19 March 2009 Thomas Mueller - Workshop AAP09 1

Spectral modeling of reactor antineutrino

Thomas Mueller – CEA Saclay Irfu/SPhN

Thomas Mueller - Workshop AAP09 219 March 2009

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 319 March 2009

Computation of reactor antineutrino spectrum

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 »

Rea

ctor

dat

a

isotopesfissilek

ki

k

timeassembliesfueli k

ki

ith

emit dttfENEtf

ttPEN )()(

)(

)()()(

Relevant degree of freedom is fuel assembly (~200 in one core) of 17x17 fuel rods

Subject of this talk

Thomas Mueller - Workshop AAP09 419 March 2009

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 519 March 2009

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 619 March 2009

… 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 719 March 2009

Microscopic approach

fpN

fp

kfp

kfp

k NYEN1

)(

bN

bbbb

kfp ZEENBREN

10 ),,()(

Fission yieldsJEFF3.1 / MURE

Fission productsENSDF + BESTIOLE

ex: branch of 13B

Microscopic approach: study of systematic effects / estimation & propagation of all sources of errors

Thomas Mueller - Workshop AAP09 819 March 2009

Comparison with Schreckenbach

No free parameter ! Good global agreement between simulation and experiment Same mean energy

Example of 235U:

Thomas Mueller - Workshop AAP09 919 March 2009

Relative comparison

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 %

R = (Ssim-Sexp) / Sexp

Thomas Mueller - Workshop AAP09 1019 March 2009

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

Improvement from more physics input (~10000 β branches)

Fit with 4 virtual β branches

Thomas Mueller - Workshop AAP09 1119 March 2009

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

Revisiting Schreckenbach’s conversion procedure (2)

effective corrections

Thomas Mueller - Workshop AAP09 1219 March 2009

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 1319 March 2009

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 1419 March 2009

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 1519 March 2009

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 1619 March 2009

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 1719 March 2009

Back up: Results for 239Pu