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

2. 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

3. Uranium, Thorium and Potassium
note: 40K also has 10.72% EC branch
QEC=1.505 MeV
10.67% to 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

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
July 21, 2008
M. Chen
OCPA Underground Science

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

6. 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

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
July 21, 2008
M. Chen
OCPA Underground Science

8. 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

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
July 21, 2008
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
July 21, 2008

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

12. 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

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.
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

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
July 21, 2008
M. Chen
OCPA Underground Science

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

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)
July 21, 2008
M. Chen
OCPA Underground Science

17. 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

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

20. Geoneutrino Flux / Crust Map
nuclear power reactor
background
from Fiorentini, Mantovani, et al.
July 21, 2008
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
July 21, 2008
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
July 21, 2008

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

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

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

28. 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