1E6) B Li C1 carbonaceous chondrite (atoms Si = 1E6)

  16. Bulk Silicate Earth • the Earth forms from accreting primordial material in the solar system, an iron metal core separates and compatible metals go into the core • but U, Th (and K?) are lithophile; they prefer to be in the silicate or molten rock around the iron core • Earth is basically “rock metal” • can thus estimate the amount of U and Th in the “primitive mantle” using chondrites, the size of the Earth, after core-mantle differentiation → this is the “Bulk Silicate Earth” model • …then, the crust becomes enriched in U, Th and K resulting in a mantle that is depleted (compared to BSE concentrations) M. Chen OCPA Underground Science

  17. K, Th & U in the Continental Crust Enriched by factor 100 over Primitive Mantle Compositional models for the bulk continental crust Enriched K, Th, U Depleted K, Th, U Cont. Crust ~ 0.6% by mass of silicate earth

  18. M. Chen OCPA Underground Science

  19. Earth Geoneutrino Models • start with the BSE • take reference values for composition of continental and oceanic crust (these come from rock samples) • subtract the crust from the BSE to get the present “residual” mantle • because continental and oceanic are so different, need to use a map of the crust (thickness and crust type) to calculate expected flux at different locations of detectors from C. Rothschild, M. Chen and F. Calaprice 1998 M. Chen OCPA Underground Science

  20. Geoneutrino Flux / Crust Map nuclear power reactor background from Fiorentini, Mantovani, et al. M. Chen OCPA Underground Science

  21. Getting Back to Geoscience Questions • test fundamental models of Earth’s chemical origin • are measured fluxes consistent with predictions based upon the BSE? • so far yes, KamLAND 2008 measurement central value equals the BSE predicted flux • test basic ideas of the composition of the crust • rock samples used to determine the composition of the crust • depth variations not easily sampled • are the basic ideas about the continents and how concentrations are enriched compared to the mantle correct? • it suggests measurements at a continental site and one that probes the mantle would be very interesting M. Chen OCPA Underground Science

  22. Geoneutrinos in SNO+ • KamLAND: 33 events per year (1000 tons CH2) / 142 events reactor • SNO+: 44 events per year (1000 tons CH2) / 38 events reactor KamLAND SNO+ geo-neutrinos and reactor background KamLAND geo-neutrino detection…July 28, 2005 in Nature

  23. Geo-n from Continental Crust crust: blue mantle: black total: red in SNO+

  24. Good Location for Continental Geo-n The Canadian Shield near SNO+ is one of the oldest pieces of continent. Extensive mining activity near Sudbury suggests that the local geology is extremely well studied. W.F. McDonough in Science 317, 1177 (2007) “One proposal is to convert the Sudbury Neutrino Observatory (SNO) to “SNO+” (4). This 1000-ton detector is sited in a mine in Ontario, Canada, and represents an optimal location for measuring the distribution of heat-producing elements in the ancient core of a continent. Here, the antineutrino signal will be dominated by the crustal component at about the 80% level. This experiment will provide data on the bulk composition of the continents and place limits on competing models of the continental crust’s composition.” M. Chen OCPA Underground Science

  25. Good Location Far from Continents • in the middle of the ocean, near Hawaii, far from continents and also far from nuclear power reactors; depth of 4 km • proposed experiment is Hanohano • 10 kton or larger • mobile, sinkable • retrievable M. Chen OCPA Underground Science

  26. Hanohano Geoneutrino Sources M. Chen OCPA Underground Science

  27. Hanohano • moveable geoneutrino detector that probes the chemistry (U, Th) of and the radiogenic heat in the deep Earth • geologists want to know: • lateral variability • mantle plumes • upwelling from the core-mantle boundary • mantle convection models • synergy with crust geo-n detectors M. Chen OCPA Underground Science

  28. Concluding Remarks • geoneutrinos prospects • transformative science! • probe fundamental, big questions in geology • geoneutrino detection, like the Earth itself, is a work in progress! n n n M. Chen OCPA Underground Science

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