1. Cumulated beta spectrum measurements of fission products
at ILL and FRM II
How precisely do we know the antineutrino source spectrum from a nuclear reactor?
Klaus Schreckenbach
(TU München)
Klaus Schreckenbach
SNAC11 Sept.2011

2. 1) Origin of reactor antineutrinos
2) Experimental determination of the source spectrum
Measurements of Nß in at ILL with ’BILL’ (235U, 239Pu, 241Pu thermal neutron induced fission)
Re-examination of the ILL results
Measurement of Nß from fast neutron fission of 238U at FRM II (2010)
Conversion into the correlated Nν spectrum
7) Discussion of the recently revised conversion procedures
8) Conclusion

3. Origin of the reactor antineutrinos
-> antineutrinos from the beta decaying fission products 0…10 MeV
antineutrinos/s for 1MW reactor power
Research reactors with uranium highly enriched in 235U: pure spectrum from 235U fission
Power reactor: composition of several fissile material
Typical composition of a PWR fuel
example Bugey reactor (average over measurement periode „Bugey 4“ (PL B338(1994)383):
fissile isotope
fraction of fission fk
σf (v p -> e+ n) per fission in cm2
fk ∙ σ
235U
54 %
6.6
58.4 %
239Pu
33 %
4.3
23.2 %
238U
7.8 %
10.1
12.9 %
241Pu
5.6 %
6.0
5.5 %
Illustration of the detected rates for fission neutrinos using the v p -> e+ n reaction
(From Th.A.Mueller,D.Lhuillier et al, PRC63(2011)054615)

4. 2) Experimental determination of the Nv source spectrum
measurement of the cumulated beta spectrum in units of betas per fission and MeV for individual fissile isotopes (online measurement with fissile sample exposed to neutrons)
deduction of the correlated Nv spectrum from the measured beta spectrum
Emitted antineutrinos from the reactor core according to the known composition and power of the reactor core
Challenge of the procedure:
filter the betas from the gamma and fission neutron background
absolute calibration of the spectrum per fission in the range 2- 9 MeV
intensity response function of the detection device
conversion Nß -> Nv ; depends on shape of beta branches involved (nuclear charge ,…)
Details of the reactor core
Precise experiments with the BILL magnetic spectrometer at ILL and conversion into Nv, used as base till recently
Recent experiment on 238U with a beta telescope at FRM II (2010)
Revised conversion procedure Th.A.Mueller, D. Lhuillier et al, PR C63(2011) P. Huber, PRC in press, arXiv: v3

5. 3) Measurements on beta spectra with the magnetic spectrometer ‘BILL’
at the ILL High Flux Research Reactor
Exposure time to constant neutron flux more than 12 …36 h
235U at full reactor power of 57 MW PL 99B(1981)251
(235U at 8 MW) test
235U at 4 MW PL 160B(1985)325 (final on 235U)
239Pu full power PL 118B(1982)162, final Nv PL 218B(1989)365
241Pu full power PL 218B(1989) (final)

6. target site of the BILL spectrometer, ILL
measurements at full (57 MW) and 4 MW reactor power

7. Magnetic BILL spectrometer at ILL, 1972-1991
(Electron detector in focal plane: multi chamber proportional counter in transmission, rear mounted scintillator in coincidence)

8. Achieved absolute precision for 235U: 3% at 90% CL
Absolute calibration Nß in betas per fission and MeV via internal conversion electron lines of known partial cross section per neutron capture:
Achieved absolute precision for 235U: 3% at 90% CL
(for instance 207PB calib.point, 90% CL: line fit 2.7%, masses 0.5%, cross sections 1…1.5%, ICC 1%, n flux stability 1.3%)
207Pb(n,γ)208Pb σ absolut
115In(n,γ)116mIn (ß decay) σ absolut
113Cd(n,γ)114Cd relativ
(50 keV bins)

9. Ratio of beta spectra from different isotopes following (nth,f)
(This work: ILL data)

10. 4) Re-examination of the ILL results
Calibration standards used -> no significant changes in cross section values since the 80ties
Contribution by internal conversion electrons from gamma transitions and gamma self absorption in the target -> less than 0.2% of Nß
Different reactor power -> different target and different signal to background ratio
Comparison to other measurements

11. Signal to background conditions for the two different 235U experiments at 57 MW and 4 MW reactor power at ILL
Ratio of Nß between the two measurements
Uncertainty for the ratio from absolute calibrations: 4 % at 68% CL
Statistical uncertainty <1% below 7 MeV

12. Comparison with other experiments for 235U fission
Carter: Phys.Rev 113(1959) beta telescope (wire chamber+scintillator)
Tsoulfanidis: Nucl. Scien. Eng. 43(1971)42 beta telescope (2 scintillators)
Borovoi: Sov.J.Phys. 37(1983) rotating target + beta telescope (2 scintillators)

13. 4) Measurement of Nß from fast neutron fission of 238U at FRM II (2010)
(Nils Haag et al., 2010/2011)
Technique (beta telescope similar to Carter et al.!):
Irradiate natural U-foil (0.7% 235U) with fast neutrons and detect on-line the beta spectrum with a beta telescope: U fission
Irradiate identical foil with thermal neutrons in identical setup: 235U fission
Compare 235U fissions spectrum with the BILL measurement
Extract the detector response function from this comparison
Measure relative amount of fissions in both targets via off-line
gamma-spectroscopy of fission products in the irradiated foils
(For instance 1596 keV gamma line in 1.7 d 140La beta decay, well known fission yields)
Most systematical uncertainties vanish (detection efficiency, absolute neutron flux, fission cross section, …)
Envisaged absolute precision 5% (work in progress)
Set-up of the beta telescope at the FRM II fission neutron beam
(target 2.5x2.5 cm2 in 107 fission neutrons/cm2s beam)

14. 235U thermal neutron fission
Events rates with the beta telescope at the FRM II experiment 2010
238U fast neutron fission

15. 5) Conversion into the correlated antineutrino spectrum
method used with ILL data:
Description of the measured beta spectra by about 30 virtual branches (allowed beta decay shape) of different endpoint energy and intensity, endpoint energy E0 dependent mean Z values and radiative correction
antineutrino spectrum of the individual virtual branches by mirror in E0 = Eß + Ev
Sum-up of these antineutrino branches
Smoothing over 250 keV
Global correction due to coulomb and weak magnetism term (P. Vogel PRD 29(1984)1918)

16. Illustration of one step in the deconvolution of the experimental Nß into virtual branches (as Kurie plot)
- Pi(Eo(i)) :

17. 235U data
conversion 1985
(total error in Nv 90% C.L.)

18. 6) Discussion of the revised Nv
as published recently: a)Th.A.Mueller, D. Lhuillier et al., PR C63(2011)054615
G.Mention et al.,Phys. Rev. D83(2011)073006
b) P.Huber, PRC in press, arXiv: v3
The conversion results of were well reproduced with those earlier Z distribution and beta shapes.
New in a) and b):
Weak magnetism term revised in beta shape
Better knowledge of the Z distribution of the involved beta decays
In addition: by a)
Description of the measured BILL integral beta spectrum to about 90% with real branches (tremendous work!) and 10% virtual branches to fill up the spectrum to equal the BILL result.
Nuclear charge Z, radiative correction and A_CW term included at the level of each branch
Correction for long irradiation times (about 1%)
New prediction for 238U (10% higher)
In addition by b)
detailed error propagation in the conversion
Detailed discussion of the weak magnetism term
Radiative correction in neutrino spectrum included (rather small effect)

19. Result by (a) and (b) similar:
differences to previous conversion, although using the same ILL beta spectra:
Few percent higher Nv intensities
increasing difference with energy
predicted cross section per fission for v p -> e+ n :
higher by 2.5…3.7 % for 235 U, 3.2 … 4.2 % for 239Pu, 3.9 … 4.7 % for 241Pu

20. Personal view for the bugey 4 result
(integral cross section per fission for ):
from event rate in neutrino detector; 15 m distant from reactor core
σmeasured. = ± 1.4% cm2 per fission (Y.Declais et al., Phys.Lett. B338(1994)383)
Predictions for no-oscillation for bugey4 reactor core, using beta spectra+conversion:
old: by bugey4 group; ref(a) Mueller al.; Mention et al; ref(b) P. Huber
σold = 5.82 ± 2.7% (bugey group 1994)
σ(a) = ± 2.7% with new neutron lifetime value (2011) :
σ(b) = ± 2.7% with new neutron lifetime value (2011) :
As I understand: claimed changes on σold by ref(a),(b):
A_CW … 2 % uncertain theory!
detailed Z …2 % save
n lifetime ,2% (with 2010 value); 0,64% with save
238U % only from nuclear libraries
off equilibrium …1.0% save, although from libraries
___________________________________________________________________________________________________________________________________________
total change to 1994 including new : with ref(a) %; with ref(b) +6.4%
Rold = %
Ra = %
Rb = %
Ratio measured in bugey4 to predicted:
(1 σ errors)

21. 7) Conclusion
beta spectra:
calibration σcalpartiell of BILL response function and fission cross sections used:
no significant changes since the 80ties (confusion 207Pb(n,γ)?)
238U missing data for experimental integral beta spectrum:
FRM II result will come up; precision?
Conversion in antineutinos:
Beta shapes due to Z dependence well treated by new approach!
uncertainty of weak magnetism term may be underestimated (see (b) P. Huber )
previous conversion results changed by about the quoted uncertainty
Neutron life time in antineutrino cross section: use new PFG 2011 value 881.7(1.4) s
=> Reasonable new evaluation for the predicted source Nv spectra from fission, still based on the BILL Nß results of the 80ties