prediction of reactor neutrino spectra david lhuillier cea saclay - france

Prediction of Reactor Neutrino Spectra

David LhuillierCEA Saclay - France

D. Lhuillier - CEA Saclay 2

General Considerations

Honolulu - AAP 2012


Origin of Reactor Antineutrinos

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• b-decay of the fission products of U and Pu isotopes

• Ab initio approach: build up the total spectrum with individual contribution of ~800 nuclei.

• Reference spectra approach: measure the mean fission b spectrum of U an Pu isotopes and weight them by the predicted fission rates.

e-

e-n

n

All predictions require a reactor simulation with core geometry, initial fuel composition and power history.


Energy Spectrum

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Exponential decrease of emitted spectrum

-b inverse detection reaction

Detected SpectrumRelevant E range [1.8 – 8] MeV


Contributing Nuclei

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Detection reaction enhances the contribution of high energy b-transitions:- Lot of very unstable nuclei, high above valley of stability- Short lived, reach equilibrium fast w.r.t. experiment time scale Reference fission spectra approach favored w.r.t. ab initio approach.

© F. Durillon, animea


Evolution Along a Reactor Cycle

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Typical evolution of the isotopic composition of a commercial rector

At Pth= constant, 239Pu fission leads to 60% less detected neutrinos than 235U fission.

• Monitoring of reactor n flux with sensitivity to plutonium content.

ONON239Pu

235U

238U241Pu

Refu

elin

g w

ith fr

esh

235U

Total Spectrum:


Fission Spectra uncertainties & Non-proliferation

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V. Bulaevskaya and A. Bernstein, nucl-ex/1009.2123

73 kg of 239Pu removed

• Non proliferation relies on evolution of the neutrino rate along a reactor cycle; Relative variations and/or reference anchor point.

• In the case of converted spectra, the normalization and shape uncertainties are mostly correlated for all fissioning isotopes their impact is suppressed.


Re-evaluation of Reference Spectra

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ILL data: reference b- Spectra

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Target foil (235U, 239Pu, 241Pu)in thermal n flux

Magnetic BILLspectrometer

ILL research reactor(Grenoble, France)

e-

Emitted b spectra per fission

A. A. Hahn, K. Schreckenbach et al., Phys. Let. B218,365 (1989)+ refs therein


b- n Conversion

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1- Need to break down the total e- spectrum into single b-branches fit the data with 30 virtual branches:

2- Convert each virtual e- branches to n branches

3- Sum all converted n branches to get total n spectrum


Theory of b-decay

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• W = total energy• W0 = end-point• p = momentum• Z = Nuclear charge

Fermi theory:

Corrections:

Fermi function

Phase space

Shape factor of forbidden

Finite size of nuclear electric charge

Finite size distrib. of decaying

neutron

Screening of Atomic e-

QED radiative

correction

Weak magnetism

n branch obtained by replacing: WW0-W, GbGn


Corrections to Fermi theory

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Example of a single branch with

Z=46A=117

E0=10 MeV

• Total fission spectra is a sum over a quasi continuous end-point distribution +/- cancellation at low E, adds up at high E.

P. Huber

b spectrum corrections

n spectrum corrections


Re-evaluation of Converted Spectra

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• Maximize the use of nuclear data: 90% of the total b spectrum is described by the sum of measured b-decays * Fission yields.

• Use virtual branches only to fill the remaining 10% gap.

Nuclear Data

T. A. Mueller et al., Phys. Rev. C83,054615 (2011)

• Initiated by the need of accurate prediction for the far detector of Double Chooz


Re-evaluation of Converted Spectra

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T. A. Mueller et al., Phys. Rev. C83,054615 (2011)

• True distribution of Z from all b-decays of databases.

• Apply all corrections at the branch level instead of an effective global slope.

Old effective correction

Corrections applied at branch level

“true” Z distribution from nuclear databases

• Combined effect at low and high energy leading to a global +3% shift


Reactor Anomaly

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Atmospheric Oscilation

SolarOscilation

New Oscilation to

sterile n?

G. Mention et al., Phys. Rev. D83, 073006, 2011


Confirmed by Complementary Work

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P. Huber, Phys. Rev. C84, 024617(2011)

• Confirms global increase of predicted spectrum• Fixes remaining oscillations of first re-evaluation• Extra slope correction

New conv. with “extra correction”New conv. P. HuberNew conv. T.A. Mueller et al.ILL conversion

• Revisit conversion procedure of ILL data with minimal use of nuclear data.


Two Predictions in Agreement

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• L0 correction not properly implemented in Mueller et al. re-evaluation.• Different expressions of C(Z,W) term.

The two published predictions contain the same physics.Difference in global slope is now understood:

T.A. Mueller, D. Lhuillier


Updated Reactor Anomaly

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White paper: K. N. Abazajian et al. , hep-ph/1204.5379

7% deficit~4% from reactors1% from tn, 1% from off-eq1% from previous deficit

Reactor + Gallium anomaly


Normalization Uncertainty

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Normalization of ILL data

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207Pb(n, g)208Pb115In(n, g)116mIn

113Cd(n,g)114Cd

• Dominant systematic error : 1.8 % (1 s)

• Absolute calibration via internal conversion electron lines of known partial cross section per neutron capture

• Correlated across all energy bins of all isotopes directly propagates into the converted antineutrino spectra.

& : absolute normalization

: relative normalization


Independent Norm. at 2% level?

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2. 10-2 b/fission in 250keV bin centered on 6 MeV:

• The required few 106 fissions could be recorded with less intense neutron source (away from reactor background reduction).

• Same calibration using internal conversion line + Precise counting with evt by evt fission tagging?

• Magnetic design for e- detection in large solid angle, and well controlled energy resolution.

• No off-equilibrium corrections

Potential large impact on reactor anomaly…


Shape Uncertainty

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Weak Magnetism

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Measurement of dWM through transitions in isobaric analog states:

Approximate expression neglecting nuclear structure:

CVC symmetry

Magnetic dipole M1 g decay width dWM


Weak Magnetism

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Contribution of large ft’s in fission neutrino spectra?

Experimental slope in good agreement for transitions with low ft values

, assume 100% error

P. Huber, Phys. Rev. C84, 024617(2011)


Weak Magnetism

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• High log(ft) values have a sizeable contribution to fission n spectra across all E range (reflects the large contribution of forbidden decays)

• CVC still valid for very large ft transition which have tiny overlap of wave functions in b-decay?

Relative contribution


Weak Magnetism

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A factor 4 increase of dWM would compensate the +4% deviation from ILL spectra, but also induce a large negative tilt.

Reanalysis of Bugey3 data may already constrain the weak magn slope at this level.

Upcoming data from DayBay, Reno and Double Chooz near detectors have the potential to confirm the current shape uncertainty! • Estimated shape uncertainty with ~3.5 105

accumulated neutrinos, dEscale = 0.5% and ~4% 9Li background.


Shape Uncertainty

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• Correlated across all energy bins and isotopes

favorable to non-proliferation and oscillation search


New Data

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Updated Ab Initio Predictions

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• Correct for Pandemonium effect by using Total Absorption g-ray Spectrometry (TAS)

• Short list of fission products contributing by more than 4% to 2-6MeV bins.

A. Algora et al., Phys. Rev. Lett. 105, 202501 (2010)


Updated Ab Initio Predictions

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M. Fallot et al. , nucl-ex/1208.3877

• Inclusion of 7 new Tc, Mo and Nb isotopes.

• Sizeable correction of predicted spectra within ±10% of new converted spectra.

• Analysis and new measurements on-going.

• Same uncertainties on slope factors beyond databases systematics.

• Toward a competitive prediction of absolute normalization?


New 238U data

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• Anchor point of 235U measurement:- Background well understood. - Comparison to ILL spectrum cancels most uncertainties.

N. Haag, K. Schrekenbach et al.

New 238U data @ FRMII reactor, Munich

n flux

Natural U foil b

MWCPlastic

Scintillator


New 238U Data

Honolulu - AAP 2012

• Spread of ab Initio flux predictions in the ±10% range Possible update of the reactor anomaly at 1% level with sligthly improved total uncertainty

• N. Haag Ph.D. thesis to be published by end of this year.• Final measurement of 238 U ranges from 2.0 MeV to 6.5 MeV with relative

errors of 5% - 15%


Conclusions

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Demonstrated bias in the conversion procedure of the ILL data.

+4% increase of detected rate is confirmed by two independent works.

+1% off-equilibrium correction of reactor simulation + 1% updated neutron life-time + 1% previous mean shift with old spectra

~7% deficit of the reactor anomaly


Conclusions

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Application to reactor surveillance: - Main uncertainties are common to all isotopes suppressing their impact on relative monitoring - Short baseline oscillations to be tested soon. n rate at one location is affected through U-Pu shape difference only.

Challenging independent Xcheck of spectra normalization

New data coming soon to consolidate the current error budget:

- 238U Spectrum from FRM II- New shape envelope to be measured by near detectors of currently running reactor experiments.


Back Up Slides

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Complementary Approaches

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Ab initio calculations Conversion of total b spectra

Complete simulation of core evolution

- Fuel loading, geometry, n-capture and fission physics Fission product inventory

Description of all b-decays

- Nuclear databases- Fermi theory + corrections- Nuclear models

b and n total spectra from some 104 b-branches.

Total b spectra of fissile isotopes measured at ILL in the 80’s

Accurate reference electron spectra

Conversion to antineutrinos

- Use of “virtual” b-branches- Fermi theory + corrections- Control of approximations

Reference n spectra per isotope to be combined with prediction of fissions rates.


Ingredients

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Sum of all fission products’ activities

Sum of all β-branches of each fission product

Theory of β-decay


Status of Spectra Rate

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Linear trend with slope ≤1%/MeV enhances this increase in detected rate

Emitted Flux Detected Rate

Shift (%) Signif. (s) Shift (%) Signif. (s)235U 2.4 3.7 3.7 2.4239Pu 2.9 3.9 4.2 2.8241Pu 3.2 4.0 4.7 3.0

Final difference with respect to ILL n spectra :

Global rate increase


Finite Size Correction of Weak Interaction

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• Include the envelope of various calculations as extra uncertainty?• Would reach ~2% at 7 MeV.

D. Wilkinson, Nucl. Phys. Inst. and Meth. A 290, 509 (1990)

Different expressions of C(Z,W) term:

P. Vogel, Phys. Rev. D29, 1918 (1984).


Uncertainties

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Finite size corrections could be further studies by combining nuclear model and lepton scattering data. Not dominant uncertainty.

Conversion of b spectra

Current treatment of weak magnetism neglect the nuclear structure.- Dominant shape uncertainty, 100% error assumed.- Underestimated uncertainty?

Normalization: - Common to all predictions.- Currently 1.8% at 1 .s- A new measurement should target 1%. Not likely to happen.

Quadratically stacked errors


Shape Evolution Along a Reactor Cycle

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• Expected shape rotation on top of flux reduction • But significant measurement requires very large statistics on month time scale.


Total Reactor Spectrum

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ak = fission fraction, Sk(W)=Reference spectrum

• Prediction of ak(t)

• 238U couldn’t be measured at ILL because it undergoes fission in fast neutron flux.

• Extrapolation of the ILL spectra to commercial reactors?

Half way through:

Ab initio approach addresses it all


Ab initio

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• Total error in the 10-20% range. Dominated by systematics of nuclear databases.

• Build as complete as possible nuclear database coupled to MCNP Utility for Reactor Evolution (*) full core inventory

• 235U spectrum matches the ILL data at ~10% level

Deviation from Vogel et al.,Phys. Rev. C24, 1543 (1981)

• 238U prediction 10% higher than previous estimate

ENSDF onlyPandemonium corr.Add gross theory

Phys. Rev. C83,054615 (2011)

(*) http://www.nea.fr/tools/abstract/detail/nea-1845.


Ab initio

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ILL reference data are photos of b decays after 12-36h irradiation time.

Long-lived isotopes, dominant at low energy, keep accumulating over several weeks.

Sizeable correction to ILL data in below 3 MeV; +1% total detected flux

ILL conditions

See poster 146, A., Reactor and antineutrino spectrum calculation for the Double Chooz first phase result

Off-eq. correction as computed by the MURE

C. Jones et al. arXiv:nucl-ex/1109.5379v1

ILL conditions

Study of relative spectrum variations to reach equilibrium

[53] = Chooz paper


Error Budget

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235U 239Pu 241Pu

Phys. Rev. C83,054615 (2011)

D. Lhuillier - CEA Saclay 46Honolulu - AAP 2012

Error Budget

Phys. Rev. C84, 024617(2011)


Bugey

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prediction of reactor neutrino spectra david lhuillier cea saclay - france

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