Past Experience of reactor neutrino experiments

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

2. Contents • Reactor neutrino sources • Reactor neutrino detection • Past experiments • Summary

3. Daya Bay

4. Reactor: Source of neutrinos

5. How Neutrinos are produced in reactors ?

6. Systematic Error: Power

7. Reactor thermal power Known to <1%

8. Fission rate in the Reactor

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

10. Total error on neutrino spectrum

11. Reactor neutrino detection

12. Observed neutrino spectrum

13. Background - Correlated Background - Uncorrelated: environmental radioactivity

16. Precautions for a reactor n 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, …

18. CHOOZ 5t 0.1% Gd-loaded scintillators Shielding: 300 MWE 2 m scintillator + 0.14m Fe 1km baseline Signal: ~30/day Eff. : ~70% BK: corr. 1/day uncorr. 0.5/day

20. Attenuation Length l vs time Acceleration of l aging

21. Neutron energy spectrum Gd capture Edge effect Proton capture

22. Energy cut Position cut

23. Systematics

24. Closer look -- Detection efficiency

25. Experience gained • Not stable Gd-loaded scintillator (l~ 5-2m) • PMT directly in contact with scintillator  too high uncorr. Background  too high Eth(1.32 MeV) • Good shielding  low background • Homogeneous detector  Gd peak at 8 MeV • 2m scintillator shielding gives a neutron reduction of 0.8*106.

26. Bad performance of reactor is a good news for neutrino physics

27. R=1.01 2.8%

28. 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

30. Very stable Gd-loaded liquid scintillator

31. Two trigger thresholds

32. Power method: Neutrino signal follows the variation of reactor power Prompt (e+) and delayed (n) are asymmetric But background (g-g, n-n) are symmetric N1=Ngg+Nnn+Nnp+Nn N2=Ngg+Nnn+(1-e1)Nnp+(1-e2)Nn Two method used in Palo Verde Y.F. Wang et al., PRD 62(2000)013012

34. Error on n selection cuts obtained from multi-variable analysis Systematics

35. Experience gained • Good Gd-loaded scintillator(l ~ 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 106 (lower energy, complicated event pattern).

37. Summary • Reactor neutrino experiment is not trivial • Chooz and Palo Verde give a limit of sin22q13<0.1