Past Experience of reactor neutrino experiments Yifang Wang Institute of High Energy Physics, Beijing Nov. 28, 2003

Contents Reactor neutrino sources Reactor neutrino detection Past experiments Summary

Daya Bay

Reactor: Source of neutrinos

How Neutrinos are produced in reactors ?

Systematic Error: Power

Reactor thermal power Known to <1%

Fission rate in the Reactor

Prediction of reactor neutrino spectrum Three ways to obtain reactor neutrino spectrum: –Direct measurement –First principle calculation –Sum up neutrino spectra from 235 U, 239 Pu, 241 Pu and 238 U 235 U, 239 Pu, 241 Pu from their measured  spectra 238 U(10%) from calculation (10%) They all agree well within 3%

Total error on neutrino spectrum

Reactor neutrino detection

Observed neutrino spectrum

Background - Correlated Background - Uncorrelated: environmental radioactivity

Precautions for a reactor experiment Cosmic-ray induced correlated background: –Enough overburden and shielding –Active shielding, small enough and well known ineff. Environmental radiation(uncorrelated background): –Clean scintillator –PMT with Low radioactivity glass –Clean surrounding materials –Rn free environment –Enough shielding –Gd-loaded scintillator, good for bk. But aging Calibration –Many sources at different positions –Birk’s law, (Cerenkov) light transport/re-emission, …

CHOOZ 5t 0.1% Gd-loaded scintillators Shielding: 300 MWE 2 m scintillator m Fe 1km baseline Signal: ~30/day Eff. : ~70% BK: corr. 1/day uncorr. 0.5/day

Attenuation Length vs time Acceleration of aging

Neutron energy spectrum Proton capture Gd capture Edge effect

Position cut Energy cut

Systematics sourcesRelative error (%) Reaction cross section1.9 Number of protons0.8 Detection efficiency1.5 Reactor power0.7 Energy released per fission0.6 total2.7

Closer look -- Detection efficiency

Experience gained Not stable Gd-loaded scintillator (  m) PMT directly in contact with scintillator  too high uncorr. Background  too high E th (1.32 MeV) Good shielding  low background Homogeneous detector  Gd peak at 8 MeV 2m scintillator shielding gives a neutron reduction of 0.8*10 6.

Bad performance of reactor is a good news for neutrino physics

R=1.01  2.8%

Palo Verde 12t 0.1% Gd-loaded scintillators Shielding: 32 MWE /1m water 0.9 km baseline Signal: ~20/day Eff. ~ 10% BK: corr.: ~ 15/day uncorr. ~ 7/day

Very stable Gd-loaded liquid scintillator

Two trigger thresholds

Two method used in Palo Verde Power method: –Neutrino signal follows the variation of reactor power Prompt (e+) and delayed (n) are asymmetric But background ( , n-n) are symmetric N 1 =N  +N nn +N np +N N 2 =N  +N nn +(1-  1 )N np +(1-  2 )N Y.F. Wang et al., PRD 62(2000)013012

Systematics Sources power methodSwap method e+ trigger efficiency2.0 n trigger efficiency2.1  flux prediction 2.1  selection cuts Background variation2.1N/A (1-  1 ) B pn N/A3.3 Total Error on  selection cuts obtained from multi- variable analysis

Experience gained Good Gd-loaded scintillator(  ~ 11m) Not enough shielding  too high corr./uncorr. Background Segmentation makes Gd capture peak <6MeV  too high uncorr. Background Rn may enter the detector, problem ? Veto eff. is not high enough(97.5%) Swap method to measure/cancel backgrounds  key to success 1m water shielding gives a neutron reduction of 10 6 (lower energy, complicated event pattern).

Summary Reactor neutrino experiment is not trivial Chooz and Palo Verde give a limit of sin 2 2  13 <0.1