Non-equilibrium Antineutrino spectrum from a Nuclear reactor

1
Non-equilibrium Antineutrino spectrum from a
Nuclear reactor

• We consider the evolution of the reactor
  antineutrino energy spectrum during the periods
  of reactor ON and OFF.
• The calculations show that antineutrino spectrum
  never comes to equilibrium state during reactor
  operation run.
• The existing method of experimental data analysis
  can be improved to increase an accuracy of
  predicted values in an experiment.

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Antineutrino energy spectrum
Magnetic moment
70
30
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Soft part of antineutrino spectrumE lt 1.5 MeV
4
Recoil electrons spectra for magnetic and weak
scattering of reactor antineutrinos
3
2
1
Digits are values of magnetic moment
5
Standard approach to get a reactor spectrum and a
cross section for inverse beta-decay (E gt 1.8
MeV)

• Traditionally assumed that energy spectrum
  comes to saturation in one day after reactor
  starts and falls down to zero in one day after
  reactor shuts down. In this approach the
  saturated spectrum is found as a sum of partial
  spectra ri (E) of the four fission isotopes
• r(E) S airi (E), i labels all
  isotopes
• (235U, 238U, 239Pu, 241Pu)
• K.Schreckenbach et al., Phys.Lett.B160, 325
  (1985)
• A.Hahn et al., Phys.Lett.B218, 365 (1989)
• P.Vogel et al., Phys.Rev. C24, 1543 (1981)
• sV-A S aisi , where si ?s(E)ri (E)dE,
• ai - is a part of fissions.

• The best measured cross section (Bugey, 1994 y.)
• smeas 5.750?10-43 cm2/fiss.?1.4, (68 C.L.)
• with a5 0.538, a9 0.328, a8 0.078, a1
  0.056
• smeas /sV-A 0.987 ?1.4, (exp.) ?2.7, (V-A)

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Inclusion of non equilibrium effects

• In reality the spectrum does not reach the
  equilibrium due to two effects
• Long lived fission fragments accumulating.
• Formation of beta-emitters due to neutron
  radiative capture in fission fragments in the
  reactor core.
• r(E,t) Fr(E,t) Cr(E,t)
• For calculation we used the data base including
  571 isotopes with yields more than 10-6 per
  fission.

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Ratios of the current spectra to that at the end
of the 330-day run for reactor ON and OFF periods
235U
8
Ratios of the current recoil electronspectra to
the spectrum at the end of the 330-day run for
reactor ON and OFF periods
235U
Solid lines for weak, dashed for magnetic
scattering
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The time evolution of spectra during reactor ON
period.
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Residual Antineutrino emission during reactor
OFF period
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Some of the Long lived fragments
In first 10 days 97Zr(Emax 1.922 MeV), 132I
(Emax 2.140 MeV) and 93Y(Emax 2.890 MeV)
become in equilibrium and later increase will be
defined by halflifes of 106Ru and 144Ce
Last isotope is 90Y(T1/2 64 h, Emax 2.279
MeV), that is in equilibrium with its long lived
predecessor 90Sr (T1/2 28.6 years)
Yields of long lived fragments ()
 
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The increase of inverse beta-decay cross
sections for uranium and plutonium spectra
during reactor ON period
235U
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The decrease of inverse beta-decay cross
sections for uranium and plutonium antineutrino
spectra during reactor OFF period (2 years of
irradiation)
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The increase of C?(E) component of antineutrino
spectrum due to the capture of neutrons by
fission fragments in PWR-1000 reactor.
103Rh, 147Pm
109Ag
99Tc
1 the beginning, 2 the middle and 3 the
end of reactor run.
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Conclusion

• In contrast to the standard approach

• The antineutrino spectrum in the region E gt 1.8
  MeV has a non equilibrium component with
  relaxation time exceeding the reactor
  operational time.
• It is necessary to take into account the
  additional neutrino emission due to accumulation
  of long lived fragments and neutron capture by
  fragments in a reactor core.
• One needs to know the prehistory of the fuel to
  account precisely residual antineutrino emission.
• The resulting corrections are relatively small
  but not negligible. In the antineutrino energy
  range 1.8 - 3.5 MeV the relative contribution of
  the additional radiation is about 2-4 that is
  greater than the ILL spectra uncertainty.