Spectral modeling of reactor antineutrino

1. Spectral modeling of reactor antineutrino Thomas Mueller – CEA Saclay Irfu/SPhN Thomas Mueller - Workshop AAP09

2. Purpose of these simulations • Provide the reference antineutrino energy spectrum emitted by reactor with: • Control of the systematics • Gain in sensitivity • Oscillation analysis: Double Chooz, Daya Bay • Feasibility of nuclear reactor monitoring: • Power measurement • Non-proliferation studies (IEAE) • The Nucifer project: • CEA-DSM/Irfu-DAM & IN2P3 • see M. Fallot’s talk Thomas Mueller - Workshop AAP09

3. Computation of reactor antineutrino spectrum Relevant degree of freedom is fuel assembly (~200 in one core) of 17x17 fuel rods fissions / s ν / fissions Pth : total thermal power αi : fraction of power per fuel assembly fik : fraction of fissions per fissile isotope and fuel assembly Nνk : neutrino E spectrum per fission for isotope « k » Reactor data Subject of this talk Thomas Mueller - Workshop AAP09

4. Antineutrino energy spectra references • Information on the antineutrino flux from 235U, 239Pu & 241Pu obtained through conversion of experimentally measured β spectra: • K. Schreckenbach et al., Phys. Lett. B160, 325 (1985) [235U] • A. A. Hahn et al., Phys. Lett. B218, 365 (1989) [239Pu & 241Pu] • No measurements for 238U (11% of total ν rate) but theoretical calculations: • P. Vogel et al., Phys Rev C24, 1543 (1981) • H. V. Klapdor and J. Metzinger, Phys. Lett. B112, 22 (1982) • Measurements of 238U β spectrum is ongoing by K. Schreckenbach & N. Haag (PhD thesis) @ München (Germany) Thomas Mueller - Workshop AAP09

5. From Schreckenbach’s measured β spectra… • β spectra from fission products in thermal-neutron induced fission of 235U, 239Pu and 241Pu have been measured on line @ ILL research reactor using electromagnetic spectrometer BILL • Very accurate electron reference data from 2 to 8 MeV: • Negligible statistical error → less than 1% up to 7 MeV • Negligible calibration error → momentum resolution of Δp/p ~ 3×10-4 • Normalization error → ~ 1.8% Thomas Mueller - Workshop AAP09

6. … to converted ν spectra • For each measured β spectrum, neutrino was obtained using a conversion procedure: • Fit of the β spectrum with 30 virtual β branches • Conversion of these branches into neutrino branches through energy conservation • Sum of the 30 neutrino branches to obtain the final spectrum • The conversion procedure induces a 1.8 to 3% additional error « Accurate conversion can be obtained only if […] the optimum nuclear charge Z is independently known as a function of the endpoint energy E0 » P. Vogel, Phys. Rev. C76 (2007) Thomas Mueller - Workshop AAP09

7. Microscopic approach Fission products ENSDF + BESTIOLE Fission yields JEFF3.1 / MURE ex: branch of 13B Microscopic approach: study of systematic effects / estimation & propagation of all sources of errors Thomas Mueller - Workshop AAP09

8. Comparison with Schreckenbach Example of 235U: • No free parameter ! • Good global agreement between simulation and experiment • Same mean energy Thomas Mueller - Workshop AAP09

9. Relative comparison R = (Ssim-Sexp) / Sexp • Pandemonium effect → TAGS measurements (Greenwood, Tengblad) • Very short-lived high-Qβ unknown nuclei → GS to GS approximation, gross-theory, toy-models • we cannot control residues better than few % Thomas Mueller - Workshop AAP09

10. Revisiting Schreckenbach’s conversion procedure (1) • Starting point: all experimental data i.e. ENSDF database + TAGS measurement (blue curve) • 95% of the experimental spectrum is reproduced • The remaining part is fitted using virtual branches Fit with 4 virtual β branches • Improvement from more physics input (~10000 β branches) Thomas Mueller - Workshop AAP09

11. Revisiting Schreckenbach’s conversion procedure (2) effective corrections • Correction beyond Fermi theory of β decay • QED corrections • Weak magnetism • Higher order Coulomb • Microscopic approach: more physics inputs e.g. true endpoint E0 & nuclear charge distribution Z • Better implementation of the corrections Thomas Mueller - Workshop AAP09

12. Consequences on neutrino residues • Systematic +2% bias below 6 MeV • Important for oscillation analysis • Important for flux to power comparison Oscillation range Thomas Mueller - Workshop AAP09

13. Principle of the crosschecks • Several methods have been tested to confirm this + 2% bias • The goal is to fit Schreckenbach electron data by « tweaking » database’s parameters and to check the consequences on neutrino residues • 3 independent methods: 1) BR → BR × ( 1 + αi ) 2) E0 → E0 × ( 1 - αE0 + βE02 ) 3) BR modifications + GS constraints • Requirements: • Reduce set of parameters • Only few % modification in physical distributions Thomas Mueller - Workshop AAP09

14. Comparison of the different methods • Can achieve < 1% electron residues with few % modifications • 4 independent methods are stable @ level of Schreckenbach’s error bars • + 2% bias in neutrino residues is confirmed ! Thomas Mueller - Workshop AAP09

15. Conclusions • Preliminary error budget • Schreckenbach normalization ~ 1.8% • Conversion procedure ~ 1% • Corrections to Fermi theory of β decay < 0.25% / MeV • Systematic bias of + 2% below 6 MeV • Next: • Final systematics studies • Off-equilibrium effects Thomas Mueller - Workshop AAP09

16. Back up: The pandemonium effect • Overestimation of the high energy part of the spectrum due to experimental technique - detection in coincidence of an electron and a photon • Solution: TAGS measurements with a 4π-detector Thomas Mueller - Workshop AAP09

17. Back up: Results for 239Pu Thomas Mueller - Workshop AAP09