july 21, 20081 geoneutrinos mark chen queen’s university ocpa workshop on underground science hong...

July 21, 2008 1

Geoneutrinos

Mark ChenQueen’s University

OCPA Workshop on Underground ScienceHong Kong, China

M. ChenOCPA Underground Science

2July 21, 2008

What are Geoneutrinos?

Image by: Colin Rose, Dorling Kindersley

radioactive decay of uranium, thorium and from potassium-40 produces antineutrinos

assay the entire Earth by looking at its “neutrino glow”

e

the antineutrinos produced by natural radioactivity in the Earth


3July 21, 2008

note: 40K also has 10.72% EC branch QEC=1.505 MeV

10.67% to 1.461 MeV state (E = 44 keV)

0.05% to g.s. (E = 1.5 MeV)

thus also emits e

Uranium, Thorium and Potassium

from G. Fiorentini

0.0117% isotopic abundance


4July 21, 2008

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 = 0.2 ms, neutron capture on H detect delayed 2.2 MeV rejects backgrounds

e+ and n correlated in time and in position in the detector

nepe threshold

figure from KamLAND Nature paper


5July 21, 2008

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±13.3Observed ： 152

KamLAND First Detection in 2005

reactor neutrinos

geo-

Number of Geoneutrinos:＋ 19－ 1825


6July 21, 2008

Preliminary

KamLAND 2008 Geoneutrino Results

factor two more data 13C(,n) background error reduced improved reconstruction (off-axis

calibration) larger fiducial volume accounting for reactor background

time variations

(U+Th geo-) = (4.4 ± 1.6) 106 cm−2 s−1

from S. Enomoto


7July 21, 2008

Geoscience from KamLAND 2008

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

Preliminary


8July 21, 2008

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


9July 21, 2008

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 10

Earth’s Total Earth’s Total Surface Heat FlowSurface Heat Flow

• Conductive heat flow measured from bore-hole temperature gradient and conductivity

Total heat flow Conventional view 46463 TW3 TW Challenged recently 31311 TW1 TW

Data sources

July 21, 2008 11

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

July 21, 2008 12

• 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

Urey Ratio and Urey Ratio and Mantle Convection ModelsMantle Convection Models

Urey ratio =radioactive heat production

heat loss

July 21, 2008 13

Discrepancy?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.


14July 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 15

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

Sol

ar p

hoto

sphe

re(a

tom

s S

i = 1

E6)

C1 carbonaceous chondrite(atoms Si = 1E6)

H

CN

Li

B

O


16July 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 17

K, Th & U in the Continental Crust

Enriched by factor 100 over Primitive Mantle

Compositional models for the bulk continental crust

DepletedK, Th, U

EnrichedK, Th, U

Cont. Crust ~ 0.6% by mass of silicate earth


18July 21, 2008


19July 21, 2008

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


20July 21, 2008

Geoneutrino Flux / Crust Map

from Fiorentini, Mantovani, et al.

nuclear power reactorbackground


21July 21, 2008

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, 2008July 21, 2008

• KamLAND: 33 events per year (1000 tons CH2) / 142 events reactor• SNO+: 44 events per year (1000 tons CH2) / 38 events reactor

Geoneutrinos in SNO+Geoneutrinos in SNO+

SNO+ geo-neutrinos and reactor background KamLAND geo-neutrino detection…July 28, 2005 in Nature

KamLAND

23July 21, 2008

Geo- from Continental Crust

crust: bluemantle: blacktotal: red

in SNO+


24July 21, 2008

Good Location for Continental Geo-

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.”


25July 21, 2008

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


26July 21, 2008

Hanohano Geoneutrino Sources


27July 21, 2008

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- detectors


28July 21, 2008

Concluding Remarks

geoneutrinos prospects transformative science! probe fundamental, big questions in geology

geoneutrino detection, like the Earth itself, is a work in progress!

july 21, 20081 geoneutrinos mark chen queen’s university ocpa workshop on underground science hong...

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