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Abstract:
The reactor antineutrino experiment Double Chooz aims to provide a precise measurement
of the neutrino mixing angle θ13. In the analysis with one detector,
accuracy in the predicted neutrino spectrum from simulation is a necessity with
regard to normalization and energy shape. The detection efficiency of neutron
events, which are part of the coincidence signal created by neutrinos, introduce
the largest uncertainty contribution of the normalization of the experiment related
to the signal detection. In order to accomplish a matching of the efficiencies observed
in data and simulation, a correction of the Monte Carlo normalization and
an associated systematic uncertainty are inputs in the θ13 analysis. Calibration
source deployments in the inner two detector volumes allow for a measurement of
the neutron detection efficiency using 252Cf fission neutrons. New methods enable
to compute the correction integrated over the whole volume and the corresponding
uncertainty of the selection cut related efficiency. With these revised approaches a
factor two improvement in the detection efficiency uncertainty was achieved. The
correction of the neutron capture fraction – the capture fraction quantifies the
proportion of captures on a particular element – is evaluated and tested for its
robustness. Furthermore, a crosscheck of this quantity is discussed using neutrons
produced by cosmic muon spallation. Finally, the uncertainty on border effects,
emerging from neutron migration at the fiducial volume boundaries, is estimated
by means of different Monte Carlo configurations with varying parameters and
neutron physics modelings.
