Geoneutrinos

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Geoneutrinos

• Mark Chen
• Queens University
• OCPA Workshop on Underground Science
• Hong Kong, China

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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
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Uranium, Thorium and Potassium

• note 40K also has 10.72 EC branch
• QEC1.505 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
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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
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KamLAND First Detection in 2005

• Expected Geoneutrinos
• U-Series 14.9
• Th-Series4.0Backgrounds
• Reactor 82.37.2
• (a,n) 42.411.1
• Accidental2.380.01BG total 127.413.3
• Observed 152

reactor neutrinos
geo-n
Number of Geoneutrinos
19 -18
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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(UTh geo-n) (4.4 1.6) ? 106 cm-2 s-1
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Geoscience from KamLAND 2008
Preliminary

• measured flux consistent with the Bulk Silicate
  Earth model
• 99CL upper limit to the geoneutrino flux, fixing
  the crust contribution, gives heat lt 54 TW

from S. Enomoto
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Switch Gears

• first part was about neutrino detection
• what does this tell us about geoscience?
• no so much yetthe 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 (RD, proposed)
• second part will be about the geoscience that we
  want to learn from geoneutrinos

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Important Questions in Geosciences

• what is the planetary K/U ratio?
• cant 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 Earths chemical
  origin
• test basic models of the composition of the crust

material in subsequent slides from W.F. McDonough
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Earths 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
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this is what we think gives rise to the
measured heat flow
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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

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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.

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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

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

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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
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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
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Geoneutrino Flux / Crust Map
nuclear power reactor background
from Fiorentini, Mantovani, et al.
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Getting Back to Geoscience Questions

• test fundamental models of Earths 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

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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 detectionJuly 28, 2005 in
Nature
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Geo-n from Continental Crust
crust blue mantle black total red
in SNO
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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 crusts
  composition.

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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

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Hanohano Geoneutrino Sources
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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

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Concluding Remarks

• geoneutrinos prospects
• transformative science!
• probe fundamental, big questions in geology
• geoneutrino detection, like the Earth itself, is
  a work in progress!

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