Transcript Geoneutrino - Daya Bay Reactor Neutrino Experiment in Hong

Geoneutrinos
Mark Chen
Queen’s University
OCPA Workshop on Underground Science
Hong Kong, China
July 21, 2008
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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”
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M. Chen
OCPA Underground Science
Image by: Colin Rose,
Dorling Kindersley
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Uranium, Thorium and Potassium
from G. Fiorentini
 note:
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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
0.0117% isotopic abundance
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How to Detect Geoneutrinos
 inverse beta decay:
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n e  p  e  n
good cross section
threshold 1.8 MeV
liquid scintillator has a lot of
protons and can easily detect
sub-MeV events
delayed coincidence signal
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t = 0.2 ms, neutron capture on H
detect delayed 2.2 MeV g
rejects backgrounds
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threshold
figure from KamLAND Nature paper
e+ and n correlated in time and in
position in the detector
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OCPA Underground Science
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KamLAND First Detection in 2005
reactor neutrinos
geo-n
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
Number of Geoneutrinos:
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OCPA Underground Science
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KamLAND 2008 Geoneutrino Results
Preliminary
 factor two more data
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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
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OCPA Underground Science
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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
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OCPA Underground Science
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Switch Gears
 first part was about neutrino detection
 what does this tell us about geoscience?
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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
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Important Questions in Geosciences
 what is the planetary K/U ratio?
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can’t address until we can detect 40K geoneutrinos
 radiogenic contribution to heat flow?
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geoneutrinos can measure this
 radiogenic elements in the core?
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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
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Earth’s Total
Surface Heat Flow
• Conductive heat flow
measured from bore-hole
temperature gradient and
conductivity
Data sources
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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?
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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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OCPA Underground Science
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1.E+08
H
1.E+07
O
Solar photosphere
(atoms Si = 1E6)
C
N
1.E+06
1.E+05
B
1.E+04
Li
1.E+03
1.E+02
1.E+02
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1.E+03
1.E+04
1.E+05
1.E+06
C1 carbonaceous chondrite
(atoms Si = 1E6)
1.E+07
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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
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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 coremantle 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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OCPA Underground Science
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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
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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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OCPA Underground Science
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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 Earth’s chemical origin
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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
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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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OCPA Underground Science
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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
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KamLAND geo-neutrino
detection…July 28, 2005 in Nature
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 crust’s composition.”
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OCPA Underground Science
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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
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10 kton or larger
mobile, sinkable
retrievable
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OCPA Underground Science
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Hanohano Geoneutrino Sources
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OCPA Underground Science
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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:
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lateral variability
mantle plumes
upwelling from the core-mantle
boundary
mantle convection models
 synergy with crust geo-n detectors
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OCPA Underground Science
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Concluding Remarks
 geoneutrinos prospects
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transformative science!
probe fundamental, big questions in geology
 geoneutrino detection, like the Earth itself, is a
work in progress!
n
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OCPA Underground Science
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