Kamland, a culmination of half century of reactor neutrino studies

KamLAND, a culmination of half century of reactor neutrino studies.

• Petr Vogel, Caltech

Selected references to this lecture:

• Kamland papers:

• K.Eguchi et al., Phys. Rev. Lett.90, 021802 (2003),

• K.Eguchi et al., Phys. Rev. Lett.92, 071301 (2004),

• T. Araki et al., hep-ex/0406035 .

• General review:

• C. Bemporad, G. Gratta, and P. Vogel., Rev. Mod. Phys. 74, 297 (2002).

Pontecorvo already in 1946 suggested to

• Pontecorvo already in 1946 suggested to

• use nuclear reactors in order to perform

• neutrino experiments.

• Indeed, in 1953-1959 Reines and Cowan

• showed that neutrinos are real particles

• using nuclear reactors as a source.

• Since then, reactors, powerful sources

• with ~6x1020 /s electron antineutrinos

• emitted by a modern ~3.8 GWthermal reactor,

• have been used often in neutrino studies.

• The spectrum is well understood….

Electron antineutrinos are produced by the  decay of fission fragments

Reactor spectrum:

• Reactor spectrum:

• 1) Fission yields Y(Z,A,t), essentially all known

• 2) decay branching ratios bn,i(E0i) for decay branch i,

• with endpoint E0i , some known but some (particularly

• for the very short-lived and hence high Q-value)

• unknown.

• 3)  decay shape, assumed allowed shape, known

• P(E,E0i,Z) or for electrons Ee= E0 – E

• dN/dE = n Yn(Z,A,t) i bn,i(E0i) P(E,E0i,Z)

• and a similar formula for electrons.

• If the electron spectrum is known, it can be `converted’

• into the antineutrino spectrum.

Spectrum Uncertainties

Reactor spectra

Detecting reactor antineutrinos; low detection threshold required

The survival probability of electron

• The survival probability of electron

• antineutrinos of energy E produced

• at the distance L from the detector is

• Pee(E,L) = 1 – sin2(2)sin2(m2L/4E

• The experiment become sensitive to

• oscillations if m2L/E ~ 1,

• proof of oscillations is Pee(E,L) < 1.

Discovery of oscillations of atmospheric

• Discovery of oscillations of atmospheric

• neutrinos implies m2 ~ (2-3)x10-3 eV2,

• thus indicating that reactor experiments

• with L ~ (1-3) km should be performed

• (Chooz and Palo Verde).

• Also, the preferred `solution’ to the solar

• neutrino deficit implies m2 ~ (5-10)x10-5 eV2,

• thus indicating that reactor experiments

• with L ~ 100 km should be performed

• (KamLAND)

Note: The best background in 76Ge  decay

• Note: The best background in 76Ge  decay

• detectors is at present ~0.2 counts/(keV kg y).

• Expressing the background in the liquid scintillator

• in KamLAND in the same units, and for

• energies 2-3 MeV, one finds value ~10 times

• smaller going out to 5.5 m radius and ~20 times

• smaller for 5 m radius

Decay chain leading to 210Po:

• Decay chain leading to 210Po:

• 222Rn (3.8d) 218Po (3.1m) 214Pb (27m) 214Bi,

• 214Bi (20m) 214Po (164s) 210Pb (22.3y) 210Bi,

• 210Bi (5d) 210Po (138d) 206Pb(stable)

• The long lifetime of 210Pb causes its accumulation.

• The from 210Po decay then interact with 13C in

• the scintillator by 13C(,n)16O making unwanted

• background. There is only ~10-11g of 210Pb in

• fhe fiducial volume, enough however to cause

• 1.7x109 decays in 514 days.

What’s next?

Future Reactor Measurements

A high sensitivity search for e from the Sun and other sources at KamLAND

Thanks to Atsuto Suzuki, Patrick Decowski,

• Thanks to Atsuto Suzuki, Patrick Decowski,

• Gianni Fiorentini, Andreas Piepke and

• Giorgio Gratta who made some of the

• figures used in this talk.