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Patrick Huber: Reactor Neutrinos: Fluxes and US Plans

March 13, 2013

The speaker discussed in detail the issue of determining the flux of neutrinos from reactor cores, and his studies on the matter.

He started with an overview of the basic processes undergoing in the reactor. Neutrinos come from the fission products of 235U. These have average masses of A=94 and 140, Zr and Ce. Together there are 142 neutrons, so six beta decays will occur, yielding six antineutrinos. Two will be above the inverse beta decay threshold energy which is the point above which one can detect them by this method. The problem is computing the number and spectrum above that threshold.

Direct beta spectroscopy of single nuclei will never be complete, and even then one has to untangle the various branches. Gamma spectroscopy yields energy levels and branching fractions, but this is with some limitations. There are degeneracies and this leads to the so-called “pandemonium effect”.

So you do electron spectroscopy with a magnetic spectrometer of everything coming out of a 235U foil inside the high-flux reactor at ILL, you can get the energy spectrum of beta rays measured precisely. The energy uncertainty is very small. If this result were wrong, all reactor neutrino results would be wrong, because they rely on it heavily. The spectrum is the one shown below.

betaspectrum

The total beta spectrum is a sum of all the decay branches. It is a Fredholm integral equation of the first kind. Solutions tend to oscillate, and this needs a regulator. The approach is the basis for “virtual branches” and it is used in the modern calculations as well. It allows to determine the end points of the spectra. Then however there are many corrections to the shape of the beta spectrum; they have a linear slope in energy, and they need to be accounted for. Many depend on the nuclear charge Z, which you can take from a nuclear database. For forbidden decays many of these corrections are not known, so they lead to a potentially large uncertainty.

The result of the analysis shows a flux which is larger than other measurements at high energy. The differences come mainly from the nuclear charge distributions.

Although the uranium flux is the most important, other fluxes also play a role. The ones from plutonium are less well determined. For uranium 235 the theory errors below 3.5 MeV are below 0.5%; experimental errors can be improved by repeating the ILL measurements.

The breakdown of uncertainties as a function of energy can be shown in the graph shown below.

tunnel_uncertainties

The speaker summarized that the reactor anomaly is a 6-7% effect. A fourth of that effect stems from the neutron lifetime. Another quarter from non-equilibrium corrections, which depends on detailed fuel history, and well-understood nuclear physics. Half, though stems from the new fluxes. If we want to work on that problem we need to improve the beta spectra for all four fissile isotopes. 25x statistics is required, while energy resolution at 50 keV is sufficient. Future reactor shape measurements are difficult but worthwhile.

Then he discussed the US plans. Research reactors are favoured because of the smaller core dimensions. One has lower rates, partly compensated by the shorter baseline; and the operation is at the surface. Three sites are under consideration: ATR, NIST, and HFIR.

It would be best to use multiple detectors. They can cover a bigger range in baselines. Backgrounds will be reactor related gammas and neutrons. Also cosmic radiation induced neutrons. The strategies for mitigation include shielding, segmentation, and 6Li doping, because the neutron capture signature spatially contained. Simulations are in place to study these issues. A sample experimental layout was shown. A near detector at 5 m, the far one at 18 meters from the core.

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