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 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%

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 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 vs time Acceleration of aging

21. Neutron energy spectrum Proton capture Gd capture Edge effect

22. Position cut Energy cut

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

24. Closer look -- Detection efficiency

25. 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.

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. 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

34. Systematics Sources power methodSwap method e+ trigger efficiency2.0 n trigger efficiency2.1  flux prediction 2.1  selection cuts 4.52.1 Background variation2.1N/A (1-  1 ) B pn N/A3.3 Total6.15.3 Error on  selection cuts obtained from multi- variable analysis

35. 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).

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