Title: Probing the depths of the Earth with neutrinos
1Probing the depths of the Earth with neutrinos
2Outline
- What are neutrinos?
- What are geoneutrinos?
- What can we learn from geoneutrinos?
3History
- Neutrinos first postulated by Pauli in 1930 to
conserve energy and momentum in beta decay. - They have no charge and very little mass so
should be almost impossible to see. - They were first observed in 1956 by Reines and
Cowen.
4Standard model
- Three neutrino flavours have been observed.
- In the standard model of particle physics it is
assumed they have zero mass.
5Neutrinos from the Sun
p p ? 2H e ?e p e- p ? 2H ?e
99.75 0.25 2H p ?
3He 86 14 3He 3He ? ?
2p 3He ? ? 7Be 99.89
0.11 7Be e- ? 7Li ?e 7Be p ? 8B 7Li
p ? ? ? 8B ? 8Be e ?e 8Be ? ?
?
6Solar neutrino spectrum
Solar Neutrino Flux at the surface of the Earth
with no neutrino oscillations. Uses the solar
model, BS05(OP).
7Solar neutrino measurements
8Neutrino oscillations
- The weak interaction neutrino eigenstates may be
expressed as superpositions of definite mass
eigenstates - The electron neutrino survival probability can be
estimated as a two flavor oscillations
9Matter effects
- In matter all neutrino flavours undergo
- Only electron neutrinos undergo
- This adds an extra phase which changes the
oscillation probability
Possible sin22?
10Reactor neutrinos
- KamLAND is surrounded by nuclear power plants
which produce electron anti-neutrinos.
1 km high Mt Ikenoyama
11KamLAND neutrino oscillation measurement
- KamLAND saw antineutrino disappearance and
spectral distortion. - KamLAND results combined with solar experiments
precisely measured the oscillation parameters.
12Outline
- What are neutrinos?
- What are geoneutrinos?
- What can we learn from geoneutrinos?
13Structure of the Earth
- Seismic data splits Earth into 5 basic regions
core, mantle, oceanic crust, continental crust,
and sediment. - All these regions are solid except the outer core.
Image by Colin Rose and Dorling Kindersley
14Convection in the Earth
Image http//www.dstu.univ-montp2.fr/PERSO/bokelm
ann/convection.gif
- The mantle convects even though it is solid.
- It is responsible for the plate tectonics and
earthquakes. - Oceanic crust is being renewed at mid-ocean
ridges and recycled at trenches.
15Total heat flow from the Earth
Bore-hole Measurements
- Conductive heat flow measured from bore-hole
temperature gradient and conductivity - Deepest bore-hole (12km) is only 1/500 of the
Earths radius. - Total heat flow 44.2?1.0TW (87mW/m2), or 31?1TW
(61mW/m2) according to more recent evaluation of
same data despite the small quoted errors.
Image Pollack et. al
16Radiogenic heat
- U, Th, and K concentrations in the Earth
- are based on measurement of chondritic
- meteorites
- Chondritic meteorites consist of elements
- similar to those in the solar photosphere
- U, Th, and K concentrations in Bulk Silicate
Earth (BSE) are 20ppb, 80ppb, and 240ppm,
respectively - This results in U, Th, and K heat production of
8TW, 8TW, and 3TW, respectively. - Th/U ratio of 3.9 is known better than the
absolute concentrations
17Discrepancy?
- The measured total heat flow is 44 or 31TW.
- The estimated radiogenic heat produced is 19TW.
- Models of mantle convection suggest that the
radiogenic heat production rate should be a large
fraction of the measured heat flow. - Problem with
- Mantle convection model?
- Total heat flow measured?
- Estimated amount of radiogenic heat production
rate? - Geoneutrinos can serve as a cross-check of the
radiogenic heat production.
18Radiogenic isotopes
- Beta decays produce electron antineutrinos
19Geoneutrino signal
- KamLAND is only sensitive to antineutrinos above
1800keV - Geoneutrinos from K decay cannot be detected with
KamLAND.
20U and Th in the Earth
- U and Th are thought to be absent from the core
and present in the mantle and crust. - The core is mainly Fe-Ni alloy.
- U and Th are lithophile (rock-loving), and not
siderophile (metal-loving) elements. - U and Th concentrations are highest in the
continental crust. - Mantle crystallized outward from the core-mantle
boundary. - U and Th prefer to enter a melt phase.
21Reference Earth Model Flux
- The expected 238U and 232Th geoneutrino fluxes at
KamLAND are 2.34?106 cm-2s-1 and 1.98 ?106
cm-2s-1, respectively - Multiplying by the cross-section the expected
238U and 232Th geoneutrino rates are 3.0?10-31
per target proton year and 0.85?10-31 per target
proton year, respectively
22Have geoneutrinos been measured before?
Fred Reines neutrino detector (circa 1953)
By Gamow in 1953
23Were they geoneutrinos?
30TW
24Outline
- What are neutrinos?
- What are geoneutrinos?
- What can we learn from geoneutrinos?
25KamLAND detector
1km Overburden
Electronics Hut
Steel Sphere, 8.5m radius
Inner detector 1325 17 PMTs 554 20 PMTs 34
coverage
1 kton liquid-scintillator
Transparent balloon, 6.5m radius
Buffer oil
Water Cherenkov outer detector 225 20 PMTs
26Inside the detector
27Determining event vertices
- Vertex determined using the photon arrival times
at PMTs. - Calibrated using sources deployed down the center
of the detector.
28Determining event energies
- The visible energy is calculated from the
amount of photo-electrons correcting for spatial
detector response. - The real energy is calculated from the visible
energy correcting for Cherenkov photons and
scintillation light quenching.
29Tracking muons
Monte Carlo (line) and Data ()
30Detecting anti-neutrinos at KamLAND
- KamLAND (Kamioka Liquid scintillator
Anti-Neutrino Detector)
- The positron loses its energy then annihilates
with an electron.
- The neutron first thermalizes then is captured by
a proton with a mean capture time of 200ms.
31Selecting geoneutrino events
Delayed
- Dr lt 1m
- 0.5ms lt DT lt 500ms
- 1.7MeV lt E?,plt 3.4MeV
- 1.8MeV lt Edlt 2.6MeV
- Veto after muons
- Rp, Rd lt 5m
- rdgt1.2m
Prompt
2.2 MeV g
0.5 MeV ?
e
0.5 MeV ?
These cuts are different from the reactor
antineutrino event selection cuts because of the
excess background events for lower geoneutrino
energies.
32Reactor background
KamLAND
- KamLAND was designed to measure reactor
antineutrinos. - Reactor antineutrinos are the most significant
background. - Reactor antineutrino signals are identical to
geoneutrinos except for the prompt energy
spectrum.
3313C(a,n)16O Background
- Alpha source, 210Po?206Pba.
- Natural abundance of 13C is 1.1
- 13C(a,n)16O.
- n loses energy creating a prompt event, and is
later captured creating a delayed event.
np scattering
13C(a,n)16O
n(12C,12C)n
34Background Event Summary
- The following is a summary of the expected
numbers of background coincidence events.
35Recent results from KamLAND
- From March, 2002 to October, 2004.
- 749.10.5 day of total live-time.
- (3.46 0.17) ? 1031 target protons, 5m radius
fiducial volume. - 0.6870.007 of the total efficiency for
geoneutrino detection. - Expect 14.8 0.7 238U geoneutrinos and 3.9 0.2
232Th geoneutrinos. - 152 candidate events
- 12713 background events.
Nature 436, 499-503 (28 July 2005)
36Candidate energy distribution
Expected total
Candidate Data
Expected total background
Expected reactor
(?,n)
Expected U
Random
Expected Th
37Likelihood analysis
- Uses un-binned likelihood analysis.
- Uses the expected prompt event energy
distribution. - Uses the neutrino oscillation parameters
determined from results of KamLAND reactor
antineutrino and solar neutrino experiments.
38KamLAND geoneutrino result
Expected ratio from chondritic meteorites
Best fit 3 U geoneutrinos 18 Th geoneutrinos
Expected result from reference Earth model
Central value 28
39Reality Check
- Could all geoneutrinos come from an
undiscovered uranium deposit? - Not likely
- The antineutrino flux from a 100kton uranium
deposit (the worlds largest) located 1km away
from KamLAND would be only 3 of expected
geoneutrino flux.
40Future geoneutrino measurements with KamLAND
- The reactor background is irreducible for
KamLAND. - We are working on purifying the liquid
scintillator, which will reduce the (?,n)
background events. - More accurate (?,n) cross section can lower the
error on the (?,n) background rate. - S. Harissopulos et al. submitted to Phys. Rev. C
calculated new (?,n) cross sections with more
accuracy. - G. Fiorentini et al. arXivhep-ph/0508048
recalculated the number of geoneutrinos using the
above cross sections and our data. They claim
that we detected geoneutrinos, 2.5?
above 0.
41Future locations
- No nearby nuclear reactors
- On oceanic crust to probe mantle
- On continental crust to probe continental crust
- Needs to be shielded from cosmic muons
- Low radioactive background
- Hawaii or CuraƧao (oceanic crust)
- Borexino, Sudbury, DUSEL (Homestake or
Henderson), or Baksan (continental crust)
42Future U prospecting
Conclusion Nature Journal says that Future
observations at KamLAND, and at the Borexino
detector under the Gran Sasso mountain in central
Italy, which begins operation in 2006, will
generate more data and provide greater
sensitivity in testing the nature and sources of
geoneutrinos. "Before the revolution really
comes to fruition, I think it'll take some time,"
Gratta told Live Science, "I would imagine one or
two decades, before we have more of those
detectors and maybe larger ones built in the
appropriate place for geophysics.'' Clearly this
is still some time off commercial application in
the mining sector, and these scientists probably
arent too concerned with how this could improve
exploration success like 3-D seismic has done for
oil and gas. They are looking at things like the
heat of the earths core. But when the BHPs of
this world get their hands on this, they could
well put it to good use, as long as it doesnt
have the unintended consequence of scaring up too
many new deposits and depressing commodity
prices. Either way, it will make for an
interesting future. Science buffs can access the
tremendously complicated results of the KamLAND
study here.
43Conclusions
- This is the first experimental investigation of
geoneutrinos. - We observed 4.5 to 54.2 geoneutrinos with 90
C.L. - Scaling concentrations in all regions of our
reference Earth model, the 99 upper limit on
geoneutrino rate corresponds to radiogenic power
from U and Th decays of less than 60TW. - The measurement is consistent with the current
geological models.
44KamLAND collaborators
45Acknowledgement
- Prof. E. Ohtani (Tohoku University) and Prof. N.
Sleep (Stanford University) - Japanese Ministry of Education, Culture, Sports,
Science, and Technology - United States Department of Energy
- Electric associations in Japan Hokkaido, Tohoku,
Hokuriku, Chubu, Kansai, Chugoku, Shikoku, and
Kyushu Electric Companies, Japan Atomic Power Co.
and Japan Nuclear cycle Development Institute - Kamioka Mining and Smelting Company