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Low-energy neutrino physics with KamLAND
Tadao Mitsui (Research Center for Neutrino Science,
Tohoku U.)for the KamLAND collaboration
Now2010, Grand Hotel Daniela,
Conca Specchiulla, September 4-11, 2010
Nuclear reactors
Continental crust
Continental crust(island arc)
KamLAND location and sources of electron antineutrinos
Oceanic crust
Mantle
“Peak” at ~180 km
KamLAND
Water tank
Buffer oil
Liquid scintillator
Delayed signalPrompt signal 0.9~8MeV
Kevlar ropes
Balloon
Phototubes
Balloon 13m
2.2MeV
time
Nhi
t
Kamioka Liquid Antineutrino Detector
Data-2008 PRL100,221803
Dm2=7.59+0.21-0.21×10-5 eV2
All data (2002 – 2007 May)
before scintillator
purification
Dm2 uncertainty:
about 2/3 of
data-2004
(PRL94,081801)
(Solar + KamLAND)
Reactor result (2008)
Spe
ctra
l diff
eren
ceDm2 uncertainty due to energy scale
Energy scale uncertainty is the largest source
of Dm2 uncertainty
Energy scaledifference of 1.37%(systematic uncertainty)
1 s (statistical only)difference of Dm2
KamLAND data contributing to q13 search
G. L. Fogli, E. Lisi, A. Marrone, A. Palazzo, and A. M. RotunnoPRL 101, 141801 (2008)
JHEP04(2010)056
M.C. Gonzalez-Garcia,a;b Michele Maltonic and Jordi Salvado
KamLAND data contributing to q13 search
Our own analysis is also on going,
with stimulated by those groups
Electron antineutrinos produced in the Earth’s interior (crust and mantle) by decays of 238U, 232Th, and 40KDecays of 238U, 232Th, and 40K :
~40% of Earth’s powerEarth’s power: plate tectonics, earthquakes, volcanoes, geomagnetism, …Origin and history of the EarthPointed out since discovered
(1950’s, G. Gamow, …)
Geoneutrinos
C
The expected 238U, 232Th, and 40K decay chain electron anti-neutino energy distribution. KamLAND can only detect electron antineutrinos to the right of the vertical dotted black line; hence it is insensitive to 40K electron antineutrinos.
Nature 436, 28 July 2005
KamLANDcan detect
Calculation of geo-n energy spectrum
Data-2008: PRL100,221803 (reactor + geo)
Data-2005: Nature436, 499
Data-2010: Neutrino2010 (preliminary)
Experimental investigation of geoneutrino(step by step: investigation → hint → …)
Zero geonu disfavored at: ~2 s
> 4 s
Zero geonu “rejected?” at: ~2.7 s
Nature 436, 28 July 2005
Data-set:749.1 days(Mar. 9, 2002-Oct. 30, 2004)Fiducial:5 m radius
13C(,n)16O4211 rea
ctor 8
0.4
7.2
Total BG 1
27.0
13.1
152 events observed“signal” 25 +19 18
232Th238U
Systematicuncertainty
(E=Eprompt+0.8MeV)
Data-2005
Data-2008E
ven
ts /
0.42
5 M
eV
Reactor-n
geo-n
Data-2005: 7.09×1031 proton yrData-2008: 2.44×1032 proton yr (×3.4)
Data-2008
geo-n (U+Th, ratio fixed):
4.4±1.6×106 cm-2 s-1 (73±27 events)
Finite signal: 2.7s (~2s for Data-2005)
U+Th: 69.7 events expected
in Reference model (Enomoto et al.)
Georeactor at the center of the Earth
< 6.2 TW (solar + KamLAND data)
Data-2008 v.s. Data-2010Data-2008: PRL100,221803 Data-2010: Neutrino2010
preliminary
In data-2010, Th only is disfavored for the first time, due to higher-energy peak contribution
Dilemma of near-field
contribution
At Kamioka, about one half of geonu is from Japanese island crust (continental crust)
This contributes much for the “non-zero geonu significance”, but a background if we are interested in more deep mantle contribution
To understand and “cancel” the near-field contribution, multi-point observation is more and more important (now Kamioka + Gran sasso!)
“As many antineutrino detectors as seismograph.”
(A. Suzuki 2002)
Contour ofpercentage ofthe contributionto geonu fluxat Kamioka
Noble gas: can be dissolved into liquid scintillator up to ~ 3 wt%, with little effect (damage) on the scintillator character, such as light yield, transparency, and density.
Slow 2 n decay (T1/22n > 1022 yr): modest requirement
for energy resolution, suitable for liquid scintilltor experiment (KamLAND: 6.3%/√E[MeV])
Up to 90% enrichment has been established
136Xe and liquid scintillator experiment
PRL 72, 1411 (1994) R.S. Raghavan
Modification of KamLAND
Develop “mini-balloon”: to reduce cosmogenic bg (mainly 10C), bg from scintilltor (208Tl etc), smaller balloon should be installed, in which Xe is loaded up to maximum concentration
Develop Xe storage, and dissolve/extraction system (design almost fixed, to construct in a few month)
Develop dead-time free electronics to tag 10C by m-n-10C triple coincidence (installed, now running and trying to detect neutrons after a muon)
BG and mini-balloon design(by MC simulation)
214Bi can be reduced by a factor ~10 by tagging, so 238U, 232Th < 10-12 g/g is the requirement for the balloon film (now we searching for clean film)
Tagging 214Bi and complicated battles
214Bi-214Po tag: short coincidence time (T1/2=164.3 ms) is good, but a is easily stopped in the balloon film (15 mm film: 90% tagged, 25 mm: 80%, 50 mm: 60%, according to MC)214Pb-214Bi tag: long coincidence time (T1/2=19.9 min.), so reduction of bg of b+g from 214Pb (0.5 ~ 1 MeV) is further challenge: 40K in the film, 210Bi in the scintillator (reduction by distillation?) and balloon film (222Rn control during the balloon fabrication)
Test balloon of 15-mm thick
Quarter-scale balloon was fabricated
Very fragile, we gave up, then design thickness is now 25 mm
Test balloon of 80-mm thick
We understand this is too thick, but to perform installation test etc, this full-scale test balloon was fabricated (March 2010)