Geoneutrinos Mark Chen Queen’s University OCPA Workshop on Underground Science Hong Kong, China July 21, 2008

ne 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 July 21, 2008 M. Chen OCPA Underground Science

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 July 21, 2008 M. Chen OCPA Underground Science

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 July 21, 2008 M. Chen OCPA Underground Science

KamLAND First Detection in 2005 Expected Geoneutrinos U-Series： 14.9 Th-Series：4.0 Backgrounds Reactor： 82.3±7.2 (α,n) ： 42.4±11.1 Accidental：2.38±0.01 BG total： 127.4±13.3 Observed： 152 reactor neutrinos geo-n Number of Geoneutrinos: 25 ＋19 －18 July 21, 2008 M. Chen OCPA Underground Science

KamLAND 2008 Geoneutrino Results Preliminary 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 July 21, 2008 M. Chen OCPA Underground Science

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 July 21, 2008 M. Chen OCPA Underground Science

Switch Gears first part was 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 July 21, 2008 M. Chen OCPA Underground Science

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 July 21, 2008 M. Chen OCPA Underground Science

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 July 21, 2008

this is what we think gives rise to the measured heat flow July 21, 2008

Urey Ratio and Mantle Convection Models radioactive heat production Urey ratio = heat loss Mantle convection models typically assume: mantle Urey ratio: 0.4 to 1.0, generally ~0.7 Geochemical models predict: mantle Urey ratio 0.3 to 0.5 July 21, 2008

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. Some mantle convection model suggests that the radiogenic heat produced should a larger fraction of the total heat flow. So at least one of the following have some problems: the total heat flow measured, the estimated amounts of U, Th or K, or the mantle convection model. So, by measuring the antineutrinos from beta decays of daughter nuclei in the U and Th decay chains, we can maybe serve as a cross-check of the radiogenic heat production rate. chondritic meteorites -> 19TW, mantle convection model->more >70% should be from radiogenic heat. Need to subtract continental crust contribution (6.5TW) -> only ~35% (44TW) or ~51%(31TW). July 21, 2008

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 July 21, 2008 M. Chen OCPA Underground Science

C1 carbonaceous chondrite Solar photosphere (atoms Si = 1E6) B Li C1 carbonaceous chondrite (atoms Si = 1E6) July 21, 2008

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) July 21, 2008 M. Chen OCPA Underground Science

Enriched by factor 100 over Primitive Mantle 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 July 21, 2008 Cont. Crust ~ 0.6% by mass of silicate earth

July 21, 2008 M. Chen OCPA Underground Science

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 July 21, 2008 M. Chen OCPA Underground Science

Geoneutrino Flux / Crust Map nuclear power reactor background from Fiorentini, Mantovani, et al. July 21, 2008 M. Chen OCPA Underground Science

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 July 21, 2008 M. Chen OCPA Underground Science

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 July 21, 2008

Geo-n from Continental Crust crust: blue mantle: black total: red in SNO+ July 21, 2008

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.” July 21, 2008 M. Chen OCPA Underground Science

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 July 21, 2008 M. Chen OCPA Underground Science

Hanohano Geoneutrino Sources July 21, 2008 M. Chen OCPA Underground Science

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 July 21, 2008 M. Chen OCPA Underground Science

n n n 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 July 21, 2008 M. Chen OCPA Underground Science