Title: Experimental%20Investigation%20of%20Geologically%20Produced%20Antineutrinos%20with%20KamLAND
1Experimental Investigation of Geologically
Produced Antineutrinos with KamLAND
- Stanford University
- Department of Physics
- Kazumi Ishii
2Outline
- Geologically Produced Antineutrinos
(Geoneutrinos) - KamLAND
- Background Events
- Results
3Structure 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
4Convection 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.
5Total 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
6Radiogenic Heat
- 238U, 232Th and K generate 8TW, 8TW, and 3TW of
radiogenic heat in the Earth
- Beta decays produce electron antineutrinos
7Urey Ratio and Mantle Convection Models
- Urey ratio indicates what fraction of heat
dissipated comes from radiogenic heat. Urey ratio
can be defined as - Some mantle convection models predict
- Urey ratio gt 0.7.
8Discrepancy?
- The measured total heat flow, 44 or 31TW, and the
estimated radiogenic heat produced in the mantle,
13TW, gives Urey Ratio 0.3 or 0.5. - Problem with
- Mantle convection model?
- Total heat flow measured?
- Estimated amount of radiogenic heat production
rate? - Geoneutrino can serve as a cross-check of the
radiogenic heat production.
9Geoneutrino Signal
- KamLAND is only sensitive to antineutrinos above
1800keV - Geoneutrinos from K decay cannot be detected with
KamLAND.
10U and Th in the EarthChondritic Meteorites
- U and Th concentrations in the Earth are based on
measurement of chondritic meteorites. - Chondritic meteorites consist of elements similar
to those in the solar photosphere. - Th/U ratio is 3.9
- Th/U ratio is known better than the absolute
concentrations.
11U and Th Distributionsin 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 the highest in the
continental crust and continental sediment. - Mantle crystallized outward from the core-mantle
boundary. - U and Th prefer to enter a melt phase.
12Reference Earth ModelConcentrations of U and Th
- Total amounts of U and Th in the Earth are
estimated from the condritic - meteorites.
- Concentrations in the sediments and crusts are
based on the samples - on the surface, seismic data, and tectonic
model. - Concentrations in the mantle are estimated by
subtracting the amounts in - the sediments and the crusts.
13Geological Uncertainty
- We assigned 10 for the observable geological
uncertainty. - This does not include uncertainties in the total
amounts or - distributions of U and Th.
U concentrations
U and Th concentration variations due to various
crustal types contribute 7 error in the total
flux.
Variations in local U and Th concentrations
contribute 3 error in the total flux.
14Neutrino 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
15KamLAND Neutrino Oscillation Measurement
- KamLAND saw an antineutrino disappearance and a
spectral distortion. - KamLAND result combined with solar experiments
precisely measured the oscillation parameters.
16The Expected Geoneutrino Flux
- Given an Earth model and neutrino oscillation
parameters, the antineutrino flux per unit energy
at KamLAND is given by
- The decay rate per unit mass
- The number of antineutrinos per decay chain per
unit energy
- The mass concentration as a function of position
in the Earth
- The density as a function of position in the Earth
- A survival probability due to neutrino
oscillations, -
for geoneutrino energy range.
17Reference Earth Model Flux
- Expected geoneutrino flux at KamLAND
- 238U geoneutrinos 2.34?106 cm-2s-1
- 232Th geoneutrinos 1.98 ?106 cm-2s-1
18Expected Geoneutrino Detection Rate
- By multiplying the expected geoneutrino flux and
cross-sections, detection rates for geoneutrinos
from U and Th at KamLAND are - 238U geoneutrinos 3.0?10-31 per target proton
year - 232Th geoneutrinos 0.85?10-31 per target proton
year
19Geoneutrino Map of the Earth
Simulated origins of geoneutrinos detectable with
KamLAND using the reference Earth model
KamLAND
20Geoneutrino References
- G. Marx, Menyhard N, Mitteilungen der Sternwarte,
Budapest No. 48 (1960) - M.A. Markov, Neutrino, Ed. "Nauka", Moscow, 1964
- G. Eders, Nucl. Phys., 78 (1966) 657
- G. Marx, Czech. J. of Physics B, 19 (1969) 1471
- G. Marx and I. Lux, Acta Phys. Acad. Hung., 28
(1970) 63 - C. Avilez et al., Phys. Rev. D23 (1981) 1116
- L. Krauss et al., Nature 310 (1984) 191
- J.S. Kargel and J.S. Lewis, Icarus 105 (1993) 1
- R.S. Raghavan et al., Phys. Rev. Lett. 80 (1998)
635 - C.G. Rothschild, M.C. Chen, F.P. Calaprice,
Geophys. Rev. Lett. 25 (1998) 1083 - F. Montovani et al., Phys. Rev. D69 (2004) 013001
21Have Geoneutrinos Been Measured before?
Fred Reines neutrino detector (circa 1953)
By Gamow in 1953
22Were Fred Reines Background Events from
Geoneutrinos?
30TW
23Outline
- Geoneutrinos
- KamLAND
- Background Events
- Results
24KamLAND 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
25Inside the Detector
26Determining Event Vertices
- Vertex determined using the photon arrival times
at PMTs. - Calibrated using sources deployed down the center
of the detector.
27Determining 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.
28Tracking Muons
Monte Carlo (line) and Data ()
29Detecting Antineutrinos with KamLAND
Delayed
Prompt
- KamLAND (Kamioka Liquid scintillator AntiNeutrino
Detector)
2.2 MeV g
0.5 MeV ?
e-
e
0.5 MeV ?
n
p
- Inverse beta decay
- ne p ? e n
- E? Te 1.8MeV
p
d
ne
- The positron loses its energy then annihilates
with an electron.
- The neutron first thermalizes then captures a
proton with a mean capture time of 200ms.
30Selecting Geoneutrino Events
Delayed
Prompt
2.2 MeV g
0.5 MeV ?
- ?r lt 1m
- 0.5µs lt ?T lt 500µs
- 1.7MeV lt E?,plt 3.4MeV
- 1.8MeV lt Edlt 2.6MeV
- Veto after muons
- Rp, Rd lt 5m
- ?dgt1.2m
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.
31Outline
- Geoneutrinos
- KamLAND
- Background Events
- Results
32Reactor Background Introduction
KamLAND
- KamLAND was designed to measure reactor
antineutrinos. - Reactor antineutrinos are the most significant
background.
33Reactor Background Measurement
- Reactor antineutrino signals are identical to
geoneutrinos except for the prompt energy
spectrum. - To calculate the reactor antineutrino interaction
rate per target proton per year, we need to know
the neutrino oscillation parameters, the
detection cross-section (0.2) and each
reactors - Location
- Reactor thermal power (2.1)
- Fuel composition (1.0)
- Antineutrino spectrum (2.5)
34Long-lived Reactor Background
Fractional Increase in energy spectra
- Fission fragments with half-lives greater than a
few hours (97Zr, 132I, 93Y, 106Ru, 144Ce, 90Sr)
may not have reached equilibrium. - The reactor antineutrino spectrum is based on the
measured ß spectrum after 1day exposure of 235U,
239Pu, and 241Pu to a thermal n flux. - Long-lived isotopes occur in the core and spent
fuel. - Spent fuel is assumed to be at the reactor
location.
235U fission products
239Pu fission products
Antineutrino EnergyMeV
Kopeikin et al. Physics of Atomic Nuclei 64
(2001) 849
3513C(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
36Cosmic Muon Induced Background
- Muons produce unstable isotopes and neutrons as
they go through the detector. - 9Li and 8He ?-decay producing n, mimicking
inverse ?-decay signals. - Any events after muons are vetoed.
- 2ms after all muons
- 2s within 3m cylinder of the muon track
- 2s whole detector for muons with high light yield
37Random Coincidence Background
- There is a probability that two uncorrelated
events pass the coincidence cuts. - The random coincidence background event rates are
calculated by different delayed event time window
(10ms to 20s instead).
38Background Event Summary
- The following is a summary of the expected
numbers of background coincidence events.
39Pulse Shape Discrimination
- Antineutrino prompt event is caused by e whereas
13C(a,n)16O prompt event is caused by n. - These different prompt events produce different
scintillation light time distributions allowing a
statistical discrimination.
From AmBe source
Neutrons
Gammas
40Pulse Shape Discrimination Part 2
- This study assumes similarities in time
distributions of positrons and gammas. - This method yields consistent 13C(a,n)16O
background event rate.
From AmBe source
Neutrons
Gammas
41Outline
- Geoneutrinos
- KamLAND
- Background Events
- Results
42Data-set
- From March, 2002 to October, 2004.
- 749.10.5 day of total live-time.
- (3.46 0.17) 1031 target protons.
- (7.09 0.35) 1031 target proton years.
- 0.6870.007 of the total efficiency for
geoneutrino detection. - 14.8 0.7 238U geoneutrinos and 3.9 0.2 232Th
geoneutrinos are expected.
43Geoneutrino Candidate Energy Distribution
Expected total
Candidate Data
Expected total background
Expected reactor
(?,n)
Expected U
Random
Expected Th
44Rate Analysis
- 152 candidate events
- 12713 expected background events.
- geoneutrinos.
- / (target proton-year)
detected geoneutrino rate.
-
45Likelihood 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.
46Log Likelihood Equation
For given NU and NTh, log L is maximized by
varying the other parameters.
47How Many Geoneutrinos Did We See?
Expected ratio from chondritic meteorites
Best fit 3 U geoneutrinos 18 Th geoneutrinos
Expected result from reference Earth model
48How Many Geoneutrinos Did We See, Part 2?
??2 2(logLmax - logL)
Expected result from reference Earth model
Central Value 28
49Reality 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.
50Conclusions
- This is the first experimental investigation of
geoneutrinos. - This is the first chemical analysis of the mantle
of the Earth. - 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.
51Future of Geoneutrino Measurement 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.
52Future Geoneutrino Experiment Considerations
- Location and geoneutrino data purity
- 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
- People are talking about
- Hawaii (oceanic crust with no reactors)
- Canada, South Dakota, Australia, the Netherlands,
and South Africa (continental crust with no
reactors) - Geoneutrino Meeting in Hawaii, December 2005
53Acknowledgement
- 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
54KamLAND Collaborators
55Geoneutrino Results in Nature
Nature 436, 499-503 (28 July 2005) doi
10.1038/nature03980
http//www.nature.com/nature/journal/v436/n7050/fu
ll/nature03980.html